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TITLE Aerographers Mate 1 & C: Rate Training Manual. INSTITUTION Naval Training Command, Pensacola, Fla. REPORT NO NAVEDTRA-10362-B PUB DATE 74 NOTE 665p. AVAILABLE FROM Superintendent of Documents, U. S. Government Printing Office, Washington, D. C. 20402 (Stock No. 0502-051-8110)

EDRS PRICE MF-$ 1.08 HC-$33.64 PLUS POSTAGE DESCRIPTORS Climatic Factors; Equipment Maintenance; *Job Training; *Manuals; Measurement Instruments; *Meteorology; *Oceanology; Study Guides

ABSTRACT One of the series for enlisted Navy and Naval Reserve personnel, the training manual prepares students for advancement within their rating, aids in on-the-job training, or serves as a review source. Charts, graphs, tables, maps, and illustrations supplement detailed text covering atmospheric physics and world climatology, air masses, circulation, and frontal systems. Several chapters focus on forecasting upper air systems, surface systems, cloud conditions, precipitation, temperature, storms,icing, fog, and so forth. Tropical conditions are givenspeical treatment, and several chapters deal with weather briefing, flightforecasting, sea surface and ocean thermal structure forecasting, and otherspecial forecasts. Three chapters examine meteorological equipment and maintenance, and a final chapter covers administration,training, and communications. (i4DW) AEROGRAPHEL, MA.E 1 &, NAVAL EDUCATION & TRAINING COMMAND RATE TRAINING MANUAL NAVEDTRA /0'62-B

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OF HEALTH. U S DEPARTMENT&WELFARE EDUCATION OF NATIONAL INSTITUTE EDUCATIONBEEN REPRO UMCNT HAS FROM (WS DCA TLY AS RICEIVE0 OUCED EXAC ORORDANI/ATiONORIDIN rr.E PERSONPOUT s or VIEWOR OPINIONS A (IND It NECESSARILY REPRE STATED DO NO, Or NATIONAL INSTITUTE SENT OFFICIALPOI,' I ION 0 RPOLICY ( OUT Alton PREFACE 6E13 171975

This Rate Training Manual is one of a series of training manuals prepared for enlisted personnel of the Navy and Naval Reserve who are studying for advancement in theAerographer's Mate (AG) rating. As indicated by thetitle,themanual is based on the professional qualifications for the rates AG1 and AGC, as set forth in the Manual of Qualifications for Advancement, NavPers 18068(Series). Combined with the necessary practical experience and a thorough knowledge of the material contained in AG 3 & 2, NavTra 10363-D, completion of the correspondence course based on this manual will greatly assist the AG2 and AG1 in preparing for advancement examinations. This training manual should also be valuable as a review source for the AGC who is studyingfor advancement to AGCS and the AGCS who is studying for AGCM.In addition, its everyday use in on-the-job training is highly recommended. Thistraining manual was prepared by the NavalEducation and Training Program Development Center, Pensacola,Florida, for the Chief of Naval Education and TrainingSupport.Technical review of the manuscript was provided bypersonnel of the AG(B) School, NATTC Lakehurst,N. J. Technical review and assistance wasalso provided by personnel of the Naval Weather ServiceCommand.

1974 Edition

Published by NAVAL EDUCATION AND TRAINING SUPPORTCOMMAND

Stock Ordering No. 0502-051-8110

UNITED STATES GOVERNMENT PRINTING OFFICE WASHINGTON, D.C.: 1974

3 THE UNITED STATES NAVY

GUARDIAN OF OUR COUNTRY

The United States Navy is responsible for maintaining control of thesea and is a ready force on watch at home and overseas, capable of strong action to preserve the peace or of Instant offensive action towin in war. It is upon the maintenance of this control that our country's glorious future depends; the United States Navy exists to make itso. t. WF SERVE WITH HONOR

Tradition, valor, and victory are the Navy's heritage from thepast. To these may be added dedication, discipline, and vigilance as the watchwords of the present and the future.

At home or on distant stations we serve with pride, confidentin the respect of our country, our shipmates, and our families.

Our responsibilities sober us; our adversities strengthen us.

Service to God and Country is our special privilege. We serve with honor.

THE FUTURE OF THE NAVY The Navy will always employ new weapons, new techniques, and greater power to protect and defend the United States on the sea, under the sea, and in the air. Now and in the future, control of the sea gives the United States her greatest advantage for the maintenance of peace and for victory inwar. Mobility, surprise, dispersal. and offensive power are the keynotes of the new Navy.The roots of the Navy he in a strong belief in the future, in continued dedication to our tasks, and in reflection on our heritage from the past. Never have our opportunities and our responsibilities been greater.

ii 4 CONTENTS

Chapter Page

1. Aerographer's Mate rating 1

2. World climate and weather 8

3. Atmospheric physics 40

4. Atmospheric circulation 62

5. Air masses, fronts, and cyclones 117

6. Surface weather map analysis 171

7. Upper air analysis 212

8. Forecasting upper air systems 261

9. Forecasting surface systems 278

10. Cloudiness, precipitation, and temperature forecasting. 306

11. Forecasting thunderstorms, fog, tornadoes, icing, and contrails 357

12. Tropical analysis and forecasting 413

13, Weather briefing and flight forecasting 470

14. Sea surface forecasting 493

15. Ocean thermal structure forecasting and ASWEPS 545

16. Special observations and forecasts 553

17. Maintenance of meteorological equipment 590

18. Maintenance of augmenting meteorological oceanographic equipment 604

19. Radiosonde and rawinsonde equipment maintenance and calibration 632

20. Administration, training, and communications 643

Index 652

iii 5 CREDIT LIST

Credit is given to the followingsources for permission to use copyright material contained as indicated in the chaptersof this manual.

I. American Meteorological Society, Boston, Mass.,and authors Wayne Herring and Wayne Mount for permissionto use the technique for predicting 30-hour movement of northeastward movingcyclones from "Evaluation of T;chniques for Predicting the Displacementof Northeastward Moving Cyclones," publishing in the February 1960issue of the Bulletin of the American Meteorological Society. This informationappears in chapter 9, pages 286 through 288.

2. Academic Press, New York, N.Y., for permissionto use information on the prediction of air mass thunderstorms, predictionof intensity of possible turbulence in thunderstorms, fronts, and line squalls,and forecasting surface gusts from thunderstorms contained in chapter II, pages 361 and 365. This information is based on material from "WeatherForecasting for Aero- nautics," J. J. George and associates, Academic Press,New York, N.Y.

3. F. Singleton and B. G. Wales-Smith, BritishMeteorological Service, and the 2nd Weather Wing, Headquarters USAF forpermissiol to use "A Cirrus Forecasting Technique," from "The MeteorologicalMagazine," No. 1,053, Vol, 89, April I 960 issue. The information extractedfrom this publication is contained on pages 333 through 336 of chapter 10.

4. Further credit is given to Professor W. D. Duthiefor background material for chapters 5, 6, and 7 from "Noteson Analysis of Weather Charts," by W. D. Duthie, Department of Meteorology, U.S.Navy Postgraduate School, Monterey, Calif.

iv CHAPTER 1

AEROGRAPHER'S MATE RATING

This training manual is designed to aid the Subdivisions of certain general ratings are AG2 in preparing for advancement to AG I and identified as SERVICE RATINGS. These service the AG Iin preparing fa advancement to AGC. ratings identify areas of specialization within the It is based primarily on the professional require- scope of a general rating. Service ratings are ments or qualifications for AG1 and AGC, as established in those general ratingsin which specifiedin the Manual for Qualifications for specialization is essential for efficient utilization Advancement, NAVPERS 18068 -C. of personnel. Although service ratings can exist In preparing for the advancement examina- at any petty officer level, they are most com- tion, this manual should be studied in conjunc- mon at the P03 and P02 levels. Both Regular tion with the manual, Military Requirements for Navy and Naval Reserve personnel may hold Petty Officers 1 & C, NavTra 10057 -C. service ratings The intent of this chapteristo provide information on the enlisted rating structure, the AEROGRAPHER'S MATE RATING AG rating, requirements and procedures for advancement, and references that will help you The Aerographer's Mate rating is divided into in performing your duties as an Aerographer's six rates or pay grades. It consists of just one Mate. This chapter also includes information on ratingthe general rating. The general rate is the how to make the best use of Rate Training pay grade level within the general rating. There Manuals. It is therefore strongly recommended is no service rating provided for Aerographer's that you study this chapter carefully before Mates. beginning intensive study of the remainder of Figure I-I illustrates all paths of advancement the manual. for an Airman Recruit (AR) to Master Chief Aerographer's Mate (AGCM), or toLimited ENLISTED RATING STRUCTURE Duty Officer (LDO). Shaded areasindicate career stages where qualified enlisted men may The present enlisted rating structure includes advance to Warrant Officer (W-1), and selected two types of ratings. general ratings and service Warrant Officers may advance to Limited Duty ratings. Officer. GENERAL RATINGS are designed to provide Through the AG2 level, Aerographer's Mates paths of advancement and career development. are generally utilized in a variety of billets but A general rating identifies a broad occupational are for the most part considered to be observer field of related duties and functions requiring or plotters. With the advancement to the AG1 similaraptitudes and qualifications. General level their duties will change to the extent that ratingsprovidethe primary means usedto they will act in the capacity of section leaders or identify billet requirements and personnel quali- assistants to the forecaster. It is also at this level fications. Some general ratings include service that personnel, under supervision, will produce ratings; others do not. Both Regular Navy and extended forecasts and other related products. Naval Reserve personnel may hold general rat- With advancement to AGC, personnel are ings. expected to be proficient forecasters, ajob they

I AEROGRAM IER'S MATE 1 & C

It should be pointed out that the successful completionof Aerographer's Mate, LOO Class B METEOROLOGY school is a requirement for advancement to AGC and for authorization to be a meteorological/ oceanographic forecaster. Forecaster qualifica- WARRANT OFFICER C w 0 \ CWO CWO tionsare coveredinNaval Weather Service W-1 W - 2 4VM W-3 W-4 Command Instruction 3140.5. \\\ The AG School located at the Naval Air Technical Training Center, Lakehurst, N.J. has a large number of instructor billets at both the AG 1 and AGC levels. Instructor billets are in the "A", "B", and "C" scUools. Billets for support personnel such as in the testing unit are also available. Instructor billets are normally assigned on a voluntary basis and may be requested by qualified personnel via normal procedures. A small number of AGC's may be assigned as Chief PettyOfficer-in-Charge(CPO-IN-C) at smaller NWSED's. Interesting and rewarding billets for personnel arefound atthe National Climatic Center, Asheville, N.C.; as aerial ice observers; and in a number of independent duty assignments on various staffs, ships, or stations. It is impossible to list all the variety of duties that AG l's and AGC's can expect or have the opportunity of filling. It is anticipated, however, AIRMAN E- 3 that with the intended shift to an all volunteer serviceincreasedauthority and responsibility will be placed on senior enlisted personnel. It AIRMAN APPRENTICE willbecomenecessarytoplace even more E- 2 emphasis on the selection of highly qualified personnel for advancement, and billet assign- AIRMAN RECRUIT ment. E-I The Aerographer's Mate detailer, assigned to AG.1 the Bureau of Naval Personnel, through the use Figure 1-1.Paths of advancement. of official newsletters, notices, personal contact, and the command master chief petty officer, strives to keep personnel informed of changes will continue to perform. They will also gain thatareanticipated(orthatoccurun- increased responsibility in the supervising of a expectedly), which will have an effect on per- larger number of personnel, or as in the case sonnelassignment.Inorder for your duty aboard ship be the division chief petty officer. preference to be known to your detailer, it is essentialthatyour Enlisted Duty Preference BILLET ASSIGNMENT form (NavPers 1306/63) be properly filled out, coded and submitted. In this manner personal A wide variety of assignments, in addition to considerations may be made by the detailer in the normal shore and sea duty billets, becomes filling the various billets available with qualified available at the AG1 and AGC level. personnel.

2 8 Chapter 1AEROGRAPHER'S MATE RATING

NAVY ENLISTED CLASSIFICATION In General Order No. 21, the Secretary of the Navy outlined some of the most important The Navy Enlisted Classification (NEC) codes aspects of navalleadership. Naval leadership are utilized to a small extent within the Aerogra- means the art of accomplishing theNavy's pher's Mate rating. mission through people. It is the sum of those The NEC 7414 is used to indicate the holder qualities of intellect, of human understanding, has completed the course of instruction of the and of moral character that enable a man to Rawin/Radiosonde Set Operators School, Class inspireandto manage a group of people C. This NEC can also be obtained through successfully.Effective leadership, therefore, is on-the-job training. based on personal example, good management The most important NEC at the AG2 and practices, and moral responsibility. The term AG 1level is 7412, which indicates the holder is leadership includes all three of these elements. a graduate of Aerographer's Mate Class B school. The current Navy Leadership Programis Holders of this code can be qualified as meteoro- designed to keep the spirit of General Order No. logical/oceanographicforecasters withintheir 21 ever before you. If the threefold objective is commands. carried out effectively in every command, the Navy Enlisted Classification codes are also program will make of you a better leader of men used to assign personnel to billets that require in your presentbilletandinyour future the special skills they have obtained. assignments. As you advance up the leadership ladder, more and more your worth to the Navy RESPONSIBILITY will be judged on the basis of the amount of efficient work you obtain from your subordi- At the AG 1 and AGC level personnel become nates rather than how much of the actual work acutely aware of the importance that weather you do yourself. plays in naval operations. It will be necessary to For information on the practical application provide or assistinproviding climatological of leadership and supervision, study Military statisticsto personnel or staffstasked with Requirements for Petty Officer1 & C, NavTra preparing operational plans and orders. As time 10057-C. for the commencement of operations draws As you study this material containing leader- near, the actual forecasts of expectedconditions ship traits, keep in mind that probably none of will be heavily relied upon for dictating the our most successful leaders possessed all of these manner in which the operations will be con- traits to a maximum degree, but a weakness in ducted. some traits was more than compensatedfor by Itisatthislevel, AG 1and AGC, that strength in others. Critical self-evaluation will individualsbegin tofeel how responsiblea enable you to realize the traits in which you are position theyhave. They now have a more strong, and to capitalize on them. At the same complete viewing of the overall picture. More time you must constantly strive to improve on complex questions than "What's the temperature the traits in which you are weak. out?"aredirectedtoward them. They are Your success as a leader will be decided, for accepted as the experts in their field. the most part, by your achievements in inspiring otherstolearnandperform. Thisisbest accomplished by personal example. LEADERSHIP Since you have been a petty officer for some ADVANCEMENT time, you realize that more leadership is required of the higher sates. Not only are you By this time, you are probably well aware of required to have superior knowledge, but you the personal advantages of advancementhigher are also required to have theability to handle pay, greater prestige, more interesting andchal- personnel. This ability increases in importance as lenging work, and the satisfaction of getting you advance through the various rates as apetty ahead in your chosen career. By this time, also, officer. you have probably discovered that oneof the

9 AEROGRAPHER'S MATE 1 C

most enduring rewards of advancement is the itdoes not guarantee, his advancement.Such training you acquire in the process of preparing factors asthescore made onthewritten for advancement. examination, length of time in service, perform- The Navy also profits by your advancement. ance marks, and quotas enter into the final Highly trained personnel are essentialto the determination of who will actually be advanced. functioning of the Navy. By advancement. you increase your value to the Navy in two ways: HOW TO PREPARE First, you become more valuable as a person FOR ADVANCEMENT who can supervise, lead,. and train others and second you become more valuable asa technical Preparation for advancement includes study- specialist and thus make far-reaching contribu- ing the qualifications, working on the practical tions to the entire Navy. factors. studying the required Rate Training Since you are studying for advancement to Manuals, ind studying any other material that P01 or CPO, you are probably already familiar may be specified. To prepareyourself for with the requirements and procedures for ad- advancement ortohelp others prepare for vancement. However, you may find it helpful to athanLement, you will need to be familiar with read the following sections. The Navy does not ( I)the "Quals" Manual, (2) the Record of stand still. Things change all the time. and itis Practical Factors, Na vEdIra 1414/1, (3)a NavEdTra possible that some of the requirements !taw publicationcalledBibliographyfor Advance- changed since the last time you went up for ment Study NavEdTra 10052 (Series), and (4) Rate advancement. Furthermore, you will be respon- Training Manuals. The following sections de- sible for training others for advancement. there- scribe these materials and give some information fore, you will need to know the requirements in on how to use_ them to the best advantage. some detail. "Quals" Manual HOW TO QUALIFY FOR ADVANCEMENT The Manual of Qualifications for Advance- ment, NavPers 18068 (Series), gives the mini- To qualify for advancement, a person must. mum requirements for advancement to each rate within each rating. This manual is usually called I.Have a certain amount of time in grade. the "Quals" Manual, and the qualifications 2. Complete the required military and profes- themselves are often called "quals." The qualifi- sional training manuals. cations are of two general types: (1) military 3. Demonstrate the ability to perform all the requirements, and (2) professional or technical PRACTICAL requirements for advancement by qualifications. Military requirements apply toall completing applicable portions of the Record of ratings rather than to any one rating alone. Practical Factors, NavEdIra 1 f 1 1 '1 AG. Professional qualifications are technicalor pro- 4. Berecommendedbyhiscommanding fessional requirements that are directly related officer. to the work of each rating. 5. Demonstrate his KNOWLEDGE by passing Both the military requirements and the pro- a written examination on (a) military require- fessional qualifications are divided into subject ments, and (b) professional qualifications. matter groups. Then. within each subject matter group, they are divided into PRACTICAL FAC- Remember that the requirements for advance- TORS and KNOWLEDGE FACTORS. ment can change. Check with your educational The qualificationsfor advancement and a services office to be sure that you know the bibliography of study materials are available in most recent requirements. your educational services office. The "Quals" When you are training lower rated personnel, Manual is changed more frequently than Rate it is a good idea to point out that advancement Training Manuals are revised. By the time you is not automatic. Meeting all the requirements are studying this training manual, the "quals" makes a person ELIGIBLE for advancement, but may have been changed. Never trust any set of Chapter IAEROGRAPHER'S MATE RATING

"quals"until you have checked the change thelate,reportthis fact to the supervising number against an UP-TO-DATE copy of the officer so that an appropriate entry can be made in the Record of Practical Factors. Quals" Manual. of In training others for advancement. emphasize When you are transferred, the Record these three points about the "quals": Practical Factors should be forwarded with your service record to your next duty station.It is a I. The "quals" are the MINIMUMrequire- good idea to check and be sure that this formis ments for advancement. Personnelwho study actually inserted in your service recordbefore MORE than the required minimum will have a you are transferred. Ifthe form is not in your great advantage when they take thewritten record, you may be required to start all over have examinations for advancement. again and requalify in practical factors that 2. Each "qual" has a designated rate level already been checked off. You should also take chief, first class, second class, or third class. You sonicresponsibilityforhelping lower rated factors are resporsible for meetingallquals" specified personnel keep track of their practical for theratelevelto which you arc seeking records when they are transferred. advancement AND all"quals" specifiedfor lower rate levels. NavEdTra 10052 3. The written examinations for advancement will contain questions relating to thepractical Bibliography for Advancement Study, NavEdTra factors AND to the knowledgefactors of BOTI 10052 (Series) is a very important publication the military requirements and theprofessional for anyone preparing for advancement.This qualifical ions. publicationlistsrequiredand recommended Rate Training Manuals andother reference for Record of Practical Factors material to be used by personnel working advancement. NavEdTra 10052(Series) is revised of Naval A specialform known as the Record of and issued once each year by the Chief Practical Factors, NavEdTra 1414/1, isused to Education and Training Support. Each revised record the satisfactory performanceof the prac- edition is identified by a letter following the tical factors. This form lists all mil;raryand all NavEdTra number. When using this publication, professional practical factors. Whenever a person be SURE you have the most recent edition. demonstrates his ability to perform apractical The required and recommended references are factor, appropriate entries must be madein the listed byratelevel in NavEdTra 10052 (Series). It DATE and INITIAL columns. As a POI orCPO, is important to remember that you are respon- levels, as you will often be required tocheck the practical ale for all references at lower rate factor performance of lower rated personneland well as those listed for the rate to which you are to report the results to yoursupervising officer. seeking advancement. As changes are made periodically tothe Rate Training Manuals that are markedwith "Quals" Manual, new forms of NavEdira1414/1 an asterisk (*) inNavEdTra 10052 (Series) are are provided when necessary.Extra space is MANDATORY at the indicated rate levels. A allowed on the Record of PracticalFactors for manila; y training manual may becompleted entering additional practical factors asthey are by (I) passing the appropriateNon-Resident publishes' in changes to the "Quals"Manual. Career Course that is based on the mandatory also provides training manual; (2) passing locallyprepared The Record of Practical Factors given in the space for recording demonstratedproficiency in tests basal on the information skills which are within the general scopeof the mandatory training manual; or (3) in some cases, rate but which are notidentified as minimum successfully completing an appropriateNavy this in school. qualifications for advancement. Keep do mind when you aretraining and supervising When training personnel for advaixement, other personnel. ;f a person demonstrates pro- not overlook thesection of NavEdTra 10052 ficiency in some skill which is notlisted in the (Series) whichlists the required and recom- "quals" but which is within the general scopeof mended references relating to the military re-

5 AEROGRAPHER'S MATE I & C quirements for advancement. All personnel must lar plan, looking at the illustrations complete the mandatory military and reading requirements bits here and thereas you see things that interest training manual for theappriate rate level you. before they can be eligibleto advance. Also, 3. Look at make sure that personnel thetraining manual inmore working for advance- detail, to see how it ment study the references which is organized. Look at the are listed as table of contents again. Then,chapter by chap- recommended but not mandatoryin NavEdTra ter, read the introduction, the 10052 (Series) may be used headings, and the as source material subheadings. This will give for the written examinations, you a pretty clear at the appropriate picture of the scope and levels. content of the manual. 4. When you have a generalidea of what is in the training manual and how it Rate Training Manuals is organized, fill in the details by intensivestudy. In each study period, try to cover There are two general a complete unitit may be a types of Rate T.aining chapter, a section ofa chapter, or a subsection. Manuals. Manuals (suchas this one) are prepared The amount of material for most enlisted rates and you can cover at one ratings, giving infor time will vary. Ifyou know the subject well, oration thatis directly related or to the profes- if the material iseasy, you can cover quite a lot sionalqualificationsfor advancement. Basic at one time. Difficultor unfamiliar material will manuals give information thatapplies to more require more study time. than one rate and rating. 5. In studying each unit, writedown ques- Rate Training Manualsare revised from time tions as they occur toyou. Many people find it to time to bring them up to date.The revision of helpful to make a written a Rate Trair'ng Manual is identified outline of the unit as by a letter they study, or at least to writedown the most following the NavEdTra number.You can tell important ideas. whether a Rate Training Manual is the latest 6. As you study, relate the information edition by checking the NavEdTra in the number (and the training manual to the knowledgeyou already letter following the number) inthe most recent have. When yo:: read abouta process, a skill, or edition of List of Training Manualsand Corres- a situation, ask yourselfsome questions. Does pondence Courses, NavEdTra 10061(Series). this information tie in.withpast experience? Or Rate Training Manualsare designed for the is this something special purpose of helping naval new and different? How does personnel pre- this information relateto the qualifications for pare for advancement. By this time,you have advancement? probably developed yourown way of studying 7. When you have finished these manuals. Some of the studying a unit, personnel you train, take time out tosee what you have learned. however, may need guidance in the use of Rate Look back overyour notes and questions. Training Manuals. Althoughthere is no single Without looking at the training "best" way to study manual, write a training manual, the down the main ideasyou have learned from following suggestions haveproved useful for studying this unit. Do not just many people. quote the book. If you cannot givethese ideas in your own words, the chancesare tha'ou have not really 1. Study the militaryrequirements and the mastered the information. professional qualifications foryour rate before 8. Use Non-Resident CareerCourses when- you study the training manual, andrefer to the ever you can. The non-resident "quals" frequentlyas you study. Remember, career courses are based on the Rate Training Manualsor other you are studying the trainingmanual primarily appropriate texts. As mentionedbefore, com- to meet these "quals." pletion of a mandatory Rate 2. Before you begin to study Training Manual any part of the can be accomplished by passing training manual intensively. a Non-Resident get acquainted with Career Course based the entire manual. Read the on the twining manual. preface and the You will probably find it helpfulto take other table of contents. Check throughthe index. non-resident career courses, Thumb through the manual without as well as those any particu- based on mandatory trainingmanuals. Taking a

6 Chapter 1AEROGRAPHER'S MATE RATING non-resident career course helps you master the subject to revision, make sure that you havethe information given in the training manual, and latest edition. When using any publication thatis also gives you an idea of how much you have kept current by means of changes, be sure you have learned. have a copy in which all official changes been made. A list of training manuals and publications SOURCES OF INFORMATION thatwill be helpfulasreferences and for additional study in preparing for advancementis As a P01 or CPO, you must have an extensive included in the reading list at the beginningof knowledge of the references to consult for this text. Additional training manuals that are accurate, authoritative, up-to-date information applicable areavailable through your educa- onallsubjects relatedtothe military and tional services officer. professional requirements for advancement. In addition to training manuals and publica- Publications mentioned inthis chapter are tions, training films furnish a valuable sourceof be subjectto change or revision from time to supplementary information. Films that may timesome at regular intervals, others as the helpful are listed in the U.S. Navy FilmCatalog, need arises. When using any publication that is Nav Air 10-1-777.

: 13 CHAPTER 2

WORLD CLIMATE AND WEATHER

When a forecaster limits himself toshortrange Physical Climatology considerations of the weather he is at thesame time limiting his forecast. Hemust not only This approach to climatology seeks to explain consider the present synoptic indications,but the causes of the differences in climate must carefully study the large-scale climatic in the light of the physical processes influencingcli- features and seasonal climatic changesas well. mate, and the processes producing the various The relationships between the currentmeteoro- kinds of physical climates suchas marine, deserts, logicai structure and the climatic backgroundof mountain, etc. the area are of great importance to thecritical forecaster. Descriptive Climatology This chapter will give you an introductionto climate, and the elements whichare uszd to Descriptive climatology typically orients itself assemble the data obtained and to classifythis interms of geographic regions andisalso data into climatic zones and types. Many other referredtoasregionalclimatology. A de- excellent texts and references are available fora scription of the various types of climates is made further study of the characteristics of climates onthebasisof analyzedstatistics from a and their importance in weather forecasting. particular area. A further attempt is madeto describe the interaction of weather and climatic CLIMATE AND CLIMATOLOGY elements upon the people and theareas under consideration. CLIM ATE

Climate is defined as the averageor collective Dynamic Climatology state of the earth's atmosphere at any given location or area within a specified period of This study of climate attempts torelate time. We think of weather as the day-to-day characteristics of the general circulation of the changes in the atmosphere. On the other hand, atmosphere to climate. the climate of an area is determinedover periods of manyyears andrepresents the general weather characteristics of an area or locality. Climatology as Related to Other Sciences CLIMATOLOGY One of three prefixes is often added to the Climatology is the scientific study of climate. word "climatology" to denote a scaleor magni- Itis a branch of meteorology, or simply the tude. Micro, meso, and macro indicate small, study of the atmosphere. There are three princi- medium, and large scales, respectively. pal approaches to the study of climatology. Microclimatologicalstudiesoftenmeasure They are physical, descriptive, and dynamic. contrasts between hilltop and valley, city and

14 Chapter 2WORLD CLIMATE AND WEATHER surrounding country, or they may be of an HYDROMETEORS (PRECIPITATION) extremely small scaleone sideof a hedge contrasted with the other, a plowed furrow This is the second most important climatic versuslevelsoil,or oppositeleaf surfaces. element. In most studies, this includes all water Climate in the microscale may be effectively reaching the earth's surface by falling either in modified by relatively simple human efforts. liquid or insolid state. The most significant Macroclimatology is the study of the large- forms are rain, snow, and hail. Precipitation has scale climate of a large area or country. Climate a wide range of variability over the surfaceof of this type is not so easily modified by small the earth, and because of this variability a longer human efforts. series of observations is generally required to Mesoclimatology embraces a rather indistinct establisha mean or an average. Since two middle ground between macroclimatology and stations could have the same amount of annual microclimatology. The areas are smaller than precipitation, but itcould occur in different those of macroclimatology and larger than those months, or days during these months, and the of microclimatology and may or may not be intensitycouldalsovary,itoften becomes climatically representative of a general region. necessary toinclude such factors as average These terms (micro, meso, and macro) are number of days with precipitation, average also applied to meteorology. amount per day and other factors. Climate has become increasingly important in Further,sinceprecipitationamountsare other scientific fields. Geographers, hydrologists, directly associated with amount and type of and oceanographers use quantitative measures of clouds, cloud cover must also be included along climate to describe or analyze the influence of withaprecipitationstudy. Cloud coveris our atmospheric environment.Climate classifica- usually expressed in tenths of sky cover. Precipi- tion has developed primarily in the field of tation is expressed in most studies in inches, but geography. The basic role of the atmosphere in centimeters may be used in some studies which the "hydrologic cycle" is an essential part of the are based on data derived from themetric study of hydrology. Parallel to this, both air and system. Cloud climatology also includes such water measurements are required to understand phenomena as fog and thunderstorms. the energy exchange between air and ocean. WIND

CLIMATIC ELEMENTS Climatologists are mostly interested in wind in terms of wind direction, speed, and gustiness. The weather elements which are usedto Frequently itis expressed in terms of "pre- describe climate are discussed in the following vailing" wind direction, average speeds, and section. A further discussion of the effects upon maximum gusts. Some climatological studies use some of these elements are coveredlater in this "resultant" wind which is the vectorial average chapter. of all wind directions and speeds for a given level, at a specific place, and for a given period. The vectorial averageis obtained by dividing each wind observation into components, making TEMPERATURE a summation for a given period,then obtaining averages and converting the average components This element is undoubtedly the most impor- into a single vector. tant of all the climatic elements. The temperature of an area or locality is dependent upon latitude, or the distribution of incoming and CONDENSATION outgoing radiation; nature of the surface (land or water); altitude; and theprevailing winds. The This climatic element includes such deposits temperature normally used in climatologyis the as dew, frost, and rime ice. It is notof particular surface temperature. importance in most general studies where there

9 15 AEROGRAPIIER'S MATE 1 & C

is sufficient rain to support life. Insome areas, NORMAL however, it can be important. In climatology, the term "normal" isapplied to the average value which in the EVAPORATION course of a period of time any meteorological elementis found to have ona specified date or during Althoughthisclimaticelement does not specified times. These timesmay be a particular receive the attentionitdeserves,itcan be month or other portion of theyear. They may extremely important when itis considered in refer to a season or to ayear as a whole. The relation to the formation of weatherphenomena normal serves as a standard with which values over water bodies or oceanic areas. It can bean occurring on a date or during a specified time important factor in the formation of fogsover may be compared. such areas. ABSOLUTE EXPRESSION OF CLIMATIC ELEMENTS The term "absolute" usuallyis applied in As an Aerographer's Mate, you must under- climatology to the extreme highest and lowest stand climatological terms and the methods used values for any given meteorological element to derive these terms, in order for the climato- which has been recorded at the place ofobserva- logical data to have definite meaning to you. In tion. Assume, for example, that theextreme this section the most commonly usedterms are highest temperature ever recorded ata particular defined, and where applicable, theirusage in station was 106°F and the lowest recordedwas expressing climatic elements is explained. 15°F. These are called the absolutemaximum and absolute minimum, respectively. MEAN OR AVERAGE EXTREMES The mean is the most commonly used climatological parameter. The term "mean" normally The term "extreme" is applied to the highest refers to the arithmetic mean which is obtained value and the lowest valuefor a particular in the same manner as theaverage. This is an meteorological element which have occurred average obtained by adding the values of all over a period of time. The termis usually factors or cases and then dividing by the number applied to months, seasons,years, or a number of items. For example, the average daily temper- of years. The term may be used fora calendar ature would be the sum of the hourly tempera- day only, for which it is particularly applicable lures divided by 24. The mean, as computed in to temperature. For example, the highest and this manner, is generally optimum for both the lowest temperature readings fora particular day expected value and the center of the distribution are considered the temperature extremes for for temperature. that day. At times it is applied to theaverage of Other methods are used for computing vari- the highest and lowest temperatures and termed ous meteorological elements. For example, the mean monthlyextremes and mean annual mean temperature for the day has been derived extremes. by simply adding the maximum and minimum values for the day and dividing by 2. Assume the RANGE ,maximum temperature for a certain day is 75°F andthe minimum temperature is 57°F; the Range is the difference between the highest mean temperature for the day is 66°F. andlowest values andreflectsthe extreme Unfortunately the term "mean" has been variations of these values. This statistic is not usedin many climatological records without recommended except for very crude work, since clarification as to how it was computed. In most it has a high variability. The range is related to cases, the difference in results obtained is slight. the extreme values of record andcan be useful In analyzing weather data, the terms "average" in determining the extreme range for the records and "mean" are often used interchangeably. available.

10 16 Chapter 2WORLD CLIMATE AND WEATHER

FREQUENCY the use of the median instead of the mean or averagefor some of theclimatic elements Frequency is defined as the number of times a because the extremes of these elements are certain value occurs within a specified period of averaged and combine in some cases to give a time. When a large number of variate values need wholly unrepresentative picture of distribution to be presented, a condensed presentation of and probability of that particular element. How- data may be obtained by means of a frequency ever, a longer period of record might berequired distribution. to formulate a median. STANDARD DEVIATIONS MODE

The mode is defined as the value which occurs In many analyses of climatological data, it is with the greatest frequency, or the value about desirable to compute deviation of all items from which the most cases occur. a central point. This may beobtained from a The mode cannot be determined readily from computation of either the mean (or average) unorganized data; therefore, the data must be deviation or the standard deviation. These are grouped in a frequency distribution before its termed measures of dispersion and are used to location can be determined accurately. It is not determine whether the average is truly represent- always well defined or possible to locate prop- ative or to determine the extent by which data erly. The maximum density point may be more vary from the average. than one point so that the point determined as The average deviation is obtained by com- the mode depends upon the judgment or desire puting the arithmetic average of the deviations of the person interpreting and using the mode. from an average of the data. First, we obtain an In general, the mode is not recommended for average of the data, then thedeviations of the climatology. However, it can be useful in local individual items from this average are deter- climatological studies or in determining the most mined, and finally the arithmetic average of common or frequentlyattained value for a these deviations is computed. The plus or minus prediction technique. For example, in the objec- signs are disregraded. The formula for computa- tivetechnique, The Prediction of Maritime tion of the average deviation is as follows: Cyclones, the deepening prediction graph for Zd cyclones,has amodal valueaswell as a Average Deviation = maximum value. n the Greek letter E (sigma) means the sum of MEDIAN "d" which are the deviations and "n" is the The median is the value at the midpoint in an number of items. array. For determining the medianall items have The standard deviation, like the average devi- to be arranged in order of size. Roughestimates ation, is the measure of the scatter or spreadof of the median may be obtained by taking the all values in a series of observations. Toobtain middle value of an ordered series or if there are the standard deviation, each deviation fromthe two middle values, they may beaveraged to arithmetic average of the data is squared. Next obtain the median. The position of themedian determine the arithmetic average of the squared may be found by the useof the following deviations, finally extracting the square root of formula: this average. This is also called the root mean square deviation, in that it is the square rootof n+I the mean of the deviations squared. Median = 2 The formula for computing standard deviation is given as follows: where n is the number of items. /zd2 The median is not widely used in climato- Standard Deviation = logical computations. Some writers recommend n

11 17' AEROGRAPHER'S MATE I & C

Ede is the sum of the squared deviationsfrom The average deviation of temperature during the arithmetic average, and "n" is the number of the month of January for the period of record, items in the group of data. 10 years, is 2.6°F. An example of the computation of both the To compute the standard deviation: average deviation and the standard deviation is 1. Squarethedeviations from the mean given in table 2 -I and in the followingpara- (column 3). graphs. 2. Total these squared deviations. In thiscase Suppose, on the basis of 10years of data the total is 104. (1954-1963), for the month of Januarywe 3. Apply the formula for standard deviation: wished to compute theaverage deviation of mean temperature and the standard deviation for this period. First arrange the data in tabular Standard Deviation =ii Id2 11104 form (such as in table 2-1), giving theyear in the n 10 first column, the mean monthly temperature in 10.4 = 3.225 or 3.2°F the second column, the deviations froman arithmetic average of the mean temperature in the third column, and the deviations from the Thus the standard deviation of temperature mean (column 3) squared in column 4. for the month and period in question is 3.2°F (rounded off to the nearestone tenth degree). Table 2-1.Computation of average and Naturally a question arises. Just whatuse is standard deviation. this and how can I apply the computationsmade in table 2 -I for everyday forecasting purposes? With the standard deviation just determined it is January Mean DeviationsDeviations readily apparent that the implication year isthat temperaturefrom meansquared thereisa small range of mean temperature 1954 47 -4 16 during this month. If we had availablea fre- 1955 51 +0 0 quency distributionof temperatureforthis 1956 53 +2 4 station for each day of the month,we could 1957 50 -1 1 readily determine the percentage of readings 1958 49 -2 4 which would fall in the 6.4-degree spread (3.2 1959 55 +4 16 either side of the mean). From these datawe 1960 46 -5 25 1961 could then formulate a probability forecastor 52 +1 1 the number of days within this 1962 57 +6 range that we 36 could expect the normal or mean temperature to 1963 50 -1 1 occur. This study could further be broken down Totals 510 26 104 Mean 51 2. 6 3.2 into hours of the day, etc.

To compute the average deviation: CLASSIFICATION OF CLIMATE I. Add all the temperatures in column 2 and divide by the number of years (10 in this case) The climate of a given regionor locality is to get the arithmetic average of temperature. determined by a combination of severalmeteor- 2. In column 3, compute the deviation from ological elements, and not justone element the mean or average determined in step I. (The alone.For example, two regions may have mean temperature for the 10-year period was similar temperature climates but very different 51°F.) precipitation climates. Their climatic difference 3. Total column 3 (disregarding the negative therefore becomes apparent only ifmore than and positive signs). This total is 26. one climatic factor is considered. 4. Apply the formula for Average Deviation, Since the climate of a region is composed of all the averages of the various climatic elements, d -=--/6 = 2.eF such as dew, ice, rain, temperature, wind force, n 10 and wind direction, itis obvious that no two

12 18 Chapter 2 -WORLD CLIMATE AND WEATHER locations have exactly the same climate. Hoyt Temperate Zone of the Southern Hemishpere is ever, itis possible to place similar areas into a bounded on the north by the Tropic of Capri- grouping known as a climatic zone. corn and on the south by the Antarctic Circle, which is located at 66 1/2° S lat. The two Polar CLIMATIC ZONES Zones are the areas in the polar regions which have the Arctic and Antarctic Circles as their The basic grouping of climatic zones consists boundaries. The Polar Zones are sometimes of classifying climates into five broad belts based called the Frigid Zones. on astronomical or mathematical grounds. Actu- ally they are zones of sunshine, or solar climate. A glance at any chart depicting the isotherms The five basic regions or zones are the Torrid or over the surface of the earth will show that the Tropical Zone, the two Temperate Zones, and isotherms do not coincide with latitude lines. In the two PolaiZones. The Tropical Zone is fact, at some places the isotherms parallel the limited on the north by the Tropic of Cancer longitude lines more closely than they parallel and on the south by the Tropic of Capricorn, the latitudelines. The astronomical or light which are locatedat23 1/2° N and Slat. zones therefore differ from the zones of heat. A respectively. The Temperate Zone of the North- closer approach to the understanding of climate ern Hemisphere is limited on the south by the can be made if the climatic zones are limited by Tropic of Cancer and on the north by the Arctic isotherms rather than by parallels of latitude. Circle, which is located at 66 1/2° N lat. The (See fig. 2 -I.)

A Jo 7 ORTH POLAR ZONE L1,11114°1 50°F ISOTHERM (Warmest Month)

60 p 41ZONE70 NORT TEMPERATE

40 1 I MEAN ANNUAL ISOTHERM OF 68° F 20

HOT BELT

II 40 MEAN ANNUAL SOUTH TEMPERATE ZONE IISOTHERM OF 68° F

4P1 50 ° F ISOTHERM t--- --r-- FS:OUTH :POLAR ZONE (Warmest Month) 180 i60 140 120 100 80 60 40 20 0 20 40 606I AG.382 Figure 2-1.Temperature zones.

13 19 AEROGRAPIIER'S MATE I & C CLIMATIC TYPES 3. Topography. 4. Ocean currents. Any classification of climate depends toa large extent on the purpose of the classification. LATITUDE A classification for the purpose of establishing air stations, for instance, where favorable flying Perhaps no other climatic control has sucha conditions are important, would differ consider- marked effect upon climatic elementsas does ably from one for establishing the limits ofareas the latitude, or the position of the earth relative that are favorable for the growing of crops. to the sun. The angle at which rays of sunlight There are two classifications that particularly reach the earth and the number of "sun" hours merit your attention. They are the classifications each day depend upon the distance from the of Koeppen and Thornthwaite. Equator. (See fig. 2-2.) Therefore, the extent to In Koeppen's classification thereare five main which an air mass is heated is influenced by the climatictypes. They are TROPICAL RAIN, latitude.Latitude influences the sources and DRY, HUMID MESOTHERMAL, HUMID MI- direction of air masses and the weather they CROTHERMAL, and POLAR. These main types bring with them. are further divided into climatic provinces. The Koeppen classification is based mainly on tem- Influence on Air Temperature perature, precipitation amount, and season of maximum precipitation.Numerical values of Regions under direct or nearly directrays of these elements constitute the boundaries of the the sun receive more heat (per unit of time) than above types and were selected primarily accord- those under oblique rays. The heat brought ing to their effect on plant growth. about by the slanting rays of early morningmay Thomthwaite'sclassificationofclimates be compared with the heat that is caused by the places a great deal of emphasis on the effective- slanting rays of winter. The heat which is due to ness of precipitation. Effectiveness of precipita- the more nearly direct rays of midday may be tion means the relationship between precipita- compared with the heat resulting from themore tionandevaporationatacertainlocality. nearly direct rays of summer. Thornthwaite classified climates into five cli- The length of the day, like the angle of the matic provinces, which are WET, HUMID, SUB- sun'srays,influencesthe temperature. The HUMID, SEMIARID, and ARID. To each of the length of the day varies with the latitude and the provinces is given a precipitation effectiveness season of the year. A place near the Equator has rating. about 12 hours of daylight every day in the year. Because of this, and because the sun at CLIMATIC CONTROLS noonday is always high in the sky (giving nearly directrays), equatorialregions do not have The variation of climatic elements from place pronounced seasonal temperature changes. to place and from season to season is caused by During summer in the Northern Hemisphere several factors called climatic controls. The same all places north of the Equator havemore than basic factors that cause weather in the atmos- 12 hours of daylight. This situation is reversed in phere also determine the climate of an area. thewinter;latitudesnorth of the Equator These controls, acting in different combinations received less than 12 hours of daylight. and with varying intensities act upon tempera- Great seasonal variation in the length of the ture, precipitation, humidity, air pressure, and day and the seasonal difference in the angle at winds to produce many types of weather and which the sun's rays reach the earth's surface therefore climate. cause seasonal temperature differences in middle Four factors largely determine the climate of and high latitudes. In the far north, long hours every ocean and continental region. They are as of winter darkness produce cold temperatures follows: that breed powerful polar air masses; conversely I.Latitude. long hours of summer daylight weaken the polar 2. Land and water distribution. air masses.

14 20 Chapter 2WORLD CLIMATE AND WEATHER

6 MDNTHS 6 MONTHS NIGHT TANGENT AT N. POL SUN RAY

PERPENDICULAR SUN RAY

TANGENT Okor *446 MONTHS NIGHT AT S. PDLE 6 MONTHS SUN RAY DAY AT S. POLE

22 DECEMBER- WINTER SOLSTICE 21 JUNE - SUMMER SOLSTICE

12 H TANGENT -- ARCTICA CIRCLE 12 H

TROPIC OF CANCER POSITION OF PERPENDICULAR AND ER TANGENT SUN RAYS DETERMINES

1 TROPICS OF CANCER AND CAPRICORN eQUATDR_ 4---.PERPENDICULAR AND ARCTIC AND ANTARCTIC CIRCLES 12H SUN RAY POSITION OF DAYLIGHT CIRCLE DETER- TROPIC OF'CAPRICORN MINES LENGTH OF DAY AND NIGHT. i r2H \\..... 1 ANTARCTIC CIRCLE TANGENT 12 H UNRAY

22 SEPTEMBER- AUTUMN EQUINOX 21 MARCH- SPRING EQUINOX

AG.383 Figure 2-2.Latitude differences in amount of insolation.

The hot and humid climates of equatorial with the earth's surface that the energy received Africa and South America are good examples of per unit area is extremely small and the sun's the influence that latitude has on climate. At no effectiveness is minimized even though it may time during the year are the sun's rays at much shine for days without ceasing. of an oblique angle. Therefore, there is little The average world surface temperatures are difference between mez.n temperature for the represented on two world charts for July and coldest and warmest month. Contrast this pic- January as shown in figure 2-3.It must be ture with the opposite extreme, where the sun is remembered that these are mean charts and are either below the horizon for a great deal of the not meant to be an accurate portrayal of the time or is only slightly above the horizon at any temperatures on any one particular day. Note time. The sun's rays in reaching the earth's that in general the temperatures decrease from surface in polar regions make such a small angle low to high latitudes. This is the latitude factor. -2115 1.4K;41ii 7-ri - .11; 4 , MIii. 1 4' kiir iniiiil hilibiLiv liff

111111107 ' : 'INN ;:rir1. ,, Iiritt-i AMENACTIIIIL \ j\ 7 111111111r '15ilhe..1.:111111111 ' 1118114ilriNt, . 111.1111111111 1111ML:. EN ' BLIvin iNININIMII maw ijaMict LEIL. 111149 '1111 ...ANT-7PITomitsrammar . 1111111011111111 11_,API,111:1 41111111111111i11111111111111111 ' 11111

I IIAMEMIIII MIN eic 1.614 NMI 4j, It% 'C` ir 4 jimmy', i .-.....011 alll 47109/ /JAL. . .41\ita arairIme-111111.!-P '46.111Cal

64",!!!Filkow.:1111111111111 CliOiirilltr.. all 111111111 OPAL: ENII imam 'emr mar_AgArr1111116orrtu jllitilM10 611;r1P'illiiiMeillilalrr AM . monfiltel101.11141111111111111F.1=1711 IIMMUMIIIIUMIIIIUIPildennum I'm milbill111101111111111111111111111111111 Chapter 2--WORLD CLIMATE A.4) WEATIIER

LAND AND WATER DISTRIBUTION sphere has less land and more water surface than the Northern Hemisphere, the change due to the Because land heats and cools about four times greater water surfaceisless with consequent faster than water the location of continents and more nearly uniform isotherms. Too, the con- oceans greatly alters the earth pattern of air tinentsoftheSouthernHemispheretaper temperatureandinfluencesthe sources and toward the poles and do not extend to as high a direction of movement of air masses. latitude as in the Northern Hemisphere. The nature of the surface affects the local Influence on Air Temperature heat distribution. Color, texture, and vegetation influence the rate of heating and cooling. Gener- Coastal areas take on the temperature charac- ally, dry surfaces heat and cool faster than moist teristics of the land or water to their windward. surfaces.Mowedfields,sandy beaches, and In latitudes of prevailing westerly winds, for paved roads become hotter than surrounding example, west coasts of continents have oceanic meadows and wooded areas. During the day, air temperatures and east coasts have continental is warmer over a plowed field than over a forest temperatures. The temperatures are determined or swamp; during the night the situationis by the windflow. reversed. The distribution of water vapor and cloudi- Since the upper layers of the ocean are nearly ness is another important factor influencing air always in a state of violent stirring, heat losses or temperature. Although areas with a high per- heatgains occurringattheseasurfaceare centap of cloudiness have a high degree of distributed throughout a large volume of water. reflectivity, the energy which is not reflectc:l is This mixing process sharply reduces the temper- easily trapped in the lower layers due to the ature contrasts between day andnight and greenhouse effect. Thus, it must be expected that between winter and summe over oceanic areas. areas of high annual moisture content will have Over land, there is no redistribution of heat relatively high annual temperature. by turbulence, and the effect of conduction is negligible. Thus violent contrasts between sea- Air Circulation sons and between day and night are created in the interiors of continents. During winter, a The higher mean temperature of the Northern largepartof the incident solar radiationis Hemisphere is not only an effect of its greater reflected back toward space by the snow cover land cover, but the oceans are also warmer than that extends over large portions of the northern in the Southern Hemisphere. This is partly due continents.Forthesereasons,thenorthern to the movement of warm equatorial waters continents serve as manufacturing plants for dry from the Southern Hemisphere into the North- polarair.Thepolaraircapisno longer ern Hemisphere, caused by the southeast trades symmetrical, but is displaced far to the south, which cross the Equator. Another factor con- particularly over the interior of Asia. duciveto higher mean temperaturesinthe The large temperature difference between the Northern Hemisphere is the partial protection of land and water surface, which reverses between its oceans from cold polar waters and Arctic ice the two seasons, determines to a great extent the bylandbarriers.Thereisno suchbarrier seasonal weather patterns. between the Antarctic region and the southern oceans. You willnoteinfigure2-3thatinthe Northern Hemisphere the isotherms are more closely spaced and parallelinwinter. In the TOPOGRAPHY Southern Hemisphere, the temperature gradient does not have as great a seasonal change as it Over land, climates may vary radically within does in the Northern Hemisphere. This is due to very short distances because of altitudes and the the unequal distribution of land and water on variations in land forms. In this section we will the two hemispheres. Since the Southern flemi- discuss these two general effects.

17 23 AEROGRAPIIER'S MATE I & C

Altitude OCEAN CURRENTS

The height of an area above sea level exerts a The currents of the oceans have considerable considerable influence of itsclimate. For in- influence on the climate of certain areas and stance, a place located on the Equator in the must be considered when preparing operational high Andes of South America would have a forecasts. However, since a discussion of ocean climate quite different from a place located a currents is presented in chapter 22 of AG 3 & 2, few feet above sea level at the same latitude. NT 10363-D, they are not discussed inthis A change of all climatic values is observed as a manual. function of elevation. OTHER FACTORS Land Forms INFLUENCING CLIMATE Vegetation and human activity, though their A powerful influence on climates is mountain- roles may be minor inthe ous terrain, especially the long high chains of overall climatic mountains thatact as climatic divides. These picture, do have an effect, at least in a local area, obstacles deflectthe tracks of cyclones and on the climate of that locality. block the passage of air masses in the lower Itis believed that large human settlements and industrial plants have a large influence on levels.Ifthepressuregradientsarestrong enough to force the air masses over the moun- climate. The atmospheric pollution is increased and the radiation balance in the vicinity of the tains, the forced ascent and descent will modify pollution the air masses to a great extent, thus modifying ischanged. This affectsthe daily the climate on both the windward and leeward maximum and minimum temperatures inside sides. cities. They are generally higher than'.4.1 the The alinement of the mountain range may suburbs. The higher concentration of hygro- block certain air masses and keep them from scopic condensation unclei in the cities results in getting to the lee side of the mountains. For an increased number of fogs. Too, with the example, the Himalayas and the Alps, with an largerheatsourceconcentratedover cities, increasedconvectiongives east-west risetogreater orientation, prevent fresh polar air amounts masses from advancing southward. Therefore, of cloudinesswithslightlyhigher the climates of India and Italy are warmer in amounts and frequencies of rain. winter than other locations of the same latitude. Evaporation and transpiration (breathing) of plants are very important influences on climate. The coastalrangesof mountainsinNorth America, running in a north-south line, prevent Fallingprecipitationcaughtintrees before the passage of unmodified maritime air masses reaching the ground may be evaporated. Precipi- to the lee side. tation which reaches the ground does not readily Probably the most noted effect of mountains evaporate, nor run off easily, due to the spongy is the distribution of precipitation. The precipi- structure of solid forests that can absorb and tation values, level for level, are much higher on store considerablequantities of water. Snow in the windward side. forests is protected from direct radiation by the In regions where the prevailing circulation trees and may stay on the ground for much flows against a mountain barrier, the amounts of longer periods than over open, exposed surfaces. Inside forests temperature maximums and mini- precipitation increase more or less uniformly mums are higher than over open land at the toward the tops of the mountains. This occurs same latitude. Relative humidities are higher in on the windward sideof the mountains to forests and wind speeds are considerably lower elevations of about 10,000 feet. However, in the than outside. trade wind zone, such as at the Hawaiian Islands, precipitation amounts increase only to about CLIMATOLOGICAL DATA 3,000 feet and then decrease gradually. Even withthis decrease in amounts, more rainis Climatological records are based on the mete- received at 6,000 feet than at sea level. orologicalobservationsthataretaken ata

18 24. Chapter 2WORLD CLIMATE AND WEATIIER particularlocality. This information may be METHODS OF PRESENTATION presented in a number of ways. Climatologicalinformationispresentedin Temperature records generallyinclude the many different ways. Tables are frequently used. following temperature values. Daily maximums Maps are particularly useful in presenting cli- and minimums by months, the extremes, the matic information in cases where geography is average temperature by year and month; the an important factor, Wind data can often be mean monthly and annual temperature, and the given by means of a device called a wind rose, mean monthly maximum and minimum. These which presents information on the prevailing values or elements were discussed earlier in this wind directions. For an example of a wind rose, chapter. Sometimes included are the monthly see figure 2-4, and seasonal degree days. A degree day is the derarture of the mean temperature from a daily average temperature of 65°F. For instance, if 30 the mean temperature for a particular date were N 50°F., the number of degree days for that date is15. Of great climatic significance is the range 13 between the mean temperature of the warmest month and the coldest month. Other tempera- turedataaresometimesgiven. These may include the number of days with the following temperatures: Maximum of 90°F and above; maximum of 32°F and below; minimum of 32°F and below; and minimum of 0°F and below. 10 13

Precipitation records include the mean annual 7. Scale and monthlytotals. The range between the highest and the lowest annual rainfallfor a locality isthe best indication of the dependa- 0 so °/. bility of the precipitation. The records often show the absolute maximum rainfall and snow- AG.385 fall for a 24-hour per, xl by months, as well as Figure 2.4.A wind rose. the maximum and minimum precipitation for each month. Graphs are usually divided into bar and line graphs, or the graph may be a combination of Climatic recorub usually show data on winds. the two. Figure 2-5 shows an example of a bar Such informationindicates the mean hourly graph and a line graph of the same information. speed and the prevailing direction by month. Also shown are the speed and direction of the fastest mile of the wind for the 12 months and AVAILABILITY OF DATA the year in which it occured. Every naval weather service office should have Data on cloudiness, humidity, thunderstorms, climatological records in usable form availably and heavy fog are often included. Other helphful for the area in which the office is located, and data would be the frequency and distribution of for such other areas as may be necessary to carry cyclones and anticyclones, passage of fronts, the out its mission. Various climatological records proportion of rainfall andsnowfallreceived that are available for Naval Weather Service units from cyclonic storms and local, airmass thunder- from the NWSED, National Climatic Center, storms; and climatological data on upper air Asheville, NOrth Carolina include: the Summary conditions. ofMeteorologicalObservations.Surface

19 25 ALEROGRAPIIER'S MATE I & C

(SMOS): Local Climatological Data (LCD) for selected ::rations: Cross-Wind Summary: Sum- PRECIPITATION mary of Synoptic Meteorological Observations 50 (SSW)); and Winds Aloft Summaries. The first two are routinely issued and revised: 40 the last three are prepared and issued only on request. 30 Frequency SMOS 20 These standard summaries are prepared firm

10 Navy Monthly Meteorological Records (MMR). Each SMOS isfor a specific station.Fre- quency distributions for various parameters are 0 19 57 1958 1959 1960 1961 1962 1963 presented by time of day, month, and year. (A) Each SMOS is revised every 5 years.

PRECIPITATION so Local Climatological Data The Local Climatological Data summary con- 40 sists of means and extremes (temperature, pre- MI :; , cipitation, wind, etc.) by month, mean tempera- 30 Mir,a tureandtotalprecipitation by months for

i' 1 ' I 1 I 1 specificyearsof record,and monthly and I' II 1 o 20 III;II 1 . seasonal degree days. It is revised annually.

II : I II 1 I 1 I. 1 1 IIilli I. . I 1 , 1 I s II I0 I Cross-Wind Summary 1 i I II I I III I 1 I II I I I " I I I 1 I I I 1 I I I This summary presentsthe precentage of II i I I I 0 1957 1958 1959 1960 1961 1962 1963 occurrence of cross-winds for a given nmway. (B)

PRECIPITATION Summary of Synoptic Meteorological Observations so These standard summaries are similar to the 40 Summary of Meteorological Observations, Sur- face (SMOS) in that they present a number of 30 useful monthly and annual tabulations of surface climatological data and various combina- 20 tions of the includedparameters. They are specifically designed for compatibility with the

10 variousweather reporting codes offoreign nations and of ships at sea.

0 1957 1958 1959 1960 1961 1962 1963 (C) Winds Aloft Summary

AG.386 Thesesummariesareeitherseasonalor Figure 2-5.A comparison of the bar and line graph monthly and are generally prepared for various methods of showing the variable annual precipitation in constant heightlevels and constant pressure a time series. (A) Bar graph; (B and C) line graphs. levels. The summaries contain winds aloft data

20 26 Chapter 2WORLD CLIMATE AND WEATHER giving speed and directions over the period containing isobars of normal pressure and princi- covered. The legend on each chartisself- pal storm tracks. The upper air section includes explanatory. chartsof upper wind graphs,temperatures, relative humidity, and heights for selected levels; Worldwide Airfield Summaries tropopause data; and refractive index gradients. 3. The local Area Forecaster's Handbook. These summaries provide climatological data This handbook as required by NAVWEASER for airfields and geographical areas throughout COM Instruction 3140.2( )contains valuable the world. There are 10 volumes, some pub- information on local and area weather as fol- lished in two or more parts. lows: A description of the local topography and The svmmaries are prepared at the National terrain,generalsynopticcharacteristicsof Climatic Center (NCC) Asheville, North Caro- weather occurrences inthe localarea, mean lina. The Headquarters, Naval Weather Service storm tracks for your region, a limited amount Command, controls the Navy participation in of climatological data, and local forecasting rules the jointactivity of the Asheville operation. or techniques.Itcanserveasa composite Requests for these summaries should be for- summary of expected weather events and the warded via official channels to: effectofcertainparametersonthelocal weather. Commander, Naval Weather Service Command Stations that are preparing their first hand- Washington Navy Yard book or revising their old one can make ex- Building 200 cellent use of their Summary of Meteorological Washington, D. C. 20390 Observations, Surface (SMOS). Individual tables or graphs thereof may be placed in the hand- with a copy to the Officer in Charge, NWSED, book, or the entire SMOS may be used as a NCC, Asheville, North Carolina. separate appendix to supplement the descriptive text. Extra copies of SMOS are available from CLIMATOLOGICAL REFERENCES the Headquarters, Naval Weather Service Com- mand for this purpose. If the. SMOS is used as a There are many references which can be used separate appendix,itneed not be forwarded in climatological work, so many in fact that they with the handbook to the Headquarters, Naval would be too numerous to list all of them here. Weather Service Command. Many excellent sug- Most of the Navy climatological references are gestions for climatological preparation can be listedinthe Navy Stock List of Forms and found in NW 50-IC-536, Climatology at Work. Publications, Nav Sup Pub:" ration2002, Cog- This is also an excellent statistics primer. nizance Symbol I, Part C, Section VIII. Navy The handbook, to be of the utmost use and Climatological publications are found under the value to the station forecasters, must be updated NA/NW 50-IC series. A few of those available and revised periodically. are listed as follows: I. Guide to Standard Weather Summaries, CLIMATOLOGICAL SERVICES NavAir 50-1C-534. This publication contains an index of allthe standardmachine-tabulated The Naval-Weather Service Command pro- summaries available and on file at the National vides climatological services to the Navy through Climatic Center. its weather units, Fleet Weather Facilities and 2.U. S. Navy Marine Climatic Atlas of the Centrals, and the NWSED, NCC, Asheville. World, Vols.I through VIII. These publications Weather units at shore stations must provide contain climatic data for all the principal ocean climatological studies for their local areas based areas of the world. It has both a surface and all upon locally available weather data and climato- upper air section. The surface section contains logical publications. Requests for climatological data presented by graphs, tables, and isopleths studies beyond the capabilities of the local on such elements as surface winds, gales, visibil- weather unit should be submitted to the nearest ity,precipitation, etc.Italso includes charts Fleet Weather Central.

21 27 AEROGRAPIIER'S MATE1 & C

Fleet Weather Centrals provide, in addition to caster who has personal experience at a particu- tool area studies, climatological studies for their lar station can use climatology as a refresher for areas of responsibility, climatological studies for theoverallweatherpatterns whichcanbe other weather units and forLes afloat, including expected for the ensuing season. This knowledge the preparation of the climatological portion of can help him to be more perceptive inhis weather annexesforoperationplans when everyday analyses,tobealertfor changing requested. They also conduct active research in patterns with the seasons, and to produce a applied climatology. higher quality forecast. The inexperienced forecaster who has had no INTERPRETATION experience at a particular station must rely on climatology as a substitute for this experience. Climatological records are almost worthless Forecasters cannot be expected to become unlesstheyare interpreted correctly. Proper familiar overnight with the weather peculiarities interpretation requires that all of the meteoro- of their new area of responsibility. The station logical elements be studied so as to present a indoctrination period can be greatly reduced if composite picture. One meteorological element the new forecaster is furnished with "packaged alone means verylittle to a meteorologist. For experience" in a form which can place him more instance, itis possible to conclude that Cairo nearly on a par with those forecasters already Egypt, and Galveston, Texas. have about the experiencedatthat station. The Local Area same kind of weather from a study of only the Forecaster's Handbook is a good example. temperature,sincetheyearlyand monthly means and annual range are approximately the USES AND LIMITATIONS OF same. However, Galveston has about 40 times as CLIMATOLOGICAL DATA much precipitation. Thus, their weather conditions over the year must differ greatly. The Naval Weather Service makes many uses To properly interpret just one meteorological of climatological data. In using the data, how- element requires a study of several factors. The ever,it must be clear that climatology has its temperature of a particular locality must be limitations in the field of meteorology. It may studied from the stanpoint not only of the mean be putthis way. Climatologyis an essential but also of the extremes, and the diurnal and supplement to meteorology, but it must never annual ranges. The effectiveness of precipitation be considered a substitute for the meteorological depends on severalfactors, such as amount, situationwhichconstitutes currentweather distribution, and evaporation. The mean precipi- conditions. tation for a particular month for a locality may TheNavy makes use ofclimatological be several inches, but the interpreter may find research in determining the locations for naval from a study of the locality's records that in bases and naval air stations. The prevalence of some years the precipitation for that month is fogs, the prevailing wind directions, the occur- less than an inch. possibly not even a trace. rence of low ceilings, and the passage of fronts, along with the frequency of thunderstorms, are APPLICATION OF CLIMATOLOGY some of the important considerations in estab- TO WEATHER PREDICTION lishing naval air stations in particular. In the locating of naval air stations, fogs are Climatology is introduced where operational especially important as a consideration. Ground planning is required for a length of time beyond fog will tend to drain into an area that is located the range covered by weather forecasting tech- in a topographical depression. When ground fog niques. A study of the climate of that area or occurs, a station located at such a site would be region may wellforetellthe general weather the first to be fogged in,and it would be the las: pattern to be expected. to havefog dissipation.If advection fog is A more direct application of climatology can predominantinthearea,itseffect canbe be made by boththeexperienced and the minimized by selecting a site where the pre- inexperienced forecaster. The experienced fore- vailing wind during such fogs has adownhill

22 Chapter 2--WORLD CLIMATE AND WEATHER component so that adiabatic compression, with following paragraphs, concluding with a discus- its associated temperature rise, will dissipate fog. sion of United States weather asit relates to If the airstationisto be located in an world weather. industrialarea,itshouldbe located to the windward side of the industrial area to minimize the reduction of visibility due to smoke. Air WORLD AIR MASSES AND FRONTS stations should not be located near the lee side The initial characteristics of an air mass is ofhighobstructionswhich couldproduce troublesome eddy currents. determinded by the surface (land or water) over which the air mass originates (its source region). Also,inthe establishment of a naval air station the occurrence of low ceiling must be However, these initial characteristics undergo a considered. A study of the climatological data of variety of changes once the air mass begins to move out of the source region. an area willreveal the prevalence of ceilings which would adversely affect flight operations. The regions of high pressure from which The passage of fronts is another important principalairflowsoriginatearethe primary source regions for the air masses that move over consideration.Itis very essential to know the frequency of frontal passage, the normal speed the world. The relative intensity of the continen- of the movements of fronts, seasonal fronts, and tal and maritime highs determines the location the usual weather conditions that accompany of the zones of convergence between them. In the different types of fronts that move across these zones of convergence, where air masses the area. with different properties meet, liethe polar fronts. The transient low-pressure cells of the The distribution and frequency of thunder- extratropicalcyclone move along the polar storms are important climatically in the location fronts. of naval air stations. This is due to the great destructibility of gusty winds accompanying Air-Mass Source Regions thunderstorms, and the disruptions that they and Classifications can cause in the flight program. The orientation of the runways at an air Figures2-6 and 2-7 show the worldair station can be properly made only after due masses, fronts, and centers of major pressure consideration of the prevailing winds of the area. systems in January and July, respectively. Prevailing winds must be studied carefully. It is A more detailed discussion of air mass source necessary toconsiderseveral directions that regions and classifications is presented in AG 3 occur frequently. Wind directions associated & 2, Chapter 6, NT 10363-D. with low wind speeds may be somewhat ignored ifthe speedislow enough that crosswind Air-Mass Characteristics landings may be made safely. In using climatological records for study, it is The weather characteristics of a particular necessary to consider all elements and values month in a given locality are governed by: that are available.Itisa mistake to rely too 1. The intensity of the airstream flowing over heavily on the normal values that are attached to that particular region. certain weather elements,since one year or 2. The origin of the air mass within the month may vary a great deal from the normal stream. for the year or month. 3. The previous history of the air mass before arriving over the region. WORLD WEATHER 4. The effects of local topography upon the several meteorological processes. Factors to be considered in analyzing weather 5. The proximity of the region to one of the in different areas of the world include world air zones of atmospheric convergence. masses and fronts,the polar front, regional The rate of transformation depends upon the circulations, oceanic weather, and arctic and degree of difference in temperature and mois- antarctic weather. These are discussed in the ture, the speed of airflow, and the amount of

23 29 AEROGRAPIIER'S MATE I & C

:10t)Plik '..,..n,' ),,,t: =.: . :,.4','./ '11? ATI, '1 Mdif;:-: ''.--t' / rJ, 102 ./ -s",,IIIItv A ,.: -vm,,,,WIrlIkt ....s Id ISUNIIIIN%.... ii.,.." 1:11M/11 sizz.grisaliZt.'..2i?' UPZIIIIIIII Mil 11.11k2C1 1 '41.11.0:1:14AFZINNI NINE WIWI& ;:limorrtrevin arm /IV me' 11UE.Biiiigir. il 1III1111 .. Imo JANUARY

AG.387 Figure 2-6.Chart showing world air masses, fronts and centers of major pressure systems in January. turbulence in the lower layers.Discussion of the reat.h only a limited height. Under such condi- modification process is facilitated if the follow- tions. a shallow layer of unstable air exists at the ingthree classes of conditions are considered. surface with a stable inversion layer above. This I. Warm air moving over a colder surface. In prevents the distribution of moisture to high this case, the surface layer of air undergoes rapid levels. transformation,but.because of the stability produced by this surface cooling, the effects do POLAR FRONT not extend to great heights. 2. Cold air moving over a warmer surface. In There are four principal types of air masses in such a situation, the surface layer of air under- eachhemisphere, named according to their goesless rapid modification, but the effects source regions. arctic. polar. tropical, and equa- extend to much higher levels because of the torial. Arctic air is separated from polar air by turbulent and convective exchange of heat and the Arctic front. The polar front exists between moisture. tropical and polar air masses. The ITCZ lies in 3. Warm. dry air moving over a warm, moist the equatorial air mass region. surface.Inthis case.the air mass picks up The polar front does not constitute a single, moisturerapidly,but, because of the small more or less continuous, band of convergence amount of heating at the surface, the effects and bad weather. It appears. rather, as a series of

24 30 Chapter 2- WORLD CLIMATE AND WEATHER mmmuWill

\--\,c6 disinkiE 2,!01211

411111.511111rr''''.4111

--0101noftlymosimmommal001a11114MNENIN

JULY

AG.388 Figure 2-7.Chart showing world air mattes, fronts, and centers of majorpressure systems in July.

active,convergentzones. which move in a theadvancing wave, thus forming the warm generaldirectionfromwesttoeast.These front of the cyclone. The warm, light air behind convergent zones arethe migratory low or the warm front tends to ride up and over the traveling cyclones of the middle latitudes. It has denser cold air ahead of the front. The waves been shown that. in middle and high latitudes. it develop into occlusions, and atypical extra- isimpossibleforairmasseswith different tropical cyclone is the result. densities(temperatures)tolieside by side The formation of a cyclone along the polar continuously in equilibrium, but that waxes will front is first noted as a slight indentation of the form alongthe discontinuityor frontthat front. This indentation or wave deepens and separatesthe two.If the wavelength of the moves along the front, rapidly at first, and then disturbances are of the order of 300 to 1.800 becomes progressively slower. The waves nearly miles, the waves are said to be unstable and, always move from west to east for two reasons. their amplitude will increase with time. In this In the first place, this is the prevailing direction case, the cold air in the rear of the waves is of the windstream in which they form. Sec- pushed toward the Equator, where it tends to ondly, according to wave theory, a wave type underrun the warm air. thus forming the cold cyclone should move in such a way that the front of the cyclone. A compensating polarward warmer airis on the rightinthe Northern flow of warm air occurs in the front portion of Hemisphere and on the left in the Southern

25 31 AEROGRAPHER'S MATE I & C

Hemisphere. Since the warmer air is usually to cause a complete seasonal reversal of winds, as the south of the polar front in the Northern does Asia, but they produce partial monsoons, Hemisphere, and to the north of the polar front which have a significant effect on the amount of in the Southern Hemisphere, the wave move- seasonal rainfall. ment also contributes to the west-to-east motion Regions with strong monsoon tendencies usu- of cyclones. Thus, the zones of active con- ally are on the eastern sides of continents. This vergence along the polar fronts in each hemi- is especially true in the middle latitudes, since sphere appear as migratory lows which travel the ;western or windward coasts are distinctly from west to east, generally in the region which marine in character, with only small changes in is more than 20° from the Equator. temperaturefromwinter to summer.Itis, therefore, only on the more continental eastern, REGIONAL CIRCULATIONS or leeward, sides that sufficiently large seasonal extremes of temperature can develop to produce Many regions have unusuallocal weather a wind reversal. phenomena caused directly by the temperature Monsoon air,as observed over India and difference between land and water or by local Burma, consists of unmodified equatorialair topographical features. Knowledge of the re- during the summer monsoon and of highly gional circulations which have significant effects modified cP air during the winter season. The on operating conditions is essential for naval weather conditions during the summer monsoon operations. consist of cloudy weather with almost continuous rain and widespread shower activity. High Land and Sea Breezes temperaturesandhumiditiesaretherule. Weather conditions during the winter monsoon Ocean breezes generally occur on a summer are dominated by the dry, adiabatically warmed afternoon near a coastline. When an air mass is polarairflowing toward the Equator.Itis stationary over a coastline and the land surface during this time that generally pleasant weather is warmer than the sea surface as a result of prevails over most of the area influenced by radiation heating, an onshore breeze is produced monsoon conditions. at the surface, with an outward drift aloft. At The reverse of this condition occurs along the night, the radiation cooling of the land is greater east coast of Indochina, particularly the north- than that of the sea; the horizontal temperature erncoastal sections of Vietnam. During the gradientis, therefore; normally reversed. This winter the cP air flowing southward over the produces the offshore breeze, which reaches its East and South China Seas becomes sufficiently maximum about dawn. The land and sea breeze modified to produce extensive cloudiness and phenomenon is purely local. Its influence does drizzle to this area. This phenomenon is called not extend far from the coastline and seldom the "Crachin." exceeds a height ci 3,000 feet. Ravine Winds Monsoon Winds CHARACTERISTICS.Strong windsoften Monsoon winds are characterized by a ten- blow along deep valleys or ravines that break dency towardareversalinprevailing wind across mountain ranges. Some of the highest direction between winter and summer. observed wind velocities have occurred in such Because of the great size of the continent, the valleys. The mistral of southern France and the monsoon is most developed over eastern and bora of Trieste (located on the Adriatic Sea in southern Asia. However, monsoons in modified northeastern Italy) are among the best known of form, or monsoon tendencies are characteristic a large group of such winds. All ravine winds are of other regions as well. Southeastern United characterized by their violence, and most of States, northern Australia, Spain, and South them have a marked seasonal variation. It is Africa are regions with monsoon tendencies. believed that ravine winds are due to a pressure These land areas are not extensive enough to gradient acting along the ravine. The wind blows

1 32 Chapter 2WORLD CLIMATE AND WEATHER directly from high pressure to low, along the Oceanic Weather Control ravine; the wind speed increases with distance from the source, reaching a maximumasit It has been long recognized that the influence emerges at the low-pressure end of the ravine. of the ocean plays an important part in climate The wind occurs more frequently in winter and weather, particularly in the realm of temper- thaninsummer.Itisstrongestand most ature, humidity, and precipitation. This is only frequent in the early morning. natural, since three-fourths of the earth's surface is covered by water. RAVINE WINDS OF THE UNITED STATES.Many examples of ravine windsare The two climatic extremes that relate to found inthePacific Coast Ranges and the water and land distribution over the earth are coastal areas of the United States. The best MARITIME and CONTINENTAL. Continental known include the following: climate is generally evidenced by a wide range in 1. The galesinthe Columbia River Gorge annual and diurnal temperatures, little cloudi- across the Cascades. ness, and little precipitation. Continental climate 2. The southeasterly winds through the San is a product of a minimum influence by the Joaquin Valley in California. oceans. Maritime climate, on the other hand, 3. The Santa Ana wind in the Los Angeles prevails over the oceans, which spawn it. Charac- region. The Santa Ana blows through the Cajon teristicallyitstemperature range both annual Pass between the San Gabriel and the San and diurnal, is small and its precipitation and Bernardino ranges. At the high-pressure end over cloudiness are great. The great inlandareas of the Mojave Desert, speeds are low, but 15 miles North America and Eurasia, for instance, show farther south, speeds often exceeds 50 mph. At the effect of being a great distance from the the low-pressure end, it may exceed 60 mph for oceans. Much of the moisture has been precipi- many hours. The Santa Ana blows only in the tated by the time the air masses have moved to winter half of the year, as the temperature the interiors from the oceans, leaving little for discontinuity responsible for the pressure gradi- those areas. The humidity is less over areas away ent along the Cajon Pass is absent at other times. from oceanic influence, allowing more of the This is generally true of the Pacific Coast ravine sun's insolation to reach the earth's surface and winds. causing the earth's surface to lose more of its heat through terrestrial radiation; these processes OCEANIC WEATHER help to cause a large diurnal range in temperature. In the summer the relatively cool Since the naval vessels of the United States air masses from the oceans on the west coast of the continents are warmed as they move inland now operate regularly in virtually all the oceanic areas of the earth, you as Aerographer's Mates and lose their maritime effect. On the other hand, therelatively warm air masses inthe must be acquainted with oceanic weather. Some winter are cooled. The northwest coastal area of general considerations of the weatherencoun- theUnited States shows theeffect of the tered over ocean areas are given in this section. maritime influences of the Pacific Ocean. The Because land and water heat and cool at ocean brings to that region a small daily and different rates, the location of continents and annual range in temperature, high humidity, and oceans greatly affects the earth's pattern of air abundant rainfall. temperatureandthereforeinfluencesthe weather. Water vapor is considered one of the most Since the upper layers of the ocean are nearly important variables in meteorology. The state of always in a state of motion, heat lossor heat the weather is largely expressed in terms of the gains occurring at the sea surface are distributed amount of water vapor present and what is throughout large volumes of water. This mixing happening to the water vapor. Two principal process sharply reduces the temperature con- elements of climate, which are precipitation and trasts between day and night and between humidity, are dependent upon water vapor. The winter and summer. oceans are the main source of water vapor. Thus,

27 _ 33 AEROGRAPIIER'S MATE I & C

Aerographer's Mates should see readily that the weather is largely controlled by the oceans. 90° Effects of Air-Sea Interchange 80°

The atmosphere and the oceans have tremen- 700 dous effects on each other. These effects are principallyinthe realm of temperature and 60° water vapor. For a fuller picture of the interchange of the atmosphere and the oceans. you as 50° Aerographer's Mates must consider the various processes that control the heat balance of the air u) w40° and sea. cr The heat balance of the oceans is maintained § by the processes of radiation, the exchange of z 30° sensible heat, and the evaporation and conden- 620° sation of water vapor on the sea surface. The amount of radiant energy absorbed by I- the sea depends upon the amount of energy at, 10° reaching the surface and the amount of reflec- 0° tion. When the sun is directly overhead, the 40% 35% 30% 25% 20% 15% 10% 5% 0 amount of its energy that is reflected amounts PERCENTAGE OF REFLECTION to only about 3 percent. Even when the sun is 30° above the horizon, the amount of reflection AG.389 is just 6 percent. There is a reflection of about Figure 2-8.Percentage of reflected radiation. 25 percent of the energy when the sun is 10° above the horizon. (See fig. 2-8.) The amount of atmosphere is reflected from the surface of the radiation which penetrates the sea surface de- oceans, most of itis absorbed in a very thin pends upon the reflection loss from the total layer of the water surface, since at long wave- amount of incoming radiation. Reflection loss is lengths the absorption coefficients are great. The especially great in the presence of waves when difference between the incoming long-wave at- the sun is low. mospheric radiation and the outgoing long-wave Most of the energy is absorbed in the first radiation from the sea surface is known as the meter of sea water. This is true of the clearest EFFECTIVE BACK RADIATION. The effective water as well as of quite turbid water. In water back radiation depends primarily on the temper- that is extremely turbid the absorption is in the ature of the sea surface and the water vapor very uppermost layers. Foam and air bubbles are content of the atmosphere. The time of day and two major causes of a proportionately greater the season have little effect on effective back amount of absorption in the uppermost meter of radiation, since the diurnal and annual variation the sea. However, due to vertical mixing the heat of the sea surface temperature and of the absorbed in the upper layer is carried to great relative humidity of the air above the oceans is depths of theocean, which actsasa great slight. storage reservoir of heat. In order for conduction to take place between There is an exchange of energy between the the oceans and the atmophere, there must be a oceans and the atmosphere. The surface of the difference in the temperature between the ocean oceans emits long-wave heat radiation, the en- surface and the immediately overlying area. On ergy of this radiation being proportional to the the average the temperature of the surface of the fourth power of the surface's absolute tempera- oceans is higher than that of the overlying air. It ture. The sea surface at the same time receives might be expected that all of the ocean's surplus long wave radiation from the atmosphere. Al- of heat is either radiated or conducted to the though some of this incoming radiation from the atmosphere. This is not the case. Only a small

28 '34 Chapter 2WORLD CLIMATE AND WEATHER

.percentage of the ocean's surplus heat is actually is equal to that of the atmosphere above the conducted to the atmosphere. About 90 percent liquid, there is little or no apparent interchange of the surplus is used for evaporation of ocean of moisture. In other words, at equal vapor water. pressure, as many molecules escapefrom the Due to the processes of radiation and mixing, liquid to the atmosphere as reenter the liquid the oceans act as a thermostat relative to the from the atmosphere. This would be true of air atmosphere. The energy stored at one place that had become saturated. The saturation vapor during one season may be given off at another pressure increases with increasing temperature. locality and during a later season. Hence, there If the temperature of the surface water of the seems to be a constant effort by the atmosphere ocean is warmer than that of the air, the vapor and the oceans to keep their temperature in pressure of the water atits surface is greater balance by an interchange of heat. than that of the air. When this condition exists, STABILITY.As you know, the deciding there can be abundant evaporation from the factor of most weather phenomena is the stabil- ocean surface. This evaporation will beaided by ity of the atmosphere. Air masses may become the turbulence of the air brought on by the more stable or less stable as they move over unstableconditionof thelowestlayers.It ocean surfaces. The temperature contrastbe- follows, then, that the greatest evaporation takes tween the ocean surface and the lowest layers of place when cold air flows over warm ocean the overlying air determines whether the ocean waters. will cause stability or instability. Let us consider the opposite cond,tionwarm When the air moving over the ocean has a air flowing over a relatively cold body of water. highertemperaturethanthat of the ocean When this happens, there is stable stratification surface,the lower layers of the air become in the lowest layers of the atmosphere. The stable. As the air cools and hovers over the vapor pressure of the air soon reaches a state of ocean, conduction of heat from the air tothe equilibrium withthat of the water surface. ocean diminishes rapidly with the sharpreduc- Evaporation stops. However, if the warm air is tion in the convective activity of the air. quite moist, it is possible for the moisture in the On the other hand, when the air mass is air to condense on the water surface. Contact of colder than the ocean surface over which itis the warm air with the cold water may result in moving, thereisresulting instability, as the theformationof fog by loweringthe air colder air is warmed by the ocean, and convec- temperature to the dewpoint. tive activity increases. If the warming by the The direct interchange of moisture from the in the nature of oceanissufficientlyintense,the convective atmosphere to the oceans is currents may rise rapidly to such heights that precipitation and, to a minor extent, of conden- thunderstorms develop. sation of moisture upon the ocean surface. The direct interchange, however, is not as meteoro- MOISTURE CONTENT.The interchange of logically important as the indirect interchange. moisturebetweentheatmosphere andthe features of The indirect interchange is in the nature of the oceans is one of the most important runoff from the land surfaces, which follows a the whole meteorological picture. Without this interchange, weather as we know it could not sequence of events beginning with the evapora- exist: there would be no clouds and no precipitation of water from the ocean surfaces and tion. The oceans are by far the greatest source of ending with the subsequent condensation and moisture for the atmosphere. Other moisture precipitation over land areas. It may be thought sources are negligible in comparison. that there is more precipitation over the oceans Whether the atmosphere gives up some of its than over the land. This is not always the case, moisture to the ocean or vice versa, depends as the ocean surfaces do not favor precipitation greatly upon the vapor pressures of the two. nearly as much as land surfaces. You may recallthat vapor pressureisthe The relatively low evaporation in the tropical pressure exerted by the moleculesof water latitudes is attributed to the high relative humid- vapor in the atmosphere or overthe surface of ities of the atmosphere and the low wind speeds liquid water. When the vapor pressure of a liquid of the equatorial belt. In the middle and high

, 35 AEROGRAPIIER'S MATE I & C

latitudes the lower relative humidities and higher figures 2-6 and 2-7 for the location ofthe wind speeds favor the evaporationfrom the following frontal zones. warm surfaces to the relatively cold air. I.Polar Fronts in the Atlantic. In the Atlan- The moisture supplied to the atmospherein tic, the polar front of winter swings back and the higher latitudes varies greatly fromwinter to forth between the West Indies and theGreat summer. During the summer months, the air Lakes area, with a maximum of intensitywhen over the sea surfaces is usually aswarm as or the front coincides with the coastline. Waves warmer than theseasurfaces.Inthis case, with cold and warm fronts form along thispolar evaporation is at a minimum. On the otherhand, front and move northeastward alongthe front. the air is colder than theocean surfaces during Likeallcyclonic waves,they develop low- the winter, and evaporation is ata maximum. pressure centers along the frontal trough. They may growintoseveredisturbances and go Equatorial and Tropical Weather through the usual stages of development: formation, growth, occlusion, and dissipation. In the temperate zone, where westerlywinds These cyclonic waves occur in families. Each predominate,pressurepatterns moveinan family of waves is associated witha southward easterlydirection.Inthetropics,however. surge, or outbreak, of cold polar air. The polar weather moves inthe opposite direction. The front commonly extends approximately through normal moist layer is 5,000 to 8,000 feetdeep. the Great Lakes area. As the polar air advances, rising above12,000feetduring periods of itpushes the front southward. The outbreak unfavorableweather.Convergence occursin occurs, and polar air, joining the trade winds, opposing trade wind streams, northward flowing spills equatorward. air, and cyclonic curvature. The deep,moist There is no regular time interval for the large layer andthe convergence accountforthe outbreaks of polar air, but theaverage period weather in equatorial and tropical regions. between themisabout5 1/2days. Under Tropical cyclones, waves in the easterli. average conditions, there are from three to six the cyclonic waves on the polar front between each intertropical convergence zone, and other bad outbreak of polar air. The first of these usually weather zones are discussed in chapter 12 of this training manual. travels along the front that lies farthest to the north. As the polar air accumulates north of the front, the frontis pushed southward, and the North Atlantic and last wave therefore followsa path that starts North Pacific Oceans farther south than the path followed by the first wave. These, families of polar front cyclones POLAR FRONTAL ACTIVITY.In winter, appear most regularly over the North Atlantic themostfavorableconditionsforvigorous and North Pacific in winter. frontal activity are concentrated along theeast During the summer months, the polar front of coasts of North America and Asia. Cold air the Atlantic recedes to a locationnear the Great masses from continental sources meet warm Lakes region, with the averagesummer storm moist air from over the oceans. Thewarm ocean track extending from the St. Lawrence Valley, currents along these coasts greatly accentuate across Newfoundland. and on toward Iceland. the frontal activity. The great difference in the Polar outbreaks, with their accompanying family temperature of the air masses, caused by the groupings of cyclones, are veryirregularin contrastingcharacteristicsandproximityof summer and often do not exist at all. Frontal their sources and the great amount of moisture activityis more vigorousinwinter than in that feeds into the air from thewarm ocean summer becausethepolar andtropicalair currents, accounts for the intensity and persist- masses have greater temperature contrastsin ence of these frontal zones off east coasts in winter, and polar highs reach maximum develop- winter. Modification of the air massesas they ment in winter. Both of these factors increase sweep eastward acrossthe ocean causes the the speed of winds pouring into fronts. Over modified frontal activity of west coasts. See oceans of middle latitudes, a third factor helps

30 36. Chapter 2WORLD CLIMATE AND WEATHER to make winterfronts more vigorous than quite vigorously and advance to the American summer fronts. In winter, continental air be- coast, generally occluding against the mountains. comes very unstable when it moves over the When this second polar front exists, two systems comparatively warm ocean surface; in summer, of cyclonic disturbances move across the Pacific. it remains relatively stable over the compara- Becauseof their greater sources of energy, tively cool ocean. Summer frontal activity (in however, storms that originate over the Japan middle latitudes) is therefore weak over oceans as Current and move toward the Aleutians are well as over land. The high moisture content of always more severe. In the Atlantic, a second maritime air causes much cloudiness, but this polar front, similar in nature and cause to the moisture adds little energy to frontal activity in second polar front of the Pacific, sometimes the relatively stable summer air. though rarelydevelops. 2. The polar front activity of the Pacific is During the summer months, the Pacific polar similar to that of the Atlantic, except that in front lies to the north of Kamchatka and the winter there are usually two fronts. When one Aleutians and shows no rhythmic polar out- high dominates the subtropical Pacific in the breaks. winter season, the Pacific polar front forms near AIR-MASS WEATHER.Flying weatheris the Asiatic coast. This front gets its energy from usually at its best in tropical maritime air at its the temperature contrast between cold northerly source, within the subtropical highs. Scattered monsoon winds and the tropical maritimeair cumulus and then patches of stratocumulus masses they meet, and from the warm,moist clouds may develop, but the sky is almost never Japan Current. In moving along this polar front overcast and the scant precipitationfallsin of the Asiatic North Pacific in winter, storms scattered showers. Variable, mild winds prevail. occlude before reaching the Aleutian Islands or The excellent flying weather of these mT the Gulf of Alaska.Because of its steady source regions commonly extends through the cyclonic circulation, the Aleutian low becomes a moving airmasses some distance from the focal center, or a gathering point, for cyclones. sources. Cloudiness in the mT air increaseswith The occluded fronts move around its southern increase in distance from the source. On flights side like wheel spokes. This frontal movement is from Hawaii, or from the Azores northward limited to the southern side of the Aleutian low through northward-moving mT air, stratiform because mountains and the North American clouds increase to the north. On flights from winter high-pressure center prevent fronts from HawaiiortheAzores southwardthrough passing northward through Alaska without con- southward-moving mT air(or the northeast siderable modification. trades), cumuliform clouds increase. (We are here When cold season cyclones reach the Aleu- considering only Northern Hemisphere situa- tians and the Gulf of Alaska, cold continental tions. A comparable pattern exists in the South- winds from the Arctic feed them from the ern Hemisphere.) (See figure 2-9 for winds and north, and cool mP winds from the ocean feed stabilityconditionsaroundthesubtropical them from the south. Here, where Arctic air high.) meets maritime air over relatively warm water, is A typical breakdown of the weather condi- the Pacific Arctic front of winter. Although tions which may be encountered in air masses many occluded storms dissipate in theGulf of around the subtropical highs is as follows: Alaska, others become, strongly regenerated with 1. North of a Subtropical High. Any mT air waves developing on the occluded fronts. that moves northward becomes chilled over the When tne Pacific subtropical high divides into cool ocean surface, A stratus overcast may form, two cells or segments (as it does 50 percentof and drizzle may fall. Farther north, low ceilings the time in winter and 25 percent of the time in (usually below 1,000 feet) may reach the sur- summer), a front forms in the vicinity of the face, producing fog. The mT air surges farthest State of Hawaii. Along this front, the Kona north in summer, for then subtropical highs are storms develop and move northeastward.Those best developed and polar fronts liefarthest that succeed in moving beyond the realm of the north. This mT air brings most of the summer northeast trades, which stunt them, may develop fogginess to northern seas and coasts. It brings

31 : 37 AEROGRAPHER'S MATE 1 & C

AIR STABLE (POSSIBLE FOG a STRATUS)

SUBSIDENCE INVERSION (STRONG)AIR STABLE SUBSIDENCE (RELATIVELY DRY WEAK CLIMATE) AIR NEUTRAL OR UNSTABLE S(DRY LITT (WET) RAINFALL (PRECIPITATION AIR UNSTABLE ABUNDANT) (WET)

AG.390 Figure 2-9.Winds and stability conditionsaround the subtropical high. greatest fogginess in the Atlantic where it blows flows over warm water all ofthe way, the air from over the warm Gulf Streamto over the cold Labrador Current (near neither chills nor heats. Over theocean near the Newfoundland), Philippines (and nearFlorida and the West and in the Pacific where it blows fromover the Indies),this warm Kuroshio tradewind brings goodflying Currenttooverthecold weatherclear,orwith Oyashio Current (near Kamchatka). scatteredcumulus 2. East of a Subtropical clouds. When it movesover land, this warm, High. Along the moist air becomes unstable andturbulent and is California coast, and along theAtlantic coast of a source of thunderstorms. When it North Africa, the mT air blows moves over from the west cold land (for example, southeasternUnited and the northwest. This air tendsto remain States in winter), it becomes stable stable for the following and produces reasons: stratus clouds or fog. Over coldocean surfaces, a.It is coming from the northern,cooler such as the Sea of Japan and the portion of the source region. Kamchatka and Labrador Currents, it develops thepersistent low b. Its surface layers remain cool, because it stratus and fogs characteristic of theseareas. moves over cold ocean currents. c.Its upper portionswarm adiabatically ARCTIC AND ANTARCTIC because of subsidence. WEATHER Throughout theyear, airways are smooth. Geographically we think of the Arctic The skies are clear to partly zone as cloudy. Clouds are being north of 66.5° N lat andthe antarctic genet Lily patches of stratocumulus, and rain is zone south :f the Antarctic Circle at 66.5° S lat. rare. The chief flight hazard in this air is coastal Due to increased operation and fog, which often hides the California importance of or Euro- these zones,itis incumbentupon the Aero- pean coastal land. Stratus and stratocumulus grapher's Mate to familiarizehimself with the clouds may cause the sky to beovercast, develop prevailing weather and peculiaritiesof these low ceilings, and produce drizzle thatreduces regions. Only a brief visibility. resume of the weather conditions of thesezones is given here. Many 3. South of a Subtropical High. Wherethe recentlypublished texts and research mT air moves southward papers or southwestward (as provide additional informationon these zones. trade winds), its lower layersare warmed by the tropical ocean surface. This producesscattered Arctic Weather cumulus. Near the Equator, afterab3orbing much moisture and being heated, thisair may Today the Arctic is rapidly develop cumulonimbus. becoming the aerial crossroads of the world.This is not only 4. West of a Subtropical High. ThismT air due to the shorter Arctic blows from the east and the southeast. routes between some Since it of the major cities of the world,but also because 32 38 Chapter 2--WORLD CLIMATE ANDWEATHER flying weather over the Arctic is generally better ture characteristics of the underlyingsurface. than that encountered over the familiar ocean Thus, these areas are air-mass source regions, and routes. As we learn more about Arctic weather they are particularly effective as source regions and its effects upon flying, Arctic flying will during the winter when the surface is covered become even more common. with snow and ice. To understand some of the important prob- The Greenland icecap is essentially a moun- lems of the Arctic, the broad, underlying causes tain range and rises to a height in excessof of the Arctic climate must be understood. 10,000 feet above mean sea level. It restricts the movement of weather systems, oftencausing SEASONAL TEMPERATURE VARI- low-pressure centers to move northward along ATIONS. --From our previous discussion of cli- of the most the west coast of Greenland, resulting in some matic controls we have seenthat the largest rates of falling pressure (otherthan important factor that determines the climate of hurricanes and tornadoes) recorded anywhere in receives from any area is the amount of energy it the world. The deep low centers that movealong the sun. Look back to figure 2-2 and you will note the west coast of Greenland are, in the main, winter much of the Arctic primarilyresponsible for the occasional high thatduring the winds that are recorded in that area. receives little or no direct heat from the sun. In On some occasions, the winter temperatures comparison notice the amount of sunlight re- ra, the Arctic areunusually high. This situation is ceived at different latitudes. The cold, winter b ought about by deep low centers moving into temperatures common in the Arctic result from the Arctic, coupled with compression of air(the a lack of the sun's energy. foehn effect) as it often blows down off the Since the energy from the sun is not the only icecaps,primarilythe the climate, we must slopingedgesof the factor responsiblefor Greenland icecap. considertwo other factors:theland-sea-ice distribution and mountain barriers. All three ARCTIC AIR MASSES.The moisture con- contribute to the tremendous variation in cli- tent of air masses that originate overland is low mate at different points at the same latitudein atall altitudes in winter. The distinctionbe- during the the Arctic. tween air masses almost disappears summer because of the nearlyuniform surface . Land-sea-icefeatures.Inthe Northern Hemisphere, the water features are the Arctic, conditions over the Arctic and subpolar regions. of the North Atlantic, and North PacificOceans. The frozen surface thaws under the influence These bodies of water act as temperature mod- lengthened or continual daylight: the snow melts melts in erators since they do not have large temperature from the glaciers and pack ice; the ice variations. The major exception to this is when the lake areas in the Arctic; and the water areas large areas are covered by ice in winter. The land of the polar basin increase markedly. Thus, the features are the northern bulk of Eurasia. the polar area becomes mild, humid, and semimari- smaller continent of North America, the island timeincharacter. Temperatures areusually area of Greenland. and theCanadian Archi- between freezing and 50°F. Occasionally, strong pelago. As opposed to the water areas. the land disturbances from the south increase the temper- areas tend to show thedirect results of the ature for short periods. Daily extremes,horizon- extremes of seasonal heating andcooling by tal differences and day-to-day variabilities are their seasonal temperature variations. slight. 2. Mountains. T:Arctic mountain ranges of During the winter months the air masses are Siberia and No" 'America are factors which formed over an area that is completely covered contribute to tht. Jimate and air-mass charac- by ice and snow. The air masses arecharacter- teristics of the regions. These mountain barriers, ized by wry coldsurface air and alarge few thou- as in midlatitudes, restrictthe movement of air. temperature inversion in the lowest During periods of weak circulation the air is sand feet. Since the amount of moisturethe air blocked by the ranges and remains more orless can hold is directlydependent on the temper- stagnant over the area. It is duringthese periods ature, the cold Arcticairis very dry (low that the air acquires the temperature andmois- absolute humidity). The air mass thatoriginates 33 39 AEROGRAPHER'S MATE 1& C

over oceans does not have a surfacetemperature each day. There are large differences inversion in winter; the surface in tempera- air temperature ture between the interior and coastalareas. is warmer and there isa corresponding increase Intheinterior during the in the moisture content of theair. It is during summer days, movement inland of moist air from the temperatures often climb to the mid 60'sor low warmer 70's and frequently riseto the high 70's or low water that most of the rather infrequentArctic 80's, cloudiness and precipitation occasionally evenintothe90's.Fort occurs during this Yukon, Alaska, which is just season. north of the Arctic Circle has recordedan extreme high temperature During the summer months,the large expanse of I00°F, while Verkhoyansk of open water and in north central warmer temperatures result in Siberia has recorded 94°F. a somewhat more abundant supply ofmoisture. Consequently, the largest amount of During winter, the interiorareas of Siberia, cloudiness northern Canada, and Alaska and precipitation occurs duringthese summer act as a source months. region for the cold Arcticair that frequently moves southward into the middle latitudes. ARCTIC FRONTS.The weather The associated coldest temperatureson record over the North- with a front in the Arctic hasmuch the same ern Hemisphere have been establishedin Siberia. cloud structure as with midlatitudefronts, ex- Oimekon, Siberia holds the cept that the middle and high cloud record low with a types are temperature -108°F. Snag, in the YukonTerri- generally much lower, and theprecipitation is tory of Canada, witnessed the coldest usually in the form of tempera- snow. ture in North America witha record low of Periods of maximum surfacewind usually -83°F. occur during and just after a frontal passage. In the northern This strong windflow oftencreates hazards, such areas of the interior regions duringthewinter months, as blowing snow and turbulence whichmake temperature are operational flying difficult. usually well belowzero. In fact, during these long hours of darkness, thetemperature nor- The best flying weatherin the Arctic over mally falls to -20°F land is most likely to or -30°F, and in some occur in midsummer and isolated areas the normal dailyminimum tem- midwinter; the worse (low ceilingsand visibili- perature may drop to -40°F. In north ties) in the transition period central between the two Siberia the normal minimum dailytemperature seasons. Winter ischaracterized by frequent in the winter is between -45°F storms and well-defined frontal and -55°F. passages, but The Arctic coastal regions, because of the dryness of the air,cloudiness and which include the precipitation are at a minimum. Canadian Archipelago,are characterized by rela- In summer, tively cool, short there are fewer stormpassages and fronts are summers. During the summer weaker; however, the increased months the temperatures normallyclimb to the moisture in the 40's or low 5.0's and occasionally air results in more widespread cloudsand pre- reach the 60's. cipitation. Over the There is almost no growingseason along the seaareasthe summer coasts, and the temperatures will weather is wry foggy, but windsare of lower fall below speeds than in winter. freezing during all months ofthe year. At Point Barrow, Alaska, the minimum During the transition periods of temperature fails spring and to fall below freezing on only about42 days a fall, operational flying conditionsare usually the year. worst. Frontal systems are usuallywell defined, Over the Arctic Ocean the active, and turbulent. Icingmay extend to high temperatures are levels. very similar tothose experienced along the coast; however, the summertemperatures are TEMPERATURES IN THE ARCTIC.Tem- somewhat colder. peratures in the Arctic, as one might expect, Winter temperatures along theArctic coast are very cold most of the year. But contrary to are very cold but not nearly common belief, the interior areas of Siberia, so cold as those observed in certain interiorareas. Only on rare Northern Canada, and Alaskahave pleasantly occasions does the temperature warm summers with many hours of sunshine climb to above freezing during the winter months.The coldest 34 .40 Chapter 2WORLD CLIMATE AND WEATHER readings for these coastal areas are in the-60's climate over the Arctic Ocean and adjoining desert and -70's (degrees Fahrenheit). coastal areas is as dry as some of the These figures may seem surprising, since at regions in the United States. Most of the annual first one might think that the temperatures near precipitation falls as snow on the ArcticOcean the North Pole would be colder than those over and adjacent coastal areas andicecaps. On the the northern continental interiors.Actually the other hand, most of the annualprecipitation flow of heat from the water under theice has a falls as rain over the interior. moderating effect upon the temperature. RESTRICTION TO VISIBILITY.Two con- CLOUDINESS.Cloudiness over the Arctic is flicting factors make the subject of visibility in at a minimum during thewinter and spring and the polar regions very complex. Arcticair, being at a maximum during the summerand fall. The cold and dry, is exceptionally transparentand average number of cloudydays for the two extreme ranges of visibility are possible.On the 6-month periods on climatic charts shows a other hand, there is a lack of contrastbetween alldistinguishable generaldecreaseincloudiness inthe entire objects,particularly when Arctic area during the winter months. objects are covered by a layer of new snow. During the warm summer afternoons inthe Limitations to visibility in the Arctic are pri- interior regions, scattered cumulusform and marily blowing snow, fog, and localsmoke. occasionally develop into thunderstorms.The Local smoke is serious only in the vicinityof thunderstorms are normally scatteredand sel- larger towns, and often occurs withshallow dom form continuous lines.Along the Arctic radiation fogs of winter. coast and over the Arctic Oceanthunderstorms occur infrequently. Althoughtornadoes have 1. Blowing snow. Blowing snow constitutes a been observed near the ArcticCircle,their more serious hazard toflying operations in the occurrence is extremely rare. Arctic than in midlatitudes, becausethe snow is WINDS.Wind speeds are generally lightin dry and fine, and is easily picked upby gentle the continental interior during theentire year. and moderate winds. Winds in excessof 8 to 12 the interior normally knots may raise the snow severalfeet off the The strongest winds in obscure occur during summerand fall. During winter, ground, and the blowing snow may the interior continental regions are areasof surface objects, such as runway markers. produce only 2. Fog. Of all the elements that restrictflying strong anticyclonic activity which is light surface winds. in the Arctic regions, fog in most respects along the paramount. The two types of fogmost fre- Strong winds occur more frequently advection Arctic coast than in the continentalinteriors. quently found in the polar regions are The frequency with which thosehigh winds and radiation fog. in fall and winter Fogisfound most frequently along the occur in coastal areas is greater parallel to than during summer. These windsfrequently coastal areas and usually lies in a belt the shore. In winter, the sea is warmer thanthe cause blowing snow. air Wind speeds greater than 70 knotshave been land. The moisture in the relatively warm Strong moving from the sea condenses over the cool observed at many Arctic coastal stations. persist- winds are infrequent over theicepack, but the land causing fog. This fog may be quite wind blows continuously,sincethereis no ent. hindrance offeied by natural barrierssuch as 3. Ice Fog. A fog condition peculiar toArctic hills and mouniains. As a result ofthe combina- climatesisicefog.Ice fog is composed of the minute ice crystals rather than water dropletsof tion of wind speed and low temperatures, when Arctic coastal area and the area overthe icepack ordinary fog and is most likely to occur the temperature is about - 40°F or colder. areveryuncomfortable and limitoutdoor human activity. 4. Sea Smoke or Steam Fog. The cold tem- PRECIPITATION.Precipitation amounts are peratures in Arctic may have effects which seem small, varying from 5 to 15 inchesannually in peculiar to people unfamiliar with the area. the continental interior and 3 to 7inches along During the winter months, the inability of the unusual the Arctic coastal area and over theicepack. The airtoholdmoisturecausesan

35 41 AEROGRAPHER'S MATE?, I & C phenomenon called sea smoke. This is caused by The amount of lightfrom a snow-covered open bodies of comparatively warm water occur- surfaceis much greater than theamount re- ring simultaneously withlow air temperatures. flectedfrom darker surfaces of the middle Actually, this phenomenon is about the same as latitudes. As a result, useful illuminationfrom thefamiliar one of steam formingover hot water. equal sources is greater in the Arcticthan in lower latitudes. When thesun is shining, suffi- In the case of sea smoke, thetemperature of cientlightis often reflected from the both the air and the water is snow much lower, but surface to nearly obliterate shadows.This causes the air temperature is stillthe much colder of a lack of contrast, which in turn results inan the two, causing steam to risefrom the open inability to distinguish outlines of water to form a fog layer. This fog terrain or occurs over objects even at short distances.The landscape open water, particularly over leads in the ice- may merge into a featureless grayish-white field. pack, and is composed entirely ofwater drop- Dark mountains in the distance lets. may be easily recognized, but a crevasse (rift ina glacier or 5. Arctic Haze. This is a condition of reduced mass of land ice) immediately in front ofone horizontal and slant visibility (but goodvertical may be undetected due to lack of contrast. The visibility) encountered by aircraftin flight over situation is even worse when theunbroken snow Arctic regions. Color effectssuggest this phe- cover is combined with a uniformly overcast sky nomenon to be caused by very small ice parti- and the light from the sky is aboutequal to that cles. Near the ground it is calledArctic mist or reflected from the snowcover. In this situation a frost smoke; and when thesun shines on the ice person loses all sense of deptham orientation particles, they are called diamond dust. and appears to be engulfed ina uniformly white glow; the term for this optical phenomenonis ARCTIC WEATHER PECULIARITIES.---The whiteout. strong temperature inversions presentover the Pilots have reported that the lightfrom a Arctic during much of wintercauses several half-moon over a snow-covered field issufficient interesting phenomena. Sound tendsto Larry for landing purposes. It is possiblecn occasions great distances under these inversions.On some to read a newspaper with the illuminationfrom days, when the inversion isvery strong, people s a full moon in the Arctic. Even the illumination voices may be heardover extremely long dis- from the stars creates visibilityfar beyond what tances as compared to the normalrange of the one would expect elsewhere. It is only during human voice. Light raysare bent as they pass periods of heavy overcast that the nightdarkness through the inversion at low angles.This may begins to approach the degree ofdarkness in cause the appearance above the horizon of lower latitudes. In lower latitudes, south of 65°N objects that are normally belowthe horizon. latitude, there will be long periods ofmoonlight. This effect, known as "looming," isa foim of a The moon may stay above thehorizon for mirage. Mirages of the type whichdistort the several clays at a time. apparentshape of the sun, moon,or other objects near the horizonare common under Antarctic Weather inversion conditions. One of the most interesting phenomenain the Many of the same peculiarities prevalent Arctic is aurora borealis (northern over lights). These theArcticregions arealsopresentinthe lights are by no means confinedto the Arctic Antarctic. For instance, theaurora borealis has butarebrightestatArcticlocations."1 heir itscounterpartin the Southern Hemisphere. intensity varies from a faint glowon ,ertain There the lights are calledaurora australis. The nights to a glow which illuminates the surface of same restrictionstovisibilityexist over the the earth with light almost equalto that of the Antarctic regions as thoseover the Arctic. A few light from a full moon. of the other characteristics of theAntarctic The reactions resulting in theauroral glow regions are as follows: have been observed to reacha maximum at an Precipitation occurs inall seasons, with the altitude of approximately 300,000 feet. maximum probably occurring insummer. The 36 42 Chapter 2WORLD CLIMATE ANDWEATHER

decreases poleward along the trailing edge of an oldfront. The low amount of precipitation intensifies in certain from the coast. pressure along these fronts Temperatures are extremely cold. In winter, areas as the front surgessouthward ahead of a they decrease from the coast to the pole,but moving mass of cold air from the polarregions. Much of the weather in the TemperateZone is a there is some doubt that this is true in summer. effect of these storms The annual variation of temperature asindicated directresultof the in (especially in winter). In addition to theweather byLittle America shows the maximum is air-mass January and three separate minimums, withthe from theeffect of these storms stations weather. Air-mass weather is the name given to lowest in early September. Other coastal weather in the show similar variations. A peculiar. and todate all weather other than the frontal temperate region. Air-massweather is the net unexplained, feature of temperature variations and during the Antarctic night is the occurrenceof effect of local surface circulation, terrain, maximum temperature on cloudless days inthe the modifying effect of significant waterbodies. detail in chapter early hours after midnight. On cloudy days,the This effect is discussed more in day is warmer than the night. 6. Thereare many subdivisionsof weather regions in the United States. For the purposeof UNITED STATES WEATHER thisdiscussion, we have dividedthe United States into the regions as indicated infigure with minor The weather in the United States, 2 -10. exceptions. is typical of all weather typeswithin the temperate regions of theNorth American, European, and Asiatic continents.The general Northwest Pacific Coast Area the United States, as in the air circulation in has more entire temperate zone of theNorthern Hemi- The northwest Pacific coast area sphere is from the west to the east.All closed precipitation than any other region inNorth surface weather systems (highs and lows)tend to America. Its weather isthe result of frontal occlusions move with this west-to-eastcirculation. How- phenomena, consisting mainly of circulation which move in over the coast fromthe area of 'ever. since this is only the average and orographic and weather systems move withthe general the Aleutian low to the coast, migratory lifting of moist stable maritime air.Predominant flow, the fronts associated with the fog which are lows move southward if' they arecold fronts and cloud forms are stratus and if they are warm fronts.Surface common to all seasons.Rainfall is most frequent northward in winter and least frequent in summer. low-pressurecenters,withtheirassociated weather and frontal systems. areoften referred to as cyclones. Knowledgeof the mean circulation in the temperate region makesit possible to Southwest Pacific Coast Area observe and plot average stormtracks and to future movement with a reasonable This region experiences a Mediterraneantype forecast from any degree of accuracy. climate andis distinctively different Certain geographical and climaticconditions other North American climate.This type climate tend to make certain areasfavorable for the occurs exclusivelyinthe Mediterranean and development and formation of storm centers.In SouthernCaliforniaintheNorthern Hemi- the United States these areas are westTexas. sphere: in the Southern Hemisphereit occurs Cape Hatteras, central Idaho, and thenorthern over small areas ofChile, South Africa, and Once a storm Southern Australia. portions of the Gulf of Mexico. by warm to has formed, it generally followsthe same mean This type climate is characterized formed in that hot summers. tempered by seabreezes, and by track as the previous storm that the temperatures area. The average or meanpaths are referred to mild winters during which seldom go below freezing.Little or no rainfall as storm tracks. and only light to moderate These storms are outbreaks on thepolar front occurs in the summer or the generation orregeneration of a storm rain in the winter. 37 43 AEROGRAPIIER'S MATE 1 & C

1 I u,,WEST CENTRAL N'""' IINTERMOUNTAIN PACIFIC I if, COAST I CENTRAL PLAINS / i /NORTH \ \ / S.WEST \ \ i ATLANTIC PACIFIC \ /COASTAL COAST \ S WEST DESERT\ .1--- \\ AND MOUNTAIN .,.." SOUTHEAST \ AND GULF STATE \ ... Y

AG.391 Figure 2-10.United States weatherregions.

Cold fronts rarely penetratethe southwest In midwinter a cold high Pacific coastregion. The weatherover this is generally centered in region is due to the circulation this region and generallyprevents the possibility of moist Pacific of storm passages. Annual air from the west being forcedup the slope of precipitation is light. the coastal range. Insummer, air is stable, and Southwest Desert and MountainArea stratus and fog result. In winter,unstable air, which is forced over the mountain ranges, causes This area includes Lower showers and sometimessnow showers in the California and some mountains. of Southeast Californiaand all the southern half of Arizona and NewMexico and west and southwest Texas. It isan area almost completely Intermountain West CentralArea surrounded by high mountainsand is either all very arid or actual desert. Annual rainfallseldom This area includes the GreatPlains region. exceeds 5 inches. The This region is locatedeast of the Cascade and more northerly sections have cold winters, and allparts have extremely Coastal Ranges and west ofthe Mississippi hot summers. The chief Valley, and north of the flying hazardis the southwest desert area. predominance of summer and The climate is generallycold and dry in the spring thunder- winter, and warm and dry storms caused mainly bymaritime tropical air in summer. Most of penetrating into thisarea with the expansion of the regionis semiarid. The westernmountain the Bermuda high in range, which acts as a climatic summer and the air being barrier, has an forced aloft at the mountains.For this reason extreme drying effecton the air in the westerly nearlyall circulation. significant peaks andranges have thundershowers buildingover them in spring and The maximum rainfalloccurs in the spring summer. and Thethunderstormsaregenerally is due mainlytothe predominance of scattered but always cyclonic storm track severe; however, they may passages during this season. generally be avoided by circumnavigation.

38 44 Chapter 2WORLD CLIMATE ANDWEATHER the gulf Central Plains Area squall lines, air-mass thunderstorms, and stratus occurring in variouscombinations make (,;. the This area includes the continentalclimate this area an especially difficult one regions of the Great Plains, MississippiValley, forecaster. Rocky Frontal passages may be expected only inthe and Appalachian Plateau between the winter, Mountains to the west and the Appalachians to late fall, winter, and early spring. In the when the circulation near the surfaceis south- the east and the Gulf States on the south. cooled from In the western section itis rather dry but erly, the warm moist gulf air is below to saturation. When this occurs,fog and moisture increases eastward. The main weather the area for hazards arecaused by wintertime outbreaks stratus may form and persist over along the polarfront andassociated wave several days. The southerly circulation in sum- phenomena. Also, the development of convec- mer causes warm moistair to be heated from thunderstorms which are below, and convective thunderstorms are com- tivetype air-mass generally quite moist and prevalent over this area in summer. mon. Since the air is Frontal passages, both cold and warm,with unstable, the storms are generally severe. associate,: weather are common in this area. Thunderstorms are usually of convective origin and are most violent if they havedeveloped in North Atlantic Coastal Area maritime tropical air. This occursoften in the spring, and tornado activity becomes aclimatic This is an area of storm track convergence, accompanying feature due to its frequency. and cyclonic storm activity with weather is frequent in winter. Moreover,this is intensified by Southeast and Gulf States Area an area in which these storms are heating and addition of moisture overthe Great This area includes all the states bordering on Lakes. The lake effect is directlyaccountable for and the great amounts of snowfalloften found over the Gulf of Mexico and South Carolina weather Georgia. A circulation phenomenon known as this area in the winter. Generally good gulf stratus affects this area. The stagnationof prevailsinsummer due to thedominating southward moving cold fronts, rapidlymoving influence of the Bermuda high.

39 CHAPTER 3

ATMOSPHERIC PHYSICS

The science of physics isdevoted to finding, Vectors defining, and reaching thesolutions to problems. Physics, therefore, not onlybreeds curiosity of the surrounding environment, Problems arise in which it isnecessary to deal but it provides a with one or more forces actingon a body. To means of acquiring answers to questionswhich continue to arise. solve problems involving forces,a means of representing forces must be found. The basic physical subjectsrelating to motion, A force is completely described when itsmagnitude, direc- force, and energy as they pertainto meteorology tion, and point of applicationare given. Because are briefly presented in chapter3 of Aerog- a vector is a line which represents rapher's Mate 3 & 2, both magni- NavTra 10363-D. Itis tude and direction,a vector may be used to recommended that this materialbe reviewed describe a force. The length of prior to continuing with the line repre- the more complicated sents the magnitude of the force, the processes to be presented in this direction manual. of the line represents thedirection in which the force is being applied, andthe starting point of the line represents the pointof application of the force. For example,to represent a force of 10 pounds acting due VECTORIAL MOTION east on point A, drawa line10 units long, startingat point A and FORCE extending in a direction 90°clockwise from north. (See fig. 3-L) Many people think that allforce comes from muscular effort, suchas the effort required to push a box resting on the deck.However, water 10 LB in a can exerts forceon the sides and bottom of the can and an upward forceon any object on the surface of the water. Atug exerts force on A the ship it is pushingor pulling. In all of these examples, the body exerting the force isin N contact with the body on whichthe force is exerted; forces of thistype are called contact forces. There are also forces whichact through i empty space without contact. Theforce of gravity exerted on a body bythe earthknown AG.392 as the weight of the bodyisone example of Figure 3-1.Example ofa vector. this type of force. Forces which act through Composition of Forces empty space without contactare called action- at-a-distance forces. Electric and magneticforces If two or more forcesare acting simultaneously are action-at-a-distance forces. at a point, the same effectcan be produced 40 _ 46 Chapter 3ATMOSPHERIC PHYSICS by asingleforceof thepropersize and direction. This single force, which is equivalent to the action of two or more forces, is calledthe resultant. Putting component forces together to -..., find the resultant force is called composition of .....-., forces. The vectors representing the forces must --....-..... be added to find the resultant. Because a vector (R) RESULTANT represents both magnitude and direction, the method for adding vectors differs from the procedure used for scalar quantities, which have no direction. Consider this example: Find the resultant force when a vertical force of 5 pounds and a horizontal force of 10 pounds are applied at point A. (See fig. 3-2.) AG.394 Figure 3-3.Graphic method of the composition of forces.

r Let AB and AC (fig. 3-3) represent the two X C C forces drawn accurately to scale. From point draw a line parallel to AB and from pointB draw alineparallelto AC. The lineswill intersect at a point X, as shown in figure3-3. forces 5 LB. The force AX is the resultant of the two AC and AB. Note that the two dashed linesand thetwo given forces make aparallelogram ACXB. Solving for the resultant in this manner is called the parallelogram method. The sizeand AG.393 direction of the resultant may be found by Figure 3-2.Composition of two measurement from the figure drawn toscale. right-angle forces. This method applies to any two forcesacting on a point whether theyact at right angles or not. Note that the parallelogram becomes arectangle The resultant force may be found asfollows: for forces acting at right angles. Represent the given forces by vectors ABand Consider the following problems as practical AC drawn to a suitable scale, as infigure 3-2. At examples of combining forces: perpendicular 1. A supply bargeis anchored in a river points B and C draw dashed lines it to AB and AC, respectively.From A draw a line during a storm. If the wind acts westward on to the point of intersectionX of the dashed with a force of 4,000 pounds and the tide acts lines. Vector AX representsthe resultant of the southward on it with a force of 3,000 pounds, what is the direction and the magnitudeof the two forces. Thus, when twomutually perpendic- 34 ular forces act on a point, the vectorrepresent- resultant force acting upon the barge? Figure ing the resultant force isthe diagonal of a shOws the forces acting upon the barge. rectangle. The length of AX, on the samescale If the force vectors have been drawn toscale, original forces, is the size of the magnitude and direction of theresultant as that for the two the length of the the resultant force; the angle0 gives the direc- may be obtained by measuring diagonal of the completed parallelogram andthe tion with respect to the horizontal. the When it is desired to find the resultantof two angle the diagonal makes with a side 'of forces which are not at right angles, thefollow- parallelogram. The resultant can be found also ing graphic method may be used: by geometry and trigonometry.

41 AEROGRAPHER'S MATE1 & C

reverse operation of subtraction. Considerthe WIND 4 000 LB. problem of subtracting force ACfrom force AB. (See fig. 3-5.)

TIDE 3,000 LB.

AG.395 Figure 3.4. Problem using the composition of forces.

R2 = 30002+ 40002 R2 = (3 X 1,000)2+ (4 X 1,000)2 R2 = (32 X 1,0002)+ (42 X 1,0002) AG.396 Figure 3.5.Parallelogram method of R2 = (1,0002)X (32 + 42) subtracting forces. R2 = (1,0002) X(25) First, force AC is reversed indirection giving AC. Then, forces- AC and AB are added by the R = 01,0002) X (25)= (1,000) X (5) parallelogram method, giving theresulting AX, which inthis caseisthe difference between R = 5,000 lb forces AB and AC. A simplecheck to verify the results consists of adding AXto AC; the sum or From trigonometry: resultant should be identical with AB. MOTION tan 0 opposite side adjacent side Everything about us in the worldmoves. Even a body supposedly at rest on the surface ofthe 3,000 tan 0 =3,-0.750 earth is in motion because it isactually moving 4,000 with the rotation of the earth andthe earth is turning in its orbit around the The angle 0 can be found from sun. Therefore, tan 0 by using the terms "rest" and "motion"are relative either trigonometric tables or the slide rule. terms. The change in position ofany portion of matter is motion. The atmosphere, being 0 = 36.9° a gas, is subject to much motion. Temperature,pressure, and density act to produce themotions rif the The resultant is a force of 5,000 poundsacting atmosphere. The motionsare subject to well- at an angle of 36.9° with the directionof the defined physical laws. wind or in a direction of 233.1° (270°36.9° = 233.1°), Newton's Laws of Motion 2. With a slight modification,the parallelo- Newton's first law of motion gram method of addition applies also deals with the to the principle of inertia and states thatevery body 42 48 Chapter 3 ATMOSPHERIC PI IYSICS continues in a state of rest or of uniform motion vrt= V'T, or it can be written =17 in a straight line unless acted upon by sonic external force. The second law states that whenter a net V initial volume force acts on a body, it produces an aeceleation T initial temperature (absolute) in the direction of the net force. Theaccelera- V' new volume tion is directly proportional to the net forceand T' new temperature (absolute) inversely proportional to the mass of the body. Newton's third law of motion concerns the The UNIVERSAL GAS LAW is a combina- principle of action and reaction, and statesthat tion of Boyle's law and Charles' law. It states to every action there is an equaland opposite that the product of the pressure and volume of a proportionalto the.,solute reaction. gasisdirectly temperature. The formula is as follows: GAS LAWS PVT' = P'V'T In order for you as Aerographer's Mates to have ageneral understanding of the earth's P initial pressure atmosphere, itis necessary for you to have a V initial volume working knowledge of the gas laws.From these 'I' initial temperature (absolute) you can readily discernthat the behavior of any P' new pressure gas depends upon thevariations in temperature, V' new volume pressure, and density. Sincethe atmosphere is a new temperature (absolute) mixture of gases, its behavior isgoverned by well-defined laws. Although these gas laws were EQUATION OF STATE they are discussed in AG 3&2, NT 10363-D, a general presented briefly in the following paragraphs to The EQUATION OF STATE is priortodiscussingtheir formula which gives the same information as providecontinuity Boyle's law and Charles' law. It involves a gas application. A real and ideal gas is a gaswhich obeys constant, which is a value assigned each gas.For Charles' and Boyle's laws perfectly.The atmos- instance, the gas constant of air is 2,870 when phere very nearly obeys these laws,and meteor- the presure is expressed in millibars, and the ologists consider it as a perfect gas in most cases. densityis expressed in metric tons per cubic centimeter. The BOYLE'S LAW states that if the temperature meter or in grains per cubic depend- remains constant, the volume of anenclosed gas constant may be expressed differently, ing on the system of units used. Thefollowing isinversely proportional toits pressure. The formula for Boyle's law is as follows: formula is an expression of the equation: VP = V'P' P = pRT

P pressure in millibars V initial volume p density P initial pressure V' new volume R gas constant new pressure T temperature (absolute) DALTON'S GAS LAWS CHARLES' LAW states that if the pressure remains constant, the volume of a gas isdirectly John L.Iton, an English physicist,formulated the pres- proportionalto its temperature. Experiments two fundamental gas laws relative to show that the volume will increase by1/273 for sure of a mixture of gases.One of the laws states in temperature. The formula for that a mixture of several gases whichdo not a 1°C rise equal to the Charles' Law is as follows: react chemically exerts a pressure

43 AEROCRAPHER'S MATE 1 &C

sum of the pressures which theseveral gases laws are essentially the would exert separately ifeach were allowed to same, there are two occupy the entire space alone variables which must beconsidered whenap- at the given plied to the atmosphere.They are temperature temperature. The other law statesthat the total and density. pressure exerted by the mixture ofgases is equal From Charles' law it to the sum of the partialpressures. was learned when the Water vapor in the atmosphere, temperature increases, the volumeincreases and for instance is the density decreases.Therefore, the thickness independent of thepresence of other gases; the of alayer of air vapor pressure is independent of the will be greater when the pressure of temperature is increased. To find the dry gases inthe atmosphere. The the height of a vapor pressure surface in the atmosphere(such as in pressure for any given temperature hasa maxi- working up an adiabatic chart) mum limit, reached when the air is these two vari- saturated. ables must be taken intoconsideration. The total atmosphericpressure is found by In the computation ofa radiosonde observa- adding the pressures of thedry air and the water vapor. tion, a set of tables has beencomputed and the density has been incorporatedin these tables. AVOGADRO'S NUMBER The thickness of a layercan be determined by the following formula: It was the hypothesisof Avogadro thatequal volumes of allgases under the samepressure and Z = (49,080 + 1070Po- P temperature contain thesame number of mole- Po + P cules. The number ofmolecules ina gram molecule of gas is known as AVOGADRO'S Z altitude difference in feet NUMBER: a gram moleculeof any gas contains 6.02 X 1023 molecules (thickness of layer) and at 0°C and 760mm t mean temperature in degrees pressure occupies a volume of22,421 cm3. A Fahrenheit gram molecule is the mass ofa compound equal Po pressure at the bottom point numerically to the value ofits molecular weight; of the layer likewise, a gram atom is the mass ofan element P pressure at the top point of numerically equal tothe value of its atomic the layer weight. STANDARD CONDITIONSFOR GASES For example, let usassume that a layer of air between 800 and 700 millibarshas a mean The conditions under whichgases must be compared, densities determined, temperature of 30°F. Applying theformula, the and gas con- following value is obtained: stants derived, are knownas the STANDARD CONDITIONS FOR GASES. Thestandard conditions are a pressure of 760mm of mercury and Z = (49,080 + 107 X 30) 800 - 700 a temperature of 0°C. 800 + 700 HYDROSTATIC EQUATION Z = (49,080 + 3,210)100 1,500 The HYDROSTATIC LAWstates that the difference in pressure betweentwo points at Z = (52,290) is different levels in amass of fluid at rest is equal to the weight of a column of the fluid of a unit Z = 3,486 feet (1,063 meters) cross section reaching vertically fromone level to another. (1 meter = 3.28 feet) For theatmosphere, the hydrostatic law APPLICATION OF THE GAS LAWS states that the difference inpressure between two points in the atmosphere, one above the In the field of aviation, andparticularly in the other, is equal to the weight ofthe air column Naval Weather Service, it is between the two points. Although necessary to be able these two to apply the gas laws tomany situations. For 44 50 Chapter 3- ATMOSPHERIC PI IYSICS example, in the use of weather balloons we mix ENERGY CONSIDERATIONS- helium with a small quantity of air. As we add STABILITY AND INSTABILITY the gases to the balloons, the pressure increases on the walls, but at the same time it is becoming ATMOSPHERiC ENERGY lighter and lighter until it floats. Since helium k CONSIDERATIONS lighter than air, the mixture has become lighter and less dense. This isa direct application of Energy has previously been defined as the Dalton's law. capacity to perform work. There are two basic The hydrostatic law states that the difference kinds of energy. They are KINETIC and PO- in pressure between two points is equal to the TENTIAL. Kinetic energy is ability to perform weight of the air column between these two work due to motion. Potential energy is ability points. We saw in our application of this law to perform work due to position or condition. that the thickness of the column in question is The term "kinetic" is used for energy due to dependent upon the mean virtual temperature of present motion, whereas the term "potential" the column. Virtual temperature is defined as applies to energy stored for later action. the temperature the air would have if all water The kinetic theory of gases is very helpful in vapor were removed from it.It is expressed by understanding the behavior of gases. Gases, like the following formula: other substances, consist of molecules. Unlike solids, molecules of gas have no inherent tendency to stay in one place. Instead, gas molecules, Tv since they are smaller than the space between 1 (0.379S-) them, move about at random (but in straight lines until they collide with each other or with where other obstructions). Their movement has an average speed at a given temperature. When gas Tv is the virtual temperature is enclosed, its pressure depends on the number is the temperature in degrees Celsius of times the molecules strike the surrounding 0.379 is a constant walls in a unit of film,. The number of times the e is the vapor pressure molecules strike per unit of time against the p is the atmospheric pressure wallsremains constant aslong asthe temperature and the volume remain constant. If the When the thickness is decreased, the mean volume (the space occupied bythe gas)is virtual temperature decreases. and the density decreased, the density of the gas is increased, increasesintheair column. Thisi;lustrates and the number of blows against the wails is Charles' law. For layers of equal pressure differ- increased, thereby increasing the pressure. When ence, the thickness of the layers increase as we the temperature is increased, there is a corre- go up into the atmosphere due todecreased sponding increase in the speed of the molecules; pressure on thelayers, and consequentlya they strike the walls at a faster rate, increasing decrease in the density of the strata. This is an the pressure. provided that the volume remains illustration of Boyle's law. constant. Since there is a definite relationship of thick- According to the kinetic theory of gases, the ness to temperature, this relationshipis put to temperature of a gas is dependent upon the rate work in a system called differential analysis. The at which the molecules are moving about and is height or difference in thickness between two proportional to the kinetic energy of the moving isobaricsurfacesisfoundbyapplying the molecules. The kinetic energy of the moving formula or by subtracting the height of the molecules of a gas is the internal energy )f the lower level from the height of the upper level. gas. and it follows that an increase in tempera- These lines of equal thickness are considered and tureisaccompanied by anincreaseinthe couldbelabeled mean isotherms forthese internal energy of the gas. Likewise, an increase layers, showing advection of cold or warm air. in the internal energy results in an increase in

-51 AEROGRAPIIER'S MATE 1 & C the temperature of the gas. parcel of air is lifted in the free atmosphere, it An increase in the temperature of a gas or in encountersareasof decreasing pressure. To itsinternal energy can be produced by the equalize this pressure, the parcel must expand. addition of heat or by performing work on the In expanding, it is doing work. In doing work, it gas.A combination ofthesecanlikewise uses heat. This results in a lowering of tempera- produce an increase in temperature or internal ture, as well as a decrease in the pressure and energy. This is in accordance with the first law density. When a parcel of air descends in the free of thermodynamics. atmosphere,it encounters areas of increasing. In the application of the first law of thermo- pressure. To equalize the pressure, the parcel dynamics to a gas, it may be said that the two must contract. In doing this, work is done on main forms of energy are internal energy and the parcel. This work energy, which is being work energy. Internal energy is manifested as added to the parcel, shows up as an increase in sensible heat or temperature; work energyis temperature. The pressure and density increase manifested as pressure changes in the gas. In in this case, too. other words, work is required to increase the pressure of a gas and work is done by the gas when the pressure diminishc;.It follows, then LAPSE RATES that if internal (heat) energy is added to a simple gas this energy must show up as an increase in The rateof cooling thataparcel of air either temperature or pressure, or both. Also, if undergoes as it ascends in the free atmosphere work is performed on the gas, the work energy (or the rate of heating as it descends) is known must show up as an increase in either pressure or as the ADIABATIC LAPSE RATE. For unsatu- temperature, or both. rated air the rate of change is 1°C per 100 Consider air in a cylinder, which is enclosed meters or 5 1/2°F per 1,000 feet. This is known by a piston. In accordance with the first law of as the DRY ADIABATIC LAPSE RATE. For thermodynamics, any increase in the pressure saturated air the rate of change is different. exerted by the piston results in work being done When a parcel of saturated air ascends in the free on the air. As a consequence, either the tempera- atmosphere, the rate of change is known as the ture and pressure must be increased or the heat SATURATION ADIABATIC LAPSE RATE. equivalent of this work must be transmitted to The saturation adiabatic lapserateisthe the surrounding bodies. In the case of a plain result of the condensation that takes place ina compressor,this work done by a pistonis saturated parcel of air as it ascends above the changed into an increase in the temperature and condensationlevel.For each gram of water the pressure of the air. It also results in some condensed,about 600 caloriesof heatare increase in the temperature of the surrounding liberated. The latent heat of condensation is body. absorbed by the air. Consequently, the lapse rate If the surrounding body is considered to be becomes less than the dry adiabatic lapse rate. insulated so that it is not heated, there is no heat The mean slope of the saturation adiabat may transferred, and the air must acquire this addi- be taken as approximately 0.55°C per 100 tional energy as an increase in temperature and meters or 3°F per 1,000 feet. The term MEAN pressure. The process by which a gas, such as air, SLOPE is used because the saturation adiabatic is heated or cooled without heat being added to lapse rate increases with altitude. This is a result the gas or taken away is called an adiabatic of the decrease of water vapor with altitude, process. consequently a decrease in the total heat of In the atmosphere, adiabatic and nonadiabatic condensation which is liberated. processes are taking place continuously. The air The normal or average decrease of tempera- near the ground is receiving heat from or giving ture with heightis known as the normal or heattothe ground. These are nonadiabatic average lapse rate. The normal lapse rate is about processes.However,inthefree atmosphere 3 1/2°F per 1,000 feet up to the tropopause. somewhat removed from the earth's surface, the However, the actual lapse rate in the atmosphere short-periodprocesses areadiabatic. When a atanygiventime depends on turbulence,

46 52 Chapter 3ATMOSPHERIC PHYSICS radiation processes, conduction of heat near the descent, the same stages will occur in reverse wound, or the transport of air by horizontal order and the parcel arrives at the original level advection in the upper layers. Condensation of (pressure) with the same temperature az: before. moisture or evaporation also affect the lapse rate by the addition orremoval of the heat of Irreversible Process condensation. Figure3-6 shows some of the various types of lapserate which may be found The IRREVERSIBLE PROCESSisbased upon the assumption that all excess water which in the atmosphere. is condensed from the air will fall immediately as precipitation. The stages in this process are

KM nearly similar to the reversible process. 1. The dry stage is the same as the reversible. 3 2. The rain stage is the same as the reversible except that moisture falls as rain. 3. The hail stage is eliminated, since there is . . ISOTHERMAL no liquid to change into ice. 4 2 0. 4. The snow stage is the same as the revers- 171 ible except that excess moisture condenses and

DRY falls as snow. ADIABATIC The parcel is dry by the time it reaches the 1 M top of the atmosphere; therefore, it must de- INOVRSIONE scend dry adiabatically, and the temperature is GROUsgr INVER therefore much higher upon reaching the origi- 0 nal level. .10° 0 10 20 °C The irreversible process closely approximates TEMPERATURE the actual conditions in the atmosphere; this is reflected on the Skew T diagram by the satura- AG.397 tion adiolbats showing no hail stage. Figure 3-6.Lapse rates in the atmosphere. Figure 3-7 shows the various stages during the Reversible Process ascent and descent of air in the irreversible process. ,The REVERSIBLE PROCESS is based upon the assumption that no condensed water falls as precipitation, but is carried with the ascending parcel. The ascending parcel undergoes several stages as follows: 1. The dry stage, where the parcel is lifted dry adiabatically to saturation. 2. The rainstage,whereallwater vapor exceeding the saturation amount is condensed to I liquid water. 3. The hail stage, occurring at 0°C when the parcel rises isothermally because the expansional cooling is offset by the heat of fusion being released when the liquid water is frozen. 4. The snow stage, when the excess moisture is changed directly from a vapor to a solid (snow). The processisreversible, since the parcel reaches the top of the atmosphere with the same AG.398 water content with which it started. Upon its Figure 3.7. The irreversible process. 47 AEROGRAPHER'S MATE 1 & C

If a parcel of air (A) follows the dry adiabatic reduced to near the dewpoint, or the moisture lapse rate to the 1,000-mb line, the temperature content must be increased so as to increase the at that point (P) is the potential temperature. If dewpoint to near the temperature. In laboratory the parcel continues up the dry adiabat to the experiments, it has been proved that even these saturation point (S), condensation will begin and conditions will not induce condensation if the the parcelwill follow the saturation adiabat airispure. It was proved that in pure air a through the rain stage (S41). When the tempera- supersaturation of 400 percent was possible ture falls below freezing, the condensation will before condensation occurred. be in the form of snow (H-D). Actually in nature Therearealsoseveralother observations supercooled waterdroplets may form even which do not conform to these theories. Drizzle below the freezing point. When all the moisture may fall out of stratus or stratocumulus layers has condensed, all of the latent heat of conden- that do not extend into the freezing tempera- sation has been added to the air, and if the tures, particularly at sea and in coastal districts parcelisthen brought back to the 1,000-mb with onshore winds. In the Tropics, and also in level,it will follow the chy adiabatic lapse rate warm maritime air masses in temperate latitudes, (D-E) and arrive back atthislevelwith a cumulus has been observed to yield light, moder- temperature of E. This hypothetical temperature ate, and even heavy rain though the tops did not is called the equivalent potential temperature. extend above the freezing level. The explanation for the latter phenomenon CONDENSATION AND PRECIPITATION appears to be that in the Tropics, where the freezing levelis high, cumulus can develop to The CLASSIC CONDENSATION THEORY is such great depths without reaching 0°C, that the theory used when making thermodynamic coalescence between clouddroplets becomes computations, such as on the Skew T diagram. It effective enough to result in appreciable rain. is perfectly valid for these computations. In this When the cloud droplets are of initially different theory itis assumed that water is entirely in sizes and hence have different settling rates, vapor form until 100 percent relative humidity some of them will collide and coalesce. This is reached, and then it changes to liquid or ice. It increases their settling rate, and consequently, assumes that liquid drops do not exist at a the number of collisions per unit of time. By temperature below freezing and below freezing sucha chain reaction, the droplets grow to only direct crystallization or sublimation occurs. sufficient size tofall out as precipitation. A When attempting to explain the actual process cloud of great vertical extent is needed for this ofcondensation and precipitationin the at- process to resultin heavy precipitation. This mosphere.theCLASSIC CONDENSATION conditionis satisfied in the Tropics. The high THEORY is no longer completely valid. These temperatures found there give a high liquid- arc some of the defects in the theory water content, which also furthers the coales- I. Clouds, and especially fog, are likely to cence process. occur at less than 100 percent relative humidity. The drizzle which falls out of nonfreezing The whole process of formation of a droplet is a layer cloudsisa more gentle display of the continuous one that, however, is most rapid at coalescence process. Drizzle appears to be more 100 percent humidity. frequent over the sea and Along the sea borders, 2. Liquid droplets supercooled to tempera- other things being equal; this fact favors the part turesseveraldegreesbelow freezing areso played by salt nuclei. Precipitation from non- common in the atmosphere as to be regarded as freezing clouds has not been noted in continen- the rule rather than the exception. tal air masses. 3. Liquid drops not only exist at tempera- From these observations we may reach the tures below freezing, but new condensation following conclusions. The coalescence process occurs at these temperatures as welLas direct may accountfor much of the precipitation sublimation. which falls in the Tropics; the classic theory, on Before condensation can occur in the free the other hand, applies to most of the precipita- atmosphere, the temperature of the air must be tionoccuring inmiddle and highlatitudes. 5448 Chapter 3-ATMOSPHERIC PHYSICS

Whenever moderate or heavy rain falls in the ice saturation. Sublimation nuclei are not very temperate or Arctic regions, it originates mainly active between 0°C and -10°C; in fact, when in clouds that, in the upper portions at least, considering the entire atmosphere at all tempera- have reached negative Celsius temperatures. tures down to about -40°C, there are more A further conclusion based on many studies liquid droplets than ice particles. of temperature of the cloud top as a condition Neutral or nonhygroscopic particles are part- forprecipitationrevealsthatwhen rain or icles such as ordinary dust. These particles may snow-continuous or intermittont reaches the act as condensation nuclei, but seldom do. ground from stratiform clouds, the clouds-solid The atmosphere has been described as an or layered-extendin most cases to heights AEROSOL, which is a colloidal system in which where temperatures are below -12°C or even the dispersed water vapor is composed of either -20°C. solidorliquidparticles, andinwhich the dispersing medium is the air. A cloud has been Nuclei described as colloidally unstable by virtue of its position in a turbulent atmosphere. The foreign particles in the air may be divided There have been many theories presented on into three classes: theprocesses leading tocolloidalinstability I. Hygroscopic nuclei. within a cloud which would cause the growth of 2. Sublimation nuclei. raindrops. A few of the more feasible ones are as 3. Neutral or nonhygroscopic particles. follows: When air is cooled so that it approaches the 1.Electrical attraction. dewpoint, the hygroscopic nuclei begin to ab- 2. Hydrodynamical attraction. sorb water from the air. The larger nuclei will 3. Vapor pressuregradient from smaller to then cause condensation to occur, even before larger drops. the saturation point is reached. However, as the 4. Introductionof extremelyhygroscopic drops grow in size they become so diluted that nuclei. they become less and less active as hygroscopic 5. Collision due toturbulence. material. The condensation process can then 6. Vapor pressuregradient from warmer to proceed only when the air is cooled slightly colder drops. below its dewpoint so that a slight amount of 7. Vapor pressure gradient from liquid to ice supersaturation is present.It can be seen that (Bergeron-Findeisen Theory). condensation on hygroscopic nuclei is a contin- 8. Nonuniformdropsinthegravitational uousprocess, beginningatlowrelative field. humidities. The last two theories are considered to be the The most common hygroscopic nuclei are sea most important in the formation of raindrops. salts, sulfuric acids, and nitric acids. Hygroscopic The liquid-to-ice theory is the most important nuclei vary in number, with the maximum over during the initial formation of the droplet, but cities, decreasing in number over rural areas, and once they have grown to such size that they at a minimum over the oceans. Annual variations begintofall,the gravitationalfieldtheory in amounts of hygroscopic nuclei are caused by becomes the most important. theincreased amount of combustion taking place during the winter months. Diurnal varia- Cloud and Weather tion is caused by the lack of sunlight at night to Modification Methods oxidize sulfur and nitricdioxides. Since the source region for hygroscopic nuclei is at the Activity in cloud and weather modification surface of the earth, they decrease in number has been on a sound and realistic basis only since with altitude. 1958. Sublimationnucleiare much smaller and Cloudmodificationmethodshavebeen fewer in number than hygroscopic nuclei. They mainly in the use of dry ice, silver iodide, and are shaped like an ice crystal. Ice forms on these water to increase the cloud amount and to nuclei below water saturation, but at or above possibly trigger precipitation.

549 AEROGRAPHER'S MATE I & C

The object of seeding with dry ice is to cause if it is lifted to saturation. An understanding of the coexistence of ice and water. Seeding with stability and instability is therefore essential to a dryice may be used to dissipate clouds, to forecaster. precipitate clouds, or to make existing clouds STABILITY isthe state of equilibrium in more persistent. which a parcel of air has a tendency to resist The use of silver iodide has been found to be displacement from the level at which itisin the most effective source of ice crystal nuclei equilibrium withitsenvironment or,if dis- found to date and is most effective at tempera- placed, to return to its original position. IN- tures below10°C. STABILITY is the state of equilibrium in which The use of water attempts to employ the a parcel of air when displaced has a tendency to principle of nonuniform drops in the gravita- move farther away from its original position. tional field. It may be used on actively convec- The stability and instability of air depend a tive portions of large cumulus clouds. great deal on the moisture content of the air. According to Schaefer, the most favorable Therefore, a discussion of equilibrium of air atmospheric conditions for precipitating clouds should be separate with respect to dry air and by seeding are large cumulus clouds without saturated air. precipitation already occurring, abundant moisture, a large lapse rate, a low concentration of Equilibrium of Dry Air ice nuclei, the absence of wind shear, and either no inversions or few inversions, or weak ones. The method used for determining the equi- In practical application of the weather modifi- librium of air will be the parcel method, wherein cation program the prevention of thunderstorm a parcel of air is lifted and then compared to the formation, hail, windstorms, and torrential rain surrounding air to determine its equilibrium. may be accomplished by either dissipating or The dry adiabatic lapse rate is always used as a overseeding, or increasing precipitation by seed- reference to determine the stability or instability ing the clouds with the proper amounts. No of dry air. evidence appears to exist that clouds can be ABSOLUTE INSTABILITY.Consider a col- milked of their moisture in flat regions, but umn of airin which the actual lapse rateis indicationsarethatseeding, especially with greater than the dry adiabatic lapse rate (the silver iodide smoke, can increase precipitation as actuallapserateistotheleft of the dry much as 10 to 15 percent. adiabatic lapse rate on the Skew T diagram). Another application of this processisthe (See fig. 3-8.) If the parcel of air at point A were so-called CLOUDBUSTERS of the Air Force. displaced upward to point B, it would cool at They have found that holes or windows can be the dry adiabatic lapse rate. Upon arriving at punched in certain clouds which hinder aircraft pointB,it would be warmer than the sur- landings and takeoffs, parachute drops, and rounding air. The parcel would therefore have a rescue and reconnaissance missions. Windows more than 3 miles wide have been created by overseeding such clouds with dry ice pellets. Twos R TORy A01A 9ATICI PTO Tr( uOISTAINAIIATICT The dissipation of certain types of fogs can be NAP Mt. THAI SURROUNOiNG MR accomplished by seeding. This is more effective for cold fogs and has little effect on warm or ice crystal fogs. ACTUAL LAPSE CA:leAsBATiC, SA AT 0 IRAT ATC RATE \ LAPSE RATE \ RATE STABILITY AND INSTABILITY \\ Mostweatherphenomenadependupon whether the air masses are stable or unstable. As .10 0. to. stated before, moisture content plays an impor- AG.399 tant part in weather. A parcel of air may be Figure 3-8.Absolute instability (any degree stable when dry and then may become unstable of saturation).

50 Chapter 3ATMOSPHERIC PHYSICS tendency to continue to rise, seeking air of its would cool at the dry adiabatic lapse rateif own density. Consequently the column would displaced upward. It would at all times be at the be unstable. From this, the rule is established same temperature and density as the surround- that if the 'apse rate of a column of air is greater ing air, and would have a tendency neither to than the dry adiabatic lapse rate, the column is return to nor to move farther away from its in a state of ABSOLUTE INSTABILITY. The original position. The column of dry air there- term "absolute"is used because this applies fore, would be in a state of NEUTRAL STA- whethertheairisdry or saturated,asis BILITY. evidenced by displacing upward a saturated parcel of air from point A along a saturation Equilibrium of Saturated Air adiabat to point B'. The parcel is more unstable than if displaced along a dry adiabat. When saturated air is lifted, it cools at a rate STABILITY.Consider a column of dry air in different from that of dry air. This is due to which the actual lapse rate is less than the dry release of the latent heat of condensation, which adiabatic lapse rate (the actual lapse rate is to is absorbed by the air. The rate of cooling of the right of the dry adiabatic lapse rate on the saturated air is known as the saturation adiabatic Skew T diagram). (See fig. 3-9.) If the parcel at lapse rate. This rate is used as a reference for point A were displaced upward to point B, it determining the equilibrium of saturated air. would cool at the dry adiabatic lapse rate, and ABSOLUTE STABILITY.Consider acol- upon arriving at point B it would be colder than umn of air in which the actual lapse rate is less the surrounding air.It would therefore have a thanthe saturation adiabatic lapse rate (the tendency to return to its original level. Con- actual lapse rate is to the right of the saturation sequently, the column of air would be stable. adiabatic lapse rate on the Skew T diagram). (See fig. 3-10.) If the parcel of saturated air at point A were displaced upward to point B, it would cool at the saturation adiabatic lapse rate.

(POINT B COLDER THAN I and upon arriving at point B it would be colder THE SURROUNDING AIR than the surrounding air.

!POINTS 8 (DRY ADIABATIC) AND 8.(MOIST ADIABATIC11 %BOYER NAN SURROUNDING AIR DRY ADIABATIC LAPSE RATE A ACTUAL LAPSE RATE DRY SATURATION ACTUAL ADIABATIC ADIABATIC LAPSE LAPSE \ LAPSE \ RATE RATE RATE \

0.t0° ° 10°

AG.400 Figure 3-9.Stability (dry air).

From this,the rule is established that if the -10° 10° actual lapse rate of a column of DRY AIR is less than the dry adiabatic lapse rate, the column is AG.401 stable. Figure 3-10.Absolute stability(any degree of saturation). NEUTRAL STABILITY.Consider a column of DRY AIR in which the actual lapse rate is The layer therefore would be in a state of AB- equal to the dry adiabatic lapse rate. The parcel SOLUTE STABILITY. From this, the following

5I 57 AEROGRAPIIER'S MATE 1 & C

ruleis establishedIf the actual lapse rate for original level. It therefore would be in a state of a column of airisless than the saturation NEUTRAL STABILITY. The rule for this situa- adiabatic lapse rate, the column is absolutely tion is that ti the actual lapse rate for a column stable. Dry air cools dry adiabatically and also of saturatedairisequaltothe saturation wouldbecolderthanthe surroundingair. adiabaticlapserate,the columnisneutrally Therefore,thisruleappliestoallair,asis stable. evidenced when an unsaturated parcel of air is displaced upward dry adiabatically to point B', where the parcel is more stable than the parcel Conditional Instability displaced along a saturation adiabat. INSTABILITY. -Consider now a column of In the treatment of stability and instability so air in which the actual lapse rate is greater than far, only air that was either dry or saturated was thesaturationadiabatic lapse rate.(Seefig. considered. Under normal atmospheric condi- 3 -I I.) If a parcel of saturated air at point A were tions natural air is unsaturated to begin with, displaced upward to point B, it would cool at but becomes saturated if lifted far enough. This the saturation adiabatic lapse rate. Upon arriving presents no problem if the actual lapse rate for at point B the parcel would be warmer than the the column of airisgreater thanthe dry surrounding air. For this reason, it would have a adiabatic lapse rate (absolutely unstable) or if tendency to continue moving farther from its the actual lapse rate is less than the saturation original position. The parcel therefore would be adiabatic lapse rate (absolutely stable). However, in a state of INSTABILITY. The following rule if the lapse rate for a column of natural air lies isapplicableIfthe actuallapse ratefor a between the dry adiabatic lapse rate and the column of SATURATED AIR is greater than the saturation adiabatic lapse rate, the air may be saturation adiabatic lapse rate. the column is stable or unstable, depending upon the distribu- unstable. tion of moisture. When the actual lapse rate of a column of air lies between the saturation adia-

POINT 8 WARDER THAN batic lapse rate and the dry adiabatic lapse rate, THE SURROJNOING AIR the equilibrium is termed CONDITIONAL IN- STABILITY. because the stability is conditioned by the moisture distribution. The equilibrium of SATURATION this column of air is determined by the use of ADIABATIC LAPSE RATE positive and negative energy areas. The deter- A mination of an areaas positive or negative ACTUAL depends upon whether the environment is colder LAPSE RATE \ or warmer than the ascending parcel. Positive areas are conducive to instability; negative areas are conducive to stability. TYPES OF CONDITIONAL INSTA- BILITY.Conditional instability may be one of AG.402 threetypes. The REAL LATENT type isa Figure 3-11.Instability (saturated air). condition in which the positive area is larger than the negative area (potentially unstable). The PSEUDOLATENT type isa condition in NEUTRAL STABILITY. Consider a column whichthepositivearea issmaller than the of saturated air in which the actual lapse rate is negative area(potentially STABLE). The stable equal to the saturation adiabatic lapse rate. A ty pe is a condition in which there is no positive parcel of air displaced upward would cool at the area. saturation adiabatic lapse rate and would at all ENERGY AREAS FOR MECHANICAL times be equal in temperature to the surround- LIFTING. A negative area is the area on a Skew ing air. On that account, it would tend neither -rdiagram) bounded by the temperature curve, to move farther away from nor to return to its the dry adiabat from the surface point to the

52 58 Chapter 3ATMOSPHERIC PHYSICS lifting condensation level (LCL): and the moist (LFC)).Infigure3-12, the negative areais adiabat from the LCL to its intersection with shaded with clashed lines. the temperature curve (this pointon the temper- A positive areais the area (on a Skew T ature curve is termed the level of free convection diagram) to the right of the temperaturecurve,

500 /

A UPPER NEGATIVE AREA

600

POSITIVE AREA eIl 1-.I Vs 0et, onr1 r i) i i .. NEGATIVE , O AREA s r i) , , 4' , , 850 LCL r

POINT A

0/ ISOBARS mb 1000

AG.403

Figure 3-12.LCL, LFC, and negative and positive areasmechanicallifting.

53 59_ AEROGRAPHER'S MATE 1 & C bounded by the temperature curve and the ENERGY AREAS FOR CONVECTIVE saturation adiabat extended upward from the LIFTING. If lifting by convection is expected, LFC. In figure 3-12, itis shaded with a cross the negative area is determined by locating the pattern. intersection of the temperature curve, the average

AG.404

Figure 3-13.CCL, and negative and positive areasconvective lifting.

54 60 Chapter 3ATMOSPHERIC PHYSICS mixing ratio, and a dry adiabat. The average etc., and are usually found during periods of mixing ratiois chosen because more than just intense surface heating. surface parcels are involved in convective activity. The standard practice isto average the mixing ratio for a 100-mb stratum above the Convection Stability surface, or to average the mixing ratio for the and Instability moist surface layer when itis less than 100 millibars in vertical extent. In addition, each In the discussion so far of convection stability locality should add a small factor, determined andinstability, PARCELS of air have been locally, to allow for an increase in moisture; this considered. Let us now examine LAYERS of air. isespecially true on coastlines or near large A layer of air which is originally stable may rivers and lakes. This procedure insures use of a become quite unstable due to moisture distribu- realistic moisture content for the lower layer tion if the entire layer is lifted. Convectivestabilityis (which will be thoroughly mixed in thecon- theconditionthat vectiveprocess). This levelis known as the occurs when the equilibrium of a layer of air, con- ;ive condensation level (CCL). The area because of the temperature and humidity dis- downward from this intersection and bounded tribution, is such that when the entire layer is by the temperature curve and the dry adiabat is lifted, its stability is increased. Convective is the negative area. It represents the energy which instability the condition of must be supplied in order that a parcel of air will equilibrium of a layer of air occurring when the rise from the surface to a level where it will temperature and humidity distribution is such continue to rise without a supply of energy from that when the entire layer of air is lifted, its instability is increased. an outside source. The intersection of the dry adiabat drawn from the CCL with the surface CONVECTIVE STABILITY.Consider a level determines the surface temperature neces- layer of air whose humidity distribution is dry at sary for free convection. Notice that in this the bottom and moist at the top. If the layer of situation the negative area is to the right of the air is lifted, the top and the bottom will cool at temperaturecurve, whereas withmechanical the same rate until the top reaches saturation. lifting the negative area is to the left of the Thereafter, the top will cool less rapidly than temperature curve. (See fig. 3-13.) the bottom. The top will cool saturation adia- The positive area in a situation of convective batically; the bottom will still continue to cool lifting is the area to the right of the temperature dry adiabatically. The lapse rate for the layer curve, bounded by the temperature curve and then will decrease. The stability will increase. the saturation adiabat extended from the inter- The layer must be unstable at the beginning section of the mixing ratio and the temperature and may become stable when lifting takes place. curve; that is, the CCL. (See fig. 3-13.) In a layer thatis convectively stable, the equivalent potential temperature increases with elevation. Au toconvection CONVECTIVE INSTABILITY.Consider a layer of air in which the air at the bottom is AUTOCONVECTION isa condition which is moist and the air at the top of the layer is dry. If started spontaneously by a layer of air when the this layer of air is lifted, the bottom and the top lapse rate of temperature is such that density willcooldryadiabaticallyuntilthe lower increases with elevation. For density to increase portion is saturated. The lower part will then with altitude, the lapse rate must be equal to or cool saturation adiabatically while the top of the exceed 3.42°C per 100 meters. (This is the layer is still cooling dry adiabatically. The lapse AUTOCONVECTIVE LAPSE RATE.) An ex- ratethen beginstoincrease,theinstability ample of this condition is found to exist near increasing. the surface of the earth in a road mirage or a In a layer of convectively unstable air, the dust devil. These occur over surfaces which are equivalent potential temperature DECREASES easily heated, such as the desert, open fields, with elevation.

55 61 AEROGRAPHER'S MATE I & C

In order to determine the convective stability important in determining certain stability condi- or instability of a layer of air, youshould first tions in the atmosphere. We know, too, that the know why you expect the lifting of a whole difference between the temperature and the layer. The obvious answerisanorographic dewpoint is an indication of the relative humid- barrier or a frontal surface. Next, determine how ity,andthat whenthe dewpoint and the much lifting is to be expected and at what level temperature are the same, the air is saturated does it commence, for you need not necessarily and some form of condensation cloud may be have to lift a layer of air close to the surface of expected. Thislendsitselftoa means of the earth. The amount of lifting will, of course, estimating the height of the base of clouds depend on the situation at hand. In determining formed by surface heating; that is, cumuliform the convective stability or instability of a layer type clouds, when the surface temperature and of air at a particular locality proceed as follows: dewpoint are known. We know that the dew- I.Lift the lowest end of the lapse rate along point will decrease in temperature at the rate of the appropriate adiabat(s) (dry, moist, or dry then moist upon saturation) for a predetermined number of millibars. NORMAL LAPSE MOIST ADIABATIC 2.Lilt the upper end of the lapse rate along RATE the appropriate adiabat(s) for an equal number of millibars. ADIABATIC 3. Connect the upper to the lower point thus ISOTHERMAL formed with a straight line, representing the new SUPER lapse rate. ADIABATIC INVERSION Stability Determination from Existing Lapse Rate We can very simply test the stability conditions of the plotted sounding by observing the observed temperatures and lapse rates in reference to superimposed linesrepresenting the dry STABLE and moist adiabatic lapse rates on the Skew T CONDITIONALLY UNSTABLE diagram. ERINUNSTABLE For instance. if we observe that the lapse rate on the actual sounding is to theright of the moist adiabatic rate, the air is absolutely stable. AG.405 If itlies between the moist and dry adiabatic Figure 3-14.--Degrees of stability in relation lapse rates.its stabilityis dependent on the to temperature changes with height. moisture present. and it is called conditionally stable. If the lapse rate is greater than the dry 1°F per 1,000 feet during a lifting process. The adiabatic lapse rate, we have absolute instability ascending parcel in the convective current will inthe air and can expect vertical currents to experience a decrease in temperature of about cause turbulence in that area. 5 1/2°F per 1,000 feet. Thus the dewpoint and Figure 3-14 illustrates the varying degrees of the temperature approach each other at the rate air stability which are directlyrelated to the rate of 4 1/2°F per 1,000 feet. As an example, at which the temperature changeswith height. consider the surface temperature to be 80"17 and the surface dewpoint 62°F, a differenceof Determining Bases of 18°F. This difference, divided by the approx- Convective Type Clouds imate rate that the temperature approaches the dewpoint (4 1/2°F per 1,000 ft) indicates the We have seen from our foregoing discussion in approximate height of the base of the clouds an earlier section of thischapter that moisture is caused by this lifting process (18 4 1/2 =

56 62 Chapter 3ATMOSPHERIC PHYSICS

4,000 feet). This is graphicallyshown in figure Stability in Relation 3-15. to Cloud Type

When a cloud is formed, the stability THE AIR of the atmosphere helps to determine thetype of cloud formed. For example, if the air isvery stable and GIST ADIA 'T 5,000 31111111PRIal itisbeing forcedto ascend the side ofa mountain '(fig. 3-16), the cloud formedwill be 4,000 4 ~ral layerlike with little vertical developmentand W with little or no turbulence. If, however,the air z 3,000 is unstable in the situation, the cloudsformed would have vertical development and 2,000 turbulence atkinkVRA.Y\DIA would be expected with them. Thebase of this LAPSENR TAT E type of cloud would be determined by mechani- 1,000 , mm cal lifting and by the LCL.

40'ISINiMbi,50' 60' 80* DEGREES FAHRENHEIT ELECTROMAGNETIC RADIATION

AG.406 Electromagnetic energy as related to lightand Figure 3-15.Determination of cloud's basewhen the heat were discussed in AG 3 & 2,NavTra dewpoint and temperature are known. 10363-1). The various types ofelectromagnetic energy comprising the electromagnetic spectrum are radio, infrared, visible, ultraviolet,x-rays, The Aerographer's Mate shouldremember gamma rays, and cosmic rays. (See fig. 3-17.) that this method cannot be appliedto all cloud The term spectrum, as it is used here, refers types,butis 'limited to to clouds formed by the whole range of electromagneticradiations. convection currents, such as summertimecumu- Although the basic nature of electromagnetic lus clouds, and only in the locality where the waves is the same (for example, they all travelat clouds form. It is not valid around maritimeor 186,000 miles per second), they do mountainous areas. differ in their wavelength. The wavelengthdetermines

AG.407 Figure 3-16.Illustration showingthat very stable air retains its stabilityeven when it is forced upward, forming a flat cloud. Air which is potentially unstable when forced upward becomes turbulent and formsa towering cloud.

57 63 AEROGRAPHER'S MATE 1 & C

io-3 10-10 52

EXTREME U.V. NEAR VISIBLE INFRARED RADIO AND RAYS U.V.

SCHEMATIC DIAGRAM OF THE DISTRIBUTION OF ENERGY IN THESOLAR SPECTRUM. (NOT TO SCALE). THE NUMBERS AREPERCENTAGES OF THE SOLAR CONSTANT. THE FIGURE FOR THE RADIO ENERGY IS FOR THE OBSERVEDBAND FROM 15 TO 30,000 MHZ.

WAVELENGTHSIN MILLIMICRONS 1012 1014 1016 108 10.4 10.2 1 10 2 104 106 106 1010

COSMIC GAMMA X-RAYS ULTRA- INFRA- HERTZIAN RADIO LONG WAVES ELECTRICAL RAYS RAYS VIOLET RED WAVES RAYS RAYS OSCILLATIONS

VISIBLE SPECTRUM 400 me 700 mp

AG.408 Figure 3-17.Electromagnetic spectrum. whether they will be categorized asinfrared, militaryunits are currently monitoring solar ultraviolet, radio, etc. activity, and reporting, analyzing, and predicting solar activity. The U.S. Navy has developed a During periods of significant solaractivity, system for space environmental monitoringand radiative energy increases of as much as1,000 relatedforecastingtechnologySOLRAD-HI percent may occur in variousfrequency ranges (Solar Radiation-High Altitude) Satellite Experi- scheduled for andx-ray).This energy ment. Launch of the satellites is (radio,ultra-violet, Solar contributiontothetotalemission over the the mid-70's; a Naval Weather Service entire spectrum is very, very small becausethese Forecast Center is scheduled for establishment frequency ranges account for such a small part in the latter half of the 70's. From this, itshould of the total energy, as shown in the upper part be apparent that the Aerographer'sMate must in have some knowledge of solar activityand its of figure 3-17. However, this energy increase general these particular frequencies is extremelysigr;- effects. This chapter will provide some activities; in information on the sun, its atmosphere,and ficant in its effect on some of man's notable features of the solar particular, it is significant to militaryoperations some of the more surveillance disk. A later chapter, SpecialObservations and which are concerned with satellite information. and with communications. Consequently, some Forecasts, will provide additional

58 Chapter 3ATMOSPHERIC PHYSICS

SOLAR ATMOSPHERE

SOLAR SURFACE TEMPERATURE APPROX. 6,000°K

RADIATIVE ZONE

CENTRAL CORE (THERMONUCLEAR PHOTOSPHERE REACTIONS) APPROX. 14,000,000° K

AG.409 Figure 3-18.One-quarter cross-section depictingsolar structure.

FEATURES OF THE SOLAR DISK convective action as within the earth'satmos- The term solar disk refers toa visual image of phere. The gases are cooledto approximately the outer surface of thesun as observed from 6,000 K (Kelvinor absolute)atthe sun's outside regions. surface. The sun may be describedas a globe of gas The main body of thesun, although com- heated to incandescence by thermonuclearreac- posed of gases, is opaque and hasa well-defined tions from within the centralcore. (Sec fig. visible surface referred toas the photosphere. 3-18.) This is the source we see from which all the light The tremendous heat (or energy) generated and heat of the sunisradiated. Above the from within the sun's core is transported bythe photosphere is a more transparentgaseous layer radiative transfer of photons (ameasurement of referred to as the chromosphere witha thickness gamma radiation), which bounce from atom to of about10,000 km. Itishotter than the atom similarto bouncing ballsthrough the photosphere. Above the chromosphere isthe radiativezone.Withintheconvective zone, corona, a low density high temperature region, which extends very nearly to the sun's surface, which is extended far out into interplanetary the heated gases are raised buoyantly upwards spaceby thesolar winda steady outward with some cooling occurring and subsequent streaming of the coronal material. Much of the 59 AliRoGRApHER's MATE I& c

PLAGE SOLAR PROMINENCES

FLARE SUNSPOTS

AG.410 Figure 3-19.Features of the solar disk.

When solar x-ray and solar radio emission originates in floating free with no visible connection. the corona. viewed against the solar disk as illustrated in Within the solar atmosphere certain mole ligute 3-19. ;`try appear as lone, dark ribbons transient phenomena (referred to as solaraetk- and are called filaments: when viewed against ity) occur just as cyclones. frontal systems, and the solar limb, they appear bright and arecalled thunderstorms occur within the atmosphereof prominences. They display a variety of shapes, sizes, and activity which defy generaldescrip- theearth.This activity may consist of the inthefollowing para- tion. they have a fibrous structure and a pear to phenomena discussed 30,000 to graphs which collectively describe thefeatures resist solar gravity. They may extend of the solar disk. (See fig. 3-19.) 40,000 kin above the chromosphere. The more active types appear hotter than thesurrounding at mospherewithtemperaturesnear Solar Prominences/Filaments 10,000.000° K.

Solar prominences/filaments are injectionsof Sunspots gases from the chromosphereinto the corona. They appear as great clouds of gas, sometimes Sunspots are regions of strong localized mag- in resting on the sun's surface and at other times netic tick's and indicate relatively cool areas Chapter 3ATMOSPHERIC PHYSICS

the photosphere. They appear darker than their chromosphere and often completely or partially surroundings and may appear singly or in more obscure an underlying sunspot. complicated groups dominated by larger spots near the center. (See fig. 3-19.) Flares Sunspots bzgin as small dark areas known as pores. These pores develop into full-fledged spots Solar flares are perhaps the most spectacular ina few days, wi.., maximum development of the eruptive features associated with solar occurring in about Ito 2 weeks. Decaying of the activity. (See fig. 3-19.) They appear as flecks of sunspots consists of the spot shrinking in size light which suddenly appear near activity cen- with an accompanying decrease in the magnetic ters,appearinginstantaneouslyas though a field. This life cycle may consist of a few days switch were thrown. They rise sharply to peak for small spots to near 100 days for larger brightness in a few minutes, then decline more groups. The larger spots normally measure about gradually. The number of flares may increase 120,000 km. Sunspots appear to have cyclic rapidly over an area of activity. Small flarelike variations in intensity, varying through a period briglitenings are always in progress during the of about 8 to 17 years. Variation in number and more active phase of activity centers. In some size will oc,:ur throughout the sunspot cycle. As instances flares may take the form of prom- a cycle commences, a few spots are observed at inences, violently ejecting material into the solar high latitudes of both solar hemispheres, in- atmosphere and breaking into smaller highspeed creasing in size and number. They gradually drift blobs or clots. Flare activity appears to vary equatorward as the cycle progresses, reaching a widely between solar activity centers. The great- maximum in about 4 years. After this period, estflare productivity seems to be during the decay will set in and near the end of the cycle week or 10 days when sunspot activity is at its only a few spots appearing in the lower latitudes maximum. (5° to 10°) will be left. Flares are classified according to size and brightness. In general, the higher the importance Plages classification, the stronger the geophysical effects. Sonic phenomena associated with solar Plages such as those indicated in figure 3-19 flares have immediate effects; others, delayed are large irregular bright patches which surround effects (15 minutes to 72 hours after flare). sunspot groups. They normally appear in con- Such phenomena as communications problems junction with solar prominences or filaments and plans for moving toward a solution to these and may be systematically arranged in radial or problems are briefly discussed in chapter 16 of spiral patterns. Plages are features of the lower this manual.

61 67 CHAPTER 4

ATMOSPHERIC CIRCULATION

The direction of motion air follows withinthe force involved. Let us consider, then, a parcel of atmosphere may be either horizontalor vertical, air from the time it begins to move until it or it might move both horizontally and verti- develops into a geostrophic wind. cally. It is very important for the Aerographer's As soon as a parcel of air starts tomove due Mate to be able to determine the types of to the pressure gradient force, the Coriolis force motion which exist and understand the effects begins to deflect it from the direction of the this motion may have on weather changes within pressure gradient force. The Coriolis force is the the atmosphere. It must be remembered that the apparent force exerted upon a parcel of air due existingatmosphericcirculations(air move- to the rotation of the earth. This force acts to ments) will continually cause changes inpres- the right of the path of motion of the parcel of sure, temperature, and moisture content as well air in the Northern Hemisphere and to the left in as various other factors comprising the total the Southern Hemisphere. It always acts at right atmosphere. angles to the direction of motion. In the absence of friction, the Coriolis force will change the BASIC WIND THEORY direction of motion of the parcel until the Coriolis force and the pressure gardient forceare The basic rules pertaining to the relationships in balance. When the two forcesare equal and between pressure gradient and windwere dis- opposite, the wind will blow parallel to the cussedin chapter 5 of AG 3 & 2, NavTra straight isobars. It may be well to remember that 10363 -D, along with a brief discussion ofgeo- the Coriolis force affects the direction but not strophic and gradient wind. the speed of the motion of the air. Normally, In this section we will take a closer look at Coriolis force would not be greater than the these circulations, the forces involved, and the pressure gradient force. In the case of super- effect they have on the general circulation. gradient winds, Coriolis force may be greater than the pressure gradient force. This will cause GEOSTROPHIC WIND the wind to be deflected more to the right in the NorthernHemisphere, ortowardhigher GEOSTROPHIC WIND is a steady horizontal pressure. air motion along straight, parallel isobars inan Figure 4 -Ishows from lefttoright the unchanging pressure field, with gravity the only changes, in three stages, that take place from the external force, and in a direction perpendicular time the parcel of air begins to move until the to that in which the Coriolis force and the final development of a geostrophic wind. The pressure gradient force as acting equally and vectorsintheillustrationindicate direction oppositely. only, not magnitude. In the above definition you may have noted Geostrophic wind is dependent to a certain that gravity is the only external force. Thus, it extent upon the density of the atmosphere and shouldbe apparentthatinorder to have the latitude.If the density and the pressure geostrophic wind, there must be no frictional gradientremainconstantandthr)latitude

62 68 Chapter 4ATMOSPHERIC CIRCULATION

W&PG PG PG4 1 D

i 10041008 1012 10041008 1012 10041008 1012

PG= Pressure Gradient D = Deflecting or Coriolis Force W = Directionof Parcel of Air

AG.411 Figure 41.Geostrophic wind. increases. the wind speed decreases. On the other Centrifugal Force hand, if the latitude decreases, the wind speed increases. If the density and the latitude remain When a parcel of air moves in a curved path, a constant and the pressure gradient decreases, the force acts on the parcel of air and tends to wind speed decreases. If the pressure gradient throw the parcel outward from the center about and the latitude remain constant and the density whichitis moving. This is CENTRIFUGAL decreases,the wind speedincreases.If the FORCE. density increases, the wind speed decreases. True geostrophic wind is seldom observed in nature, but the conditions are closely approx- Movement of Air Parcels imated on upper-level charts. Around Anticyclones

GRADIENT WIND The movement of gradient winds around anticyclones is affected in a certain manner by the GRADIENT WIND is the wind that flows pressure gradient force, the centrifugalforce, parallel to the curved isobars in an UNCHANG- and the Coriolis force. As you would expect, the ING PRESSURE FIELD. when the centrifugal, pressure gradient force acts fromhigh to low Coriolis, and pressure gradient forces balance. As pressure; and the Coriolis force actsopposite to in the case of geostrophic wind, thereis no the pressure gradient force and at right angles to frictional force acting. True gradient winds arc the direction of movement of the parcel.The rarely observed in nature, but rather a mutual centrifugal force acts at right angles to the path coexistence of supergradient and subgradient of motion and outward from the center about winds. (In the case of supergradient conditions, which the parcel is moving. (See fig. 4-2.) In this the wind would be too strong for the existing case, the pressure gradient forceand the' cen- pressure gradient and would beevidenced by a trifugal force balance the Coriolis force. (It may component of thewindacross the isobars be expressed in the following manner: PG +CF toward higher pressure.) = D.)

63 69 AEROGRAPHER'S MATE I & C

PG+A

ANTICYCLONE CYCLONE 1024 1004

1003 1020 1012

1016 D=Ceriolis Force PG:Pressure Gradient CF= k-entrifu 9 al Force W=Dirction Of Parcel Of Air

AG.412 Figure 4-2.Forces acting on pressure systems.

Movement of Air Parcels TUM. This law states that the angular velocity of Around Cyclones a parcel multiplied by the radius of rotation squared, is equal to a constant. In equation form As in the case of anticyclones, gradient winds this may be written wr2 = R, where o) is the around cyclones are affected by the pressure angular velocity of the parcel. (The angular gradientforce, the centrifugal force, and the velocity of a parcel is the value of the angle Coriolis force, but the balance of the forces is through which a parcel moves per unit time; it is different. (See fig. 4-2.) In a cyclonic situation not linear distance.) The radius of rotation, that the pressure gradient force is balanced by the is, the perpendicular distance the parcel is from. Coriolis force and the centrifugal force. (It may the axis of rotation of the earth, is represented be expressed in the following manner: PG = D + by r. The constant is represented by R. CF.) If a parcel of air moves northward in the Centrifugalforce acts WITH the pressure Northern Hemisphere, its radius of rotation gradient force when the circulation is anticy- decreases. If r becomes smaller, w must become clonic and AGAINST the pressure gradient force larger in order that the product of r2 and w when the circulationis cyclonic. Therefore, remain the same. If w becomes larger, the parcel wind velocity will be greater with an anticyclone will have a tendency to move eastward relative than with a cyclone of the same isobaric spacing. to the earth's surface. That this actually happens can be demonstrated on a globe. For example, Conservation of Angular Momentum let us assume that a parcel of air is moving northward along a particular longitude at point The apparent deflection of a parcel of air A in figure 4-3. By the time the longitude has moving from south to north or from north to reached point A' the parcel of air has moved to south may be explained on the basis of the law B because the angular velocity must increase due of CONSERVATION OF ANGULAR MOMEN- to the decrease of the radius of rotation. The

64 70 Chapter 4ATMOSPHERIC CIRCULATION distance from A to A' must equal the distance curved, differ from that determined by a geo- from B to C. C is the point the parcel would strophic wind scale. It should be borne in mind have reached if the earth were tationary. that if the flow under consideration is around a high-pressure cell, the isobars will be farther apart than indicated by the geostrophic wind scale. And, if the flow under consideration is around a low-pressure cell, the isobars will be closertogetherthanindicated by the geostrophic wind scale.

Geostrophic and Gradient Wind Scales There are a number of scales available for measuring both geostrophic and gradient flowof both surface and upper air charts. For a detailed discussion of the theory and construction of these scales consult Meteorological Wind Scales, AG.413 NavWeps 50-1P-551. An explanation of the use Figure 4-3.Parcel of air moving northward of these scales is covered in chapter 6 of this in the Northern Hemisphere. training manual. Some of the weather plotting charts in use in the Naval Weather Service have geostrophic wind If a parcel of air moves southward in the scales printed on them for both isobaric and Northern Hemisphere, the radius of rotation contour spacing. A number of other scales are increases and the angular velocity decreases. available from other publications and sources. However, the parcel of air is apparently still The two common scales in general usage are deflected to the right. for sea level (4-mb isobars) and the pressure In the Southern Hemisphere the apparent contour scale. The firstisa scale usedin deflection of a northward or southward moving determining the geostrophic wind speed, or parcel of air is to the left. isobar spacing on surface (sea level) charts, and the other is used for determining the geostrophic Variations wind speed for contour spacing on constant/ pressure charts. In tropical regions, the geo- In view of the foregoing discussion of the strophic wind scales become less reliable due to movement of air parcels around cyclones and the fact that pressure gradients are generally anticyclones, it can be seen that with the same rather weak. density, pressure g.adient, and latitude, the wind will be weaker around a low-pressure cell than a CYCLOSTROPHIC WIND high-pressure cell. This can also be seen from a comparison of the two types of =client winds In some atmospheric conditions, the radius of with the geostrophic wind. The wind we observe rotation becomes so small that the centrifugal on a synoptic chart is usually strongeraround force becomes quite great in comparison with low cells than high cells because the pressure the Coriolis force. This is particularly true in low gradientis usually stronger around the lowlatitudes where the Coriolis force is quite small pressure cell. to begin with. In this case, the pressure gradient In summary, under given conditions of lati- force is nearly balanced by the centrifugal term tude, density, and pressure gradient, the geo- alone. When this occurs, the wind is said to be strophic wind is stronger than the gradient wind cyclostrophicinflow. By true definition, a around a low and is weaker than a gradient wind cyclostrophic wind exists when the pressure around a high. It is for this reason that isobar gradient forceis balanced by the centrifugal spacing and contour spacing, when the flow is force alone.

65 71 AEROGRAPFIER'S MATE I & C

This exact situation rarely exists, but isso ing potential energy as represented by heat nearly reached in some situations that the small differences into kinetic energy of motion. The Coriolis effect is neglected and the flow is said wide equatorial belt between latitudes 35° N to be cyclostrophic. Winds in a hurricane or and 35° S, representing over one-half the earth's typhoon, and the winds around a tornado are surface, with a net gain of heat, and the higher thought to be cyclostrophic. latitudes with a net loss of heat, superimposed by a mean atmosphere where temperature de- FRICTIONAL EFFECTS creases with elevation; adiabatic contribution of The surface wind is usually about two-thirds temperature; the expansion of the atmosphere of the geostrophic wind. This reduction in wind over equatorial regions: and the contraction of speed is caused by friction. Since the Coriolis the atmosphere over the poles form a perfect force varies with the speed of the wind, a situation for the formation of a solenoidal field. reduction in the wind speed by friction means a A solenoid is a tube formed in the atmosphere by the intersection of isotimic surfaces (isotimic reduction in the Coriolis force. This results ina momentary disruption of the balance and when surfaces are surfaces in space on which the value the new balance, including friction is reached, of a given property is everywhere equal) of two the air blows at an angle across the isobars from scalar quantities. In figure 4-4, the solenoidal high pressure to low pressure. The angle varies field is representative of the mean conditions in from 10° over the ocean to as much as 45° over the Northern Hemisphere. (Note the concentra- Wigged terrain. Frictional effects on the air are tion of solenoids in the middle latitudes.) greatest near the ground, but the effects arc also A solenoidin figure 4-4 is formed by the carried aloft by turbulence. Surface friction is intersection of isolines of specific volume and effective in slowing down the wind up to about isobars and is shown by the enclosed area a, b, c, 1,500 to 3,000 feet above the ground. Above and d. It can be mathematically shown that the this level the effect of friction decreases rapidly work done by a unit mass of air in a circulation and may be considered negligible for all parctical arising from solenoid-producing thermal proc- purposes. Therefore. air at about 3,000 feet or esses can be determined from the solenoidal more above the ground usually tends to flow field. The acceleration of such a circulation parallel to the isobars. depends on the number of solenoids in the atmosphere. Without solenoids, the circulation THE GENERAL CIRCULATION comes to a standstill. The concept of a circulation arising from the formation of solenoids is The interplay of the factors which accomplish not limited to the large-scale general circulation, the balance of heat in the atmosphere is random, butit has equal application in explaining the though always in response to cause. The atmo- formation of secondary and tertiary circulations sphere as a whole is constantly in the process of as well. tryingtoestablish equilibrium.In this Solenoids have little application in everyday equilibrium-seeking process. a general pattern of meteorological work, howeve they have great windflow is establishedthe general circulation. application in research work, when new con- The general circulation of the atmosphere is cepts are formulated or new or improved fore- preciselythat. Irregularities within that circula- casting methods are devised. Solenoidal repre- tion are the rule rather than the exception and sentation has particular application in numerical forthis reason the general circulation is not forecasting work when dealing with the fields of covered at great length. pressure, density, and temperature in the atmosphere. In connection with this particular appli- THEORY cation. two other terms may prove of value to the Aerographer's Mate because of the ever- The basisfortheprimarycirculationis increasing use of them. They arc: baroclinity thermally driven motion of air expressed in and barotropy. A BAROCLINIC fieldis one terms of a solenoidal field. The earth's atmo- where thepressure/densityand temperature sphere is a huge, inefficient heat engine convert- field are nonparallel, that is,it is a solenoidal

66 Chapter 4 ATMOSPI IERIC CI r ':ULATION

AG.414 Figure 4-4.A solenoidal field. field with pressure/ density and temperature as known as centers of action. There are also closed coordinates. A BAROTROPIC field is one where circulations that migrate and play an important isovalues of pressure;density and temperature role in the daily and seasonal changes of the are parallel to each other: that is, it contains no weather. solenoids. There are two factors which cause the pres- The solenoidalpresentation does not EX- sure belts of the primary circulation to break up PLAIN the general circulation.itis only the into closed circulations of the secondary circula- BASIS for the general circulation. For an ex- tions. From our previous discussion in chapter 2 planation of the general circulation with the of this training manual, we know that the earth zonal winds, the consequent formation of the is composed of both land and water surfaces. centers of action, and the influence of migratory The surface temperature of the ocean changes disturbances. the physics of motion must be very little during the year, while in winter the considered. Advanced treatment of the physics land is cold and in summer itis warm. In the of motion is given in chapter 3 of this training winter, highs form over cold continents and the manual. The general circulation. secondary cir- lows over adjacent oceans. The reverse is true in culation with the centers of action, monsoon the summer except in the area over the warm circulations, migratory disturbances. and tertiary water beltnear Greenland which tends to circulations are adequately treated in chapter 5 maintain low pressure. This effect of the differ- of AG 3 & 2. The general circulation also ence in heating and cooling of land and water includes the special circulations, consisting pri- surfaces is known as the thermal effect. marily of the jetstream and zonal flow. Zonal Circulation systems are also created by the flowisthe basis for the zonal index. Both interaction of wind belts or systems, or the specialcirculations,thjetstrzam and zonal variation in wind in combination with certain flow, receive treatment later in this chapter. distributions of temperature and/or moisture. This is known as the dynamic effect. This effect SECONDARY CIRCULATIONS rarely, if ever, operates alone, in creating secondary systems, as most of the systems areboth Since the earth does not have a uniform created and maintained by a combination of the surface, the general circulation. as represented thermal and dynamic effects. by more or less continuous pressure belts around the earth at the various latitudes. is not a true CENTERS OF ACTION picture of actualconditions. Certain effects When diagrams showing the distribution of cause an organized systemof highs and lows mean surface temperature arecompared with 67 73 AEROGRAPHFR'S MATE I R C

those showing mean sea levelpressure.it can winter and the Asiatic low in readily be seen that the pressure belts of the summer. In winter, theAsiatic continentisaregion of strong primary circulation are brokenup and take a cooling and therefore is dominated more cellular stnicture. The break corresponds by a large high-pressure cell. In summer.strong heating is with regions showing differences intemperature present and the high-pressure cell becomes from land to water surfaces. These cells which a large low-pressure cell. This seasonalchange in tend to persist in a particulararea are called pressure cells gives rise to the monsoonal flow of centers of action: thatis.they are found at the India-S.E. ASIAarea. nearly the same location with somewhat similar Another cell which sonic considerto be a intensity during the same month eachyear. center of action isthe polar high. However, There is a periaanent belt of rel,niely low recent explorations into both the Arctic andthe pressure along the Equator and another deeper Antarcti, have revealed considerablevariations ring of low pressure aroundtheAntarctic. in pressure in these regions and thepresence of Permanent belts of highpressure largelencircle many traveling disturbances in summer. The the earth, particularlyLwei the °Leans. in both Greenland high. for example, dueto the Green- the qthern hemisphere and &nit hem lemi- land icecap. is a persistent feature,but it is not a spher.., with a number of centers ofmaximums well- defined high during allseasons of the year. about 30 to 35 degrees from the Equator. It often appears to bean extension of the polar high, or vice versa. Other continental regions showseasonal varia- SEASONAL VARIATIONS tions from low pressure insummer. but are generally of small size. and theirlocation is There are also regions where thepressure is variable. Therefore, theyare not considered to predominantly low or high at certainseasons, be centers or action. but not throughout the year. The annual averagepressure distribution chart In the vicinity of Iceland. pressure is low most for the year. (fig. 4-5) revealsseveral important of the time. The water surface iswarmer than characteristics. theicecaps of Greenland and Iceland. The First, along the Equator there isa belt of Icelandic low is most intense in winter, whenthe relativellow pressure encircling the globe greatest temperature differences with occur. but it barometric pressure of about 1,012millibars. persists with less intensity through thesummer. Second. on either side of thisbelt of low The Aleutian low is most pronounced whenthe pressure is a belt of high pressure. That in the neighboring areas of Alaska and Siberia are snow Northern Hemisphere lies mostly betweenlati- covered and colder than the adjacent ocean. tudes 30° and 40° N with threewell-defined These lows are not a continuation ofone and centers of maximum pressure--one thesame cyclone. They are over the regions of low eastern Pacific. the second over the Azores.and pressure. where lows frequently form or arrive the third over Siberia. all about 1,020millibars. from other regions to remain statilnaryor nune Thebelt of high pressureinthe Southern sluggishly for a time, after which they pass on or I lemisphere roughly follows parallel 30°S.It die out and are replaced by others. Occasionally also has three centers of maximumpressure -one these regions of low pressureare iinaded b!, in the eastern Pacific, the second inthe eastern traveling high-pressure systems. Atlantic, and the thirdin the Indian Ocean, There is a semipermanent high-pressurecenter again about 1020 millibars. over the Pacific to the westward of California. A third characteristic to be noted fromthis Another overlies the Atlantic. near the Azores d'art is that beyond the belt of highpressure in and ott the coast of Africa. Pressure is also high. eitherhemispherethepressurediminishes but less persistently so. westward of theAcores toward the poles. In the SouthernHemisphere to the vicinity of Bermuda. (Botharchest the decrease in pressure toward the SouthPole is developed in the suninnr season.) regular and very marked. Thepressure decreases The largest individual circulation wells in the from an average somewhat above 1,016millibars Northern Hemisphere are the Asiatic high in along latitude 35° S to anaverage of 992 4'8 Chapter 4 ATMOSPI IL:RIC CIRCULATION

rz ez-fisiimmog t A4,,wo 1020 ABA 20 YIMODI-4.016.0*-4. 10 2

0 011111*Igatalletlitin011iaggi

1012

sisieram016 10 6 10 :020 aid2 102011MIENStellairg*I31016 106 1012 102 1111101111WRINTIlliii

40 80 mommimmomm160 120 mom

AG.415 Figure 4-5.Average annual pressure distribution chart

and are millibars along latitude 60° S. In theNorthern millibars. while in July these lows fill up Hemisphere, however, the decrease in pressure almost obliterated. toward the North Pole is less regularand not so The subtropical highs reach their greatest great. intensity during the summer and the area over While. the pressure belts which stand out on which they extend is much greater andthe the average annual pressure distributionchart pressure higher in summer.In winter these represent average pressure distributionfor the subtropical highs are found at lowerlatitudes year, these belts are rarelycontinuous on any and migrate poleward with the onsetof the given day. They are usuallybroken up into summer season andequatorward with the winter detached areas of high or low pressureby the season. secondary circulation of theatmosphere_ In 1 he polar high in winter is not a cellcentered either hemisphere, moreover, the pressure over directly over the North Pole, but it appears tobe the land during the winter seasonis decidedly an extension of the Asiatichigh and often above the annual average, during the summer appears as a wedge extendingfrom the Asiatic season decidedly belowit.the extreme varia- onthient. The cell is displaced towardthe area tions occurring in the case ofcontinental Asia of coldest temperatures, the AsiaticContinent. where the mean monthly pressure rangesfrom In summer, this high appears as anextension of about 1,033 millibars during January toabout the Pacific high and is again displacedtoward 999 millibars during July. Over the northern the area of coolest temperature,which in this oceans, on the otherhand, conditions are case is the extensive water areaof the Pacific. reversed, the summer pressure there being some- Icelandic and In winter over North America.the most what higher. Thus in January the by Aleutian lows intensify to a depthof about 999 significantfeature is tin,domination

69 75 AEROGRAPHER'S MATE I & C high-pressure cells. These cells are also due to North Vietnam experience precipitationand low cooling but are not as intenseas the Asiatic cells. visibilities due to fogas a result of periodic In summer, the most significant feature is the interruptions of the monsoon flowas the out- so-called heat low over the southwesternpart of breaks of polarair the aremodified by their continent whichiscaused by extreme trajectories over waterareas to the northeast. It heating in this region. has a steadiness similar to that ofthe trade winds and often attains the forceof a near gale. TRAVELING DISTURBANCES The southwest orsummer monsoon occurs from May to September. It breaks These disturbances are part of thesecondary with severity on circulation,but do not show upon mean some coasts, accompanied by heavy squalls and pressure charts. They are migratory and include thunderstorms. As the season advancesand the such systems as the lows which southwest monsoon becomes established,squalls form along the and rain become less frequent. In polar front and the migratoryhighs which some places it accompany polar outbreaks intothe middle blows as a light breeze, unsteadyin direction, latitudes. These traveling disturbancesplay an while in other places it prevails with freshspeeds important role in the day-to-day changesin the throughout the season, being infrequentlyinter- weather. rupted by calms or by windsfrom other directions. TERTIARY CIRCULATIONS LAND AND SEA BREEZES The preceding sections deal with thegeneral circulation and the secondary circulationand Corresponding with the seasonalcontrast of the main forces causing windflows, andthe wind temperature and pressure over land andwater patterns found in high- and low-pressureareas. (such as the monsoon effect) there isa diurnal However, the concepts of thelarge-scale move- contrast exercising a similar. thoughmore local, ments of the wind do not take into considera- effect. An example of this type ofinfluence on tion the effects of local conditionswhich fre- the existing wind flow is the "land breez-,"or quently causedrastic modifications in wind its reverse condition commonly referredto as direction and speed close to the earth'ssurface. the "sea breeze." These two diurnalbreezes Thus, from the point of view ofthe meteor- were presented previously in chapter 5 of "iG3 ologist a knowledge of the factors whichaffect & 2, therefore, only brief mentionof them is local winds is desirable. made at this time. Tertiary circulations are localized circulations The sea breeze usually begins in themorning directly attributable toone of the following hours, from 9 toI Io'clock local time, and is causes or a combination of them: local cooling, usually onlya few hundred feetthick. By local heating, adjacent heatingor cooling, and midafternoon, when it has reached itsmaximum induction (dynamics). speed, it can extend upward to 3,000feet in moderately warm climate and 4,500 feetin MONSOON WINDS tropical regions and to as muchas 30 miles both on shore and offshore. Also, by midafternoon it A discussion on monsoon windswas pre- may be strong enough to be influenced by the sented in chapter 5, AG 3 & 2,as well as in Coriolis force, causing it to flow atan angle to chapter 2 of this training manual. Therefore the the shore. following paragraph is confined to a brief mention The land breezes, when compared to thesea of the general type of weather whichmay be breeze, are less extensive and notas strong. Land expected where monsoon conditionsarein breezes are at maximum developementlate at existence. night, in late fall, and early winter. The northeast (winter)monsoon blows in the In the Tropics, the land and South China Sea from October sea breezes are to April. Itis repeated day after day with great regularity.In marked by dry or fair weather with theexcep- high latitudes the land andsea breezes are often tion that the Gulf of Tonkin and thecoast of masked by winds of synoptic features.

70 76 Chapter 4ATMOSPHERIC CIRCULATION winds are also The occurrence or nonoccurrence ofthe sea of windis "anabatic." These breeze and the return land breezeflow is of presented later in this section. critical importance inthe evaluation of the effects of the release of airborne pollutants on Glacier Winds individual harbors. The lapse rate within the sea These winds are caused by air comingin breeze layer is approximately adiabaticin the contact with ice or some othercold surface, as temperate latitudes. The moderateto strong explained in chapter 5 of AG 3 &2. This will wind speeds combined with this lapse ratewould cause faster cooling thanis normal and the air result in excellent dispersal of materialthrough- willbesetinmotion by a strong pressure out the sea breeze layer.However, the top of the gradient or density difference. Also,if this wind sea breeze layer islikely to be marked by an is funneled through a pass orvalley, it may be inversion or at least a decrease in lapse rateand very strong. This typewind may form during the wind velocity so that a fairlyeffective upper day or night, while the mountainbreezes occur barrier to the diffusion of material wouldexist. only at night due to radiationalcooling. The In addition, the sea breeze decreasessteadily in glacier wind is most commonduring the winter depth with its continued penetrationinland, so when more snow and ice are present. that the top of the leading edge maybe only a Glacier winds, as mentioned previously, are few tens of feet above theground and thus cold katabatic winds descendingfrom glacier confine material very effective'),o a shallow regions and are localized in nature. layer. Figure 4-6 shows how light winds canoften The land breezeis quite stable, the wind become much stronger when they areforced to speeds are light, and diffus:on is at aminimum. converge and funnelthrough a narrow mountain On accasion the land breeze isinsufficient to pass. The Santa Anawind of southern California carry the material morethan a mile or so (described later in this chapter)is due in part to offshore and, with the reestablishmentof the sea this type of phenomenon. breeze, this material may berecirculated onto the land.

WINDS DUE TO LOCAL COOLING Drainage Winds Drainage winds, also called mountain or gravity winds, are caused by coolingof air along the slopes of a mountain. Consequently,the air becomes heavy and flows downhillproducing the MOUNTAIN BREEZE. These mountain breezes, which were previously discussed in chapter 5 ofAG 3 & 2, may reach several hundredfe 'tin depth and in extreme cases may attainspeeds of 50 knots or more. The downhillmotion of these and other breezes with similar motion is termed"katabatic." They may be duetolocalterrain configuration or large scale causessuch as air AG.416 mass characteristics.Although drainage winds Figure 4-6.Strong wind producedby funneling. and the glacier winds discussed inth,- following paragraphs are cold katabaticwinds, in some WINDS DUE TO LOCAL HEATING instances katabatic winds maybe warm. The circulation this section are an There are three types of tertiary foehn winds described later in heating: valley breezes, dry example of warm katabaticwinds. The term caused bylocal used to describe the opposite orupslope motion thermals, and wet thermals. 71 77 AEROGRAPER'S MATE 1 & C Valley Breeze are topped with a cumulus type cloud withthe base at the condensation level. Valley breezes or anabatic The tops are low winds are produced under stable conditions, butmay develop into when the daytime airnear a mountain is heated cumulonimbus under unstable conditions. by contact while airat some distance from the mountain surface is not affected.This produces INDUCED OR DYNAMIC light air near the slope whichtends to move TERTIARY CIRCULATIONS upward to high levels andthereby produces the valley wind. Theyare generally restricted to Induced or dynamic tertiary slopes facing the south circulations are or the more direct rays of three types: eddies,foehn winds, and jet of the sun and aremore pronounced in southern winds. latitudes. They are diurnallystrongest during late afternoon andare seasonally strongest in Eddy Winds summer. They are deeper andstronger than mountain or katabatic winds. Eddies derive theirenergy from the windflow It is conceivable that thistype of circulation overtopographical could be found in an barriers or obstructions. area where there would be They are on a much largerscale when the very little net outflow of air fromthe valley obstacle is a mountain barrier. The system,If this were the eddies are case,a pollutant generally found on the lee side ofmountains, contained within the valley airmass would be but with low wind speeds, mixed by this circulation until stationary eddies or most of it was rotating pockets of airare produced and remain diffused throughout the depthof circulation. As on both the windward and leeward sides a result, a much heavier ground concentration of of obstructions, such as buildingsover which the pollutants might exist in thisregion than would air may be passing. (See fig. 4-7.) be expected witha more rapid change of air mass within the valley system.

Dry Thermals

Dry thermals are small local verticalcircula- tions caused by theuneven heating of the air Unto due to the fact thatsome kinds of surfaces are more effective than others in heating theair directly above them. Plowed ground,sand, rocks and barren land give offa great deal of heat, whereas water and vegetation donot. When the dry thermal tops do not reachthe condensation level, they are commonly referredto as dust devils. They are best developedon clear days with light winds and hightemperatures. Dust devils are most frequentlyseen over desert areas and can reach as muchas 5,000 feet under extremely dry air conditions. Whenoccurring LOW WIND SPEED HIGH WIND over water areas, dust devilsare called water- (BELOW 20 MPH) (ABOVE 20 MPH) spouts. AG.417 Figure 4-7.Eddy currents formed whenwind flows Wet Thermals over uneven ground or obstructions.

Cumulus type cloudsoccur when rising wet When the wind speeds exceed thermals reach their condensation about 20 miles level. They per hour, the flow may be brokenup in irregular 72

78 Chapter 4ATMOSPHERIC CIRCULATION entirely to eddies which arc carried along with awind some to grow larger. This is due almost distance downsteam from the obstruction.These eddy motions. Molecular motion also produces eddies may cause extreme and irregularvaria- diffusion,but comparedto eddy diffusion, tions in the wind anJ may disturb thelanding molecular diffusion is relatively insignificant. area sufficiently to be ahazard. It is important to be alert for these turbulenteddies when a Foehn Winds landing fieldis located near large hangars or When air flows downhill from a high eleva- other buildings. tion,itstemperature israised by adiabatic A similar and much disturbed windcondition compression. Foehn winds are katabatic winds occurs when the wind blows overlarge obstruc- caused by adiabatic heating of air as itdescends tions such as mountain ridges. In such casesthe on theleesides of mountains. The rise in wind blowing up the slope on the windwardside temperature result , in a lowering of the relative is usually relatively smooth. However, onthe humidity. They arrive at the bottom of the;ee the leeward side the wind spills rapidly down side of the mountain causing temperaturerises slope, setting up strong downdrafts and causing as much as 50°F. the air to be very turbulent. Thiscondition is Foehn winds occur quite frequently in the illustrated in figure 4-8. western MountainStates.In Montana and Wyoming the chinook is a well known phenomenon, and in SouthernCalifornia the Santa Ana is known particularly for its high-speedwinds. Generally speaking, when the Santa Ana blows through the Santa Ana Canyon, a similar wind simultaneously affects the entiresouthern California area. Thus, when meteorological conditions are favorable this dry northeastwind will blow through the many passes and canyons, over all the mountainous area, includingthe highest peaks, and quite often at exposed placesalong the entire coast from Santa Maria toSan Diego. Therefore, the term Santa Ana refers to the general condition of a dry northeast wind over southern California. AG.418 Although these wir 's may on occasion reach Figure 4-8.Effect of windflow over mountains. destructive velocities, one beneficial aspect to consider is that when they coincidewith the release of airborne pollutants over an area,the The situation can be compared to water material would be quickly dispersed andcarried flowing down a rough streambed. Thesedown- away from the area affected. drafts can be very violent, and theyhave been A complete study and discussion of theSanta the cause of a number ofaircraft accidents, Ana may be found in Climatology andLow- causing the aircraft to crash into thesides of Level Air Pollution Potential From Shipsin San mountains. This effect is also noticeablein the Diego Harbor, NWRF 39-0462-056. case of hills andbluffs, but is not as pronounced. Eddy winds may also resu''n a change of Jet Winds location of pollutant material vtin an air mass, the m, = of clean air Jet winds, also known as mountain gap or or they may result in squall type winds, with the polluting material. Thetter decreases canyon winds, are extensive the concentration of material perunit volume whose speed is increased through a channeling mountain and is of primary concern. A puffof material, if effect. See figure 4-6 for the effect of actually injected into a volume of air,will seem gaps. 73 79 AEROGRAPHER'S MATE 1 & C

LARGE-SCALE VERTICAL down into a much more complicatedpattern WAVES (MOUNTAIN WAVES) with downdrafts predominating. An indication of the possible intensitiescan be gained from This type of phenomenonoccurs on the lee verified records of sustained downdrafts (and side of topographical barriers andoccurs when also updrafts) of at least 3,000 feetper minute the windflow is strong, usually 25 knotsor with other reports showing drafts well inexcess more, and the flow is roughly perpendicular to of this figure. Turbulence in varying degreescan the mountain range. The structure of the barrier be expected as indicated in figure 4-9and is and the strength of the wind determinesthe most likely to be particularly severe in the lower amplitude and the type of thewave. The levels. Proceeding downwind,some 5 to 10 miles characteristics of a typical mountainwave are from the summit, the airflow begins to ascendas shown in figure 4-9. part of adefinite wave pattern.Additional waves, generally less intense than the primary Figure 4-9 shows the cloud formationsnor- wave, may form downwind (in some cases six or mally found with wave development andillus- more have been reported). These are not unlike trates schematically the airflowin a similar the series 31 ripples that form downstream from situation. From the illustration in figure 4-9it a submerged rock in a swiftly flowing river. The can be seen that the air flows fairly smoothly distance between successive waves usuallyranges with a lifting component as itmoves along the from 2 to 10 miles, depending largelyon the windward side of the mountain. The wind speed existing wind speed and the atmospheric stabil- gradually increases reaching a maximumnear the ity, but wave lengths up to 20 miles have been summit. On passing the crest the flow breaks reported.

[STRONG WINDS somMIIMM .111. TURBULENT LAYER

TUULENT

ROLL ,Aetr-tt CLOUD

IBMs CAP CLOUD %

AG.419 Figure 4-9.Schematic diagram showing airflow and clouds ina mountain wave. 1111111111111171111111111111111.1111111111111hapter4 ATMOSPHERIC CIRCULATION From the meteorologist's standpointitis jetstream and some of the forecasting rules important to know how to identify a wave which are covered in later chapters. situation, and having identified it to advise the pilot how to plan his flight so as to avoid the MEAN TEMPERATURE AND wave hazards. Characteristic cloudforms pecu- VERTICAL SPACING OF ISOBARS liar to wave action provide the best means of visual identification. The lenticular (lens shaped) In chapter 3 of this training manual, Atmo- clouds inthe upper right of figure 4-9 are spheric Physics, and application of the gas laws smooth in contour. These clouds may occur shows that volume is directly proportional to singly or inlayers at heights usually above temperature. Stated another way, we might say 20,000 feet, and may be quite ragged when the that the thickness of alayer between two airflow at that level is turbulent. The roll cloud isobaric surfaces is directly proportional to the forms at a lower level, generally near the height mean virtual temperature of thelayer. Thus, of the mountain ridge, and can be seen extending thickness lines are also isotherms of mean virtual across the center of figure 4-9.The cal., cloud temperature. The higher the mean virtual tem- shown partially covering the mountain slope to perature, the thicker the layer, or vice versa. The the left of figure 4-9 must always be avoided in thickness between layersiscurrently being flightbecause of the turbulence, concealed expressed in geopotent$al meters. The shift in moutain peaks, and on theleesidestrong location, as well as the change of shape and downdrafts. The lentJars, like the roll clouds intensity upward of atmospheric pressure sys- and cap clouds, are stationary, constantly form- tems,is dependent on the temperature dis- ing on the windward side and dissipating on the tribution. lee side of the wave. The cloud forms themselves An example of the foregoing is shown by the are a good guide to the degreeof turbulence fact that if two columns of air are placed side by with generally smooth airflowin and near side, one cold and one warm, the constant smooth clouds, and turbulent conditions if these pressure surfaces in the cold columnwill be clouds appear ragged or irregular. closer together than those in the warm column of air.Figure 4-10 illustrates an increase in While clouds ate generally present to forewarn thickness between two given pressure surfaces the presence of wave activity, it is possible for for an increase in mean virtual temperature. wave action to take place when the airis too Also, note the increase in the distance between dry to form clouds. This makes the problemof the constant pressure surfaces; P, P1, etc., from identifying and forecasting more difficult. A to B. The thickness between two pressure surfaces can be derived by integratingthe hydrostatic VERTICAL EXTENSION OF equation, or by means of the hypsometric HIGH- AND LOW-PRESSURE CELLS equation. (See ch. 3.) Thickness may also be constructed from known variables by the use of In order to better understand the nature of tables, graphs, etc. the pressure centers of the secondary circulation Within the troposphere, the horizontal tem- which was covered previously in this chapter, it perature gradient is directed poleward fromthe is necessary to consider them from athree- lower few thousand feet of the troposphere up dimensional standpoint, not only length and to the level of the tropopause. Themaximum width, but vertically as well. With the aidof gradient isin middle latitudes and reflects a upper air charts, you will be able to seethe three seasonal shift southward in winter and north- dimensions of these pressure systems, as well as ward in summer. This maximum gradient also the circulation patterns of the secondary circula- has its highest numerical value in winter. The minimum temperature occurs at the tropopause. tionasestablishedat:nigherlevelsinthe troposphere and lower aratosphere. With the The subarctic tropopauseis much lower in knowledge gained through this study you will be winter than in summer and is always lowerthan better able to understand the mechanics of the the subtropical tropopause.

75 81 AEROGRAPIIER'S MATE I & C

VERTICAL STRUCTURE Or: HIGH-PRESSURE CELLS

T The topographic features which indicate the AH circulationpatternsat500milibarsinthe atmosphere correspond in general to thoseat lower and higher levels. However,they may P4 experience a shift in locationas well as a change in intensity and shape. Forexample, a closed high on a surface synoptic chart TB P3 may be reflected by a ridge aloft. In addition,upper air circulation patterns may takeon a wavelike P2 structureincontrast to the alternate closed lows, or closed high patterns at the surfacelevel. PI The smoothing of the circulationpattern aloft is typical of atmospheric flow patterns.The verti- P cal isobaric spacing of the followingtypes of highs is illustrated in chapter 21,AG 3 & 2, NavTra 10363-D.

AH IS THE INCREASE IN THICKNESS (3...711.1EEN Cold Core Highs TWO GIVEN PRESSURE SURFACES FOR AN The cold core high isso characterized due to INCREASE IN MEAN VIRTUAL TEMPERATURE its low tropospheric coldness and existsprinci- FROM TA TO TB. TB IS A HIGHER MEAN pally because of the weight of thecold air near VIRTUAL TEMPERATURE THAN TA. the surface. The cold high disappearsrapidly with height. If it becomes subjectedto warming AG.420 from below and subsidence aloft,as it moves Figure 4-10.Thickness of two strataas a function southward from its source and spreadsout, it of mean virtual temperature. diminishes rapidly in intensity with time(unless some dynamic effect sets in aloft over the high to compensate for the warming). Since Within the lower stratosphere duringsummer, these highs decrease in intensity with height,thick- the temperature gradient is directed fromthe pobis to the Equator. During winter, the nesses are relatively low. If the thickness pattern tem- issufficientlystrong,the wind perature gradient is directed from approximately circulation around the center canreverse its direction at 55° lat to the Equator, and from 55°lat to the high levels. poles. This gives rise to a wind condition called Usually the temperature field around the polar vortex. 1Vithin the a cold- upper stratosphere core anticyclone is asymmetrical. The center of during summer, the temperature gradient is still low thickness will be found displacedfrom the directed from the poles to the Equator, though 1,000 -mb high toward the regionof lowest much weaker than in the lower stratosphere. temperatures. Thicknesses are relativelygreater During winter, the gradient is directed from the where middle cloud and cirrusare present 0.. Equator to the poles in the upper stratosphere. precipitation is falling from layer clouds(except In the mesophere in summer, the temperature stratocumulus). gradient is directed away from the Equatorto Figure 4-11 (A) showsan extremely cold and the poles, while in winter it is from the poles to intense sea level anticyclone andcorresponding the Equator. In the thermosphere, the distribu- thickness pattern for the 1,000 to 500-mblayer, tion is unconfirmed, though it is believedthat thelatter for avery deep cold pool (low during summer the gradient is from the polesto thickness) centered directlyover the 1,000 -mb the Equator and vice versa in winter. high.

76 82 Chapter 4 ATN1OSPHERIC CIRCULATION

I 'PeE I / I7000 / i / / / ...".. 240 // I I / /

000 we CONTOURS - -1000-500 MB THICKNESS

SL HIGH 4300

500 MS CONTOURS 500 MB CONTOURS

AG.421 Figure 4-11.Two examples of cold core anticyclones in relation to 1,000- and 500-mb contours and 1,000- to 500-mb thickness.

Figure 4-11 (B) shows the associated 500-mb lies over the center of the sea level anticyclone. pattern, containing a closed 500-mb low directly An illustration of a cold core high (and warm over the 1,000-mb high. This model charac- core low) is shown in figure 4-12. terizes extreme cases over continental areas of middle and high latitudes. It is based on actual observations of the Siberian winter anticyclone. Warm Core Highs Less intense Siberian winter anticyclones and most North American winter anticyclones of the The warm high is so characterized because sub-Arcticregionsarecharacterizedby the throughout the troposphere the warmest air is at models illustrated in figures 4-11 (C) and (I)). thecenter of thehigh.This high has an Figure 4-11 (C) contains 1,000-mb contours and anticyclonic circulation at all levels; hence the 1,000- to 500-mb thickness lines. A cold trough term "high level anticyclone." In the strato- (lowthickness)liesover the sea level anti- spherethistype of highhasa cold core cyclone, with the lowest thickness values to be associated with the center of the anticyclonic found in polar areas well to the north. Figure circulation. 4-11 (D) shows the corresponding 500-mb con- In the warm core high the thickness values tour lines. which form a weak trough whose axis overthecenterarehigher than along the

77 83 AEROGRAPHER'S MATE I & C

periphery and the thickness center is displaced toward the warmest air in cases of an asymmetric temtterature distribution. These highsare characteristically found inthe form of large warm, oceanic highs. The subtropical highs are eood examples of this type of high. In middle

and high latitudesitis found in the form of CONTOURS OF warm blocking highs. 1000 M8 As with other models, weather manifestations SURFACE in a warm high provide clues to the thickness distribution. Convective activity usually isasso- ciated with somewhat lower thickness values: thus the thickness center (high) is displaced to the side of the 1,000-mb anticyclone away from the areas of convection.

M VERTICAL STRUCTURE OF 8900 8840 LOW-PRESSURE CELLS 8778 6888 8656 HEIGHT OF Warm Core Lows 300 M8 8534 SURFACE THICKNESS FROM 1000 TO A \ irm core low is similar to a cold core 300 MB anticyclone in that it decreases in intensity with C-C height. This type low is characterized by virtue of its low tropospheric warmth. In low tropospheric thickness charts, a warm core or tongue of maximum thickness is found. It is apparent HEIGHT OF 1000 M8 thatif the thickness patternis pronounced SURFACE M 120 enough, the sum of the 1,000-mb height at the 60 A 0---A low center and the thickness over the low can -60 produce an anticyclone at the upper surface of the layer. They rarely affect the stratospheric circulation. The warm low is frequently station- AG.422 ary, such as the heat low over the southwestern Figure 4-12.Idealized warm core cyclone with United States in the summer, and is a result of symmetrical temperature distribution. strong heating in a region usually insulated from intrusions of cold air which would tend to fill it This figure shows how the thickness values or cause it to move. The warm low is also found indicate the presence of an anticyclone aloft in its moving form as a stable wave moving along over the low. The upper diagram represents a frontal surface. It moves by virtue of the low idealized contours of the 1,000-mb surface. level advectior in advance of it and the cold Curve BB shows the profile of the 1,000-mb advection to its tear This type of wave cyclone surface along lien AA, curve CC shows the lacks any dynamic mechanism aloft to cause it 1,000- to 300-mb thickness profile along the to deepen and occlede. Itusuallyfills as it same line, and curve DD is the sum ofEBand moves eastward, and tie frontal wave flattens CC, or the profile of the upper pressure surface out. There is no warm low aloft in the tropo- alo..g AA. In this idealized example the thick- sphere. The tropical cyclone is believed to be a ness(temperature)and coutourfieldsare warm low; its intensity diminishes with height. mutually symmetrical. Figure 4-12 shows an idealized warm core In general, however, the temperature field is cyclone with symmetrical temperature distribu- quite asymmetrical around a warm core cyclone. tion. Usually the southward moving air in the rear of 78

84 Chapter 4ATMOSPHERIC CIRCULATION the depressionwill not be as warm as that evidence in the surface pressure field that an moving northward in advance of it. Here the intense circulation exists aloft. The cyclonic maximum thickness values will be found on the circulationaloftisusuallyreflected on the warmest side of the cyclone, whose axis will surface in an abnormally low daily mean temper- slope upward away from the thickest region of atureandin precipitationandunstable the layer. An examination of the thickness field hydrometeors. associated with this warm core low shows the At high latitudes the cold pools and their largest thicknesses are found in the warm sector associated upper air lows show some tendency just to the east of the wave crest. The thickness for location in the northern Pacific and Atlantic circulation pattern with the warm core low is Oceans where, statistically, they contribute to opposite the circulation pattern at the surface, the formation of the Aleutian and Greenland, and the low decreases in intensity with increas- lows. At lower latitudes the assoicated upper air ing height. The winds, as might be expected, in cyclones are frequently called CUTOFF LOWS, thisregionshiftfrom northeasterly atthe which usually occur when blocking highs exist surface to southwesterly aloft. The surface low far to the north. is replaced by a ridge aloft. Figure 4-13 illustrates the displacement of a Weather and cloudiness found in warm core cold core low southwestward aloft where the cyclones can yield clues to the thickness. Where lowest thickness values are displaced toward the evidence of convective activity, such as showers, region of lowest temperatures. Also note the is greatest, the thickness values are relatively low position of the 500-mb low. although they .se in a region of generally high The thickness circulation pattern over this values. Where the weather is fine and clear, low has the same direction as the surface except for perhaps a few cirrus, the thickness circulation pattern. This indicates an increase in values will be relatively high. winds aloft. At 500 millibars the winds are seen to be considerably stronger than at the surface Cold Core Lows and the wind direction does not shift appreciably with height. The cold core low in its purest form contains the coldest air atits center throughout the -DYNAMIC LOW troposphere; that is, going radially outward in any direction, at any level in the troposphere The dynamic lowis a combinaion of the warmer air is encountered. The cold core lows warm surface low and a cold upper low or increase in intensity with height. Relative mini- trough, or a warm surface low in combination mums in thickr..st, values, called cold pools are with a dynamic mechanism aloft for producing a found in such cyclones. When the temperature cold upper low or trough. It has an axis which distribution is asymmetric, the thickness center slopes toward the coldest tropospheric air. In will lie in the direction of the lowest tempera- the final stage, after occlusion of the surface tures.When the temperature distributionis warm low is complete, the dynamic low be- symmetric, the axis of the low is nearly vertical. comes a cold low with the axis of th,; low Tropospheric thickness charts show closed mini- becoming practically vertical. mum thickness lines symmetrical with the pressure center. In other words, in the cold low, the lowest temperatures coincide with the lowest DYNAMIC HIGH pressures. Inthe lower stratosphere, there is a warm' The dynamic highi5 a combination of a core vertically over the low center. Here the surface cold high and an upper-level warm high highest temperatures are associated with the or well-developed ridge, or a comb" ation of a lowest pressures. surface cold high with a dynamic mechanism The cold low has a more intense circulation aloft for producing high-isvel anticyclogencsis. aloft from 850 to 400 millibars than at the Dynamic highs have axes which slope toward the surface. Some cold lows show but %ery slight warmest tropospheric air. In the final stages of

79 85 AEROGRAPHER'S MATE I & C

AG.423 Figure 4-13.Illustration of a cold core low showing 1,000 -mb contours (solid lines), 500-mb contours (dashed lines), and thickness pattern (dotted lines). warming of the cold surface high, the dynamic high. Therefore, anticyclones found in tropical high becomes a warm high with its axis practi- air are always warm cored. Anticyclones found cally vertical. inArcticairare always coldcored, while anticyclones in polar air may be warm or cold SUMMARY cored. A cold core low is accompanied by a low A warm core high is accompanied by a high warm tropopause. Since the pressure surfaces are cold tropopause. Since the pressure surfaces are close tother, the tropopause is reached at low spaced far apart, the tropopause is not reached altitudes where thetemperature isrelatively until great heights. The temperature continues warm. Good examples of cold core lows are the to decrease with elevation and is very cold by Aleutian and Icelandic lows. Occluded cyclones the time the tropopause is reached. The sub- will generally be cold cored because of the polar tropical highs are good examples of this type of or Arctic air that has closed in on them. Chapter 4ATMOSPHERIC CIRCULATION

Warm core lows decrease in intensity with chart is to determine the representativeness of height or completely disappear and are for the surface winds and temperatures, to determine most part replaced by anticyclones aloft. The depth of moisture patterns in winter, and to heat lows of the southwestern United States, replace the surface chart in mountainous and and over Asia and Africa are good examples of plateau areas where the mean elevation is around warm corelows. Newly formed waves will 5,000 feet. Both temperature (frontal) analysis generally be warm cored because of the wide- and moisture analysis should be carried out on open warm sector. this chart, and its analysis should always be Systems which retain their closed circulations made in close conjunction with the surface char t to appreciable altitudes are called dynamic lows whenever possible. or highs. A complete and carefulisotherm analysis made at this level in conjunction with wind and USE OF pressure analysis will lead to the correct place- CONSTANT PRESSURE CHARTS ment of fronts at this level and by implication location of the surface front. A thumb rule to The analysis of and a general description of guide ,xerographer's Mates in location of most the types of constant pressure charts is pre- fronts is to look for the 850-mb warm front sentedinchapter 21, AG 3 & 2, NavTra roughly 2 1/2° to 3°latitude ahead of the 10363-D. Therefore thi., section will be confined surface front and cold fronts from 3/4° to 2° to a brief description of the application of the latitude behind the surface front. various constant pressure charts. The intervalis the same as the 1,000-mb chart-60 meters. Radiosonde and moutain sta- THE 1,000-MB CHART tion reports of the height of the 850-mb level should be used in plotting and analyzing this Thischartindicatestheheightofthe chart. The 850-mb isotherms serve as a good 1,000-mb pressure surface above or below sea indication of the 1,000- to 700-mb thickness level. When the 1,000-mb surface lies below sea patterns. level, it is indicated by negative height values. In mountainous areas caution must be observed in using I ,000-mb height values. THE 700-MB CHART The chart is normally constructed from the surface chart by assuming that 7 1/2 millibars The approximate height of the 700-mb chart equal 60 meters (200 fecf) for temperatures is 3,010 meters, (9;880 feet) above mean sea between 0° and 20° C. For temperatures below level. This chart is used mostly to determine the C 8 1/2 to 9 millibars equal 60 meters, and verticalextent and structure of fronts and for temperatures above 20° C, 6to 61/2 pressure systems, or to play the role of the millibars equal 60 meters. These are a valid 850-mb chart over elevated areas. It also plays approximation of the 1,000-mb heights. Heights the role of the 850-mb chart in moisture analysis from radiosonde soundings shouldbe used in summer when moist tongues extend to greater whenever available. heightsthaninwinter, due toconvective The principal use of the 1,000-mb chart is activity. Other uses of this chart are in fore- made in constructing space differential (thick- casting (steering currents for certain shallow ness) charts. It serves as the base level for the pressure systems are determined at this level) 1,000- to 700-mb and the 1,000- to 500-mb and in differential analysis. The contour interval differential charts. is the same as the 850-mb chart. Short waves are a predominant feature of this THE 850-MB CHART chart. They play a great role in the weather. For this reason the wave features of the 700-mb The approximate height of the 850-mb level chart are carefully studied and tracked. Short according to U.S. Standard Atmosphere is 1,460 waves have great influenee on cloudiness, fronti meters (4,780 feet). The principal use of this intensity, precipitation areas, etc.

81 87 AEROGRAPIIER'S MATE I & C

ME 500-MB CHART indispensable toolin planning jet operations. Contour intervalisnormally120 meters; a Th. ipproximate height of the 500-mb level is 60-meter interval may be used in areas where a 5,5-i, meters. (18,280 feet) above mean sea finer degree of delineation is required. level. Primary features of this chart are the warm highs and cold lows with associated troughs and THE 200-MB CHART ridges. Long waves may also be identified at this level although most short waves have lost their The approximate height of the 200-mb sur- identity. face is 11,790 meters, (38,660 feet). This chart 1....kart is the most wi'iely used of all upper is used principally as an adjunct to the 300-mb air _..:its for prognostic purposes. For various analysis. In summer, it plays the same role with reasons. this chart comes closest to representing respect to the jetstream as the 300-mb chart the mean state of the atmosphere at the time of does in winter. In wirer, its principal synoptic observation.In additionto representing the use is for estimation of advective temperature wino structure at a common flight altitude(for changes in the stratosphere. The 200-mb temper piston-engine aircraft), it is also used extensively ature analysisisparticularly used in isotach in forecasting the movement and development analysis at 300 millibars. Operational use and of fronts and pressure systems at low levels, sea contour interval is the same as for the 300-mb level in particular. chart. Sincethislevelapproximately divides the atmosphere with respect to mass, it is often used THE 150-, 100-, 50-, inconjunction withthe1.000-mb chartto AND 25-MB CHARTS provide layer analysis in the lower half of the atmosphere. When no 200- or 100-mb chart is These charts are normally prepared only by available, it gives a fair approxima*on to the activities with large staffs and are used primarily horizontal position of the jetstream. Because the for research purposes. The amount of data amount of data available decrease rapidly above available at these levels is so scanty that analyses thislevel, the 500-mb chart provides an im- are likely to be greatly over-simplified and more portant base unon which to construct high-level than one logical analysis of the same data is SCS. The ccatour interval is the same ,isfor easily possible. It can be anticipated that as the the 700-mb chart. operation-al ceilings of jet aircraft increase, so will the practical uses of the 150- and 100-mb THE 300 -MI) CHART charts.

The apprc.ximate height of the 300-mb level is SPACE DIFFERENTIAL about 9,100 meters (30,050 feet). The primary (THICKNESS CHARTS) features of this chart are. the permanent and semipermanent highs and lows, certain dynamic Space differential charts are commonly re- lows. lone waves, the jetstream in winter, and ferred to as thickness charts, since they repre- the tropopause, especially the Arctic and midlat- sentthedifferenceinheight between two itude tropopauses in winter. constant pressure-surfaces. The most commonly The primary uses of this chart are forecasting. used space differential charts are the 1,000- to determination of thecharacteristics of long 700-mb, thz 1,000- to00-mb, and the 500- to warn,;; analysis and forecasting of the jetstream 300-mb charts. moth associated isotach analysis), analysis of the They are usually constructed by graphically tropopauseinwinter, determination of the subtracting the heights on one analyzed constant vortteity distribution; and in the case of tropical pressurechart from those on another. The cyclones which do not show a closed circulation constructionofthesechartsiscoveredin atthislevel,steering currents.Jet engines chapter 7 of this training manual. operate more efficiently at altitudes upward of Since vertical spacing between two pressure 300 millibars; therefore, this chart, too, is an surfaces isdirectlyrelatedtothevirtual

82 88 Chapter 4--ATMOSPIIERIC CIRCULATION

temperature of the layer in question, the difference in height between two pressures is directly related to the mean virtual temperature of the layer. Consequently liens of constant thickness on a space differential chart are also lines of constant mean virtual temperature for the layer.

Space differential charts are an aid in the analysis LONG -' *E and construction of prognostic charts. RIDGE The variation of wind with height is determined by the horizontal gradient of temperature. This difference in wind direction and AG.424 speed from one level to another is referred to as Figure 4-14.Illustration of a long and short wave and the vertical wind shear or the thermal wind. The the measurement of length and amplitude of a long significance and determination of the thermal wave. wind are covered in chapter 7.

Long Waves CIRCULATION PATTERNS ON UPPER AIR CHARTS Long waves, sometimes referred to as major troughs, provide general guidance in prognosti- The patterns on constant pressure charts take cating surface events, such as cyclogenetic areas on much the same appearance as the patterns and in determining steering currents for systems delineated by isobars on a surface chart. How- on surface synoptic charts. A long wave is a ever, the pattern becomes smoother and more wave in the major belt of the westerlies which is simple with an incr,.ase in elevation, since many characterized by large wave length and ampli- of the pressure systems decrease in intensity tude. Long waves vary in length between 50° with height. The 1,000-mb chart, for example, and 120° of longitude. The amplitude of long may show many closes!, centers, but by the time wavesislarge andincreases upward inthe the 200-mb level is reached there are few if any troposph- re. Therefore, they are associated with closed centers. Instead, the contours present a cold troughs and warm ridges. Normally, the wavelike pattern. These waves, like ocean waves number of long waves around the hemisphere is have definite properties t)1which they may he 4 or 5, but there may be as ,-,any as 7 or as few identified. as 3. Long waves move slowly with a normal movement at 40° N lat. on the order of about 2° long. per di y during the spring to slightly less LONGANDSHORT WAVES than 1°duringthefall,butthey can ba stationary or even retrogress. Wave patterns are classified according to their Long waves ar.: persistent; that is, :hey do not wave lengths as long and short waves. Wave appear or disappear rapidly. Formation generally length is the distance from one trough to the takes place in the same geographical area when next trough, or from, one ridge to the next ridge, new waves are forming from short waves or from or from a point on one crest to the correspond- a changing synoptic situation. Long waves are ing point on the next crest. Wave lengthis easily identifiable from 5-day mean charts. measured in degrees longitude. Wave amplitude As related to synoptic events, long waves are is the north-south distaacc from point of inflec- associated with surface cyclone families. Over tiontopeak orto lower extremity. Wave the eastern portion of the cyclone family may amplitude is measured is degrees latitude. Figure be found a major (warm) rid&, over the western 4-14 illustrates the measurement of the length portion, a cold trough associated with du, polar and amplitude of a long wave. Also note the outbreak. Therefore, major waves mirror the appearance of a short wave trough on the long cyclone series inits entirety, while-iinor or wave. short waves may be associated with individual

83 89 AEROGRAPIIER'S MATE 1 & C surf.:..e 14 clones. Long w aves art. best identifi- ofsurface pressure systenis but Jo have a great able on the 300-mb .hart. though during the elieLt on long waves. Short waves intensify as colder se.. ,on the 500-m: Ulan is frequently they approaLh long waves, and weaken as they used for this purpose. leave the long wave behind. They flatten a long Figure4-15illustrates thy. relationship wave ridge w hen superimposed on one. between long waves and other meteorological B,,,mse of the sparseness of upper air data in events. many areas. short waves may be easily over- lookedorcarelesslysmoothedout.of the analysis. In isolated areas, the principal clue to the passage of a short wave trough is a slight veering of the wind for a brief time. Too, the long wave troug) intensifies when overtaken by a short wave trough andthis intensification often results in surface cyclogenesis. One of the 12i)1." best synoptic clues to the existence and location 4,tlirot" oattC14.LCAG ..4 14C pnT of short waves in denser networks is the 12- or 24-hour coincidence of small closed height fall

ttSS aac 0.0.4n(:eaDcro centers with short'ZIVC troughs and the 12- or LeSS MC.RabCP. .etssanr4OrAVGCS ,133Poi Cnte4fsiSts 24-hour small closed rise centers with short wave :,:tgf.CPS u(.4.0.; tAT.C.CUMS +.1.4,:aCS 102(ST ATSThta %Me ridges. (PL4rirxe..

i---.04t ger:. ?KW.Stert1 Isotherm-Contour Relationship

AG.425 Certain generalized patterns of isotherms in Figure 4-15.Long waves and related weather. relation to ,ontours are of significance in that they are indicative of present conditions, hence, future conditions also, especially in relation to Short Waves movement of troughs and ridges. Isotherms on constantpressure charts are Superimposed on the long wave contours of a considered m phaseor out of phase with given upper air chart, say 500-mb, are numerous contours. When isotherms arein phase, they troughs and ridges cf small dimensions called form a pattern in relation to the contours that is short wave troughs and short wave ridges. Short exactly coincident with the troughs and ridges. wave troughs are progressive waves of smaller Theyarealso deemedinphase when the amplitude and wave length than the long waves, isotherms are of greater or lesser amplitude than moving in the same direction as the basic current the contours. Isotherms are out of phase when in which they are embedded. They often dis- the trough and ridge lines of the isotherms are appear with height. and may not be detectable not coincident with the contour trough and above the 500-mb level. Short waves are more ridge lines. (See fig. 4-16.) numerous than long waves, with10 or more The rules for the movement of waves deter- active waves being present in the hemisphere mined by the isotherm contour patterns are as most of the time. They are associated with warm follows: troughs and coldridges.Short waves move rapidlyandareprogressive.Their eastward 1.If the isotherms are in phase and parallel motion is very Lear that of the 700-mb flow, to the contours, the wave is stationary, and an average speed for these waves may be 2.If the isotherms are in phase but of less from 8° of longitude per day during the summer amplitude then the contours, the wave will have to 12° of longitude rex day during the winter. retrograde motion. In wmparison with long waves, short waves 3.If' the isotherms are in phase but of greater appear anddisappearrapidly.Shortwaves amplitude than contours, the ware is slowly appear to be of little consequence in the steering progreFAve.

84 90 Chapter 4-ATMOSPHERIC CIRCULATION

/.....-- /T / / T .... / de P ,,----...... e / / ...... / ---- /

(A) IN PHASE-IC3THERMS PARALLEL (B) IN PHASE-ISOTHERMS LESS AMPLITUDE- STATIONARY RETROGRADE /...- \ ..--, ..--.. ,../- /-\ \ N. /i\\ de% \ \ / T ,_ \ XT P\., de \\ .. / T ...... -/ (C) IN PHASE-ISOTHERMS GREATER AMPLITUDE-(D) OUT OF PHASE-ISOTHERMS 90° OUT- SLOWLY PROGRESSIVE EQUAL TO GRADIENT WIND

(E) OUT OF PHASE- ISOTHERMS 180° OUT - FAST - MOVING

AG.426 Figure 4-16.-Isotherm-contour patterns.

4. If the isotherms are 90° out of phase. the equatorwardside or the strongest westerlies. wave moves with the speed of the gradient wind. These lows and troughs generally arc, but not 5. If the isotherms are 1800 out of phase. the necessarily. associated with cyclones at lower wave is fast moving. levels (sea level in particular). If the situationisreversed,thatisa low appears on the equatorward side of the jet and UPPER HIGHS AND LOWS highs are poleward, they are said to be in an abnormalposition.Highs andlowsinthis The most common pattern which appears on abnormal position are sometimes referred to as upper air charts consists of alternate troughs and CUTOFF CENTERS. They are most common in ridges.withoccasionalclosedlowsinthe the spring and least common in the fall.In tro ghs and dosed highs in the ridges, forming seasons when they.iremostfrequent, they ser,es of waves which girdle each hemisphere. repeatedly occur in the same geographical loca- The lows are normall} on the poleward side of tion. Cutoff lows appear most frequently over the strongest westerlies, and the highs arc on the the southwestern United States and along the

85 91. AEROGRAPHER'S MATE I R C northwestern Lutist of Alri La. Cutoff highs are Under zonal conditions, the long waves have most ficquent in northeastern Siberia, Alaska, smaller amplitudes and longer wave lengths than and Greenland. they have under meridional flow. Another way The formationoftheseLuton' entersis of characterizing the long wave pattern is by disLussed laterinthis chaptei. The pet in wave number. Zonal flow has a small wave %%Ina these oLLin is believed to be part of the number, meridional flow a large wave number. index cyLle,taking sonic 4tob weeks to Surface systems are comparatively shallow (in acLomplish. When luglis and lows areinthis vertical extent) and move rapidly west to east in position, we appear to have our topsy-turvy zonal patterns, while in meridional patterns they weather. FOI instanLe, the tempciature may be are deeper in vertical extent and move poleward warmer in Alaska di:al in Florida. or equatorward, or remain quasi-stationary.

ZONAL INDEX SPECIAL CIRCULATION FEATURES The zonal index is a measure of the strength During the discussion of secondary circulation of the midlatitude westerlies, expressed as the you noted that centers of action migrate and horizontal pressure difference between 35° and change in intensity. These changes in location, 55° latitude, at the surface or aloft. The upper orientation. andintensity of thecenters of level most frequently used (and the most con- influence the movement of migratory action venient to use) is the 700-mb level. systems. Since special circulations play a large A quantitative expression of the indexis role in influencing the secondary circulations, it associated with certain weather phenomena. The is importantthatyoulearnthe significant zonal index does not, however, account for features of these circulations and how they can dynamic processesinthe upper air such as influence the daily, and sometimeq the seasonal, convergence and divergence: consequently the weather. zonal index is now vnerally used only as an indicator and is seldom computed, but merely ZONAL AND MERIDIONAL FLOW estimated. The zonal index may be computed atthe N,'Hiation in the speed and direLtion of the surface by averaging the reported and estimated west wind is one of the most significant aspects sea level pressures at latitudes 35° and55° at 5° of the atmospheric Lirculation. Probably, the longitude intervals over a span of at least 90° first, and certainly the simplest, of all methods but preferably120°, longitude. The mean sea of classifying the flow patterns is acLording to level pressure at 55° latitude is then subtracted whether west-east or north-south components of from the mean sea level press- ire at 35° latitude. flow dom mate. The former Lase is LharaLterized If the sea level pressure difference is at !east +8 as zonalflow. the Lit terIs meridional flow. millibars, nigh zonal index prevails: and if the Figure 4-17 shows typo al examples of zonal and difference is +3 millibars or less, low zonal index meridional flow on the 500-mb chart. prevails. When computing the zonal index at 700-m1 level, the actual contour values are used, and the final difference is then converted to millibars by letting 60 meters equal 7 1/2 millibars.

High Zonal Index

When the pressure difference between 350 and 55° latitude is more than 8 millibars, high zonal index prevails. Iligh zonal index is asso- AG.427 ciated with long waves in the upper air, having Figure 4-17.Zonal and meridional flow patterns. small amplitudes, warm troughs. and cold ridges

86 92 Chapter 4ATMOSPHERIC CIRCULATION

With high zonal index the Icelandic and Aleutian low zonal indexisnot immediately a static lows are in normal position, or slightly north of value. The weather situation changes irregularly their respective normal positions and well devel- from high to low zonal index and vice versa. oped. The axis of these lows will be east-west, Changing zonal index is therefore an important and the associated troughs will have an east-west aspectto consider in estimating the current orientation. The Atlantic and Pacific subtropical trends. A changing zonal index may be a falling highsarenorth oftheirrespective normal or rising zonal index. positons and their orientation is strongly east- FALLING ZONAL INDEX.The Icelandic west. The Great Basin High is present, as is the and Aleutian lows move southward and begin to Siberian High, but it has little westward exten- split. The Atlantic and Pacific subtropical highs sion. Highs are absent inthe high latitudes. follow course. Migratory systems slow down, Frontal zones move from west to east, are at especially in the higher latitudes, resulting in a higher latitudes, move rapidly, and havea gradual shiftin frontal orientation to north- prominent east-west orientation. There are few south. Polar highs develop and begin to break polar outbreaks, and the moderate highs in the out. low latitudes move rapidly. The midlatitude RISING ZONAL INDEX.The eastern cells temperatures are moderate, and the weather is of the semipermanent lows and highs begin to generally fair. The polar regions become colder fill and weaken, respectively, and move east- and at 60° N lat. the weather is stormy. ward, finally disappearing completely. The western cells of the centers of action move northeast- Low Zonal Index wardintotheir normal positions.Migratory systems begin to intensify and speed up, espe- Low zonal index prevails when the sea level cially in the higher latitudes, resulting in a more pressure difference between latitudes 35° and east-west orientation of the associated frontal 55° is +3 millibars or less. An extreme low zonal systems. The polar high stagnates over the Great indexisat hand when the pressure at 55° Basin; cutoff systems begin to fill. latitude is higher than that at 35° latitude. On The seasonal change in the zonal index is of the 700-mb chart short waves are a predominant great magnitude. Summer changes are small and feature. The short waves have relatively large winter changes are extreme. A particular index amplitudes and are associated with warm ridges may remain nearly stagnant for several weeks, and cold troughs. Cutoff centers are common. especially in tin winter, or manifest itself only The Icelandic and Aleutian lows have split into for a few days. two weak cells each, and they have a strong north-southorientation.TheAtlanticand BLOCKS Pacific subtropical highs are weak, and in a more southerly position than normal with a north- Blocking is defined as the development of a south orientation. Each of the highs has split warm ridge or cutoff high aloft at high latitudes intotwocells.The polar region highsare which bt.:omes associated with a cold high at strongly developed, and joinedin the Arctic the surface. Blocks form most '1-equently in the regions. There are more fronts during low zonal northeastern portion of the Atlantic and the index; thereis sharp contrast in them; their Pacific. They move very slowly and tend to orientation is north-south. Storms and cyclones move westward during intensification and east- with heavy precipitation are frequent in the low ward during dissipation.Atlantic blocks fre- latitudes. Temperatures occur in extremes in the quently move as far westward as the Maritime mid and low latitudes. The high latitudes are Provinces of Canada. The blocking high is the storm-free with mild temperatures. situation when a warm core upper high has become situated over a cold core surface high, Changing Zonal Index splitting the westerlies. The cutoff high is a particular example of the The zonal index does not change from high to general class of high ailed blocking highs. low overnight; however, the degree of high or A normal accompaniment sufficientfor

87 93 AEROGRAPHER'S MATE 1 & C anticyclogenesis is the existence of high-speed tion from zonal to meridional flow must exist in wind approaching anticyclonically curved and the region of the split; upstream the flow is weaker contour gradients downstream. The high- zonal and downstream meridional. est pressure to the right of the high-speed (jet) There are other characteristics for recognition winds tends to be propagated downstream to fill and detection. The high is centered in middle or and cause to move east any trough in its path high latitudes (poleward of 40° lat.). There is a downstream. strong jetor branch of the jetstream going around the poleward side of the high. They tend Three general types of blocks are said to exist. to occur persistently in the same season or not One iscalled the OMEGA block due toits atall.They occur most frequently in late winter resemblance to the Greek letter OMEGA. Figure 4-18 shows the three types of blocks as they and early spring. occur in the atmosphere. Isotherm-Contour Relationship

Identifying Characteristics In a typical blocking pattern, a core of warm air is present in the vicinity of the cutoff high; if For identification purposes, blocks have the a low is also present, a cold air core is present in followingcharacteristics:The basicwesterly the vicinity of the cutoff low. flow splits into two branches, and each branch transports an appreciable amount of mass; about Formation half the contours go north and the other half south. Each branch must eAtend over at least The genesis of a typical blocking situation is 45° long. before they recombine; a sharp transi- normally preceded by intense cyclogenesis to

(OMEGA)

AG.428 Figure 4-18.Types of blocks.

88 94 Chapter 4ATMOSPHERIC CIRCULATION

the west and the development of a coldcon- fashion and are also dynamic. Because of the tinental anticyclone eastward of the blocking obvious analogy, some meteorologists have sug- area. This brings about induction of warm air gestedthat a surface high associated with a into the ridge, causing the warm ridge to cut off cutoff high aloft be called an occluded anti- and split the westerlies. Several days are nor- cyclone. mally required to accomplish this. Cutoff lows occur most frequently Jong the Blocks should be looked for in late winter or southwestern coastal area of the United States early spring in certain preferred locations; that is and the northwestern coastal areas of Africa. over the eastern portions of oceans in high and They may form just south, but mostly south- middle latitudes. During intensification, blocks west, of a blocking high. A very sharp ridge is haveatendencyto move slowly westward usually to the west of them, oriented southwest (retrograde). This movement is very slow. to northeast at 500 millibars. The cutoff low forms a pcol of cold air.Itis a completely Dissipation isolated cyclone of cold air. The conditions, as illustrated in figure 4-14, leading to the forma- The reestablishment of the strong zonal fly tion of a cutoff low are high-speed winds and by whatever processitis induced causes the heightfallsdownstream approaching weak dissipation of blocks. During such dissipation cyclonically curved contours of a low in the processes, blocks tend to move eastward at a midlatitudes. The direction of these high-speed very slow rate. winds is such that in deepening, the low is also displacedfarthersouthorremainsquasi - stationafy (at least in the formative stages). CUTOFF LOWS THE JETSTREAM Waves in the contour patterns on upper air charts can be either stable or unstable, the same A jetstream is a band or belt of strong winds as waves on a front. depending on whether their of 50 knots or more with a strong westerly amplitudesincreasewithtime.Figure 4-19 component which meanders vertically and hori- shows the typical stages in the development of a zcatzlly aroundthe hemisphereinwavelike CUTOFF LOW inthe trough of an unstable patterns. It is not found at the same latitude or wave. elevation around the earth, but has a sinusoidal Stages 3 and 4 corresp9nd to occlusion in the pattern (a wavelike trajectory). At times it is a wave cyclone andare often associated with continuous band, but more often it is broken up surfaceocclusions.Cutoff lowsarealways into several discontinuous segments, or it is split dynamic. Cutoff highs are defined in a similar at several points. Jetstreams are characteristic of

,. COLD ...--...... -- lerpe -- L....4;;%""-..,"7, ....."../ oe ,./".., / \ t .."",,.25 %....'. %. . / % ... .4' ... I I , \ I ll 1 I e/ % iI \,....%. 0 I I 11 I/ WARM . .4...0 %...... / ( I) (2) ( 3) 4) ---ISOTHERMS CONTOURS FORMATION OF A CUTOFF LOW

AG.429 Figure 4 'I9.Development of a cutoff low.

89 95 AEROGRAPHER'S MATE 1 & C both the Northern Hemisphere and the Southern forecasting a still finer delineation is necessary. Hemisphere, however, much more is known For instance, the polar front jetstream may be about the ones inthe Northern Hemisphere, either a single or multiple system, depending on since there is more data a%ailable about them. certain meteorological factors. Listed below are The general characteristics mentioned here are the principal divisions of these jets: based on what is known of jetstreams in the I. Polar Front Jetstream (Single System). In Northern Hemisphere. both cases of the polar front jetstreamitis An observation of high wind speed does not associated with the principal frontal zones and by itself warrant use of the term "jetstream." It cyclones of middle and subpolar latitudes. The isnecessary for large vertical and horizontal polar front jetstream as a single system is a shears to exist which limit vertical and lateral characteristic occurrence during a high index extent of the strong winds. in order for a "core condition. to exist." Moreover, the word "stream" implies 2. Polar Front Jetstream (Multiple System). that the core must possess considerable length. During lowindexconditions,theair mass For the sake of definition, the World Meteor- discontinuity associated with the polar front is ologicalOrganizationin1958 proposedthe modified, and as a result a branch of the polar following definition. "Normally a jetstream is front jet system is established north and west of thousands of kilometers in length, hundreds of the migratory high in addition to the old polar kilometers in width, and also some kilometers in front branch that remains south of the high. depth. The vertical shear of wind is of the order This is especially true over the Pacific through of 5 to 10 m/sec per km :Ind the lateral shear is the action of a migratory polar high from Japan of the order of 5m/sec per100 km. An which stagnates in the Pacific. The northern arbitrary lower limit of 30 misec is assigned to branch of this jet system has been labeled the the speed of the wind along the axis of the interpolar front jet; the other, the old polar jetstream." The rate of change of wind direction front let. Once the formation of the interpolar and speed with either altitude or on a horizontal front is initiated, the old polar front jetstream is plane is termed shear. apt to disappear rather rapidly and the inter- If one visualizes a current with dimensions as polar front jet becomes the dominant one. given by the WMO. he finds that the jetstream 3. Subtropical Jetstream. This jetstreamis can be likened to athin, narrow ribbon. A found solelyin the lower latitudes over sub- system of such shape cannot be portrayed on tropical high cells and marks the poleward limit true scale on vertical cross sections where a of the trade wind cell of the general circulation. description of vertical and 1, rrizon tal gradients is Generally, the transfer of angular momentum is desired. On such sections the vertical distance is assigned the leading inle in the formation and always exaggerated, usually about 100.1. This maintenance of this jetstream.. Thermal contrasts scale distortion must be kept in mind when cros:, are in evidence on the left of the stream(tooling sections are examined. downstream), though they are not as intense as One jetstreamrarely occurs aloneinthe those contrasts between air masses. atmosphere; multiple jets are more the general 4. Arctic or "Polar Night" Jetstream. This rule. They have been found to have a pro- jetstream is situated high in the stratosphere in nounced association with the subtropical high and around the Arctic or Antarctic Circles. and frontal systems that occur between selected Although there may be other currents re- air masses: the Arctic or "Polar Night" jetstream sembling jetstreams atstill higher !eve's, our is not associated with any of thesephenomena, knowledge about themisstill too scant to but is situated at high altitudes in the Arctic at warrant treatment of them here. Although the an altitude where the stability of the atmosphere three types of currents occur in widely differert increases discontinuously upward. geographic locations,their basic structureIs In general, jetstreams are labeled as the polar similar. The basic structure of the jetstream is front jet and the subtropical jet; a later dis- discussedin thischapteranddifferences coveryistheArctic or "Polar Night" jet. between the polar jet and the other two pointed However, for the purpose of identification and out where appropriate.

90 96 Chapter 4ATMOSPHERIC CIRCULATION

Synoptic Structure of the Jetstream certain conditions this zone of concentration may be traced continuously around the hemi- The model of the jetstream in the upper air in sphere, but not often. The degree of concentra- common usage shows a strong concentration of tion will vary along the front, being weaker, in high wind speed lines (isotachs) both vertically general, through ridges and stronger in troughs. andhorizontallyinthe upperairthat are Since a concentration of temperature repre- organized into fairly narrow ribbons embedded sents the frontal intersection, more than one in the relatively slow speed wind fields of Cie zone of concentration would exist if More than surrounding atmosphere. The concentration is one front extends to any one plane. This is located just to the right of the barocliniczones particularly evident at low levels. 500 millibars (looking downstream) between relatively cold and below. Above 500 millibars only the major and warm air. A complete representation of the frontal systems are likely to appear. wind structure shouldtakeinto account its A jetstream is usually associated with each variationsinthreedimensionsvertically thermal concentration in excess of 10°C in 200 (depth), latitudinally (width), and longitudinally miles,expeciallyifthethermal contrastis (length).Sincethewindfieldinathree- maintained through a deep layer. This thermal dimensional pictureis hard to portray, a two- contrast comprising the frontal zone is usually dimensional picture is used. They are the vertical evident atalllevels up to and including the and horizontal fields. (See fig. 4-20.) 400-mb level, and, on occasions as high as the 300-mb level. A positive thermal gradient (warm Thermal Field Around the Jetstream air to the south and cold air to the north) is present in these levels. Between 300 and 200 A jetstream (other than the subtropical jet- millibars their thermal contrast is greatly dimin- stream) is associated with the thermal discon- ished or disappears. Above the 200-mb level tinuitybetween air masses. The subtropical their thermal contrast is reestablished, reflecting jetstream does not depend upon air mass con- the appearance of warm stratospheric air to the trast but rather on momentum transfer. left of the jetstream (looking downstream) and The magnitude of the thermal discontinuity cold tropospheric air to the right. Thus in these between the air masses depends upon the initial high levels, the direction of the thermal gradient properties of the individual air masses and the is now reversed (warm air to the north and cold degree of subsequent modification of these two air to the south). The level at which the reversal pro perties. of temperature gradient occurs is termed the One of the most valuable clues in locating the Z-SURFACE. The foregoing temperature dis- jetstream stems from the relationship of the tribution is necessary if the wind is to increase jetstream to the thermal discontinuity (front) to a maximum value and then decrease. (See fig. between two air masses. The core of the jet- 4-20.) stream is located directly above, or nearly so, Thus, it may be said that the jet core will lie the thermal concentration on the 500-mb sur- between 200 and 300 millibars directly above face. Since recent studies have revealed that the strongest meridional temperature gradient at troughs and ridges slope very little from 500 to 500 millibars. A strong meridional temperature 300 millibars and are almost vertical from 300 would be indicated on the 500-mb chart by a to 50 millibars, a high degree of correlation is packing of the isotherms. evident in locating the jetstream on charts above the 500-mb level. Location on Upper Air Charts Frontalintersectionsaloft appear on the horizontal planes as areas of highly concentrated From these observations we can evolve the temperature lines or zones of maximum concen- followingrules:Withtheexception of the tration of temperatures. This concentration of subtropical jetstream, expect jetstream cores isotherms may reach a magnitude as great as (especially midlatitude jets) to be found directly 10°C in 45 miles, but usually ranges between above,ornearlyso,azone of maximum 10°C in 90 miles and 10°C in 150 miles. Under concentration of temperatures on the 500-mb

91 97 AEROGRAPEIER'S MATE 1 & C

50,000

.P COLD 40,000' 30G MB SFC

I . .

,,1 ANTWING1 rliET harlill 30,000 volRm lip pr WARM poLARTRopoPAUSE 500 Ple SFC

_ ,00W 300 MB SFC i

20,000' COLD 500 MB SFC ` .... `...... c. .. c 10,000' 0 ' R !, FRO . iWARM 44 AIR MASS COLD ASS l -r. EQUATOR NORTH ( A ) POLE 1 I -50° -60° -70°

C OLD TROPOPAUSE -70°

WARM

-40° ...- ...... - ...-...... - ....- -30 0 ...... - ...... IninEm.),mmommanot ...... TROPOPAUSE .....-.- ...... -- -20° -40° 10° - ...OP -30° WARM COLD -o 20°---- - 4,10° 10° .. -- "- " 440° 0 - - - - SURFACE EQUATOR ( DEGREES CELSIUS) NORTH POLE (B) AG.430 Figure 4-20.Vertical cross section of a model of the jetstream. (A) Wind andfronts; (B) temperature distribution. Chapter 4 ATMOSPHERIC CIRCULATION surface. O'Connor states that the jetstream is spacing of these isotachs around the core such frequentlywelldefinedatthislevel andis that a concentrationisgreatest through the usually found in the 17° C to -20° C isotherm frontal area and above the core. ribbon with a most frequent concentration along With the exception of the shear through the the17° C isotherm. The contour channels most frontal zone, the shear above the jet core (in frequentlyassociatedwiththe jetstream are closeproximitytothe tropopause)is much between 10.200 (5.548 meters) and 18.600 feet greater than the shear below the jet core. (5.670 meters). with a concentration along the VERTICAL SHEAR.Within the boundaries 18,400 -foot meters) contour.Project (5,608 of the jetstream are found the strongest vertical AROWA found in a research study that the jet wind shears observed in the belt of the wester- percentofthetime maximums occur 82 lies. In vertical cross sections, the isotachs north between29.400 -foot(8.960meters)and and south of the core are very close to being 30,300-foot(9,236 inters)contours.This vertical. indicating little variation of wind speed should be primarily a wintertime rule, since the with height in these areas. In general. the highest height would vary with area and season and values of vertical wind shear are encountered specifiedclimatic conditions. Also, from the above the 500-mb level. In summer some- foregoing we can expect the width of the core to timesinwinter.thestrongestvertical wind be approximately equal to the width of the zone shears may be located above 300 millibars and of maximum concentration of temperature on become negligible near 500 millibars. Thus at the 500-mb surface. 500 millibars, winds at this level may not give an indication or winds at high elevations. Relation of the Jet to the Polar Front HORIZONTAL SHEAR. -Vertical cross sections of the jetstream also indicate holizontal Since the jetstream is located above the polar wind shear. There is a rapid decrease on either front at 500 millibars, we could expect the core side of the jet core. In extreme cases. as much as of high speeds to be almost directly overhead at 100 knots in 100 miles on the north side and a station where the frontal intersection is found 100 knots in 300 miles on the south side of the close to the 500-mb level at that station. If the corewilloccur. The greatestcyclonic and frontal intersection is found below 500 millibars anticyclonic shears are located at the level of over a station. the core of high speeds will be maximum winds. Cyclonic shear is found pole- north of the station. When the frontal inter- ward of the jet axis; anticyclonic shear, equator- section is found above 500 millibars above the ward. station, the core of high speeds are found south The jetstream follows the configuration con- of that station. tours on upper levelcharts.In view of the While a thermal concentration is usually quite variation of thelevel of the jetstream with apparentatlevels up to and including the latitudes (low at high and high at low latitudes) 350-mb level, there is one important exception. andits known tendency to shiftto higher the Arctic front. Due. to the shallowness of the elevations in the warmer season of the year. the air masses involved, this particular frontal dis- 300-mb chart is considered suitable for locating continuity is not often apparent on the 500-mb the jetathighlatitudesin winter and the surface. 200-nib chart for middle latitudes. This is due to thefactthepolar jetstream coreisusually Distribution of the Wind located between 300- and 200-mb levels. A Field through the Jetstream higher chart is used for locating the subtropical jetstreamasitisusually located somewhere The wind field comdrising a jetstream is so between the 200- and 100-mb level, and is often distributed that a cross section taken normal to near the 150-mb level. the wmdflow revealsi system of annular speed Since the jetstream varies with elevation along lines(isotachs).(Seefig.4-20.)Ina well- the line of flow, it does not always appear as a organizedjetstreamthereis a nonuniform continuous flow on a constant pressure chart.

93 99 AEROGRAPHER'S MATE 1 & C

General Remarks About the Jetstream sphere and mid-troposhpere. These jets axes then, have a tendency to attain their maximum Jetstreams undergo a distinct life cycle con- strengthinmidlatitudes. Individual jetaxes sisting of a period of organization and one of often show a net equatorward shift of about disorganization. Their horizontal structure, as it one-half degree of latitude per day. New axes appears on upper level charts, depends on the form in high latitudes as the older ones move phase of this life cycle. When the jet is well out.Theequatorwardmigrationisusually organized,thelongitudinal axis follows the accompanied by a rise of the core to higher pattern of long waves, and its amplitude is elevations.Although the separate jetbands comparable to that of the 300-mb contours. A usually retain their identity around the hemi- lack of uniformity of wind speed along the axis sphere, they occasionally combine into one or is very evident: there are intervals where the more regions. Conversely, a single well-defined winds are comparatively weak. This creates a jet is often observed to split into two or more rathersharpwind wind gradient along the branches. In this case, the northern branch often longitudinalaxis,for the difference between shows a temporary northward trend. This split is wind speeds at maximum and minimum points associated with blocks. may amount to over 100 knots. During disorganized periods, the well-defined concentration of speeds is missing, and these Relation of Jetstream west winds either break up in circular flow, or to the Tropopause show fairly uniform speeds throughout the mid latitudes. It is during this period that numerous Since the level of reversal of the temperature splits or JET FINGERS appear. These fingers are field often coincides with the tropopause, atall comparatively weak and sometimes separated tempts have been made to locate the level of by as little as 5° latitude. strongest winds from tropopause analysis. This Centers of high and low wind speed alternate has proved a difficult undertaking. The tropo- along the axis of a well-developed jetstream. The pause often lies above the level of maximum maximum speeds in these centers may reach well wind and is therefore not a perfect indicator for over 250 knots. The distance between successive the maximum wind core. Besides, tropopause maximums isnot constant, but varies from analysis is very complicated because a single and about 10° to 25° longitude. Variations of height simple tropopause is rarely observed in middle of these maximums also exist along the axis. latitudes.Moreoften,thetropopauseisill These maximums are associated with short wave defined or thereisa multiple structure, and troughs in the westerlies and move with a speed many assumptions and definitions of an arbi- proportional to the 700-mb flow. trary character are necessary in drawing tropopause analyses. Multiple Jets One method of associating the jetstream with the tropopause has received rather wide accept- The concentration of west wind in the middle ance, but itis not completely representative of and upper troposphere does not occur along a allcasesforthe reasons statedabove. The single axis. On almost any day maps reveal the jetstreamis assumed to be associated with a presence of at least two bands of speed concen- break in the tropopause. North of the jet the tration. Occasionally three or four distinct bands tropopauseis low, and south of the jet the are in evidence for short periods. The individual tropopause is high. If the tropopause is contin- jet bands tendtolieparallelto the upper uous through the jet, it has a very sleep slope in contours, but appreciable departures from this the region of maximum wind. Oftentimes, the alinement are often observed, especially east of two tropopauses overlap with the jet located the trough. The different jets are not of equal between them. The midlatitude or polar front intensity. The most intense is usually the mid- jetis associated with the break between the latitude jet and is associated with the zone of subarctic and midlatitude tropopauses (30° to greatest temperature gradient in the low tropo- 50° N lat.). A second jet, the subtropical jet, is

94 100 Chapter 4 ATMOSPHERIC CIRCULATION frequently associated with a break between the over a short period of time. They show streaks midlatitude and subtropical tropopauses (25° to inthedirection of the wind and occur in 30° N lat.). coherent patterns. The use of the cloud method in identifying or detecting the presence of the Associated Clouds and Weather jetstreamisrestrictedto periods when lower clouds do not obscure the sky and when enough Statistical and observational studies of cloud moisture is present in the jetstream to permit patterns associated w ith the jetstream reveal that cloud formation. These cloud patterns can be most of the time there are no clouds at or above attributed to the convergence occurring down- the level of the jetstream core. In these stream from the jet maximums and to the lifting however.chillsgenerallyreachedtowithin created by the undulatory motion in the areas of several thousand feet of the tropopause on the Wind shear along the jet axis. equatorward side of the axis and no clouds were Inview of the wide variety of jetstream observed above the tropopause there. structures which occur. precipitation occasion- In one study. clouds were most frequent in ally will be encountered in almost any position the upper troposphere with the center of cloudi- with respect to the high speed center. ness 10.000 to 15.000 feet below the core and But as with cloud formations, the occurrence from 4° to 5° poleward of the core. Also. it was of precipitation associated with the jetstream is revealedthatanothercenterof cloudiness controlled primarily by the distribution of wind existed at about the same distance equatorward shear and curvature along the jetstream. at about 5.000 to 10.000 feet below the level of Several studies have been made relating pre- strongest wind. On many occasions the crew cipitation to the jetstream. In all. there is general members of research aircraft engaged in a study agreement that the highest incidence of pre- of cloud patterns associated with the jetstream cipitation almost straddles the jet axis, with a reported a sharp discontinuity in the cirrus near slight bias toward the poi,and side. However. the core with cloudless skies to the immediate inone of these studiesitwas found that north. The observers also reported that in cases precipitation occurred only 50 percent of the where the cirrus extended to the north of the time near the axis, and that large portions of the core there is often a narrow break in the cirrus core were without precipitation. at the core itself. The thickness of the cirrus averaged about 500 feet. but individual layers Climatology of the Jetstream ranged from a hundred to several thousand feet in depth. From day to day the jetstream can move in a There are generally recognized to be four direction at right angles to its axis, with a speed cloud patterns associated with the jetstream. of approximately 10 knots. This motion can be They are as follows: eithertowardthepolarair or toward the tropical air. 1.Lines of cirrus in bands (H4, 5, and 6). The mean position of the jetstream shifts 2. Patches of cirrocumulus (119) or altocumu- south in the winter and not th in the summer, lus castellanus (M8). with the seasonal migration of the polar front. 3.Lenticular clouds in waves (M4 or M71. The average latitude of the jetstreamin the 4. Waves of altocumulus (M3 or M5). Northern Hemisphere in summer is 42° N lat; in winter itis 25° N lat. Additionally, there is a At least three of the four patterns must be climatological variation in longitude, especially present for a jetstream to be justified. and they in winter. High wind speed concentrations are must present a pattern. These cloud patterhs found near the east coasts of continents and often extend from the horizon to the horizon, lower wind speed concentrations are found on and have waves at right angles to the airflow. the west coasts of continents. The rule is that From the ground, they are observed to move at speed concentrations increase as the westerlies high speeds (except the lenticular type), often pass over the continental anticyclonic circula- resulting in rapid local changes in cloud cover tions and weaken as they pass over the oceans.

95 101 AEROGRAPHER'S MATE I & C

WIALMS. LIZTLIWZ ....

06 ...... , -...... ,..---...... i ...' ;- : 7.":...... : 4%%% ,b-.1 " %%,s'Z s":% 0% Oe. . .. - i .. ". .- -'.' % 0 'I - %. t t . .% \ t % tx%, t% t % t/t -4 V %\ \ t 1. t N t 1, I % i 1 :%:: , i ..--....0 -1 * . . I j . :.if i i '.'').,:sie-5+.-...y -- `e it': ,..,." -/ , \ ,,, ',.if :".,- tr. : . it #701V ')Nli " - --?i - - r -3 4. %.....,1!. ' ,..' / t - i, .,I A.1" I- CO0 % i \ A .. , ,,...0 4 .00 , se ,.. . , ;..../: 4;;,..:. / i. Ii., . (A) '

ams

AG.431 Figure 4-21.Mean seasonal jetstream distribution. A) January.

Seefigure 4-21(A) and(B)forthe mean and extensive bad weather areas tend to be seasonal positions of the jetstream during Jan- connected with jetstreams. This is especially true uary and July. for weather associated with a warm front in the Latitudinal fluctuations of the jetstream are formative and mature stages of a cyclone. greatest on the west coast of continents. We have stated previously that the midlatitude jet usually lies directly above the polar Relation of the Jetstream front at 500 millibars. Using an average frontal to Fronts and Cyclones slope and anaverage height of the 500-mb surface (18,280 ft), you can place the jetstream Jetstreams pass over thousands of miles of some 300 miles behind the surface cold front subtropical desert areas where there is little or and around 600 miles in advance of a warm or no cloudiness and where the surface circulation stationaryfront. However,in estimating the is anticyclonic. They also lie above the strongest location of a front at 500 millibars in mountain- cyclones in the westerlies with extensive sheets ous areas, the height of terrain must be con- of clouds and precipitation extending along the sidered. For example, if the height of the land is axis. Thus, the mere existence of a jetstream in 10,000 feet, the jetstream should be placed only any area, even a very strong one, does not of about 300 nautical miles in advance of the itself imply bad weather there. The converse, surface warm front. however, holds toa much better degree of A strong surface front does not guarantee a approximation, especiallyin winter; cyclones jetstream aloft, as a strong temperature contrast

96 102 Chapter 4--ATMOSPIIERIC CIRCULATION

%1LAT.ICA tiOTT:NG CHART.,

...... ''''''''''''''' sv.,,,, , , 7--..,, \. ;;, ,-- ...,-.----,--:,--.. : , ... A.^ ..,' ,:.,...... 1'1 ---. \ 1t'...... - ' 1 t ' . ,i ."- 'It . 1 1 1 1 Cr ...... Z ;pt:Gv:: ,i I, s s i A'O., .;.;:-/ ./..1t8. I N1 i N , .,.... SI S. ....':-rS co' ,1. lart ' ".., S l' \ ),..42.5-1-;;::"' s'. 4 4; ,. t * :-...-s.so ,... i, I , .14' a A p A A e., ...,:er,-' St / /

1 .%. % ..: 0 ..° a a. f0. 0 /7 . ,i S \ % ... 0.,.:,c, . . ., ,., . , -.. ...__:. :...... ---...- ,', . 41 ,. ...^.....-,---.:.----.- , . ,- ...... ,...... " _ , 'aQa...... if. (B)

AG.432 Figure 4.21. Mean seasonal jetstream distributionContinued. (B) July. throughout a deep layer isrequired. In some 2. The jetstreamwill remain north of an cases, a wedge of cold air behind a front will be unoccluded wave cyclone. overrun by a warm southwesterly current, pro- 3. The jetstream will be south (near the point ducing extensive cloudiness and continuous pre- of occlusion) of the low associated with an cipitation. This shouldbetakenas definite occluded front. evidence that the cold wedge is shallow. The 4. In a series of lows of a cyclone family each jetstream behind such a front will be displaced low will be associated with a jetstream maxi- well to the north, usually about 600 miles. mum. (See fig. 4-22.) If there is enough moisture to produce typical cloud patterns with fronts, the location of the front at 500 millibars may be estimated as being The two preferred positions of a low center near the transition from high to middle clouds. with respect to a moving jetstream maximum are The jetstream will usually lie directly above this given in figure 4-22. As the wave cyclone in the transition zone. rightrearquadrant(lookingdownstream) The relationship of the jetstream to surface deepens, the associated tightening 01 the thermal lows and fronts is given in the following section. wind (and contour) gradient produces a new wind maximum. Further deepening and occlu- I. The jetstream will be perpendicular to an sion displace this maximum south of the low occlusion and to coldfronts oriented north- placing the low in the left front quadrant of the south with no associated warm front. jet maximum.

97 1.03 AEROGRAPIIER'S MATE I & C

amplitude as the cyclone deepens. At first the jetstream lies north of a young cyclone at the surface, moves southward, and inthe latter stages of development is found south of these deep surface cyclones. At first, the jetstream does not intersect the frontal system, but the jet does cross the occluded front il. the latter stages of cyclone development.

Subtropical Jetstream

Subtropical jet streams (STJ) are persistent features of the tropical general circulation. In the northern hemisphere during winter, the STJ AG.433 shows asimplebroadscale current whichis Figure 4-22.U.;ual position of surface lows in continuous aroundthe worldwithabasic relation to moving jet maximums. three-wave pattern of ridges and maximum wind speeds over the east coasts of Asia and North America. and the Middle East. The mean lati- 5 Although not every jetstream maximum tude is 27.5° N, ranging from 20° to 35° N. The will have an associated sea level low, each low core is located near the 200 mb level, and speeds embedded in the westerlies will be associated of 150 to 200 knots are not uncommon. with a jet maximum. Lows embedded in the In the northern hemisphere during summer, westerliesarethose migratory lows that are the westerly jet is not in evidence, instead, the under influence of the basic westerly flow. tropical easterly jet (TEJ) is a persistent feature 6. The jetstreamaloftshould parallelthe over extreme southern Asia and northern Africa. direction of the warm sector isobars of a surface It extends over the lay er from 200 to 100 mb in low, since these isobars are alined along the the latitude belt of 5° to 20° N, core speeds over upper level flow. WO knots are often observed. 7. The jetstream aloft will roughly parallel In the southern hemisphere, a subtropical jet the isobars around the southern periphery of a stream exists in both summer and winter as a cold (slow moving) surface low. broadscalecontinuouswesterlycurrent.Its 8. The jetstreamwillroughly parallel the mean latitude varies from 26° S in winter to 32° isobars along the northern periphery of :: warm S in summer. The highest average core speeds (slow moving) surface high. range from 140 knots in July to 70 knots in 9. When a cold, moving, polarhigh stagnates January-February. The greatestvariabilityin acidbeginsto warm up,itcanalter quite location and speed occur in summer and the markedly the jetstream configuration to its rear. least in winter. Asthethermalcontrastin thesouthwest quadrantis destroyed and warm air advected northward, the original jetstream dissipates and Polar Night Jetstream a new jet forms to the north. This is explained bythefactthatthelow or troughaloft The polar night jetstream has been suspected associated with the surface cold high dissipates for a number of years, but because of the lack of and the whole system becomes a warm cored observational data in the Arctic and Antarctic high. regions, the actual existence of such a jetstream has been difficult to confirm. Chapter VI, titled In summary, thepolarfront jetstreamis "The Polar Night Jet Stream" from NavWeps associatedwithsurfacefrontsand pressure 50-IP-549,Jet Streams of the Atmosphere, systems ina very simple way. The jetstream summarizes the data available on this jetstream wave associated with the surface low increases in to date.

98 104 Chapter 4ATMOSPHERIC CIRCULATION

The polar night jetstream is said to be located where the flow of air is directed outward from high in the stratosphere in and around the Arctic the center, causing a downward motion there. and Antarctic Circles. This jetstream does not Divergence, too, is classified as either horizontal appear to be associated with tropopauses as with or vertical. the jetstream of the middle and lower latitudes, The simplest form of convergence and diver- but rather with heating and cooling inthe gence is the type that results from wind direc- ozonosphere. which takes place over the long tionalone. Two flows of air need not be winters and summer peculiar to that region. opposing each other to create convergence or Nevertheless, this jetstream core is associated divergence but may be at any angle to each with one of the features of the other jetstreams other to create a net inflow of air for conver- in that this core is situated at an altitude where gence or a net outflow for divergence. the stability of the atmosphere increases discon- Wind speed in relation to the wind direction is tinuously upward. The principal research and also a valuable indicator. For example, on a study on this stream has been for winter. streamline analysis chart we can analyze both wind direction and wind speed, the field result- CONVERGENCE AND DIVERGENCE ing from variations in wind speed along the streamlines or the convergence or divergence of You are, no doubt, familiar with the two the streamlines. The following are some of the terms "convergence- and "divergence," as they combinations or variations of wind speed and have come to your attention when used in streamlines. relationtosurface lows andhighs.Inthis section.thetermsareredefined, andthe 1. Inafield of parallel streamlines, if the motions involved in convergence and divergence, wind speed is decreasing downstream (producing and the relationship of these processes to other a net inflow of air for this layer) convergence is meteorological processes are covered. taking place.If the flow is increasing downstream (a net outflow of air for the layer), CONVERGENCE AND DIVERGENCE divergence is occurring. (SIMPLE MOTIONS) 2. In an area of uniform wind speed along the streamlines, if the streamlines fan out (diverge), Convergence divergence is occurring; if the streamlines converge, we can say that convergence is taking Convergence is defined as the increase of mass place. within a given layer of the atmosphere. In order 3. Normally, the convergence and divergence for this to take place, the winds must be such as components are combined. The fact that stream- to result in a net inflow of air into that layer. We lines converge or diverge does not necessarily generally associate this type of convergence with indicate convergence or divergence. We must low-pressure areas where convergence of winds also consider the wind speedswhether they are towardthecenter of the low resultsin an increasing or decreasing downstream in relation increase of mass into the low and an upward to whether the streamlines are spreading out or motion. In meteorology, we distinguish between coming together. two types of convergence as either horizontal 4.If,whenlooking downstream on the or vertical convergence, depending upon the streamlines, the wind speed increases and the axis of the flow. streamlines diverge, divergence is taking place. On the other hand, if the wind speed decreases Divergence downstream, and the streamlines come together, convergence is taking place. Divergence is defined as the decrease of mass within a given layer of the atmosphere. Winds in There are other cases where it is difficult to this situation are such that there is a net flow of tell whether divergence or convergence is taking air outward from the layer. We associate this place, such as when the speed of the wind typeof divergencewithhigh-pressurecells, decreasesdownstreamandthestreamlines

99 105 AEROG RAPI I ER'S MATE I & C

spread apart, and when speed increases down- SIGNIFICANT horizontalDIVERGENCE. or stream and the streamlines converge. A special CONVERGENCE stratum. Also the advection evaluation then must he made to determine the stratum may be thought of as the zone in which net inflow or outflow. compensation of the dynamic effects of the upper stratum occurs. DIVERGENCE AND CONVERGENCE At about 8 km the density is nearly constant (COMPLEX MOTIONS) in time and space. This level, which isnear the 350-mb pressure surface,iscalledthe ISO- In this discussion stress is placed on high-level PYCNIC LEVEL. a level of constant density. convergence and divert/A:net: in relation to con- This levelis the location of the upper tropo- tour patterns downstream, and the advective spheric maximum of MU:I-diurnal pressurevaria- patterns associated therewith. Low ow tropospheric tion, with mass variations of opposite sign above advection(andalsostratospheric advection) and below this level. certainly plays a large role in pressure change Since the densityat 200 millibars is only mechanisms, but in the majority of cases itis four-sevenths the density at the isopycnic level, thought to compensate the higher level mass the height change at 200 millibars would be changes produced bytheunbalancedforces almost twice that a 350 millibars for thesame associated with the inertia of parcels of air aloft. pressurechangeat corresponding geometric Sincetheterm "divergence"ismeant to levels. Thus height changes in the lower strato- denote depletion of mass, while convergence is spheretend to be a maximum even though meant to denote accumulation of mass, the pressure changes are a maximum atthe iso- prognostic analyst is concerned with the MASS pycnic level which is usually below the nudlati- divergence or convergence in estimating pressure tude tropopause. or height changes. Mass divergence in the entire The large pressure variations at the isopycnic column of air produces pressure or height falls, levelrequire TEMPERATURE variationsfor while convergence in the entire column of air constancy of density. Since the density is nearly produces pressure of height rises at the base of constantin space atthislevel,the required the column. temperature variations must result from vertical Mass divergence and convergence involve the motions. When the pressures are rising at this densityfield aswell asthevelocityfield. level, the temperature must also rise to keep the However, the mass divergence and convergence densityconstant. A temperaturerise can be of the atmosphere are believed to he largely produced by descending motion. Similarly fall- stratified into two layers as follows. ing pressures at this level require falling tempera- turesto keep the density constant.Falling I.Below about 600 millibais. velocity diver- temperatures in the absence of advection can be gence and convergence occur chieflyinthe produced by ascent through this level. friction layer. which is about one-eighth of the Thus, rising heights at the pressure level of weight ofthe 1,000-to 600-mb advection constant density are associated with subsidence stratum, and may he disregarded in comparison and falling heights of this surface are associated with density transport in estimating the con- with upwelling. tribution to the pressure change by the advec- Subsidence at 350 millibars can result from tion stratum. horizontal CONVERGENCE above this level and 2. Above 600 millibars, mass divergence and upwelling here would result horn horizontal convergence largely result from horizontal diver- DIVERGENCE above this level. gence and convergence of velocity. I lowever, on Since rising heights in the upper troposphere occasion, stratospheric advection of density may are also known to be associated with rising of the be a modifying factor. tropopause and lower stratosphere, the maxi- mumhorizontal convergencemust occur The stratum below about the 400-mb level between the constant densityleveland the may be regarded as the ADVECTION stratum, averagelevelof the tropopause (about 250 above approximatelythe400-mb level,the millibars). This is due to the reversal of sign of

100 106 Chapter 4 ATMOSPHERIC CIRCULATION the vertical motion between the tropopauseand eventually becoming zero at the level of maxi- the isopycnic level. Thus, the level ofmaximum mum horizontal divergence.Above this level, descending motion is believed to set in. In the horizontal velocity convergence must be between 300 millibars and 200 millibars andis rear of the low the ceveseistrue: thatis, the causative mechanism for pressure orheight descending motioninthe surface divergent stratum and ascending motion in the upper rises inthe upper air. Similarly, upper height falls are produced by horizontal velocity diver- troposphere above the level of maximum hori- gence with a maximum atthe same level. The zontal convergence. In deepening systems the maximum divergence occurs near orslightly convergence aloft in the rear of thelow is small above the tropopause and closer to 200millibars or may even be negative (divergence).In filling advance of the thantc300 millibars.It isprobably more systems the divergence aloft in realistic therefore to define a LAYERof maxi- low is small or even negative (convergence). mum divergence and convergence asoccurring Thus. in the travel and development of highs involved, between the 300- and 200-mb pressuresurfaces. and lows, two vertical circulations are This is also the layer in which the core of the one below and one abovethe 300-mb level approximately. The LOWER VERTICAL CIR- jetstreamis usually located. At this level the cumulative effects of the mean temperaturefield CULATION is upward in the cyclone, thence hori- toward the anticyclone. and then downwardin of the troposphere produce the sharpest CIR- zontal contrasts in the wind field. the anticyclone. The UPPER VERTICAL CULATION involves subsidence in the strato- The chart best suited for determinationof sphere of the developing cyclone and ascentin convergence and divergenceis the 300-mb chart. of Because of the sparsity of reports at the300-mb the upper troposphere and lower stratosphere fig. 4-23.) level, it is frequently advantageous todetermine the developing anticyclone. (See divergence at Divergence and upper-heightfalls are asso- the presence of convergence and weak the 500-mb level. These chartswill ordinarily ciated with high-speed winds approaching cyclonically curved. yield good results. contour gradients which are Figure 4-24 illustrates schematically some con- The usualdistributionof divergence and tour patterns associated with contourfalls. convergence relative to moving pressuresystems Convergence and upper-height rises are asso- is believed to be as follows: (Also s2e.ch.I I.) ciated with: In advance of the low, convergence occurs I, I. Low-speed winds approaching straight or at low levels and divergence occursaloft, with cyclonically curved strong contour gradients. the level of nondivergence at about 600millibars 2. high -speed windsapproachinganticy- on the average. 2. In the rear of the low, there is usually clonically curved weak contour gradients. aloftanddivergencenearthe convergence Figure 4-25 illustrates schematically the con- surface. tour patterns associated with eachof these cases The low-level convergence ahead ofthe low of contour rises. The height rises and falls occur dow, ,tream and to the left of the flow. occurs usually in the stratumof strongest warm of advection, and the low-level divergencein the The technique for determining the areas divergence consists in noting the areas where rearof the low occursinthe stratum of strongest cold advection.The low-level diver- winds of high speed are approaching weaker cyclonic curva- inhe friction layer, pressure gradients of straight or gence occurs principally carries a high- and itis thought to be of minorimportance in ture downstream. When inertia modifying estimates of thickness advection com- speed parcel of air into a region of weak pressure pared with heating and cooling from theunder- gradient, it possesses a Coriolis for:e too large to be balanced by the weaker pressuregradient lying surfaces. is thus deflected to the right by the In advance of the low, the air risesin response force.It with the maximum resultant unbalanced force to the right.This to the low-level convergence the ascending motion at the level ofnondivergence results in a deficit of mass to the left due to

101 107 AEROGRAPILER'S MATE 1 R C

DYNAMIC WARMING DYNAMIC COOLING UPPER HEIGHT FM.I.S UP R HEIGHT RISES

.NCOMPLETE INCOMPLETE INFLOW OUTFLOW STRATOSPHERIC ADVECTION STRATUM 200 MB.

MAXIMUM HORIZONTAL MAXIMUM HORIZONTA.. DIVERGENCE STRATUM DIVERGENCE 300 Ka. LEVEL OF CONSTANT DENSITY' 350 MBADKM) 400 MB.

ZERO HORIZONTAL DYNAMIC DIVERGENCE --GOO MB. --COOUNG DYNAMIC TROPOSPHERIC WARMING ADVECTION STRATUM

CONVERGENCE SURFACE FRICTION DIVERGENCE LAYER SURFACE FRICTION 1,000 MB. NIGH LAYER

AG.434 Figure 4-21Generalized vertical circulationover developing his and lows.

influx of high-speed wind iseven more marked due to the additional effect ofcentrifugal forces (on the other hand, the effectof centrifugal forcesof anticyclonicallycurving high-speed parcels is of extreme importancein producing overshooting of high-speedair from sharply curvedridgesintoadjacenttroughs causing pressure rises in the west side of the troughs.) If the influx of high-speedparcels toward diverging cyclonically curvedcontoursis sustained, large contour falls willoccur downstream to the left of the inertial path ofthe strong winds. Eventuallya strong pressure gradient is AG.435 propagated downstream tothe Figure right of the 4-24.Divergence illustrated. high-speed winds, chiefly by'ressure falls to the left of the direction of hig. peedwinds in the cyclonically curved contours with failure of incoming parcels to weak pressure compensate the gradient. Usually the deflection ofair toward longitudinaldeficit of mass by the parcels higher pressureisso slightthatitis hardly leaving the particular area. The parcels whieaare observable in individual wind observations. deflectedto How- theright must penetrate high ever, when the pressure field is very flat to the pressure and are thus slowed down until they are right of the incoming high-speedstream, notice- in balance with the weakerpressure gradient. able angles between the wind and Then they can be steered along the contours may existing be observed, especially at lowerlevels. due to isobaric or contour channels. transport of momentum downwardas a result of If the weak gradients downstream are cycloni- subsidence, where the gradientsare even weaker. cally curved, the divergence resulting fromthe This occurs sometimes to suchan extent that

102 108 Chapter 4ATMOSPHERIC CIRCULATION

Itismore usual, however, for thewind component toward high pressure to be very slight, and unless the winds and contours are drawn with great precision, the deviation goes unnoticed. High-speed winds approaching sharply cured ridges resultin large height rises downstream froM the ridge due to OVERSHOOTING of the highspeedair.Asiswell known fromthe gradient wind equation, for a given pressure gradient thereisa limiting curvature to the trajectory of a parcel of air moving at a given speed.Frequently onthe upper aircharts, sharply curved stationary ridges are observed with winds of high speed approaching the ridge. (A) The existence of a sharply curved extensive ridge usually means a well-developed trough not far downstream, and frequently a cold or cutoff low existsinthis trough. The high-speed parcels approaching theridge, because of centrifugal forces,are unable to make the sharpturn necessary to follow the contours.These parcels overshoot the ridge an ticyclonically, but with less curvature than the contours, resulting in their plunging across contours toward low pressure downstream fromtheridge, andtheir accelerating. This may result in any one of a number of consequences for the trough downstream, depending on the initial configurationof the ridge and trough, butallare based on the convergence of mass into the trough as aresult of overshooting of air from the ridge. Four of the most frequent consequences may be enumer- (B) ated as follows: I. Filling of the portion of the trough down- AG.436 stream from theridge. This happens if the Figure 4-25.Convergence illustrated. contour gradient is strong on the east side of the trough: that is, a blocking ridge to the east of the streamlines are considerably morecurved the trough. anticyclonically than the contours. In rare cases 2. Acceleratingthecutoff low out of its this results in anticyclonic circulation centers stationary position. This usually occurs in all out of phase with the high-pressure center.This cases. is, of course a transitory condition necessitating 3.Radicallyreorientingthetrough.This a migration of the pressure centertoward the usually happens where the trough is initially circulationcenter, andthisis of prognostic NE-SW, resulting in a N-S and in some cases a value. In cases where the high-pressure center NW-SE orientationafter sufficienttime (36 and anticyclonic streamline center are outof hours). phase, the pressure center will migratetoward 4.It may actually cut off a low in the lower the circulation center (which is usually a center end of the trough. This usually happens when of mass convergence). the high-speed winds approaching the ridge are

103 109 h. AEROGRAPHER'S MATE 1 & C southwesterly andapproachtheridgeata west side of the downstream trough comparativelyhighlatitude results in relativetothe lifting of the tropopause withdynamic cooling, trough. This frequently reorientsthe trough line and upper-level contour rises. towards a more NE-SW direction. Usually, the Low-speed parcels of air froma weak pressure reorientationofthetroughoccurssimul- gradient which enter taneously with 1 and 2. an area of stronger gradient become subject to an unbalancedgradient force toward the left due to the weakerCoriolis force. Closelyrelated to the above situationare These sub-gradient winds cases of sharply curved ridges where the are deflected toward pressure lower pressure, crossingcontours and producing gradient in the sharply curvedportion (usually contour rises in the area of the northern portions of cross-contour flow, a north-south ridge) has This cross-contour flow acceleratesthe air until momentarily built up toa strengththatis itis moving fast enough to be incompatible with the anticyclonic balanced by the curvature. stronger pressure gradient. Due to theaccelera- Such ridges often collapse withgreat rapidity tion of the slower oncoming parcels subsequent to the development of of air, the such excessive contour rises propagate much fasterthan might gradients, causing rapid fillingof the adjacent be expected on the basis ofthe slow speed of trough downstream and large upper contour falls the air as it initially enters thestronger pressure where the ridge now exists. Thegradient wind gradient. relation implies that subsequent trajectories of The following two rules summarizethe above the high-speed parcels generatedin the strong discussion: ridge line gradient must be lessanticyclonically curved than the contours in theridge. 1. High-speed winds approachinglow-speed Itcanbe shown from the gradientwind winds with weak cyclonicallycurved contour equationthattheanticyclonic curvaturein- gradients are indicative of divergenceand upper- creases as the difference between the actual height falls downstream andto the left of the wind and the geostrophic windincreases, until current. the actual wind is twice thegeostrophic, when 2. Low-speed winds approachinghigh-speed the trajectory curvature isa maximum, This fact winds with strong cyclonicallycurved contour can be utilized in determining the trajectory of gradients or high-speed windsapproaching low- high-speed parcels approaching sharplycurved speed winds with weakanticyclonically curved stationary ridges, or sharply curvedstationary contour gradients are indicative ofconvergence ridges in which the contour gradientis excessive. and upper height rises downstreamand to the By measuring the geostrophic windin the ridge, left and right of the current, respectively. the maximum trajectorycurvature can be ob- IMPORTANCE OF CONVERGENCE tained from the gradient wind scale.This trajectory curve is the one whichan air parcel at the AND DIVERGENCE origin point of the scale will follow untilit Convergenceanddivergence have intersects the correctioncurve from the geo- apro- strophic speed to the displacement nounced effect upon the weatheroccurring in curve of the atmosphere. Vertical motion,either upward twice the geostrophic speed. or downward, is If actual wind speed observations recognized as an important are available parameterinthe atmosphere. For instance, for parcels approaching the ridge,comparison extensiveregionsofprecipitationassociated can be made with the geostrophic winds (pres- with extratropical cyclones sure gradient) in the ridge. If the actual speeds are regions of large- are more than twice the measured geostrophic scale upward motion of air throughmost of the troposphere.Similarly,thenearlycloud-free wind in the ridge, the anticycloniccurvature of regions in large anticyclones these high-speed parcels will be are regions in which less than the air is subsiding througha large portion of the MAXIMUM TRAJECTORYcurvature obtained troposphere. Vertical motions alsoaffect tem- from the gradient wind scale, andeven greater perature, humidity, and other overshooting of these high-speed parcels meteorological will variables either on a small scale (locally)or on a occur across lower contours. Convergence in the large scale as pointed out above.

104 110 Chapter 4--ATMOSPHERIC CIRCULATION

Changes in Stability ahead of such centers. Divergence exists in the other two quadrants. Below theregions of Whenconvergenceordivergenceoccurs, divergence theair rises; below those of con- whether on a large or small scale, it can have a vergence there is subsidence. very pronounced effect on the stabilityof the air. For example, when convection is induced by These general rules of thumb are not perfect, convergence, air is forced to risewithout the and only yield a very crude idea about distribu- addition of heat.It' this airis unsaturated, it tion of vertical motion in the horizontal. Par- coolsfirstatthe dry adiabaticrate; orif ticularly over land in summer, there exists little saturated, at the moist rate. The end result is relationbetween large-scaleweather patterns that the air is cooled, which will increase the and vertical motion. Rather, vertical motion is instability of that air column by increasing the influenced by local features and shows strong lapse rate. Clouds and weather often result from diurnal variations, the phase which may be quite this process. different in different regions. Large-scale vertical motion is of small magnitude at the ground Conversely if air subsides, and this process is (zero if the ground is flat). Above the ground, it produced by convergence or divergence, the air increase in absolute magnitude to at least 500 sinking will heat at the dry adiabatic lapse rate. millibars and decreases in the neighborhood of The warming at the top of an air column will the tropopause. Reversals in sign in a vertical increasethe stability of that air column by column are rare in the troposphere, but are reducing the lapse rate. Such warming often likely to occur in the stratosphere. There have dissipates existing clouds or prevents the forma- been several studies of therelation between tion of new clouds. When sufficient warming frontal precipitation and large-scale vertical ve- due to the downward motion takesplace, a locities, computed by various techniques. In all subsidence inversion is produced. cases, the probability of precipitationis considerably higher in the 6 hours following an updraft than following subsidence. Clear skies Effect on Weather are most likely with downdrafts.On the other hand, it is not obvious that large-scale vertical The full aspects of the relationship of vertical motion is related to showers and thunderstorms motion to weather are explored inVertical caused during the daytime by heating. However, Motion and Weather, NWRF 30-0359-024.The squall lines, which are formed along linesof most improtant application of verticalmotion is horizontalconvergence show that large-scale the prediction of rainfall probability andrainfall vertical motion may also play an important part amount.In addition,vertical moiton affects in convective precipitation. practically all meteorological properties, such as temperature, humidity, wind distribution,and Vertical velocity charts are currently being particularly stability. In the following section transmitted over the facsimile network by the the distribution of large-scale and small-scale National Meteorological Center and are com- vertical motions are considered. putedbyNumericalWeatherPrediction methods. The charts have plus signs indicating Since cold air has a tendency to sink, sub- upward motion and minus signs indicating sidenceis likely to be found to the west of downward motion. The figures indicate vertical upper tropospheric troughs, andrising air to the incentimeters per second (cm/sec). troughs. Thus, thereisa good velocity east of the With the larger values of upward motions (plus relation between upper air meridional flow and values) the likelihood of clouds and precipita- vertical flow. tion increases. However, an evaluationof the In the neighborhood of a straight Northern moisture and vertical velocity should be made to Hemisphere jetstream, convergence is found to get optimum results. Obviously, upwardmotion the north of the stream behind centersof in dry air is not as likely to produceprecipita- maximum speed as well as to the southand tion as upward motion in moist air. AEROGRAPHER'S MATE I & C

ROTATIONAL MOTION AS IT not spin as it moves downstream. If it doesspin, AFFECTS THE ATMOSPHERE the chip has vorticity. Whenwe try to isolate the cause of the spin, we find that two properties of Many forecasters and meteorologistsstarted the flow of water cause the chipto spin: ( I) If their careers years ago when the main concern the flow of water is moving fasteron one side of was with small areas and short-time intervals. the chip than the other, thisis shear of the Thegeographicalextension of observations, current; (2) if the creek bedcurves. the path has especially those of the upper air, and the study curvature. Vorticity always applies toextremely of these observations by dynamicmeteorologists small air parcels: thus,a point on one of our have proved (although ithad long been sus- upper air charts may represent sucha parcel. We pected) that the cycloneor anticyclone is not can examine this point and say that the parcel independent of developments in otherparts of does or does not have vorticity.However, for the hemisphere: rather it constitutesa cogwheel this discussion, larger parcels willhave to be in a larger mechanism. The relationship of the usedtomoreeasilyvisualizetheeffects. cyclone to the larger-scale flow patterns must Actually, a parcel in the atmospherehas three therefore be a parr of the daily forecastroutine rotational motions at thesame time: (I) Rota- if progress is to be made in forecasting the tion of the parcel about itsown axis (shear). (2) motions of cyclones and associatedupper wind rotation oftheparcel aboutthe fields. axis o` a pressure system (curvature), and (3) rotation of One of the studies that has gainedprominence the parcel due to the atmosphericrotation. The in recentyears has beenthat of vorticity. sum of the first two components is knownas Unfortunately many forecasters havea tendency relative vorticity, and thesum total of all three to shy away from the subject of vorticity,as is known as absolute vorticity. they considerittoo deep a subjecttobe mastered. This means theyare neglecting an Relative Vorticity important forecasting tool. Theprinciples of vorticity are no more complicated thanmost of Relative vorticity is the sum of the rotationof the principles of physics, andcan be understood the parcel about :he axis of thepressure system just as easily. During the last fewyears vorticity (curvature) and the rotation of the parcel about has become a prominent tool in meteorology. its own axis (shear). The vorticity ofa hori- especiallyinthefield of Numerical Weather zontal current can be broken down intotwo Prediction. For this purpose,we will consider in components, one due to curvature of thestream- thissectionof the chapterthe meaning of lines and the other due to shear inthe current. vorticity. its evaluation. and its relationshipto SHEAR.--First,letus examine the shear other meteorological parameters. effect by looking at small air parcels inan upper air pattern of straight contours. Herethe wind shear results in each of the three parcels having VORTICITY different rotations. (See fig. 4-26.)

Vorticity measures the rotation ofvery small airparcels. A parcel has vorticity when the parcel spins as it moves along its path. On the Z I (I) other hand, a parcel which does not spin is said Z to have zero vorticity. The axis of spinningor Z +3 3 rotation can extend in any direction. but forour Zr4 purposes, we are mainly concerned with the rotational motion about an axis that isperpen- DIRECTION OF WIND 0 dicular to the surface of the earth. For example, we could drop a chip of wood into a creek and AG.437 watch its progress. The chip willmove down- Figure 4-26.Illustration of vorticity due stream with the flow of water but it may or may to the shear effect.

106 11.2 Chapter 4ATMOSPHERIC CIRCULATION

1. Parcel No.I has stronger wind speeds to Place a small parcel at the trough and ridge its right. As the parcel moves along, it willbe lines and observe the way in which the flow will rotated in a counterclockwise direction. spin the parcel, causing vorticity. The diameter 2. Parcel No. 2 has the stronger speed to its of the parcel will be rotated from the solid line left;therefore,itwill rotate ina clockwise to the dotted position (due to the northerly and direction as it moves along. southerly components of the flow on either side 3. Parcel No. 3 has speeds evenly distributed. of the trough and ridge lines). This parcel has equal speeds at opposite ends of Note that we have counterclockwise rotation the diameter. It will move, but it will not rotate. at the trough (positive vorticity), and atthe It is said to have zero vorticity. ridge line we have clockwise rotation (negative vorticity). At the point where thereisno curvature (inflection point), there is noturning Therefore, to briefly review the effect of of the parcel, hence no vorticity. This isdemon- sheara parcel of the atmosphere has vorticity strated at point P in figure 4-27). (rotation) when the wind speed is stronger on COMBINED EFFECTS.To find the relative one side of the parcel than onthe other. vorticity of a given parcel we must consider both Now let's define positive and negative vortic- the shear and curvature effects.Itis quite ity in terms of clockwise and counterclockwise possible to have two effects counteract each rotation of a parcel. The vorticity is positive other; thatis, where shear indicates positive when the parcel has a counterclockwise rotation vorticity but curvature indicates negative vortic- and the vorticity is negative whenthe parcel has ity, or vice versa. (See fig. 4-28.) clockwiserotation(anticyclonic,Northern Hemisphere).

Thus, in figure 4-26, parcel No. 1 has positive vorticity, and parcel No. 2 has negative vorticity. Zt2 2.1 CURVATURE.Vorticitycanalsoresult from curvature of the airflow or path.In the case of the wood chipflowing with the stream, the chip will spin or rotate as it movesalong if the creek curves. To demonstrate the effect of curvature, let us (8) consider a pattern of contours having curvature (A) but no shear. (See fig. 4-27.) AG.439 Figure 4-28.Illustration of shear effect opposing the curvature effect in producing vorticity. (A)Negative shear and positive curvature; (B) positive shear and negative curvature.

To find the net result of the two effects we would measure the value of each and addthem algebraically. The measurement of vorticity will be discussed in the next section. Itmust be emphasizedherethat relative vorticity is observed instantaneously. It is not necessary for a parcel to rotate alarge amount AG.438 to have vorticity. In summary,relative vorticity inthe atmosphereisdefined as the instan- Figure 4.27. Illustration of vorticity due The to curvature effect. taneous rotation of very small particles.

107 113 AEROGRAPIIER'S MATE 1 & C

rotation results from wind shear and curvature. An is the direction along whichwe measure We refer tothisvorticity as being relative, theshear vorticityandrepresents the because all the motion illustrated was relative to distance between the two points for which the surface of the earth withlatitude and v was determined. longitude lines used as reference points. MEASUREMENT OF RELATIVE VOR- To illustrate how this equation isevaluated to TICITY. The various mathematical expressions find geostrophic vorticity, for vorticity are merely "shorthand" a sample contour ways of field hthirig both shear andcurvature is pre- sayingthatvorticity may be determined by sented in figure 4-29. shear and curvature. One common expression To find the vorticity at P, proceed in for relative vorticity is: exactly the same manner regardless of where P islocated in the pattern. It must be noted, however,that Zr = v -2 R is positive for cyclonic curvature andnegative R An for anticyclonic curvature, and the Anis perpendicular to the flow and measured from high where: to low contours, or from leftto right when looking downstream. Zr indicates vorticity of the windfield at a Intheexample,figure 4-29, R may be constant level or constant pressure surface determined from a template. The geostrophic (the so-called vertical component ofvor- wind at P gives the value ofv. These two values ticity). This is expressed in radians/sec. give the curvature vorticity, v/R. If 5 is 500,000 v isthe wind speed, in terms of knotsor meters and v is 50 meters/sec, the meters/second, and represents the speed of movement of the air parcel. 50 m/sec = 10-4 sec- I R is the radius of the windflow. R ispositive R 500,000 in 10,000 sec for cyclonic curvature and negative for anticyclonic curvature. To investigate the shear term Av representsthe changein speed of the airflow along a line perpendicular to the Av v2 -v1 con tours. - An An

500 KM

AG.440 Figure 4.29.- Evaluation of vorticity using thevorticity equation.

108 114 Chapter 4ATMOSPHERIC CIRCULATION at point P, we measure the wind speed at points The total vorticity, that is, relative vorticity v1 and v2, and An. (Zr) plus that due to the earth's rotation, is known as the ABSOLUTE VORTICITY. As was v,= 20 in/sec stated before, for practical use in meteorology, only the vorticity about an axis perpendicular to v1 = 70 m/sec the surface of the earth is considered. In this An = 50,000 m case, the vorticity due to the earth's rotation becomes equal to the CORIOLIS PARAMETER. This is expressed as 2w sin 0, where w is the Therefore, angular velocityof the earth and 0 isthe latitude. Therefore, the absolute vorticityis Av v2 vt 20-70 m/sec equal to the Coriolis parameter plus the relative An An 50,000 in vorticity. Writing this in equation forms gives: (Za = absolute vorticity) = I 10,000 sec Za = 2 co sin 0 + Zr Thus, the sum of the shear and curvature Current and prognostic charts of absolute vorticities for the selected point P is. vorticity are currently prepared by Numerical Weather Prediction methods and transmitted by

v Av 1 1 the National Meteorological Center on the fac- LZr = = r An 10,000 sec 10,000 sec simile network.

CONSTANT ABSOLUTE VORTICITY 2 / X 10-1 sec-1 10,000 sec TRAJECTORIES (CAVT)

Vorticityis usually expressed as radians/sec. Ithas been proved mathematically (under Radians are used to measure angles in the same some rigid assumptions) that parcels moving in manner as degrees. Whereas a circle has360°, it the atmosphere conserve their absolute vorticity has 6.28 radians; so a radian is equal to about as they move from place to place; that is, the 57°. These units, radians/sec, demonstrate the value of Za does not change. The most impor- idea of vorticity as an instantaneous rotational tant of the assumptions is that no convergence speed, so that for radians/sec we merely write or divergence occurs. Actual observations under 1 /sec or sec -1. There are numerous other ways conditions of no convergence or divergence bear that vorticity can be measured. This is only one out this theory. example. Absolute vorticityremaining constant, the In summary, we must keep in mind that motion of a parcel of air as it moves from cyclonic vorticityis positive and anticyclonic latitude to latitude in the Northern Hemisphere vorticity is negative. Therefore, when vorticity is will describe a definite pattern called Constant said to increase,itis becoming more cyclonic Absolute Vorticity Trajectories (CAVT's). Let and less anticyclonic. When it decreases, the us now consider the movement of some of these converse is true. air parcels.

Absolute Vorticity Coriolis Parameter

When the relative vorticity of a parcel of air is As a parcel moves from south to north, the observed by a person completely removed from latitude increases; there the CORIOLIS PARAM- the earth, he observes an additional component ETER increases. If the value of 2 w sin 0 of vorticity created by the rotation of the earth. increases, the value of Zr must decrease in order Thus,this person sees the total or absolute that the sum remains the same. If Zr decreases, vortici:y of the same parcel of air. it becomes less cyclonic or, in other words, more

109 115 AEROGRAPHER'S MATE 1 & C anticyclonic. This being the case, the parcel of The points A,B, C, and D, where the air would move in a path which becomes less curvature changes from cyclonic to anticyclonic, and less cyclonic with increasing latitude, and or vice versa, are known as inflection points. eventually becomes anticyclonic. Then the anticyclonic curvature increases until the parcel is moving from west to east. Continuing tomove in an anticyclonic path, the parcel beginsto turn back toward the south. (See fig. 4-30.)

AG.443 Figure 4-32.Sinusoidal vorticity path. r Amplitude and Length AG.441 The amplitude andlength of the waves Figure 4-30.Coriolis parameter increasing. depend upon the initial speed and direction of the parcel and its initial latitude. This fact is As a parcel moves from north to south, the fundamental in determining constant vorticity trajectories.If the speed and direction of latitude decreases, and hence the Coriolisparam- a eterdecreases. parcel can be determined at the inflection point If the value of 2 w sin 0 and decreases, the value of Zr must increase in order thelatitudeof the inflectionpointis that their sum remain the same. If Zr increases, known, the future path of the parcelcan be it becomes more cyclonic or less anticyclonic. In determined. When the inflection angle (the angle atthe other words, the parcel of air wouldmove in a inflection point between the latitude path which becomes less and less anticyclonic, circle and the direction of motion of the parcel) and eventually becomes cyclonic. Then the is small, the waves tend to be flat and elongated. cyclonic curvature increases up to a critical Both the amplitude and the wavelength increase latitude where the parcel will move from west to with an increase in speed. For values of the east and begin to turn back toward the north. inflection angle greater than 90°, but less than (See fig. 4-31.) 130°, from east, the path becomesa figure eight. In figure 4-33 the parcel of air starts out at 39° N lat. with a speed of 25 knots,with an inflection angle of +90°. (Heading: due north.) The Coriolis parameter increases, and relative vorticity (Zr) decreases in order that 2 w sin 0 + Zr = Za. The parcel begins to turn slowly anticyclonically until it reaches its critical latitude. The ciritical latitude of a parcel of air with a certain speed and inflection angle, starting from a certain latitude, is that latitude at which AG.442 it has reached maximum amplitude in its sinus- Figure 4-31.Coriolis parameter decreasing. oidal wave pattern, and must now, due to the forces acting upon it, reverse its direction of travel. All parcels with different initial inflection angles,speed,andlatitudehave their own With these forces and no others acting on the criticallatitude, that latitude being dependent parcel,itisevidentthat the parcel would upon all three factors: The inflection angle, describe a sinusoidal path as it moves through speed, and latitude of the parcel. The parcel of space. This is illustrated in figure 4-32. air in figure 4-35 reached its critical latitude at Chapter 4ATMOSPHERIC CIRCULATION

SAMPLE C A V TRAJECTORY. (STROKES AND NUMERALS ALONG THE PATHS MARK POSITIONS AND DATES FOR 24- HOUR INTERVALS FROM THE STARTING POINT ON THE 2300 EST MAP OF THE INITIAL DATE.) AG.444 Figure 4-33.Sample CAV trajectory.

50° N. We see the parcel heading eastward, and clonic curvature. This point is referred to as an in so doing, the Coriolis parameter decreases, the inflection point and will later be an important relative vorticity increases, and the parcel then factor in forecasting upper air long-wave move- heads southward in an ever lessening anticy- ments. clonic path until the path is straight. Inasmuch The parcel continuesits southward move- asthe Coriolis parameterisstill decreasing, ment, increasing its relative vorticity because the relative vorticity is still increasing, and the parcel Coriolis parameter is decreasing. At 28° N lat., begins to travel in a cyclonic path. When the the parcel has again reached its critical latitude. parcel was in straightline flow at 39° N lat, it The Coriolis parameter now begins to increase was midway between a cyclonic and anticy- again and the parcel flows northward in a path

117 AEROGRAPHER'S MATE 1 & C

that is becoming less and less cyclonic, until, Latitude once again, at 39° N lat.,it has reached its inflection point, having been displaced in this If the vorticity theorem is used, the parcel's process over 2,000 miles to the east. Without relative vorticity must decrease if the latitude other considerations entering the picture, this remains unchanged and horizontal divergence process would then repeat itself interminably. takes place. This is because D decreases with horizontaldivergence;therefore,Zrmust In the above, discussion, it was assumed that decrease in order that 2 co sin (/) plus Zr/D will no horizontalconvergence occurs. However, equal the same valueas Zr did before the parcels depart from the CAV trajectoriesin decrease in D. When Zr decreases, it becomes regions of horizontal convergence and diver- lesscyclonic or more anticyclonic, and the gence. If a column of air, as shown in figure 4-34 parcel moves in a more anticyclonic path than it of height D is considered and it is alsumed that would if no divergence occurs. no change in the volumn occurs, it is found that If the latitude remains unchanged and hori- the height increase with horizontal convergence zontalconvergence takes place,the. parcel's and decreases with horizontal divergence. relative vorticity must increase. This is because D increases with horizontalconvergence; therefore, Zr must increase in order that thecon- ditions of the vorticity theoremmay be satisfied.When Zrincreases,itbecomes more D D a cyclonic or less anticyclonic, and the parcel --Ir.-4-- moves in a more cyclonic path than it would if no convergence occurs.

AG.445 Southward Movement Figure 4-34.Height changes with horizontal convergence and divergence. If aparcel moves southward, thereisa decrease in 2 co sin (/), which requires that Zr increase if D remains the same, in order that 2co VORTICITY THEOREM sin(/) plus Zr/D remain constant. However, if horizontaldivergenceistakingplace, D is It can be shown mathematically that relative becoming less, so that with horizontal diver- vorticity is related to horizontal divergence and gence and southward motion, the denominator convergence in the following manner: and 'ne term in the numerator of the vorticity theoremaredecreasing.Consequently,the decrease of these two values must be such that 2 2 co sin (/) + Zr Constant G.) sin (1) plus Zr/D remains constant, or there D must be a compensating decrease in Zr. The latter is the normal case since, as explained in This is known as the VORTICITY THEOREM the previous two paragraphs, it has been shown and shows that the sum of the Coriolis param- that Zr and D increase or decease together ata eter and the relative vorticity of the parcel given latitude. This fact also eliminates thecase divided by the height of the column must equal where there might be such a large decrease in 2 a constant value. This means that if one of the co sin (A and such a small decrease in D that Zr values changes, one or both of the others must must increase. A decrease in the relativevor- change so that the quotient will remain the ticity of the parcel results in its movingmore same. anticyclonically. In this case, the parcel would follow the dashed line as shown in figure 4-35 How does the vorticity theorem explain the rather than the solid line (assuming that the motion of a parcel of air as the parcel undergoes divergence is taking place in the vicinity of the horizontal divergence or convergence and as it point of inflection). The solid line represents the moves from latitude to latitude? path if no convergence or divergenceoccurs.

112 1.1.8 Chapter 4ATMOSPHERIC CIRCULATION

decrease if D remains the same. However, if horizontal convergenceistaking place,I.)is becoming larger. With horizontal convergence and northward motion, the denominator and one terminthe numerator of the vorticity i theorem are increasing. Consequently, the in- AG.446 crease of these two values must be such that Figut.4-35.Parcel moving southward. 2 (,) sin ¢ plus Zr D If horizontal convergence is taking place while a parcel moves southward, there is a decrease in 2(...)sin(I) and an increase in D. Consequently, remains constant, or there must be a compen- there must be a compensating increase in Zr. sating increase in Zr. The latter is the normal Therefore, the parcel acquires cyclonic vorticity case and resultsinthe parcel moving more morerapidlythanifno convergence were cyclonicallythanitwould if there were no occurring and curves eastward more rapidly. convergence.Inthiscase,the parcel would This is illustrated in figure 4-36 by the dashed follow the dashed line as shown in figure 4-37 line. rather than the solid line. .'-

,, AG.447 Figure 436.Horizontal convergence while a AG.448 parcel moves southward. Figure 4-37.Parcel moving northward.

From the above deductions, it is obvious that If horizontal divergence is taking place while a when parcels moving southward curve more anti- parcel moves northward, there is an increase in 2 cyclonically than their CAV trajectories, hori- (...) sin (I) and a decrease in D. Consequently, there zontal divergence is occurring; when horizontal must be a compensating decrease in Zr. There- divergence is expected in a current moving from fore, the parcel acquires anticyclonic vorticity north to south, the path of the parcel will be more rapidlythanifno convergence were more anticyclonic than expected from the CAV occurring and curves eastward more rapidly. trajectory. Also, when parcels move southward This is illustrated in figure 4-38 by the dashed and curve more cyclonically than their CAV line. trajectories, horizontal convergence is occurring; From this discussion, it is evident that when when horizontal convergence is expected in a parcels moving northward curve more cycloni- current moving from north to south, the path of callythantheir CAV trajectories, horizontal the parcel will be more cyclonic than expected convergence istaking place. When horizontal from the CAV trajectory. convergenceis expected ina current moving south to north, the path of the parcel will be Northward Movement more cyclonic than expected from the CAV trajectory. Also, when parcels moving northward If a parcel moves northward, thereis an curve more anticyclonicallythantheir CAV increase in 2 (A)sin 0 which requires that Zr trajectories, horizontal divergence is occurring. AEROGRAPHER'S MATE 1 & C

east than CAV positions. For ridges, the actual contours will be flatter than CAVT's and farther east as well. The net result of convergence is that actual troughs will have less amplitude and will be located P.rther west than CAV troughs as shown in figure 439.

AG.449 HORIZONTAL DIVERGENCE HORIZONTAL CONVERGENCE Figure 4.38. Horizontal divergence while a parcel moves northward. \ / ,,.. I../ When horizontal divergenceis expected in a northward moving current.the path of the parcel will be more anticyclonic than expected from the CAV trajectory. -- - - ACTUAL PATH CAV PATH USE OF CAVT'S AG.450 TheappendixofPracticalMethods of Figure 4-39.Modification of CAVT's because WeatherAnalysisandPrognoses.NavAer of convergence and divergence. 50-i P-502. contains CAVT tables for intervals of 5° of latitude. The tables are entered according to the initial latitude. and the wind speeds and Evaluation of Vorticity winddirectionsoftheairparcelsattheir inflectionpoints. The tabular values arein In addition to locating the areas of conver- one-fourth wavelengths. The results of these gence and divergence in order to adjust the tables are a series of waves in which the upper CAVT's we must also consider the effects of and lower portions of the waves are symmetri- horizontal wind shear as it affects the relative cal, though not necessarily of equal size. The vorticity, aid hence the movement of the long primary probleminmoving long waves by waves and deepening or tilling associated with CAVT's is the determination of a valid inflection this movement. point. The definition of the inflection point The two terms "curvature" and "shear," requires straightlineflow (thatis,midway which determine the relative vorticity, may vary between the cyclonic and the anticyclonic flow) inversely to each other. Therefore, it is necessary and no shear. to evaluate both of them. Figures 4-40 through In the case of a jet max being embedded in 4-43 illustrate some of the possible combina- the westerly flow, the CAVT's cannot be used at tions of curvature and shear. Solid lines are facevaluebecause of theconvergence and streamlinesorcontours;dashedlinesare divergence in the currents. It must be decided isotachs. whether there will be convergence or divergence Figure 4-40 represents a symmetrical sinus- and whether the CAVT must be readjusted oidal streamline pattern with isotachs parallel to accordingly. If the path of the parcel is toward contours. Therefore.thereis no gradient of diverging cyclonic contours, the actual path will shear along the contours alone. In region I, the be to the right of the CAV path. If the path of curvature becomes more anticyclonic down- the parcel is toward converging contours, the stream; therefore,relativevorticity decreases actual path is to the left of the CAV path. The downstream and the whole region is favorable netresultof divergenceisthattheactual for deepening. The reverse is true west of the amplitude of the troughs will be greater than trough, region II, and the region is unfavorable CAV amplitudes and trough lines will be farther for deepening.

1;4

120. Chapter 4ATMOS1'IIERIC CIRCULATION

III ---t---7:-Z--. I -----

\ %..\...... 57.,

_ 1 __\ Iv 1 --'r

AG.454 Figure 4-43.Contourisotach pattern for AG.451 shear analysis. Figure 4-40.Contourisotach pattern for shear analysis. Infigure4-41 there isno curvature of streamlines; therefore, the shear alone determines the relative vortieity. The shear downstream in regions I and IV becomes less cyclonic; in regions II and 111, it becomes more cyclonic. RegionsI and IV are therefore favorable for deepening downstream. In region I of figure 4-42 both cyclonic shear .. and curvature decrease downstream and this . .---* __IV's -1. II regionishighly favorablefor deepening. In .., , regionIII both cyclonic shear and curvature ....--"°. increase downstream and this region is unfavorable for deepening. In region II the cyclonic curvature decreases downstream, but the cy- AG.452 clonic shear increases. This situation is indeter- Figure 4-41.Contourisotach pattern for minate without calculationunless one term shear analysis. predominates. If the curvature gardient is large and the shear gradient small, the region is likely to be favorable for deepening. In region IV, the cyclonic curvature increases downstream, but the cyclonic shear decreases, so that this region is also indeterminate unless one of the two terms predominates.

In regionIof figure 4-43 the cyclonic shear decreases downstream and the cyclonic curva- tureincreases. The regionisindeterminate; however, if the shear gradient is larger than the curvature gradient, deepening is favored. Region IIhas increasing cyclonic shear and curvature downstream and is quite unfavorable. In region AG.453 III,the shear becomes more cyclonic down- Figure 4-42.Contourisotach pattern for stream and the curvature becomes less cyclonic. shear analysis This regionisalso indeterminate unless the

115 121 AIEROGRAPIIER'SMATE I& c curvature term predominates. In region IV, the One rule using vorticity in relation to cyclone shear and curvature become less cyclonic down- development stems from the observation that stream and the region is favorable for deepening. when cyclone development occurs, the location The whole point of these illustrations is to almost without varianceisin advance of an point outthat decreasing cyclonic shear and upper trough. Thus, when an upper level trough curvature are favorable for deepening of the with positive vorticity advection in front of it frontalwave andthereverseforincreasing overtakes a frontal system in the lower tropo- cyclonic shear and curvature and that when the sphere, there is a reliable indicator of cyclone situation is indeterminate, the perdominant term development on the surface chart. This is usually will determine whether or not there will be accompanied by deepening and intensification deepening. Deepening of a trough will increase of the surface system. Also, the development of the amplitude of the wave trough and shorten cyclones at sea level takes place when and where thewavelength,andthe CAVT's mustbe an area of appreciable positive vorticity advec- adjusted accordingly. In fact, the shear analysis tion situated in the upper troposphere overlies a evaluation should be appliedto the extrapo- slow moving or quasi-stationary front on the lation of long wave movement and the move- surface chart. ment by Petterssen's wave speed nomogram as well, for the reasons just stated. (This is dis- It has been shown before that surface pressure cussed in chapter 8 of this training manual.) changes that take place aloft. The relationship Relation of Vorticity between convergence and divergence can best be illustrated by the shear term. !f we consider to Weather Processes a flow where thecyclonic shear is decreasing Vorticitynotonlyaffectsthe genesis of downstream (stronger wind to the right than to cyclones and anticyclones, butitalso has a theleftof thecurrent), more airis being direct bearing on cloudiness, precipitation. and removed from the area than is being fed into it, pressure and height changes as well. Vorticity is hence a net depletion of rm.ss aloft, or di'ier- usedprimarilyinforecasting cloudiness and gence. Divergence aloft is associated with surface precipitation over an extensive area. One rule pressure falls, and since this is the situation, the statesthat whenrelativevorticity decreases relative vorticity is decreasing downstream. We downstream in the upper troposphere. conver- may statethatsurface pressurefalls where gence is taking place in the lower levels. When relative vorticity decreases downstream in the convergence takes place, cloudiness and possibly upper troposphere, or where advection of more precipitation will prevail if sufficient moisture is cyclonic vorticity takes place aloft. The converse present. of this is in the case of convergence aloft. CHAPTER 5

AIR MASSES, FRONTS, AND CYCLONES

In the preceding chapters you haveseen that turecontrasts and produces a homogeneous regions of the earth have special properties of mass of air. The energy supplied to the earth's temperature and humidity. Under favorable con- surface from the sun is distributed to the air ditions these regions impart these properties to mass by convection, radiation, and conduction. the overlying air, thereby forming air masses. Another necessary condition for air mass These air masses have different properties deformation is equilibrium between ground and pending upon whether they form over land or air. This is established by a combination of the water surfaces or over polar or tropical regions. followingprocesses:( I)turbulent-convective When two air masses with different properties transport of heat upward into the higher levels are brought together a line of discontinuity or of the air; (2) cooling of air by radiation loss of front is formed between them. When a warm air heat; and (3) transport of heat by evaporation mass lies adjacent to a cold air mass and either and condensation processes. of these is caused to be accelerated along a part of the front, there is a tendency for a wave By far the most effective process involved in motion to be set up along the front. The result establishing equilibriumis turbulent-convective of this interaction is called a wave cyclone. The transport of heat upwards. It is also the fastest wave cyclone may be either stable, that is,it process whereby equilibria r .1 is established. The travels along the front ina horizontal plane leasteffectiveand slowest process whereby without appreciably intensifying; or it may be equilibrium is established is radiation. unstable, thereby going through a life cycle Evaporation and condensation serve to con- occluding and dying. tributegreatly,duringbothradiationand The material contained in this chapter is an turbulent-convective processes, in conserving the extension to that of chapter 6, Air Masses and heat of the overlying air by the water vapor Fronts,Aerographer's Mate 3 & 2,NavTra permitting radiation only through transparent 10363-D. A review of the basic material would bands in the case of radiation cooling and in be helpful in understanding some of the more releasing the latent heat of condensation in the advanced material in this chapter. case of turbulent-convective processes. For this reason, Aerographer's Mates can readily see that AIR MASSES the tropical latitudes form air masses rapidly primarily through the upward transport of heat CONDITIONS NECESSARY by the turbulent-convective process and the FOR FORMATION polar regions form air masses slowly primarily by the loss of heat through radiation. Two primary factors for the production of air The Aerographer's Mate will further recognize masses are as follows: First, a surface whose that the underlying surface is not uniform in its properties, essentially temperature and moisture, thermal properties throughout the year and the are relatively uniform (it may be a water, land, distribution of land and water is unequal, and or snow coveredarea); and second, a large that for these reasons special summer or winter divergent flow which tends to destroy tempera- air masses may be formed. The rate of air mass

117

123 AEROGRAPHER'S MATE I & C formation will further vary with the intensity of ducive to air mass formation. These conditions insolation. were described in the preceding paragraphs. Generally speaking, there are two primary air Effects of Air Mass mass source regions. One is associated with the Circulation stagnant cold polar regions where the cold air Itis recognized that there are three circula- masses develop; and the other exists along the tion types over the earth. Some of these are lower latitudes near 30° where the warm air favorable for air mass development, while others masses develop. Between these two areas there do not fulfill the requirements. They are: exists a zone of transition over which mixing of the cold air masses from the north and warm air 1. The anticyclonic systems of the earth.. masses from the south takes place. They have stagnant or slowly moving ail which A more detailed description of air mass source allowstimeforairto adjustitsheat and regions may be found in AG 3 & 2, chapter 6. moisture contenttothatof the underlying surface. These anticyclones possess a divergent CLASSIFICATION OF AIR MASSES airflow which spreads the properties over a large area horizontally, with turbulence and convec- The source region has been found to be a tion distributing these properties vertically. Sub- useful criterion as a first consideration when sidence, another property of anticyclones,is classifying air masses. The letter designations favorablefor lateral mixing which resultsin used to indicate the various source regions are as horizontal or layer homogeneity. follows: Warm highs extend to great heights due to a lesser density gradient aloft and thereby produce A Arctic/Antarctic air masses. an air mass of relatively great vertical extent. P Polar air masses. Two examples of warm highs are the Bermuda T Tropical air masses. high and the Pacific high. Cold highs are of E Equatorial air masses. moderate or shallow vertical extent and thereby S Superior air masses. produce air masses of moderate or shallow height. The second criterion is made by prefixing an 2. Cyclonic systems are not conducive to air "m" or "c" to one of the above to indicate mass formation because they are characterized whether the air mass has maritime or continental by greater wind speeds than anticyclonic sys- characteristics. tems. These wind speeds prevent cyclonic sys- It should be noticed that, due to the pre- tems from stabilizing the necessary amount of dominance of land masses or ice fields in the ;:ale an air mass should remain over a surface to Arctic, maritime Arctic (mA) would be a rare adjust itself. An exception is the heat low type. In a like manner, equatorial air (E) is 3. Belts of convergence are not conducive to found exclusively over the ocean surface in the air mass formation since they have essentially vicinity of the Equator and is designated neither the same properties as cyclonic systems. "c" nor "m" but simply E. A third letter is added to indicate whether the There are two areas of convergence which are air is colder (k) or warmer (w) than the surface exceptions to this rule. These are the area over over which itis moving. This is the thermo- the north Pacific oetween Siberia and North dynamic classification. The "k" type air mass America and the area over the Atlantic off the will be warmed from below, and convection coast of Labrador and Newfoundland. These currents will form. Some characteristics of this two areas act as source regions for maritime type of air mass are turbulence up to about polar air. 10,000 feet; unstable lapse rate (nearly dry adiabatic); good visibility except in showers and SOURCE REGIONS dust storms; cumulonimbus clouds such as cumu- Source regions are areas of the earth's surface lus and cumulonimbus; and showers, thunder- whichpossess thenecessary conditions con- storms, hail, ice pellets, and snow flurries.

118 Chapter 5AIR MASSES, FRONTS, AND CYCLONES

The "w" type of air mass is cooled from The lower layer of continental polar air in its below. This type tends to maintain its original source region is characterized by a strong surface properties and is modified only in the lower few inversion, an isothermal middle layer, and a thousand feet as it moves. Some characteristics steep lapse rate aloft. of this type are smooth air (above the friction MOISTURE DISTRIBUTION.Moisture level); stable lapse rate; poor visibility (smoke adapts to temperature so as not to exceed and soot are heldin the lower few thousand saturation.Moistureinthelower layersis feet); and stratiform clouds and fog and occa- withdrawn by frost and ice crystal fogs. sionally drizzle. According to Wexler,allpolar air masses above 10,000 to 15,000 feet are alike. The Properties of Air Masses in essential difference between continental polar Their Source Regions and maritime polar is the modification up to that level, and modification is dependent upon The properties an air mass will acquire in its the snow or water source regions. Above this source region are dependent upon a number of level, a steep lapse rate is exhibited in both air factorsthe time of year (winter or summer), masses. This air aloft is sometimes referred to as the nature of the underlying surface (whether polar superior air. land, water, or ice covered), and the length of The mixing ratio is low, usually less than 1 timeitremains overits source region. The gram per kilogram. The humidity is high at the properties described in the following section are, surface (85-95 percent), decreasing to the 50's in in general, applicable to all areas of the world, the isothermal layer and lower aloft. The effect except when the differences are pointed out. of cooling, however, seldom exceeds 10,000 feet You must remember that these indicate only inthe cA air masses thatreach the United average or normal properties and conditions, and States. thosecharacteristicsand properties may be In general, the skies will be clear or have different on any particular day. flattened stratocumulus clouds, if any. Visibility willbe good, exceptif icecrystal fogs are Arctic and Polar present(intemperatures -30°C and below). Continental Air Masses Refer to figure 5-1 for typical lapse rates of cP air in its source region. Since these two air masses have similar charac- Continental polar air has the greatest annual teristics,they are discussed together. In the variation of any air mass, and its properties show UnitedStates continental polarair(cP)is a large variation from season to season. The usually a modification of cA, although it may be warming may result in a weaker inversion in more unstable in the eastern part of the United summer that may completely disappear during States. In addition, cP air is the air mass that the day only to be reestablished at night due to usually interacts with mT air along the polar cooling. European continental polar air is more front, and resultant rains and snows make up a moist than North American or Siberian conti- large proportion of the winter precipitation nental air. Over Asia, cP air is colder than other from the Rocky Mountains eastward. cP air masses. In the winter, its North American location is Occasionally we have intrusions of maritime from the Atlantic Ocean to the Bering Sea from polar airintothecontinentalpolar source 50° north to he Arctic Circle. Over Siberia, it is regions. A low in the Beaufort Sea region will from the Pacific Ocean to the edge of Europe induce maritime polar air into the Mackenzie and the Arctic Ocean south to the Plateau of River Valley by bringing it from the Bering Sea Tibet. Over Europe, a more distinct separation across northern Alaska. Temperatures will rise between Arctic and continental polar air is due from 20°to 40°F, butthisairisquickly to the cold easterly currents toward the Ice- modified to continental polar air. Another case landic low from the Arctic regions and the is that of the Nova Scotia wave. A strong low in advection of warmer air from the Atlantic Ocean the vicinity of Nova Scotia will induce ml' from easterly toward the Barents Sea. the North Atlantic into the continent in the

119 125 AEROGRAPHER'S MATE 1 & C

12 12 11111111IL VERY SMOOTH FLYING WEATHER

lEgRATURE 10ELIE

u- 8ILAILAk z O I-z 6

IS: 4 NOTE POSSIBILITYOF ICE CRYSTAL FOG OR POOR VISIBILITY DURINGNIGHT

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-400 -30'C -20C -10C -40F -22F -4F 14F 32F TEMPERATURE AG.455 Figure 5-1.Continental polar air in its source region (winter). Labrador-Hudson Bay region. Temperatures in are subject to wide daily variations due to their this region will rise 30° to 40°F and the air will characteristic dryness aloft. During afternoon be modified quickly to cP, though not as rapidly hours, adiabatic or superadiabatic lapse rates as air in the Mackenzie Valley. may develop near the ground andproduce In summer, due to extremely long days and turbulence as high as 6,000 to 10,000 feet. the absence of a snow cover over a large portion Occasionally, after the air has stagnated for a of the source region, cP air masses are very few days over the southeastern United States, different from those observed during the winter sufficient moisture may accumulate to result in months. The warmth of the underlying surface the formation of local thundershowers, usually produces unstable layers near the ground in confined to the mountainous districts. Over contrasttothe extremely stable conditions Europe, the source region is essentially the same during winter. as for the winter months, but itis predomi- When an air mass originating over the Arctic nantly a dry air mass and generally produces fair Ocean in summer moves southward, it is impos- weather, except for haze and smoke present in sible to distinguish it after it reaches the United the lower layers. States from one originating over continental North America. During this season, even mP air Maritime Polar Air from the North Pacific Ocean becomes practically identical with the cP masses after crossing Maritimepolarair masses are formed in the western mountains of the United States. transition zones, and the properties the air mass Continental polar air masses in summer are will have are dependent upon its original source characterized by relatively low temperatures and and its trajectory through the transition zone. 120 1146 Chapter 5- -AIR MASSES, FRONTS, AND CYCLONES

The air may arrive at the transition zone either tures and marked subsidence render this portion by a path from over land to the water, which of the high stable. The western portion is the may be either cyclonic or anticyclonic, or from most unstable due to warmer surface conditions an overwater path from the subtropical regions. and a lesser effect of subsidence. Most often, mT If the air arrives at the source region in winter air is modified mP air, but all air masses are from a continental source over the transition continually acquiring new properties in primary zone and the trajectoryis cyclonic, the air is andsecondarysourceregions.Subsidence, warmed from below, heat is added by radiation especially over the eastern limbs of the high, -and eddy exchange from the surface, and mois- prevents the distribution of moisture to high ture is added which is carried aloft by convec- levels. (See figs. 2-6 and 2-7 in chapter 2 of this tion. Subsidence is not operativeas the air is manual.) Therefore, the air is moist below the usually dominated by the low which is associ- inversion and dry above. (See Trade Inversions ated with the outbreak. Modification usually in chapter 12.) Cloudiness is more prevalent in extends up to about 12,000 feet. When the air the western portion, and the amount of-precipi- arrives over the eastern portion of the mPsource tationwillbesmallunless associated with region, its stability increases because the rate of frontal convergence or some other type. of heating from below is reduced. The air is usually convergence. Typical properties of mT air over dry and cold aloft with a continental character. the Atlantic in winter in its source region are a On the other hand, if the air moves out from moderately steep lapse rate with the air convec- its continental source and passes through the tively unstable to about 9,000 to 11,000 feet transition zones with an anticyclonic trajectory, with a mixing ratio of 13 to15 grams per theairis more stable, particularly atupper kilogram. Relative humidity is high in the lower levels, due to the subsidence effect. levels and decreases aloft. Surface temperatures Also, in winter, air may arrive at the mP average around 75° to 80°F and the 800-mb source region from the subtropics due to an temperature about 12° to 13°C. Therefore, the elongated trough extending southward; the air air is characterized in its source region by high has a long overwater trajectory and is cooled by temperature, high moisture content, conditional its passage over colder water on its path north- instability, and a few stratocumulus clouds. ward. In this case, the air is cooled in its lower Summer conditions are very similar to those layers and therefore more stable. This is the of winter except that the increased surface warmest air that arrives in this region. temperature in the western portion causes in- In general, maritime polar air inits source creased instability and air mass showers and region in the winter is convectively unstable in thunderstorms. In the eastern portion, the axis its lower layers, dry and cold aloft and has little and latitude of the seasonal shift of the sub- or no diurnal temperature variation. Mild con- tropical high-pressure cells cause a flow which vective type clouds and precipitation predomi- results in upwelling along the western coasts of nate. continents, with increasedstability and the Just as there is an appreciable difference in prevalance of coastal stratus and fogs. the structure of cP air masses in summeras compared with winter, so we find pronounced differences in the structure of polar maritime Continental Tropical air. In summer, the maritime air masses are more stable than winter. In winter this air mass is characterized by In summer, air arriving at its mP source region moderately warm temperatures, but in summer from continental regions is cooled from below, they are extremely warm. This is the i.s.,armest air resulting in extreme stability in the lower layers. mass found in North America. Due to the usual Fogs and stratus are common. extension of a high-pressure cell over these regions in the winter, the flow is anticyclonic Maritime Tropical and the air consequently stable. In summer the The subtropical highs are the source regions dry land is strongly heated. There is usually a for mT air. On the eastern side, colder tempera- thermal low present at the surface with a high

121 127 AEROGRAPHER'S MATE 1 & C level anticyclone aloft. This air mass is character- path. The surface over which this path takes the ized by strong daytime convection in the sum- air mass after leaving its source modifies the air mer with a strong and steep lapse rate, often dry mass. The type trajectory, whether cyclonic or adiabatic to high levels. anticyclonic, also has a bearing on its modifica- During winter and summer the air has a high tion. The time the air mass has been out of its condensation level, due to the lack of moisture, source region will determine to a great extent and clouds are rare. A few small cumulus may thecharacteristics of theair mass when a form late in the afternoon. The air, especially in thermodynamic classification is attempted. summer, has a large diurnal temperature variation, often as high as 30° to 40°F. Surface Conditions

Equatorial Air The first modifying factor on an air mass as it leaves its source region is the type and condition There is little or no seasonal variation in the of the surface over which it travels. Here, the properties of this air mass. It is characterized by factors of surface temperature, moisture, and a great uniformity in the distribution of temper- topography must be considered. ature and humidity. The temperatures are high TEMPERATUREThe temperature of the and instability is the rule. Humidity is also high surface relative to that of the air mass will and uniformly distributed, resulting in a low modify not only the temperature, but its stabil- condensation level. When occurring over land or ity as well. For example, if the air mass is warm when forced aloft by some kind of lifting action, and moves over a colder surface, such as tropical the weather is characterized by convective insta- air moving over colder water, the cold surface bility, producing showers or thunderstorms. cools the lower layers of the air mass and its stability is increased. This stability will extend Superior Air to the upper layers in time, and condensation in the form of fog or low stratus normally occurs. Superior air is a high level air mass and has (See fig. 5-2.) little direct relation to the air mass which it is If the air mass moves over a surface that is overlying. It develops above an inversion layer. warmer, such as polar continental air moving out Superior air is created by an anticyclonic flow from the continent in winter over warmer water, and subsidence, and is very stable and dry. It is the warm water heats the lower layers of the air warmer than any other air mass for its altitude, mass,increasinginstability and consequently with little seasonal variation in its properties. spreading to higher layers. Figure 5-3 illustrates the movement of cP air over a warmer water AIR MASS MODIFICATION surface in winter. The changes in stability of the air mass give When an air mass moves out of its source valuable indications of the cloud types that will region, there are a number of factors which act form, as well as the type of precipitation. Also, upon the air mass to change its properties. These the increase or decrease in stability gives further modifying influences do not occur separately. indications of the lower layer turbulence and For instance, in the passage of cold air over visibility. warmer water surfaces, thereisnot only a MOISTURE.The air mass may be modified releaseof heattotheair,but also some initsmoisture content by the addition of moisture. The Aerographer's Mate must be moisture by evaporation or by the rcmoval of aware of the changes that take place once an air moisture by condensation and precipitation. If mass hasleftits source region in order to the air mass is moving over continental regions, integrate these changes into his forecast. the existence of unfrozen bodies of water can greatly modify the air mass and, in the case of MODIFYING FACTORS an air mass moving from a continent to an As an air mass expands and slowly moves out ocean, the modification can be considerable. In of its source region,ittravels along a certain general, dependent upon the temperature of the

122 128.' Chapter 5 AIR MASSES, FRONTS, AND CYCLONES

/AIR TEMPERATURES

LIGHT WINDS

14. 1111117/111/11 Ilf/11/11/////f/7 7filf/f/////111 WARM BECOMING .AIR TEMPERATURES -, OVER LAND OR WATER

MOD. WINDS

1ff iAlTiife

WARM BECOMING COLD OVER LAND OR WATER

AG.456 Figure 5-2.Passage of warm air over colder surfaces.

COOL CONTINENT WARM OCEAN

AG.457 Figure 5-3.Continental polar air moving from cool continent towarm ocean (winter).

two surfaces, the movement over a water surface For example, the passage of cold air over a will increase the moisture content of the lower warm water surface will decrease the stability of layers,andtherelative temperature of the theair with resultant vertical currents. The surface. passage of warm moist air over a cold surface

123 129 AEROGRAPHER'S MATE 1 & C increases the stability and could result infog as affected by this process. On the other hand, if is cooled and mositure is added by the trajectory is anticyclonic, its stabilityin the the air of subsidence evaporation. upper levels is increased as a result TOPOGRAPHY.The effect of topography is associated with anticyclonic relative vorticity. evident primarily in the regions of mountains. Age The air mass is modified on the windward side by the removal of moisture through precipita- Although the age of an air mass in itself tion with a decrease in stability; and as theair cannot modify the air mass, it will determine, to descends on the other side of the mountain, the a great extent, the amountof modification that stability increases as the air becomes warmer and takes place. For example, an air mass that has drier. recently moved from its source region will not have had time to become modified significantly. Trajectory However, an air mass which has moved into a After an air mass has left its source region, the new region and stagnated for sometime, and is trajectory it follows, whether cyclonic or anti- now old, will be found to havelost many of its cyclonic, has a great effect on its stability. If the original characteristics. in air follows a cyclonic trajectory, its stability Summary the upper levels is decreased; this instability is a reflection of the cyclonic relative vorticity. The In figure 5-4 the two modifying influences are stabilityof the lower layersisnotgreatly classified thermal and mechanical,

. THE PROCESS HOW IT HAPPENS RESULTS

A. THERMAL

1. Heating from below. Air mass passes from over a cold surface to Decrease in stability. a warm surface, or surface under air mass is heated by sun.

2. Cooling from below. Air mass passes from over a warm surface to Increase in stability. a cold surface, OR radiational cooling of surface under sir mass takes place.

3. Addition of moisture. By evaporation from water, ice, or snow Decrease in stability. surfaces, or moist ground, or from raindrops or other precipitation which falls from overrunning saturated air currents.

4. Removal of moisture. By condensation and precipitation from the Increase in stability. air mass.

B. MECHANICAL

1. Turbulent mixing. Up- and down-draft. Tends to result in a thorough mixing of the air through the layer where the turbulence exists.

2. Sinking. Mcvement down from above colder air masses Increases stability. or descent from high elevations to lowlands, subsidence and lateral spreading.

3. Lifting, Movement up over colder air masses or over Decreases stability. elevations of land or to compensate for air at the same level converging.

AG.458 Figure 5-4.Air mass changes.

124 130 Chapter 5AIR MASSES, FRONTS, AND CYCLONES

The tigule indicates the modifying process, Path No. I (cyclonic) is usually indicative of a what takes place, and the resultant change in strong outbreak of cold air accompanied in the stability of the air mass. It must be reiterated lower few thousand feet by high winds and that these processes do not occur independently, turbulencewithgustyflyingconditions. A buttwoof more processesareusuallyin strong inversionpersists between 5,000 and evidence at the same time. 10,000feet,and fractocumulus and strato- It should be pointed out that within any cumulus clouds often form under this inversion. single air mass the weather is controlled by the As the air mass, following this path, moves over moisture content,stability,andthe up-and- the Great Lakes,itis heated from below and down slope movements of air. It must also be moisture is added, especially during early winter stressed that the conditions indicated are only before the lakes have frozen over. This results in average conditions and that each individual case either rain or snow showers on the lee side of may be quite different. the Great Lakes and on the windward side of the Appalachians. (See fig. 5-6.) CHARACTERISTICS OF East of the mountains, relatively clear skies MODIFIED AIR MASSES prevail. The cloud bases are at 500 to 1,000 feet on the lee side of the Great Lakes and at or near North American Air zero over the mountains. Cloud tops are at Masses (Winter Season) about 7,000 feet in the Great Lakes region and around 14,000 feet in the mount.'region. (Sce CONTINENTAL ARCTIC (cA) AND CONTI- fig. 5-6.) NENTAL POLAR (cP).Figur! 5-5 illustrates In the Middle West, clouds associated with some of the paths taken by cA and cP air this type of air mass continue for 24 to 48 hours entering the United States. after the arrival of the cold mass, while along the Atlantic Coast rapid .passage of the leading edge of a cA air mass produces almost immediate clearing. When polar air follows path No. 2 (anticyclonic) over the central United States, generally smooth flying conditions exist, except in the lower 4,000 feet where contact of the air with the warmer surface may cause turbulence. Surface visibility is good, except during the early morning hours when the air mass stagnates over a region and subsidence occurs aloft. Path No. 3 does not occur very often. This is essentially a wintertimephenomenonand usually occurs when an intense high in the cP source region (1,035 to 1,040 mb) results in a strong pressure gradient forcing air over the mountains. When the trajectory is similar to path No. 3, the air arrives over the central and southern coast of California as a cold, convectively unstable current with mbsequent squalls, rain showers, and even snow occurring as far south as southern California. MARITIME ARCTIC (mA) AND MARITIME POLAR (mP) (PACIFIC).When an outbreak of AG.459 polar air moves over only a very small part of Figure 5.5. Paths of cP and cA air the Pacific Ocean before reaching the United (North American winter). States, it usually resembles mAk. It' its path has

125

131 AEROGRAPIIER'S MATE I & C

CLEAR c P 10 °F TEMP 2000' 10°F AT RTEMP_4°F

TEMP MOISTURE AND DIFFERENCE 410F WARM AIM LAIR RISING $ 1 1 APPALACHIAN MOUNTAINS SURFACE TEMP 0°F SURF 911ffEs NW SE

AG.460 Figure 5-6.cP air moving over the Great Lakes (winter). been far to the south, it is typically mP. Figure pulled out over the Pacific Ocean by a low close 5-7 shows some of the trajectories by which mP to British Columbia in the Gulf of Alaska. This air reaches the North American coast during the air has a relatively short overwater path and winter. brings very cold weather to the Pacific North- Path No. Iis a cyclonic trajectory. The air west. The airis convectively unstable in the originates in Alaska or northern Canada and is lower layers; and when the air is lifted by the coastal ranges, showers and squalls are common with 2,000-foot ceilings along the coast and near zero along the mountain ranges. The visibility is

130 good exceptinprecipitation.Rough flying weather prevails in the turbulent air. Icing is most noticeable in the mountains and may be 70 severe. Maritime polar (mP) air with a longer over- so water trajectory path No. 2 dominates the west coast of the United States during winter months. When there is rapid west to east motion and so small north to south motion of pressure systems, inP air may influence the weather over most of

40 the United States. Due to a longer overwater trajectory,this mP airisheated to greater heights, and convective instability is present up to about 10,000 feet. This air has typical "k" 30 characteristics-turbulent gusty winds, steep lapse rate, good visibility at ground except in precipitation, and cumulus and cumulonimbus clouds 20 leo Ko 130 120 with showers. These showers are not as intense as those produced in the shorter trajectory mP AG.461 air, but the total amount of precipitation is Figure 5-7.Paths of mP air over the greater. With a very long overwater trajectory, Pacific coast in winter. path No. 3, mP air tends to become rather

126 132 Chapter 5AIR MASSES, FRONTS, AND CYCLONES stable, with one or two subsidence inversions air mass which itis overrunning and is thus and stratus type clouds with base about 1,000 associated with frontal activity. feet and tops about 4,000 feet. In the Southern States, mT air is mild with Maritime polar (mP) air sometimes stagnates much lowcloudinessatnight andahigh between the mountains in the Great Basin region frequency of fogs. During the day convective of the Western United States. This coldair activityraisesceilings;flying conditions are becomes modified, and subsidence inversions, good. As mT air moves northward, extensive low stratus clouds, and fog form. This process cooling in the lower layers takes place, causing causes the Pacific coast valleys to be among the much fog and low stratus, which sometimes foggiest places in the country during winter. precipitates a fine drizzle. True mT air in winter When this air moves eastward across the Conti- has a dewpoint in excess of 60°F and should not nental Divide and downslope, it generally brings be confused with modified cP returning from clear skies, mild temperatures, and low humidi- the Gulf of Mexico. Northward movement of ties. mT occurs only with a very low zonal index Maritime polar air that moves eastward with- situation. out stagnating has much of its moisture condensed out during the lifting necessary to cross Summer Air Masses the mountains. It then warms dry adiabatically as it descends, forming a very stableair mass with good flying conditions. This air has large Most of the United States is dominated by daily temperature ranges. When mP air crosses either Sor mT air,whereas Canada (and the Rocky Mountains and encounters a deep cP Northwestern United States) is dominated by airmass which forcesitaloft,very severe polar air. Occasionally, tropical air is transported snowstorms or blizzards may occur. to the Canadian tundra and Hudson Bay legion. MARITIME POLAR (ATLANTICORI- MARITIME POLAR (PACIFIC ORIGIN). GIN).The rarest air mass in the United States is The whole of the Pacific coast is usually under mP air of Atlantic origin, owing to the normal the influence of mP air in the summer. This air westeast movement of systems. The air mass is seldom extends east of the Rockies; and when it confined to the east coast of the United States, does, it has acquired properties almost identical New England, and the Maritime Provinces of to cP air. Coastal weather is generally clear with Canada. The air mass is originally cP which has scattered cumulus. Typical unstable conditions had a shorttrajectory over water, but the prevail along the Pacific coast as far as northern heating, addition of moisture, and steepening of California with the influx of such mP air. lapse rateis not as great as in the mP air of MARITIME POLAR (ATLANTIC ORI- Pacific origin. GIN).The eastcoast of the United States MARITIME TROPICAL (PACIFIC ORI- occasionally experiences an influx of mP air GIN).Maritime tropical air of Pacific origin is from the Atlantic, but due to the lesser tempera- seldom observed along the west coast of the ture contrast between continent and oceanin United States, especially near the ground. On summer, this influx of mP air creates nohazard those occasions when true mT air moves over to flying. The temperature drops quite a bit, the lower coast of California, the air is pushed bringing relief from heat waves. Precipitation aloft inthe rapidly occluding frontal systems, from this air is rare. producing the heaviest winter rains on record CONTINENTAL POLAR.Polar air of Cana- there. dianoriginoccasionallyinvades the United MARITIME TROPICAL (ATLANTIC ORI- Statesin summer under a low zonal index GIN).The temperature and moisture content situation. When it does, it preserves to a large are higher than in any other air mass onthe extent its temperature characteristics. Convec- North American Continent in winter. Maritime tive activity in the United States is extensive but tropical air is responsible for the greatest portion mild, confined in height to 700 mb or less. of precipitation both in summer and in winter. MARITIME TROPICAL (ATLANTIC ORI- Most of the precipitation falls through a polar GIN).The most extensive air mass in summer

127 133- AEIZOGRAPHER'S MATE I & C over theI astern United States. High tempera- co:NTINENTAL ARCTIC.--Continental Arc- lue:11d great deal of moisture dhiracterize tic (cA) air comes into Europe from the north- this air, with a summer dewpointnear or in east. Itis much more frequently felt in eastern Excess of 70)F at the surface. Movement of mT air oter warmer ground Europe.Inmildwintersitaffects western increases its convectite Europe verylittle.Severe winters in western instability. Low stratform cloudsare the rule in Europe are due to cA air being abnormally the mornings. especially along theeast coast, displaced to the west. In the wanner months it becoming convective clouds during the day,with occurs very infrequently in western Europe. fropient thunderstorms at night. Flyingcondi- tion,, in this aIr mass are not hazardousdespite The cA air in Europe undergoesa great deal the thunderstorms because theyare easily en-- of modification asitspreads out over the an,riat igatcd, Ground fogs are frequent with continent, especially in the west, where the air, thw ard movementof mTair over land, and even in winter, shows the results of a great deal Ica fogsith movement over water. The famous of mixing and stirring. However, in andnear the rugs or thc Grand Banksare typical of ml' air source region there is a great deal of stability in over a cold ocean current. the low layers. Occasionally during late summer and early MARITIME ARCTIC.Maritime Arctic (mA) fall, the Bermuda high formsa westward exten- air masses spread into Europe from the north. sion going as far west as Lower California. The Due to its rapid movementover warm seas, it intense surface heating and orographic lifting becomes quite unstable (especially in winter) makes thissituationintensely unstable with with a lapse rate close to the saturated adiabatic thunderstormsofcloudburstintensity rate inall layers. The air is not greatly different (SONORA weather). from maritime Polar air, whosesource region is CONTINENTAL TROPICAL. Thisair is to the west.Ithas "k"characteristics only. 1'und only during the summer forming°ter 41 Outbreaks of this air (mAk)occur mostly in small area of northern Mexico, western Texas, winter, with only occasional outbreaks of such New Mexico, and eastern Arizona.Itcan be air in the summer, and then it is quite shallow 1,1i:untied its extremely high surface tempera- and undergoes modification quickly. tures very lowhumidities,largediurnal- In temperatureranges, and infrequent precipita- winter, cold rain or snow showers accompany fresh outbreaks of mA air, along with tion.Flyingconditionsareexcellentwith a great deal of turbulence. Where orographic respecoto weather,but clear air turbulence lifting takes place, the snowfall makes ro a rough ride. is extremely heavy. These mA air masses are sometimes the l'igure 5-8(A) and (13) show the properties of cause of the MISTRAL in southern France. North Americanairfrom the standpoint of i13., lug during the winte and summerseasons. MARITIME POLAR. Maritime polar (mP) air masses predominate in western Europe. They arc quitevaried, dependent on their source kir Masses of Europe regions and dynamic effects. With cyclonic circulation they show noticeably the effects of rho air masses of Luiope do not fit neatly stirring and convergence and a steep lapse rate; nib,the same categoriesas those of North withanticyclonic circulation subsidence and Anit.riLa. Prinicipally, this is due to the fact that divergence tend to cause the airmass to have a the atmospheric separations occur ata different more stable lapse rate. rune 01 values of temperature. This is bi,cause Mild showers accompany fresh outbreaks of g,.ographical environments of the two conti- mP air with "k" characteristics. However,as the t),nts differ greatly. The continent cf North air spreads over the continent, it becomes stable America widens toward the north, the opposite inwinter.Itbecomes more unstable in the i' true of Europe The great mountain systems summer, but due tocontinentalheating,it 01 North America nn north and south; those of becomes too dry to ,:ause appreciable shower Europe arc oriented east and west. activity.

128 134 VISIBILITIES TURBULENCE TURETEMPERA-SURFACE F. (A) cP(near source AIR MASS region) NoneCLOUDSStratocumulus and cumulus UnlimitedCEILINGS ExcelIent(except1trial - 5 miles, 0 nearin snow indus- than 1 - 4 silos). ModeratehighSmoothup windsto except 10,000turbulence velocities. with feet. -10 to -60.0 to 20. mP(oncP(southeant Pacific of Coast)Great Lakes) 20,000Cumulustops 7,000-10,000 feet tops above feet. 0500-1,000 over1,000-3,000 mountains. feet, feet, GoodExcellentandflurries. except in showers. 0except over mountainsnear indus- SmoothModerateturbulence. except to strong in lever 3045 to 55.40. mP(eastsP(east coast)of Rockies) StratocumulusNonetops 6,000-8,000 and stratum feet. 0-1,000Unlimited feet tationFairtrial except area. areas, 0 in then precipi- 1 - 4 miles. SmoothRoughlevels in lowerwith highlevels. winds. 30 55to to40. 60. mT(east ofT(Pcific Rorkies)Coast) StratumStratum or orstratocumulus. stratocumulus. 500-1,500100-1,5000 feet feet Good Smooth 60 to 70 TURBULENCE TURETEMPERA-SURFACE F. (B) cP(near source AIR MASS region) CLOUDSScattered cumulus CEILINGSUnlimited GoodVISIBILITIES1/2 - 10 miles SlightlyModerate10,000 roughturbulencefeet. in clouds. up to 55-6050-60 n P(Pacificlqesst of Pacific)coast) None2,000-5,040Stratuslux etcept neartops, mountainsfeetscattered cumu- Unlimitedlimited100 feet-2,500 during dayfeet,un- Excellent GenerallySmoothSlightlydesert above. regionssmooth rough exceptinup afternoon.to 15,000over 75-6560-70 S(MississippimT(esst of Valley) Rockies) NonecumulonimbusStratocumulus afternoon. early morning; 3,000-4,000500-1,00Unlimited feet feet a.m.: p.m. ExcellentExcellent bulence.storms,Smoothfoot. exceptthan strong in thunder- tur- 75-85 Figure 5-8.Properties of air masses over North America from the standpoint of flying, (A) Winter; (B) Summer. AG.462 r:- AEROGRAPHER'S MATE

Occasionally iuP air spreads across France and air from the extensive land mass or from the into the Mediterranean Sea between the Pyr- large warm ocean areas to the southwest and eness and Ur Alps and travels eastward as far as south comes in contact with the cold waters of the northwestern coast of India without losing the Oyashio. its identity. WINTER AIR MASSES.The great winter air CONTINENTAL POLAR.--The continental masses of Asia show the effects of more vertical polar (cP)air masses of Europe are sort of mixing than do their counterparts in North neutral air masses between the Arctic air and the America. Thisis tropical air. Quite frequently they due to the more general are simply distribution of mountains in Asia and to the modification of cA air. The cP air is especially strong winds of the widespreadwhen a winter monsoon. The retrograde or stationary towering Himalayas and their eastern extensions anticyclone exists. Such anticyclones are prev- prevent 'the continental air masses from mixing alent in northern Europe in winter. In spring with the tropical air from the south; therefore, and summer they occasionally settle over the the Asian air masses are less variable than those Caspian and Black Sea area, bringing cP air into in North America. Europe through the Balkans and the Ukraine. Similar to North America, the temperatures MARITIME TROPICAL.Due to the rela- are lowest near the ground in the interior of the tivelylow temperatures of the water areas continent of Asia, but near the coast at southwest of Europe, the occurrence of mari- com- parable latitudes the temperatures are lower time tropical (mT) air is not as extreme as in the aloft. United States. The winter air masses usually have moved The mar:time tropical (mT) air of Europe across vast areas of water before they encounter originates south and east of the large semipermanent anticyclones of the Atlantic. The mT tropical air. On reaching the warmseas they air then flows around the center of the anti- become moisture laden and unstable, bringing to cyclone and enters Europe from the north or the lee side of mountains on the principal islands northwest. much shower activity. These air masses assume tropical characteristics by the time they reach CONTINENTAL TROPICAL.European con- the tropical seas. tinentaltropical(cT)airmasses have their source regionsin SUMMER AIR MASSES.As in winter, the North Africa and in Asia air masses are similar to those in North America. Minor, where the air is unstable but very dry. In summer, the monsoon in Asia brings tropical These air masses appear in summer and winter, air into the interior from the southeast, permit- although they are rare in winter. As they move ting considerable interaction of air masses. The intoEurope, much moistureis added and tropicalair that invades India and China in instabilityshowers result. Continental airis summer is extremely warm and moist. Some air confined to southern Europe, in the Mediter- invades India and the Far East fromacross the ranean region. Equator from the Southern Hemisphere. The monsoon air may often be classified as equa- Air Masses of Asia torial.

As expected, over the vast landmass of Asia, the air masses exhibit even more extremeprop- Air Masses of the erties than those of North America. The coldest Southern Hemisphere winter air masses (with the exception of Antarc- tica) and the warmest summer airmasses are The air masses of the Southern Hemisphere spawned in Asia. The area of most frequent are predominantly maritime. This is due to the cyclogenesis in the world is the Asiatic-Pacific overwhelming preponderance of ocean areas. area, where the extremely cold Arctic air in Great meridional transports of air masses as they winter comes into contact with one of the are known inthe Northern Hemisphere are warmest ocean areas. In summer, one of the absent because the westerlies are much more foggiest regions in the world is where the warm developed in the Southern Hemisphere than in

130 Chapter 5AIR MASSES, FRONTS, AND CYCLONES the Northern Hemisphere. Except for Antarc- continues to function as a source for cold cA air. tica, there are no large land masses in the high In both winter and summer the air mass is latitudes in the Southern Hemisphere; this pre- thermallymodifie I,asitflowsnorthward vents sizable invasions of' Antarctic air masses. through downslope motion and surface heating The large land masses near the Equator, on the and as a result becomes less stable. It assumes other hand, permit the extensive development of' the characteristic of maritime Antarctic air. The warm air masses. leading edge of this air mass then become:, the The maritimetropicalairmasses of the northern boundary of the Antarctic front. Southern Hemisphere are quite similar to their To the north of the Antarctic front is found a counterparts of the Northern Hemisphere. In the vast mass of maritime polar air which extends large area of Brazil, there are two air masses for around the hemisphere between 40°S and 68°S consideration. One is the regular air mass from in summer and betweer 34°S and 65°S in the Atlantic which is composed of unmodified winter. At the northern limit of this air mass is mT air. The other originates in the Atlantic; but foundthe Southern Hemisphere polar front. by the timeit spreads over the huge Amazon During summer this mP air is by far the most Riverbasin, itundergoestwoimportant important cold air mass of the hemisphere due changes: the addition of heat and moisture. As a to the lack of massive outbreaks of cold conti- result of' strong heating in summer, a warm dry nental air from Antarctica. air mass, continental tropical (cT) is located Different weather conditions occur with each from 30° south to 40° south. type air mass. The maritime polar air that invades South America is quite similar to its counterpart in the I. The cA produces mostly clear skies. United States. Maritime polar air occupies by far 2. The mA air masses are characterized gen- the most territory in the Southern Hemisphere, erally by an extensive overcast of stratus and encirclingitentirely and providing the mo- stratocumulus clouds with copious snow show- mentum for the great west wind drift. ers within the broad zone of the Antarctic front. Australiaisa source region for continental 3. Anareaoftransitionwhichextends tropical air. It originates over the vast desert area mainly from the coastline to the northern edge in the interior. Except along the eastern coast, of the consolidated pack ice is characterized by maritime tropical air does not invade Australia broken to overcast stratocumulus clouds with to a marked degree. This air is brought down somewhat higher bases and little precipitation. from the north, particularly in the summer, by thecounterclockwise circulation around the FRONTAL CHARACTERISTICS South Pacific high. Antarcticaisa great source region for in- A FRONT is defined as the point of inter- tensely cold air masses. They have continental section of a frontal surface with a horizontal characteristics, but before the air reaches other surface. A FRONTAL SURFACE is a surface of land areas, it becomes modified and is properly separation of two adjacent air masses of differ- called maritimepolar. The temperatures are ent densities, temperatures, and moisture con- colder than inthe Arctic regions. Results of tent. It is taken to be the surface adjacent to the Operation Deepfreeze have revealed the coldest warm air. In reality, the discontinuity between surface temperatures for the world to be in the adjacent air masses is not abrupt enough to Antarcti'. warrant suchanarrow definition.Forthis During the polar night the absence of insula- reason the concept of a FRONTAL ZONE was tion causes a prolonged cooling of the snow introduced. A FRONTAL ZONE is the transi- surface which makes Antarctica a permanent tion zone between two adjacent air masses of source of very cold air. It is extremely dry and different densities, bounded by a frontal surface. stable aloft. This polar air mass is referred to as Since the temperature distribution is the most Continental Antarctic Air, (cA). important regulator of atmospheric density, a In summer the continent is not as cold as in front almost invariably separates air masses of winter duetoconstant solar radiation but different temperatures. AFIZOGRAPI WIZ'S MATE 1 & C

CONDITIONS NECESSARY between the high and low which bring the air FOR FRONTOGENESIS particles toward the neutral point. The "x" axis lies between the high and low which take air Frontogenesis (the formation of a new front particles away from the neutral point and is or the regeneration of an old one) takes place known as the axis of dilation. only when two condition are met. (I) Two air The distribution and concentration of iso- masses of different densities exist adjacent to thermsinthe deformationfield determines one another, and (2)a prevailing wind field whether frontogenesis willresult.If the iso- exists, bringing them together. therms formalargeangle with the axis of There are three basic situations which are contraction, frontogenesisresults;ifasmall conducive to frontogenesis and which satisfy the angle, frontolysis results. It has been shown that two basic requirements. They are as follows. in a perpendicular deformation field. isotherms must form an angle of 45° or less with the ails 1. The windflow is cross-isothermal as illus- of dilationforfrontogenesis to occur (fig. trated in figure 5-9, decreasing in speed down- 5-10(A) and (B)). In a nonperpendicular defor- stream and flowing from cold air to warmer air. mation field, the critical angle changescorre- The flow need merely be cross-isothermal. not spondingly as illustrated in figure 5-11(A) and perpendicular, but the more perpendicular the (B). In most cases. frontogenesis will occur along cross-isothermal flow, the greater the intensity the axis of dilation. At any rate, frontogenesis of frontogenesis. will occur where there isa concentration of isotherms with the circulation t, sustain that concentration.

FRONTOLYSIS

Frontolysis, or the dissipation of afront, occurs when either the temperature difference between the two air masses disappears or the -tT4 wind carries the air particles of the air mass away from each other. Frontolytical processes AG.463 are more common in the atmosphere than are Figure 5-9.Frontogenetic cross-i sothermal wind flow. frontogeneticalprocesses.This comes about because there is no known property of the air whichisconservative with respect to allthe 2. The winds of opposite air masses move physical or dynamical processes of the atmos- towardthe same point orlineinacross- phere. The theory of frontogenesis is based on isothermalflow. A classicexample of this the assumption that there is such a property and situation is the polar front where cold polar air thatitistemperature. The nonconservative moves southward toward warmer temperatures (principally nonadiabatic) influences on temper- and warm tropical air moves northward toward ature must therefore be added to all the fronto- coldertemperatures. The boundary between lytical processes. them is the polar front. Frontolytical processes are most effective in 3. The windstreams have formed a deforma- the lower layers of the atmosphere since surface tion field similar to the ones illustrated in f:gures heating and turbulent mixing are the most 5-10 and 5-11 intense of the nonconservative influences on temperature. A deformation field consists basically of a col It should be pointed out that fog frontolysis area of flat pressure with two opposing highs to occur, only one of the two conditions stated and two opposing lows. It has two axes which above need be met. The simultaneous happening have their origin at the neutral point in the col. of both conditions results in more rapid frontol- The "y"axis,or axisof contraction,lies ysis than if only one factor were operative.

132

13E1 Chapter 5AIR MASSES, FRONTS,AND CYCLONES

71 r 74713 712 TI 72\ 76 75 73\ H IH iII 1. I H \ fi 1 1 TI 74\, 75\ \ -\ 1 . I I \ 1 72- 76\ \ \ \\ \\\\\ i 73- \\ ,,\ \N X X I X XT4- ---- X\ \\\\\\ \ I I 15- \ -,.. H L 41\ L I L. H \ I T6 \\\ 1 I Y Y Y \ (C) IDEALLYFRONTOLYTIC (A ) IDEALLY FRONTOGENETIC (B 1 CRITICALLY FRONTOGENETIC

AG.464 . Figure 5-10.-Perpendiculardeformation field.

I TI TI .....-. /7 12 ,:------,- ..7' 72 n ."---- -...... _____.----"H - TH 3 Y -.... Ti ,/ -,,4 T3 ...... -- ,-"" -, X __---._ X ..- .,...... 74------.--=----, -. ---L T2 - ....- L 76 .... .-- - - L ..- 13L. . T6L --- -- ..--?--- 75 yl H,- Y .,--- H---- /- --- 76/.'._--- (C) IDEALLY FRONTOLYTIC (A) FRONTOGENETIC (B) CRITICALLY FRONTOGENETIC .

AG.465 Figure 5-11.Nonperpendiculardeformation field.

the Arctic Circle ZONES the North Pacific, and near WORLD FRONTOGENETICAL north of Europe. In summer,this front mainly exhibit a high disappears, except north ofEurope. Certain regions of the world other hand, is present frequency for frontogenesis.These regions are The polar front, on the temperature con- the year round, although not asintense in the coincident with the greatest in the winter, due to alessening trasts. Two of the mostimportant frontal zones summer as the North Pacific and theNorth temperature contrastbetween the opposing air are those over forms wherever the windflow Atlantic Oceans as discussedin chapter 2 of this masses. This front the Arctic front. a and temperature contrastis favorable. Usually it training manual. In winter, tropical and polar air, boundary between polar andArctic air, forms in is the boundary between but it may form betweenmaritime polar and high latitudes overNorthwest North America,

133 136. AEROGRAPHER'S MATE 1 & C

continental polar air. It alsomay exist between Temperatures modified polar air anda fresh outbreak of polar air. This type is common over North America in Sincea the continental regionsin winter in the vicinity front separates two differentair of 50°N lat. masses, a frontal discontinuity shouldappear on 1 The polar frontin a thermodynamic diagramasa straight line winter is found most joining two different lapse frequently off the eastern coasts ofcontinents in rate curves. The top the neighborhood of 30° to 60° of the inversion or changein slope of the lapse latitude. It is rate would indicate the lower limit also found over land; but since thetemperature of the warm contrasts are greater between the air, and the base of the inversionwould indicate continent and the depth of the cold air. The the oceans, especially in winter,the coastal areas thickness of the are more favorable for formation and intensifica- frontal zone would be indicatedby the thickness tion. of the inversion layer. However,each air mass The intertropical convergence has characteristics whichwere acquired inits zone, though source region, but have been modified through not truly a front but a field ofconvergence between the opposing trades,forms a third changes caused by verticalmotions, surface semipermanent frontal type. This region temperature influences, addition ofmoisture shows a from seasonal variation just as do thetrades. The evaporationofprecipitationfalling intertropical(or convergence zone)isthor- through the cold airmass, and other factors. oughly discussed in chapter 12 ofthis training Frontal zones thereforeare often difficult to manual. identify on upper air soundings. The frontal discontinuity isnot always an SLOPE OF A FRONT inversion of temperature. The degreeto which the frontal zone appears pronouncedis proportional to the temperature differencebetween When we speak of the slope ofa front, we are two air masses. speaking basically of thesteepness of the frontal Therefore, the primary indication surface, usingahorizontal dimension anda of a frontal vertical dimension. The vertical zone on a thermodynamic diagram isa decrease dimension used in the lapse rate somewhere is normally one mile. A slopeof 1:50 would be in the sounding considered a steep slope anda slope of 1:300 a below 400 mb. The decreasein lapse rate may be a slightly less steep lapse gradual slope. Factors favoringa steep slope are rate for a stratum in alarge wind velocity differencebetween air a weak frontal zone to a very sharp inversionin masses, small temperature difference, and high strong fronts. In additon toa decrease in the latitude. lapse rate, there is usuallyan increase in mois- The frontal slope thereforedepends on the ture (a concurrent dewpoint inversion)at the latitude of the front, the wind frontal zone. This is especiallytrue when the speed, and the front is strong and abundant temperature difference between theair masses. cloudiness and precipitationaccompanyit.Figure5-12(A) shows theheightof theinversionin two DISTRIBUTION OF ELEMENTS different parts of a IN A FRONTAL ZONE frontal zone, and figure 5-12(B) shows a strong frontalinversion with a consequent dewpoint inversion. From our previous discussion and definitions The extent of the discontinuity,the strength of fronts, it was implied thata certain geometri- of the inversion cal and meteorological consistency or isothermal layer, and the must exist thickness of thezone of discontinuity depend between fronts at adjoining levels. Itcan also be on the discontinuity between the inferred that the data at air masses; no one particular level that is, the frontal intensityand the width of the is sufficient to locatea front with certainty in frontal zone. Fronts every case. In this section of the chapter are drawn on weather maps the as sharp lines of discontinuity. Thisis done be- horizontal and vertical distribution ofweather cause of the scale of the charts. I f the elements in a frontal zone will be air masses discussed. were separated by a distinct line, thelapse rate 134 140 Chapter 5--AIR MASSES, FRONTS, ANDCYCLONES

ATMOSPHERIC SOUNDINSS IN THE COLD AIR MASS TEMPERATURE

ACCUMULATION OF MOISTURE WARN AIR MASS F11.1 DEW POINT

COLD AIR MASS SURFACE POSM OF FRONT

WARM AIR MASS

FRONTAL INVERSION (A) HEIGHT OF INVERSION (B) INDICATES SLOPE OF FRONT

AG.466 Figure 5-12.(A) Height of inversionand thickness of inversion indicate frontal slope and intensity;(B) frontal inversion. through the front would appear asshown in POTENTIAL TSWERATIp/ trV( TIP4 figure 5-13(A). OWINTOPITY is considered the ideal lapse This lapse rate i e ----° 0, is impossible one 1, rate, butit an obviously OEAS VIIK.1).4/3 because two pressures andtemperatures of IA) different value cannot coexist at the samelevel side by side. Actual lapse ratesthrough frontal -- e -- 42 zones therefore will liebetween those as illus- STAInG ) _ 12 451 0, -- PI''"" trated in figure 5-13(B), (C), and(D). ; adjacent to the The edge of the frontal zone el the frontal surface, and ; --"el 0, warm air is referred to as P w<,1,(Ort .2 I 1 0 is actually the intersectionof this frontal [CI . it I ei ------surface with the weather map whichis indicated C at the line of discontinuity. 62 P A cold front usually shows amarked tempera- r 0I,,-- e, ; ' 0, P, higher relative humidity APP .e ture inversion. Also, or - -- P2 (dewpoint and mixing ratio) isindicated in both the air masses near the front. In soundings through coldfront transition ordinarily is lower in the AG.467 zones, the dewpoint frontal zone. upper air mass if thatair mass has the larger Figure 5-13.Discontinuities through a 135 141 AEROGRAIIIIER'S MATE I C houlontal wind component towardthe surface If the potential position of the fiont and thereforehas a new temperature of a frontal dow /AN ard motion. surface has been determinedfrom the soundings and no representative temperatures that highcan 1 he air above the coldfront surface hasan upward be found at the groundin the area wherethe omponent when the horizontalwind frontal surface should ;omponent normal to the frontal be expected to intersect surface de- the ground, the frontisdesignated on the t.reasesith height through thetransition zone. surface chart In these cases, the dewpoint as an "upper front" if thestratum usually increases of air over whichthis upper front lies is upward through a relativelywell-marked transi- 1,500 ;ii,n zone. feet or more in thickness.True fronts, therefore, would not be crossedby potentialtemperature ('old fronts generally showa stronger inver- lines. In nature, however, sion than warm fronts, and the inversion true discontinuities do will not occur: mixing takesplace in the frontal appear at successively higher levelsas the front zone. There is usually mocs past a station. The a crowding of potential reverse is true of warm temperature lines at fronts. hunt Occluded fronts generally show a double The potentialtemperature of the top of the ;ion,However, as the occlusionprocess frontal zone is practically k.ontines, an amalgamation of the independent of eleva- air masses tion, except for a slightincrease at higher levels. takes place, and the inversionsare wiped out, or they fuse. For example, in the coolerseason the potential temperature (0) of the polar front It N very important may increase in raob analysis notto from 298°K near the surfaceto 302°K at the eonfuse the subsidenceinversion of polar and 500-mb level. The 0 ofthe frontal surface \i etic air masses with may frontal inversions. Ex- also vary in the horizontal,depending upon the tremely cold continental Arcticair. for instance, distribution of 0 in thewarm air mass. Fronts 1;asi \irony, inversion which extendsto the tend to have their highestpotential temperatures "i 00-mb level. in the Southwest UnitedStates, when theyare Somevimes itis difficult to finda significant next to continental tropicalair which has a 0 of stable zone on a particularsounding, though it is about 308° to 312°K inthe warmerseasons. ti own that a front intersectsthe column of air Fronts in contact withmaritime tropicalair over o given station. Thismay be because of (polar fronts) have lowervalues of potential warming of the descendingcold air just temperature, about 300°K.During the winter thefrontalsurface or excessive local months the 0 of the polarfront is about 298°K. Nerti, al mixing in thevicinity of the frontal In the warmerseason the polar front is foundat fy Under conditions of subsidenceof the a 0 of 302°K. The Arctic frontis generally found air beneath the frontal surfacethe sub- at a potential temperature ofabout 286°K. sklence inversion withinthe cold air may be more markedthanthefrontalzoneitself. Wind Vertical wind shear criteriathen become even more important in the analysis. Sometimes fronts on a raob sounding which Since winds near the earth'ssurface blow might show a strong inversionoften are accom- mainly along theisobars with a slight drift panted by little weather activity.This is due to toward lower pressure, it subsidence follows that the Wind in thewarmair,whichwill direction in the vicinity ofa front must conform strengthen the inversion. The activity at the with the refraction ofthe isobars. Thearrows in float increases only whenthere is a net upward figure 5-14 indicatethe winds that correspond vertical motion of thewarm air mass. to tlpressure distribution. From thisit can be seen thata front is a Potential Temperature WIND SHIFT LINE andthat wind shifts ina cyclonic direction. Since thefront moves in the direction of the windcomponent normal to the Potenftil temperatureisa conservative air mass property. front (heavy arrowon diagram), we can evolve thefollowing rule:IF YOU STAND WITH 136 Chapter 5 -AIR MASSES, FRONTS,AND CYCLONES

throughthe layer, the front would be the warm YOUR BACK AGAINS"1"11-113WIND IN AD- WIND WILL type. VANCE OF Tim FRONT, THE valuable indi- SHIFT CLOCKWISE (VEER) AS THEFRONT The thermal wind can also be cator for the location offrontal zones. The PASSES. magnitude of thethermal wind for a layer The speed of the winddepends upon the indicates the strength of the horizontalgradient pressure gradient. Thus,in figure 5-I4(A) the of mean temperature of thelayer.Since a speed will be about the same inboth air masses; frontal zone defines a layer ofmaximum hori- in(B) and (C) arelativelystrong windis zontal thermal gradient, it also represents a zone (D) a weak followed by a weaker wind, and in of maximum thermal wind.Furthermore, since wind is followed by a strong wind. . the thermal wind blows parallel tothe mean An essential characteristicof a frontal zone is isotherms for the layer (with coldair to the a wind discontinuitythrough the zone. The left), the direction of the thermal windvector is wind normally increases or decreasesin speed roughly representative of the directionalorienta- with height through afrontal discontinuity. tion of thefront. The winds aloft for the Backing usually occurs through a coldfront and sounding shown in figure 5-16 are seenplotted veering through a warm front. Thesharpness of as a hodograph infigure 5-17. the wind discontinuity isproportional to the The thermal wind is indicatedby the`" shear temperature contrast across thefront and the vector connecting the wind vectorsfor the base pressure fieldin the vicinity of the front (the maximum thermal the two air and top of' each layer. The degree of convergence between windisfoundbetween 700 and 650 mb, streams). With the pressurefield constant, the supporting the previous frontalindications. The sharpness of the frontal zone isproportional to direction of the thermal wind vectorfrom 800 the temperature discontinuity (notemperature to 650 mb is roughlynorth-south; the frontal discontinuitynofront; thus,no winddis- zone could thus beassumed to be oriented along variation continuity). The classical picture of the a generally north-southtine. in wind along the vertical through afrontal zone In conclusion, the verticalwind shift through is shown in figure 5-15. a frontal zone depends onthe direction of the An example of a frontal zoneand the winds slope. In cold fronts the windbacks with height, through the frontal zone is shownin figure 5-16. and cold advectionistherefore indicated. In above the warm fronts the wind veerswith height, indicat- On this sounding the upper winds surface, the wind surface layer which show the greatestvariation ing warm air advection. At the This ALWAYS veers across the frontand the isobars are those inthe 800- to 650-mb layer. toward higher indication coincides closely with thefrontal have a cyclonic kink which points dewpoint pressure. Sometimestheassociatedpressure indications of the temperature (T) and the front; in such (Ta) curves. Since the wind veerswith height trough is not coincident with

LOW LOW 1004 LOW LOW

1008 9% 1000 1000 /-9° 1000 . 100 HIGH HIGH HIGH HIGH .I. (D) (A) (B) (C) AG.468 Figure 5.14.Types of isobars associatedwith fronts.

137 143 AEROGRAPHER'S MATE 1 & C

W+E S

AG.469 Figure 5-15.Vertical distribution ofwind direction in the vicinity of frontalsurfaces. cases there may not be an appreciable wind shift over the potentially colder across the frontonly a speed discontinuity. air. This isalso demonstrated by actualupper air soundings When the lowestpressureinthe associated where it will be found that trough is found ahead of the front, a specific potential the winds are temperature is always found at a higher altitude of the same direction, butthe speed is greater in the cold air than in the BEHIND the front. When the warm air. Stated in a lowest pressure in different manner, the potentialtemperature in the associated trough is found behindthe front, the warm air is higher, level for level. the wind direction is thesame on either side of the front, but the wind speed isgreater AHEAD of the front. The reason for all ofthe foregoing Clouds and Weather is that a front must havea cyclonic shear across itto be a front. When the cyclonic shear Cloud decks are usually in thewarm air mass disappears, so does the front. Thereasoning due to the upward vertical behind this cyclonic shear requirement movement of the can be warm air. Clouds forming in the cold airmass found in Margules equation of frontalslopes. are due to evaporation of moisture from vrecipi- The direction of the slope is determined by the tation from t'.2e overlyingwarm air mass and/or wind speed difference betweenthe cold air and by vertical the warm air. The sign of the lifting. Convergence atthe front difference deter- results in a lifting of bothtypes of air. The minesthedirectionof the slope with the stability of the air qualification that the slope must be masses determines the cloud toward the and weather structure at the frontsas well as the cold air. The potentiallywarmer air always lies weather in advance of the fronts.

138 144 Chapter 5AIR MASSES, FRONTS, ANDCYCLONES

800

-20 -10

AG.470 frontal zone. igure 5-16.Distribution ofwind and temperature through a warm FRONT. This type CLASSIFICATION OF FRONTS 3. QUASI-STATIONARY front is one along which oneair mass does not The frontal characteristics described sofar are appreciably replace the other. generalcharacteristics of fronts and may be 4. OCCLUDED FRONT.An occluded front applied as to whether cold air replaces warmer is one where the cold frontovertakes the warm air or warm air replaces colder air.Some specific front and the warm air issqueezed upward. The cases were mentioned toillustrate frontal char- occluded front may be either aWARM FRONT acteristics. However, depending uponthe move- TYPE, one in which the coolair behind the cold ment of the front and thestability conditions of front overrides the colderair in advance of the the air masses, a number ofadditional character- warm front, resultingin a cold front aloft; or a istics must be considered. Fronts areclassified as COLD FRONT TYPE, one inwhich the cold air follows (motion relative to the warmand cold behind the cold frontunderrides the warm front air masses is the criterion): and the v rm front is aloft. I. COLD FRONT. A coldfront is one that moves in a direction inwhich cold air displaces FRONTAL TYPES warm air at the surface. front is one Whether afront isto be classed asquasi- 2. WARM FRONT. A warm stationary, warn, or cold isdetermined on the along which warmer air replacescolder air.

139 143 AEROGRAPHER'S MATE 1 & C

0° . . . --. -.-. . . F.- .--- . ., . ..., ...... , I ./..- ...... -. / L -...-- ..-- I . ..- / . / / 1 \\ 1 / ...... -- . - N // ..- 1 -..N \\ / , 1 \ , *"... / / -''' .. tA \ \ / . .... I tr ' \ / // ..,....-r, \ , , I , / / . , . ibo'c'800,0\ 1 I 1 , / ,--t 1 ; I I 1 I i / / I 750mb , , , 270.1__I___.4.._ _i___ _.1__L_J_.---:÷gglege_____t_ _ ...t. goo 60 50 40 30 20 -r _ 700mb 1 (KNOTS) V I 1 11 a / k 1 `.....i..... / I 1 \ \ \ `,,,. i \ I // / fish \\\ --, 1._- -- / / --, mb \ \ / / °Oomb /1 \\ \ N. I .,/ / i - ..1_ / ------/ / / \\ --II- OBSERVED WIND / / \ N - THERMAL WIND /// . N 1 / \ - .0.// --- I / \\ -...... ______...--- / -... -I -.. ./ -... ..--./ -...---II- ----...- 180°

AG.471 Figure 5.17. - Hodograph of observed andthermal winds for sounding in figure 5-16. basis of the instantaneous or known direction of designation will usually bea warm front, be- movement. Past movements for 3 to 6hours cause cyclonic curvature means ordinarily that may beusedas a guide. The instantaneous there is a component of motion of thecold air direction of motion is determinedinsofar as away from the front. In this case the direction possible on the basis of actual winds inthe cold of motion of the front is consistent air rather than on geostrophic winds. If with that repre- indicated by the pressure gradientalong the sentative wind observations in the coldair are front. (See fig. 5-18.) not available, the direction of the frontcan be inferred from the pressure gradient andpast 2. If isobars with easterlytype flow are history of evidence ofpassage of the front at anticyclonic in the colder airmass, the designa- individual stations. tion will usually be coldor quasi-stationary, as If isobars intersect the front withlow pressure illustrated in figure 5-19. to the north (westerly winds) and colder airis to the west, the front is usuallya cold front. The following examples illustrate this point: This case occurs only when thepressure gradient along the front is slight,or the isobars 1.If isobars with easterlytype flow are cross the front only at relatively long intervals of cyclonic or straight in the colder airmass, the distance.

NC 146 Chapter 5--A1R MASSES, FRONTS, AND CYCLONES

the warm air. If the cold air wind component toward thefrontisvery small, the quasi- stationary character of the front may be indicated. A problem sometimes arises during the winter seasons when such a cold wind component toward the warm air exists; but because of daytime heating, the leading edge is continually destroyed and the front appears to remain stationary and may even retrograde during the day. The policy is to continue to indicate this front as cold, since the cold air component is toward the warm air. Actually, the cold air will make progress toward the warm at night, in keeping with its designation as a cold front. This AG.472 conditionisencountered especially with inP Figure 5 -18. Warm front in an easterly flow. fronts from the Pacific moving into the interior of the Pacific Northwest States in summer. The only criterion for classification of a front as cold is its direction of motion, requiring that the component of true horizontal surface wind in the cold air, perpendicular to the front aod adjacent to it, be directed toward the warm air. Thetruewind may be different from the corresponding component of the geostrophic wind, as illustrated in figure 5-19. With southward moving cold fronts along the east slope of theRockies, true winds in the cold air arc sometimes nearly perpendicular to both the front and the sea level isobars.

AG.473 Occluded Fronts Figure 5-19.Cold and quasistationary fronts The Aerographer's Mate should designate as in an easterly flow. "occluded fronts" only those fronts which result Warm Fronts from the classic occlusion process that takes place when an unstable wave develops along Normally, theeasternpart of the frontal what was originally a single front. system of any nonoccluded cyclone or open For example, if a cold type occlusion over- wave is properly designated as a warm front, but takes a warm or stationary front along the north there may be some cases of open waves where Pacific coast and isforced aloft, the coastal caution is necessary because high-pressure condi- warm front overtaken is not then indicated as a tions on the cold air side mad prevent movement warm type occlusion because it is not the result of the kohl, thedesignation such cases is of a normal occlusion process from an unstable usually quasi-stationary. wave. There are several reasons for this convention, one being that the process illustrated by Cold Fronts this example isusually not accompanied by cyclonic development, and anotherthat the A surface front is indicated as "cold" if the potential temperatures along the two frontal low-level wind direction in the cold air along any surfaces may be widely different; whereas, with appreciable segments of the front has a compo- the normal occlusion process, the temperatures nent, however small, toward the front or into along the two fronts are initially about the same.

141 147 AEROGRAPHER'S MATE 1 & C

A situation similar to the overtaking ofa overtaken the surface warm front, provided the coastal warm front occurs when maritime polar process is that of an unstable occluding wave air from the Pacific overtakes a quasi-stationary cyclone. Cold occlusions aremore frequent than Arcticfrontalongtheeastern slope of the warm occlusions. The lifting of the warm front Continental Divide. In this case the best practice asitis underrun by the cold front implies isto continue to designate the portion of the existence of an upper warm front to therear of Arctic front overtaken by the new surge of mP the cold occlusion; actually sucha warm front air as quasi-stationary, cold, or warm,as appro- aloft is rarely discernible or significant. priate, and not as a warm type occlusion. In this Most fronts approaching the Pacificcoast of case the two fronts have markedly different North America from the westare cold occlu- potential temperatures, and it is possible under sions. In winter these fronts usually encountera proper circumstances for a wave to then form shallow layer of surface airnear the coastline, alongthe Arcticfrontand progress to the from about Oregon northward, that is colder occluded state. than the leading edge of cold air to therear of WARM OCCLUSIONS. --A warm occlusion, the occlusion. Sometimes, especially off British or warm-front type occluded front, develops Columbia, this coastal layer of cold air extends when the temperature of the leading edge of the far enough outwardfromthecoastlineto cold front, as it overtakes the warm front, is warrant its delineation by a surface stationary higher than that of the warm front at the point front. The usual practice in thesecases is to of contact. The term "warm occlusion"is continue to designatethe cold occlusion as applied to that portion of the surfacewarm though it were a surface front because of the front which has been overtaken by the cold shallowness of the layer over which it rides. front, provided the process is that of an unstable Ordinarily the portion of the coastal stationary occluding wave cyclone. The warm occlusion front which has been overtaken by the occlusion usually develops inthe Northern Hemisphere may be dropped from the analysis and should be when conditions to the north of the warm front, completely dropped after the entire occlusion such as the existence of an anticyclone, maintain has moved inland. The coastal stationary front low temperatures north of the warm front while itself is prevented by topographic features from the trajectory of the cold air behind the cold moving inland,atleastbeyondthe higher frontis such as to cause warming. The cold mountains which are generally within 100 miles front, in this case, continues as an upper cold of the coastline. The passage of the cold type front above the warm front surface. occlusion over the coastal layer of colder air presents a difficult problem of analysis in that A special case arises when the center ofa low having a warm type occlusion moves along the no surface wind shift will ordinarily occur at the exact occlusion or when a new low develops along the time of passage. However, alineof occlusion or at the apex of the warm sector. stations reporting surface pressure rises is the Whenthishappens, the original warm type best criterion of itspassage. This should be verified by reference to plotted raob soundings occlusion necessarily reverses direction and begins to move toward the west or south as a cold where available. When a Pacific cold occlusion front.In order to avoid confusion, since the moves farther inland, it may encounter colder front no longer moves as a warm front and is not air of appreciable depth over the Plateauor by history Western Plains areas, in whichcase it should be a cold type occlusion, the front redesignated as an "upper cold front." should be redesignated as a "simple cold front." COLD OCCLUSIONS.A cold occlusion,or Quasi-Stationary Fronts cold-front type occluded front, develops when the temperature of the leading edge of the cold A front is classed as "quasi-stationary" if front, as it overtakes the warm front, is lower there is no movement of cold air normal to itor than that of the warm front as the point of if the amount of movement is too small for its contact. The term "cold occlusion" is applied to direction to be determined, as may be the case that portion of the surface cold front which has with minor undulations along the front.

142 148 Chapter 5AIR MASSES, FRONTS, AND CYCLONES

The quasi-stationary designation tends to be otherwise be classifiedas weak is considered used too frequently. Many of the fronts so moderate if turbulence and gustines' are prev- designated move slowly but steadilyin one alent along. it, and an otherwise modeote front direction or the other. For example, a front is classified as strong. The term gustiness for this moving 10 miles per hour between stations 70 purposeistakento include convective phe- miles apart could be found on three successive nomena such as thunderstorms and strong winds 3-hourly synoptic maps still between the same regardless of the amount of wind shear. stations. Such fronts arc actually slowly moving cold fronts or slowly moving warm fronts, and Temperature Gradient should be thus designated. Fronts which are often designated "quasi-stationary" by successive analysts on many map sequences may be Temperature gradient, rather than true differ- found to have traversed great distances in 12 to ence of temperature across the frontal surface, is 24 hours, not infrequently 200 a 300 miles. used in defining the frontal intensity in terms of A special type of quasi-stationary front which temperature. The temperature differencebe- is frequently very well marked by wind and tween the two air masses does not occur across a dewpoint discontinuity as the surfaceis the surface of zero thickness as required for the true so-called dewpoint front extending north-south definition of a front. Rather, the difference is usually through Texas and Oklahoma; for ex- distributed across a transition zone which is ample, during the warmer months. This line of usually some 25 to 50 milesinwidth. In demarcation between hot cT to the west and addition, there is normally a temperature gradi- cooler mT air to the east either slopes upward to ent away from the front in the cold air mass. the east or is nearly horizontal. The hot dry cT Occasionally, autographic records of frontal air of the Southwest United States is therefore passages indicate that in addition to the transi- usually continuous with warm dry air aloft over tionzonethereissometimes achange of the Central Plair.s area and southern United temperature at the instant of arrival of the cold States. Since this discontinuity rarely moves east air, which suggests a nearly perfect discontinuity of about the longitude of Dallas, Texas, and is a of density across the front. This is usually in climatological feature of the.region, Analysis addition to a greater net decrease of temperature Centers at times unfortunately do not designate across the transition zone. it as a front. Temperaturegradient,whendetermining frontalintensity,is defined as the difference betweentherepresentative warm air imme- FRONTAL INTENSITY diately adjacent to the front and the represeta- No completely acceptable set of criteria is in tive surface temperature 100 miles from the as to the determination of frontal front on the cold air side. By convention, the existence transition zone is taken to be part of the cold air intensity,asitdepends upon a number of variables. Some of the criteria which may be mass. helpful in delineating frontal intensity are dis- A suggested set of criteria based on the cussed in the following paragraphs. horizontal temperature gradient just described has been devised. It defines a weak front as one Turbulence where the temperature gradient is less than 10°F per100 miles;a moderate front where the Except when turbulence or gustiness may temperature gradient is10° to 20°F per 100 result, weather phenomena are not taken into miles; and a strong front where the gradient is account when specifying frontal intensity, be- over 20°F per 100 miles. cause a front is not defined in terms of weather. The 850-mb level temperatures may be used A front may be intense in terms of discontinuity in lieu of the surface temperatures if representa- of density across it, but may be accompanied by tive surface temperatures are not available and no weather phenomena other than strongwinds the terrain elevation is not over 3,000 feet. Over and a drop in temperature. A front which would much of the western section of the United

143 149 AEROGRAPILER'S MATE 1 C

MO) level temperatures can b. cussed in a later section) and upon weather the -.Ltrface temperatures. along the front from which intensity is taken as a measure of the convergence. cs,!1EAR In actual practice, the average thermal wind over a 5-degree band on both sides of the front may be either the vector is determined by aveniging the thermal winds or , .11 components of surface geo- computing them from the thickness gradient. aid parallelto and immediately on The thermal wind sly:.ar is converted into fnital of the front or the 1,000-500 mb intensity using the following relationships. xvind shear. If the thermal wind shear is equal to or less than 25 knots, no front exists or frontolysis is in k;l'itae y;ind Shear progress: if the thermal wind shear is greater than 25 knots, but equal to or less than 50 ! .ontrast (shear) is normally taken as knots, itis a weak front or frontogenesis is in between the components progress, if the thermal wind Shear is greater ostiophie P;'Idparallelto and than 50 knots but equal to or less than 75 knots, uneither side of thefront,as the front is of moderate intensity; and if the Jilt tirure 5-20. thermal wind shear is over 75 knots, the front is "C inthefigureare vectors strong. the computed geostrophic wind in NMC ,,lso modifies the int,,nsities of fronts on .old air, respectively, and the "w" the basis of observed weather associated with

! the rLspective components parallel the front. Clear skies and weak or no surface , In (A). the wind contrast is w- (- wind shift decrease the intensity one category. 4 ts,tall i(II) it is %%/.e, Moderate to heavy precipitation or pronounced surface wind shifts increase the intensity one thud Shear ea t ego ry,

\, most important properties ascribed POLAR FRONT THEORY ti boundries by the National Meteor- ' (NMC) a,a pressure gradient The polar regions are dominated by gold an '. .11,da three-dimensional tempera- masses and the Tropics by warm air masses. The Jheentinuity. The frontal intensity middlelatitudesarert..gions where Loid and directly upon the intensity of warm air masses continually interact with each gradient discontinuity (to be dis- other, the cold air moving southward and the

(A) (B)

AG.474 ;IA.re5-20.Determination of wind shear across a front for determination of frontal intensity. (A) Wind ,ornponents in opposite direction; (B) wind components parallel to front in the same direction.

144 150 Chapter 5AIR MASSES. FRONTS, AND CYCLONES warmair movingnorthw aidinalteinating cold front. The cold front is moving faster than tonitues or waves, The zone which separates thewarmfront(D). When the coldfront theseairmassesisthepolarfront. When overtakes the wain' front and closes the warm conditions are favorable, extratropical cyclones sector. an occlusion is formed (E). This is the develop onthe polarfront.Theyform as time of maximum intensity of the wave cyclone. wavelike perturbations on thefront and go As the occlusion continues to extend out- through a life Lycle. This cycle can be either of ward,thecycloniccirculation diminishes in two patterns, that of stable waves or unstable intensity(the low-pressure area weakens), and waves. the frontal movement slows down (F). Some- times a new frontal wave may now begin to STABLE WAVES form on the westward trailing portion of the cold front. In the final stage, the two fronts A stable wave is one that neither develops noi become a single stationary front again. The low occludes, but appears to remain in about the center withits remnant of the occlusion has same state. Stable waves are usually of small disappeared (G). amplitude and have afairly regular rate and direction of movement. CYCLONES

UNSTABLE WAVES DEFINITIONS AND TERMINOLOGY

This type wave is by far the more common The term cyclone is used to denote any area that is experienced with development along the of closed counterclockwise circulation inthe polar front. The amplitude of this we increases Northern Hemisphere.Because of the basic with time until the occlusion process sets in. and wind-pressure relationships, the center of such when the occlusion processis complete, the an area will also be a relative minimum in the polar front is reestablished. pressurefield. The term lowisusedinter- Figure 5-21illustrates the development and changeably with the term cyclone. A distinction occlusion of an unstable wave cyclone. isalso made between those cyclones forming Inits initial stage of development the polar outside the Tropics (extratropical) and those frontseparates thepolar easterlies from the forming in the Tropics (tropical). The latter are midlatitude westerlies (A), the small disturbance discussed in chapter 12 of this training manual. due to the steady state of the wind is often not In this section the word cyclone applies only to obvious ontheweather map. Uneven local those forming outside the Tropics. If the central heating, irregular terrain, or wind shear between pressure of d cyclone is decreasing with time, it the opposing air currents may start a wavelike issaidtobe deepening:filling denotes the perturbation on the front (B), if this tendency converse situation. An unstable wave cyclone is persists and the wave inLreases in amplitude, a usually developing, whereas a stable wave is not. counterclockwise (cyclonic.) circulation is set up. When the more rapidly mop mg cold front of a One section of the front begins to move as a wave cyclone overtakes the warm front, the warm front while the adjacent scLtiuns begin to cyclone is said to be an occluded cyclone. The move as a cold front (C). This deformation is typical stages of development of an occluded called a frontal wave. wave cyclone were illustratedin the previous The pressure at the peak of the frontal wave section. falls, and a low-pressure center is formed. The cyclonic circulation becomes stronger, and the CYCLOGENESIS wind components arc now strong enough to move the fronts, the westerlies turn to south- There isa systematic relationship between west winds and push the eastern part of the cyclones and fronts, in that the cyclones are frontnorthwardasa warm front,andthe usually associatedwith waves along fronts- easterlies on the western side turn to northerly primary cold fronts, Cyclones come into being winds and push the western part southward as a or intensify because pressure falls more rapidly

145 151 AEROGRAPI1ER'S MATE 1 & C

FRONTAL

WAVE

/VARM

AG.475 Figure 5-21.The life cycle of an unstable frontalwave.

146 Chapter 5AIR MASSES, FRONTS, AND CYCLONES at one point thanit does in the surrounding 1. Marked increase in katallobaric gradients, area. The resultant increase in the intensity of (falling pressure) especially in the cold air. thecounterclockwisecirculation(Northern 2. Marked location changes in wind direction Hemisphere) is called cyclogenesis. It can occur or speed. anywhere, but in middle and high latitudes it is 3. Increasing cloudiness north of the frontal most likely to occur on a frontal trough. In this area. instance, the cyclogenesis begins at the lowest 4. Start of precipitation on the cold air side level and works gradually upward as the cyclone of the front. deepens. The reverse also occurs; closed circulations aloft sometimes work downward until they The above criteria do not always give ade- appear on the sur.ace chart. These cyclones quate warning; in many cases the cyclone has rarely contain fronts, and are quasi-stationary or already formed by the time these changes appear drift slowly westward and/or equatorward. The on the surface data. The upper air indications surface process however is usually a combination must be taken into account with simultaneous of two factors: perturbations along the polar occurrences on the surface. front in conjunction with atmospheric waves The shape and curvature of the isobars also aloft. give valuable indications of frontogenesis and Wave cyclones, as opposed to those which therefore possible cyclogenesis. work downward from upper air charts, normally On a cold front anticyclonically curved iso- progress along the polar front with an eastward bars behind the front indicate that the front is component at an average rate of 25 to 30 knots, slow moving and therefore exposed to fronto- although 50 knots is not impossible, especially genesis. Cyclonically curved isobars in the cold air behind the cold front indicate that the front in the case of stable waves. Both the speed and direction of movement of the surface cyclone is fast moving and exposed to frontolysis. On the warm fronts the converse- istrue. provide good clues as to the nature and vertical Anticyclonically curved isobars in advance of extent of cyclogenesis. It should be noted here the warm front indicate the front is fast moving that cyclogenesis and deepening are not synony- and exposed to frontolysis and with cyclonically mous terms. Deepening very often accompanies curved isobars the warm front is retarded and cyclogenesis, but not necessarily. For example, a exposed to frontogenesis. Figure 5-22 illustrates certain amount of deepening or filling results the frontogenetical conditions around an ideal solely from the diurnal variation of pressure. wave. FG denotes frontogenesis and FL fron- In considering the likelihood of cyclogenesis, tolysis. the forecaster should keep in mind the basic definition of a relative minimum in the pressure field as the intersection of two or more troughs. Whenever a faster moving upper trough overtakes a surface front and its associated trough, cyclogenesisislikely.Thisis an especially important occurrence when a progressive shortwave trough from the Rockies moves over a quasi-stationary front along the Gulf Coast in winter. Wave formationismore likely on slowly moving or stationary fronts than on rapidly moving fronts, and certain oragraphical areas are preferred localities for cyclogenesis. The Rock- ies, the Ozarks, and the Appalachians are examples in North America. AG.476 The most common indications of wave cyclo- Figure 5-22.Ideal wave with indications of frontogenesis genesis are: and frontolysis from the curvature of the isobars. L5V7 AEROGRAPHER'S MATE 1 & C

Some of the major causes for perturbations Hatteras low is a pnme example of this type of along fronts, thatis,the formation of waves, cyclogenesis. are: 5. Winds aloft flowing parallel to fronts at the surface. When winds aloft up to about 700 mb flow parallel to fronts on the cold air side, I. Disturbance in the nearby westerlies aloft. A cyclonic flow or higher wind speeds in the vorticity considerations often lead to the devel- coldairinthewesterliesaloft produce a opment of waves aiong such fronts and the cyclonic shear and therefore tend to set up a formation of new cyclones. cyclonic circulation which builds downward to the surface. The surface low thus formed will The vast majority of cyclones, it has just been continue to develop and exist as long as itis in pointed out, develop as a result of wave accion association with this disturbance in the wester- along fronts. There are additional causes for lies aloft and a depletion of mass or divergence is cyclogenesis; they are as follows: taking place in the system. I.Barrier of cold air. (The barrier theory of 2. Superposition of a jet maximum over a cyclogenesis.) Cyclogenesis may occur when a front. Due to the steep gradient of temperature slow moving mass of cold air blocks a rapidly in the cold air aloft, wind speed gradients are eastward moving mass of warm air by dynamic alsosteeperinthecoldairaloftand the pressure reduction to the "lee" of the cold air. circulation takes on a strong cyclonic shear. This The lee in this case is the return flow meeting shear in turn leads to a cyclonic circulation aloft the advancing warm high. This type of cyclo- which will build downward to the surface. The genesis is similar to that resulting from mountain introduction of the cyclonic circulation in the waves. front causes formation of waves along the front 2. Convection. (The convection theory of and marks the beginning of the life cycle of the cyclogenesis.) The convection theory suggests cyclone. The mechanism for the reduction of that widespread intense convection may cause pressure in the center of the cyclone is identical cyclogenesis as a result of the influx of air at the with that for disturbances inthe westerlies. surface if the convection is of sufficient duration Some meteorologists make no distinction be- and intensity. tween these two causes of cyclogenesis because of their identical function in cyclogenesis. Barrier and convection cyclogenesis do not 3. Mountain waves. These mountain waves contribute greatly to the formation of new lows. often generated to the lee of the Rocky Moun- The last type of extratropical cyclogenesis to tains and the Appalachian Mountains can set up consider is the formation of thermal lows. These cyclonic circulations aloft independent of other lows develop on a large scale over the conti- influences. Mountain waves may be only influen- nental subtropics in summer. The local heating tialin weakening a high, generating an inde- lifts the isobaric surfaces in the air over the heat pendent cyclone, or, as is more often the case, source. This results in an upper level high and an generating a cyclonic circulation on a front in outflow of air aloft and inflow at the surface. their proximity. The Texas and Colorado lows Thesethermallows,althoughtheypossess areprime examples of the role played by cyclonic flow in the low levels, are more akin to mountain waxes in cyclogenesis along fronts. stationary anticyclones in their behavior and in When an independent cyclone is generated, it their influence on the weather. soon draws nearby fronts into its circulation. 4. Coastlines. Rapid changes in friction acting CYCLOLYSIS on the wind can deflect the wind in direction and speed and initiate a cyclonic circulation. When pressure at the center of a cyclone Sudden changes in surface temperatures over increases at a greater rate than the surrounding short distances along the east coast of continents area, the intensity of the cyclonic circulation can cause a concentration of solenoids which decreases. This process is called CYCLOLYSIS. can easily generate a cyclonic curvature. The It can occur even before occlusion begins, in the 154 Chapter 5AIR MASSES, FRONTS, AND CYCLONES case of a stable wave, and always takes place in kinetic energy of the surrounding air and the mature occlusions.In the case of frontal cy- energy from the latent heat of condensation. clones, frontolysis also occurs during cyclolysis. The kinetic energy of the surrounding air is Since air mass contrasts are least near cyclone derived from the concentration of air over a centers, frontolysis of occluded fronts proceeds smaller area as a result of horizontal convergence from the center outward. toward the center and a resultant increase in the wind speeds. ENERGY OF CYCLONES The latent heat of condensation is released when the moisture in the air condenses as a Cyclones transform a great deal of potential result of lifting.Itisa large contributor in energy into kinetic energy in the atmosphere. extratropical cyclones and the major source of Their greatest source of energy is the potential energy in tropical cyclones. energy of horizontal mass distribution; that is, the energy of air masses of different temperature lying side by side. When the frontal slope is not TYPES OF WAVE CYCLONES an equilibrium slope, potential energy exists. This energy is converted to kinetic energy as the The change in wind velocity along a hori- air masses try to adjust themselves to equilib- zontal axis perpendicular to the direction of rium. The solenoids aretilted and therefore flow iscalled horizontal wind shear. The six accelerate the circulation both in the vertical possible combinations of vector pairs which and inthe horizontal. This acceleration con- involve shear are illustrated in figure 5-23. tinues as long as the favorable solenoidal field The upper row shows the three pairs which exists. Cyclones thus increase in energy until give cyclonic shear, while the lower row shows they reach maximum intensity at occlusion. the opposite or anticyclonic shear combinations. Two other major sources of energy in devel- Needless to say, only the A, B, and C types are oping cyclones vie for second place, depending possible in frontal troughs. upon the origin of the air masses, the moisture There is some doubt as to whether type C content, and their relative stability. They are the wave cyclones actually occur, especially in middle

A B C

A' B'

AG.477 Figure 5-23.Six possible combinations of vector pairs involved in shear.

149 155 AEROGRAPHER'S MATE 1 & C latitudes where synoptic networks are dense Of less frequent occurrence, but fairly com- enough to make positive identification possible. mon, is the type A wave cyclone. This type The most likely areas of formation of type C cyclone is illustrated in figure 5-25(A). wave cyclones would be the trailing ends of polar fronts in the subtropics and the Arctic front. Winter is therefore the only season when (A) such disturbances couldoccur. Cyclones do develop out of type C shear, but they are ..------/-*-----, nonfrontal.Tropicaldisturbancesassociated with waves in the easterlies are of this type. Type C wave cyclones, if they occur, would never occlude, since their life cycle must be short and their closed circulations, if any, are of limited extent. The most common, called the classical wave cyclone, is type B. Its life cycle as illustrated in (B) figure 5-21 shows the occlusion process. This cyclone often moves, deepens, and occludes with great rapidity. It is most likely to exhibit the class:cal frontal characteristics. Figure 5-24 shows anidealized model with cyclonically curved isobars in the, cold air mass and straight parallel isobars in the warm sector.

AG.479 Figure 5-25.Type A wave cyclone. (A) Typical isobaric configuration; (B) typical appearance of type A wave cyclone in a synoptic situation.

It moves more rapidly than the type B wave but seldom deepens or occludes. Its cold air isobars are anticyclonically curved and its pressure field, as a whole, is symmetrical. Fronto- genesis along the coldfront and frontolysis along the warm front are common(note isobaric curvature). This type of wave cyclone normally occurs in the synoptic situation, shown in figure 5-25(B) as the second or third member of a AG.478 frontal cyclone family. Rapid cyclogenesis often Figure 5-24.Idealized type B cyclone. occurs when the wave overtakes the occluded The warm front generally increases in intens- system ahead of it. ity while the cold front becomes more diffused. The isobar kinks, especially alorig the cold front, OCCLUSION OF UNSTABLE are slight. The warm sector is quite homogene- WAVE CYCLONES ous with respectto temperature. The most frequent modifications and exceptions are dis- The Polar Front Theory and the resultant cussed later in the chapter. occlusion of unstable waves was discussed in a

150 156 Chapter 5AIR MASSES, FRONTS, AND CYCLONES

previous section of this chapter. The position, over the warm sector. The thickness lines are orientation, and season of the year have a great nearly parallel to the sea level fronts, and their bearing on whether the cyclone will occlude or strongest gradients lie at some distance on the remain a stable wave. cold sides of the fronts. The 500-mb contours The general orientation of the frontal system show a pronounced ridge just ahead of the sea along which awave cyclone moves usually level low center. A belt of maximum 500-mb determines whether or not it will occlude and if geostrophic winds, which appears in the 500-mb so, what type of occlusion is most likely to contour pattern, coincides with the strongest form, provided the underlying surface is uni- concentration of 1,000-500 mb thickness lines; form; that is, all ocean or all land. that is,itlies on the warm side of the frontal In figure 5-26, the wave on the left is unlikely zone at 500 mb. to occlude atall, the one in the center will A vertical cross section of thickness along line become a cold type occlusion if it occludes, and X-Y is given in the middle portion of figure the wave moving poleward is apt to become a 5-27. For simplicity, the 1,000-mb surface is warm type occlusion. represented as a horizontal line. There is very little thickness gradient above sea level in the warmsector. Strong thickness gradients are observed between sea level and 500-mb locations of both cold and warm fronts. The lower portion of figure 5-27 illustrates the height variations above sealevel of the 1,000-, 70G, and 500-mb surfaces. The trough and ridge axes slope northwestward with height. A strong 500-mb contour gradient appears in the AG.480 zone bounded by the positions of the front at Figure 5-26.Likelihood of occlusion in relation to orientation of the wave cyclone. 1,000 and 500 mb. In general, frontal slopes are steepest, hence Type A waves, if occlusion occurs, would thickness gradients are greatest near the warm show very little contrast across the front, with sector apex. slightlygreater change of colder air tothe westward. However, actual data may occasionally show conditions to the contrary and the actual SECONDARY CYCLONES observations should take precedence. Occlusions are discussed in detail in chapter 6 The second and younger member of a family of Aerographer's Mate 3 & 2 and some of the of cyclones on a polar front is called a secondary characteristics of occlusions are discussed later cyclone. According to our previous classifica- in this chapter. tions of types A, B, and C, the first two were discussed in a previous section of this chapter RELATION OF CYCLONES and are classifiedas secondary cyclones. An- TO UPPER MR FEATURES other type of secondary cyclone was named by its Norwegian discoverers for the area where it Figure 5-27 illustrates a typical model un- was most likely to occur. This type is known as co.:hide,: wave cyclone in relation to the thick- the Skagerrak. It was originally thought to have ness lines, upper troughs, and upper contours. been caused by the orography of the southern The simple model of a warm sector frontal Scandinavian peninsular. Similar occurrence at depression is given inthis figure. The upper other orographically preferred areas of the world portion rcpiesents the 1,000-500 mb thickness have further confirmed this theory, but itis field and the fields of the 1,000 and 500 mb known to occur even over the open ocean areas contours. A pronounced warm tongue of thick- where some of the factors must be operative. ness is associated with the warm sector depres- Typical stages in the process are shown in figure sion, and the thickness lines are widely spaced 5-28.

15I 157 AEROGRAP!IER'S MATE 1 & C

1000 500 MB THICKNESS LINES 500 MB CONTOURS 1000 MB CONTOURS

/Y 41-1A,"gide

THICKNESS AXIS OF 700 -500 MB THICKNESS C010 --..TROUGH THICKNESS PP

1 1000-700 MB . THICKNESS 1000 MB SURFACE Y

GEOPOTENTIAL N 500 MB r 18,400 A 1- 18,200 I- 18,000 700 MB AXIS_ r +200 0 %000 t49 SEA LEVEL -200

AG.481 Figure 5.27. A simplified 3-dimensional model of a wave cyclone showing relationship of upper air features.

152 158 Chapter 5 -AIR MASSES, FRONTS, AND CYCLONES

(I) (2)

(3) (4)

AG.482 Figure 5-28.Stages in the development of a Skagerraking cyclone.

Prin ipal surface indications of this type of operative. In some cases, a rapidly ino cyclogensis are: A wave which overtakes the slow movin,:,oc:_lt,- sion may bethetriggering mechan:In for I. Spreading of isobars north of the peak of a cyclogenesis. warm sector. Whatever thr! exact nature of its stt 2. Large negative tendencies near the peak, type of cyclogenesis proceeds with especially in the warm air. ity. The new occlusion forms inimetliato").. ,end 3. Diverging isobars in the warm sector. soon overshadows its predecessoi ui 11, ,t ai 4. Increasing intensity of precipitation near andintensity.However, theWkioo i:on. the peak, having greater vertical extension, exert., ,1 iertain 5. Increasing cross-isobaric flow around the control on the movement of the new c.entk. peak, which at first follows the periphery of the old center. Later, the two centers pivot cyclonically Orography, without a doubt, plays a great about a point somewhere on the axis Joining role in certain preferred areas of Skagerraking, them until the old center has filled awl loses its but over the ocean some other factors must be separate identity.

153 159 AEROGRAPHER'S MATE 1 & C

Skagerraking can take place with either the Data given are to be used only as a general warm or cold type occlusions. If it occurs near a guide and principally in areas where reports are west coast in winter, there is a good chance the sparse. Actual conditions may deviate greatly new occlusion will be of the warm type. from these averages. This table applies only to wave cyclones which are occluding. Nonfrontal WARM SECTOR TROUGHS cyclones Skagerraks behave in an entirely differ- One of the most common ways in which ent fashion. frontal cyclones differ from the idealized model The usefulness of this table is evidenced by is through the presence of a nonfrontal trough in thefactthatif one or two of thelisted the warm air. Two examples are illustrated in characteristics are known, then the table may be figure 5-29(A) and (B). used to fill in the mission features. The front in (A) can be quite intense, but the For an example of the use of this table, cold front in (B) is generally weak and diffused assume thatalow-pressure center has been since the prefrontal trough, is more marked than moving in a northeast direction at about 25 the frontal trough. The trough in (A) can on knots for the past 18 hours. The table shows occasion contain a cold front, in which case its that it should be considered an occlusion with extension north of the rrincipal front should be the central pressure somewhat under 1,000 mb. carried as an upper cold front. Cyclones in low latitudesinparticular, commonly have pre- THE COLD FRONT frontal troughs of the type shown in (3).Although most of the definiteexamples of this type Cold fronts usually move faster and have a ofwavecycloneare foundinthe dense steeper slope than warm fronts. Cold fronts synoptic networks of populated areas, there is which move very rapidly have very steep slopes no reason to believe that they occur only over in the lower levels and narrow bands of clouds land. Here, close attention to indications of a which are predominant along or just ahead of prefrontal trough must be given when making an the front. Slower moving cold fronts have less oceanic analysis. steep slopes and their cloud systems may extend far to the rear of the surface position of the SUMMARY fronts. Both fast moving and slow moving cold Table 5-1presents average values of certain fronts may be associated with either stability or numerical characteristics of various stages in the instability and either moist or dry air masses. life cycle of an unstable wave cyclone. The typical characteristics of the fast and slow

(A)

AG.483 Figure 5-29.Two examples of a trough in the warm sector. (A) Cold front oriented E-W; (B) cold front oriented NNE-SSW.

154 160 Chapter 5AIR MASSES, FRONTS, AND CYCLONES

Table 5- 1. Numerical characteristics of the life cycle ofan unstable wave cyclone.

Wave Cyclone OcclusionMature OcclusionCyclolysis Time (hours) 0 12-24 24-36 36-72 Central Pressure (mb) 1,012-1,000 1000-988 984-968 998-1,004 Direction of Movement NE to SE or NNE to N N to NNW (toward) (quad) (arc) (arc) c Speed of Movement 30-35 20-25 10-15 (knots) 0-5 The symbol indicates that the fillingcenter drifts slowly in a counterclockwise directionalong an approximately circular path about a fixedpoint.

moving cold fronts are discussed in detail in The type of precipitation observedis also chapter 6 of AG 3 & 2. We are concerned here dependent upon thestabilityand moisture chiefly with the upper air characteristics of these conditions of the air masses. fronts. The ceiling is generally low with the frontal Both slow and fast moving cold fronts de- passage, and gradual liftingis observed after velop when cold air bulges southward along the passage. Visibilityis poor in precipitation and frontal boundary. The inclination of the frontal may continue low for many hours after frontal slope depends chiefly upon the wind direction passage as long as the precipitation occurs. When and velocity arrangement across the front. the cold air behind the front is moist and stable, a deck of stratus clouds and/or fog may persist SLOW MOVING COLD FRONT for a number of hours after frontal passage. Characteristic Upper Air Features The slope of this type front is usually in the TEMPERATURE.The temperature inversion neighborhood of 1:100 miles. Near the ground on this type front is usually well marked. In the the slope is often much steeper due to surface precipitation area the relative humidity is high in friction. both air masses. Farther back of the front, The type of weather experienced with this subsidence may occur, giving a second inversion frontis dependent upon the stability of the closer to the ground. warm air mass. When the warm air mass is stable, Upper air contours are usually parallel to the a rather broad zone of altostratus and nimbo- front as well as the mean temperature (thickness stratus cloud systems follow the front by several lines). The weather will usually extend as far in hundred miles. If the warm air is unstable (or back of the front as these features are parallel to conditionallyunstable),thunderstormsand it. When the orientation changes, this usually cumulonimbus clouds may develop in this cloud indicates the postion of the upper air trough. bank and may stretch for some 50 miles behind WINDS.The wind usually backs rapidly with the surface front. These clouds all form within height (on the order of some 60 to 70 degrees the warm air mass. In the cold air theremay be between 950 and 400 mb), and at 500 mb the some stratus or nimbostratus formed by the wind direction is inclined at about 15 degrees to falling rain but generally outside the rain areas the front. The wind component normal to the there are relatively few low clouds. This is due front decreases slightly with height, and the to the descending motion of the cold air which componentparalleltothefrontincreases produces, at times, a subsidence inversion some rapidly. The thermal wind between 950 and 400 distance behind the front. mb is almost parallel to the front.

155 161 AEROGRAPHER'S MATE 1 & C

Surface Characteristics frontalpassage. Isobars may be curved anti- air. This type front The pressure tendency associated with this cyclonically in the cold uv.ally moves at an average speed between 10 type frontalpassageisusually indicated by either an unsteady or steady fall prior to frontal and 15 knots. passage, while the rises behind this type front Figure 5-30 illustrates the typical characteris- areweak. Temperature and dewpoint drop tics in the ve:.ical of a slow moving cold front sharply with the passage of a slow moving cold (upper half) and typical upper air flow in back front. The wind veers with the cold frontal of the front and accompanying surface weather passage, reaches its highest speed at the time of (lower half).

CIRRUS CIRRO- , - STRATUS POSSIBLE ALTITUDE FT CUMULONIMBUS ,- 15,000 TEMP CURVE

FRONTAL 10,000 INVERS!ON I /"NeTh \ AS 1...) Al:FOCH/AMOS, db 0°C NIMBOSTRATUS 5,000 MND PRECIP. IPMEA4tf AREA

COLD AIR

POINT A

200 150 10 0 50 50 100 NILES

AG.484 Figure 5-30.Typical vertical structure of a slow moving cold front with upper windflow in back of the front.

156 16Z Chapter 5AIR MASSES, FRONTS, AND CYCLONES

Since thisis only one typical case, many resultant adiabatic warming, the temperature variations to this model can occur in nature. change across the frontis often destroyed or may even be reversed. A sounding taken in the FAST MOVING COLD FRONT cold air immediately behind the surface front The warm air above the frontal slope is wouldindicate only one inversion with the moving faster than "-cold air. This type front characteristic inversion and an increase in mois- is characterized by a narrow band of weather ture through the inversion. Farther back of the associated with the downslope motion of the front, a double inversion structure would be in warm air above the frontal slope. The descend- evidence. The lower inversion would be due to ing air is warmed adiabatically and the tempera- the subsidence effects in the cold air. This is ture contrast across the front is increased, thus ofttimes confusing to the analyst as the subsid- strengtheningthefrontal discontinuity. This ence inversion is usually more marked than the front has an average slope of 1:40 to 1:80 and frontal inversion and may be mistaken for the moves with an average speed of about 25 knots. frontal inversion. This is the most important type of cold front. WINDS.In contrast to the slow moving cold If the warm air is moist and unstable, a line of front, wind above the fast moving cold front thunderstorms frequently develops along this exhibits only a sl:ght backing with height on the type front. Sometimes, under these conditions, a order of some 20 degrees between 950 and 400 line of strong convective activity is projected 50 mb and the wind direction is inclined toward the to 200 miles ahead of the front and parallel to front at an average angle of about 45 degrees. it.This may develop into a line of thunier- The wind components normal and parallel to the storms called a squall line. On the other hand front increase with height, and the wind compo- when the warm airisstable, an overcast of nent normal to the front exceeds the mean altostratus clouds with general rain may extend speed of the front at all levels above the lowest over a large area ahead of the front. If the warm layers. The thermal wind for the 050-400 mb airisverydry,littleor no cloudinessis layer has an average angle of about 30 degrees to associated with the front. The front depicted in the front. the following sections is a typical front with typical characteristics. Surface Characteristics Weather Cumulonimbus clouds are observed along and Pressure tendency falls ahead of the front, just ahead of the surface front. Stratus, nimbo- and there are sudden and strong rises after the stratus, and altostratus may extend ahead of the frontal passage. If a squall line lies some distance front from the cumulonimbus. These clouds ahead of the front, there may be a strong rise may extend as much as 150 miles ahead of the associated with its passage and a shiftin the front, butthey are generally the altocumulus wind. However, after the influence of the squall and stratocumulus type. The clouds just men- line has passed, winds will back to southerly and tioned are all in the warm air. Generally, unless pressures will level off. The temperature shows a the cold air is unstable and descending currents fall in the warm air just ahead of the front due are weak, there is a lack of clouds in the cold air to evaporation of falling precipitation. Rapid behind the front. Showers and thundershowers clearing and adiabatic warming just behind the occur along and just ahead of the front. The front tend to keep the cold air temperature near ceiling is low only in the vicinity of the front. that of the warm air. An abrupt temperature Visibility is poor in precipitation but improves change usually occurs far behind the front. The rapidly after passage. dewpoint and wind direction are better indicators of the passage of a fast moving cold front. Upper Air Features The wind veers withfrontal passage andis strong, gusty, and turbulent for a considerable TEMPERATURE.Owing to the sinking mo- period of time after passage. Dewpoint also tion of the cold air behind the front and the decreases sharply after passage.

157 163 AlR(A.R \PHI \IAll I

ALTITUDE FT 25,000 CIRROSTRATUS

20,000

15,000 WIND SPEED FRONTAL ALTOCUMULUS INVERSION

10,000 118SiDENCE 0°C i4VERSION

PRECIP. 5,000 CUMUL N1MB S AREA STRATOCUMULUS CUMULUS

1 4 1 200 150 103 50 0 50 !no

UPPER 'It-- WIND FLOW

AG.48F Figure 5.31.Typical vertical structure of a fast moving cold front with upper windflow across the front.

Figure 5-31 illustr, le, a vet tkal %Joss setion OMR UPPER AIR CHARACTERISTICS of alastmoving coldtrontwithiesultant OF (DL!) FRONTS IA cattier. Also indicated in the lower lull 01 the diagram is the surta,c. ,v,ather at a point a short ( Old on upper air arts are character- distance in Jth Mkt: of Oil front and upper ited by a pa...I,ing ot isothcrins behind them. The airflow above the trout more4..losel paked the isotherms. and the

1.64 ;AA Chapter 5AIR MASSES, FRONTS, AND CYCLONES

FRONTAL ZONE

WARM mT

WEST EAST

AG.486 Figure 5-32.Upper cold front.

more nearly they parallel the fronts, the stronger for several hundred miles over the midwestern the front. When isotherms cross a front, they plains. kink toward the cold air. Itis more common for a cold front on the SQUALL LINES AND surface to lie ahead of the upper air trough. This INSTABILITY LINES is illustrated in figure 5-31. A squall line is a line of active showers and UPPER COLD FRONTS thundershowers that roughly parallels the cold front and generally occurs in the warm sector of Itis generally recognized that there are two wave cyclones. This type usually develops along types of upper cold fronts. One is the upper cold or ahead of fast moving cold fronts. The term front associated with the warm type occlusion squalllineisoften confused with the term which will be taken up later in th:, chapter. The instability line. Although the two are often used other occurs most frequently in the area just interchangeably, their meaning has been defined east of the Rocky Mountains in winter when mP by agreement of the Subcommittee on Aviation air crosses the mountains behind a cold front or Meteorology of the Air Coordinating Commit- behind a trough aloft. Usually a very cold layer tee. Their definitions are as follows: of continental polar airis lying next to the ground over the area east of the mountains. I. INSTABILITY LINE. This isa line of Warm maritime tropical air moving northward incipient, active, or dissipating nonirontal in- from the Gulf of Mexico has been forced aloft stability conditions; itis an analytical term for by the cold cP air. When the cool mP air flows indicating primarily the incipient and dissipating over the mountains, it forces its way under the stages of nonfrontal squall line phenomena and warm mT air aloft and flows across the upper for the sake of continuity also includes the surface of the cP air just as if it were the surface active squall line stage. It is frequently found in of the ground. All frontal activity in this case the warm sector of an extratropical cyclone, and takes place above the top of the cP layer. Refer unlike a true front the instability line is transi- to figure 5-32 for an example of this type of tory in character, usually developing to maxi- front. mum intensity within a period of 12 hours or Weather from this type of front can produce less and then dissipating in about the same extensive cloud decks and blizzard conditions length of time.

159 165 AEROGRAPHER'S MATE 1 & C

2. SQUALL LINE. A squall line isa line of shifts cyclonically with passage. However, it' the activethunderstorms or squalls which may zoneisnarrow, the wind shift may not be extend over several hundred miles.Itis the noticeable on surface charts. There is generally a phenomenon of the mature and active stage of large drop in temperature due to the cooling of instability line development and ma} be eithera the air by precipitation. Pressure rises after the solid or broken fine of numerous thunderstorms passage of the squall line, and at times a small accompanying vertical motions of a greater micro-high may form behind it. The cold front order of magnitude than is usually present in the has little weather or clouds associated vith it. atmosphere. After passage of the squall line, the wind will back to southerly before the cold frontal pas- NOTE: The term instability line, as defined sage. above, is the more general term and includes Squall lines move along in advance of the cold squall line as a special case. The two will be used front at a speed at times exceeding that of the interchangeably in this section of the chapter. associated coldfront. A rough guide toits In this section general characteristics of squall direction and speed can be obtained from the lines occurring in the warm sector in advance of 500-mb winds. It will move approximately 40 fast moving cold fronts and the more special percent of the wind speed and long the 500-mb case of the instability line not associated with a winds. fast moving cold front over the Plains States are discussed. Vertical Structure Squall lines, develop ahead of fast moving coldfrontsinthe warm sector of a wave Theoretical explanation of the phenomenon is cyclone. This line is located, on the average, stillincomplete, but sinceitis known to be some 100 to 300 miles ahead of the cold front. associated with fast moving cold fronts, the A typical case showing the isobaric patterns is winds at upper levels are moving faster than the shown in figure 5-33. winds in the cold air beneath the frontal surface. A vertical cross section of the squall line in the warm sector is depicted in figure 5-34.

AG.488 Figure 5-34.Vertical cross section of squall line associated with a fast moving cold front.

A typical phenomenon noted in this illustra- AG.487 tionshows tothe east of the squallline Figure 5-33.Typical isobaric pattern associated pseudo-warm frontal weather. with a warm sector squall line. It should be noted that weather associated with the cold front decreases in intensity after a Showers and thunderstorms (sometimes tor- squall line has formed and increases in intensity nadoes) occur along the squall line, and the wind after the squall line has dissipated.

160

166 Chapter 5- AIR MASSES. FRONTS. AND CYCLONES

The squall line is nut .onfinecl to ind component across this ridge of at least 30 the warm sectors of wave 4.1ones. In some knots. The second mechanism is a source or dry instances it has been first id atified behind the airv. Rich the middlelevel wind carries to a cold front which itovertakes and passes as it position w here it can be cooled by precipitation mows out in the warm sector. It .an also extend from above to its wet bulb temperature which poleward of the warm and often retains must be lower than that of the moist air. its identity during the ou.lusion process. This 12. Stronglycurved200-300-nib flows are canleadto the curious situation of a .old unfavorableforsqualllinedevelopment. A occlusion with an apparent upper void front. the straightflow or slightly anticyclonic flowis squall line. more favorable.

Development Squall Line The following conditions are favorable for Figure 5-35 illustrates a generalized, vertical squall line development of this type. cross section of the formation of the formidable Great Plains type squall or instability line not I. The warm sector. or other area of suspi- associated with a fast moving cold front. cion, is watched for development of a trough or The basic requirement for this type of forma- line of cyclonic wind shear andfor actual tion is evaporative cooling of dry air as a result development of thunderstorms. of high altitude precipitation. 2. Coldair advectioninthe middle and An additional requirementisthat surface higher levels. heating be of sufficientintensityto cause 3. Minimum stability indexes are usually zero thermal currents to transport moisture from or less inthe area of squall line formation. lower to higher levels. However, the line may form east or north and Formation of this type squall or instability downwind some 100 to 300 miles from this linerequires the following conditions to be area. fulfilled: 4. Maximum dewpoint areas at the 850- and 700-mb levels should be noted becsoas.. I. Cold moist air advection. usually at 14,000 lines form in these areas t when other .onditions feet msl or higher but based lower than 20,000 are favorable) some 90 percent of the time. feet (top of fig. 5-35). 5. A favored position for squall line forma- 2. Cooling of the layers below the advection tion is on the west and central portion of moist levels by evaporation of rain or cloud particles and warm tongues. (middle of fig. 5-35). 6.Increasing moisture at low and high levels 3. An increase of surface temperature, usu- with a midlevel dry source upwind. This permits ally by insolation, to the point that will cause evaporative cooling. convective currents to reach the condensation 7.Low-level convergence and lifting. level. 8. The most favorable geographical area in 4. Low-level wind convergence to increase North Americaisthe Central and Southern updrafts ( bottom of fig. 5-35). States in late winter and spring. (Farther northward the season runs from late spring through WARM FRONTS .summer.) 9. Warm air advection at 850 nib. Inthis section some of the characteristic 10.In the cyclonic, shear of the jet at $50 ;at). features of the warm front both at the surface 1 I. A mechanism to divert the strong middle and aloft will be discussed. level winds to the surface. This mechanism is believed to produce an autoLonveLtive lapse rate CHARACTERISTICS OF WARM FRONTS by rapid evaporation of moisture in a mixing zone.Itusually ..onsists of a moisture ridge The characteristicsof a warm frontwill oriented at a large angle to the windflow with a depend upon a number of factors. One is the

161 167 AEROGRAPIIER'S MATE 1 & C

COLO, MOIST AIR

WARM, MOIST AIR

COLO, MOIST AIR

\...... /--,...... 0A--...... 0 `s....,...-

DRY AIR

COLO, MOIST AIR

AG.489 Figure 5-35.Formation of Great Plains squall line (not associated with fast moving cold front). condition which existed prior to the lifting of Movement the warm air; the stability of the air mass will determine the type of cloudiness that will form. Warm fronts move slower than cold fronts. The moisture content and the speed ofmove- Their average speed is usually between 10 and ment of the two air masses are also determining 20 knots. The rule for determining the move- factors astothetype and severity of the ment of warm fronts states that a warm front weather. will move with a speed of 60 to 80 percent of the component of the geostropic wind normal to Slope the front in the warm air mass.

The slope of the warm frontisusually Weather somewhere between1:100 and1:300, with occasional fronts with lesser s:opes. Therefore, The weather associated with typical warm warmfrontshavecharacteristically shallow fronts is given in chapter 6, AG 3 & 2. slopes which are due to the effect of surface The amount and type of clouds and precipita- friction which retards the frontal movement tion vary with the characteristics of the air near the ground. There is a gradual veering of masses involved. Three situations are described the wind through the frontal zone. in the following paragraphs:

162 168 "41 III 1, !'1\lti

I. k Ilk%) r,ioi i-, trituno Ow! alltlNtabk, 11,1i-0, lightto \ R. V.I I itIS I It S

1)..1.e, 111 ", \ I 1 \. I %. ClOtidN ion stirrace. I T411.,'1 the 1,4111 \t hen ills'' ,041 t` !t."1 0.01.111' 'IPp. 10::(0111q!,k Oh' iNIn the e, t \ LUT1.1. 2.\k ,. :111 LAI% Ittt ,kitT ,t ( r\ion thunder .torlitf r ; l'''ilt r,olir 0;11\ C.1Nes..II; sari 1, um: title to the .3.1,k hen i,. ttufhtilence. 11411e1 intiNt condeqs.,tt k 45, \ Mt :1,e'r ttiee Ill t

alto.tratti, ppct It preeipitsitit I .tai* ; e. Old to th,: Irt)ot ant, ,how ahead 1 troni.I he -aronger the lott in thi, tl1. tront.I he is 1,:kitig (.ci,t front Surface I eatmo. in.,styli; ,); 1111,4:111- I ie 01,)1 .1111`11,%,..:114...1!.,)11 not pro, 11,,;1/'I'1.1) stm"etit.it ,.11 chart I t i.usual!,. put- %Int t 120\ IS oti .1: it's 00911 en- 2I ; r 11;1,. r, -,eltiOn1 eN,,,ert r ;.11 141.1,. the in': t)rin,i- ,, (411,k.,.INC tt'hk,11 hhe w ; x.karnitr,n1T-. ' 1, ,Iirill! kriic,ith ,tit. 1,1.:,11 1, ,I19)1alt://1.111 and Nt,.1% 'gradient I front.ii 11. 111 to.er.1 thinning hi- . onthe upper t.) I .11, , , .11 ,er1)ilk.1.11-N Upper i it, ! nNina 1tlIA. oi 1 ii li' 111,1 ft 111,4'5 ahead ot the ,. . a front 174,, c't *0? 1), a'1,P, rnitr , older ,i11 141-4.4 f 111-.tt,e ; hetween ;/ I Is iln_ aa tvarin lion( . 0 ti. ; 1 . 1st anI his Is with .1 rdpid k 14) 002P1).0.i,.111311

169 AEROGRAPIIER'S MATE I & C

(A)

FRONTAL INVERSION

TURBULENCE INVERSION

(13)

AG.490 Figure 5-36.Typical upper zir soundings associated with a well-marked warm front. (A) Well-marked inversion; (B) frontal and turbulence inversions.

Mountains in winter. Warm air atheLtion is more occlusion. When the air behind the cold front is rapid and precipitationisheaviest where the colder.itwill push in under the cool air in steeperslopeisencountered.Pressurefalls advance of the warm front and produce a cold rapidly in advance of the upper warm front and front type occlusion. Vertical cross sections of leveis off underneath the horizontal portion of these two types of occlusions are contained in the front. chapter 6, AG 3 & 2.

OCCLUDED FRONTS COLD TYPE OCCLUSIONS

The structure of the occlusion depends upon Formation the temperature difference between the cold air in advance of the s:'stem and the cold air to the Cold front type occlusions form when the air rear of the system. If the air in advance of the ahead of the warm front is less cold than the air warm front is colder than the air to the rear of behind the overtaking cold front. When the cold the cold front, when the cold air of the cold front overtakes the warm front, both the warm front o%ertake.., the warm front. it will ino%e up air behind the warm front and the cool air ahead over this coil air in the form of an upper k.old are lifted by the colder air moving in behind the front,therebyforming a warm front type cold front. Thus, the warm front itself is lifted

164 170 Chapter 5 MR MASSES. FRONTS. AND CYCLONES

WARM AIR

ALTOCUMULU CIRRUS

LTOSTRATUS , 0 FRONT -'" RMFRONT COLD AIR At* COOL AIR ,II/ NIM: 1TR 1 40 STRATOCUMUL vtli

)%11

RAIN

100 150 250 200 100 150 0 50 DISTANCE MILES ( A)

(B)

AG.491 Figure 5-37.Cold front type occlusion.(A) Vertical sturcture; (B) horizontal structure. by the undercutting cold front. and it becomes 5-3703). Figure 5-37(A) shows the cold front This frontis seldom type occlusion in its initial stageswhen the cold a., upper warm front. air. Figure delineated on surface charts due to its close air has begun to underrun the warm would be proximity to the surface front. 5-37(13) shows how this occlusion VERTICAL AND HORIZONTAL STRUC- depicted horizontally on a surface chart. WEATIIER PIIENOMENA. -In the occlusions illustratesavertical TURE.Figure 5-37(A) weather and cross section through the pointsA-Ai on figure initial stages of development, the

165 171 Chapter 5--AIR MASSES, FRONTS, AND CYCLONES

The most reliable identifying characteristics of the warm sector, and bothfronts become the upper front are: progressively weaker and most diffused as they approach the center of low pressure. Note that I.A line of marked cold frontal precipitation the closed isobars arc very nearly circular; this is and cloud types superimposed on warm frontal typical of all mature occlusions undistorted by types ahead of the occluded front. orographic barriers. Itis often noted that after 2. A slight but distinct pressure trough. occlusion the center of lowest pressure tends to 3. A line of pressure tendency discontinui- roll back along the occlusion. ties. The pressure tendency shows a steady fall ahead of the upper cold front aloft and, with Upper Air Characteristics passage, a leveling off for a shortperiod of time. Another slight fall is evident with the approach TEMPERAT!lRE CURVES.The same stateof the surface position of the occlusion. After ments as thoL* made for cold type occlusions passage the pressure shows a steady rise. apply to the warm type as well. UPPER AIR CHARTS.--The warm type occlusion (like the cold type) would appear on The pressure trough and the accompanying approximately the same isobar kinks are often more distinct at the upper upper air chartsat the surface occluded levels.However, one distinct difference does front than they are at appear in the location of the warm ridge ofair front. Typical isobaric patterns, tendency fields, associated with occlusions. The warm tongue, as winds, and weather distribution are shown in evidenced by the 1,000-500 mb thickness analy- figure 5-41. sis, reveals that the warm ridge in the thickness pattern lies just to the rear of the occlusion at the peak of its development. In the late stages of development of both types of occlusions, the warm tonguebecomes quite narrow as it works its way around the surface low center. When this occurs, thereis usually a closed thickness minimum south of the LARGE N sea level low. TENDENCIES MODIFICATIONS OF FRONTS

Up tothistime we have been discussing SMALL t'-) typical characteristics of fronts with little regard TENDENCIES to the modifications they undergo whenpassing over certain types of land areassuch as mountain barriers. Mountain barriers are but oneof thefactorsthat have considerable effect on frontal characteristics. These barriers affect the speed,slope, and weather activity associated with the front. The degree of modification that a mountain barrier has cn afront depends on the size of the barrier, the orientation of the front, and the stability of the air masses in- AG.495 volved. Figure 541.Typical warm' type occlusion on a surface MODIFICATION OF WARM FRONTS

The intensity of the weather along the upper On the windward side of the mountain, the front decreases with distance from the peak of precipitation area is widened and the duration

169 175 Chapter 5 -AIR MASSES, FRONTS, ANDCYCLONES

STATION Y

B A

AG.493 Figure 5-39.Illustration of typical lapse ratechanges in a cold front occlusion.

which it Cold front type occlusions are quite common sented on all charts below the level at in the western parts of oceans inwinter. In intersects the warm front. summer this type is the rulealong the west coast of the United States, Canada, and Alaskawhere Went her air over the ocean air is colder than the warm with warm front the continents. The weather associated occlusions has the characteristics of both warm and cold fronts. The sequence ofclouds ahead WARM TYPE OCCLUSIONS of the occlusion is similar to the sequenceof clouds ahead of a warm front, whilethe cold Figure 5-40(A) and (B) illustrates the vertical front. and horizontal structure of a warm typeocclu- front weather occurs near the upper cold If either the warm or cool air whichis lifted is sion inits early stages of development. Line sometimes A-Al corresponds in each figure. moist and unstable, showers and Figure 5-40(A) shows the structure of the air thunderstorms may develop. Weather conditions change rapidly in occlusions, and areusually masses,andfigure5-40(B) shows how the most severe during the initial stages.However, occlusion would be depicted on a surface chart. higher The occlusion is represented as a continuation of when the warm air is lifted to higher and the warm front. The cold front aloft is repre- altitudes,theweatheractivitydiminishes.

167 173 AEROGRAPIIER'S NIATE I &

CUMULANIMBUS WARM AIR ---____ f- CIRRUS

NIMBOSTRATU

COOL AIR COLD AIR

STRATOCUM LL19OR STRATUS

A RAIN AND 0 50 DISTANCE MILES 100 150 200 250

(A)

(B)

AG.494 Figure 5-40.Illustration of warm type occlusion. (A) Vertical structure; (B) horizontalstructure. Showers and thunderstorms, when they occur. Surface Indications are found just ahead and with theupper cold front. Normally, there isclearing weather after passage of the upper front, but this isnot always The warm type occlusion lies in thesame type the case. of pressure pattern as the coldtype occlusion. 14 Chapter 5--AIR MASSES, FRONTS, ANDCYCLONES

The most reliable identifying characteristicsof the warm sector, and bothfronts become the upper front are: progressively weaker and most diffused as they approach the center of low pressure. Note that I. A line of marked cold frontal precipitation the closed isobars are very nearly circular; this is and cloud types superimposed on warm frontal typical of all mature occlusions undistorted by types ahead of the occluded front. orographic barriers. Itis often noted that after 2. A slight but distinct pressure trough. occlusion the center of lowest pressure tends to 3. A line of pressure tendency discontinui- roll back along the occlusion. ties. The pressure tendency shows a steady fall ahead of the upper cold front aloft and, with Upper Air Characteristics passage, a leveling off for a short periodof time. Another slight fall is evident with the approach TEMPERAT!JRE CURVES.The same stateof the surface position of the occlusion. After ments as thor,., made for cold type occlusions passage the pressure shows a steady rise. apply to the warm type as well. UPPER AIR CHARTS.The warm type occlusion (like the cold type) would appear on The pressure trough and the accompanying approximately the same isobar kinks are often more distinct at the upper upper air chartsat the surface occluded levels.However, one distinct difference does front than they are at appear in the location of the warm ridge ofair front. Typical isobaric patterns, tendency fields, winds, and weather distribution are shown in associated with occlusions. The warm tongue, as evidenced by the 1,000-500 mb thickness analy- figure 5-41. sis. reveals that the warm ridge in the thickness pattern lies just to the rear of the occlusion at the peak of its development. In the late stages of development ofboth types of occlusions, the warm tonguebecomes quite narrow as it works its way aroundthe surface low center, When this occurs, thereis usually a closed thickness minimum south of the LARGE N sea level low. TENDENCIES MODIFICATIONS OF FRONTS

Up tothistime we have been discussing SMALL (\-) typical characteristics of fronts with little regard TENDENCIES to the modifications they undergo whenpassing over certain types of land areas such as mountain barriers. Mountain barriers are but one of thefactorsthat have considerable effect on frontal characteristics. These barriers affect the speed,slope, and weather activity associated with the front. The degree of modification that a mountain barrier has en afront depends on the size of the barrier, the orientation of the front, and the stability of the air masses in- AG.495 volved, Figure 5-41.Typical warm' type occlusion on a surface I MODIFICATION OF WARM FRONTS

The intensity of the weather along the upper On the windward side of the mountain, the front decreases with distance from the peak of precipitation area is widened and the duration

169 175 AEROGRAPHER'S MATE 1 & C and intensity areincreased. Thisis due to surface tendencies and associated weather. slowing down of the front, This caused by trapping situation is common over the Rockies in winter. the cold air between the frontalsurface and the mountain. On the lee side of the mountains, themove- Itisalso due to increasedlifting ment of the front is accelerated. The afforded by the mountains. intensity of On the lee side of the front decreases due to adiabaticwarming as the mountain, the precipitationarea and intensit descends the lee of the barrier. ity are decreased. The frontal activity again increasesas the front moves away The front maymove over the lee side of the from the barrier. mountain for aconsiderabledistance before Wave formation ona cold front may occur reaching the ground again ifthe air on the lee when itpasses over a mountain barrier if a side is extremely cold. portion of the coldfrontis retarded while another portion is not. MODIFICATION OF COLD FRONTS

('nthe windward side of themountain MODIFICATION OF OCCLUSIONS barrier, the precipitation ahead ofthe front is increased due to lifting of the air over the Occluded fronts are affectedin much the mountains. The speed of movement of thefront same manner as cold and warm fronts.Cold is decreased, since the air must pushup over the front type occlusions usually retarding barrier. The frontal slope act as cold fronts, may be and warm front type occlusionsas warm fronts. steepened as it passes over the barrier; andif the Mountain barriers, however, cold front has a higher potential may accelerate the temperature occlusion process in that theyretard the warm than the valley over which itpasses, it may not front so the cold front overtakes touch the ground. In this it more rapidly case it will appear as than if the warm front continuedto move at its an upper cold front, and can be detected by normal rate of speed.

170 176 CHAPTER 6

SURFACE WEATHER MAP ANALYSIS

!,e Aerographees \late, First Class or Chid', SUBDIVISION OF TUE PROBLEM who is given the job of fOrecasting the weather Is faced w ith innumerable problems. \ the num- As a means of approaching the subject of map ber of reporting stations and the area lkAeredby analy sis the procedure should be divided into these stations mere, the success of the mete- logicalsteps.Inthis chapter the following orologist depends to a greater extent uponhis approachis used. These categories are listed ability to evaluate all the data. Because mole below in increasing order of extent and impor- informationis available at the surface, the sea tance. level chart continues to be the basis upon which other analyses are based. 1 he higher level anal- Local Analysis yses, as pointed out previously,aid in constructing a 3-dimensional model of the surface analysis Local analy sis is concerned with the examina- as well as provide charts finother meteorolog- tion of data in a small area or at a fixed spot ical purposes. over a short interval of time such as:

OBJECTIVES OF MAP AN ALYSIS I. Detection of errors in a particular report by comparison with surrounding reports. Hie problem of the analyst IN a multiple one. 2. Location of a front between two adjoining The analyst is faced wi,.1 a chart upon w Inchis reports. plotted a maze of synchronously observed data 3. Elimination of unrepresentative elements whereby he is required to perform the following of a report. 4. Examination of hourly sequences from tasks. airports or 3-hourly sequences from weather I. Delineate by accepted standard sets of ships for indications of frontal passage or of lines andior symbols the current state ofthe inconsistencies between successive reports. atmosphere and how it arrived at that state. Intermediate Analys s 2. Decide w hat particular atmospheric processes are involvedin the production of the The scope of intermediate analysis is com- various kindsofIweather reported. representing prisedprincipallyof thefitting of synoptic the processes where possible by stand models to data covering an area, roughly that of arc! 3-dimensional models. a cy clone of anticy clone. 1 hemodels of ridges, fronts, wave cyclones, cutoff lows, block- 3. Completetheanalysisinthe shortest ing highs, and occluded cyclones are elements of amount of time so thatit mayiro< used for intermediate analysis. However, such details as briefing and forecasting purposes. the kink in an isobar at a front in an occluded

I71

177 AEROGRAPHER'S MATE 1 & C'

cyclone would be an item of local analysis. For instance, it is oftennecessary to reanalyze a Where considerations of intermediate analysis -.hart in the light of later data andevents shown cannot be reconciled with those of local analy- on later charts. sis,intermediate analysis shouldtake precedence. SUMMARY Extended Analysis You can readily see thata knowledge of the The most comprehensive and importantcate- basic meteorological concepts and modelspre- gory is that of extended analysis. It includes all sented in earlier chapters of this trainingmanual time and space relationships among the elements is a natural prerequisite toa thorough, complete, of intermediate analysis. and correct analysis of a weather chart.Given a workable vocabulary of synoptic models,you Time relationships are generally groupedun- should encounter the least difficulty withinter- der the heading of history or historicalsequence. mediate analysis. Local analysis will requirean The termcontinuityisalso used with this understanding mu familiarity with the quality connotation. If ahy one element of extended and peculiarities of reports. You willbecome analysis is to be singled out for precedenceover proficient in extended analysis only afteryou all others, it must be history or continuity. This have acquired a basic understanding ofthe means that no particular weather chart can physical and dynamicprocesses which give rise properly be analyzedinisolationfrom its to the circulation patterns of theatmosphere predecessors. The first and most important step combined with sufficient experience and knowl- in the analysis of any weather chart is therefore edge of synoptic charts. the transfer of the previous positions and intensities of all significant meteorological fronts and pressure systems of tne last analyzed chart to ANALYSIS APPROACH thecurrent unanalyzed chart.Thisisbest accomplished by tracing a line (overalight The surface synoptic weather chartisthe table) with a yellow pencilor a narrow dashed principal working tool of the forecaster.The line with ink, the previous positions of fronts, first step in the analysis of data after it hasbeen troughs, and ridge lines, and by spotting the plotted on the surface weather chart is for the previouspositions and centralpressures of forecaster to be completely familiar withthe closed lows and highs. Placing past positionson quality of the data. Knowledge of theprobable current charts essentially converts time relation- errors and the types of nonrepresentative ob- ships into space relationships, and subsequent servations give the forecaster a better chance of analyses can be carried out in terms ofcon- filtering out errors by the smoothingprocess. sistent displacements. This particular feature of The types of errors thatcan frequently be analysis is extremely important. corrected prior to analysis are: continuingpres- sureerrors from successive reports of ships; Of almost equal importance are therelation- communicationsgarblesorerrors;pressure ships among the various synoptic modelson a tendencies which do notagree with reported particular chart and the geometrical and mete- pressure;and ship winds which are notin orological consistency of analyses atone level agreement with the magnitude or direction of with those at neighboring levels inspace. Only sea swell or have been plotted in the wrong then can a proper 3-dimensional picture of the quadrant of the globe. current structure of the atmosphere be obtained. When all available synoptic reports havebeen correctlyand completely entered and their It should be emphasized that the discussion of quality evaluated, the weather chart isready for relative precedence in this section impliescor- analysis. A number of different recommended rect analyses in each category. Obviously, incor- procedures are available for the analystto use. rect extended analysis should never takeprece- In this chapter, in general, weuse the procedure dence over correct intermediate or local analysis. outlined in the first portion of this chapter.

172 178 Chapter 6SURFACE WEATHER MAP ANALYSIS

EVALUATION OF DATA is most stable. Similar variations are apparent in pressuretendency,temperature, clouds, and There is always the possibility of an error in precipitation. plotted reports because of faulty instrumenta- Temperature varies much less over oceans tion,observation,transmission, or charting. than over land diurnally due to the small daily Also, even though correct, surface reports are change of the ocean surface temperature. In sometimes not representative of the surrounding general,asthedistance from water bodies areaor of conditions aloft. The analystis increases, the average diurnal temperature range therefore required to be constantly on the alert also increases. Precipitation at most inland sta- for errors or nonrepresentativeness; the quality tions and more particularly in the Tropics shows and usefulness of the analysis depending upon a maximum in the afternoon coupled with the his judgement and keeness of perception when maximum of convectional storms that time. interpreting reports. Location errors are rare for At tropical islands, oceans, and coastal stations, landstations wherethestation numberis the maximum precipitation appears at night. printed on the chart. Reports from ships, however, are frequently found to be in error for one or more of the following reasons. Erroneous Detection of Erroneous Data pressuresaredue toinaccuratereading or calibration of the barometer and by either The validity of a report or part of a report applying corrections incorrectly or by failing to becomes suspect when itis inconsistent with apply the needed corrections at all. Errors in nearby reports (in case of dense synoptic networks), containsinternalinconsistencies, or transmission by radio, errors caused by differ- leads to marked or unlikely changes in con- ence intime, and frequent errors of ship's position by 5° to10°, either inlatitude or tinuity or history (in areas of sparse data). longitude. A good remedy for the position and Thefirsttwo cases involveprincipallya other type errors from ships is to keep a running problem of local analysis. The reports should be log of reports from merchant ships. On the log, compared with neighboring data, exercising care successive reports from any given ship may be that comparative data are observed at the same plotted side by side for ready comparison. altitude and over the same type of underlying surface. The last case involves an isolated report and requires the application of all categories of Diurnal Variation of analytical tools. If possible the previous 3-hourly Meteorological Elements reports from this station should be consulted to check for continuity. Sometimes in the case of Processing of large numbers of observations an apparently erroneous isolated reportitis for each hour of the day has shown that a virtually impossible to check its validity. How- certainkind and amount of change in any ever, never disregard or discard anisolated meteorological variable can be expected simply report simply because it is difficult to fit into a because of the varying times of day of the preconceived pattern. At least one or more valid observation. This change is called the diurnal logical reasons should exist for not drawing to variation of the elements. There are also con- such a report. Many analysts have violated this siderable seasonal differences in the elements. rule only to find that subsequent charts con- For example, along coastal stations, the onset of firmed the existence of an important meteoro- the sea breeze in the late morning hours and a logical event. subsequent maximum velocity in the early or Climatology can also be used to detect errone- middle afternoon is especially evident at mid- ous reports; for example, snow in the summer or latitude and some tropical stations. Over land, an 80°F temperature inside the Arctic circle. the time of maximum wind velocity is usually Random errors are the most difficult to detect early afternoon when thermal convection has unless they are large, while systematic or re- reached its maximum and the time of minimum peated errors are more easily discovered and is the early morning hours when the atmosphere corrected.

173 1.79 AEROGRAPIIER'S MATE I R C'

Classification and are almost impossible to correct. I lowever, some Correction of Errors stations may make consistent, obvious errors in weather elementvalues and alistof these One an error is detected. the data must not stations should be maintained with the probable be discarded arbitrarily. Some attempt should be correction to be applied to the weather iepoit. made to estimate the proper correction of the Computational errors are more likely in upper data. Alter detecting the en or. y on should make air data than sea level data, since most of the a careful consideration of the possible sources of hitter are directly observed. Exceptions are sea the error. A convenient class;fication of errors levelpressurereductions. dew pointtempera- by source is thefollow ing.errors due to tures. and true wind speed and direction re- encoding. transmission. decoding. and plotting. ported by moving ships. The first two elements errors of observations. and computational error, aregenendlytaken directly[rumtables or in data not directly observed. graphs prepared especially for such calculations Errors in the first group are essentially coi- and the errors a, use principally from the Mt' of ninications errors.Mistakesindecoding or incorrector .,representativeentriestothe plotting are the most easily checked. requiring tables or graphs. Greater reliance should be only a re-examination of the original message. placed on true wind reports from ships known This should be the first step toward correction to carry well-trained observers. such as weather of inconsistent or suspect data. A transposition ships. naval vessels. and large commercial of numerical digitsforthe correct ones can Manysmallervessels are not equipped with occur bothinlandline operation and more anemometers. in which case direct estimates of nly in ('W messages which are in Morse true wind are made using the Beaufort scale to code. Substitutions of aI for a 6. or 2 for 7 can correlatthe state of sea with the wind speed. occur in the CW message if the operator transposed dots and dashes. Frequently. on teletypes REPRESENTATIVENESS OF DATA the neighboring digitissubstitutedforthe correct one. for example a S or a 7 for a 6. Even when all the probable errors in a report These errors are equally probable in any numeri- are accountedfor,parts ofit may stillbe cal group, but only those occurring at positions inconsistent with corresponding parts of other where the units being transmitted are fairly large reports nearby 01 within the same air mass. Such will they be detectable and correctable. Thus. a report is then said to be unrepresentative or errors in the tens digit of pressure or tempera- the property being delineated. Any meteorologi- ture are usually obvious. but similar transposi- cal element subject to purelylocal influences tions in the units digit cannot be identified so such asheatingorcooling,terrain,water easily. Errors of this ty pe are also common in sources. local conve:gence. and the likc. is likely wind direction and ship position reports. to be unrepresentative. Except in the vicinity of As the number of errors or inconsistencies in well-defined currents, ocean surfaces are more a single report increases. so does the probability homogeneous horizontallythan are land sur- that the remainder of the report is also errone- faces, so one would expect reports from ships ous. In the analysis of these, as well as other and small vessels to be more representative than communications errors.,t well-trained, experi- continental reports. This is generally true. but enced, and consciei .ious plotter is of inestima- important exceptions are noted. Similarly, prop- ble valuetotheanalyst. llowever, even an erties of air in the friction layer are more subject experienced plotter should never be allowed to to local influences than are the corresponding change any element ofreport. This is a job for elements of the free atmosphere. Here isthe the analyst. The plotu. should be given instruc- pn..cipal reason for the continual insistence that tions to distinguish these ty pes of data, such as no surface analysis can be considered completed placing a question mark next to the dubious until it has been shown to be geometrically and element or report. meteorologically consistent with corresponding Observational and computational errors of analyses in the free atmosphere. Since surface or most reports received from other weather units sea level data are observed at the base of the

174 180 Chapter 6SURFACE WEATHER MAP ANALYSIS

friction1.1y er. w here lot.al influences are most readily see that an error of only I percent in effective. itis easily possible to find conditions estimating the mean density of the substitute under which each element of the surface report column would therefore result in a 3-nib error in is nonrepresentative. The follow ing section con- sea level pressure. Station level pressure, or sea tains a discussio., of th representativeness of level pressure at a station near sea level, would some of the various meteorological elements. bethemostrepresentativeelement,fthe meteorological report. Sea Level Pressure (PPP) Table 6 -I shows probable errors in pressure observationsthatare used by the National Pressureatstationlevelis,by definition Meteorological Center (NMC). representative of the mass of the air column or unit cross section above the station. Station Pressure Tendency and pressures at different levels must be reduced to a Net 3 Hour Change (app) common reference level if they are to be useful. The most common level for surface charts is sea Ships without abarograph do not report level. Sea level pressure reported by a station not pressuretendency. Land stationsreportthe at sea level is a convenient fiction arrived at by most accurate 3-hourlychangesinpressure, substitutingforthe missing part of the air followed by the stationary ships such as Ocean column a column of nonexistent air with an Station Vessels. Some shipping is subject to estimated mean density and of length equal to erratic course and speed variations which make the altitude of the station. The representa- pressure tendencies and net 3-hour changes seem tiveness of such derived pressure is thus deter- inaccurate. mined by the representativeness of the mass of The characteristic of the change in pressure the substitute air column. Ot viously, the shorter and the amount of the change are very i.npor- the column. the greater isthe chance of its tant clues to developing weather situations. On a having a representative mass. Overestimates of moving ship, the pressure change indicated by the mean temperature of the column in question the barograph is due to the actual change in will result in reported sea level pressures that are atmospheric pressure plus the change in pressure unrepresentative on the low side and vice versa. as the ship moves in relation to the high- and Thus, a mountain station with a temperature low-pressure areas. For example, a ship sailing lower than its neighboring stations will report a eastward at15 kt and being overtaken by a higher sea level pressure. This is also the reason low-pressure system moving at 20 kt would why the intensity of thermal highs and lows in show a slowly falling pressure characteristic. mountain and!meat' areas is, to some extent, Thus, two ships ishe same area might actually fictitious on sea level charts. It is also the reason reportdifferentpressurecharacteristicsand why some other level than sea level, as the changes in the Dsysapp group. The forecaster is reference level, is used for constructing cht.rts in able to correct apparent pressure change to true these areas. pressure change by using the direction of move- Pressures reported by stations at or near the ment and speed of the ship (D,v,), and the reference level are the most representative of all movement of the pressure system. the meteorological elements, although (especiallyinthe case of ship reports) they are Temperature (TT) subject to error. Sealevelpressure reports from mountain Any process or wadition which tends to stations which extend well above the average produce cooling inthe lowest layers of the surrounding terrain should be disregarded in atmosphere will cause a low-level or ground drawingthe sealevelpressurepattern. For inversion, the intensity of which is a measure of example, a station such as Leadville, Colorado, is the resultant unrepreseatati,,eness of the surface 10,158 feet above sea level. Since the pressure at temperature. The most common of such proc- 10,000 feet is about 700 milibars, the correction esses and conditions are (I) nocturnal radiation, to :T.alevelisabout 300 milibars. You can (2) advection over a colder surface, (3) drainage

175 AEROGRAPIIER'S NIATE 1 & C

Parameter Probable Error Remarks

Reduced sea level pressure 4- 0.5 mb per This error results from (Land stations with 1, 000 ft nonrepresentative calibrated mercurial elevation temperature parameter barometers.) in the reduction to sea level computations. Weather ship pressures 1 mb Occasionally a weather ship will leave port with its barometer miscalibrated. Commercial ship pressures + 1 to 2 mb This error is frequently constant with a given ship. Most ships use aneroid barometers, which are less accurate than mercurial barometers.

Table6-1.Errors in pressure observations. of enld air into valleys, (4) snow on the groun..1 Because the amount of unrepresentativeness Is) evaporation from a local watersource. of surface temperature in a warm stable airmass All of these processes except (2) and (5)are may exceed the frontal contrast, the curious ineffective or irrelevant at sea, so temperatures case in v hich the temperature rises following a ire generally more representative at sea than on cold frontal passage can occur. Typical vertical land 1 he prevalence of theseprocesses on land temperature distributions in such a situation are makes temperature the least representative of all shown in figure 6-1. The solid line with ground domeits of the surface report from land sta- inversion represents conditions before the front tions. passed, and the broken line with the higher TnI,le 6-2 summarizes the relative unrepresen- frontalinversionrepresents the postfrontal tatiVelleSS of temperature under various condi- condition. tions.On land.surfacetemperatureisin Frontal temperature contrastscan also be accordance with table 6-2. masked on night and early morning charts if the In winter a snow cover chart should be used warm air mass is clear or nearly cloudless, with an :lid in evaluating surface _emperatures. The light winds, permitting a !narked radiation inver- edge ()I the sum field ()Wu appears in the siontoform. Free atmosphere temperatures temperature field as a pseudo front. shouldthenbeusedinlocatingthe front Two exceptions to the representativeness of discontinuity,for example, on the 850-mb surface temperature at sea should be noted: (1) chart, where this surface is not too near ground. larked ocean currents affect air temperatures Stations at high altitudes with unrepresenta- over (1) near them; and (2) the internal warmth tive temperatures result in incorrect reductions of most modern ships renders theproper ex- or pressure to sea level. !Ugh level stations under posure ()I' thermometers almost impossible. Even clear conditions atnight and early morning withthebestexposure, shiptemperatures report sea level pressures too high, resulting in a appear to be about 1°F too high. fictitious high in the surface analysis. Sucha

176 . 1.8Z Chapter 6 SURFACE WEATHER \1 \1' SIS

Most representative tireprosentat On afternoon maps. On early not nine; maps. *In unstable (cold) air masses. *In stable (war m )air thasst *In areas of strong winds. *In areas of calm or lightinds. Windward side of mountains. Leewardsite of molvtains. On equatorward slopes of hills and mountains.At valley bottoms and lint, *When no precipitation is occurring. 'Durino,-precipitation. In cloudy areas at night. In clear areas at night. On windward sides of lakes. On leeward sides m takes. Over grass-covered areas. In deserts. forests, and sno,/, tields. Statements marked with (I are equally true of ship reports.

Table 6-2.Surface temperature on land.

repr,:sentative. the high eerlier out 'he of Clockwise ,:irt.ttlItion an: generally ..!ri..,tieot

Dewpoint adTd I

Thedewpoint temperoture whenzver the air temperature is..toil $1',.o repreKntative %viten aitemperature i-Jolt Tti;s IN due tothe tactthat dewp, inn unaffected adrab.itte and iobaii_ esr`- adiabatic proc-.:sses ex*:ept the involv Int! ration and etmderhtillon kt Ntations %%rater source or Iterel.erpr,cipitation FRONT ring. kiewpoint tAnperattire Wive. under most Litho ;1v:,' renresentaIRC !halltcniperatiii.: and beused torind fronts thc teal! Contrast is masked. T The dewpoint di,continuit ii'.tenthe ten, means of I ,eating the boundaries ni 3n .t,bati,- AG.496 ing wedge or tongue of m'l an 1 he sc, +Rol Figure 6-1.Illustration of a temperature "dew point frontis generalh. located neat Vic rise with a cold front. 60-degree dewpoint isotherm it v interitoi\.t'rs most often ;01the Gulf Stales and fictitious high should be avoided by referen..e to identified.tierit has turned %kk ; the wind field just above the surfaLe the gulf coast, bec..ia-A.ttmc Aka% layers. The centers of clockwise cirkadation at dew point k.ontrast between a co.istal and ati theselevels(intheNorthernIlemisphere) inlan I station. duet) the s Is istal accurately pinpoint the existence of true high point, vvcskan buondary rf thi .marl, pressure centers. Where pressure reduction, ate dewpoint ,ontrast inI evaond AEROGRAPHER'S MATE I & C

Oklahoma during much of the year, but particu- The thermal deformation of local isobaric lar y in warmer seasons. This north-south line patternsis most marked along coastlines and oscillates from west to east, but rarely moves lakeshores, and results in adding to the repre- east of Dallas. It separates the low dewpoints of sentative wind the so-called sta or land breeze dry LT air over the southwest United States component. Land breeze components are gen- from the moist mT air over the southeast United erally smaller and more normal to the shoreline. States. Even though Itis so persistent as to be An estimate of the representative wind can be almost a climatological feature of this region, 1 t obtained by subtracting the sea or land breeze isnot atruefront. Since fog, drizzle, low component vectorially from the (nonrepresenta- stratus. and restricted visibilities are most likely tive) observed wind, or by consulting wind to occur in this moist tongue, itis a forecasting reports at higher levels. The winds reported from necess:ty to keep track of its boundaries. Shad- small islands often show evidence of sea breeze ing the whole tongue light red is an analytical components. convenience which avoids the crr )r of indicating Local convective activity can affect surface it as a front. winds to a marked degree in the immediate Elevationdifferences between neighboring vicinity because of the extreme local conver- stations in the same ar mass generally show a gence and divergence necessary to maintain or much smaller difference in dewpoint than in air compensate for the vertical motion. temperature. This is due to the fact that the Table 6-3 shows the reliability c,f. wind direc- dewpoint temperature diminishes vertically at tion and speed over land surfaces. only about one-fifth the rate of decrease of air All the factors discussed in table 6-3 affect temperature. both the speed and direction of the wind. There For the reasons outlined above, the dew point is further important correlation between these temperature is one of the riost reliable and two properties of air motion. direction is most useful elements of the surface report. representative when speed is 10 knots or greater. Unless there is reason to believe that it also is unrepresentative, pressure should be given prece- Wind Direction and Speed (ddff) dence over wind direction in drawing isobars in areas of light winds. Ocean wind ,eports from vessels ale in general A physical tool which is commonly used over reliable, but those fron. land stations are ottcn oceans where winds from ships can frequently affectedbylocaltopography, makine them be converted into a gradient wind level is the anrepresen.ative for a short distance above the geostrophic wind scale. This scale is discussed surface. This zone iscalled the friction layer. later is this chapter. Studies at the WC, and by Wind in tht friction layer is much more subject others, show that by increasing the surface wind to local influences over land than over water. speedby one-thirdand adding 20°toits The principa! causes are terrain, vegetative cover, direction,a rood estimate of the sealevel and local heating and cooling. In many places pressuregradientresultsfrom shipreports. terrain permitsairto move only in certain Although this method is not completely accu- directions with respect to its own orientation. It rate, this procedure in estimating the horizunal also acts as a windbreak for points on its lee pressure gradient is a valuable tool in construct- side. Frictional drag varies with vegetative cover. ing analyses over sparse oceanic areas. and local heating deforms the pressure pattern by realining the surface isobars more nearly Present Weather (ww) parallel to the surface Lotherms. For all of these reasons, a chart of winds above or near the top The weather occurring at a stationis most of the friction layer is indispensable to accurate likely to be representative if it extends over an surface analysis over land awas. Except in very appreciable length of time, but even intermittent mountainous areas, the winds .;,000 feet above precipitation can be scattered thoughout an air thesurface (not sea level) have been found mass and hence would be representative. Once adequate. the possibility of error has been eliminated, it is

178 184 Chapter 6 SURFACE wEATinz MAP ANALYSIS

Most representative Unrepresentative In open flat ar as. At coastal or island stations. At mountain or valley stations. Over bare soil. In forested areas. Windward side of orographic obstructions. ,ec ward side of mographic obstructions. In unstable air masses. *In stable air masses. In daytime. At night.

*Near S,p , 1-4; ,etc

Items marked (*) apply equally to winds at sea.

Table 6.3.Reliability of wind data. almost .iiVa)sne,cssary to asNunic that reported distances and instrumental observations at a land present %veat her is representative. station. Over the sea, visibility may be quite Smoke. dust. sand. haze. and fog are often unrepresentative due to the absence of such purelylocalphenomena. and hence can be markers or instrumental means. unrepresentative. Ilowever. fog and dust can be characteristic of large part of an air mass, GENERAL SURFACE ANALYSIS particularly advection foe and the dust storms of PROCEDURE southwestern United States. The time of obserbation. with respect to the There are several factors which influence the diurnal maximum of various types of weather. order in which a map analyst draws the various should also he considered. element:, of his analysis on the surface weather map. These factors include delayed receipt of Clouds (C1,. Cm.Cn I reports. type of map andnpending weather, historical sequrnces available, relative difficulty High and iniddle cloud; are more likely to be of analyses in speeic areas. auxiliary upper level representative than low clouds or clouds with data availal 'e, time required for complet'on, and great vertical development. As in the (-UN: of skill and experience of the analyst, Moreover, present we alter,the diurnal variation of the very often the various elements of the analysis hitter tv pes is important in the evaluation of are drawn concurrently or certain Jcments are their representativeness. sketched in lightly (or mentplly) before other A striking example of unrepresentative low elements are drawn in their final form. clouds is the vves,.-coast stratus, which rarely In general, the various steps and elements of extends inland for any appreciable distance. the analysis are usually carried oui in a manner similar to the following outline: Visibility (vv) I.Indicate in ink (or yellow pencil) at least Onlyrestricted '. )ilities arelikelyto be three previous positions of all centers which are unrepresentative. Su-ii visibilities are associated expected to he found on the current chart. Also, with unreprzsentativprc.,ent weather discussed when available, draw in dashed ink (or } dlow in a previous section. LOA visibilities are not pencil) at least two previous positions of all likely to be the result of errors of judgment, dui; fronts which are expected to be carrico forward to the existence of markers at accurately known to the current chart.

179 185 AEROGRAPHER'S MATE I & C

2. Try todelineate fronts before drawing indicating intensification or weakening of pres- isobars. This is not alw,iys possible, especially suresystems, where data are inadequate to when th. front is weak u- diffuse. In this case. directly determine their intensity. sketch in isobars in the areas where the front is 4. Care in showing the isobaric angle at a most probable. front. especially to avoid violation of data in 3. Draw isobars or continue isobaric analysis attempting to conform to a preconceived model. to delineate highs, lows, and other features of the pressure pattern. All three categories of analysis arc applicable 4. Illustrate by color the sketched-in posi- in the drawing of isobars. Local analysis is used tions of the fronts, keeping in mind that the in interpolating between two reports, neither of color of the front is determined by the instanta- which has the exact value to the particular neous motion of the cold air mass. isobar being drawn and it is also used in deciding 5. Print a red "L" near the center of each which way the isobars are to be drawn around cyclone and a blue "H" at the center of each colstroughs, ridges, etc. Intermediate analysis anticyclone. provides the synoptic models. Extended analysis 6. Label air masses. if appropriate. in the form of history or continuity is used in 7. Color in all present and past weather areas, obtaining an approximate position and general as appropriate. shape of closed centers. Relating cols to closed 8. Draw isallobarsthis is optional and de- centers by drawing intersecting trough and ridge pends upon local policy. lines is also extended analysis. The order may be varied to conform to local requirements and other factors as mentioned ANGLE OF THE WIND WITH THE ISOBARS above. Owing to friction, the wind does not blow ISOBARIC ANALYSIS along the isobars exactly but a little left or across the isobar from higher to lower pressure. At every point on an isobar the barometric Because of friction between the air and sea or pressure, reduced to sea level, is the same. If a land surfaces, the wind will blow across the free air map is drawn on a vertical cross section isobar from higher to lower pressures at an angle and points of equal pressure are connected, a of 10° to 20° over sea; the greater the wind line of constant pressure is found which repre- speedthe greateristhe angle of the wind sents the isobaric surface. The surface is usually direction to the isobar. There are exceptions to not a flat suri'ace but will have hills and valleys. this rule however, especially over land, and you should not insist too rigidly on this rule. Figure Since an accurate surface analysis is an impor- 6-2 illustrates the influence of the earth's rota- tant supplement to upper air data in determining ticm and friction of the wind direction. many details of analysis of upper air charts, the final surface analysis must also reflect general WIND SPEED AND upper air condition SPACING OF ISOBARS Before proceedingwiththeanalysis, the From the discussion of the basic wind theory analystshouldkeepinmind the following in chapter 4, several relationships between wind considerations: speed and isobar spacing are evident: I. Compatibility of the analysis with both I. The spacing of isobars is inversely propor- current did.. and continuity, so that meteoro- tional to the wind speed. Isobar spacing is a logical events portrayed on the current (-hart representation of pressure gradient, and pressure show logical deva pment from previous charts. gradient is directly related to wind speed. Winds Consistency between the actual windlields arestronginareas of large/strong pressure and the movement of iron',, and air masses. gradient and isobars are close together, winds are 3. Regard for reasonable continuity on the weak in areas of small/weak pressure gradient basis of accepted models or experiencein and isobars are far apart.

180 186 Chapter 6SURFACE WEATHER MAP ANALYSIS

HIGH LOW HIGH LOW HIGH LOW

(A) (B) (C)

AG.497 Figure 6-2.Angle of wind direction to isobars. (A) If the earth did not rotate; (B) influence of earth's rotation; and (C) influence of both earth's rotation and friction. For a given wind speed, the spacing be- tN% een isobars decreases with increasing latitude. Table 6-4 shows the spacing of isobars at 4-mb Wind intervalsforgeostrophic wind speed versus speed Approximate distance (in nautical miles) between isobars drawn for latitude. observed every 4 millibars From table 6-4, it may be that at latitude (knots) 40° with wind of 20 knots, the isobars at intervals of 4 millibars should be separated by 30° 40° 50° 60° 179 miles. At 60° with 20 knots, the isobars should be 133 miles apart. 10 461 3. For a given wind speed, the space between 358 301 266 isobars will be greater with anticyclonic curva- 15 307 239 200 177 ture than with cyclonic curvature. With a given 20 230 179 150 133 wind speedandstraightisobars.the space between isobars will be less than with anti- 25 184 143 120 106 cyclonic curvature and more than with cyclonic 30 154 119 100 89 curvature. 35 132 102 86 76 Sometimes in areas where data are scarce, you may havetodraw isobars with verylittle 40 115 90 75 66 information available. Isobars are crowded more 50 92 72 60 53 closelytogetherinareas where the windis strong but farther apart where the wind is light. 60 77 60 50 44 Thisprinciplehelps when observations are scanty as illustrated in figure 6-3(A) and (B). Table 6-4.Geostrophic wind distance between isobars When thereisa large difference of pressure over ocean at 4-mb intervals for various wind speeds between two ships on the chart, several isobars and latitudes. must be drawn.If there are no intervening entries it is a good idea to determine the number of isobars to be drawn and space a series of dots GEOSTROPHIC AND GRADIENT between the ships to use as a guide. If one of the WIND SCALES ships has a considerably stronger wind than the other, the isobars should be more closely spaced Because the pressure gradientis closely re- near the ship with the strong Nind. lated to one of the principal components of the

181 187 AEROGRAPIIER'S MATE 1LC C

radius of curvature of the path of the air parcel istakeninto consideration. Corrections arc made for anticyclonic and cyclonic curvature. One type of scale has as its bivic unit of lengthin construction the degree of latitude. This makes this type scale independent of the scale of aparticular map projection andis equally applicable to all chrrts. The most convenient type scale is one which can be placed on (A) the chart where the wind report is plotted or where wind measurements are to be made. This requires that the wind scale be constructed to fit the map scale, which is constant only on the standardparallelsoftheprojection.Such geostrophic wind scales have been constructed LOW for most meteorological charts, and are either printedin one corner of the chart or on a separate transparent overlay. In most circumstances the gradient wind is a GO better approximation to th .!true wind than is the geostrophic wind. The gradient wind scale cm should,therefore, be more usefulthan the geostrophicscale. butinpracticeitisnot, principallybecause of the complicationsin- (B) volved in determining the radius of curvature of the path followed by the air parcel. Since itis AG.498 such a laborious job to correct for curvature, Figure 6-3.Isobaric spacing. (A)In accordance with qualitative estimates of the CAMla lure correction wind speed; (B) in accordance with the geostrophic are generally made. wind scale. Windanalysisinthefreeatmosphereis somewhat different. Gradient wind scales which, wind, and in fact detz!rmines the general Lliret..- in effet..t, Lompute path t..urvature from contour tion of the windtlow, a powerful additional tool cut-vature and Lontour movements have been is afforded the analyst in delineating the pres- developed for use on upper air charts. sure field. Where wind observations are available, Further details on th,geostrophic, and gra- isobaric analysis is more easily and accurately dient wind problem and various scales developed accomplished than is the analysis of any other for use by the Navy can be found in Meteorolog- scalar quantity of the atmosphere. From Buys ical Wind Scales, NW 50 -I P -55 I. Ballot's law, relating wind and pressure, approximate location of high and low centets can be found without any pressure data what- Use of the Geostrophic Wind Scales ever, if sufficiently reliable wind data are available. Two types of geostrophic wind scales which There are many sizes, types, and shapes of may be used for surface analysis are discussed geostrophic wind scales but all have one thing in here. The first type is the one that is commonly commonthey are either a graphical or tabular printed on the base map: the other is an overlay solution of the geostrophic wind equation. The type. computations made on the;eostrophic wind USE OF GEOSTROPIfIC WIND SCALE scale are for straight isopleths above the fria' )n (PRINTED ON BASE MAP). An illustration of layer. Gradient wind scales are graphical sdu- the Lyt,^of geostiophk wino scale for sea level :ons of the gradient wind equations where ti.e ourfaLe v eather maps is shown in figure 6-4.

182 1 SS Chapter 6SURFACE WEATHER MAP ANALYSIS

GEOSTROPHIC WIND SCALESEA LEVEL SURFACE

100 50 30 20 15 10 7.5 85-

70 MAP SCALE = True at lat 65 ISOBARIC INTERVAL:4MB DENSITY AT SEA-LEVEL

\ 150100 \\. 60 50 40 3530 25 20 15 10 7.5 6 901170 80 75 VELOCITY (KNOTS)

AG.499 Figure 6-4.Use of geostrophic wind scale commonly printed on base maps.

1 o determine the wind speed at a point over curved line from point B, you can then read off the ocean, you would proceed as follows: First, at the base of the scale the geostrophic wind place the edge of a sheet of paper at right angles speed, which in this case is0 knots. If point B to two successive isobars at the place on the of the line representing the distance between weather map where you want to find the wind isobarsfalls between two curved wind speed speed. Mark the distance between the isobars on lines on the scale, you can obtain the wind speed the edge of the sheet and also note the latitude. by interpolation. Suppose the distance between the isobars as If the distance you measured between two marked on your sheet of paper is equal to the isobars on the weather map extended from 35°N line AB as shown at the right in figure 6-4; also to 40°N, for example, you should use the mean that the line AB was at latitude 40° on your of these figures, that is, latitude 37.5° to find chart. Starting at the left of the scale, measure the wind speed. off the distance AB on fline for latitude 40°. Similarly, you can also obtain from the scale It can be seen that point B coincides with a (fig.6-4) the spaci between two isobars if curved line inthe scale. Looking down this both the wind speed and latitude are known. As

183

189, ROGRAPIIER'S NIATE I & C

1st :vs d' .alai's till the wind theabsenceofdatatothe contrary. The 0 nolo and latitutk (40-I as in the frictional effects which cause the cross isobar Stalii,11.;atthe bottom of the flow also reduce the speed of the observed wind up trl,1 line marked 10 knots as compared to the gradient wind. So many 0,1111 latit,,,k10 line, point B. factorsareinvolvedasto make any really \ , the ,orred sFaLing for accurate estimate of frictional effects on wind ,,011 as .it Lona& 40' fora wind speed virtually impossible. Followingthe t.,1tot, practice used by the NMC, stated previously, It important to remembei that the will be helpful in determining gradient wind sail alas shown in figure (-4 is speed and direction over oceans. it .'.xopl and should not be used tbr 11:C2 ;'4,1 0.!,..k1 and the spacing of isobars FLAT MAPS stirw...a0:0- charts You should use the tl..a, printed on Nom map On the flat map, winds are light and variable

' . , ,1111 isobarsforthesame and pressures in certain regions, where isobaric " Sk.die analysis is attempted, are relatively uniform. The IN1) SCALE wind movement is irregular with no systematic

+ I Pi 1 I nt typewind scale differences between barometric pressure read- ; ,,, I, ut thost presented ings to the right or left of the wind. When the tl.ttt,;,.1 A Male,. \W 50-1P-551. map, or any large section of it,isflatitis 13k ittu ,.aid d \ppelldlk helpful to study the previous map prepared 12 A . t' aociyou dil obtain wind se,,les or 24 hours earlier. A well-defined cyclone on h 1 ainb,rt Confounal, the previous map islikelytopersist,ina .ffid\Icr,atr Pt'1{t*.tt, a WON Iwo tangs are modified form, and may be possible to identify ti Ai& NtathLird hap stale, one for within the flat section of the current map. Also, 111t other tom Lipper air winds. keep in mind that there are certain permanent or ,,Ne this type ,tale tot suriat,t winds, select semipermanent features of the pressure distribu- trim `o..al. 101filart;ptand map tion and wind circulation of the oceans such as givenas dude. plat the latitude the Aleutian Low and the Azores High. Even numbei from Ow t., ad on an ',ohm line. and when the map isflatitis usually possible to tul,.ltfist'nuinbcr iniilibars between this trace wind circulation and the characteristic, X interpolate where though slight, variations that exist in the vicinity ) Multiply the number of millibars by of the centers. it)to oht,ou the gcostropht. wind for that Tsokis It that latitatie, (See fig()-5.) In COMPUTER PRODUCTS fflustration given in figiae G 5 place the 45° k t,tt tl ,aitt on an isol),11 (10121 and Since the advent of computer produced prod- tothe 1V)1 N t2 mbi uctsthe accuracy of meteorological analysis tltiply 1-)N 10 tot knots 120 kncki. and prognosis has increased considerably. The availability of computer products has provided a 00-.i.:i,red Versus Gewarophic unique flexibility in the utilization of personnel and raw data. The reliance of the forecaster on ding s,,tion was de-wt.:kJ Judi}, to computer produced charts is normally propor- .,1 isobat, Ih,.lr k;ireLtion should he tional to the time frame he is working under and aia.d .1,,,ktining a,t', fain anwunt of the availability of personnel to plot and analyze oi, :10,ttkiv.akilito,ckpre .A.1fc by conventional means. In many instances com-

A ttik aft all .11411, hctklttin surfat.c puter products are used entirely, either due to la;ituae,varN,f from a personnel limitations or because the computer

, ,AI' 1:%el o...t.,1,1,1. as ninth as 45° product is actually more accurate. The computer

. ,o.tr ...A1 ttrt,an,\11 aterai.rt: taint: of is capable of digesting a vast amount of data in a 1+.1 land .an be usecl limited amount of time andpresenting the

184 190 Chapter 6SURFACE WEATHER MAP ANALYSIS

,.INDEX

60 940 15 30 -o- -- 25

.A G.500 Figure 6.5.Use of geostrophic wind scale (overlaytype).

185 191 AEROGRAPI1ER'S MATE I & C forecaster with a finished analysis or prognosis 4. The Conservation of Water Vapor Equa- otherwise impossible to achieve. There are many tion. advantages to the machine produced product 5. The Equation of State. Pressure, density, such as the elimination of many human errors. and temperature of a perfect gas are related. However, the close attention to detail necessary 6. The Hydrostatic assumptionis usedin inperforming hand analysis 'hay make this place of the vertical momentum equation. procedure desirable especiallyforlocalarea analysisifitispractical and circumstances The PE-Model vertically divides the Northern permit. The forecaster must focus his attention Hemisphere atmosphere into 5 layers, six sigma on many details he might otherwise overlook. surfaces. (The sigma surfaces are at the bottom, Accuratelocalareaforecasting is dependent between layers. arid at the top of the atmos- upon the number of factors considered in the phere). The basic inputs for the model are: forecast. including such factors as geographic influence,localupper airpeculiarities,etc., 1. Virtualtemperatureanalysesforthe which the forecaster is capable of imposing upon Northern Hemisphere at12 constant pressure the large scale flow. levels distributed from 1000 mbs to 50 mbs. 2. Height analyses at seven constant pressure Computerproductspresentanexcellent levels. means of determining the macroscale meteoro- I Moisture analyses at 4 levels from the logical features. They also fill in the gaps in data surface to 500 mbs. scarce areas (oceanic. desert, mountain. etc.). It 4. Terrain field. (Height of mountains). must be rememberee however that computer 5. Sea level pressure analyses. products also have their weaknesses and that 6. Sea surface temperature analyses. their vglidity should be verified to the degree 7. Albedo field, which is computed using the possible and practical. monthly mean surface temperatures. U. Nondivergent wind components,obtained Numerical Prognoses Tec;miques from the solution of linear balance equations at pressure surfaces with interpolation to sigma The first numerical prognoses techniques used surfaces. either a "Barotropic Model," FNWC Monterey, ora"BaroclinicModel,"National Weather During the calculation to solve the PE Model Service. With the use of these techniques the a number of items are considered such as: machine product equalled or exceeded in accu- racythe product proL aced by skilledfore- 1. Surface friction in the lowest layer. casters. However, the early model still missed 2. Radiation processes. both solar and terres- strong development frequently because of the trial. limited number c1 meteorological variables con- 3. Sensible heat exchange and evaporation. sidered in the prognoses. 4. Condensation and the release of the latent A new model has been developed which uses beat of condensation. Newtonian Equations and is called the Primitive 5. Moist convective processes. Equation (PE) model. The Primitive Equations The results of the solution of the PE Model used in the model are: aresurface and upper air prognoses for the I. Newton's Second Law of Motion. The northern hemisphere for times out to 72 hours, individua motion of a particle of air is the result as well as additional information such as: of the sum of all the forces acting on it. I.Large scale precipitation prognoses. 2. The Thermodynamic Equation. Changes in 2. Sea and swell height prognoses. the potential temperature of a particle are the results of heating and cooling. The verification of the PE Model prognoses 3. The Continuity Equation. Mass is con- shows a definite improvement in the FNWC served. Monterey's product. 192' Chapter 6 SURFACE WI-A ElIFR MAP ANALYSIS

\t t.isurface shoos attempt is made to model the pressure distribu- aDining the a intcr thePI,was abort' tion using other elements of the reports. For tw .Nskillfula thethickness-ackection example, isobars crossing a front will be kinked at, Id: only where reports are sufficiently dense to so hDuring the summer theimp nnement indicate, a heavy rain report will not becon- w as less. %ow iner. there ate few er baroclinic sidered in the an:qysis of a frontal wave. situations in summer- 2. Analysis flaws are usually the result of one _'.At thc501 ml' lot!f the results 0". the or all of the following problems: %critic-A 1n are excellent. lhe model ago sLOWS a. Lack of data t!oodr..-saltswithit'bait!.to simulate the h. Erroneous data. growth ot baroelinic mares. c. Extreme atmospheric change. 3. SURF (.1- PRESSE. IZI- In regions of no reported data, the final \ \Al YSIS.[he analysis will surt.icepressureanalysis is accomplished be a combination of thefirst through the follow mg program Input: approximation and climatology.Initiallythe contribution made by climatological values is small but increases with the number of synoptic I Reported surface observations (approximately 4000-5000 reports). periods in which no reports are received in the 2. Hourlylustorytape of surface pressure area. prognosis derned [p.m the preceding 0000Z or 4. Surface observations reported in error may '00/ analysis. be used in the analysis. No internal consistency 3. Hourly history tape ot 500 nib prognosis checking of reports is presently attempted. A denied from the preceding 00001Li- 12001 ship reporting ten degrees out of position or ten analysis. millibars off the actual pressure due to transmis- 4. Surface pressure climatology field for the sion error. For example, will not be corrected. present month. The datawilleither pass the error checks, 5.I femispheric surface covet:me for the pre- resulting in an incorrect analysis, or it will be ceding 00001 or I 2001 analysis. rejected with the possible loss of some significant information. PROGRAM ( OMPUTATIONS. 'Construction 5Surface pressure and ship mind observa- of the first 3..tuess is as lollows. The first guess tions reported in error w ill normally be rejected to the surfa,:c pressure analysisis a modified ..i the Gross Error Check (GEC). If the errone- surface pressure prognosis v Inch enfies at map ous report should pass the GEC tolerances, it time. This map is a result of combining the will contaminate the analysis. 6-hour old surface analysis and 3 hour surface 6.if the bad report. accepted in the GEC. is prognosis %ert13,ing 3 hours prior to map time located in a high data density region (e.g.. over for the procedure of updating and reanalysis. A land).itwill probably be rejected when com- description of the procedures used in reanalysis pared with its neighbors in the lateral fitting ispresented inchapter 3 of the Computer check.Itmay, however, have already had a Products Manual. Na% Air 50- I G-522. slight adverse influence on the final analysis. 7.IF the bad reportis located in a sparse Program Limitations region (e.g., over oceans), it may pass the lateral fitting check as well as the Gross Error Check. In fointelligently Litilita: the computer products this case the analysis will depict with utmost desired mini the computer program it is impor- precision the pressure distribution represented tant rhat the Forecaster understand the program by the erroneous data. limitations as well as its advantages. Some of R. Occasionally extreme atmospheric change tliesc limit:dim:, arc presented in the following will manifest itself as an error in the analysis. In paragraplis- this case, strong development results in valid datafailing toe gross error check tolerance. I the surface pressure analysis makes use of Gross error check tolerances are predicated ona reportedpressures andship winds only. No reasonable forei.ast being used in construction of

187

193_3 AEROGRAM IER'S MATE I & C

the first approximation. If the forecastis ab- messages in cede form are also transcribed onto normally poor and off-time and late reports do appropriate charts following the coordinate plot- not correct it sufficiently. this type of error may ting procedure. occur. Although rare, it represents a serious flaw and should be guarded against. Isobaric Consistency Once the validity of the computer product Detection of Errors has been determined the isobaric patterns of the various charts should be compared for uniform- Errors of the types previously discussed can ity. The adjustments made between charts will easily be discerned at major network centers. involve variables such as the forecaster's experi- Data density upon which the analysis is based is ence, chart scales, operational objectives, etc. In provided by the Hemispheric Surface Coverage any case. the computer products should be and Gross Error Check Reject Coverage Charts. compared prior to finalizing the hand analysis. Data lists. including all ship reports used in the The types of computer products available and analysis as well as those rejected by the Gross transmissiontimesarepresentedinvarious Error and lateral fitting checks are available. It is publications and directives including H.O. 118; expected that major network centers will use Computer Products Manual, NavAir 50-IG-522; this information to determine validity of the NMS Forecasters Handbook No. I. surface pressure analysis in their areas of responsibility. Corrections, 11 necessary, will be ini- ADDITIONAL RULES AND tiated or notices of error will be disseminated to CONSIDERATIONS fleet users, by these centers. Errors may be detected by the operational Mountainous terrain presents a distinct and forecaster who receives the analysis directly on difficult problem in surface analysis because of the high -speed communication links. The total the roughness of terrain and the uncertainty in number of reports used, a simple indicator of the reduction of pressures to sea level. In this analysis quality, appears in the title of all surface case some higher level, either 850- or 700-mb, pressure analysis. The 24 hour surface pressure should be consulted as a guide to the reality of change chart or message can be used to deter- pressure centers and to obtain reasonable corre- mine areas of abnormal surface development. spondence between the pressurepatterns as Nonmeteorological or new disturbances should represented on the surface map. Along the be cross checked with available observations. mountains you may have a packing of the isobars with an unrealistic gradient and winds may even blow at right angles to the isobars. Verification You should become familiar with the local peculiarities of mountainous terrain and make A quick method of verifying the computer allowances for them when analyzing the surface analysisistotake the hand drawn analysis chart. having the same time and lay one over the other Along coastlines, especially the Atlantic and on a light table. More complex methods involv- Gulf Coasts of the United States during winter, a ing the use of acetate overlays, ozalid machines, problem of isobaric analysis often arises when and overheadprojectors, are sometimes em- cold air of polar or Arctic origin flows over ployed at larger weather units. much warmer coastal water. Since it is found by Weather units may receive computer products observation that a relatively smooth flow pat- via facsimile or teletype message. The facsimile terncontinuesaloft(850-or 700-mb and may be difficult to verify using the light table higher), it follows that the configuration of sea method due to variations in transparency. In this level isobars at coastlines should be affected case it may be necessary to verify by replotting markedly by the warming of air in lower levels, coordinates from one charttothe other to because the warming produces lower pressures at depict the significant chart features. Teletype the surface over the water than otherwise would

188 194 Chapter 6-- SURFACE WEATHER MAP ANALYSIS be the case. This type of distortion is illustrated pressures and the map. Consequently,the iso- is infigure6-6. The dashedlines represent a bars have a wavy appearance which not smooth extrapolation of the pattern over the corroborated by the wind olservations. Errors in ocean, and the solid lines show the true pattern. barometer reading may be responsible for the The magnitude of this effect varies with the difficulties in fitting the isobars to the wind temperature differenze between land and sea, system. At sea, where readings from aneroid and presumably with the lapse rate in the coin barometers are not highly accurate, there is a air when over land. This effect can be antici- tendency for inexperienced analysts to draw pated off any coast during winter. wavy isobars on maps. While youshould make Highly irregular isobars are not impossible but an attempt to smooth cut the lines onthe chart, they are improbable. Itis a common error for you should not try to smooth outirregularities beginners to draw isobars with many irregu- which are judged to be real and convey impor- larities in order totit them precisely to the tant information. At sea, the effect of frictionis

AG.501 Figure 6-6.Effect of warm coastal waters on isobars.

189 1 65 Al ROGRAPIIFR'S \IA IT

minimized and the w Ind observations are largely not be divalent on the map. Location of the dependable. Pressures aie not as accurate.so %allot's observation stations and localeffects more weight should he givento winds than play a great Nit In YYLighing the elements which pressu res. are used in locating fronts. The front may be weak.itmay be strong. oritmay be of SUMMARY OF RULES FOR modeiate intensity.II may be well defined in DRAWING ISOBARS the wham. pattern and easy to locate through the surface weather changes peculiarto that The order given Is not arbitrary andmay be type of hunt. On the otlici hand. the front may varied to suit local needs or procedures. This be w cak of indistinct and therefore difficultto summary includes some of the suggestions for locate from stilt-t: paiameters. Consideration of drawing isobars on surface charts. frontal structures from upper air informationis helpful I. Choose the isobar interval according to the inlocating all types of fronts. These teatures()I' previous instructions. fronts on upper aircharts an discussed in a later sect ion. Select isobars which will outline pressure ceviers best and draw. these centers first. using When a Front approaches andpasses a land history and winds to help locate them. station. the setilteiKe of eventsmay be antici- 3. Sketch lightly the orientation and spacing, pated, but atsea the movement of the ship of isobars inthe y1Linit3ofisolated reports, combines with the moyellent of the front to elsewhere the}maybe sketchedin alittle produce chanties which arenotso easy to heavier. foresee. For example, on a ship traveling west- 4. Draw isobars at largest interval commensu- ward the front passes oukkly,,for the front and rate with ability and experience. Inexperienced the ship are moving in opposite directions. If the analysts willfindthe ;slid) interval about the shipis moving eastward,in aboutthe same limit.Experiencedanalysts areabletouse direction as the front, the change of wind and !ii -nth iiitcrvals in then first approximation of attendant weather conditions take place slowly. the isobaric pattern. Intermediate ti -nib isobars In exceptional cases a fast shipmay overtake a art. thenadded.follow edby4-mb isobars. front and pass through it from west to east, thus Liasures and adiustincnts to the initial patterns reversing the sequence of changes. k'ariations in are usually necessary and to be expected in the the type of SC.thei encountered with frontal final analysis. passages must also be taken into account. The 5. Along an axis rowing two highs or two typical models available only representtypical two isobars with the same label must conditions and other factors such as strength of occur. alone an axis joining d high and a low, the front and nature of the terrain over which it two isobars cannot hay< Ilk same Libel. has passed or k passing. and subsequent modifi- O. Serious errors nl cin,uktion patterns can cations to the air masses. speed if the front, and often be avoided by drawing isobarsas ifthe type of front must be taken into consideration. pencil were traveling with the wind, counter- In this section of the chapter, some of the rules clockwkearound lows and clockwise around andaidsinlocatingfronts on the surface highs.['hiswill help prevent placing a small weather map are discussed. closed center on the wrong side of an isolated wind report. 7. Resolve conflicts between wind andpres- LOCATION OF FRONT sure data and smooth isobarsbyevaluating the ON A WEATHER MAP reliability and representativeness of the data. Afterthe datahas been entered andits reliability evaluated on the map, one of the FRONTAL ANALYSIS major tasks of the analyst is locating the fronts. Some of the factors which enter in the determi- When a front is drawn on the weather map nation of fronts on a weather chart are as the analyst consideis many factors which may follows:

190 196 Chapter 6- SURFACE WEATHER MAP ANALYSIS

I.Fronts should showlogicalcontinuity In summary, in the analysis of a weather map from precious charts.Corrections should be for fronts, the foregoing steps should be con- taken into account. sidered together. Along a single front the data 2. Fronts normallylieintroughs of low used to locate a front may vary considerably. pressure. While a trough of low pressure in the For example, at one point a temperature differ- pressure pattern is a necessary condition for a ence may give the strongest indication, while front. it is not a sufficient condition. The front wind may be locally influenced and not give a must 'no% e ina manner consistent with the representative picture of the frontal location. motion of the colder of the two air ,rasses. In Other parts of the fronts may be more easily other words, a frontal surface is con --ed of identified by pressure tendency or wind. No one particles of air which move with the wnids. If item should be used as a complete guide to the trough line does not move with approxi- analysis. mately the component or the winds normal to it. the trough line cannot contain a front. This is ISOBARS IN RELATION TO not, however. sufficient by itself, for a trough THE LOCATION OF FRONTS line could move at the proper speed and still not contain afront. Other factors must be con- The isobaric pattern reflecting the traditional sidered. kink of isobars at fronts is often one of the best 3. A moving front will have a pressure tend- indicators of the existence of a front on the ency difference across the front. It' the front is weather chart, The amount of kink is governed stationary or slow moving, there will be little or by the strength of the front, its movement, and no tendency difference across the front. If' the the particular stage of development. front has passed the station within 3 hours of With due consideration to past history and map time the pressure tendency will show a the other factors mentioned above,a good kink. method, especiallyatse:, is that of drawing 4. Winds will shift cyclonically across the down the wind along observations where a front front. is suspected to exist. For example, in figure 6-7, 5. Dewpoint differences will exist across the suppose you start to draw the isobar for the front. Precipitation and water surfaces affect 08.0 (1,008 millibars). Drawing down the wind dewpoint values so.attimes,they are not in proper relation to the barometer readings, representative of air masses. with the wind atsmall angles (aboard ship) 6. Temperature, at times. is a good indicator across theisobars toward low pressure, you for locating a front; however, the temperature of come to observation point B. Here you will note the air near the ground is so dependent upon that the next observation ahead shows a change insolation and radiation thatitis often not in the wind, and it is now necessary to draw the representative of the air masses, If two stations isobar toward C.Itis evident that thereis a at nearly the same elevation are cloudy and have discontinuity, or front, along the dashed line strong winds, then a temperature difference may from D toE.If you drawall the isobars carefully, studying the winds as you go; the not be significant. isobars would look like those in figure 6-8. patterns 7. The cloudsandprecipitation This shows you that there are irregularities or should be representative of the particular front discontinuities which are not a result of error in in question. observations, for they are systemically arranged. 8. Fronts sho:ild move withthe cold air The best fit of the isobars is obtained by making winds and can be located by historical sequence. an angle in the isobar with the vertex at the If the cold air :s moving toward the front it will front, or wind shift line. be a cold front; if the cold air is moving away Figure 6-8 shows the location of fronts in a from the front it will be a warm front. cyclonic system approaching the west coast. The 9. The line representing a front should be fronts in this situation are clearly identified by placed on the warm air side of the transition the reports. Fronts should be marked on the zone and along a line of cyclonic wind shear. map when they can be clearly identified.

191' 197 AEROGRAPIIER'S MATE I & C

04 DI

031

I 041 \4....6368 3

I 0618

I 075

AG.502 Figure 6-1 Isobaric discontinuity ata front. At left, drawing down the wind from A, a discontinuity is found along line DE. At right, the isobars showa systematic arrangement

CONSIDERATION OF ELEMENTS 5. Check the displacement of the frontal surface from its last known position forcon- Assume that a cold frontis to be located. sistency with movement of the colder airmass Local analysis may revealthatthe pressure during the interval. trough, the wind shift, the line of showers,and discontinuitiesintemperature, moisture, and Step 5is by far the most important and tendency coincide at most in pairs. Whatline should be applied twice, once in the beginning should be chosen to represent the front?The following procedure is recommended: to determine approximately where the cold front begins and again at the end fora final check. I. Eliminate erroneous and unrepresentative It is convenient to delineate only the leading data in accordance with previous instructions in edge of a cold frontal zone and the trailing edge this chapter. of a warm frontal zone with their respective 2. Locate the front at selected points where frontal symbols. This puts all the strong gradient the maximum number of identifying character- of temperature in the cold air istics occur simultaneously. mass. 3. Intheabsenceof known orographic INTERPRETATION OF SATELP obstructions,interpolateorextrapolatethe LITE CLOUD PHOTOGRAPHS front in doubtful or confused areas. 4. Check the 3-dimensional geometrical and Satellite cloud photographs have proven to be meteorological consistency of the frontal surface a most valuable aid in the location of fronts on at other levels. the weather map. It is important for the analyst

192 198 Chapter 6 SURFACE wEATIIER MAP ANALYSIS

1008

1012

45 03 9

961)047

53..261

57 26 8

60 240

57261 64 99

AG.503 Figure 6-8.Surface weather map showing location of pressure centers and fronts from isobars.

to keep in mind, however, that the satellite is compared withsurface analyses is shown in presenting a view from the top down. This figures 6-9 and 6-10. means that features which appear only in the These two figures illustratea sequence of lower layers of the atmosphere might be hidden eventsover a 24 hour period.Notice the by extensive cloud coverage aloft. While satellite similarity in the configuration of the clouds and pictures are undoubtedly a tremendous step fronts depicted in the analyses and photographs. forward for the analyst, some analysts fall into The center of circulationis outlinedin the the trap of blindly accepting them as infallible. photographs by the entrance of the cold dry air The satellite photograph must be viewed in in the upper levels during the occlusion process. proper context, with as many other pertinent Notice the breaking clown of the organization in factors as available being added tothe final the cloud structure as the occlusion or dissipat- analysis.ft cannot be overemphasized that the ing process progresses during the time period analyst must use all of the tools at his disposal if represented by the two figures. The utility of heisto acquire consistent accuracyinhis these photographs in identifying the :loud bands analysis. along the periphery of the storm area is quite evident. This is especially true in sparse data Identifying Cloud Systems areas. An example of the appearance of frontal The analyst must bear in mind the normal cloudstructure insatellite cloud pictures :is slope of fronts and pressure systems aloft when

191 199 1/

'love .fir S e" -1, Figure 6.9.Comparison of surface analysis and cloud pictures on 31 May 1969. L AG.504 0 t. Cre' "`"*.:4/ 1/4 ref Figure 6-I0.Comparison of surface analysis and cloud pictures on 1 June 1969. AG.505 \ROGR \PHER'S \IA1'E 1 fi C

making comparisons between surfaceanalysis liontableisprovidedfir each radiometer, and the satellite pictures. Morediscussion re- allowing the analystto assign a temperature lated to this subject is presentedin the chapter value to each shade of gray in the following this one pertaining to step wedge. upper air anal- This allows the analyst to interpretthe infrared ysis as well as in chapters 8 through12 of this display in terms of temperature. Aforecaster manual pertaining toprognosis. Disaission of shouldconsider operationalgoals andlocal the location of surface frontal positionson the conditions to enhance the informationthat can satellite photograph mayseem somewhat pre- be obtained from a DRIR display. mature at this point in the manual due to the The typicalradiometer has a temperature considerations which must begiven to upper air range extending fromat proximatelyI 80°K systems. However, their usefulness in estimating ( )3°C)to 320°K ( 47°C).Ingeneral, the frontal positions must be consideredfrom the coldest expected cloud temperatures outset of the analysis. are reached at the tropopause(approximately 2.20° K More detailed information pertainingto the (-53°C) in mid latitudes). The utilization of satellite photographs warmest tempera- in analysis ture of interest will be about 293°K (20°C). and forecasting may be found inthe Direct Therefore. for meteorologicalpurposes, the en- Transmission System Users Guide publishedby tire range of the sensor is not required. the National Oceanic and Atmospheric AP'[ sets Adminis- should be adjusted so that all tration (NOAH): the Guide For Obseiving step wedges for the temperatures colder than the tropopause Environment With Satellite Infrared tem- Imagery, perature (220°K) appear white. Similarly, all NWRF F-0970-158; chapter 8 ofthe NMS step wedge, warmer than a nominal surfaceair Forecasters Handbook No. I.Facsimile Prod- temperature (293°K) should be adjustedto ucts: and other NWRI: booklets as listed in Navy appear black. With this adjustment, cloud fea- Weather Research Facility Reports andPublica- tures will appear in finer detail. tions, NWRF 00-0369-143. Inasimilar fashion,the DRIR-scan step wedges can beadjustedtohighlight other Direct Readout Infrared features of interest, basedon their temperature (DRIR) Interpretation characteristics, as shown in figure 6-11.

The foregoing discussion has been withrefer- DISTINCTIVE DRIR FEATURES.Interms ence to the commonly received t.amcra sensLd of the earth and its atmosphere,on a nominally satellitephotographs.Ilowerei,theinfrared adjusted readout set the various features should radiometer is now frequently usedas a satellite appear as follows: sensor and the analyst or forecaster must en- I. deavor to properly interpretthe information 11ot deserts should appear black. received. 2. Lakes andoceans should appear near black. An infraredsensordiffersfrom a visual 3. Clouds with low tops (Cu, Sc. St) (camera type) sensor inthat should the instrument appear dark gray. measuresradiatedheatrather than reflected light.Infrared 4. Clouds with medium tops (As, Sc)should sensors therefore measure the appear light gray. temperature of cloud tops (or the earth's sur- S. Clouds with high tops (Ch. face)rather thantheir albedo.I he infrared Ci, Cu) should appear white. display is arranged so that warm bodies (earth, low clouds) appear dark. while cold bodies (high 6. Snow and ice should appear lightgray. clouds) appear bright. Thus. infrared displays of Because of the nature of infrared sensing, clouds have an appearance thatis similar to however, an operator who understands infrared visual pictures of clouds, but have an entirely display can adjust his readout equipmentto different meaning. enhance features of interest. SET ADJUSTMENTS FOR DRIR. Current Figure 6-12 illustrates two typical infrared satellite radiometers display a 7-step calibration passes from Nimbus IV over the Western United wedge along with the DRIR picture. A calibra- States and Mexico.

196 202 Chapter 6SURFACE IVEATIR MAP ANALYSIS

Figure 6-i 2(A) is a daytime view. whilefigure 6-12(13) is the nighttime view of the same area. Note the desert area of BajaCalifornia. In the afternoon (figure 6 -I 2(A)), the desert is hotand looks much darker than the surroundingwater. In the early morning (figure 6-12(B)),the land has cooled so much that it is now slightlycolder, and therefore lighter than the water.

NORMAL SETTING FRONTAL WAVES AND DEVELOPING CYCLONES.--A surface wave, which isindicated in the visual mode by a broadeningof the frontal band, nas a similar appearancein the IR +44 +60 COLD -57 -19 +6 +26 mode, but with important differences.Cold temperature radiation from the coresof large convective clouds appears as globsof intense TROPICAL SETTING white. While the wave cloudiness usuallyhas a uniform white appearance in video pictures,the IR presentation exhibits several shadesof gray, -57 -19 +6 +26 +44 indicating clouds of varying height andthickness. Frontal zones contain stable cloud formswith MID -LAT SETTING low cloud tops, as shown in figure6-I3(A), or unstable cloud forms with high cloudtops, as shown in figure 6-13(13). Since the stability of the air massesina -19 +6 +26 frontal zone may be inferred from theheight of the clouds,itfollows that stability may be inferred from gray shadesin theIR display. POLAR SETTING Figure 6-14 illustrates the differences between the stable and active frontal zones asnormally seen in a DRIR display. COLD -57 -19 +6 Stable frontal bands (fig. 6-14(A)) appear gray(warm)withisolatedcloudbuildups (white) along the band. Activefrontal bands ICE RECCE SETTING (fig.6-14(B)) appear offwhite, withlines of convective activity (white) within theband. The inability to assess stabilityfrom visual cloud pictures can result in falselyidentifying -19 +6 cloud bands as frontal bands, while infact they may only associated with low-leveleddies. A the low-level SEA SURFACE / LAND TEMP classic example of this is seen in vortex shown in figure 6-15. In this IR seam two approximatelycircular 13 anda +6 +26 +44 graypatterns appear near A and comma-shaped pattern at C. Vortex centersare barely visible at A and B. Thesetwo circular vortices made up of AG.506 gray areas represent cloud spiraling cumuliform cloud lines.These do not DRIR step wedges Figure 6-11.-Idealized display of represent frontal characteristicsand arc shown with readout calibrated for specificinterpretation equivalent as lows on the surfaceanalysis. With intensifica- tasks. Numbers below gray shades are frontal. Notice scene temperatures indegrees Celsius. tion these systems could become

197 203 \I Rut \ PI II WS \I VI I

AG.507 Figure 6 12 -DRIB picture. (A) Nimbus IV daytimeinfrared view of Baja, California; (II) Nimbus IV nighttime infrared viewof Baja, California. thatthisisa irelitinne leadout. Due to the Sonic. characteristic~ 01 IR mode which increasedfete petat contrastinthe lower favor wave development are: nightiun- (10,11)1eS the opportunity for early of new disturbances, and I I. videncc a signilitanl increase in the provides continuo!, oil existingones. area of cold leinperattne, (high clouds). 2. Evidence of a jet stteam near the poleward A frontal waw tthitIiappeals gray in the boundary of a bulge on the frontal band. infraredcan be (Ant:el ell1u he stable and 3. Positive Vorticity Advection unlikely to do el4.p kSlieNas (PVA MAX) an area of extensive or strong von lay center, suchas those de- white cloudiness (coldtemperatures) indicates scribed in chapter 7 of this manual, that the wa% upstream is unstable and is in the process of from the frontal wave.I Ins would indicate the development I otiklilious further illus- presence of a short-wave it10-mb trough trated in figure t. upstream from the developing cyclone. Chapter 6 SURFACE WEATHER MAP ANALYSIS

WHITE

BLACK

AG.508 Figure 6 -13. -DRIB cloud depiction.(A) Stable cloud form; (B) unstable cloud form.

AREAS OF LITTLE VERTICAL ACTIVE FRONTAL BAND DEVELOPMENT ( GRAY) (OFF WHITE)

AREA OF CLOUD BUILDUP ACTIVE WEATHER AREA (WHITE) (WHITE )

( A )

AG.509 Figure 6.14-DRIR frontal depiction. (A) Inactive frontal zone; (B) active frontal zone.

When anunstable wave (cloudbulge)is overturning. In areas ofcold air advection (dry developing into a vortex, (figure 6-17) the first tongue), low cumulus cloud tops (gray) indicate evidence is the beginning of a dry tongue on the stable conditions and a small air/seatemperature trailing edge of thecloudband. The cells (open) difference. Highcumulus cloud tops (white) normally seen in the dry tongue over the ocean suggest alarge air/sea temperature difference during the Northern Ifemispheresummer appear andstrong cold air advection. Due to the higher mottled gray in the IR mode. Cold air flowing cloudtops of the cumulus congestus,cells over warm water creates low-level instabilityand (open) have a similar appearance in the IR and convective overturning. The greaterthe air/sea visual modes.Ifthe field of cells (open) is bright temperature difference, the greater the degree of inthe IR mode, this indicates intense cold air

199 205 4

Figure 6-15.Surface analysis and accompanying DR IR picture showing low level vortices. AG.510 Chapter 6- SURFACE wEATIIER MAPANALYSIS

ISOLATED AREA OF LARGE AREA OF CLOUD BUILDUP (WHITE) ACTIVE WEATHER (WHITE)

( A )

AG.511 Figure 6-16.DRIR frontal depiction. (A)Stable frontal wave; (B) unstable frontal wave.

BEGINNING DRY TONGUE `-.--'-- CELLS (OPEN) WHITE

MOSTLY WHITE

CELLS (OPEN) WHITE

( A) (B)

AG.512 cyclone. Figure 6.17.DRIR frontal depiction.(A) Developing frontal wave; (B) occluding

injection into the cyclone, and increasedlikeli- COMMON ERRORS IN hood of deepening. FRONTAL ANALYSIS the uses Additional .information pertaining to frontal of DRIR readouts in forecastingwill be pre- Some of the more common errors in sented in chapter 9 of this trainingmanual. analysis are listed below:

201 207

MN AEROGRAPHER'S MATE I & C

1.Inconsistent displacements from previous definite line of longitudinal positions. convergence in the trough, the result ofa "surge" in the north- 2. Cold fronts improperly designatedas warm westerly or westerly winds of the polar fronts and vice versa. or arctic air mass moving in behind theoccluded cyclone. 3. Too many fronts, particularlysecondary fron ts. 4. Isobars too sharply kinkedat fronts or SATELLITE DEPICTION OF kinked improperly toward lowpressure, SURFACE RIDGE LINE 5. Frontal patterns in thehorizontal which have an impossible 3-dimensionalstructure. Among the nonfrontal meteorologicalfea- 6. Use of unrepresentative data(particularly tures which may be confirmed by thesatellite temperature) in locating fronts. photograph is the surface ridge line.Although 7. Dropping of fronts inareas of sparse or no they may not beas well defined as frontal systems, reports without designating frontolysison pre- ridgelines may beidentifiedwith ceding chart or charts. increasing accuracy commensuratewith the experience of the analyst. Threecharacteristiccloud STATIONARY AND patterns found with surface ridge lineson satellite photographs NONFRONTAL TROUGHS are illustrated in figure 6-18. The cloud pattern of type A infigure 6-18 is Stationary troughs can contain frontsonly characterized by long cloudlines or fingers momentarily. The most common such trough in which extend, in a continuousfashion, from a North America occurson the lee (east) side of frontalband.Thesefingersgenerallyare the Continental Divide.Unless the air mass oriented in a more north-south following the front is dense enough direction than to fill the the frontal band. The end of thesecontinuous trough,afront moving eastwardacross the cloud fingers serve as indicators Rocky Mountains may or points for occupy this trough at a positioning the surface ridge lineon the south- particular map time, but willmove on through western side of a subtropical anticyclone. while the trough remains fixed. Needless to say, The type B(fig.6-18) surface ridge line warm fronts moving through such a trough will generallyis found on the western side ofa not affect its stationary character. These troughs subtropical high where the clouds occur on the lee side of any mountain change in range character from cumuliform to stratiform.Notice orientated across a deep current of air.Another in figure 6-18 that the points drawn kind of stationary trough for the ridge occurs in southwestern line are drawn so that theylie along the line UnitedStates,andis most pronouncedin where the cloud type changesfrom cumuliform summer, the season of the "Heat Low," when it to stratiform. This change in cloud characteris sometimes extends the full length of thewest generallyfound where coast. the low level wind changes from a southeasterly toa southwesterly The most common nonfrontaltroughsin direction. As the middle latitude ocean air moves poleward more areas occur in the wake of rapidly; stability increasesnear the surface and well developed occluded cyclones.(They also occur over land areas.) This trough produces the observed change in cloudform. is often The type C pattern (figure 6-18)is associated incorrectly analyzed as a secondary coldfront with a sharp migrating surfaceridge that lies because it has a well defined line ofshowers, between two large cyclones in close cumuliform clouds, wind changes, proximity and pressure to each other. Along this ridge, thereis a wind tendencies across the trough line similar to those shift from a general southwesterlydirection to of the cold front. Nevertheless,it moves at the northwest. While an air parcel speeds very much less than that of isin the the often southerly flow, it usually is beingcooled from very strong winds which accompanyany deep below and thus becomes cyclone. If this phenomenon more stable with time. occurs in an area Air that isin the northerly flow normallyis with a sufficiently dense networkof reports, an heated from below and becomes unstable, analysis of the wind field shows that This there is a difference in stability producestwo distinctly 202 208 Cha Pter 6 SURFACE WEATIIFR MAP ANALYSIS

0 SHP:. ri-m-446azie.se...g...0/*VA7)

AG.513 Figure 6-18.Surface analysis superimposed on satellitepictures showing surface ridge lines.

203 "09 AEROGRAPIIER'S MATE 1 & C

different cloud patterns thatcan be recognized in mountain areas. Froma moving ship, they are in satellite pictures, The cloudsin the southerly of no value unless flow are primarily stratiform, a dependable correction for while those in the the movement of the ship is made. northerlyflowarecumuliform. Hence, the Pressure tendency characteristicsmay be of surface ridgeis located in thenarrow region little help, particularly in between the stratiform and cumuliform summer, Character- clouds. istics of the barographtrace (3-hour) may be SUMMARY OF SURFACE FRONTAL helpful when the magnitudes ofthe changes are ANALYSIS PROCEDURES large by comparison with diurnaleffects, dynamic effects of convective activity,etc. The test Probable Location of the of usefulness is if theparticular characteristic is Front (Extended Analysis) organized along a line. Actual values oftemperature and dewpoint are likely to be unrepresentative If one previous location ofthe frontis near a front. known, the current position isdownwind, (refer- Temperature or dewpointgradients are more ring to the wind in the coldair) at a distance reliable with weak gradients(homogeneity) in depending roughly on the speedof the wind in the warm air mass andstrong gradients on the the cold air. cold air side of the front. Dueto horizontal Generally, cold fronts are foundto the east mixing, heating or coolingfrom below, and and/or southeast of previouspositions, warm evaporation from belownear the shallow edge of fronts tothe north and/or east of previous the cold air mass, temperatures anddewpoints positions, and quasi-stationary frontsin about on the edge close to the cold air side of thefront the same position as before. may closely approach those of thewarm side. In When the isobars these circumstances the station'swind direction, on the previous map are if parallel to the front very little, ifany, movement representative,isusuallythe determining is expected. factor. Surface temperaturesare generally of little value in frontal analysis When two successive previouspositions are on morning maps, known, an approximatepast rate of frontal and are most representativeon afternoon or evening maps. motion is established. In thiscase the quickest method of locating the probablecurrent posi- tionis tolay off the previous displacement, AIR MASS ANALYSIS usingthepencil andfingertechnique. The precise!ocationisthen obtained by minor While an Aerographer's Matefamiliar with a given weather situation adjustments based on the criteriabelow. may have little need for Do not make major adjustmentsin location air mass classification, the distinctionbetween from what would be expectedby previous air masses of differenttypesisnevertheless displacements unless the wind and/orpressure important to weather analyses and isa means of gradient in the cold air has changeda propor- conveying a more complete pictureto a user of tional amount. the weather map, suchas a pilot. If masses of air of differentcharacteristics are isolated from one another, thetransition zones Criteria for More Precise Location between them can be determined of the Front (Local Analysis) as the first step in frontal analysis. Thereversz procedure may be used to advantage in examinationof Probably the most important criterionof a vertical temperature soundings, in frontinflat which case terrain (oceans included) is the the air masses over a stationmay be isolated cyclonic windshift.Do not be misled by from one another by locating thetransition nonrepresentative winds such as along thecoast, zones or frontal discontinuities large on the sound- lakes,in valleys, etc., and directions of ings. light winds (under 10 knots)on morning maps. On the basis of hydrometeors, such For a moving front, 3-hour as fog or pressure tendency drizzle and cloud types,a stable air mass can differences over land are very helpful, especially often be differentiated froman unstable air mass

204 Zio Chapter 6SURFACE WEATHER MAP ANALYSIS across a narrow zone. In considering thecharac- pnce with standard practices. The edges of an teristicsof air masses according to accepted altostratus shield and fog areas may be outlined. classification, many modifications have to be Warm-front rain, or a corresponding steady taken into account, particularly in the warm air rain area in advance of an occluded front,is mass where, for example, stratus mayform with usually assumed in the absence of data to extend rapid movement over a cold surface while fog over a band of approximately 300 to 350 miles will tend to form if the flow is sluggish. In warm ahead of the front. This typical model may of seasons, the warm air mass can changefrom course be modified by geographical factors or foggy at night to unstable in the daytime. At sea complicated frontal structure. A known bound- the diurnal influence is slight, with a tendency ary of solid altostratus will often indicatethat for greater stabilityin the daytime in lower the edge of the precipitation area is near. Also, layers. the presence of "mid 7" clouds frequently Characteristics of the same general type of air shows thatarainareais approaching. The mass vary to some extent with their source existence of this cloud type, or of an altostratus region, Arctic air, for example varying some- layer, may be the first indication of a front what, depending on whether it has developed in approaching a coastline. Siberia, over the polar cap, or in Canada. Pressure falls in an area of precipitation may No definite values of surface temperature or indicate that the precipitation is a warm-front dewpoint can be stated for continental polar air type, the fall of pressure tending to be greatest masses, since these depend upon sourceregion, where the rainfall is heaviest. length of time over the source, depth, trajectory, If the band of precipitationis appreciably heating and cooling from below, and the like. wider than 300 to 350 miles, a double front Often there are marked horizontal gradients of structure may exist, or there may be a smaller temperature within the air mass. The amount by than normal slope to the warm-front surface. which a cold air mass is warmed, both surface Sometimes there is an increase of slope well in aloft when passing over a warmer surface, is advance of the surface position, in which case greater when there is subsidence aloft. the position of the line along which the front Arcticairatmidlatitudes generally has a begins to increase in slope may, if considered surface temperature of about 0°F or below. The significant, be shown as a warm front aloft. only air mass which in midlatitudes has approxi- Where relatively dry air ascends over a warm mate homogeneity at the surface and to some front,precipitationis often lacking. or may extent aloft is maritime tropical air. In summer, occur only some distance ahead of thesurface mT air over the contiguous United States has a position of the front. representative (maximum) surface temperature Shower symbols are used to give form to the of near 90°F and a dewpoint of around70°F. region where present weather, weather in the Corresponding values in winter are roughly 75°F last hour, past weather, precipitation amounts, and 60°F. or the presence of cumulonimbus suggestsshow- ery conditions, or where, in the absenceof data, PRECIPITATION ANALYSIS shower conditions are expected on the basis of continuity or model. Ordinarily, precipitation analysis on surface In cold air masses, showers will be prevalent charts is not completed before transmission of where there is rapid warming over water surfaces the coded maps (at weather offices that transmit as over the Great Lakes in winter; over moun- canned maps), because of insufficient time. It is, tainous country, such as the western slope of the however, carried out later and serves as an aid to Appalachians, and in general where the isobars the subsequent analysis for which precipitation are curved cyclonically and the coldair contains reports may then be more quickly interpreted. sufficient moisture. Steadyprecipitation isshadedingreen. Drizzle is looked for principally where warm, Shower areas are marked with shower symbols: moist air moves rapidly over a cooler surface, thunderstorm area, with thunderstorm symbols; especially in the warm sector of a cyclone. It and drizzle area, with drizzle symbols, in accord- mayoccurin coldair,especiallyunder

205 211 AEROGRAPIIER'S MATE I & C

anticyclonic or stagnant conditions if there is geographical -.welts during specified periods of sufficient moisture and instability in the lower theyear are favorablefor the formation or levels and a stable layer or inversion above the dissipation of fronts. Also, certain portions of low clouds. the frontal system itself are favorable for fronto- The principal cloud or precipitation systems genesis, such as the trailing edges of long cold looked for in a typical ex tratropical cycloneare: fronts whose orientation has become east-west and the isobars either anticyclonically curved I. or Warm-front rain areas, and theprewarm- parallel to the front. front cirrus and altostratus preceding them. Indicatetheareaswith 2. Warm sector drizzle, low clouds, or fog. theappropriate symbols: "FG" for frontogenesis and "FL" for 3. Warm sector showers: in many cyclones frontolysis. most showers occur in the warm sector rather than along the cold front. 4. Cold-front (squall line) cumulonimbus and APPLICATION OF SATEL- showers. LITE PHOTOGRAPHS 5.Post-cold-frontcumulus and showers (usually only where flow is cyclonic or showers As mentionedpreviouslyinthischapter, are favored by topographic features). satellite photographs are a valuable aid to the analyst. The photographs should have been All systems are subject to modifications by referred to when initially positioning the fronts, geographical features, moisture content of the lows, troughs, and ridges on the map. Prior to air, and other characteristics pertaining to the finalizingthe map these pictures should be given cyclone. compared with the analysis to insure that the completeanalysisis a consistent,smooth- ISALLOBARIC ANALYSIS flowing picture. It may be desirable to make last minute adjustments inadvertently overlooked The isallobaric field can be useful in early during the initial phase of the analysis. detection of frontogenesis, and also in determining the direction of movement of a front where weak wave activityis taking place, where the APPLICATION OF actual wind at gradient levelis different from COMPUTER PRODUCTS that indicated by the isobars, or where questionable reductionfactors make sea level isobars The application of computer products was unrepresentative. In addition, the instantaneous presenteJ earlier along with references for ob- movement of high and low centers is apparent taining the various charts. Just as it is important when isallobars are drawn. The isallobaric field is to compare the sateilite pictures, the computer frequently of value in indicating various types of products used during the initial phase of the developments. such as the formation of squalls analyses shouldalsobe compared with the or genesis of the instability line in the warm finished map priortofinalizing. As stated sectorofa cycloneand, more important, earlier,in many instances these charts will be changes in the structure or movement of pres- utilized in place of the hand drawn analysis due sure sy ',terns. Three hourly tendencies are more to personnel limitations and limitations in the significant if the diurnal correction is applied. availability of data.

FORMATION AND DISSI- FINALIZING THE CHART PATION OF FRON1 S Some of the steps outlined below will already The map should be scanned for indkations of be completed or in the final stages by the time frontogenesis and frontolysis in accordance with you have reached this step in the analysis. You the information contained in chapter 5 of this should either complete or check for completion training manual. Keepinmindthatcertain the following elements of the analysis:

206 21.2 Chapter 6SURFACE WEATI1ER MAP ANALYSIS

ISOBAR LABELING FUTURE MOVEMENT OF FRONTS AND PRESSURE SYSTEMS After all the isobars have been sketched and the position of the fronts located, the final Expected positions of fronts and pressure analysis is made. Isobars should be erased and systems may be indicated by the use of arrows redrawn, smoothing out irregularities which do pointing to the position they are expected to not show logical consistency and other irregu- move during the next 6, 12, or 24 hours. larities which may be due to errors in reported pressure values. ISALLOBARS

As pointed out previously, isallobars are most FRONTS representativeifthe diurnal characteristicis removed.Isallobarsare usually drawn for 1 After completing the isobaric analysis, color millibar intervals,I, 2, 3, etc., plus and minus inthefronts, using the appropriate standard tendency values. Dashed blue lines are used for symbols and indicating areas of frontogenesis plus or positive tendencies and dashed red lines and fro n to lysis. for minus or negative tendencies. Some stations prefer to begin with either the plus 2 or minus 2 value, thereby eliminating some of the diurnal change. The fallacy of this is that while this may HIGHS AND LOWS be representative during certain periods of the day when either plus or minus diurnal changes Label the centers of highs with a blue "H" correspond with these changes in the synoptic and the centers of lows with a red "L." If the tendency, you may be removing a perfectly valid center of a high is weakening (central pressure tendencychange.Itisbest to use diurnal falling) or the center of a low is deepening tendency tables or charts as mentioned in a (central pressure falling), suffix the letter "L" or previous section of this chapter. "H" with a minus sign. If the central pressure is rising, use a plus sign. SOUTHERN IIEMISPHERE ANALYSIS

Mostofthe Aerographer's Mates havea WEATHER PHENOMENA Northern Hemisphere orientation anu all too often completely ignore the peculiar aspects of Precipitation areas and other areas of signifi- the Southern Hemisphere until they are called cant weather should be indicated by using the upon to make analyses in this area. It requires a appropriate color shading or symbols. considerable amount of effort to reorient yourself to the fact that cyclonic circulations in this hemisphere have a clockwise rotation of the AIR MASS LABELING winds and anticyclonic circulations are counterclockwise. Also, the pressure patterns are more This is usually an optional entry and many regular in this hemisphere due to the absence of weather offices no longer enter the types of air the large land bodies, present in the Northern masses on theirbasecharts.If this stepis Hemisphere. However, certainsimilarities do desired, label the air masses in accordance with existbetween both hemispheres inthatall the standard air mass symbols. Sometimes an atmosphericfeatureswhich dependfunda- alternate method isused to indicate the air mentally on static relationships, the hydrostatic masses. This method either shades the cold and equation in particular, are the same. For exam- warm air masses in light blue and red, respec- ple,in both hemispheres fronts, troughs and tively, or is indicated by a large arrow, shaded in lows slope toward the coldest air, ridges and lightly and labeled appropriately, showing the highs toward the warmest air, isobar kinks point direction of movement of the air mass. towardhigherpressure,andstaticstability

207 213 AEROGRAPUER'S MATE 1 & C

criteria in the form of lapse rate and moisture Southern Hemisphere. Northmeans equator- relations are unchanged. The basic differencesof ward in this hemisphere and southmeans pole- hemispheric analysis can be subdivided intotwo ward. groups, geographical (including topographical), and d nam ical. DYNAMICAL CONTRASTS GEOGRAPHIC CONTRASTS All dynamical differences stem fromone fact; Aside fromthegreater landarea of the the coriolis parameter is negative in theSouth- Northern Hemisphere, the distribution of ern Hemisphere. For example, the basic relation- conti- ship between wind and pressure nental areas (including ice surfaces which donot as expressed in break at any season) is quite different. Only BuyBallot'slawbecomes inthe Southern Hemisphere: Facing in within 10°-25° of the Equatorare the landmass the direction toward areas of the two hemispheres comparable. The which the wind is blowing, lowpressure is found gleat landmasses of the Northern Hemisphere on the right. Consequently, the sense of rotation extend from subtropical to subarctic latitudes, about lowsisclockwise and about highsis counterclockwise in this hemisphere. However, with the Arctic Ocean covering most of thearea north of the 65th parallel.In the Southern lows are called cyclones and highs anticyclones Hemisphere only one-fifth of the 30th parallel in either hemisphere. crosses land and only one twenty-fifth of the Cyclonic vorticity is negative and anticyclonic vorticity is positive in the Southern Hemsiphere. 10th is not maritime. From 45° S to 65° Sthere virtually no unbroken water, with the edge of The sign conventions for divergence andconver- gence are the Antarctic icecap oscillating seasonallynear unchanged, but relations between the latter circle. divergence and vorticity must be modified. All statements relating temperature and changes in In polar regions, the Arctic icecap seldom windwithheight must be alteredfor the rises more than a few feet abovesea level, and Southern Hemisphere since they involve applica- areas of open water appear during the summer tions of Buy Ballot's law to thermal winds and season. The topography of the Antarctic icecap temperature fields, rises to altitudes over 13,000 feet. Most of the areais a plateau of mean elevation of about GENERAL CIRCULATION 10,000feet,dropping sharplytosealevel around the periphery. Only a few isolatedareas Hemispheric differences, in general, decrease of open water have ever been observedin with increasing altitude. All the jet streams of summer Greenland is the nearest North Ameri- the Northern Hemisphere have their southern can analog. analogues. Southern jets are more intenseon the The longitudinaldistribution of land and average with smaller amplitudes, reflecting the eater is a:so different, with two major conti- greater zonal indices (over double in magnitude) nents and oceans in the Northern Hemisphere of this hemisphere. The stratospheric cycloneof and three ea..11 in the Southern Hemisphere. The the polar night transforms intr the anticyclone principal topographical features of 50° S lati- of the polar day in both hemispheres, butthe tude are the Andes of South America and the "explosive warming" prior to the onset of the plateau of South Africa, with itsmean elevation polar day has not yet been observedover the about 5,000 feet, dropping rather abruptly to Antarctic. sea level all around the periphery as does the Blocks arecomparativelyrare and occur Antal\ tic icecap. Australia is practically without southeast of continents in late winter and early marked topographicalfeatures, and nowhere spring, and drift slowly eastward, again empha- south of the Equator are thereany features sizing the importance of middle latitude conti- comparable to the Alps, Urals, or Himalayas. nents on features of the general circulation. There is sonic doubt as to whether counter- North, as we think of itin the Northern parts of the Aleutian and Icelandic lows exist in Hemisphere, has a different connotation in the the mean circulation ofthe Southern Chapter 6-- SURFACE WEATHER MAP ANALYSIS

Hemisphere. If they do, they are located in the pressureisrelativelyhigh over .and, separate Ross and Weddell Sea areas, on the edge of subtropicalanticyclones canstillbedistin- Antarctica, and are most sharply defined at about guished. The only pronounced seasonal variation 700 millibars. Navy meteorologists with experi- of pressure ni midlatitudes is the shift of the ence in those areas indicate that many lows under- axes of the subtropical highs, moving only a few going cyclolysis move into these regions but degrees of latitude northward during the North- disappear rapidly at low levels, retaining their ern Hemisphere summer and a few degrees of identity as cold lows aloft for longer periods. latitude southward in the Northern Hemisphere Tentatively,itcan he said that the Ross and winter. Weddell Sea lows exist aloft in both the general and mean circulation, but near sea level only in AIR MASSES the general circulation. Not all lows are stopped on the periphery of Antarctica, they penetrate The semipermanent highs produce tropical ail to the South Pole. similar to the highs in the Northern Hemisphere. The subtropical highs of the South Atlantic, There is no continental polar air in the Southern South Pacific, and South Indian Oceans differ Hemisphere. The air over the snow and ice from their northern counterparts in number, covered regionsis Antarctic air butitrarely permanence, seasonal migration. and seasonal leaves this area as true Antarctic air. It becomes intensitychanges. They are more migratory rapidly modified to mP air as it moves over the zonally, less migratory merle ionally, and they water. Maritime polar air is the most predomi- decrease in intensity from winter to summer. nant air of the Southern Hemisphere. Air forming in the central dry regions of South Arica MEAN PRESSURE CHARACTERISTICS and Australia has allthe properties of continental tropical air and, inits source region, is The mean pressuredistributionoverthe dry and cloudless with a steep lapse rate. When Southern Hemisphere shows three large semi- it moves to neighboring oceans. a strong inver- permanent highs exist in the midlatitudes over sion is formed in the lower layers and with the the oceans. (See fig. 4-5.) One is in the eastern addition of moisture from the ocean. sheets of South Pacific extending from about 140 degrees stratocumulus and stratus are formed. westto the west coast of South America; a second one almostcompletely covering the MAJOR FRONTAL ZONES South Atlantic Ocean; and the third more or less centrally placed over the South Indian Ocean. In figures 2-7 and 2-8 you can see the major Along the mean axes of these highs, about 30°S frontal zones of the Southern Hemisphere. Areas and between these highs, lie indifferent regions of frontogenesis are found in the semistationary of cols. South of this high pressure belt, pressure polar troughs that exist along the western border decreaseisregular and marked to about 65° of each of the subtropical highs. Wave distur- south where a continuous low pressure trough bances form along these fronts much the same as known as the "Antarctic Trough" encircles the those forming along the polar fronts in the Antarcticcontinent.Farther southisfound Northern Hemisphere. Usually they develop into higher pressure, over Antarctica. North of the families of from two to six. This wave sequence high-pressure belt, from 30° south, is the region ends when the final wave member of the series of low pressure, located over the equatorial has run together and occluded in a large central regions, that separates the high pressure belts of cyclone to the southeast of the semipermanent both hemispheres. In summer heat lows appear highs. south of the Amazon Basin in South America, over the whole of the eastern part of South SYNOPTIC CHARACTERISTICS Africa, and over northern Australia. The latter is OF THE PRESSURE PATTERN thelargestofthese heatlows and covers northern Australia, the Dutch East Indies, and Between the regions of semipermanent highs the central Indian Ocean. In winter, although are trains of warm migratory anticyclones. They

209 215 AEROGRAPHER'S MATE I & C

move steadily at about 10 degrees of longitude however,thatthe orientation of the cloud perclayfrom westtoeast.Between these systems in relation to the meteorological fea- migratory highs are the upper parts of inverted tures must be adjusted in accordance with the V-shapeddepressionsthat move along with reversed flow. them. These V-shaped depressionsare the northern parts of larger cyclonic systems to the south. APPLICATION OF COMPUTER PRODUCTS APPLICATION OF SATELLITE CLOUD PHOTOGRAPHS As mentioned earlier inthis chapter, computer products will frequently be used in place The application of satellite cloud photographs of the hand drawn analysis. This practice is to analysis andforecastinginthe Southern rapidly becoming standard procedure, andis Hemisphereutilizesthe same procedures as even more applicable to the Southern Hemi- presented earlier in this chapter for the Northern sphere due to scarcity of data. itis therefore Hemisphere. The analyst must bear in mind, imperative that the AG learn as much as hecan

6;3

1016 /a140 1012 56193 68K, 7903,32e, SI 51 .210 63 2b_t_41. 30 \/ 67 2 /3.7 550°Z6 18b57(\6\7.) 1016 65 45240 20a 5 1 1: 3N,8 41221 51 221 \ 30 45 C\G.)4921)> ( 41227 as8C)280 St(:CV 48 S 199,4655 1" 31 45 4911I53 1020 47 230 43,0 1016 99a00 45 233 6\31222 44 283 49/.,..z,3284 43 58:82 54 44e .0 47 220111 48.02.0 H H 1'71c- 4632-282., 1;48 a 2' 3 19

49 ISO 7 1020

1012 016

AG.514 Figure 6-19.Weather map for a portion of the Southern Hemisphere.

210 4r,4 144.L131 Chapter 6- SURFACE wEATIIER MAP ANALYSIS about the application or computer products in cold fronts. Fronts are usually weaker in density analysis and forecasting. Their application will contrastthanintheNorthern Hemisphere. be discussedinneater detailin chapters 8 Figure 6-19 shows a typical analysis of surface through 12 pertaining to forecasting. The refer- chart in the region of Australia. Note that the ences mentioned earlier in this chapter as well as feathers are drawn on the right side of the shall any later or more current publications should be looking toward the station circle. carefully examined for additional information. Some basic aids to analysisover the Southern flemisphere are listed below: SYNOPTIC ANALYSIS I. Extensive use of wind scales should be utilized over the water areas. Basically, the techniques of Southern Hemi- 2. A knowledge of terrain and geographic sphere analysis do not differ appreciably from characteristicsisimportant when considering those of its Northern Hemisphere counterpart, island and other land reports. but because the Southern Hemisphere is largely 3. Seasonal variations that may indicateerro- made up of water, oceanic analysis is of greater neous heat lows must be handled with caution. importance. There are some notable exceptions. 4. Because of the rapid altitude changes in Since warm type occlusions develop principally short distances on the edges of South Africa and off the west coasts of continents in middle Antarctica, no attempt should be made to latitudes,itisextremely unlikely that such connect the pressure fields of the continental occlusions ever occur in the Southern Hemi- and adjacent ocean areas. Discontinuous isobars sphere. The so called "meridional (rows" are are in order wherever terrain heights differ by usually cold occluded fronts or sometimes just 5,000 feet or more.

211 217 CHAPTER 7

UPPER AIR ANALYSIS

Pra Lticall!, all weather as we know and experi- sy noptic useif observers are fully trained to enceitoccursinthe troposphere. For this exercise the proper care in the evaluation of reason the Aerographer's Mate must have data soundings. The compatibility tests sho...ed that from levels other than the surface to provide in practice an individual radiosonde will report himself and other users with the most complete temperature to within 1°C. 91 percent of the picture possible of the 3-dimensional distribu- time. tion of wind and temperature in the atmosphere. Upper air observations yield most of these data Pressure and when plotted and analyzed on upper air charts anddiagrams, these analyses canbe The average instrumental error in pressure is relatedto the surface and other upper air from 2 to 4 mb with an absolute ma,:imum of manifestations to gain a more accurate picture about 5 mb. This maximum error. however, is of the existing conditions. found in only I out of 100 instruments. The mean virtual temperature estimate of an isobaric EVALUATION OF DATA layer of any appreciab;:.: thicknessis. for all practical purposes, unaffected by the relatively DATA CONSIDERATIONS small instrumental error.

The analyst should be aware of. and alert for, discrepancies between reported data and that Humidity actually existing at the particular level. The same type of errors and unrepresentativeness of data In the average upper air sounding the humid- can exist at upper levels as at surface levels and ity reports are about 5 percent in error at the depend to a large extent on the method used in 700-mb level, and they may be as high as 20 determining the data. percent at the 300-mb level. Relative humidity values are always given with respect to a water Temperature surface and never with respect to ice crystals. This means that in passing through a cirrus type An international comparison of radiosondes cloud, the humidity element may give only a 70 sponsored by the World Meteorological Organi- to 80 percent relative :tumidity and stillbe zation in Switzerland in 1950 revealed that in 95 accurate. For example. at -20°C the humidity percent of the cases the differences between the with respect to water need only be about 83 data given by any two radiosondes on the same percent for the air to be saturated with respect flight were not greater than 2°C and 20 percent to ice. In this respect one can make a very rough relative humidity up to 700 mb. The results estimate that for every 10°C below zero, a 10 showed that while some instruments are affected percent lowering of the reported relative humid- too greatly by solar radiation and more accuracy ity can be anticipated due to the fact that the is needed in humidity measurements below 0°C. humidity is coded with respect to water rather the instruments provide satisfactory data for than with respect to ice.

212 218 Chapter 7UPPER AIR ANALYSIS

Upper Winds limited somewhat in accuracy by the types of instruments used.In general they are not as The reliability or upper wind observations accurate a; rawinsonde and radiosonde equip- depends upon the manner in which they were ment. Hov,ever, flight altitude data are reason- determined. For example, when plane triangula- able accurate and useful. Heights are usually tionis used to determine the height of the given at some mandatory pressure surface (700-, balloon, and the curvature of the earth is not 500-mb, etc.). taken into account, the balloon seems to be lower than it actually is. AIRCRAFT REPORTS (AIREP'S) Wind directionsareoften veryunreliable when light winds in the stratosphere are ob- The accuracy of AI REP data will vary radi- served above strong winds in the troposphere, cally with the method of position determina- and they may be as much as 180 degrees off in tion, the instruments available to the navigator, direction. instrument limitations, and the navigation procedures used. When extrapolations are made from Summary flight altitude to some other level, these must be corrected. In those cases where the mean wind Table 7-1is the table used by the National over the last hour is reported, the wind should Meteorological Center for the probable error in be spotted at the midpoint of the last hourly height, wind, and temperature data at various track. elevations. RESOLVING ERRONEOUS DATA ORDER OF ACCURACY OF UPPER AIR DATA Reports are occasionally received and plotted which are evidently not compatible with other The order of accuracy of upper air data is observations in the same region. In many cases listed below. such erroneous reports can be resolved into usabledata. Such error may appear for a Rawinsondes (Radiosondes) number of reasons. Common ones, which can sometimes be resolved, are communications and Rawin wind data are for the most part very plotting errors, computation errors, and in the accurate.Heights andtemperature data are case of aircraft reports, erroneous position re- subject to the same type errors listed previously. ports. Although the Aerographer's Mate must Wind data are most accurate when determined exercise discretion in correcting such errors, he by radar. should not throw away or disregard data, unless it is absolutely necessary. Pibal Data PURPOSE OF ANALYSIS The outstanding accuracy limitations of this type of observations are the assumption of a The purpose of the analysis of upper air constant ascension rate and the possibility of charts istoestimate and to representthe errors because of the timelag in readings by the continuous distribution of atmospheric condi- manual operator. Errors in pilot balloon data tions throughout three dimensions from observa- may become especially significant in the case of tions made at but few points or along a few strong winds or large vertical shears. lines. Such an analysis should provide pictorial representations of the conditions everywhere; otherwise the many interrelationships between Weather Reconnaissance Reports the various conditions to be considered would be too difficult to be useful. Pressure, temperature, and humidity observa- Inmany regions the greatest amount of tionsgathered by dropsondemethodsare observational data is available at the surface of

213 AEROGRAPHER'S MATE 1 & C

Table 7-1.Probable errors in upper air data.

Parameter Probable error Remarks

Raoh temperatures + 1° C to 400 mb 7 2° C above Raob heights + 6 meters per 305 Different temperature parameter 1,000 mb meters of station used for reduction to 1,000 mb elevation than to msl which results in inconsistencies of the order of 120 meters in extreme cases. In order to circumvent this, we directly convert the reported msl pressure to 1,000 mb. 850 mb + 9 meters 700 mb + 12 meters 500 nib + 21 meters 400 nib ± 30 meters 300 nib + 49 meters 200 nib + 70 meters 150 mb _+90 meters Thicknesses: 1,000-700 ± 12 meters 1,000-500 + 21 meters 500-300 + 27 meters Tr opopaus e heights ± 10 mb Winds aloft ± 5 degrees Due in part to rounding off to directions nearest 5 degrees in coding. Winds aloft + 10 knots up to This error increases rapidly, speeds 500 nib for especially for winds over 100 winds up to kt, usually due to low elevation 50-75kt angles of recording equipment. Also varies considerably with different types of equipment. the earth. In those cases it is desirable to start If the complete 3-dimensional picture is avail- with the surface level and work up through the able to the meteorologist, his forecast problem is successive upper levels by the use of thickness greatly simplified. Therefore, it is important that analyses. This method is known as differential data be evaluated, extrapolated, and analyzed analysis and is discussed later in this chapter. accurately to portray the best possible picture of However, in some regions data are adequate and the actual atmospheric conditions. Some of the ample attheupperlevelsand only some data useful to both the meteorologist and the intermediate points must be constructed. pilot are apparent from a preliminary study of

214 220 Chapter 7UPPER AIR ANALYSIS the map. Other data must be computed or temperatures will then be less than the "true" derived from this. Two of the most important mean temperature. Of course the analyst can considerations are the use of winds, both actual compensate for such inversions when their pres- and computed, in the analysis of contours. and ence is suggested on the synoptic map by using a the extrapolation of heights in areas where data high estimate of the upper temperatures. This are sparse. procedure could be used in the following situa- Constant pressure charts are used for: tions.

I.Locating pressure systems. I.In the vicinity of high pressure cells where 2. Steering pressure systems. subsidence inversions are present. 3. Locating dry and moist layers of air. 2. When surface inversions are indicated by 4. Determining if fronts extend to the level in stable weather phenomena such as fog. question. 3. When a frontal surface is below the level to 5. Locating cyclonic and anticyclonic flow. be extrapolated. 6. Locating areas of horizontal convergence and divergence. A recent study comparing the true thickness 7. Forecasting surface and upper air weather. of the 1,000-700-mb and 1.000-500-mb layers 8. Constructing thickness charts and advec- to the value obtained by using the arithmetic tion charts. mean of the 1.000-mb temperature and the 700 - 9. Constructing time differential charts. or 500-nib temperature. respectively, showed an 10. Jetstream and isotach analysis. error of less than 30 meters in 90 percent of the cases for the former, and less than 60 meters in EXTRAPOLATION OF HEIGHTS OF 90 percent of the cases for the latter. This CONSTANT PRESSURE SURFACES sample included several hundred soundings. Obviously, if we are to construct the height of In practice, the analysis of upper level charts the 700- or 500-mb level, the first correction may be conveniently placed in two categories that must be madeis that of correcting the the analysis of sparse data areas and the analysis current temperature and pressure to the 1,000 of dense data areas. On many occasions, upper mb level and then extrapolating the height of air observations do not reach all the way up to the upper level. thelevel where ananalysis must be made. Because of the scarcity of upper air reports over Computing Height of the ocean areas, it is frequently desirable to extrapo- 1.000-mb Surface late upper air data from surface reports. There are many methods. employing the use of tables The height of the 1,000-mb pressure surface is or nomograms. which may be used to accom- the height of this surface in geopotential meters plishthis purpose. All of these are based on above mean sea level. For computational pur- some form of the hydrostati,..: equation. With the poses. assume that 7 1/2 nib equals 60 meters known surface temperature and pressure. and for 8 meters per I millibar) when determining the assumed mean virtual temperature to the this height. desired level, it is possible to obtain the height of that level. The mean virtual temperature is Extrapolation of Upper Levels usually obtained by assuming a moist adiabatic lapse rate or by averaging the known surface One of the methods for obtaining the height temperature and an estimated temperature for of the 700- or 500-mb level is outlined below. the upper level. The latter estimated temperature is based on previous analysis compensated !.Estimatethetemperature atthe upper for any changes which may have occured during level from past analysis. the interim. In the case of marked inversions of 2. Use figure 7 -I. With a straightedge line up any type. the estimated height is less than the the surface temperature on the left scale with "true" height since the arithmetic mean of the the upper level temperature on the right scale.

215 221 AEROGRAPI1ER'S MATE 1 & C

1,000 - TO 700 -OR 1,000 - TO 500 -MILLIBAR THICKNESSIMETERS) 6,370 -49 3,261 6,340 6,309 3,231 6,279 -35 6,248 3,200 6,218 6,187 3,170 6,157 --30 6,127 3,140 6,096 6,066 -.-25 3,109 6,035 6,005 3,079 5,973 5,943 3,048 5,912 5,882 3,016 5,852 .-.-15 5,822 2,887 5,79 1 2,957 5,761 -10 5,730

2,927 5,700 5,669 -.5 2,897 5,639 5,608

2,865 5,57 6 1,000 -TO 1,000 - TO CELSIUS IEMPERATURE TOO-MB. 5,547 -0 500-MB. TEMPERATURE 30 AT 1,000 -MB. THICKNESS 2,635 5,517 THICKNESS AT UPPER LEVEL FROM 5,486 PRESSURE SURFACE REPORTS 2,80 5,456 SURFACE (ESTIMATED) -5 5,425 2,774 5, 395 5,365 --10 2,7 43 5,334 HOW TO USE 5,304 2,713 5,273 I. WITH A STRAIGHTEDGE,LINE UP- 5,243 SURFACE TEMPERATURE ON --I5 2,682 LEFT SCALE WITH UPPER-LEVEL - 5,212 TEMPERATURE ON RIGHT SCALE. 5,182 READ OFF THICKNESS OF 2,652 5,151 APPROPRIATE LAYER ON CENTER SCALE WHERE STRAIGHTEDGE --20 5,12 0 INTERSECTS. 5,090 2. TO GET HEIGHT OF UPPER 5,0605 PRESSURE SURFACE ABOVE SEA 2,591 5,029 LEVEL,ADD OR SUBTRACT -25 HEIGHT OF 1,000 -MB. LEVEL 4,999 CONVERTED FROM SEA LEVEL 4.956 PRESSURE. 4,9384 2,530 -30 4,907 4,877 4,846 -35 4,816 4,785 4,755 4,724 -,--40 °F °C AG.515 Figure 7.t- Nomogram for computing height of the 700-mb and 500mb levels.

216 222. Chapter 7-UPPER AIR ANALYSIS

Read off the thickness of the appropriate layer times 60 meters equals120 metersifthe wherethe straightedge intersects the center standard value is used). scale. Note that this is the thickness from 1,000 c. The height of the 700-mb level equals mb to the upper level and that values for the 2,957 meters plus120 meters equals 3,077. 1,000- to 700-mb thickness are on the left of the meters. center scale and values for the 1,000- to 500-mb Inareas of sparse reports andif a quick thickness are on the right of the center scale. computation of the thickness is desired, figure 3. Estimate the height of the 1,000 -mb level. 7-2 may be used to compute the thicknesses 4. Add algebraically the height obtained in betweenthe1,000-700 mb layer andthe step 3 and the thickness obtained in step 2. This 1,000-500 mb layer using the temperatures at is the height of the upper surface. For example, the 850 and 700 mb levels,respectively. A a ship reports a pressure of 1,015 mb and a column is also included in this table to show the temperature of 78° F. If the temperature of the thickness from 500 to 300 millibars based on 700-mb level is estimated to be -5° C,hat is the temperature value at 500 millibars. A word the height of the 700-mb level? of caution inusing thistable: inversions or a. The 1,000- to 700-mb thickness equals nonrepresentativetemperaturesateitheiof 2,957 meters (from fig. 7-1). these levels will result in an incorrect thickness b. The height of the 1,000-mb level equals for the layer. For a more correct computation 120 meters (1,015 mb minus 1,000 mb equals based on this method see Table 53, Smithsonian 15 mb; 15 mb divided by 7 1/2 mb equals 2; 2 Meteorological Tables, NA 50-113-521.

t850 1000-700 t700 1000-500 t500 500-300 -38 2439 -41 4694 -47 3261 -35 2469 -37 4755 -46 1292 -32 2499 -34 4816. -45 3292 -27 2560 -31 4877 -43 3322 -24 2591 -28 4938 -41 3322 -21 2621 -25 4999 -39 3353 -16 2682 -23 5060 -37 3353 -13 2713 -20 5121 -35 3413 -10 2743 -17 5182 -33 3444 -7 2774 -14 5243 -30 3475 -4 2804 -11 5304 -27 3505 -1 2835 -9 5365 -24 3536 +1 2865 -6 5425 -22 3566 +4 2896 -3 5486 -20 3597 +7 2926 0 5547 -19 3627 +12 2987 +3 5608 -17 3658 +15 3018 +5 5669 -14 3688 +18 3048 +8 5730 -11. 3719 +22 3079 +11 5791 -9 3749 +26 3139 +14 5852 -7 3780 +30 3170 +17 5913 -6 3810

LEGEND ON TABLE.

Z's - thickness in meters between specified constant pressure levels is- temperatures in degrees Celsius at specified levels

AG.516 Figure 7-2.-Thickness values from temperatures at constant pressure levels. 217 22a, AEROGRAPHER'S MATE 1 & C

EVALUATION OF WIND mal wind is not a wind which actually blows in theatmosphere: rather,itisthe difference When evaluating data during the analysis of between winds at two levels. To illustrate, if the upper winds, the analyst should differentiate 700-mb geostrophic windis southwest at 50 between that data which is related to actual knots, and the 500-mb geostrophic wind is west wind flow as compared to the flow of ge- at 80 knots, then the vector difference or the ostrophic wind. thermal wind is northwest at 58 knots. (See fig. 7-3.) Differences Between Actual and Geostrophic Winds

Itis sometimes helpfulto consider some theoretical ways in which the geostrophic wind should differ from actual winds. At low levels (below 2.000 feet above the terrain) friction with the ground is effective and actual winds should flow across contours or isobars from high tolowvalues.Abovethefrictionlayer, discrepancies should occur whenever the flow is curved rather than along straight lines. Actual 10 20 30 40 50 60 70 80 windsshould besomewhatweakerthan 500 mb WIND geostrophic winds incounterclockwise flows around lows, and somewhat stronger than geostrophic winds in clockwise flows around highs. Inlow latitudes (between 20° N and 20° S) geostrophic winds become poor estimates of the actual winds because the geostrophic wind is inversely proportional to the sine of the latitude which approaches zero near the Equator.

Wind Shear Wind shear may be either on a vertical or horizontal plane. The vectoral rate of change of wind with respecttoaltitudeiscalledthe vertical wind shear. Horizontal wind shear is the rateof change on ahorizontalplane. The 700mb vertical wind shear per unit distance within a CONTOURS layer of air may be determined by taking the vector difference between the wind reported at AG.517 the top of the layer and the wind reported at the Figure 7-3.Thermal wind between two pressure surfaces. bottom of the layer and dividing bytheir vertical separation. In this example, the mean isotherms between 500 and 700 mb would be orientedina Thermal Wind and Wind Shear northwest to southeast direction with colder air toward the north. In general, the direction of Thevectordifferencebetweenthegeo- the thermal wind is parallel to the mean iso- strophic winds at two levels is called the thermal therms in the given layer, with the colder air to wind by meteorologists. Consequently, the ther- the left; looking downstream. The magnitude of

218 224 Chapter 7---UPPER AIR ANALYSIS the thermal mind is dneetly pioportional to the system appearing in an AVCS satellite photo- mean temperature gradient of the layer. graph. In summary, the thermal wind bears the same The IIRIR picture shows the familiar cloud relationship to the horizontal mean temperature pattern (fig.7-4(A)) associated with a cutoff gradient in a layer of air as the geostrophic wind low in the eastern North Atlantic. In this case does to the horizontal pressure gradient. the cloud bands which spiralin toward the center contain some cumulus congestus, but One must distinguish between the change of thereis no distinct spiral of low-level clouds the actual wind through a layer and the thermal which would define the location of the center wind of that layer. The actual wind change is the on a surface map. In fact, there is some question vector difference of the observed winds at the whether or not a surface low center exists. two levels. On the other hand, the thermal wind is a special type of wind change vector, if the The general patternis quite similar in the winds at both levels are geostrophic, the actual visual and IR pictures. but the areas of strong wind change and thermal wind are identical. In vertical motion and active weather are more somecases,as whenthecurvatureof the clearly depicted inthe temperature field. For contours changes rapidly with height, there may example,thecloudband(fig.7-4(13) and be a difference between the thermal wind and 7,5(B)). appeared quite bright and solid in the the actual wind shift through the layer. visual pictures taken at both 12 hours before and12 hours after theIR, whereas the IR One of the most fundamental meteorological showed only small scattered areas of activity. concepts used by meteorologists in upper level The area shown in figure 7-5(C) appears quite analysis is the relationship between wind and active in the visual mode, but the IR shows that heightas expressed by the geostrophic and the cloud tops are low and flat. gradient wind equations. The preceding comparisons are intended to point out a few of the interpretations which Geostrophic Wind Scales mightbe derived fromtheuse of satellite pictures. It should be borne in mind that the The mathematical equations representing de- HRIR nighttime view also yields definitive infor- viationsingeostrophic wind are appliedto mationforflightbriefing purposes regaiding analysis through the use of scales. The procedure icing. turbulence, and type of precipitati,,n in for using these scales is presented in chapter 6 of the vicinity of the systems appearing in the this manual. The use of geostrophic wind scales pictures. makes practical application of the equations possiblewithinthetime-frame necessaryto From the preceding discussionitbecomes maintain validity in the analysis. readilyapparentthat a comparison of the analysis withthesatellitepicturesisquite beneficial to the analyst. He can not only verify the existence, intensity, and position of weather APPLICATION OF systems, but in sparse data areas this may be his SATELLITE DATA only means of completing the analysis. The cloud formations depicted on satellite Information related to the interpretation of pictures have a definite relationship to the flow satellite pictures as they pertain to map analysis patterns which appear through the analysis of may be found in the Direct Transmission System computer or hand drawn weather charts. Figure Users Guide, available from NOAA and Guide 7-4 illustrates the relationship between an upper for Observing the Environment with Satellite airanalysisandan HRIR (highresolution Infrared Imagery, NWRF F-0970-158. In addi- infrared) satellite picture showing a cutoff low tion, other publications arelistedin NavSup at the 500-mb level. Figure 7-5 shows the same 2002. AEROGRAPIIER'S MATE I & C

SURFACE (SOLID) AND .,00M8 (DASHED) ANALYSES 0000Z 9 JUNE 1969 IR, LOCAL MIDNIGHT (oliez 9 JUNE 1969)

AG.518 Figure 74.-500mb analysis and accompanying HRIR satellite picture.

REVIEW OF PARAMETERS Thickness

Contours The geometeric thickness of a layer of air between two isobaric surfaces is evaluatedfrom the mean virtual temperature of the layer. The Contours are lines joining equal heights of a thickness may be found by taking the difference given isobaric surface. These lines areusually between the heights reportedat mandatory drawn for 60 or 120 meter intervals. Occasion- isobaric levels at each station. ally they may be drawn for intermediate 30 or 60 meter intervals where the gradient isrela- Wind Shear tively flat. Wind observations are the primary source of data. along with height values,for The wind shear may be either vertical or drawing contours. Thatis.the contours are horizontalandisimportantin determining directed and spaced in accordance with observed turbulence, location of frontal zones, and other and calculated winds. factors of meteorological importance. 220 226 Chapter 7UPPER AIR ANALYSIS

I . w v.

41007r1 kk r fj, 1, ....Ake. ,

AG.519 Figure 7-5.-500mb analysis and accompanying AVCS satellite picture.

Deepening and Filling continuity. The slope of height fields and their orientations do not change radically in short areterms usedto Deepening andfilling periods of time. Consequently, a valuable aid in describe processes wherein the pressure at the contour analysis of a given pressure surface is center of a low pressure system is decreasing the past history of the contours at that surface. with time (deepening) or the system is dissipat- The study of previous maps is essential, both as ing (filling). Deepening usually indicates a con- a key to the present situation, and as a method tinued intensification of the gradients of pres- of preserving an orderly progression of move- sure systems. ment and change of atmospheric systems. The necessityformaintainingthiscontinuityis Strengthening and Weakening essential for analysis as well as for prognostic A pressure system is said to strengthen when purposes. the gradients and the associated winds become stronger. When the contour gradient becomes flatter or less intense with time, the system is COMPARISON WITH said to weaken. COMPUTER ANALYSIS

CONSIDERATION OF CONTINUITY When analyzing upper air charts it must be emphasized thatallavailable data should be One basic consideration in the approach to all utilized. A valuable and often used aidfor types of map analysisisthat of history or determining the validity of the manually prepared

221 22.7 AEROGRAPHER'S MATE I& C

analysis is thecomputer analysis.Inthe I. Determine the accuracy of the"principal not too distant future, as hardware continuesto analyses"in and upstream of the operational become more available, manually performed area of concern. analysiswill no doubt become entirelyout- The surface pressure and the 500 mb height moded. However, the analystor forecaster who analyses arethe base upon which all other understands the concepts involved inaccom- atmospheric analyses are constructed. Figures plishing the manual analysis will beable to more 7-8and7-9 show examples ofthese two intelligently utilize the computer product.Re- important charts. member, a machine cannot exercisejudgement. The accuracy of these "principal analyses" It must be provided withaccurate data and profoundly affects the validity of directions for making decisions. most meteor- Although in ological and oceanographicprognoses produced most cases the computer product will bemore within the computer program. Before accurate than the manually produced using any one, the prognosticcharttheoperationalforecaster analyst must apply his own judgement based on should satisfy himself that these principalanal- his experience and other availabledata. He must yses are reasonably correct. not blindly follow any singlesource, unless this 2. If necessary, modify the is prognoses which all he has available. A relativelyaccurate will be used in preparing required forecastsby: analysis will be accomplished if comparisonis a. Correcting for any errors detected in the made between satellite data, computerproducts principal analyses. and the locally prepared manual analysis(assum- b. Correcting for known weaknesses in- ing adequate personnel are available). herent inthe prognostic modelas discussed previously in chapter 6 of this manual. NOTE: Types of Computer The overheadprojector or lighttableisa Products Available valuable aid in accomplishing these twosteps. 3. Interpret relevant analyses andprognoses There are over 500 products currently listed forrequiredmeteorologicaland/or oceanographic forecasts, correcting for known in the General Environmental ComputerProd- local ucts Catalog. This publication is published and effects. NOTE: The Local Area Forecasters revised periodically by FNWC Monterey. Handbook, the experience a the forecaster,or textbooks providing data on thearea in question In some instances a computer chartmay be may all be brought into play during this phase of received which is completely unfamiliarto the the analysis. recipient. By using the computer productscata- log, a description of the chart andits content SEA LEVEL PRESSURE ANALYSIS.The can be obtained. For example, the GG THETA sea level pressure analysis was discussed briefly ANAL showninfigure7-6orthe CCAT in chapter 6 of this manual. To obtainthis chart, ANALYSIS shown in figure 7-7 mightbe re- computations involving stationpressure, present ceived at a remote site without explanation. weather, visibility, and ceilingareextracted The catalog would reveal that these charts from each report. Objective ana;ysis ofstation were an Atmospheric Fronts Analysis and Clear pressure values is performed using the previous Air Turbulence Analysis respectively. analyses as described in chapter 6. Theprogram output may either provide a hemispheric product as shown in figure 7-8 or, witha variation in the gross error check and the analyses used,a Procedure for Product surface pressure analysis of the UnitedStates Application may be obtained. On the analysis limited to the UnitedStates more detailedinformationis The following is a recommended procedure shown. This includes significant weathersym- for utilizing the computer productsas described bols and station flight conditions. Foran illus- in the Computer Products Manual, Nav Air tration of this chart refer to chapter 3 ofthe 50 -1 G -522. Computer Products Manual, Nav Air 50-IG-522.

222 228 Chapter 7-- UPPER AIR ANALYSIS

0,12Z CZ .GGil-113107: FOXiiMeFt04:Y

AG.520 of Figure 7-6.Atmospheric fronts 36hourprognosis. Axis of patterns depicts warm boundary strong 1000-mb temperature gradients.Strength of temperature gradient is proportional to number of isolines surrounding pattern axis.

223 229 AEROGRAM IER'S MATE 1 & C

AG.521 Figure 7-7.Clea aii- turbulence in the 400. 300-mb layer analysis. Contour interval 25 relativeunits.

224 230 Chapter 7 -UPPER AIR ANALYSIS

AG.522 Figure 7.8.Sea level pressure analysis. Contour interval 4 mbs.

225 231 AEROGRAPIIER'S NIATE 1 & C

AG.523 Figure 7.9.-500 mb height analysis. Contour interval60 meters.

226 232 Chapter 7UPPER AIR ANALYSIS

500 MB ANALYSIS. The 500 mb analysis SMOOTHING OF FLOW shown in figure 7-9is obtained through the AND GRADIENTS following procedure: It should be kept in mind that the atmos- 1.Objectiveanalysisof reported 500-mb pheric circulations of interest are of a relatively heights and heights extrapolated by the geo- largescale. Consequently, the field of flow strophic approximation from reported 500-mb represented by the contour lines should nor- winds is performed. The first guess is obtained mally be a smoothly continuous field. However, by reanalyzing the 12 hour previous 500-mb contour gradients are seldom uniform or con- analysis using all data collected during the 12 stant over large areas. Rather, at places such as hour period between synoptic observation times. near jetstreams and near fronts, both contour The "update" analysis usually contains 10 to 20 and thickness lines in the lower troposphere are percent more 500-mb data than did the original. often oriented somewhat parallel to these phe- From the update, a 12 hour 500-mb prognosis nomena and their spacing is considerablycloser verifying at the present map time is derived. This than at other areas. prog is then modified for the 12 hourchange in Small-scale perturbations which are not appar- 500- to 1000-mb thickness inferred from the ent in two or more contours and which cannot present surface pressure analysis. The prog is be associated with some surface system should be smoothed out of the 700- and alsoadjusted toward climatology in regions usually where the update analysis was constructed from 500-mb analyses. On the other hand,itis sparse data. erroneous to assume that gradients remain constant over large areas. Nevertheless, unless there 2. Three cycles through the analysis routine is definite evidence of discontinuous conditions are made with the data. Thefirst and second or important small-scale phenomena,the analy- passes with data utilize only theheight values sis should reflect a uniform progression of flow from the reports. The second pass provides and its time changes. Such a smooth analysis will lateral checking wherein the datum value must usually be more correct than if sporadic changes be within a prescribed tolerance of the value are indicated. In this aspect, youshould use the interpolated from surrounding data. classical models of fronts, jetstreams, etc., to aid 3. Upon completion of the second pass, the indepicting the most likelydistribution of reported winds are used to compute geostrophic winds and contours in regions of sparse data. gradients and derivedheight values atfour For example, it is the policy of the National surround points. The extrapolations parallel to Meteorological Center to smooth out disturb- the wind are placed 1.5 mesh lengths from the ances on surface andupper-level charts of datum and the normal extrapolations are placed wavelengths less than about 8 degrees latitude. 0.75 mesh lengths away. (South of 30N these This policy is based primarily on the belief that distances are changedto2.0 and1.0 mesh the majority of disturbances of wavelengths less lengths respectively). If another report of height than 8 degrees latitude are fictitious and the and windfallswithinthis samearea,the result of inaccurate observations and are non- distances will be modified so that the extrapola- representative of the large scale features. tions are located no closer than half the distance between the two stations. In the caseof aireps HISTORICAL SEQUENCE with ro height values reported, aninterpolation of the analysis is used to provide theworking Historical sequence is vitally important to height values. good map analysis. Some of the reasons for this 4. The third and last analysis pass is made have been pointed out in a previous section using the heights and wind extrapolations with titled"Consideration of Continuity." Conse- lateral checking as defined previously. The final quently, one of the first steps in the analysis analysis field is then modified where necessary should be to check the previous charts for so that the absolute vorticityalways exceeds accuracy, rationality, and any correctionsmade one-third of the coriolis parameter. subsequenttotheinitialanalysis.Fronts,

227 233 AEIZOGRAPHER'S MATE I & C

troughs, and pressure ,:enters do not normally representananalysis. A reasonably correct appear or disappear from one chart to another analysiscanbeobtained only by applying and any such occurrence should beicwecl with careful judgement to all factors involved in the suspicion. analysis. The following discussion of rules for The pastpositions of all pressure centers drawing contours in the Northern Hemisphereis should be entered on tl.current chart fo a illustrated by figures 7-10 and 7-11. period of atleast 24 hours and for longer periods when practicable. These positionsare normallyenteredin blackinkwith an X 324 318 312 306 circumscribed with a circle and connected witha dashed line. The time and dataare entered above the circle. The corrected positions of all fronts, troughs, and ridges should be transposedon the current chart in yellow pencil. Do not forget the HIGH short wave troughs. These troughmove around and through long wave troughs andare often important in the genesis of fronts and cyclones.

CONTOUR ANALYSIS

The rules given in this section generally apply toall upper level chart:,. The analysis of the AG.524 Figure 7-10.Contour pattern between jetstream by isotach analysis is covered ina later a high and a low (700mb heights in meters). section. The primary intervals of contourspac- ing are 60 and 120 meters. The 60-meter interval is used on charts from the surfaceup to 300 mb I. Between adjacent highs and lows, there and the 120-meter interval is used on charts at will never exist two contours of thesame value 300 mb and above. A solid black line isused to as illustrated in figure 7-10. represent these primary contours. Intermediate 2. Between two adjacenthighs there will contours may be used when greater definition is always be two contours with thesame value (A needed: a 30-meter interval is used with the and B in fig. 7-10. 60-meter primary interval anda 60-meter interval is used with theI 20-meter primary interval. Intermediatecontours arerepresented by a 306 300 306 288..,...... LOW___,288 dashed black line. 300 294 294 300 294 Before beginning the contour analysisyou 288 (/1288 306\\_____ 306 should first compute heights in areas ofsparse C data, using the principles outlined ina previous HIGH HIGH HIGH section of this chapter. Such approximationsare 0 306 306,------mostreliableover oceanareas.Plotthese 288 300 294f LOW 300 ,e------294 300 306 approximations on the chart and enclose them 306 294 tise-7----",w 288 in parenthesis. 288

AG.525 RULES FOR DRAWING CONTOURS Figure 7-11.Contour pattern between adjacent (NORTHERN HEMISPHERE) highs and lows (700-mb heights in meters).

Except where reports are closely spaced, there The windflow along contour A is oppositeto are usually many different sets of contours that theflow around contour B, and in sucha can be drawn to a given distribution of height manner that a trough is formed between two reports: however, only one set is correct. There- highs.Remember,thereisalwaysa valley fore,linesdrawnmerelytodatadonot between two mountains. The configuration of 2di Chapter 7 UPPER AIR ANALYSIS independent of the shape of carefully selected so that thesketching can be contour A is medium lead pencil contour B. The heightbetween A and 13 is less cleanly and easily erased. A than the value of .A or B, but greaterthan the is recommended. The region of Contour labels consist of numbersrepresent- value of' the next lower column. line in meters for lightest winds usually exists betweenA and B. ing the whole value of the thereare which each particular contouris drawn. For 3. Between two adjacentlows 3060 meter contour onthe always two contours of the samevalue (C and D example,the along contour 700-mb chart would be labeled306, dropping in fig. 7 -I I). The mean windflow is labeled at C is opposite to the flow around contourD and the tens digit. An open contour both ends; a closed contourshould be broken at in such a manner that a ridgeis formed between one convenient spot topermit entry of the label, two lows. Remember,there is always a moun- of the tain between two valleys. Theconfiguration of usually atthe northernmost position particular contour line. The labelsfor a series of isindependent of the shape of contour C closed, concentric contours shouldbe arranged contour D. The heightbetween C and D is of numbers running C and D, but less than to form an easily read line greater than the value of from low to high contour values.All contour thevalue of the next highest contour.The size, their bases exists between C' labels should be of uniform region of lightest winds usually should be parallelto the adjacent circlesof and D. latitude, and they should be inthe same color and with the same pencil used todraw the COMMON ERRORS IN ANALYSIS contour lines. Libelsshould be neatly printed. do not try inexperienced one, In sketching preliminary contours, An analyst, especially an to manipulate the pencilwith just your fingers, will have difficulty drawing contoursin areas of lines, bringing your illustrates some but make smooth sweeping sparsereports.Figure7-12 entire arm into motion. Have yourhand in such typical errors in contour analysis. a position that itdoes not obstruct the view of the reports with which you areimmediately SKETCHING CONTOURS concerned. Keep your eyes justahead of the pencil, thereby determining thepoints through Contours shouldfirstbe sketched lightly, This will enable with the re- which your contour should pass. using black pencil in accordance you to anticipatechanges in direction which the ported and computed heightsand winds. As these changes may parallel to the contour should take so that stated previously, contours are be smooth rather than abrupt. wind direction and spacedinversely proportional lines, closing wind scales All contours are continuous tothe wind speed. Geostrophic either within the limits of aparticular chart or should always be kept near athand. These scales beyond the margin of the chartbeing drawn. In have been discussed previouslyin this chapter other contour lines, These scales should be no cases do contours cross and in chapters 4 and 6. join contour lines of differentvalues, or become used as an aid in spacing contours,especially in broken. areas of sparse data.Some precautionary rules Wind direction and wind speed areof primary using these scales. should be kept in mind when importance in drawing contourlines. Each con- For stationary troughs andridges, cyclonically drawn parallel to nearby spacing of contour line should be curved contours have a closer winds whose speeds are 10 knots orgreater. tours for the same speedwhen the contours are pattern, the subgradient or While constructing the contour anticyclonic. Too, in regions of analyst should continually strive torepresent the supergradient winds,the winds blow across show a slight wind field accurately. contours. The contours normally for the possibil- level, but show up Allowance must also be made kink on fronts at the 850-mb drawing contours. level. ity of' errors in the data when as troughs above this The experienced analyst mustconstantly bear in The sketched contour patternis adjusted for to the variety of continuity. mind the factors which contribute reported winds and maintenance itis proper to neglect The pencil used to draw contoursshould be of errors. In general, 229 Zas AEROGRAPHER'S MATE 1& C

SAME WIND FLOW ALONG TWO 300 CONTOURS (1 1 HIGH HIGH HIGH HIGH

31211 306 306300 294 300306 300 300 306 312 ANOTHER 200 CONTOUR IS NECESSARY SAME SHAPE AND CONFIGURATION FOR TWO CONTOURS OF THESAME VALUE

300 306300 300 294

HIGH HIGH HIGH LOW

306 300

TWO 300 CONTOURS BETWEEN ADJACENT HIGH AND LOW WIND FLOW REVERSES WIND FLOW REVERSES ALONG CONTOUR B ALONG 294 CONTOUR 294 CORRECTION SHOWN 294 BY DOTTED LINE 300

L H 2881 306 306 300 300294 306 CONTOUR HAS BEEN OMITTED

LOW HIGH LOW LOW TOO GREAT A WIND SHEAR IN THIS VICINITY CORRECTION SHOWN 294 BY DOTTED CONTOURS 300 294 30o 306 306300 294 312 318 324

AG.526 Figure 7-12.Common errors incontour analysis (700-mb chart). unusual departures from the overall height distri- the contours were drawnto the absolute value bution and draw eachcontour as a smooth of each individual height curve, omitting irregularities that would exist value reported. Nor- if mally,however,thelinesshouldnotbe 230 236 Chapter 7 UPPER AIR ANALYSIS smoothed if the Irregulalities are indicated by Figure 7-13 depicts an FIRIR picture of a two or more adjacent contours. When reports frontal band and associated 300-mb short wave are sparse, however, no report should be disre- trough in the eastern Pacific. Here 'Ughest garded unless thoroughly dieUed. clouds, which radiated at the coldest temperature, appear the whitest. and the lower warmer Application of Satellite clouds appear dark gray. In this case, the high Cloud Photographs bright clouds begin abruptly at 0, just east of the The application of satellite cloud photographs point where the 300 millibar trough (dashed to the analysis of contours is somewhat limited line) intersects the frontal band. and dependent to a considerable degree upon To the west of the trough line, the frontal the experience of the analyst. however, there band (Q) appears dark gray: here the frontal are certain aspects which are helpful. Fol exam- clouds are warmer and lower than the clouds ple, the orientation of troughs and ridgs.s may be eastof thetrough. The darkestareas,(P) verified by the existence of certain dotal fea- correspondto clear sky views of the warm tures. Figures 7-4 and 7-5 illustrated to some waters of the Pacific Ocean. The contour gradi- degree tae contour pattern vs satellite presenta- ent is indicated to some degree by the spacing of tion about a cutoff low. the cloud features.

92493 °It 900 ca. R a h 6 309* 293 280*

4QN 0268* 259' -948

253' Piogow i"--960 S 247° 3001.4 239°

150.w 140°W226' NIMBUS I HRIR 210° Pass 279 0941 GMT Sept.16,1964

300 MB CHART

9 1200 GMT Sept.16,1964

AG.527 Figure 7.13. Evidence of an upper level trough and associated contourpattern in HRIR data and accompanying 300-mb analysis.

231

2;,:e7 AEROGRAPHER'S MATE 1 & C The position ofan upper level ridge and either of these conditions,the position of the associated contourpattern areillustratedin figure 7-14, upper level ridge is less clearly definedby the clouds,

APPLICATION OF COMPUTER PRODUCTS

The application ofcomputer products to contour analysis has been foundto be of such value thatinthe majority of instancesthe computer product is used inplace of the hand drawn analysis. However,as mentioned earlier in this chapter,an understanding of the concepts utilized in the hand drawnanalysis will enhance the analyst's or forecaster'sability to utilize the computer product. Given correct data in sufficientdensity the computer performs analyses withan accuracy ) exceeding that of the ,- % -Aiik instruments usedto 1 t . 50N, measure the elements being analyzed. --IT I Computa- vc 4 . \ tion of winds or pressure-height .4.- values using the geostrophic wind equationis performed wher- a lb ever b. 04 desirable. The computeriscapable of .'"./ ; handling complex interactionswith ease and eliminating unreasonablenoise andstability values. Continuity inspace and time is maintained to a degree beyondthe capability of hand methods. The computer is whollyobjective, and eliminates individual differencesin the analysis. If the program and thedata are correct, the AG.528 computer product will be correct. Figure 7-14.-500mb flow pattern superimposed on The recommended procedurefor applying the AVCS satellite photograph showing theposition of an upper level ridge. computer product to the handdrawn analysis was presented earlier in this chapter. There are many computer The accuracy with which charts available to the position of a assist in the location ofridge and trough lines. ridge line can be located isstrongly affected by These charts are listed in the the amplitude of the ridge, and Computer Products its wind field, Manual, NavAir 50-1G-522.The procedures to which determine the associatedcloud structure. be followed and considerations If the upper level ridge has to be made in sharp anticyclonic their applicationwere mentioned in chapter 6 curvature, the multilayered frontal clouds dissi- when discussing surface analysisand earlier in pate rapidly in the area of subsidencedown- this chapter when discussing stream from the point where the hand drawn upper upper level air analysis. Computerprognoses are excellent flow has its maximum anticycloniccurvature. aids for determining future Notice that in figure 7-14 the movement of these eastern edge of the features and will be discussedin chapter 8 of frontal cloudiness ends abruptlyat the 500-mb this manual. ridge. The solidlines show the 500-mb flow pattern.Ifthe windisvery strong or the anticyclonic curvature of theupper level ridge is FINISHING CHART quite gradual, the multilayeredclouds are more Finally, after a reasonably diffuse in appearance east of accurate contour the ridge. Under pattern has beenanalyzed,takinginto Chapter 7UPPER AIR ANALYSIS consideration the factors mentioned above, the sketched contours should be erased and smooth flowing lines, except where noted above, drawn on the chart. Then the contourlabels should be placed in their final form in accordance with previous instructions. The troughs, ridges, highs, and lows should be indicated by the appropriate symbols. SEA-LEVEL ISOBARS CONTOURS IN RELATION --- UPPER-AIR CONTOURS TO PRESSURE SYSTEMS As pointed out in chapter 4 of this training AG.530 manual, the most common upper air pattern Figure 7-16.Slope of surface pressure consists of alternate troughs and ridges, with systems with height. occasional closed lows in the troughs and closed Other closed lows sometimes appear on upper highs in the ridges. This is illustrated in figure air charts and work down to the surface. These 7-15. systems have relatively cold cores and are called cold corelows.Highlevelanticyclones not associated with surface highs are much rarer and never attain an intensity comparable to thatof cold lows.

CONTOURS IN RELATION TO FRONTS

Except on the 1,000-mb chart and along cold fronts on the 850-mb chart, contours kink only slightly or not at all, in crossing fronts. Above 700 mb, fronts are usually omitted from the analysis. It is unusual to find the upper portions of fronts above 850 mb located along upper AG.529 trough lines, except for very short periods of Figure 7.15.Common upper air patterns. time. Usually the upper portions of cold fronts These troughs and ridges form a series of are located somewhere inadvance of the upper waves which encircle each hemispherewith lows trough, with very slight shifts of wind across the normally placed poleward and highs equator- frontal zone and almost impreceptible kinks in ward. These lows and troughs are generally the contours. The major wind shifts and contour associated with cyclones at lower levels, sea level troughs occur in the rear of the upper portion of in particular. the frontal surface. When upper portions of cold Similarly, ridges and highs often reflect the fronts are found along these upper trough lines, vertical extent of surface anticyclones. (Seefig. the fronts are at maximum intensity. Usually, 7-16.) Note that the axes of the systems shown the upper trough moves much more slowly than in figure 7-16 are not vertical; the axis ofthe the wind, and since the cold front moves with cyclone or trough always slopes upwardtoward the wind speed, except for the slowing effect of colder air (usually westward and poleward) and subsidence in the cold mass, the front usually the axis of the anticyclone or ridge alwaysslopes outstrips the trough until it reaches the south- toward the warmest air (usually westwardand west flow in advance of the trough,where its equatorward). These spatialrelations are an eastward movement slows down. absoluterequirementfortheproper Upper portions of warm fronts are seldom 3-d i me n si o nalrepresentationofpressure ever associatedwithtroughs or perceptible systems. cyclonic kinks in the contours of the constant 233 AEROGRAPIIER'S MATE 1 & C

pressure surfaces above 800 mb, assuming the that are most important, and ground is near sea level. Above these can be about 700 mb determined only by making fulluse of all the warm front intersectionis usually found available data and by having nearthe a knowledge of the ridgeline. Not only are thereno picture normallyto be expected in 'a given apparent cyclonic kinks in thecontours crossing situation. the frontal zones, but frequently, contour curva- The isotherms must be reasonabledynami- tures and wind shiftsappear anticyclonic. At cally and kinematically. Kinematically, 700 mb the anticyclonic the iso- curvature is the rule therms should move with the windif the air rather than the exception. At700 mb and parcels conserve their temperature. above, no attempt should Usually iso- be made to kink the therms move somewhatmore slowly than the contours across upper portions of frontswhich wind speed, depending have been found in the frontal upon the amount of analysis. vertical motion and the amount ofheating or cooling. ANALYSIS OF SPECIFIC FEATURES Dynamically, the warm tongues inthe troposphere are related to ISOTHERM AND FRONTAL pressure ridges, due to ANALYSIS sinking and adiabatic warmingresulting from high-level convergence. If the ridge Drawing Isotherms attains sufficient amplitude, the northern endmay be cut off into a separate high cellat upper levels. This Isotherms (lines of equal temperature)are results in a cutoff high. Upper-level usually drawn at intervals of 5° convergence C. They are also cares ascent of air and coolingin the lower drawn as solid red lines. Isothermsshould be stratosphere at the 300-, 200-, and 100-mblevels sketched in according to reportedtemperatures, and above. Thus, the building of using past isotherm positions an anticyclone as a guide. Along aloftischaracterized by an increasing with wind and pressure, isotherms warm on the con- tongue in the troposphere andan increasing cold stant pressure charts yield cluesas to the exact tongue in the lower stratosphere. state of the atmosphere at the present timeand Cold tongues are relatedto pressure troughs aids in the construction of prognosticcharts. in the upper troposphere, due After sketching the isotherms and to upper-level checking divergence, resulting in ascent of air inthe lower their consistency, the isotherms shouldbe drawn and middle troposphere, with adiabatic in as smooth solid red lines, heavy enough cooling. to be An additional consequence of divergencein the easily legible, but not so heavy thatthey detract upper troposphere is sinking in the lower strato- from the contours. Label the isothermsnear the sphereaccompanied byadiabatic edge of the chart. For those which warming. form closed Deepening of a low aloft tends to beassociated curves, leave a small break in the curvenear the with an intensifying cold tongue inthe tropo- top of the curve for labeling. sphere. This is illustrated insome cases of cold lows aloft where the isothermsare parallel to the Isotherm Patterns contours and no advectionisindicated. As further deepening occurs, the isothermsaround Isothermpatternsatlower levelsusually the cold core are seen tomove radially outward, consist of tongues of warm and cold airthat across the wind, due to dynamic cooling. move across the map with reasonable continuity from day to day. There is a correlationbetween Movement of Isotherms the movement of the isothermpattern and the circulation, and between the form of theiso- In regions of sparse dataan estimatt, of the therm pattern and the distribution ofcontours. current contour pattern, basedon the pro ions Warm tongues generally tendto coincide with temperature field and circulation integratedwith ridges and cold tongues with troughs, particu- the latest surface pressure pattern,aids greatly in larly at higher levels, suchas 500 mb and above, determining the most nearly but this is only true as a first approximation. correct pattern It aloft. This is substantially the procedureinvolved is actually the departures from this relationship in graphical addition of thicknesspatterns to 234

' 240 Chapter 7UPPER AIR ANALYSIS lower-level contour patterns to produce con- upper-levelcharts.Reasonable continuity in sistent upper-level contour patterns. Temper- time from chartto chart and a reasonable atures at selected points are frequently usedin consistency between the various levelsfor the the computation of heights at various upper same time are sought at alltimes. It is important levels, instead of using the differe...,a1 method, in regions of scarce data that thesurface chart although the latter is used by some analysts. be examined carefully in order thatthe main Keeping this first approximation in mind as features of the upper-level isothermal pattern with the isotherms are drawn, the Aerographer'sMate will be consistent within reasonable limits finds that even in regions of scarce data,much what is expected from the sea level pressure of the departure from this relationship becomes distribution and frontal systems. evidentfrom the data. But, where data are After the isotherm analysis is completed, the scarce, itis necessary to examine further evi- movement and development of the t ,-rperature dence. In this respect, certain conditions must pattern should explain the temperaturechanges reported at individual stations. The temperature be considered, and these are discussed in some explained as the detail in succeeding paragraphs. changes at any station can be It is important tonotethat a continuous rise result of: of temperature cannot always bedistinguished 1. Advection. from a case where the temperature was rising 2. Changes due to the addition orsubtraction during most of the period between reports,and of heat, and adiabatic changes due tolifting or then began to fall shortly before the timeof the subsidence. apply to current report. Similar reasoning would Isotherms generally move with a speed some- opposite thepassage of a cold tongue with what less than the wind speed. Two of themain temperature changes being observed. Where there is a change of wind withheight, reasons are: the isotherm pattern is oriented toconform to 1. As warm air moves toward a coldregion, it the shear vector and the spacing of theisotherms tends to be cooled; cold air movingtoward is made to conform with themagnitude of the warm regions tends to bewarmed. When cold air shear vector. Over land, where there is adense moves across a warm sea surface,this retardation network, wind shear is not used explicitlyin the may be as great as 50 percent;and in the special determination of the isotherms. Over the oceans instance of Arctic or polar air fromLabrador computed shear vectors are of considerablevalue moving southeast over the warm ocean currents, Both of indrawing isotherms. Speeds of less than10 this retardation may exceed 50 percent. knots may not be significant, but when theshear these types of advection retard the movementof vector exceeds this value, it is ofparticular use isotherms, but in the case of cold airmoving in isotherm analysis. over a warm surface, theresultant instability The assumption that isotherms inconstant which develops produces an even greatereffect pressure surfaces at themiddle of an isobaric on the retardation at upperlevels. layer are closely similar to the meanisotherms 2. Cold air spreading southward over warmer (thickness lines) of the layer isborne out by a regions tends to subside, whichresults in warm- comparison of the 850-mb isothermswith the ing; warm air moving over a cold regiontends to mean isotherms of the1,000- to 700-mb layer, be lifted, which results in cooling.This latter especially at and by comparing the 700-mbisotherms with factor is the more important one, the mean isotherms of the1,000- to 500-mb the intermediate levels aloft. Bothfactors work layer. The 700-mb isotherms constitute agood againstthe advection of isotherms withthe advection chart for the layer from1,000 to 500 speed of the wind. mb. Also see chapter 10 of thistraining manual Front:. Analysis for additional information on movementof the The principal reason for theimportance of 850-mb isotherms. vicinity of The thermal winds and reportedtemperatures temperature Inalysis, especially in the verticalvariation of the are not the onlyguide to drawing isotherms on fronts,isthat the 235 241 AEROGRAPHER'S MATE 1 & C

geostrophic wind at any point inthe atmosphere Isotherms can assume other is determined by the patterns in rela- temperature field at that tion to fronts, indicating the relativestrength of point: hence, in the freeatmosphere the vertical the front. The weaker the packing shear of actual wind is largely or gradient in controlled by the the colder air, the weaker is thefront and the temperaturefield. Thus,in areas where no greater is the probability the front temperature data are available, temperature has a shallow anal- slope.Isothermsperpendicular tothe ysiscanstillbecarried out, provided the front indicate little air mass contrast andconsequently variation of wind with height isknown. For the a weak front. Northern hemisphere, the basicrelations can be You should first sketch in summarized as follows: fronts using the thermal gradient, wind shift, andtentative surface frontal positions 1. as guides. Of these three, If the wind increases (decreases)in speed thermal gradient is by far the with increasing height, but does most important. not change its Surface frontal andupper level frontal analysis direction, contours and isothermsare parallel, must always be consistent and thereforeshould with cold air to the left(right) facing downstream. never be made independent of each other. Data at both levels should be considered in 2.If neither the wind speed locating nor direction frontsatbothlevels.Raobsplotted on a change with height the air isthermally homogeneous. thermodynamic diagram areone of the best aids in locating frontal positions aloft.(See chapter 5 3. if the wind veers (backs)with increasing of this training manual.) height. the isothermscross the contours in such a way that advection of warmer (colder) The consistency of currentfrontal positions air with past positions should bechecked and then takes place. colored in.

Temperature is generally regardedas repre- APPLICATION OF SATEL- sentative in the free atmosphere whereitis LITE CLOUD PHOTOGRAPHS relativelyunaffectedby many localfactors which affect surface temperature.This makes Among the most significant contributions temperature analysis at 850 and 700 mb the an satellite has made to meteorolgoyhas been its important adjunct to surface analysisas an aid in application inthe identification, positioning, determining the representativeness of thesurface and prognosis of frontal systems.Some informa- temperature and in determining the location and tionpertaining to the utilization of satellite vertical extent of fronts. pictures during surface analysis The frontal patterns expected aloft as an aid in the are dis- positioning and identification of frontswas cussed in chapter 5 of this trainingmanual. In presented in chapter 6 of this manual. the case of wave cyclones, the frontal The same pattern generalconsiderations apply when analyzing aloftis roughly parallel to that of thesurface upper air charts. wave and displaced from it in the direction of An additional factor to considerwhen analyz- the colder air. In the absence of actualdata on ing an upper air chart is the displacement the upper air chart, this displacement should and be slope of frontal systems with altitude.Although consistent with the height of the chart andthe above 700 mb the frontsare normally indicated slope of the front. as troughs, the displacement of thesesystems Frontal analysis is usually not carriedabove must be considered at all levels. the 700-mb level. On charts which donot carry Earlier in the chapterwe mentioned certain frontal analysis, isothermsare continuous lines. characteristics of frontal cloud bands On charts on which frontal analysis which is depicted, provide information helpful in thepositioning of some of the isotherms in the vicinity of the upper level troughs. (See fig. 7-18,) Examination front may be discontinuous.Isotherms are gen- of numerous satellite pictures erallyparallelto the front with the tightest has revealed that the appearance of a frontal bandchanges ab- packing in the cold air behind the front.(See fig. ruptly where it 7-17.) is intersected by the 500-mb trough line.In figure7-18 the dashed line 236 242 Chapter 7UPPER AIR ANALYSIS

-20°

-15°

-10°

AG.531 Figure 7-17.Isotherm packing at a warm and cold front aloft. indicates the position of the upper level trough single band normally seen in extratropical cy- line. clones.(Seefig.7-5.)Sincethe IR mode The frontal clouds east of the trough appear represents cloud top temperature rather than mut) brighter and the band is broader due to brightness,it shows more cloud height detail upward motion in the southwesterly flow aloft. than the AVCS or daylight mode. (See fig. 7-4.) To the west of the trough line in the region of The formation and development of cutoff subsiding air, the clouds tend to dissipate, the lowsisoftenstrikinglyapparent from the front appears more ragged, and there are several satellite data in two general ways: cloud-free areas within the frontal band. 1. When a frontal band separates in advance Vortices of a moving 500-mb trough, the equatorward portion of the upper-level trough has cut off, Cyclonic disturbances produce a variety of and a closed 500-mb low is forming along the spiral cloud patterns, some of which are de- upstream edge of the isolated cloud mass. tached andin some cases disassociated from 2. When lines of cumulus congestus cells frontal system,. Fortunately these patterns are (open) are observed to form cyclonically curved not difficult to recognize in satellite pictures. lines downstream from an approaching frontal CUTOFF LOWS. -Vortices associated with band, an upper-level cyclone is usually forming upper-level cutoff lows display a variety of ahead of the front as illustrated in figure 7-19. patterns, but in the fully-developed state normally cover an area about 10-15 degrees of In either of the preceding cases, the location latitude in diameter. The vortex usually consists of tie upper-level center may be determined by of several narrow cloud lines rather than the examining the overall cloud pattern in the IR

237 243 AEROGRAPIIER'S MATE I & C

. I aw-- ;sok! -7":4'fITMZIO -

./-% 7. aliefirMW f" 7-17

h._

AG.532 Figure 7-18.Change in frontal clouds where an upper level troughintersects a front. picture.Unless a distinct centerappears in Cells (closed) appear as rather flatgray; cells low-level cloudiness, the forecaster cannot deter- (open) appear as a field of mottledgray which mine whether or not a closed circulation exists has white elements if cumulus congestusclouds at the surface. are present.Itis often difficult to distinguish LOW-LEVEL CLOUD PATTERNS. Low- between cells (closed) and stratus due to the level cloud patterns frequently encountered in coarse resolution of the IR and small tempera- the IR are low-level vortices and fields of stratus turecontrastwiththe surface. The overall or stratocumulus. pattern and geographic location may be usedas The centers of low-level cloud vorticesare guides. (See fig. 6-12.) difficult to locate in the IR picture, dueto the factthat thereislittle temperature contrast APPLICATION OF between low cloud tops and the earthocean COMPUTER PRODUCTS surface. (See fig. 6-15.) Centers may beespe- ciallyhard to identify with the older APT When utilizing computer products for frontal receivers, or in dissipating cloud systems. In analysis on upper air charts, thesame procedures most cases. it may be possible only to approxi- presented earlier in this chapter, and in chapter mate the geometric center of the entire cloud 6 of this manual, should be followed. Among pattern_ The centers of weak low-level cyclones the more useful computer charts for determining may be located with more confidence in the frontal position and intensity is the GG THETA visual data than in the lit. chart.

238 244 Chapter 7 UPPER AIR ANALYSIS

intense the front will be. For example. if two consecutive GG THETA charts show an increase in the number of lines in a given frcuttal area, this would indicate intensification of the front. GG patterns are very useful in determining thelocation of surface fronts inregions of reasonably dense upper air observations. Areas of frontogenesis or frontolysis will generally be depicted by the GG THETA well before any indication in the synoptic data. The following paragraphs present a number of conditions under which difficulties might arise and extreme care should be exercised inthe identification of fronts on the GG THETA chart.

1. Occlusions may not be depicted due to the lack of strong horizontal thermal gradient in the lower levels, and because the thermal gradient is not perpendicular to the occluded front. 2.If a polar high is shallow and only extends AG.533 to around 850 mbs the front preceding it may Figure 7-19.Formation of 500mb cutoff low not be evident on the GG THETA because this ahead of a front. chart is derived from the 1,000 to 700 mb layer. 3. The GG THETA may present difficulties GG THETA Chart in identification of frontal zones or the determination of frontal intensification when two fronts The GG TIIETA chart is a frontal analysis and areinclose proximity. as when one front prognosis arrived at by mathematically identify- approaches another. In some cases one front ing and analyzing the direction and magnitude may be absorbed by the other. Since all zones of discontinuities in gradient within the temper- indicated on the GG THETA over land areas are ature field. The chart is a hemispheric product notnecessarily fronts, distinguishing of the true involving calculated thicknesses between the 700 front may be hindered by the addition of a zone and 1,000 mb levels converted to mean potential caused by topography, land vs water. etc. temperature. (See lig. 7-6.) In the winter, over Asia in particular, where cold pockets of air are trapped in the lower Use of the GG THETA Chart levels, it will be indicated on the GG THETA as an area of sharp thermal contrast. Of course the When usingthe GG THETAitmustbe indicated frontal zone is in actuality the moun- remembered that the lines on the chart represent tain barriers that have pocketed the coldair. therate of change of potential temperature Also in the summer. the thermal low found over gradient. The packing of lines (increase in rate of MexiLo will indicate a frontal zone on the GG change) may not always be the result of a front. THETA but close inspection by the analyst or Some of these exceptions will be presented in forecaster will normally reveal no frontal type later paragraphs. One means of checking the weather. existence of a front in the area is to overlay the 4. The use of the GG THETA over extensive GG THETA chart with the surface chart.If, water areas should be carefully evaluated. Just as however, a front is determined to exist. it can be with other types of charts,the number of reasoned thatthe more lines you have in a surface and upper air reports available, their particular area. the stronger the contrast will be aLcuraLymd whether the reports were repre- between the two air masses. hence, the more sentative mmt be considered. If a front exists,

239 t- AsiLik) AfiROGRAPI [ER'S MATE I & C

vuld be pat king Of the isotherms indi- approximates the northern edge of the cloud ,,A1):, the Sc -nib chart. In addition. the 500- hand, spiralling into the vortex at position E in ntio nib tluckneSti analysis should show a figure 7-20. Cloud patterns in satellite photo- nt,,tion of thicknesslinesin areas of graphs correlate with the1,000- to 500-mb possthlt. frontal location. thickness patterns. Thisisbecause the cloud patterns arebasically the result of integrated 11.1.111NTING DATA motions throughout the atmosphere and not at any particular level. is vs, vvide reports may be used to further thvt,.' . frontal analysis. As mentioned previ- . vsl.t.the omputer will not draw infronts. thtcomputer will not kink isobars or s unless adjatent reports are near enough _ ...net to indicate a kink. Observing wind shift riperature Lhange on radiosonde reports a, pinpoint atturacy on frontal location.

1 \:I the International Analysis Code (IAC) itpossible for comparison of your I 11 zth the analysis of another individual, IliIt lv allowing you to take the better parts of '',sift anal ses

RI OCKS

xp:ttente has shown that about one-third of time the distortion of the westerlies is so ;neatthattherules covered in chapter 4 on rt Ind meridional flow patterns are totally 1:',111able. A condition that exists at intervals ' vottlt as the blocking ,.nation (also called tit,inefi high).I ypes of v 'locks are discussed rt chapter 4 of this training manual. Itis rather vintieult for the analyst to determine a blocking AG.534 Figure 7-20.-500mb analysis and satellite photograph ev+ntlition from a single chart. The method of showing an omega type blocking high. {kit:C:11111.! existing blocks depends primarily on the use of 2- to 3-day running means of wind and pre,sure aloft, particularly at the 500-mb Use of Computer Products

Satellite Depiction The thermal characteristics related to block- of I pper Level Blocks ing situations were describe, in chapter 4 of this manual.By studying the computer 500-mb vood example or the use of satellite analysis (figure7-9),the1,000 to 500 mb t'ft.,gt ,ph, to orient the position of a block is thickness charts, the vertical velocity analysis, d iu figure 7-20. and other tropospheric and stratospheric analy- ridge in the form of an omega block ses. considerable assistance may be obtained in ,'It has minor ridges v ithin the large scale determining whether a blocking situation exists. +lowI me AB. in both figures 7-20 and 7-21 A complete discussion of blocks and blocking illustrat thisfeature. The major ridge line situations may be found in the Navy publication viviz oth figures runs from position C to 1.), Handbook of Single Station Analysis and Fore- Ittan h,' seen that the mint)a ridge line closely casting. NA 50-IP-519.

"")40 ell A I d4" Chapter 7- -UPPER AIR ANALYSIS

charts for the purpose of delineating the moist and dry tongues in meridional flow patterns. The extent and intensity of moisture patterns are of particular important.e in areas where the atmospheric processes producing clouds and; or precipitation predominate. 70N lsodrosotherms are discontinuous atfronts. Moist tongues are most pronounced in warm er maritime air masses. while dry tongues are most markedincold continentalairmasses. For obvious reasons. an isodrosotherm cannot cross the isotherm with the same temperature Libel. The National Meteorological Center (NMC) provides a Composite Moisture Index Chart. I.. ."40..i:..17. i. .4:...1,11.1r This is a four panel chart for the conterminous 4,:&$.'.. "; '>' r, United States. The four panels present information pertaining to index conditions. precipnable water.freezinglevel, and relative humidity. -I ;-!:'?"-':7'.e. --,

15-,,,4?'_,:.4. .-.,..;.,:kolw'''tisit-;,:..:,-- Details on the construction of this computer produced chart and transmission times may be found inthe National Weather Service Fore- AG.535 caster's Handbook. WBFFI No. I. Local weather Figure 7-21.-500-mb analysis and satellite photograph showing an omega type blocking high. units may utilize this computer produced data as an aid in determining areas close to saturation. The author suggests in this publication that for single station forecasters it has been found On a working chart light green is the color adequate to define blocking solely in terms of customarily used for isodrosotherms. with light closed anticyclonic patterns which persist at the green shadingformoisttongues. becoming 500-mb level for 2 days or more north of a progressively darker towardthe axis of the certain critical latitude. In the Atlantic Ocean tongue. this was found to be 40° N; in the Pacific Ocean The troughs and ridges in the isotherm pat- 35 ° N. In both cases, when a closed anticyclone terns are wiled cold and warm tongues, respec- appeared on the 500-mb chart north of the tively, and inthe moisture pattern they are critical latitude and persisted for 2 days or more referred to as dry and moist tongues. respec- itwas attended by the familiar features of tively. blocking action. The blocking types. as developed empirically for both the Atlantic and Pacific Oceans, have a INDEXES number of common features which permit a sketch of atypic.:1blocking pattern. These The types and characteristics of zonal indexes features are shown graphkally in figure 7-22. aredescribedin chapter 4 of thistraining Blocks should be looked for during their most manual. A method of computing zonal indexes frequent seasons of occurrence inlate winter is also covered in that chapter. Indexes are best and early spring. identified on mean charts. However, there arc indications on the present upper air charts (700 MOISTURE ANALYSIS mb and above) which present identifiable features of the indexes. A brief review of the Lines of ,onstant mixing ratio or isodroso- indexes and some methods for identifying them therms may be drawn on 850- and 700-mb are presented here,

241 247 AEROGRAPHER'S MATE I & C

/Aboveitlormal ___-- ..,--- Pressur ( (----- Below Normal Below Normal

Pressure Pressure 10 35 - 40 (winter) _35'- 40 (winter) ,30 (summer) \ / 30' (suthrker,..).

Low t 5 - r Representative Streamline I

_20 - 25 (winter) - 25 (winter)__ Representative Isotherm 10 - 15 (summer)10 -15 (summer)

AG.536 Figure 7-22.Characteristic 500mb surface streamlines and dimensions for oceanic blocking (all measures in degrees of longitude and latitude).

Characteristics perturbations superimposed onthe westerly flow correspond to a family of wave cyclones at Indetermining theindex pattern. we are the surface. There is little northward or south- primarily con_erned about the fluauations of ward movement of pressure systems nor rapid the principal band of westerlies between 35 and development of centers. In short. there is no 55 degr:.es north latitude. From computations as interchange of contrasting air masses from north described in chapter 4 Itis found that zonal to south. Therefore, with a high index circula- indexes may be computed from the indicated tion. surface lows move rapidly eastward with- strength of these westerlies. out developing into major systems. A high index pattern, which is the normal A high index pattern is the normal condition pattern, connotes a westerly flow with high as may be evidenced by the west-east orientation pressures to the iouth and low pressure to the of the 700-mb contours on mean charts. An north. A primary characteristic of this pattern is additionalcharacteristicis the occurrence of the more or less regular arrangement of contours pressure near normal wit!. only small deviations. aloft in a west to east direction and the pretence of quasi-stationa.y fiat troughs of long wave- A low index pattern consists of large north- length through which moves rapidly a series of south components of flow with a general de- last, shallow lesser troughs which are warmer crease or even reversal or the pressure gradient thantheir environment and which have little of low pressure to the north and high to the vertical development. A group of these wave south as associated with the high index.

242 248 Chapter 7 -UPPER AIR ANALYSIS

The primary characteristic of this pattern is COORDINATION OF UPPER AIR large amplitude perturbations in the westerlies DATA AND SURFACE MAP and the existence of meridional flow over large areas, wh...h means that widely contrasting air Certain implications from the above data and masses are interchanged. In some cases a closed othei data obtained from Lippe' air observations low circulation mayexistaloft m the mid- can be employed to determine the configuration latitudes. Troughs are slow, deep. and colder of the current map. Primarily, the overall picture than their environment and have marked vertical of the current situation is obtained by VISLIalIZ- development. Under these conditions surface ing the index pattern. Conclusion .is to whether lows will move less rapidly, unfavorable weather high. low, or transitional index should be de- will spread over large areas, and consequently duced from consideration of the factors stated clearing will occur less rapidly than when a high in chapter 4. consideration of the above. and index circulation is present. Similarly. aloft. the from thefollowingfeatures.Either 700- or trough will have a smaller west to east compo- 500-mb levels may be used. nent of movement. I. Deviation of the 700-mb heights and temperatures from normal and of the tropopause Index Type from Radiosonde Analysis height and temperature. 2. Past weather. The use of radiosonde analysis in locating 3. Direction changes in the flow at 700 mb. fronts was discussed in chapter 5 of this training 4. The height tendency at 700 mb or some manual. The radiosonde can also be a valuable higher level. aid in identifying the index situation through 5. Types of air masses present. the variation of temperature and height of the tropopause in middle latitudes. For instance. a continued westerly flow with 700-mb heights and temperatures near the nor- The variation can be related to surface air mal and no large change would allow an assump- masses.Atapproximately 45 degrees north tion of a high index. On the other hand. large latitude the northward influx of tropical air is deviations from the 700-mb noimal height and associated with a high cold tropopause of the temperatures and large height change values order of 15 to 18 km in height and -70° to would indicate other than a high index pattern. 80° C temperature. A low warm tropopause of 7 to 9 km and temperatures -50° to -55° Cis associated with a southward surge of Arctic air COORDINATION OF SATELLITE and an intermediate tropopause of 11 to 13 kin AND COMPUTER PRODUCTS and-60° to -70° C accompanies the advent of maritime air which isthe normal case. The Satellite photographs and computer products greater the deviation of the tropopause from the such as those described earlier in this manual mean position. the more intense is the north- provide a valuable source0 : Iassistancet in deter- ward or southward push of air. mining index conditions. Since the presence of tropical or polar air in the middle latitudes is associated with a low Satellite Photographs indextype,itfollows thatthe tropopause position is a good key to the circulation index. The existence of low index situations. com- The appearance of a distinct change in lapse rate monly associated with baroclinic conditions. are atthe tropopause levelis linked wall ,1 low evident on satellite photographs by the existence index pattern. and a gradual or rather indistinct of frontal systems. vortices. and other forms of change in the lapse rate with the tropopause convergentweather. High index conditions will level above 12 km indicates the existence of high be characterizedonthephotographs by a index conditions. predominance of fair weather conditions. 416 AEROGRAPHER'S MATE 1 & C

Computer Products Figure 7-23 illustrates common occurrences of short waves. long waves, and a surface low. There are a varietyof computer products Note in this figure that the slope of the surface available. primarilyin the form of upper level pressure system varies from near zero in the charts.. will pros ide the analyst assistance Mediterranean, where the presence of cold air in determining index conditions. The use of massesisextremely rare, to 8 to 10 degrees these kihtas has been presented previouslyin longitude off the eastern coasts of the United thismanual.Itwouldproveredundant to States and Asia. attempt tolistthe various charts since they Typical wave patterns of isobars and iso- change from time to time and are listed in the therms of various levelsinthe zone of the Computer Products Manual. westerliesis shown in figure 7-24. The most significant features of this observed pattern are LONG WAVES. TROUGHS. AND RIDGES that the surface isobars and isotherms are most frequently about135 degrees out of phase. Two superimposed wave patterns can gen- Using either the hydrostatic equation or the erally he distinguished on upper air charts (700 thermal wind equation,itisseenthat this nib and above). Slow moving features designated surface distribution produces a verticaltilt to as long wave troughs and long wave ridges lie the trough and ridge lines and a decrease in the under asystem of fast moving disturbances amplitude of both the isohypses and the iso- designated as short wave troughs and short wave therms until both are in phase. This normally ridges. The identify ing features are discussed in occursatabout the height of the 600-mb chapter 4 of this training manual. surface, which is also the average height of the Long wave patterns arebest detected and level of nondivergence. About the level at which identified on mean or other type charts. All the isohypses and the isotherms are in phase. the wave patterns are classified according to their amplitude of the horizontal isohypses and iso- wavelengths as either long or short waves. Long therms will increase upward to the tropopause. waves vary in length from about 60 to 120 Troughs and ridges normally slope very little degrees longitude and their normal movement is above 500 mb. From the distribution of the from west to east about 5 degrees longitude per wind speed profile.itis seen that below the day, but long w call be stationary or even 600-mb surface, waves will normally travel faster retrogress to the west. than the zonal wind, and somewhat above that Great Lae in tracking trough and ridge lines level, the zonal wind is usually higher than the on successive maps is necessaryif proper Lon- wave speed. The corresponding fields of con- tinuityisto be maintained. This is especially vergence. divergence. and vertical motions are true of short waves. which appear as perturba- shown for reference. tions of small dimension moving rapidly along the long wave pattern at a speed of 10 to 20 Satellite and Computer degrees per day. Because of the sparseness of Products upper air data in many areas these short waves are easily overlooked or Larelessly smoothed out Satellite photographs and computer products of the analysis. In isolated areas. the principal provide an excellent means of determining the due to the passage of a short wave trough is a present as well as future positions of long and slight backing of the wind for a brief time. The short wave troughs. The application of these long wave trough intensifies when overtaken by important devices to analy sas as presented earlier the short wave trough and this intensification in this manual should not be overlooked. often results in surface cyclogenesis. The best synoptic clue as to the existence and location of short waves in denser networks is the 12-hour VORTICITY ANALYSIS height change pattern. A small closed pressure change center is often associated with a short The determination of relativevorticityis wave trough or ridge. covered in detail in chapter 4 of this training

244 250 Chapter 7UPPER AIR ANALYSIS

LONG WAVE PATTERN

SFC ISOBAR SFC ISOBAR SHORT WAVE PATTERN

8° LONG OFF JAPAN AND EAST COAST OF U.S. 0° LONG IN MEDITERRANEAN

AG.537 Figure 7-23.Short waves, long waves, and surface features.

cc1

200 v( 200

TROPOPAUSE I HTh---Kc) -- 400 500 ON r \( \( 600 _600_ I

t \ k 800 890 co c."\\c°"\ `TEMP. \\ )1 \.\11 V 1000 /". 0 5 10 15 2025 loop _ _ ZONAL WIND METERS SEC.-I

TYPICAL VERTICAL WIND SPEED AND PLAN VIEW AT VARIOUS LEVELS VERTICAL PROFILE ALONG THE TEMPERATURE PROFILE OF THE ISOTHERM- STREAMLINE WESTERLIES SHOWING THE VERTICAL PATTERN WIND AND DIVERGENCE PATTERN

AG.538 Figure 7.24. Typical wave patterns in the westerlies.

245 251 AEROGRAPIIER'S MATE I & C manual. Voiticity Limits for present and prog- with the long wave troughs located on the SL nostic conditions are currently,' being transmitted 500 MB chart, Ind the surface chart. The effects atregulai interals on the National Facsimile of vorticityon weather werepresented in Network. chapter 4 of this manual. In practice. actual vorticity values are rather The tracking of positive vorticity centers has difficult and time consuming to compute. With become a very significant aid to the forecaster the ahem of computers. this computation is when trying to determine the intensity changes pc' formed with speed and relative accuracy .In of long wave troughs and surface lows. Through practicalanalysis you should remember that the use of computer charts along with other positivevorticityisa normal occurrence in available forecasting aids this task will become advance of troughs inthe middle and upper somewhat less difficult. The utilization of vortic- troposphere. A good indication of cyclogenetic ityinforecasting will be presentedin the development at sea level can be deduced when pertinent forecasting chapters of this training an upper trough with positive cyclonic vorticity manual. in front of it overtakes a frontal system in the lower troposphere. You should also remember ISOTACH AND JETSTREAM ANALYSIS that anticyclonic vorticity is negative and the areas from the rear of the troughs to the next Where wind dataaresufficiently concen- ridge lines are favorable for anticyclonic de- trated, isotachs are drawn to locate the axis velopment. along which the wind speeds are greatest. This axisisthe ridge line connecting the various Satellite and Computer centers of relative maxima in the isotach pat- Products tern. The isotachs are almost parallel to the isohypses and the closed centers are usually SATELLITE PHOTOGRAPHS. An impor- elongated ellipses with the long axis parallel to tant means of identifying areas of positive and the isohypses. Two idealized contour and iso- negative vorticityisthe satellite photograph. tach patterns are shown in figure 7-26. The one Comma shaped areas of clouds described as on the left shows a speed maximum at the long Positive Absolute Vorticity (PVA) Maxima and wave trough line, the other shows maxima at the vorticity centers occur in the cold air to the rear ridge lines. These centers normally move from of, or around the periphery of, cloud vortices west to east with a speed less than that of the related to extra tropical systems. Figure 7-25(A) winds themselves, but greater than that of the showsaDRIRreadoutwithavortex at long waves. position A and a frontal band extending from The isotach ridges on any particular constant position .B to C. Behind the frontal band, a PVA pressure chart represent the intersection of that MAX is shown at position D rotating around the surface with a meandering current of fast mov- vortex in the cold air. ing air, known as the jetstream. Since wind As illustrated in figure 7-25(8) thickness lines speedvariesinthe verticalas well as the are generally normal to the cloud band of the horizontal, the core of the jetstream fluctuates comma. It Is important to identify these features verticallyaswellas horizontally from one since if a PVA max overtakes an active baro- meridian to another. It usually lies between the clinic zone, cyclogenesis is likely to occur. 300- and 200-mb levels, nearer to the 200-mb COMPUTER PRODUCTS. A number of dif- level in lower latitudes and nearer the 300-mb ferent computer charts (SD 500 ANAL, 500 MB level in higher latitudes. Its width varies from HT ANAL, SL 500 ANAL, 115-10 ANAL, etc.) 250 to 400 miles, being narrower during periods areavailable for use in determining vorticity of low index and wider during periods of high conditions. Amongtheseisthe"Vorticity index. Chapter 4 of this training manual con- Advection Chart.'' Through the use of this chart, tains illustrations of the jetstream in relation to a forecaster can study the progression of the the polar front and the tropopause. positive vorticity centers and carefully examine The 300- and 200-mb charts give the best the relationship of these centers in conjunction representation of the jetstream, but itis often

246 252 Chapter T.-UPPER AIR ANALYSIS

.e7'^tIr

AG.539 Figure 7.25. (A) DR IR readout showing a vortex, frontal band, and PVA Max; (B) corresponding surface chart with superimposed thickness lines. well defined down to 500 mb. Below this level maximumisotherm .oncentrationcoincides the strong meridional temperature, particularly with the jetaxis. At 500 mb, the isotherm those associated with the polar frontinthe LonLentration is often assoLiated with the polar winter. causes the intensity of the speed maxi- front. The jetaxis atthis level seems to be mum to decrease rapidly in the lower level of around the20'C isotherm on the warm side of analysis. the zone of maximum temperature gradient. Above the jet core, the vertical axis of the Inthe horizontal, the jet axis also follows jetstream tilts equatorward. From 500 to 300 quite closely the contours. At 500 mb itlies mb, it has almost no tilt, and is usually located near plus or minus 60 meters of the 5,610-meter directly beneath the line of maximum horizontal contour. Project AROWA found in one study temperature gradientat 200 mb, hence, the that the jet maximum occurs 82 percent of the 500-mb isotach analysis and the 200-mb temper- time between the 8,960- and 9,240-meter con- ature analysis are both very important aids in tours at 300 mb. locating the 300-mb jet below the jet core. The Two exceptions to the foregoing should be lack of tilt in the vertical axis of the jet implies. noted. At low wind speeds. contours and iso- by virtue of the thermal wind relation', that tachs may crossatlargeangles, and where

247 253 : AEROGRAPHER'S MATE 1 & C'

(A) CONTOUR ISOTACH JET (8)

AG.540 Figure 7.26.Common contour and isotach patterns. (A) Speed maximum at long wave trough line; (B) speed maximum at ridge line. contours converge (upstream from a speed maxi- moves south. its core rises to a higher altitude mum). the jet axis and isotachs tend to cross and, on the average, its speed increases. In the from high to low, the reverse being the case winter the jetstreams are often found as far downstream where the contours diverge. Deep- south as 20 degrees N. The core of the strongest ening and (..ydogeneis occur downstream and to winds in the jetstream is generally found be- the left of the jet maximum at a trough and tween7,620 and12,190 meters. depending upstream aid to the right of the maximum at upon the latitude and the season. ridges. The relation of contours to the jet axis also accounts to some extent for the appearance APPLICATION OF SATEL- of themeridional and easterly jets and the LITE CLOUD PHOTOGRAPHS forkedjetsassociated with cutoff lows and blocking highs. The location of the jet stream or maximum wind zone can be accuratelypositioned by LIFE CYCLE AND SEASONAL examining the cloud patterns shown in satellite POSITIONS OF THE JETSTREAM pictures. Vertical and horizontal motions in the upper troposphere in the vicinity of jet streams The period of life cycle of a particular jet have a very marked effect on the distribution of determines the characteristic structure of the jet cirrus clouds in their vicinity. (See fig. 7-27.) at a specific time over a given region. Although Because of this. it is possible to determine the it may be convenient to visualize the jetstream locationof such wind maxima from cloud asatube of high wind speeds meandering picturestakenby weather satellites.Cirrus aroundthehemisphere, it isdearthatits clouds predominate on the equatorial side of the character generally caries from region to region. jet stream and in the anticyclonically-turning The seasonal aspects of the jetstream were portion of the jet. The poleward boundary of discussed of chapter 4 of this training manual. In the cirrus is often very abrupt; it lies under or summary, it may be said that the strength of the slightly equatorward from the jetaxis, and jetstreams is greater in winter than in summer. frequently casts a shadow on the lower clouds The mean position of the stream shifts south in which is clearly visible in satellite pictures. winter and north in summer with the seasonal Over the oceans, where the jet stream curves migration of the polar front. As the jetstream cyclonically, the differences in stability on each

248 Chapter 7 -UPPER AIR ANALYSIS

fr- .414A 1/4*

itt a

-01

r. .'-201) 15W

AG.541 Figure 7-27.-200-mb analysis showing transverse lines in jetstream cirrus over Africa where lower clouds are absent. The southern edge of the !inc.:.rails off to the west due to slower wind speeds farther away from the jetcore. side of the core are reflected in theappearance fence is possible. The Guide for Observing in, of the clouds. On the left side. looking down- Environment with Satellite Infrared Imagery. stream, cold temperatures and unstable air occur NWRF F-0970-158, and Application of Meteor- resulting in great vertical development of the ological Satellite Data in Analysis And 1-0R-- convective clouds in the open cellular patterns. casting, Technical Report 212, providean ex,L1- (Sec fig. 7-28.) lent source of additional information on locating One of the major problems facing the analyst the jet stream through theuse of satellite is differentiating between jet stream cirrus and photographs. instability lines of Cb activity. In bothcases. the display may be white. representing emission APPLICATION OF from high, cold cloudtops. The forecaster COMPUTER PRODUCTS should examine the equatorward edge of the cloud shield to determine whether the boundary The most teful computer charts mailablefo, is sharp or diffuse. analyzing the jet stream arc the 200 nib. 300 nib,isotach, and windshear analysis. The The main jet-stream cloud featuresare long isotach analysis is especially useful for del,`Inun- shadow lines,large cirrus shields with sharp ing positions of jet maxima. The chinaLteristi(s boundaries, long cirrus bands, cirrus streaks. and of the jet stream were presentedin chapter 4 of transverse bands within cirrus cloud formations. this manual. By utilizing this descriptive data Once a jet stream has been identified on a cloud along with the available computer charts;rect. photograph.itis possibleto deduce other ratepositioning of the jetstreamwillhe meteorological information, such as wind direc- practicallyassured.isotachanalysiswillbe tion, wind shear, direction of horizontal temper- describedingreaterdetailintheI °flowing ature gradient, and areas where clear air turbu- paragraphs.

249 235 AEROGRAPHER'S MATE 1 & C

c 77,1S-7;440. ` -11%.

LUMPY

SMOOTH

r. 4.1113 Pt, ..- ft,A 1"14 y. .7.

s111)6 CELLULAR CLOSED 4 4. ; "1. . 13. eV' :, 1""

11.

AG.542 Figure 728.An idealized illustration showing a jet stream over the dividingline between open and closed cellular clouds. The jet stream east of the upper air trough isshown crossing over an occluded front.

250 256 Chapter 7 -UPPER AIR ANALYSIS

ISOTACH ANALYSIS 4. Check for consistency with jet axis and isotherm patterns above and/or below the level Thefirststepinisotachanalysisisthe being analyzed. computation of wind speeds in areas of sparse or little wind data. This can be accomplished by After Lompletion of the isotach analysis, the the use of gradient and geostrophiL wind stales. isotach patterns should be piLtorial. One recom- Many instances have occurred in which observed mended procedure is as follows. winds do not agree with geostrophic or gradient computations, thereby placing this method of I. At first, draw the principal jet axis as a augmenting wind speed data intothe same heavy purple line with arrow indicating the flow category as the extrapolation of data upward direction. IndiLate secondary jets and jet fingers from a lower levelitis better than no data at with dashed purple lines. all.Thefollowing items should govern the 2. Shade regions with wind speed less than 20 computation of wind speeds: knots (also marked with an S) purple and regions with wind speeds greater than 80 knots I. Use geostrophic or gradient wind scales (at 500 mb, 60 knots) green. Intense jet centers constructed for the map projection, if availble. may be emphasized by increasing the heavy 2. First select those points where the wind is shading toward the center. most likelyto be geostrophic, thatis, where contours are straight and parallel or with little Centers of wind maxima usually propagate curvature. Then measure the geostrophic wind. downstream at a rate greater than that of the 3. Next, choose areas where contours are long wave pattern, but less than that of the curved but approximately parallel and measure winds themselves, hence they will have move- contour curvature from overlay, then estimate ment relative to the wave pattern and appear to thecorrectionfortrajectorycurvature. and move throughit. much like the short wave compute the gradient wind from ay..iiable scales. patterns do. Occasionally renters will remain stationary but have never been known to retro- In the cases of number 3, a recent statistical gress upstream. study has shown that only in cases of cyclonic curvature do you needtocorrectthe geo- OTHER AIDS IN LOCATING strophic wind to the gradient wind. In anti- THE JETSTREAM cyclonic cases, the geostrophic wind seems to give as good an approximation to the true wind The following features are used atthe Na- as does the gradient wind. tional Meteorological Center to focus attention Once the wind speed data have been aug- on areas where the jetstream might be located. mentedasoutlinedabove,isotachanalysis should proceed as follows: (The recommended I. The jetstreamliesvertically above the interval for isotachs is 20 knots.) maximum temperature gradient in the midtropo- sphere as indicated for example by the 500-mb 1. Begin the analysis in an area with dense temperature field. It will be located just to the reports and draw in the 60-knot isotach. If you south of the greatest thermal gradient. are using either the 300- or 200-mb chart, use 1.. The jetstieam reaches its maximum in- the 500-mb chartas an aid;ithas greater tensityslightly below or atthe tropopause. coverage. Thermal gradient is reversed above this level. 3. The cyclonic shear to the left of the jet 2. Draw in the remaining isotachs at 20-knot (looking downstream) is stronger than the anti- intervalsandidentifythecenters of speed cyclonic shear to the right. maximum from previous charts (continuity). 4. The jetstream is also a streamline and as 3. Sketch in the jet axis. Interpolate the jet such tends to follow the same height contour. axisinareas between centers, following the From this is follows that the axis of the jet on a contour and isotherm pattern. constantpressure surfaceisdirected toward

251 tiJe AEROGRAPIIER'S MATE 1 & C lower heights when wind Nc Lk increase along temperature tropopause, and 310° Kfor the the jet and vice versa. subarctic tropopause. The characteristic poten- 5. Information from satellite pictures aids in tial temperature may be of value in locating the locating the jetstream as covered in ,hapter 14. tropopause on an atmospheric sounding when the sounding is characterized by many inversions TROPOPAUSE ANALYSIS or by an irreear lapse rate with no inversions. There are many soundings on which the The tropopause is the boundary of transition locating of a generally acceptable tropopause is between the troposphere and the stratosphere. very easy. These cases include those where a RAH No.3,RadiosondeObservations. NA single strong inversion or marked stablization of 50 -1 D -3,givescriteriaforselection of the the lapse rate occurs. However, there are many tropopause levels at pressure lower than 500 mb other soundings where itis quite difficult to from radiosonde data. A template is available. determine where the tropopause ends and the Form FMII 3-3 1131,for use in selecting the stratosphere begins. In these cases, the lapse rate tropopause level. f MIL No. 4, Radiosonde Code, may just gradually become more stable without NA 50-ID-4, gives criteru for encoding tropo- any prominent sudden stabilization of the lapse pause data. rate with height, or there may be more than one Formerlyit was believed that the tropopause point of stabilization; then it becomes a contro- was a single unbroken layer extending from the versial problem to decide which point should Lquator tothe poles and sloping downward locate the tropopause. The variation in structure toward the poles. Data gathered subsequent to inthetropopause regionhasledto many and during World War II have served to invali- different ideas on how to define and analyze a date this theory. There have been found to be at tropopause. The conflict between the definitions leastthree more or less distinct tropopauses, officially advocated cr used in different coun- which form leaflike or overlapping structures tries was finally compromised by the WMO who between which jet Lells of maximum wind are have standardized an arbitrary definition of the found. tropopause for operational use in internationally The three generally accepted tropopauses are exchanged upper-air sounding reports. the subtropical tropopause found at about 25 The present WMO definition does not attempt degrees N near 18.290 meters or around 100 to settle the question as to what the tropopause mb, midlatitude or temperature tropopause at is.whichremainscontroversial.Itdefines. 35 to 40 degrees N near 12.190 meters (around rather. an objective technique for locating the 200 mb), and the subarctic tropopause near lowest height in the atmosphere where the lapse 9,145 meters (around 300 mb). In general the ratefirst decreases to an average lapse rate of tropopauses are found atgreater heights in 2° C,'km for a 2-kilometer layer. While the physi- summer than in winter. cal significance of this lowest height remains to Each of the tropopauses slopes downward be found, it does establish an international refer- towardthe north. For a short distance the ence height to which other atmospheric phe- subtropical tropopause tends to overlap the nomena can be empirically related. The main temperate tropopause and the temperate tropo- advantage is that the definition is objective, in pause tends to overlap the subarctic tropopause. that it allows the same height to be convistently These regions of overlap are ,haracterized by selected by all technicians from a given sound- double or complex tropopauses, and the two ing. main regions of overlap or discontinuity are associated with the midlatitude and subtropical WMO TROPOPAUSE DEFINITION jetstreams. These three tropopauses are characterized not As noted above, objective criteria for deter- only by height and pressure but also by poten- mining the tropopause height(s) transmitted in tial temperature. Within approximately ±10° C upper-air observations have been internationally the potential temperature in Winter is 390° K for standardized by the WMO. Within its definition, thesubtropicaltropopause, 350° K for the provision is made for identifying two or more

252 258 Chapter 7UPPER AIR ANALYSIS tropopauses on a sounding. The WMO definition second tropopause height. This requirement is for the tropopause is as follows: met through the layer CD, and in this particular 1. The "first tropopause" is defined as the case. qualifying remark 3 (b) applies al:,o When lowest height at which the lapse rate decreases criterion 2 is met, then criterion Iis again used to 2° C /km orless,provided alsothatthe to determine the location of a higher tropo- average lapse rate between this height and all pause. Above point D, the lapse rate DE again higher altitudes....in 2 kilometers does not decreases to less than 2° C/km for the next two exceed 2° C/km. higher kilometers. Thus. a second tropopause is 2. If above the first tropopause the average determined at point D. lapse rate between any height and all higher Tropopause heights and temperatures are cur- altitudes within a I-kilometer interval exceeds rently being placed on the 35.000-foot upper 3° C/km, then another tropopause is defined by levelwind charts andtransmitted over the the same criteria as under 1 above. This second National Facsimile Network. tropopause may be either within or above the I-kilometer layer. COMPUTER PRODUCTS 3. There are two qualifying remarks attached to the definition. They are: NMC provides a computer derived tropopause a. A height below the 500-mb surface is and wind shear analysisover thefacsimile not designated as a tropopause unless itis the network which may prove valuable as an aid in only height satisfying the definition. and the positioning the tropopause. This chart is a two average lapse rate in any higher layer fails to panel presentation for the 48 states of the U.S., exceed 3 ° C/km over at least a 1-kilometer layer. southern Canada. northern Mexico. and the Further. the sounding must reach at least the contiguous ocean areas on a P20 million scale. 200-mb pressure surface. (The intention here is This numerical analysis duplicates as closely as to admit that a discontinuity in the lapse rate at possible the manual analysis. A further descrip- a height below the 500-mb surface is a tropo- tion of this chart is contained in the National pause only ifitis reasonably certain that no Weather Service Forecasters Handbook WBFH other choice is possible.) No. I b. When determining the second or higher FNWC Monterey provides a 36-hour pressure tropopauses, theI-kilometer interval with an height and temperature prognosis for the 50`:,-, average lapse rate of 3° C/km can occur at any 400-, 200-, and 100-mb levels. To obtain this height above the conventional tropopause and chart the 500- to 400-mb temperature lapse rate not only at a height more than 2 kilometers is extrapolated upward to its intersection with above the first tropopause. the 200- to100-niblapse rate extrapolated downward. The height at which this intersection RADIOSONDE ANALYSIS takes place is taken as the tropopause height. (See fig. 7-30.) Figure 7-29 illustrates how to apply the WMO By this method the tropopause is obtained as tropopause definition to a sounding. The first a continuous surface. Its actual leaf structure is tropopauseisdefined by criterion I of the not portrayed. The tropopause height can be definition. This criterion is satisfied above point usedasan approximationtothelevel of B of figure 7-29; that is. the average apse rate maximum winds. between points B and D. the next 2 hither kilometers above point 13, is less than 2° C/km SUPPLEMENTARY UPPER AIR ANALYSIS for all points of the 2-kilometer lapse rate. Thus, the first tropopause is established at point B of The basic upper air analysis is the constant the sounding. pressure analysis. In conjunction with this basic Thereisalsoa possibility of a "second" analysisitis sometimes necessary, and at all tropopause at point D. To find out, criterion 2 is times beneficial. to conduct concurrent supple- used. This criterion requires a 1-kilometer layer mentary types of analyses of upper air proper- with a lapse rate greaterthan 3° C/km below the ties in order that the fullest use be made of

253 259 AEROGRAPHER'S MATE I & C

1km

1km SECOND i TROPOPAUSE T momb 1km 1km LAPSE RATE\\ LAPSERATE ---.-2C/kM \ e-t3 C/kM

Ikm

Ji FIRSTTROPOPAUSE 200Mb 8 : 1km cc cc Iw I o1 co ISOBARSMb 250mb A -60 -50 -40

AG$43 Figure 7-29.Tropopause determinations basedon WMO definitions. upper air informationtoleadtothe end analyzed constant pressure surfaces whereby the product, the forecast. Most of these chartsare contours (height values) of the lower levelare constructed from either reported or derived data subtracted graphically from the contours (height from upper wind and upper air reports. It is not values) of the upper level (as illustrated infig. feasible to list or explain all the types ofupper 7-31) to obtain the "differential" (thickness) aircharts currently being produced by the between the levels. National Meteorological Center. Only thespace The thickness between the levels (thickness differential (thickness) analysis, time differen- charts may be constructed betw..-nany two tial, and advection charts are covered here. pressure surfaces) indicates the mean temperature of that layer; the amount and type of SPACE DIFFERENTIAL advection occurring (thermal wind); the density (THICKNESS) ANALYSIS of the layer; deepening and filling ofpressure systems; the vertical wind shear; and cyclo- Construction genesis. Space differential chartsare used primarily as a basis for advection charts; in fore- Space differential analysis is an analysis in- casting sea level pressure; in forecastingupper volving the utilization of the contours from two heights; in forecasting deepening and filling of

254 260 Chapter 7UPPER AIR ANALYSIS

AG.544 Figure 7-30.Tropopause height 36-hour prognosis.

255 261 AEROGRAPHER'S MATE 1 & C

lines iust cross the contours of both charts in 90 the same linear direction. They cross from greater heights. to lower heights or from lower heights to greater heights on BOTH charts at any given intersection. Thickness lines cross contours of either chart only at intersections and do not cross each other. Two consecutive contours of either constant pressure chart must be separated by a contour of theother constantpressure chart or by a thickness line. Two consecutive thickness lines must be separated by a contour of one of the constant pressure charts. A thickness line be- WOO-mb. CONTOURS 700-mb. CONTOURS tween two intersections is proportional to the THICKNESS (MEAN VIRTUAL TEMPERATURE) thermal wind. The thermal wind is the me: n COLD ADVECTION wind shear vector in geostrophic balance with WARM ADVECTION the gradient of mean temperature bounded by twoisobaricsurfaces. Thehermal wind is AG.545 directed along isotherms with cold air to the left Figure 731.Graphical subtraction of contours in the 2.1orthern Hemisphere. It can be shown (heights in meters). vectorially by subtracting the wind vector of the lower isobaric surface from the wind vector at pressure systems, in forecasting (in conjunction the upper isobaric surface as illustrated in figure with other things) the direction and speed of 7-3. motion of surface and upper air pressuresystems: and in checking the consistency of frontal Uses of the 1.000-500 mb analysis. The most commonly used differential Thickness Chart charts are: the 1,000- to 700 -nib thickness chart. the 1.000- to 500-mb thickness chart, the 700- The 1,000-500 mb thickness chart is one of to 500-mb thickness chart. and the 500- to the primary tools used at the National Meteoro- 200-mb thickness chart. logical Center and can be equally as important In constructing thickness charts itis best to to you. Some of the important relationships use acetates; next best is to use a clean chart. betweenthickness and betweenfronts and First. trace the lower-level contours onto either cyclones in the various stages of development the chart or acetate. and label appropriately. are given in the following sections. Figure 7-32 The lower-level contours should have a distinc- illustrates most of the important details of tivecolor (usually yellow).Next, tracethe thickness patterns in relation to fronts. upper-level contours onto the chart or acetate, The most important features stressed and also in a distinctive color (usually green). This used are: method eliminates lines, such as isotherms in the analysisarea. Then. graphically subtract the 1. Warm sectors are relatively homogenous; lower contours from the upper and connect the the concentration of thickness contours ison points of equal thickness with a dashed black the cold side of frontal systems. This will be line, and label them. better defined with moderate to strong fronts. 2. The cold trough in the thickness contours To start the analysis,it may be helpful to liestotherearof the surface depression, determine arithmetically the height difference at halfway between the surface depression and the a few points. As experience is gained by the next upstream surface ridge, or high. Aerographer's Mate, enough proficiency is ac- 3. Thickness contours are anticyclonically quired so that the hand holding the pencil is able curved in advance of warm fronts and cycloni- to follow the eye as it sees the points. Thickness cally curved behind cold fronts.

256 26 Chapter 7UPPER AIR ANALYSIS

CONTOURS (1000mb) 1000-S00 NB THICKNESS

AG.546 Figure 7-32.Illustration of the relation of thickness patterns to fronts.

4. The spacing of thickness contours behind portrays a meaningfulpictureofthe cold fronts is closer than in advance of warm 3-dimensional temperature structure. fronts.reflecting the generally accepted idea that cold fropts are more intense than warm ADVECTION CHARTS fronts. Advection chartsare essentially a part of 5. The distance between the thickness jet space differential charts. They are usually con- which is usually Ith-ated horizontally in the same structed on the space differential chart, although position as the 500-nib jet and cold fronts is less a separate chart may be used to indicate the than with warm fronts, refleaing the steeper advection patterns. Advection patterns are indi- slope of cold fronts as compared with warm catedat every point of intersection of the fronts. constant pressure charts. The advective flow is nearly perpendicular to the thickness lines and is Adherence to these important features of the indicated with a single-shaft, single-barb blue thickness model insures the proper slope of arrowfor cold advection and a single-shaft, systems between 1.000 and 500 mb. the proper single-barb red arrow for warm advection. Ad- relationshipbetween surfacefronts and the vection arrows indicate the mean direction of polar jetstream, and surfaL.e frontal analyses that flow between the two isobaric surfaces. If the

257 AEROGRAPER'S MATE 1 & C

arrow crosses the thickness lines from higher apart. of the same isobaric surface (500 mb). value to lower value. the advection is warm. If Place the older chart on the bottom, anda clean the arrow crosses the thickness lines from lower acetate or blank chart over both. Overa light values to higher values, the advection is cold. table construct lines of equal differenceat each .Advec tion charts may be constructed between of the intersections of the two charts muchin any two desired isobaric surfaces. the manner used to construct the thickness Rules of thumb for the Aerographer's Mateto charts. Start the analysis by algebraically sub- follow when constructing advection arrows are: tracting the new contour values from theold when the lower contour is to the left while contour values and connecting the points of looking downstream, advection is cold: wlien the equal difference. Areas of falling heightsare lower contouris to the right while looking indicated by drawing the height changelines as downstream, advection is warm. Refer to figure solidredlines;areas of risingheights are 7-31 for an illustration of the proper method of indicated by drawing the height change linesas constructing advection arrows. solid blue lines, drawing the zero heightchange A derivative of the thermal wind equation and line as a purple line. The centers of rising and the advection arrows is the thermal wind rule. falling heights are then transposed ontoa blank This rule simply states that in the Northern chart used solely for this purpose. Succeeding Hemisphere it' the wind backs with height. cold positions of the rise and fall centersare con- advection is indicated. and if the wind veers with nected inthe same manner as are pressure height. warm advection is indicated. That this system centers on the constant pressure charts. rule is true is shown in figure 7-33. The individual height change lines shouldbe marked inthe manner of the contourson constant pressure charts. The center values of

Tp477 .THRti 1TNRu the rise and fall centers should be enteredon the COLD FRONT WARM FRONT COLD AR MASS 1WARM AIR MASS tracking chart. For an illustration of the method > Ispeo4r."7 _ of constructing time differentialcharts, see FAT figure 7-34.

; CR COMPUTER PRODUCTS SEC (CA wT

- ANECTICii,IEND ADvECTiON WIND Space differential (thickness), and advection 13 4515 MO VEERS WARM MRS charts are available in the form of computer derived products. AG.547 The 1,000 to 700. 1,000 to 500,700 to 500, Figure 7-33.The thermal wind rule and 500 to 200 mb thickness (differential) (Northern Hemisphere). charts are all available in computer form. As discussedearlier, computer derived vorticity advection chart are available for utilization in TIME DIFFERENTIAL CHARTS the analysis process. Time differential charts may be constructed by utilizing a series of Time differential charts serve the mainpur- thickness charts and are thereforeno. available pose of tracking rise and fall centers on upper air in chart form over computer circuits. charts (usually the 500-mb chart), although other levels may be chosen for thispurpose as well. These charts are usually drawn at 24-hr SUMMARY OF UPPER AIR intervals in order to minimize diurnal effects ANALYSIS RULES which might otherwise invalidatesome of the features of these charts. Observation of many analyses by inexperienced personnel reveals some typical and often- The time differential charts are constructed in repeated errors in upper-air analysis. The follow- the following manner: take two charts, 24 hours ing list is by no means all inclusive.

258 264 Chapter 7UPPER AIR A sIALYS1S

540 ------..--- 546 ------552 558 ----

540

LEGEND: 500 mb CHART (TENS OF METERS) NEW CONTOURS OLD CONTOURS HEIGHT CHANGE LINES

AG.548 Figure 7-34.Time differentia! chart.

I. Contours (isohypses)areto be drawn 3. Wind direction cannot change discontinu- parallel to winds, where possible. This will not ously along a contour. always be possible due to errors in observations 4. In the vicinity of centers (highs and lows) and nongeostrophic and/or nongradientwinds. do not stop analysis merely because there are no Always check the dotted windshaft with the actual data. Use wind scales, history chart and number indicating u.,ection before deciding the common sense to get central contour height and observed wind is impossible to draw to. its position. If an intermediate contour helps to 2. Use history. Troughs, ridges, highs and define location of a center, put it in (with a lows may change slightly their configuration and dashed line). orientation and have differential movement. 5. Do not overemphasize the cyclonic curva- History tells what these features looked like 12 ture at troughs in the form of a kink in the and 24 hours ago and in what general area to startlooking for them on the current map. contour (i.e. ). This is absolutely Always check the previous analyzed map before starting analysis. wrong. Itimplies that the trough is a front.

259 . 'v65 AFROGRAPIIER'S MATE I & C

observing accuracy of contour analysis (use Instead dothis: Note the Buys-13a1lot's law), by drawing in a few intermediate contours. and by using the geostrophic relative maximum of cyclonic curvature at the wind scale. trough. 1 .Inusing the geostrophic wind scale to 6. Do not throw ass a} data atfaast glanLe detcimine coirect spacing of isohypses on upper- Over the ocean area the following, iepoits ale aircharts, the spacing must be less than the excellent. with the list in desLending order of observed wind indicates where the curvature of "level of the isohypses is cyclonic-, the converse is true for a. Permanentships.as M, eat he r ships anticyclonic circulation. Thisis only another November. Papa and others. tray.ling commer- way of saying that winds in cyclonic regions are cial and militar} surface craft. also. permanent subgeostrophic. in anticyclonic regions super- land stations as the Hawaiian group. Midway. geostrophic. etc.: all of the foregoing are "on-time reports. 12. In isotherm analysis keep 'in mind there is h Weather reLonnaissan,e trcLon) leports. a tendency for the lows (highs) at 500 mb to be these are plotted about a 0 w ith T. Ta.500-mb cold (warm) with adefinite closed isotherm height and usually wind: and nearlycoincidentwiththepressurecenter. c Commercial and [Millar} airaaft report- Long-wave troughs are cold. ridges warm, with ingflight-leveltemperature. wind. and some- reverse relation for short waves. times a "D value: extrapolations to 500-nib 13. Jagged. ragged or nervous lines lend an level must then be made. unprofessional appearance to an analyzed map. 7. With weather recoils and other ancraft Avoid discontinuities in the isolines. reports. time of obsersation must be takenant.' 14. Short-wave troughs move through long- account. Correct the data to map tin', time wave patterns. hence their orientation and move- interpolation or extrapolation. ment traces out long wave patterns. Troughs are S. Over the ocean. Lontinuit} and pastor} are normally concave to the direction from which asimportant as anobscrsation or extrap- they are moving. Check history map for most olation Do not lose s} stems for iaLk of data recent configuration. Marked changes in configu- over the ocean. ration should be doubted. Q. Troughs and centers ;nosing 50 kt or more 15. ('heck vertical consistency by reference to are to be doubted tlles-, overwhelming, eviden,c analyses at adjoining levels. especially the lower indicates this to be true. Mosement of troughs levels: and centers embedded in the westerlies is gen- a. Troughs and lows must slope toward erally with some meridional component. coldest air. The cold lows and warm highs tend to mose b. Ridges and highs must slope toward slowest.( usuallylessthan 0.5lat hr).On warmest air. occasion the centers of these dynanilL cold and c. Cold lows have little or no slope. warn. systems move westward with ,omitnaerad- d. Warm lows at low levels become short- ionalcomponent. At times they are stationar}. wave (warm) troughs aloft. 10. Considerabletrouble asexpcnenLed in e. Wave cyclones at sea level become short northern Canada due to lack of data. Ills is an waves aloft. area where one must re. not just draw 1. OcLluded cyclones become cold or cut- lines, Most troubles could be cleared up by off lows aloft.

260 0.%'66

I CHAPTER 8

FORECASTING UPPER AIRSYSTEMS

be To prepare prognostic charts, hot) surface chart will be considered. These methods can charts and upper air. we must first make predictionsof used in constructing your own prognostic the various weather systems found on each type where data are not available from other sources made chart. Inasmuch as the currentsurface and upper andlor to check on the prognostic charts air charts reveal in detail the current stateof the by other sources. weather, so should the prognostic chartsreveal accurately and in detail the future stateof the GENERAL PROGNOSTIC weather. Constructing prognostic chartsis no CONSIDERATIONS easy task: nevertheless it is notimpossible. The ideal approach to preparing upper airand Itisincumbent upon thefort..,asterto prognosticchartsdictatesthatthe surface considerallapplicable rules, drawupon Aerographer's Mate first begin with the tipper experience, and consult anyvalidavailable levels and then translate the progdownward in objectiveaidsto produce thebestpossible surfaceprog. The two are so terms of a forecastfromdatathatare available. This interrelated that consideration of theelements evolves into both a subjective andobjective independently of the on one should not be made approach to the prognosis and the forecast. other. of constructedatthe A forecaster has examined many aspects Prognosticchartsare and National Meteorological Center (byNumerical the weather picture from both surface upper air maps by the timehe issues his forecast. Prediction methods) and Fleet Weather soft-pedaled, Facility,Monterey. The Some conditions are overlooked or NumericalWeather forecaster resultant products are transmitted overtheir while others are emphasized. The must depend heavily upon hisexperience and respective facsimile networks. weather Overseas Fleet Weather Centrals andFacilities knowledge with past history of the yield simi- also construct and transmit progs.We are all too because similar analogous conditions decide often inclined to rely solely on thesedata with lar consequence s. Some forecasters may parameter, such as ornoactualcomputations made by to overlook a clearly defined few his experience, ourselves. Since these prognostics aregenerally sulfapressure, because through others. he may decide that for large areas, this procedurecould lead to or the experience of and mental lethargy itis not a decisive factor. In this casehe may many erroneous forecasts of tipper wind, on the part of theAerographers Mate. concentrate on the general trend For these and other reasons it is important the humidity and stability revealedby sound- Aerographer's Mate not only ings. and other factors. The mental processes that the the fore- understand the methods by whichprognostic involved in making the prognosis and forecast require- charts are constructed but theirlimitations as cast are also guided by the well. In this chapter we will discuss someof the ments. and rules for forecasting An objective system offorecasting certain more common methods and often Inthe following chapter, atmospheric parameters can parallel upper air features. An methods and techniques for proggingthe surface exceed the skill of an experienced Forecaster.

d'7 AEROGRAPHER'S MATE 1 & C objective system. also often eliminates the un- data will be extremely varied.Some stations will comfortable moments of hesitationthat every receive only teletype forecaster has undoubtedly suffered or facsimile data while when he has others will be equipped to receivedirect readout been pressed for a forecast duringnondescript of satellite photographs. conditions.However,theobjectiveprocess should not necessarily takeprecedence over the subjective method, but rather thetwo should be OBJECTIVE FORECASTING used together, where and whenapplicable, to TECHNIQUES arrive at the most accurateforecast possible in theshortestperiod of time. Atany rate, The use of objective forecastingtechniques in whichever systemisused, some systematic preparation of upper air prognosticcharts af- method should be adopted, wherebyall of the fords the forecastera better opportunity to factors possible are cataloged forready reference arrive at a more accurate chart.Experience in as a checkoff list. itself is not enough to forecastthe movement or intensity of upper air systems,but coupled with HAND DRAWN ANALYSIS basic objective techniquesprovides a sound basis for the forecaster toprepare an accurate and valid chart and forecast. The correct methods andprocedures to be utilized in analyzingupper air charts have been covered in Chapter 7. A properlydrawn hand FORECASTING THE MOVEMENT analysis provides the forecasterwith the basic OF TROUGHS AND RIDGES and most important toolin constructing an upper air prognostic chart which willattain a The techniques covered inthis section apply high degree of validity. Suchinformation as primarily to longwaves. Many of them are windspeed direction, temperature,dew point applicable to short wavesas well and it will be depression, and heights is readilyavailable for stated when and where theyare applicable. A the forecaster to integrateinto the various long wave is by definitiona wave in the major objective methods of producinga prognostic belt of westerlies which ischaracterized by large chart. length and significant amplitude.Therefore, the first step in progging longwaves is to determine their limits. Large closed lows COMPUTER PRODUCTS and highs, cutoff systems, and the like must be temporarilyset aside for separate consideration.This done, we The Fleet Numerical WeatherCentral pres- are ready to prog the movement of entlyprovides a great variety of the long charts for waves. There are several basic approachesto the dissemination to shore and fleetweather units. progging of both long and short These include analysis and waves. Chiefly prognostic charts these are extrapolation, CAVT's,Petterssen's ranging from subsurface oceanographiccharts to Wave Speed Nomogram,the Grid Method, the depiction of the troposphere,as well as a isotherm-contour relationship, and the number of specialized charts. A location number of these of the jet maximum in relationto the current in chartshave been discussed in thepreceding which it lies. chapter. A complete listing of the variouscharts is contained in the Naval WeatherService Com- Extrapolation puter Products Manual, Nav Air 50-1G-522. Knowledge of the past history ofthe systems APPLICATION OF SATELLITE affecting the area of interest is CLOUD PHOTOGRAPHS fundamental to the success of forecasting.Experience has shown that most atmospheric As a further aid, satellite systems usually change pictures, mosaics, slowly and continuously withtime. That is, a and nephanalysiscan also be used in preparing a continuity in the weather pattern prognostic chart. The availability of is exhibited in satellite a sequence of weather charts. Whena particular 262 268 Chapter 8-FORECASTING UPPER AIR SYSTEMS pressure system or distribution exhibits a tend- speed, wind direction (angle with east),and ency to continue without muchchange it is said latitude value to be used in the CAVT table. to be persistent. These concepts of persistence The inflection point is that point along each and continuity are very useful forecast aids. ridge or trough where the relative vorticity [he extrapolation procedures used in fore- changes from positive to negative, or vice versa. casting may vary from simple linear extrapola- Inother words, the actual curvature of the tion to the use of more complexmathematical contour changes from cyclonic toanticyclonic, equations and analog methods based on theory or vice versa. In this area,the point with zero orlong experience. Two such methods are wind shear (strongest windspeed) is designated presented here: one is simple extrapolation, the as the CAVT inflectionpoint. The point will other a more complex method based on kine- always be upstream from the trough orridge matic extrapolation. concerned. (See fig. 8-1.) The practicing forecaster usually extrapolates past and present conditions to obtainfuture INFLECTION conditions in accordance with scientific prin- POINTS ciples and years of practical experience. SIMPLE EXTRAPOLATION.--The simplest method of forecasting both long and short wave movement is that of extrapolation. Simple extrapolationis merely the displacement of the trough or ridge to a futureposition based on past and current movement and expected trends. It is based on the assumption that the changes in velocityofthe pressure patterns AG.549 are slow and gradual. However,there are pitfalls Figure 8-1.Selection of the inflection tothis method in that new developments fre- point for CAVT's. quently occur which were not revealed from present or pastindications upon which the extrapolation was based. However, if such dev el- Information regarding procedure to be fol- opments can be forecastb.vother techniques. lowed in using the CAVT method of forecasting allowance can be made for them. the movement of troughs and ridges maybe Extrapolationfor shortperiods on short found in various publications which arelisted in waves is generally valid. The majordisadvantage Navy Weather Research Facility Reports and of extrapolating the movement of long waves, or Publications (NWRF 00-0369-143) and updated long period movements of short waves, isthat revisions. past and present trends do not continueindefi- nitely. This can be seen when we consider a Petterssen's Nomogram wave with a history of retrogression.The retrogression will not continueindefinitelNand we Petterssen's wave speed nomogram utilizesthe must look for indications of itsreversal, that is. wavelength (L) in degrees lat tube or nautical progressive movement. miles: the windspeed at the core of the current at the trough or ridge line (U):the distance from the core of the current to locationof U where Constant Absolute Vortieity the windspeed is one-hall of U in the sameunits Trajectories (CAVT's) as L. (B) which providesthe value for LIB; the wave axis of inclination fromthe meridian of This method is used to determine the move- the trough T (gamma): and the latitude(I) (phi) ment of long wave troughs and ridges atthe where U is determined. the 500 -nib level. Using the data given in figure 8-2, enter To compute CAVT's the following data atthe nomogram (fig. 8-3) at the topunder the value of 3,000 nautical miles and go downward tothe inflection pointisrequired: maximum wind 263

Z69 AEROGRAPIIER'S MATE 1 &C

fairly symmetric velocityprofile alongthe 3,000 NM trough or ridge line isnecessary. ,,20° A test of 158 troughsand ridges indicated the I following corrections shouldbe applied to the I results of the computations: I I I. Deduct I longitude degreeper day from I each computation of troughspeed. 2. Deduct 3 longitudedegrees per day from the computed motionof each ridge east of the Rocky Mountains. 3. Deduct 5 degreesper clay from the com- Lo:500N puted motion ()leach ridgeover the Rockies.

NALITICamilES Grid Method

Ulu td8:20 This methodisusedfor forecastingthe movement of shortwave troughs and closed AG.550 lows at the 500-mb level.A grid is constructed Figure 8-2.A long wave situationfor use as a 20-degree latitudesquare, true at latitude 45 with Petterssen's wave speednomogram. degrees for the typemap projection used. The value of T (gamma). From displacent of the lowor trough is given by the the point of inter- mean geostrophic windover the area covered by section of T (gamma),go horizontally across to the the grid. (See fig. 8-4 forexample of grid.) value of (13(phi), and thence vertically First we will discuss the downward to the valueof U. From the point movement of 500-mb shortwave troughsfora24-hour forecast where you intersectedU, go horizontallyacross period. the nomogram to thevalue of L/B and thence I. Aline the grid vertically upward until so that the centerline DOC you intersect the slanting is over the trough withpoint "0" approximately line which gives themovement of the long wave in knots or degrees in the middle of the trough. latitude per day. Your result 2. Read off heightpoints from the 500-mb in this case is 2° lat/day.Other waves should be height field at all points similarly evaluated. from A to F (in this The major obstacles example meters are used). to successful use of the 3. Using the followingformula, substitute the nomogram are that L and T (gamma)are not always definitive, heights for the letters in theequation. Note plus andthatU and B, and values indicateeast movement, minus values therefore L/B are notalways accurate. Despite west movement. these drawbacks, the nomogram is of value in "X" ((A-B)(C -D) + (E-F)( = E-W MVT, that it does yielda rough indication of both the direction and extent ofmovement and this is of where E = east. value, because the Aerographer'sMate can then smooth out the result, The solutionto The best results from the the equation shouldbe use of this tech- positive to indicate eastwardmovement, since nique will be obtained if thetroughs and ridges short waves are to be computed are selected under progressive. Movement isin the following degrees of latitude. "X" isthe latitude factor conditions: Adequateupper wind data are neces- using latitude at point "0". sary: only cases should be selected See Table 8 -I for where the the "X" factors fortwo different map projec- stteamlines have a well definedsinusoidal pat- tions and the values for tern from the trough both feet and meters. or ridge in question fora (Feet using hundreds offeet, and meters using distanceL/2 upstream. Symmetryof wave- tens of meters.) lengths upstream and downstream about the For moving shortwave troughs, a systematic trough or ridgeis not required:a simple and correction of minus l degree of latitudeper 64 A.170 -11

:VOICONESIMEMN ':MMISMINVall..11...1_ VittairSt.51511.44, 4imMbirr'l 4"1:- 52'5

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-11 1111111M11131111111111111111d1111111 rune: I I mannummumaila MIN11111111 _ IMMINIMIllk I 1111111111111111111111111111111 - re111 11111111111111111111111;\X11111111111111111 1:1111 _ = = 11111111111111111111111111111111111111111 1111101MIN1111111101MOIMUM - 111I11MINIM111I11EM11EI111M11 1et. inommanacencommn 11 _ 1111111111111111111111111111111111111111 AEROGRAPHER'S MATE I & C

D results, or both, they have servedthe purpose of B F confirming previous results. Thiscan be seen in a situation where CAVT's yield smallprogressive movement and the nomogram yieldsa small retrograde movement. If theisotherm-contour relationship confirms retrogrademotion, we are G H justified in placingmore faith in the nomogram result. A number of observationsand rules are stated regarding the progression,stationary characteristics, or retrogression oflong waves. These rules are stated in the followingsection. A E C PROGRESSION OF LONG WAVES.Pro- gression (eastward movement) ofthe long waves AG.552 is usually found in associationwith relatively Figure 8.4,-500.mb grid. shortwave lengths andwell defined major troughs and ridges in the middleand upper Table 8.1. "X" factors foruse with the troposphere. At the surface, thereare usually grid equation. only one or two prominentcyclones associated with each major trough aloft.Under the forward part of each major ridge there isusually a well developed surface anticyclonemoving toward Lambert Conformal Polar theeastor southeast. The 24-hour height Stereographic changes at upper levels usually havea one-to-one association with major troughs andridges (mo- Latitude Feet Meters Feet Meters tion of maximum heightfall and rise areas associated respectively with majortrough and 25 . 57 .187 . 67 .220 30 .48 ridge motion). The tracks of theheight change .157 .54 .177 cen,Jrs depend on the movement and changes 35 .42 . 138 .45 .148 of 40 .37 .121 intensity of the longwaves, and often seem .38 .125 somewhat erratic. 45 .34 .112 .34 .112 50 .31 . 102 .30 .098 55 .30 .098 .27 .089 STATIONARY LONG WAVE PATI'ERNS, 60 .28 .092 .25 . 082 Once established, stationarylong wave patterns 70 .26 .085 .23 .075 usually persist for a number ofdays. The upper air flow associated with thelong wave pattern constitutes a steering pattern forthe smaller not be used when the contours in scale disturbances. These smallscale troughs and the neighbor- ridges, with their associated heightchange pat- hood of point "0" (initialpoint) greatly exceed terns and weak surface systems, an amplitude of 15 to 20 degrees of move along in latitude. the flow of the large scale, longwave pattern. Minor troughs intensify Isotherm-Contour Relationships as they move through the troughs of the longwaves and weaken as they move through the ridges ofthe long waves. The forecaster shouldalways examine the long waves for the The same changes in intensityoccur in those sea isotherm-contour relation- leveltroughs or pressurecenters which are ships and then apply the rulesfor the movement associated with minor troughs. Partly of long waves. These rules as a result were stated in chapter of the presence of thesesmaller scale systems, 4 of this training manual.The rules do not yield the troughs and ridges of the a very quantitative movement, but if stationary long the rules waves are often spread out and hardto locate confirm the CAVT resultsor the nomogram exactly. Chapter 8FORECASTING UPPER AIR SYSTEMS'

RETROGRESSION OF LOAIG WAVES. --A waves of large wavelength and small amplitude, continuous retrogression of long wave troughs, slowly progressive or stationary. Few extensions in which the wave troughs areconserved, is a of troughs into the low latitudes are present and rare event. The usual type ofretrogression takes inthis situation the jetstreamis weak and placeinadiscontinuous fashion whereby a disorganized. major trough weakens, accelerateseastward, and As the jet proceeds farther northward, there is transformed into a minor trough while amajor will often be a sharp break of high index with wave trough forms to the west of theformer rapidly increasing amplitude of the flow aloft. position of the old one. New major troughs are Long waves retrograde. As the jet reaches the the sub-Arctic generallyformed bythetransformation of upper midlatitudes andinto minor troughs into deep coldtroughs. Since old region, itis still the dominant feature, while a troughs are weakening and new ones are devel- new jet of the westerlies graduallybegins to oping during this process, it would be improper form in the subtropical regions. Long waves now to apply any wave velocity equation to this type beginto increase in number and thereisa Tropics. The situation. reappearance of troughs in the Retrogression is seldom a localized phenom- cycle then begins over again and repeats itself. enon, but as a rule appears to occur as aseries of With a southward migrating jet, the processes moving retrogressionsinseverallong waves. Retro- are reversed from that of the northward gression generally begins in a quasi-stationary jet.It should also be remembered that short long wave train when the stationary wavelength waves are associated with the jet maximum and shows a significant , _crease. This can happen as move with about the same speed as thesejet a result of a decrease in zonalwind speed, or of maximums. a southward shift in the zonalwesterlies. Some characteristics of retrogression are as follows. FORECASTING THE INTENSITY Trajectories of 24-hour height change patterns at 500-mb deviate from the band of maximum OF TROUGHS AND RIDGES wind. New centers appear or existing ones Forecasting the intensity of long waves often rapidly increase in intensity. Rapid intensifica- of yields nothing more than a sign of the expected tion of surface cyclones occurs to the west intensitygreater than or less than present inten- existing major trough positions. sity.For instance,if deepening or fillingis LOCATION OF THE JET STREAM. In chap- indicated, but the extent of deepening or filling ter 4 of this training manual we discussedthe is not definite, the Aerographer's Mate is forced migration of the jetstream both northward and to rely on experience and intuition in order to southward. Some general considerations can be prog the amount of deepening or filling.Numeri- made concerning this migration and the move- cal Weather Prediction Methods do forecast the ment of waves in the troposphere. intensity of upper waves with a great deal of wind In a northward migrating jet, a west success.If available, you should check your gradu- maximum emerges from the tropics and intensityand movement predictions against allymoves throughthe lower midlatitudes. these prognoses. Another maximum, initially located inthe upper midlatitudes, advances toward the ArcticCircle while weakening. Open progressive wave pat- Extrapolation terns with pronounced amplitudeand a decrease in the number of waves due to cutoff centers Patterns of upper levelchartsaremore exist. The jetis well organized and troughs persistent than those on the surface, Therefore, extend into low latitudes. extrapolation gives better results on the upper As the jet progresses northward, the ampli- aircharts than on surfacecharts.In using tude of the long waves decrease and the cutoff pressure changesaloft, the procedureisto lows south of the westerlies dissipate. By the extrapolate height change centers and add the time the jet reaches midlatitudes a classical high changes to the current height values in order to index situation exists. Too, we have weak, long get the prognostic map. Changes in theshape of

267 -273 AEROGRAIIIER'S MATE I R C the tipper air pattern may be forecast by this Consideration of CAVT's method, whereas pure extrapolationwould not indicate these variations. As mentioned earlier, bycompar:.., the amplitude of the troughs andridges on the current Use of Time Diffeuntials chart and the amplitudeas determined by the CA VT tables, intensificationor weakening can be determined. !le time differential chartis discussed in chapter 7 of this training manual. Isotherm-Contour Relationship The time differentialchart for the 500 -nib level shy" the history ofwhat changes have In long waves, deepeningtroughs are associ- taken at the 500-nib level at 24-hour ated with cold advectionto the west side of the inter,a1s. In consideringthe informationon the trough and tilling troughswith warm advection, time differential chart, those centers with a well The converseistrueforridges. Warm air defined history of movement will be of greatest advectioninto the western side ofa ridge importance. Take intoconsideration not only indicates intensification andcold air advection the amount of movement. but also the changes indicates weakening. Thisrule is least applicable in intensity of thecenters. Single centers with east of the Continental Divide and no apparent probably east history should be treatedwith of any high mountainrange where winds are caution especially with regard to their direction from the west. In shortwaves, deepening short of movement which is usually in a downstream wave troughs have cold airadvection into the position from their current position. The infor- west side of the trough,particularly if a jet mation as indicatedon the time differential maximum isin the northerly current chart should be used of the to supplement the infor- trough andfillingisindicated by warm air mation already obtainedfrom previous consider- advection on the western side. ations and when inagreement can be usedas a In the above paragraph, the guide for the amount of advection is not elmnges to the prog- the cause of the intensitychanges, but rather isa nostic contours ina given area. "sign"ofwhat is occurring.Highlevel Normally, the 24-hour heightrise areas can be convergence/divergence is thecause. moved with the speed ofthe associated short wave ridges and tine speed of thefall centers Effect of Super and with the speed of theassociatld wave troughs. Subgradient Winds Either leave the intensityof the height change centers unchanged or modify themaccording to Figure 8-c (A) through (D) showsthe effect of past developments. It must beremembered that the location of maximum windson the intensity height change centersmay be present due to of troughs and ridges. convergence or divergence factors andmay not Explanation of figure 8-5 isas follows: have a short wave troughor ridge associated I. When the strongestwinds aloft are the with them. however.normally with short wave westerlies on the western sideof the trough, the indications,a change center will appear and trough deepeus (fig. 8-5(A)). move in the direction of thecontour flow. Be 2. When the Strongestwinds aloft are the cautious not to movea height change center westerliesinthe southern quadrant ofthe with the contour flow ifitis due primarily to trough, the troughmoves rapidly eastward and convergence or divergence. does not change in intensity(fig. 8-5(B)). 3. When the strongest Once you have progged the winds are the south- movement of the westerlies between the troughand the down- heightchangecenters and determinedtheir stream ridge, the trough decreasesin intensity amplitude, apply the change indicatedto the (fig. 8- 5(C)). height on the current 500 -nub chart and use 4. Sharply curved ridges these points as guides in constructing with excessive con- prognostic tour gradients are unstable and contours. rotate rapidly clockwise, causing large heightrises any filling in '74268 Chapter 8FORECASTING UPPER AIR SYSTEMS the trough area downstream and large height changes in the intensity of troughs and ridges, falls in the left side of the strong gradient ridge we have the following rules. Refer to chapter 4 (fig. 8-5(D)). for illustrations of these rules. I. Divergence and upperheightfallsare associated with high speed winds approaching cyclonicallycurved weak contour gradients. Divergence results in height falls to the left of the high speed current. (A) 2. Convergence and upper height rises are associated with low speed winds approaching straight or cyclonically curved strong contour gradients and with high speed wind approaching anticyclonically curved weak contour gradients.

(B) FORECASTING THE MOVEMENT OF PRESSURE SYSTEMS ALOFT

Inthis section we are concerned with the movement of closed pressure systems aloft. In this category we find both anticyclonic and (C) cyclonic centers.

Movement of Highs

SEMIPERMANENT HIGHS.The semipermanent subtropical highs are ordinarily not subject (D) to much day to day movement. When a subtropical high begins to move, it will move with the speed and in the direction of the associated HEIGHT RISES ridge. The movement of the ridge (long wave) has already been discussed. Too, seasonal movement, though slower and over a longer period of AG.553 Figure 8-5.Effect of super and subgradient winds on time, should be considered. These highs tend to the deepening and filling of troughs. (A) Strongest move poleward and intensify in the summer and winds on the west side of trough; (B) strongest winds move equatorward and decrease in intensity in in southern portion of trough; (C) strongest winds on the winter. east side of trough; (D) excessive contour gradients. BLOCKS.Blocks will ordinarily persistin the same geographic Ideation unless the closed contours of the high are strongly'eccentric: that Convergence and Divergence is, they are of uniform shape and a number of Above 500 Millibars closed contours are present. Blocks will move in thedirectionof thestrongest winds when Studythe 300-mb (or 200-mb) chart to contour spacing is asymmetric: their motion is determine areas of divergence and convergence eastward when the westerlies are strongest and and note these areas and the relative strength westward when the easterlies are strongest. The and extent of each for consideration with the speed of movement of these systems can usually movement of the long wave and progging of be determined more readily and accurately by contours. extrapolation. Extrapolation should be used in Convergence and divergence are covered in moving he highs under any circumstance, and chapter 4 of this training manual. As a review of theresultsofthis extrapolation shouldbe the effects of convergence and divergence on the correlated with the other aspeLts of the move-

269 275 AEROGRAPIIER'S MATE I & C

ment and intensity prognoses of long andshort Movement of Closed Lows waves. Some indications of intensity changeswhich The permanent Icelandic and Aleutianlows are exhibitedbylowtroposphericcharts undergo little movement. These permanentlows (700-500 nth) are as follows: intensificationwill will contract or expand inarea; occasionally occur with warm air advection on the west side; splitinto two separate low cells; becomeec- weakening and decay willoccur with cold air centric; or become elongated along thehorizon- advection on the west side; and thereis little or tal during a high zonal index type of situation. no change in the intensity if the isothermsare The north-south displacement is dueprimarily symmetric with the contours. Thislow tropo- to seasonaleffects. The movement of these spheric advectionisnotthecause of the permanent lows is derived primarily fromex- intensity change but is onlya "sign." The cause trapolation. is at higher levels: for example. intensificationis caused by high-level cold advection and!ormass convergence.Under low index EXTRAPOLATION.Extrapolationcan be situationsa used at times to forecast both themovement blocking high will normally exist ata northern and the changes in intensity of latitude and will have a pronounced effect upper air closed on lows. This method should be usedin con- the systems in that area; in general it willslow junction with other methods to the movement. Under high index arrive atthe situations predicted position and intensity.Figure 8-6 there is a strong west to eastcomponent to the shows some examples of simple winds and systems will extrapolation of move fairly rapidly. both movement and intensity.Remember there

CENTRAL HEIGHT vALuE METERS

2,880 2,910 2,940/ ED300 NM ED 300N m. _10

02/12002 03/12002 FCST AT 04/12002 (A) CONSTANT MOVEMENT & FILLING ....

2,820 2,850 2,880 2 0 500 NM. 2 940 .452.4 NM 300 NM 2oly.r, KD 4ED Ow200 2 02/12002 03/1200 2 04/12002 FCsT AT 05/12002 (0) CONSTANT RATE OF CHANGE

24 HR. HEIGHT 4---cHANGE (+ 120M) (.460m) (+30141 (+15m) 2.670 2,790 2 850 53, N M. 400 NM 300N m 225 N M. .65 05/00002 06/00002 07/00002 08100002rCsT AT 09/00002 (C) PERCENTAGE RATE OF CH.' NGE

AG.554 Figure 8-6.Simple extrapolation of the movement and intensity of a closed low on the 700mb chart. (A)Constant movement and filling; (B) constant rate of change; (C)percentage rate of change.

270 276 Chapter 8 -FORECASTING UPPER AIR SYSTEMS are many variations to these patterns and each movement of long waves are determined. The case must be treated as an indiv idual one. eccentricity formula ma} be applied to derive a Figure 8-6 (A) illustrates a forecast in which a movement vector.but onl} when anearly low :s assumed to be moving at a constant rate straight eastward or westward movementis and filling at a Lonst....t rate. Sin Le the low has apparent. Migratory lows also follow the steerma moved 300 nautical miles in the past 24 hours, it principle and the mean climatolog° altracks. is assumed that it will move 300 nautical miles The climatological tracks must be used cau- in the next 24-hour period. Similarly,since the tiously for the obvious reasons. The rise and fall catral height value has increased b} 30 meters centers of the time differential charts are of in the past 24 hours. you would forecast the great aid in determining an extrapolated move- same 30 meters increase for the next 24 hours. ment vector and extrapolation is the primary While this procedure is very simple, it is seldom method by which the movement of a dosed low sufficiently accurate.Itisoftenrefinedby is determined. consulting a sequence of more than two -charts Certain cutoff lows and migratory dyilamic to determine a rate of change. cold lows lend themselves to movement calcula- This principleis illustrated in figure 8-6(B). tion by the eccentricity formula. The conditions By consulting the previous charts we find the under which this formula may be applied are: low is filling at a rate of 30 meters per 24 hours The low must have one or more dosed contours and therefore this constant rate is predicted to (nearlycircularin shape). and the strongest continue for the next 24 hours. However. the winds must be directly north or south of the rate of ino.ement is decreasing at a constant rate center. The location of the max winds deter- of change of 100 nautical miles in 24 hours. mines the direction of movement. When the Hence this constant rate of change of movement strongest winds are the easterlies north of the is then assumed to continue for the next 24 low,thelowmoves westward; whenthe hours so the low is now predicted to move just stroneest winds are the westerlies south of the 200 nautical miles in the next 24 hours. low, the low will move eastward. The low will Oftimes neither of these two situations exist also move toward the weakest diverging cyclonic and both the rate of change of movement and gradient and parallel to the strongest current. the height center change oclAtr at a percentage Systems moving eastward must have a areater rate. This is illustrated in figure 8-6(C). From a speed in order to overcome convergence up- sequence of charts 24 hours apart itis shown stream there is normally convergence east of a that the low is filling at a decreasing rate and low system. moving at a decreasing rate. The height change The eccentricity formula is written: valueis 5n percent of the value 24 hours previously on the successive charts and the rate Ec = V V'- 2C of movement is 75 percent. We then assume this constant percentage rate to continue for the Or next 24 hours so the low is forecast to move 225 nautical miles and fill only 15 meters. 2C = V - V' - Ec Accelerations may be handledin a similar manner as decelerations in figure 8-6. Also, a where sequence of 12-hour charts could be used in lieu of 24-hour charts to determine past trends. Ec is the critical eccentricity value. V is the wind speed south of the closed low. CRITICAL ECCENTRICITY.- -When a migra- V' is the wind speed north of the closed low. tory system is unusually intense, the system may C is the speed of the closed low (in knots). extendverticallybeyondthe300-mblevel. Advection considerations, contour-isotherm relationships, convergence and divergence con- In order toobtainthe value of C,itis Aerations, and the location of the jet max will necessary to determine the latitude of the center yield the movement vector. These principles are of the low and the spread (in degrees latitude) pplied in the same manner as they are when the between the strongest winds in the low and the

271 rye AEROGRAPHEICS NIATE I C

awl of the lou. tuner table S -2 with these 5. The following sources of error have been %.+1At.": and apply the tabular value thus obtained noted: to (11,2 critical eccentricity formula to obtain 2C, !tuts C a. Lows centered in troughs flanked by In determining the critical eccentricity an intense ridge. ,1 Itisnecessary to interpolate both b. Small lows whichmove southeastward tor nude and the spread. A negative value for around a well developed ridge. These ittk..:tes westward movement:apositive lows will move more rapidly than ward movement. forecast in every case. c. Lows centeredin long-wave troughs Table 8-2.-Critical Eccentricity Value. between strong ridges. d. Cases in which rapid and intensesea level development occurred just ahead ofthe low. Latitude Spread (degrees latitude) LOCATION OFTilE JETSTR As long fait;rees) 30 10 Sc I 10° 20° as a jet maximum is situated or moves on the western side of a low this low will not move out. 80 .1 .9 2.5 When the jet center has rounded thesouthern 70 .2 ! 1.8 4.919.5 80.0 periphery of the low and is not followed by (;0 .3 4 2.6 7. 127,0 115.0 another center upstream, the low willmove out 50 ; .4 3.3 9. 137.0150. 0 rapidly and fill. 40 .4 4.0 10.943.5 175.0 ISOT 11 ER M-00 NTO U R RELATIONSHIP.- 30 .5 4.5 12.350.0 - 200.0 Little movement 20 4.9 13.353.0 will occurif the isotherms and 10 5.2 14. 056.0 contours are symmetrical (no advection). The lows will strengthen and retrogress if cold air advectionoccurstothewest and progress eastward and fill if warm air advection occurs to GRID ME FlIOD. The grid methodas ex- the west. p!.iried previously for short waves may also be grog the movement of closed lows. FORECASTING THE INTENSITY 11 "". equation must be modified. The OF PRESSURE SYSTEMS ALOFT factor.; are used for both methods. I,pi,. :lure is as follows: Many of the same considerationsas explained Pi. -2 th; Center ofthe gridover the center intheprevious section for the movement of 'kk with the line DOC lined up directly closed centers aloft also apply to pronging their intensity. as in most cases intensity andmove- H.:Tilts are then read off the 500-mb ment are considered simultaneously. Extrapola- Iiitht fiL.Id at all points A through IL tion and the use of time differentials aid in ; Ilk,following formulas. substitute forecasting the change and the magnitude of -:11 Is the letters in each equation noting falls in moving lows. Again. the rise and fall ' v imlicate east:south movement indications must be correlated with advection ,-.1 !-;!-is talus west 'north movement. considerations. divergence indications, and the other factors discussed previously. \' ItA F + + F)1 = east 1%:%t movement in degrees latitude. Progging the Intensity i13 F (G- II) t (A L" north of Pressure Systems or south movement in degrees latitude. IIIGI (S.-Highs undergo littleor no change in 4 the smilecorrection of minus one degree intensity when i:optherms are symmetric with latitude per 24-hour period should be applied to contours. est component only when an eastward Ilighs intensify when warm air is advectedon ni v..inent is indicated. the west side of the high.

272 278 Chapter 8 -FORECASTING UPPER AIR SYSTEMS

Highs break down and weaken when cold air Lion considerations. divergence indications. and is advected on the west side. theindications of the contour-isotherm rela- Blocking highs usually intensify during west- tionsh ips. ward movement and weaken during eastwzrif movemen t. Progging the Formation of Convergence and height rises downstream in Pressure Systems the troughing area occur when high-speed winds with a strong gradient approach low-speed winds HIGHS.-Anticyclogenesis is ordinarily not a in an anticyclonic weak gradient. This is often problem inprogging the 500-niblevel (and the casein ridges. where the west side of the higherlevels)exceptfortheformation of ridge harbors the high-speed winds. and the ridge blocking highs. intensifies as a result of this situation due to the The shallow cold air masses of polar and accumulation of mass. Accumulation of mass Arctic origin generally give no indication of due to convergence occurring with this situation formation at the 500-nib level, and higher. and has also been termed "overshooting." This high- usually do not extend to this level either. The level anticyclogenesis can be detected at the Aerographers Mate must look for indications of 500-nib level, but the 300-nib levelis better this type of anticyclone at the lower levels. suited for this and will yield a more reliable High level anticyclogenesis is indicated when result, for it is the addition or Nmoval of mass low-level warm advectionis accompanied by at higher levels which determines the height of stratospheric cold advection. and this situation the 500-nib contours. has primary application to the formation of The rise and fall centers of the time differen- blocks,for high-levelanticyclogenesisispri- tial chart indicate the changes in intensity for marily associated with the formation of blocks extrapolation. both sign and magnitude. If other and theintensification of the ridges of the indicators agree with the rise and fall indica- subtropicalhighs. The intensification of the tions. progging the intensity (or the change in subtropical highs (the ridges) has already been intensity of pressure systems aloft) is most easily treated. and accurately accomplished by extrapolating Blocks should normally be forecast to form from the time differential. The magnitude of the only over the eastern part of the oceans in the height rises can be adjusted when other indica- middle and highlatitudes. There should be tions reveal that a slowing down or a speeding present north to northwestward warm advection up of the anticyclogenetic processes is occurring andnorthward moving heightriseareas or and expected to continue. persistent height rises at the higher latitudes. LOWS. -Lows and cutoff lows intensify when especially when southerly or southeasterly jets is advected to the west and they COLD air are present upstream. weaken (or till) when WARM air is advected to the west. The shallow anticyclones of polar or Arctic Lows weaken when a jet maximum rounds origin give indications of their genesis primarily the southern periphery of the low and when the on the surface and the 850-nib charts. The area jet max is on the east side of the low, if another of genesis will show progressively colder temper- jet max does not follow. ature at the surface and aloft: however, the drop Lows strengthen when the jet max remains on in the 850-mb temperatures does not occur at the west side of the low. The qualification here the same rate as at the surface, an indication is that the jet max to the west of the low may that a very strong inversion is in the process of not be preceded by another on the southern forming. The air in the source region must be periphery or eastern periphery of the low, for relatively stagnant. this indicates no change in intensity. LOWS. Cyclogenesis has a number of indica- The 24-hour riseandfallcenters aidin tors, and the greater the number of indicators in extrapolating both the change and the magni- agi.:ement, the greater the chance of success it tude of falls in moving lows. Again, the rise and progging cyclogenesis. They are as follows. fall indications must be correlated with advec- I. Areas of divergence exist at higher levLL.

273 A EIZOG IZA PI I I. R'S A IF,I C

2. Jet maximums on the west side ofa low Tentatively note at several pointson the indicate deepening and southwardmovement. prognostic chart the increase or decrease in the 3. Cold advection in the lower troposphere height of the constant pressure surface and warming over inthe lower stratosphere are existing height values. associated with the formation of or intensifica- 3. Move the areas of 24-hour height rises and tion of lows. falls at the speed of tlw short waves and The remaining problem note at in forecasting the several key points the amount and direction of formation of lows is the problem of progging the the height change from the current chart. formation of new cutoff lows. The indications 4. Adjust the advected height changes to the which call for progging new cutoff lowsare: height changes indicated by the rise and fall I. They generally form only off the southwestern areas and adjust these in turn for position to the coast of the United States and the position of long waves. pressure systems. and northwestern coastal area of Africa. short waves. 2. The upstream rid,,. intensities greatlyand assumes an overlapping" orientation. An inten- 5. Heights for selected pointsat 500 nib are sify ing ridge upstreamis recognized when that then extrapolated or. the basis of the 24-hour time differential indications and advection ridge contains strong. strengthening, andsus- con- tained southwesterly Clow. siderations. provided that theyare justified by the indications of high-level 3. Strong northerlies are situatedin the west convergence and side or the trough. divergence. When the contributiors from advec- 4. Height falls move south or southeastward. tion and time differentials are notin Igreement with convergence and divergence (which is 5. Strong cold advection appearson the west rarely side of the upper trough. the case), adjust the contribution of each and use this adjusted value. O. Finally, adjust the height values thus indi- Constructing Prognostic Contours cated to the forecast intensity of thesystems. These adjustments can lead toone of three The constant pressure piguostic chartis possibilities for each system: about to take form. The sters leadingto this. a. ill factors point toward intensification the final stem, have been dkeassed. Theprog- (deepening of lows- filling of nostic positk h of the longwave troughs and b. One factor washes away the contribu- ridges were deternunL.1 and plottedon the tion made by another and the system remainsat tentative [mil:no:tit. chart. The position of the or near its present state of intensity. highs. low). and Latta centers were then deter- c \II factors point toward weakening of mined and plotted on the tentative prognostic the system. chart. Slit rt waves were treatedin a similar 7. Sketchthepreliminarycontours.con- fashion Contours are constructednett, onnet,t- necting tilt. forecast positions of the longwaves, inthese systems in the fashion exhibited by the short waves,rndthe pressure systems with constantpressur.:..:harts. The pattern of the contours with the height values determined by .:ontours is largely determined by the position of steps I through S above. the long waves. short waves. and closed pressure The last step in the construction ofa constant systems. The height vilues of the contours are pressure prognost'c chart is to cheek the chart determined by actual changes in intensity of the for these following points: The chart should system and by simple advection of higher or follow continuity frc:the existing pattern. It lower values. Contwirs are drawnin accordance with the following six steps: should be vertically consistent and rational in the horizontal. It should not deviate from the I Outline the areas of warm and ,sold adv.c- seasonal patternunless substantiated beyond tionin the stratum between 500 and 200 nib doubt. and unless indicators dictate otherwise, It and move the thickness links at approximately :,110L11t1 follow the normal patterns. 50 percent of the indicated thickness gradient in Now draw the smooth contours, troughs, the direction of the thermal wind. ridges, highs, and lows, and adjust the gradients.

274 280 Chapter 8 FORECASTING UPPER AIR SYSTEMS

AG.555 Figure 8-7.-700-mb height 24 hour prognosis. Contour interval 60 meters.

275 Z81 AEROGRAM IER'S MATE 1 & C'

5003S1.1 24 If( PROC. FROM12Z 13 ad 66

AG.556 Figure 8.8.-500mb residual or longwave 24 hour prognosis. Contour interval 60 meters.

276 Z82 Chapter 8 FORECASTING UPPER AIR SYSTEMS

Application of charts. Some of the more useful upper air Satellite Products prognostic charts are the 24. 36, 48, and 72 hour constant pressure prognostic charts. Satellite cloud pictures provide the forecaster Various level upper air prognostic charts with with information that may beutilized along vary ing valid times are transmitted via facsimile with already discussed techniques in forecasting daily. Items included on the charts will vary on the movement and intensity of troughs. ridges. an individual basis. with respect to the follow- andpressure systems aloftAs discussedin ing: contours for the particular height. isotachs. previous chapters the satellite pictures should be and isotherms. compared with the analyzed products so that The forecaster may utilize these charts di- these charts reflect true picture of the atmo- rectly for preparing his forecast or in conjunc- sphere. Corrected charts utilized in preparing tion with his own prepared products. A com- prognostic charts and forecasts willinsure a plete listing of charts available with description greater degree of accuracy and validity. is found in the National Weather Service Fore- Some of the more significant features which caster's Handbook No. 1-Facsimile products. can beuseful in assisting theforecasterin The Fleet Numerical Weather Central also producing his prognostic upper air charts are prepares a large number of computer products forecasting. These include the positivevorticityadvection maximum (PVA for upper air maximum), the cloud patterns associated with various constant pressure level progs. 500-mb the upper level troughs and ridges, as well as small scale disturbance progs (SD), and 500-mb cloud patterns that are indicative of the wind residual or long wave prognosis (SR). The U.S. flow aloft. Naval Weather Service Computer Products Man- All of these features and how to utilize them ual (NAVAIR 50-IG-522) contains a detailed are discussed in Air Wevher Service Technical listing of charts available. Figures 8-7 and 8-8 are Report 212, Application 0: Meteorological Sat- examples of the computer products produced by ellite Data in Analysis and Forecasting, as well as the Navy. Chapter 7 of this manual. Naval Air Systems Command. Naval Weapons Engineering Support SINGLE STATION FORECASTING Activity (FAMOS) publication, Guide for Ob- serving the Environment with Satellite Infrared Imagery. (NWRF-F-0970-I 58), as well as Tech With the adha need methods of communica- Report 212, contains serid... of satellite photo tions and weather observing techniques most graphs and associated charts for %anous pheaom- stations will have an abundance of information ena with detailed explanations. for preparation of forecasts. However. aboard Forecasters should become familiar %kith in- ship single station forecasting techniques are formation contained in both of these pub!ica- frequently employed due to sparsity of reports tions to apply the satellite photographs most- or communications problems. effectively to their prognostic charts. Close analysis of upper air soundings and surface conditions will provide the forecaster Application of with a means of determining changes that are Computer Products occurringinthe atmosphere and utilizethis information in preparation of his forecasts. Ihe National Meteorological ('enter produces Detailed information on single station fore- and distributes via the National Facsimile Net- casting is contained in the Handbook of Single work a number of varied computer prognostic Station Forecasting Techniques (NA 50- I P-529)

277 .1:83era CHAPTER 9

FORECASTING SURFACE SYSTEMS

With the upper air prognosis completed. the categories. One is the distribution of fronts and next step is to construct the surface prognostic air masses in the low troposphere, and the other chart Since there are more data available for the is the velocity distribution in the middle and surface chart. and this chart is chiefly the one high troposphere. The rules applicable to these which the Aerographer's Mate will base his two conditions are discussed when and where forecast.prognosisofthis chartshould be appropriate. carefully constructed to give the most accurate For the principal indications of cyclogenesis, picturepossible for the ensuing period. The frontogenesis, and windflow at upper levels, surface prog may be constructed for periods up refer to chapters 4 and 5 of this training manual. to 72 hours. but normally the period is 36 hours The utilization of hand drawn analysis and or less. In local terminal forecasting, the period prognostic charts for forecasting the develop- may range from Ito 6 hours. Time ranges over mitt of new pressure systems is in many cases 36 hours are highly subjective and are of little too time consuming to be effective. For most value because of their doubtful reliability. wises the forecaster will generally rely on satel- Construction of the surface prog consists of lite photographs and/or computer drawn prog- three main tasks. They are: nostic charts to use in preparing this type of forecast. I. Progging the formation. dissipation,move- In order to most effectivelyusesatellite ment. and intensity of pressure systems. photographs itis necessary that the forecaster 2Progging the formation, dissipation, move- become familiar with satellite pictures, be able ment. and intensity of fronts. to interpret the pictures properly, and associate 3. Progging the pressure pattern; that is. the themwiththecorrespondingsurfacephe- isobaric configuration and gradient. nomena. Air Weather Service Technical Report 2)2 and From an accurate forecast of the foregoing the Navy Weather Research facility Detachment features, you should be able to forecast the Suit land. Project FAMOS publication, Guide for weather phenomena to be expected over the Observing the Environment with Satellite Infra- area of interest for the forecast period. red Imagery (NWRF F 0970-158), both contain much usefulinformation that the forecaster FORECASTING THE FORMATION should be aware of. OF NEW PRESSURE SYSTEMS The widespread cloud patterns produced by cyclonic disturbances represent the combined The central problem of surface prognosis is to effectof activecondensation from upward predict the formation of new tow pressure cen- vertical motion and horizontal advection of a ters.This problemisso interrelated to the cloud. Storm dynamics restrict the production deepening of lows,that both problems are of cloud to those areas within a storm where considered simultaneously when and where ap- extensive upward vertical motion or active con- plicahle. This problem mainly evolves into two veLticn is taking place. RA disturbances in their vig #0.784" Chapter 9 FORECASTING SURFACE SYSTEMS early stages, the limits of the vertical motion distribution comprise the most important factor controllingalouddistiibution.the comma shapedLlotidformationvvhk.liprecedes an Lippe'tiopospherk. vortmly 11 laX1111U111 isan example. Ilere, the Llouvis are Llosely related to the upward motion moduced by positive voitiv- try ity advection (PVA) In many cases this cloud = may be referred to as the PVA Max. See figure 9 -I. ,r

,, tw . ) 1,

.., t: .. -, '' ..:!,,,:pari-r,Prcit-, - ,),,-,_,I.404-:s, - ':.-,..--- r iiit... ',-.1-4.T;lei.414 ,.!/74.a.

AG.558 Figure 9.2. Frontal wave development. 40N

By recognizing the vorticity center and determining its movement from successive satellite photographs theforecaster should be able to 170W :afar prz.dictaccurately andwith confidence the formation of th,,, second low pressure system. Computer drawn chartsalsoprovide the AG.557 forecaster with another tool for forecasting the Figure 9-1.A well developed cominashaped cloud is development of new pressure systems. These the result of a moving vorticity center to the rear prognostic directlyto of the polar front. The comma cloud is composed charts may be used of middle and high clouds over the lowerlevel prepare his forecast or he may utilize a number cumulus and is preceded by a clear slot. of them to construct other charts that will aid in forecasting. On of the most useful ch,:rts in determining Wave of low development along an already, Lhanges in NW face pressure, frontogunesis, front- existing front may be detected from satellite oly sisind development of new pressure systems photographs byLoi rc,L Hyinterpreting them. In is the adveLtion chart. The normal methods of figure 9-2 a NeLondaly vurtiuty Lentei is shown LonstruLtion are time consuming, however, by approaching a frontal zone. utilizing computer chartsthechart may be Thelarge Jowl mass atA (fig.9-2)is constructed in a fraction of the normal time. associatedwithasecondaryvorticity center The 700-mb and1000- 500-mb thickness whia has moved close to the front. Inteia,..tion ,,hails should be utilized in construction of the between this voiticity center and the Iron( will adveLtion clr,ut. The 700-mb contours have been result inthe development another wave near found to approximatthe mean wind vector B. between the 1000- , 500-mb level. On the

279 :85 AEROGRAPIIER'S MATE 1 C'

1000- 500-nib (115-It)) thickness chart the con- Most forecasters prefer to move the low-pressure tours depict the following: thermal wind which area first and then the high-pressure area. blows parallel and downwind to the thickness lines, mean virtual temperature. and mean dens- MOVEMENT OF LOW- ity. For advection purposes only mean virtual PRESSURE AREAS temperature should be used. Meteorologically speaking. we know that lines olgreater thickness Lows determine, to a large extent, the frontal represent relatively warmer air than lines of less positions. They also determinea portion of the thickness. If' the established advection pattern is isobaric configuration in highs by virtue of the replacing higher thickness values with lower fact that gradients readjust between the two and thickness values, thenitmust be advecting the high seems to give way to the low. Although coolerair(convergence and divergence not highs and lows are each possessed of their considered). The opposite of this is also true. internal energy for motion, there is presenta The changing of thickness values can be deter- source of energy, derived from their interaction mined by the mean wind vector within the layer and which might be described as derived froma of air. The 700-nib contours will be used as the rotatingearth;thatis,independent of the mean wind vectors. systems. As a result of knowing the interplay of The advection chart should be construLted in energy in the systems, meteorologists were able the following manner: to evolve rules and methods for progging the movement, formation, intensification, and dissi- I.Place the thickness chart over the 700-nib pation of lows. chart and line up properly. 2. Remembering that the 700-mbcontours Extrapolation represent the mean wind vector. place a red dot indicating warm air at all intersections where the Extrapolation is also called the path method. mean wind vectoris blowing from higher to Firstand foremostin your forecast of the lower thickness values. movement of the low should be its past history. 3. Wing the same procedure place a blue dot Itisa record of movement of the pressure at all intersections where the mean wind ector centers and attendant fronts, their direction and is blowing from towel to higher thickness speed. and their intensifications. From this past history we can draw many valid conclusions as The placing of red and blue dots need not be to the future behavior of the system and its done for the entire chart but only for the area of future motion. This technique is valid for both interest. highs and lows for short periods of time. Now to utilize this advection chart it should The general procedure for the extrapolation be compared against the chat from the preLed- of low-pressure areasisoutlined below. Al- ing 12 hours. From comparison of amount of though only movement is covered, the central red and blue dots it ,.an be determiiv:d if' there pressures with anticipated trends could be added has been an increase or decrea-. in the amount to obtain an intensity forecast. of warm or cold air advection in a particular area as well as it' there has been any change in the I. Trace in at least four consecutive positions intensity of advection. of the center at equal time intervals (6-or 12-hr The advection type and amount as well as intervals). Place an encircled X over each one of change then can be applied to determine the them, positions and connect them with a dashed possibility of new pressure system development. line.(Seefig. 9-3.) Knowing the speed and direction, as obtained from past charts, it can FORECASTING THE MOVEMENT OF easily be deducted where the position of highs SURFACE SYSTEMS or lows are on the present chart and extrapolated movement for the ensuing period can be Whether you move the high- or low-pressure made on thisbasis. One word of caution area first is a matter of choice for the forecaster, straightlinear extrapolationis seldom valid

280

`,4:86 Chapter 9- FORECASTING SURFA('E SYSTEMS

largest 3-hr pressure falls. This k usually the X= EXTRAPOLATED POSITION point where the maximum warm air advection is 4 _6 taking place. One such method 4sing, the hallo- 3 2 ...(to--0- bark: maximums and minimunw is discussed ®- later in the chapter.

(Al (81 Figure 9-4 shows the movement of an occluding wave cyclone through its stages of development in relation to the surface pressure tenden, cies. This one factor cannot be used alone, )(0 Several methods including this indication should 50 be utilized to arrive at the final forecast. Too, 46 you should remember that the process depicted in this illustration takes place over several days, 6---(1- and many other factors enter into the sub- 6 (C) (0 sequent movement. Circular, or nearly circular, cyclonic centers generally move in the direction of the isallobaric AG.559 gradient; that is. in the direction of the greater Figure 9-3.Example of extrapolation procedure. pressure falls. Anticyclone centers move in the X is the extrapolated position. opposite direction. The speed is directly proportional to the isallobaric gradient. beyond 12 hours. Beyond this, modifications, Troughs move in the direction of tinskali() intensification, deceleration or acceleration, and baric gradient, and ridges move in the oppositk, changes in the path must be taken into consider- direction. (See fig. 9-5) ation.Itisextremely :mportantthatvalid history be followed from map to map. Systems Relative to Wairn Sector Isobars do not normally appear out of nowhere, nor do they just disappear. Warm, unoccluded lows move in the direction In short, the extrapolation procedure involves of the warm sector isobars if those isobav, two basic operations. First. a free hanu extrap- straight. These lows usually have straight patti.. olation of the path. and second, an adjustment (lig.9-6(A)), whereas old occluded .yLlt)r, s based on a comparison between the present usually have paths that are curved noithvvaid chart and the preceding one. For example, the (fig. 9-6(13)). The speed of the cyclones opium- prolonged path of a cyclone center must not run mates the speed of the warm air. into a stationary or quasi-stationa-ry anticyclone, Whenever either of these rules is in 0)11111,1 notably the stationary anticyclones, over conti- with upper air rules, it is better to um. the upper nents in winter. When the projected path points air rules. toward such anticyclones.itwillusually be foundthatthe speed of the cyclone center Relative to Frontal Movement decreases and the path curves northward. This path will continue northward until it becomes The movement of the pressure systL.111. parallel totheisobarsaroundthequasi- be reconciled with the movement of the as.a.4 stationary high. The speed of the center will be ated fronts if the fronts are progged least where the curvature of the path is greatest. ently of the pressure systems. Two genetali,a;, When the center resumes a more or less straight areinuseregarding the relationship 01 Liss path. the speed again increases. movement of lows to the movement of the associatedfronts:First, warm core lows are Isallobaric Indications steered along the front if the front is stationary or nearly so; and second, lows temd to move It has been a well-known fact for many years with approximately the warm front speed ,,i,.1 that lows tend to move toward the center of the somewhat slower than the cold front spt.,..1,

281 AEROCRAPIIER'S MATE I & C

(A)

DIRECTION OF MOVEMENT

ISALLOBARS

(B)

AG.560 Figure 9.4.Movement of occluding wave cyclone in relation to isallobaric Winters.

Steering of a cyclone and the velocity of the basic flow iu which it is embedded. Almostsincetheinceptionof synoptic The method works best when the flow pat- weathermaps.meteorilloidstshaw experi- tern changes very slowly or not at all. If the mented with the idea of moing pressure sys- upper flow pattern is expected to change greatly tems with an upper-level steerag current. This during the forecast period, we must first forecast principle is based on the concept that pressure the change in this pattern prior to forecasting systems are moved by the external forces operat- the movement of the ,,urface pressure systems. ing on them, just like a block of wood drifting in Do not attempt tc, steer a surface system by the water. Thus. might be said that a surface flow of an upper level that has closed contours pressure system tendsto be steered by the abovethesurfacesystem. When using the isotherms. contour lines. or streamlines aloft, by steering method, we mustfirstconsider the the warm sector isobars. or by the orientation of systems which are expected to have little or no a warm front. This principle is nearly always movement, namely, warm highs and cold lows. applied to the relationship between the velocity Then. consider movement of migratory highs

282 88 Chapter 9FORECASTING SURFACE SYSTEMS

AG.561 Figure 9-5.Movement of troughs and ridges in relation to the isallobaric gradient.

mum is followed by a closed isallobaric maximum some distance to the rear of the low. The steering flow or current is the basic flow which exerts a strong influence upon the direction and speed of movement of disturbances embedded in it. The steering current or layer is a level or a combination of levels in the atmos- WI phere which has a definite relationship to the velocity of movement of the embedded lower AG.562 levelcirculations. The movement of surface Figure 9-6.Movement of lows in relation to warm systems by this flow is the most direct applica- sector isobars. (A) Movement of warm sector lows; tion of the steering technique. Normally. the (B) movement of old occluded cyclones. level above the last closed isobar is selected. This could bethe 700-. 500-. or 300-nib level. and lows; and finally, consider the changes in However. in practice it is better to integrate the the intensity of the systems. steering principle over more than one level. The In studies using the steering technique, it was levels most often used are the 700- and 500-mb found in most cases that there was a displace- levels. ment of thelows poieward and the highs For practicalusage.the present 700- or equatorward of the steering current. Therefore. 500-mb chart should be used in conjunction expect low-pressure centers, especially those of with the 24- or 36-hr prognostic charts for these large dimension, to be deflected to the left and levels. In this way. changes bOth in space and high-pressure areas deflected to the right of a time can be compensated for. For a direct westerly steering current. Over North America application for a short period of time, transfer the angle of deflection averages about 15 de- the position of the low center to the concurrent grees, although deviations range from 0 to25 or 700-mb chart. For direction, move the center in even 30 degrees. the direction of the contours downstream and CAUTION: The steering technique should not slightlyinclinedto theleftfor low-pressure be attempted unless the closed isallobaric mini- areas. Experience with moving systems of this

283

1-89A. AEROGRAPIIER'S MATE I & C type will soon tell you how much de% iation steered. They move at a slower rate than the jet shouldbe made. For speed 01 the surface maximum and are soon left behind as the jet cyclone. average the basic current downstream progresses. over which the cyclone willpass (take into S During periods of northwesterly flow at consideration changes in direction and speed of 700 nib from Western Canada to the Eastern flow over the forecast period). Take 70 percent United Stat surface lows move rapidly from of this value for the mean speed for 24 hours. NW to SE bringing cold air outbreaks east of the Move thelow center along the contours as Continental Divide. described above for this speed for 24 hours. This 9. If the upper height fall center (24-hr) is should be your position at that time. found in the ,directionin which the surface For the500-mb chart,followthe same cyclone will move. the cyclone will move into procedure except use 50 percent of the wind the region or just west of it in 24 hours. speed for movement. If these two are not in agreement. take a mean of the two. There may Direction of Mean Isotherms be cases where the 500-mb chart is the onlyone (Thickness Lines) used. In this case you will not be able to check the movement against the 700-mb chart. A number of rules have been formulated The followingempiricalrelationships and regarding the movement of low-pressure systems rules for steering should be taken into account in relation to the mean isotherms or thickness when using the steering technique: lines. These rules are outlined as follows:

I. Warm, unoccluded lows are steered by the 1. Unoccluded lows tend to move along the current at the level to which the closed low does edge of the cold air mass associated with the not extend_ When so steered. lows tend to move frontal system which precedes the low; that is, it slightly to the left of the steeling current. tends to move along the path of the concen- 2. Warm lows (unoccluded) are steered with trated thickness lines. When using this method, the upper flow if a well-defined jet is over the remember that the thickness lines will change surface center orif thereis no appreciable position during the forecast period. If there is no flouting of the contours aloft. Low-pressure concentration of thickness lines, this method systems. especially when large, tend to move cannot be used. slightly to the left of the steering current. 2. Whenthethicknessgradient (thermal 3.Rises and falls follow downstream along wind) and the mean windflow are equal. the low the 500-mb contours, the speed is roughly half moves in a direction midway between the two. of the 500 -nib gradient. Three-hour rises and This rule is more reliable when both the thermal falls seem to move inthe direction of the wind and the mean windflow are strong. 700 -nib flow; while 24-hr rises and falls move 3. Whenthe mean windflow gradientis with the 500-mb flow. stronger than the thickness gradient, the low will 4. Cold lows, with nearly vertical axes, are move more in the direction of the mean wind- steered with the upper low (in the direction of flow. upper heightfalls).parallel to the strongest 4. When the thickness gradientis stronger winds in the upper low, and toward the weakest than the mean windflow gradient the low will contour gradient. move more in the direction of the thickness 5. Occluded lows, the axes of which are not lines. vertical. are steered partly in the direction of the 5. With warm lows, the mean isothernis show warm air advection area. the highest temperature directly over the surface 6. A surface low which is becoming associ- low,whichisabout halfway between the atedwithacyclone aloftwill slow down, 700-mb trough and ridge line. This means the become more regular. and followa strongly mean isotherm and 700-mb isoheights are 90 cyclonic trajectory. degrees out of phase. Since warm lows move 7. Surface lows are steered by jet maximums with the mean speed of the warm air above above them god deviate to the left as they are so them, they will be rapidly moving systems. Chapter 9 FORECASTING SURFACI- SYS II Mx

(). On the other hand, if the highest mean centers. It.ntetet, this fal,is ma eer.tellable temperatures occur under the700-1111)ridge because the isallobaii, it,t.often follow the low (isotherms and contours in phase). the ridge rather than lead the high itself is warm while the low is cold therefore a Steering N not used hI high ple:ssures st,tems slowly moving low. as wifely as for lows be taus:high-pressure cell: 7. Lows move with a speed of approximately donothateasgicat a c. mtic,tlextent as 50percentofthethermalwindforthe low-pressure st Howe:cc:I. stcering sem, I .000-500-nib stratum and approximately 75 to work abe lit '5 time for eold percentofthe thermal windofthe highs. 1.000-700-1111) stratum.

Movement of Lows in Relation FORECASTING MOVEMENT OE LOW, to the Jetstream BY STATISTICAL TECIINIQUES

Some of the rules for moving lows in relation Sinceitrequires maw, Nears of experience to the jetstream position were mentioned pre- and si photographic Ille111011. (0 de% clop a mental viousty under the section on steering. One basic catalog of weather pitteins. a weather tape or rule however, states that highs and lows situated normal path classakationIsa boon to the under the jetstream or very clo:e to will mostly inexperiencedmeteolologistas a means of behave regularly and follow the steering calcula- quicklt selecting analogous situations from the tion most closely. Minimum deviations occur past for application to the present. Mere .ire when the upper flow does not change with time. man), normals and crag:outfit ions to ie.sgutate behavior pattern. of future inox einem and de- MOVEMENT OF HIGH velopment. llowexer. theme are also manN cieNia- and PRESSURE AREAS non s from the north. The season of the 3, ear topographical influences are taetors to be con- The literature available on the movement of sidered. If we could catatog t at her it lies or these high pressure areas is not so plentiful as that for averagetypes. and the storms obeNed low pressure areas. In general. the methodfor rules.itwouldgre aftsimplattheartof extrapolation of low pressure areas is applicable forecasting Ilowee,er this is lust auothel tool in to the movement of high pressure areas aswell. the integrated forecast. [se it. but do not beat) The following are general considerations in too heavily on it. forecasting the movement of high pressure systems. Normal Tracks A high or the portion of the high situated under a blocking high aloft remains very nearly In191-1. Bow le and 11eightman published stationary. High situated under a jetstream or climatological tables 01 the average. bt months. very close to one are steered by the current of the 24-hr speed and direction 01 cyclonic aloft Highs ordinarily do not follow the steering centers in the United States. The storms were principle very closely. Cold shallow highs are classified with levee.' to ilk: point c); origin and steered more easily than the larger ones.The the current location of the centersAlthough Canadian and Siberian highs move little when these tables appeal to be antiquated some of there is no jet max in their vicinity or above these types resemble relatie el% ree(Ilt (.1,(11L,1- them. and they move rapidly when the jet max ale therefore of some (.due tothe with a bons is present. Progressive warm highs move present-day forecaster. speed consistent with that of the major ridges aloft. Highs (and low's) are possessed of internal The Marine Climatic tlaes also contain forces which assist in moving them. With straight average swim tiacks for cacti monthof the year westerly currents aloft, surface highs are dis- for areas over the oceans of the world Other placed equatorward. Highs tend to move in the publications are axadable wIiLIi glee average or direction of and with the speed of the isallobaric normal tracks for other areas of ( he world.

285 AEROGRAPHER'S MATE 1 C Weather Types George's Method This method of classifyingsignificant features of the weathermap with expected subsequent J. J George and Associateshave developed a movements is also called the weatheranalogue number of methods for forecastingthe move- method. The best known ofthese methods is the ment and intensity of certain categoriesof lows Elliott types. over the continental United States. Thesestudies In this typing systemthe broad features of also contain informationon the movement of theupper westerliesarerecognizedas the highsInformation on those methodsis con- steering mechanisms ofmigratory cyclones and tained in GRD Scientific ReportNo. 2, Contract anticyclones. There now existsa set of 15 to 20 No. AI:19(1 22)468,Further Studieson the types for each of 5zones extending from 135 Relationships Between Upper/LevelFlow and degreeseastthrough North Americato 45 Surface Meteorological Processesby R. J. Schafer degrees east (Western Russia). and P. W. Funke. A simpleexplanation of the The North American Continentco%ers two methods along with worksheetsand diagrams is types of zones. It is beyond thescope of this contained in each of thesepublications. training manual to discussthis method in detail. The method for moving lowsyields a displace- However, this can be anothervaluable tool and ment forecast in terms ofeastward and north- aid in formulatinga long range forecast. ward components of motion.These components are determined by graphicalcombination of (1) FORECASTING MOVEMENT the zonal flow at 700 mb,(2) the meridional BY OBJECTIVE METHODS Clow at 700 mb, (3) I 2-hrchanges in (1) and (2), and (4) a measure of the700-mb temperature Objectivetechniques arethose techniques gradient,or the amplitude of the 700-mb which employ certain predetermined parameters trough. Objective applicationis comparatively which areusedin conjunction with graphs. easy and straightforward. The methodgives a tables, nomograms.or the like to arrive at a 30-hr forecast of motion. prediction. They should onlybe used and evaluatedinthelightof all of the other Palmer Method considerations. Objective methodsdo have several attractive features. Most of the methods In 1948, W. C. Palmerdeveloped a method available can be reducedto a simplified set of for giving the 30-hr direction instructions with an attendant ray of cyclones worksheet and based on the Bowie-Weightmantables. These give the forecastera measure and degree of data are for the winter months confidencenot attained by using subjective and for storms over the Eastern United States. Thedirection methods alone. Thereare many objective meth- forecast is obtained from ods available but most which a graphical combina- are accessible to tion of the winter average 24-hrdirection based AG's appear to have beendeveloped for the upon the Bowie and Weightman tables, United States, with the geographical the past area east of 6-hr direction of movement, andthe orientation the Rockies in particular. Onlya few are covered of a line joining the katallobaric here. Credit is given to the and anallobaric American Meteoro- centers associated with the cyclone.Forecast of logical Society and to the authorsof the article 30-hr direction given by this Evaluation of Techniques for combined analysis Predicting the reducedtheaverageerror of 21.5 degrees, Displacement of NortheastwardMoving Cy- obtained fromtheoriginaltables,to16.1 clones, BAMS, February 1956,by Wayne S. degrees. Herring and Wayne D. Mount.for permission to use information on the validity ofsome of these Herring-Mount Method methods in general and foruse of the Herring- Mount technique in particularin this section of During the course of the evaluation this chapter. Some of the methods on the are covered movement of cyclone centers bythe various only briefly as they are geographicallyrestricted in application. methods, it was noted by theauthors that cyclonic centers having a past 12 -hrspeed which 286

Z9Z Chapter 9 FORI-C \STIN( ; SUN \( SN SI I \IS north of 30 was markedly slower 01taster than normal cyclone- w hose itanai position, he speed. for the time of year. tend to approachthe degrees north latitudeI urthcrthe technique November duringthesubsequent30-hr applies to the winter months only normalspeed it may he used with period. Similarly it was noted that the stormsof through Nlakiralthough this class moved in a directionbetween that some &glee of v ()Mitten, ein other months. stood indicated by a line joining the isallobane centers 1 -rum the stud:. the tollow mg (actors associated with the disturbance and the normal out as being the most important- direction of movement 52 degrees east of north. (52 degrees east of north for LambertCon- fhe location of the surface ,:yclone center formal charts and 50 degrees east of northfor with respect to the 501) -mb pattern. 2. 1 he st length of the -100-tubflow above the Polar Stereographic charts.) cyclone center. A forecast method was definedsimply by 3 the c00-mb temperature gradient tothe averaging the past 12-hr speed with the normal northwest of the surtace center. speed to give the 30-hr predicted speed. andthe isallobaric direction was averaged with the nor- Of iii locus hien intensified. the deepening mal direction of 50 or 52 degrees.depending on the 500-mb of forecast. was in geneial greatei. the stronger the chart used. to give the direction contour and isotherm gradient Thestudy also degrees The isalloharie direction is measured in indicated that the preferred location forfilling passing through east of north from the meridian cyclones is inside the closed 500-mb contours the position of the storm on thesurface chart. and that deepening cyclonesfavor the region Inthistest.these results yielded the lowest the 500-mb' of under open contours in advance of direction. speed. and position errors of any trough. the remaining portions of the pattern Figure 9-7 gives asimple therulestested. indicate areas of relatively little change, except illustration of the criteria by which this method lows located natter500-mb ridge line fill. can be applied, thetable of average speeds. and an example of its use. Recently Developed Techniques

Prediction of Maritime Cyclones Two techniques have been developedand published for the statistical prediction of cy- This method is an empirically derivedmethod clones and antivy e bones over an are tbounded by for objectively predicting the 24-hr movement 30 and (4) dee; c.e. not tit and 00 and 110degrees and change in intensity ofmaritime cyclones west. Both methods use a gridof finite intervals The technique requires only measurementof the and certain statistical parameters atthe surface 500-mb height and temperature gradientsabove and 5:0 nab to detconifit the future movement the current sea level center. anddetermination andintensityofthese two systems. This is of the type of 500-mb flow withinwhich the accomplished by a mathematical procedurein- surface system is embedded. Full details ofthis volving stepv he multiple regressionequations. methodare describedin The Prediction of Very promising results have beenobtained by Maritime Cyclones, NA 50 -I P -545. both methods. The method for lorecasting the movement It is recommended that the deepeningpredic- and intensity of the surface cyclone centersin tion be made first. as this willoften give a good the area defined above was devclovdby K.W. indication of movement. of maritime Veigas and RP. Ostby and published underthe The explosiveintensification Com/nil:lie Pre- but is title Application of a Moving cyclones is a fairly common phenomenon, Journal problems to diction Model to East Coast Cyclones, presently among the most difficult No. 2, pages forecast. Conversely, there are manysituations of Applied Meteorology. Vol. 4, 21 -3u. The statistical method ofpredicting the in which itis important to predict therapid gives an movement and changes in intensityof North fillingof cyclones. This technique American Anticyclones was based on theabove objective method for predictingthe 24-hr cen- method. It was developed at theU. S. Naval Post tralchangeinpressureof thosemaritime

287 e93 AEROGRAPIIER'S MATE I & C

HERRING gz MOUNT SYSTEM FOR TYPE IVLOWS

1. Low must be located east of 95thmeridian.

2. Low must have moved from southwestfor past 12 hours.

3. Low must be under SW flow at 850,700 and 500 nibs. (180° to 270°) Measure number of degrees east of north of line between 3-houranallobaric and 3-hour katallobaric centers. The average between this and east of north is 30-hour direction. 50 degrees (For Polar StereographicProjection Chart). (52° for Lambert ConformalCharts).

The average between past 12 -he-irspeed and normal speedfor the time of year is average speed for next 30 hours. **************************** NORMAL SPEEDS NOV 1-15 17.5 kts FEB 1-15 NOV 16-31 23.5 kts 33.0 kts FEB 15-28 30.5 kts DEC 1-15 28.0 kts MAR 1-15 DEC 16-31 31.5 kts 26.0 kt. MAR 16-31 23.5 kts JAN 1-15 34.0 kts JAN 16-31 35_0 kts * * * * * * * * * * * * * * * * * * * * * * * * * * * * * ** * * * * * *

VISUAL AID

95TH MERIDIAN FORECAST DIRECTION N 50° / -- -- KATALLOBAR IC 1 CENTER i

/ A N A LLO6ARIC I CENTER

AG.563 Figure 9-7.--Illustration of the HerringMount technique for the moving of northeastwardmoving cyclones.

288 294 Chapter 9 FORECASTING SURFACE SYSTEMS

Graduate School, Monterey. Calif.. and pub- 3. When the pressure rises extend to the rear lished under the title Statistical Prediction Nleth- of a high pressure cell or ridge line, the pressure ods for North American Anticyclones inthe system is increasing in intensity or the system is August 1963 issue of the Journal of Applied fillina. Meteorology, Vol. 2, No. 4. pages 508-5 1 6. 4. Since low-pressure systems usually move in Although the use of calculators or computers a direction parallel to the isobarsin the warm .'ouId be desirable in working both methods. sector, and since the air mass in the warm sector the grid points can be extracted and the 'aloes is homogeneous it is possible to use the pressure substituted into the equations and the computa- tendency in the warm sector as an indication of tions made by simple arithmetic. thedeepening orfilling of the system. The effects of frontal passages must be removed. Therefore.it'a low moves parallelto warm FORECASTING THE INTENSITY OF sector isobars. it can be said that the barometric PRESSURE SYSTEMS tendency inthe warm sector is equal to the deepening or filling. Also, the variation of the The changes in intens..y of pressure systems tendency in the warm sector. from the peak of atthesurfaceare determined by whatis the wave outward, can be used to indicate occurring above the surface system, since the increasing or decreasing gradient. pressure measured at the surface is a measureof the weight of the atmosphere above. Itmust be rememberedthat when using present tendency values for any of the rules, it is merely an indication of what has been happen- EXTRAPOLATION ing and not what will necessarily take place in the future. Consequently, in using tendencies for The pressure tendencies which are reported indication of deepening or filling, it is necessary on the synoptic charts indicatethe sum of the to study the past ti end of the tendencies.The pressure change due to movement of the system 12- and 24-hr pressure change charts are helpful plus that due to deepening and filling. If the in determining these values. exact amount of pressure change due to movement could be determined, it could be assumed RELATIVE TO FRONTAL MOVEMENT that the system could continue to deepen or fill at the current rate of speed during theforecast Wave cyclones form most readily on station- period, then determining future pressure would ary or slow movingfronts. A preferred position be quite simple. I lowever, it is not normally safe is along a decelerating cold front in the regionof will to assume that the current rate of change greatestdeceleration.Normally,the700-nib continue nor just how much of the pressure winds are parallel to the front along this area. changeisdueto movement. Thepressure Under conditions characteristic of the eastern tendencies may be used in a qualitative sense for Pacific, a secondary cyclone may develop with a quantitative approximation. extremerapidity. As a wave forms on the retarded portion of a cold front, the original ISALLOBARIC INDICATIONS well-marked warm front of the primary cyclone tends to fade out or become masked in the more Isallobaric indications from analyzed isallo- or less parallel flow existing betweenthe return- bars on the surface chart show the following ing cold air from the high to the eastward and relationships betweentheisallobars and the the uliginal warm feeding current. In this stage changes in the intensity of pressure systems. of development the new secondary wave is actuallyarapidly moving wave cyclone, but I. When the 3-hr pressure falls extend to the with no great deepening apparent as it moves rear of the low, the low is deepening. along the front and few, if any, indications of 2. When the 3-hr pressure rises extend ahead occluding. As the new low moves eastward along of the low, the low tends to fill. the front, even though it is a shallow wave, the AFROGRAPIIER'S NIATN l & C pressure gradien,uriounding it will tighten and tend the 5,400-meter thickness lineis now over the to sharpen the old maskedwarm front station in question and will be lying well to the east ofthe new wave itself. replaced by the Later. as the wave 5,490-meter thicknesslineinagiven time moves rapidly eastward. it interval, that is. warm advectionof 90 meters. will pick up this resharpenedwarm front and the cold front of the wave cyclone The consequence is thattheI .000-mb surface, in its field and which is now 00meters above sea level, will then occlude on this resharpenedwarm front. lower 90 meters to 30 metersbelt w sea level This occlusion exhibitswell-marked phenom- and the surface pressure willdecrease a corre- ena such as rapid deepeningas much as I0 to 15 mb in 12 hours and changing sponding amount, about llmb. Whenever the the structure of surface pressureislessthan the original wave cyclone intoone similar to 1,000 nib, the thatof the occluded cyclone. ,000-mb surface is below theground and is The resultant entirely fictitious. In view of the rapid deepening and increasein the cyclonic above description of advective pressure changes,the following circulation results ina portion of the original rules i.lay be stated: polar i'ront connection betweenthe new and the old cyclone being destroyed.Figure 9-8 illus- I. Warm advection between trates the process. 1.000 and 500 mb brings falling sea levelpressures, and the converse of this rule. INDICATIONS ALOFT FORDEEPENING 2. Cold advection betweenI .000 and 500 nib AND FILLING OF SURFACELOWS brings rising sea levelpressures.

Ad VCC live Pressure Changes Indications of Deepening From Vorticity The role of temperatureadvection (that is. the transport of air of differentdensity into a Cyclonicdevelopmentanddeepening are region) in contributing to thepressure change closelyrelatedto cyclonic flow aloft or to (or contour change) is somewhatcontroversial. cyclonic vorticity aloft, Ifyou recall from the On the one hand, low-level(usually 1,000-500 discussion of vorticityinchapter 4 of this mb) warm advectionisfrequently citedas training manual, vorticity isthe measure of the responsible for the surfacepressure falls ahead path of motion ofa parcel plus the wind shear of moving surface lows (theconverse for cold along the path of motion. Thuswe have the advection): on the other hand,warm advection following rules of the relationshipof vorticity is frequently associated with risingcontours of aloft to the deepeningor filling of surface lows: upper levels. It is well known from the tendencyequation 1. Surface pressure falls where that the pressure change at the advection of SURFACE is cyclonic relative vorticity takesplace. Thus, it equal to the pressure change atsome UPPER can be saidthat surface pressure falls where LEVEL plus the change inmass of the column cyclonic vorticity decreases (is less) of air between the two. That is, if downstream. the pressure at 2. Surface pressure rises whereadvection of some upper level remains UNCHANGED and the more anticyclonic relative vorticity takes place. intervening column is replaced withwarmer air, Thus. it may be said that surface the mass of the whole atmospheric pressure rises column (and where cyclonic relative vorticityincreases (is consequently the surface pressure)decreases, greater) downstream. and so does the height of the I ,000-mbsurface. 3. A wave will be unstable and As an example, assume that deepen if the warm advection is 700-nib wind field overit possesses cyclonic indicated below the 500-mb surface(5,460 relative vorticity. A wave will be meters) above a certain station. If stable if the no change in 700-mb wind overitpossesses anticyclonic mass is expected above this level, thepressure at vorticity. 5,460 meters will remain unchanged, and so will 4.If there are severalwaves along a front, the the height of the 500-mb level.Suppose the one with the most intense cyclonic 1,000- to 500-mb advection chart vorticity indicated that aloft will develop at theexpense of the others.

290 Chapter 9FORECASTING SURFACE SYSTEMS

PRIMARY

WAVE (SHALLOW) WITH WEAK CIRCULATION

WARM FRONT, .4-WARM FRONT WEAKENING VERY WEAK STEP 2. STEP I. AND MASKED

WARM FRONT SHARPENING

STEP 3.

OCCLUDED DEEP SECONDARY CYCLONE

WAVE CYCLONE RAPIDLY CHANGES TO AN UNSTABLE WAVE, --1"' DEEPENS AND OCCLUDES H

WARM FRONT BEING OCCLUDED STEP 5. STEP 4. AG.564 Figure 9.8.Illustration of secondarycyclone development Over the easternPacific. \I RI;R \ \ n

HUN ItilINUI,111% k 011C 11.,o,.-.i the .I si [tough. Hits technique worksonly when lows are e\pected to moe northeastwardout of a heat source such as the Southern Plains.When a low Deepening, of Lows \loving nimes Into the Southern Plains fromthe west or IZelative to Upper (onoii1 northwest. there is 1requentlyno compensatory cooling in the low troposphere.since the low is 1 he amount ot lkel)CllifieOfI .111e1 Allied moing toward the heatsource. States km, Si ni rules for Itiminqfi"I(Its',1`1"."i"It"the filling and deepening oflows \laritime Ploiii,..es ofCanada 10.401e:ilk he in relation to uppercontours are statedin the predn_te.d by estimating thenumher 01 k.oliomts following section: atthe.),00 or t) -1111,10%01that would he tiaeised by the surtat.t io\\ I Filling is indicated whena low moves into in the pet iod ot the titahead of the major ridge prognosticchat( I ow t position of the ,.:04)1Ing, c00-mb level. %%Mild .00111P011saiei Nil ni.iso,tit' 2 CI caw abotil Di Pei h iefole, the pi [ewe Surface lows tend to MI whenthe associ- of ated upper-level trough weakens. taking about 0pek entottheheight .'tit)mil 3 1 ows tendtofill when moving toward prognosticposition, slues of higher thicknesslines. illowt1 slk ek.',1111in Pk" 4 When the associated dietiii the height tharlee.it the ,clikbi tipper trough intensi- IIIIlls fies. deepening is indicated. surface low as it ;limed nomits to the 5.I ows deepen when moving prognosik 10,(tionI (11 appio 1/11 toward lower thickness values. lintitil)1the 2(H) mb ,orient heightditielenceiii tens of met els by;f and l nil liae the i Waves develop along frontswhent:.e surfaee 00 -nth wmdflow isparallelto the front or deepeninginmillibar,. II)1 ,111)111' a 240- meter height dittetence nearly so. at 200 nib iesults in a 7. Duringperiodsof plesstne change ot Iti nib at thesurface southerlyflowat 70t) -nib. along theeastcoast of the United States. secondary stormsfrequently develop in the vicinity of Cape Ilatteras.

III"\ III NA I height, aloftate indicated the ligh Tropospheric Divergence \IOUN I01 fall dues not hae to beestimated. in Developing Lows since deepening ofthesuitak elowwill he greater (Ilan :10 and g'iestet advective In the case of developing(dynamic) cyclones. cooling. assimaied withott111sronaPPC.iiNto horiiontal divergence isa maximum in the 400- to _'O0 -nth stiatunt. and the air compensate the fallsI helelow. thesaline above must sink method of appioxima Hongo,s the magnitude and waint adiabaticallyin order to maintain of the deepening continuity. Inthe deepening of ;ows 11 RISING heightsate indicated al00 owl the there must be removal of air at high levels dueto divergence in expected low position. the \\I(lU\1 \IrS I10 the 400- to 200-mbstratum, which results in the I S I 1\1 \ 11 1)niordertodeterminethe 11'.11 stiatospheric warming observed.Insufficient in- height dilleience to which therule flow atvery high will applyIn some Lases the fiewlit levels to coirr'msate the uses aloft subsidenceresultsinthe tipper -level contour over the expected position 01 the lowmay he falls. quitelarge. indicating the dolopment ota Phis is roughly the mechanismthought to be high latitude ridge aloft whichtends to block responsible for the development the eastward piogress 01 of low-pressure the low1 his may systemsIle high-level decrease in resultin rapid deceleiation of the low. with mass overcompensates the low troposphericincrease in Idling and ?or recuivature to (lie nort!In midi a density: the high-level effect thus case. the piognostit. low position determines the is roised in the reduction of pressure at thesurface when lows light of the changing circulation aloft are intensifying. Chapter 0 FORI ( \SI I \ SI \(I S1 -II \IS

H,..'Ulp, 1.10 LI1114,`t.\teal COMlolledI +\ Stratospheric and Upper tbs. uppelahnosphel 1,01 Tropospheric l)ecicase in\la \s 141, 1se1/4`1) 111011111h,11 the limel k IIWN t1,,-(1111.1\ nib loell ofthe and delllelasI he of &evening lows Is11.e 011111111 1,c`i0MC 1.011/411.1 I-hechief cause' d,,poscil, ,lithe the tippet one-thud 01 decrease in mass in the upper11(1)0,11,1e mkt \tot comic,. tion w ith the colninn bekame warmeiI he upper mass thelower stratosphere In to counteract is known thatI he dt:411.,1C1 ltN 31110U1It sillftcicllt rapidly deepening lows, it the eooling in the towel Livers plus anadditional change inIlMsin the stiatosphere contributes as change as do amount to deepenthe Lm [he pielerredregion much to the local surface presqiN to be the if not more toideepening 04 lowsWas concluded the tropospheric changesintiensit column or, ohser ed in thestrato- top thud of the Aniosplierte Warming is frequently O.A)) sphere over deepening surfacelows, pointing to the stiatospheie rS'ee stratospherePhis v.arm- subsidence n the lower Using the Curlew 500 -\lh Chart ingisaccompanied by loweringheights of constant pressure surfacesin the lower strato- contain in mass at high I,icNinide transmissionscurrently sphere, indicating a decrease prognostic ;Ntltl-mb height contourswhich can be keels.

DYNAMIC WARMING UPPER HEIGHT FALLS STRATOSPHERIC ADVECTION STRATUM INCOMPLETE INFLOW TRop r -1- -H- 004, _, ss, ..-- sys. i 2c , MB\ MAXIMUM HORIZONTAL \l/C DIVERGENCE STRATUM

MAXIMUM HORZONTAL DIVERGENCE 300 MB ISOPYCNIC LEVEL 350 MB 18 KM) 400MB TROPOSPHERIC ADVECTION STRATUM ZERO HORIZONTAL DYNAMIC DIVERGENCE 600 MB COOLING

CONVERGENCE SURFACE FRICTION LAYER 1000 MB SURFACE

AG.565 Figure 9.9.Vertical circulation overdeveloping low.

193 11MIraMIL

Al ROGRAPHER'S MATE I & C used in making predictions of advective changes, photographs show the cloudpictures over a 60 thickness patterns, andsubsequent changes to the surface pattern. hour period that depict thedeepening of a low pressure system. Deepening Relative to Figure 9-10. the AVCSpicture for noon local Weather Types time shows a large cloudmass with a low level vortex centered near A. A frontalband, B, Weather typeswere discussed previously un- extends to the southwest fromthe large cloud der the movement ofsurface lows. This method mass. The beginning of a dry tongue isevident. can also be used to forecast changes inintensity An interesting cloud band. C,which appears just of pressure systemsas each system or type has north of the cloudmass is of about the same its own averagemovement plus average deepen- brightness as the major cloudmass. ing or filling. The IIRIR scan for midnight,figure 9-1 I, shows the further development ofthe vortex Deepening of Lows in with penetration of the dry Relation to Normal Track tongue. Low-level circulationisnotvisible, but the brightness Storms whose tracks deviate (temperature) distribution differs fromthe vis- to the left of the ual picture. The detail withinthe frontal band is normal track frequently deepen.In general, the apparent. with a bright cold line, 1)13. normal track of a storm is parallel along the to the upper upstream edge of the band. This is believedto be flow. If a low deviatesto the left of normal, it the cirrus generated by the convection crosses upper contours (assuming near the an undisturbed polar jet stream. The cloud band. C.from the upper current) an'becomes superimposed by previous (daylight) picture is less mass aloft, resulting in deepening seen in the IR as of the low. composed of lower clouds than would beantici- As long as this crossing ofupper contours is pated from the video. unaccompaniedbysufficient compensatory By the second noon (fig. 9-12), cooling at the surface low center. thestorm will the vortex is deepen. clearly defined, but again thespiral arm of the frontal band is nearlysaturated. with a few Relation between Deepening shadows to provide detailon cloud layering. The Lows and Movement NMC operational surfaceanlaysis during this period shows that the cyclonehas deepened. It is believed that there is littlebasis for the Figure9-13,thepictureforthe second rule that deepeningstorms move slowly and midnight, shows the coldesttemperatures form a filling stormsmove rapidly. The speed of move- hooked shaped pattern v ith thehighest cloudi- ment of a low, whatever its intensity, isdepend- ness still equatorward of the Cortexcenter at F. ent upon the isallobaric gradient. Themagnitude The flat gray area. G.to the southwest suggests of the surface isallobaricgradients depends upon that the area is composed ofcells. The granular the low-level advection, themagnitude of the gray-to-light gray temperaturewithin the dry upper-level height changes, and the phaserela- tonguesuggestscellsconsistir of cumulus tion between the two. formed from stratocumulus.anu small white blobs indicating cumuluscongestus. Using Satellite Photographs The AVCS picture forthe third noon. Figure 9-14, shows the vortexto be tightly spiraled. High Resolution Infrared (HRIR) and Ad- indicating a mature system, Thefrontal band is vanced Vidicon Camera Systems (AVCS)photo- narrower than it was 24 hours earlier. with graphs provide the forecaster with some an aid in cloud shadows present to aid indetermining the forecasting the deepening of surfacelow pres- cloud structure. Surface analysis sure systems. intli,ates that the lowest centralpressure of the cyc!oue was In the following series of illustrations(figures reached approximately 6 hoursprior to this 9-10 through 9-15) both AVCSand HRIR picture.

294 300 Chapter 9 FORECASTINGSURFACE SYSTEMS

.4-

411;pi,:, jar

-.444.41111.111,17

AG.566 Figure 9-10.AVCS, local noon,first day.

IR data together with this series. figure 1. The use of nighttime "I'hefinalphotograph in daytime data yields12 hour continuity with 9-15. shows the coldest temperaturescompletely development or decay. vortex. The frontalband also respect to cyclone surroundthe 2. A hook-shaped cloud masscomposed of shows thesegmentednatureof theactive tops) indicates an hand. A typical vortic- cold temperatures, (high cloud weather areas within the area of strongupward vertical motionwith ity center at H showsthe cold temperaturesof cumulonimbus cloud attendant surface pressurefalls. cumulus congestus and 3. As the dry tonguewidens, the cyclone tops. The samevorticity center is apparentin picture, figure 9-14. west continues to deepen. the previous (daylight) 4. When high or middleclouds completely of the frontal band. cyclone has the following general surroundthe vortex center, the The forecaster may use reached maturity and canhe expected to fill. conclusions in adaptingsatellite photographs to the advection of cold, intensity of surface This generally indicates forecasting the change in dry air into the cyclonehas ceased. cyclones. 295 uS)01 AEROGRAPHER'S MATE 1

This cooling is dueto ascent of air, resulting from convergence inthe 400- to 200 -nibstra- tum. Incomplete outflowat very high levels causes piling up of air abovefixed upper levels, resulting in high-levelpressure rises. At the same time warmingoccurs in the lower troposphere. This warming sometimesoccurs very rapidly in the lower troposphereabove the surface levels, which may remainquite cold. A warmingof 10°C per dayat the 500-mb level isnot unusual. Such a rate of warmingis not entirely dueto subsidencebut probably hasaconsiderable contribution fromwarm advection. However, continuity considerationssuggest that the convergence in the 400- to 200-mbstratum produces some sinking andadiabatic warming in the lower troposphere. Thus, in the buildingof anticyclones. there must be a pilingup of air at high levels dueto horizontal velocityconvergence in the 400- to 200-mb stratum, whichresults in the stratospheric cooling observedwith developing anticyclones. Insufficientoutflow at very high levels results in an accumulationof mass. Thisis roughly the mechanismthought to be responsible for the developmentof high - pressurews- tems. The high-levelincrease of mass er- compensates the low troposphericdecrease of density, and the high-leveleffect thus determines the sign of increaseof pressure at the surface when highsare intensifying. (See fig. 9-16.) The development ofanticyclones appears to be just the reverse ofthe deepening of cyclones. Outside of cold source regions and frequentlyin cold source regions,high-level anticyclogenesis appears to be associated withan accumulation of mass in the lowerstratosphere accompanied AG.567 by cooling. Inmany cases this stratospheric Figure 9.11.IR, local midnight, first night. cooling may be advective,but more frequently the cooling appearsto be clearly dynamic; that is, due to ascent ofair resulting froin horizontal FORECASTING THE convergence in the upper troposphere. INTENSITY OF HIGHS Studies of successivesoundings accompanying anticyclogenesis outside coldsourceregions Anticyclogenesis Indicators show progressive warmingthroughout the troposphere. This constitutesa negative contribution Inthecase to anticyclogenesis. Inother words, outsideof of developing dynamicanti- cold source regions, cyclones, itis almost invariably during anticyclonedevelop- observed that ment, the decrease in cooling takes place atabout 200 nib and above. DENSITY in thetroposphere is overcompensatedby an increase in 296 302 Chapter 9FORECASTING SURFACE SYSTEMS

T"..

111 '4 4 It/tic_ ,_;121111tRAI

"NI

44'w" 4""

, err

AG.568 Figure 9-12.AVCS, local noon, second day.

297 30.1 Al. ROG RAPI MATE I & C

pressure systems. This factis of considerable prognostic;I tie if the dynamic processes which induce these mass and density changescan be detected on the working charts.

Progging Intensity of Highs

Intensification of surface highs is indicated and should be progged when cold advection is occurring in the stratum between 1,000 mb and 500 nthwhen either no heightchangeis occurring (or progged)at 500 mb or when convergence is indicated at and above 500 mb or both, and when the cold advection is increasing rapidly. A high also increases in intensity when the 3-hourly rises are occurringnear the center and in the rear quadrants of the high. Whena moving surface high which is not subjectedto heatingfrombelow isassociatedwitha well-defined upper ridge, the change in intensity is largely governed by changes in intensity of the upper-level ridge. Weakening of surface highs is indicated and should be progged when the cold advection is weakening or is replaced by warm advection in the lower tropospheric stratum with eitherno height change at 500 mb or when divergence is occurring (or progged) at and above 500 mbor both are occurring at the same time. When warm low tropospheric advectionis coupled with convergence aloft, or when cold lowtropospheric advectioniscoupled with divergence aloft, the contribution of eithermay be canceled by the other, and a quantitative estimate of the effects of each must be made AG,569 before a decision is made. Figure 9-13.IR, local midnight, second day. When surface pressurefalls occur near the center and in the forward quadrants of the high, mass, and generally accompanied by cooling m the high will decrease in intensity. the stratosphere. This is analogous to the deep- When a cold high which is moving southward ening of lows where the decreaseinmass, generallyaccompaniedbywarmingathigh heats from below, it will decrease in intensity, levels, overcompensates the cooling in the tropo- unless the heating is compensated by intensifica- sphere. tion of the ridge aloft. The evidence, therefore. indicates that high- The amount of intensification can be deter- levelchanges. .undoubtedlydueto dynamic mined by correlating the contributions of height mechanisms in the upper troposphere. ale largely change at the 500-mb level as progged and the responsible for deepening and filling of surface change in thickness as advected.

298 304 Chapter 9FORECASTING SURFACE SYSTEMS

P*1 if

, f1 r. )k Ntj '07444 ... 4 .. .

Pt. I

4 6 441111114-_

F. :

OE.

AG.570 Figure 9.14.AVCS, local noon, thirdday. AEROGRAPHER'S MATE 1 & C

and you are only concerned witha short interval of time. Extrapolation isperhaps the simplest and most widely used method ofmoving fronts for short periods. When moving frontsfor longer periods than a few hours, otherconsiderations must be taken into account. In this section of the chaptertwo methods for extrapolation are covered along withthe Geo- strophic Wind Method and theconsideration of upper air influences.

Extrapolation

When fronts are moved byextrapolation, they are merely moved in time andspace on the basis of pastmotion. Of course such factorsas occlusion, genesis, dissipation,change in position, intensity of airmasses and cyclones, and orographic influencesare taken into account. Adjustments to the extrapolationfrontal positionsare made on thebasis of the above considerations. Aerographer's Mates should keep inmind that fronts in themselves do notpossess the necessary energy to move, but that the adjacent airmasses and associated cyclones drive thefronts. You should also keep in mindthe upper air influences on fronts; for example,the role the 700-mb winds play in themovement and modification of fronts. Finally, it shouldbe remembered that past motion is nota guarantee to future movement, and theemphasis is on considering the changes indicated andincorporation of these changes intoa modified extrapolated movement. CONTROL LINE METHOD OFEXTRAP- OLATION.This method of extrapolationgives AG.571 a more scientific basis to extrapolation of fron- Figure 9-15.-1R, local midnight, thirdnight. tal systems. It could also be appliedto cloud systems. The method was adapted fromseveral available methods and publishedinthe Air Weather Service Manual, the LocalArea Surface FORECASTING TI1E MOVEMENTS Chart, Its Preparation and Use,AWSM 105-35. AND INTENSITY OF FRONTS Geostrophic Wind Method MOVEMENT OF FRONTS Fronts are progged by the Fronts are ordinarily progged after geostrophic wind the pres- method at two levels at severalpoints along the sure systems have been progged. However, there frontat the surface and may be cases for short range progging where at the 700-mb level. The basic idea is to determinethe component of movement of all of the systems isunnecessary the wind atthe surface and aloft which is 300 306 Chapter 9 FORECASTING SURFACE sYsTEms

DYNAMICCOOLING UPPER HEIGHTRISES 1 INCOMPLETE tRO OUTFLOW --kk-- 1i STRATOSPHERIC <16d, ADVECTION STRATUM >C< /200 MB il. t .X

MAXIMUM HORIZONTAL CONVERGENCESTRATUM MAXIMUM 300 MBi HORIZONTAL CONVERGENCE 350 MB W ISOPYCNIC LEVEL 400 MB

DYNAMIC ZERO HORIZONTAL TROPOSPHERIC 600 MB WARMING DIVERGENCE/ ADVECTION CONVERGENCE STRATUM

DIVERGENCE SURFALCAEYEFRRICTION HIGH 1000 MB SURFACE

AG.572 Figure 9-16.Vertical circulation over developing high. normal (perpeAdicular) to the front and there- front and a short distance ahead of the warm foredrives thefront.Determination of the front where a representative gradient can be component normal to the frontis made by found. The dots on the isobars in figure 9-17 triangulation. serve as a guide to the proper selection of the The procedure is as follows: geostrophic wind. 2. Draw a vector toward the front, parallel to I. Select between two to four points along the isobars from where the geostrophic wind was the front where a regular and reliable pressure determined. The vectors labeled "y" in figure gradient exists, and determine the geostrophic wind by use of the geostrophic wind scales 9-17 illustrate this step. rather than the observed wind. The wind speed 3. Draw a vector perpendicular to the front is determined a short distance behind a cold originating at the point where the "y" vector

301 "C7 AliROGRAPIIER'S MATE 1 & C

intersects the front and label this vector "x"as flow, this air mass, is the one supplying the push illustrated in figure 9-17. for the forward motion and that in thecase of 4 a convenient distance from the intersec- warm front the receding cold air mass under the tion, along the "x vector, constructa perpen- warm front determines the forward motion, dicular to the "x vector, letting it intersect the because the warm air mass merely replaces the "y" vector. This is line c in figure 9-17. cold air and does not displace it. In the foregoing discussion we have neglected the effects of cyclonic and anticycloniccurvature inthe isobars and the effect of vertical motion along the frontal surfaces. The Aerogra- pher's Mate can readily see that upglide motion along the frontal, surfaces reduces the effective component normal to the front and furthermore that cyclonic curvature in the isobars, because it indicates convergence in the horizontal (therefore divergence or mass reduction in the vertical), reduces the effective component normalto thefront. For these reasons, the component normal to the front is reduced at the surface only by the following amounts for the different types of fronts and isobaric curvature: AG.573 Figure 9'17. Geostrophic wind method. Slow moving cold front, anticyclonic curvature 0% Fast moving cold front, 5. The ankle formed at the intersection of the cyclonic curvature 10-20% "y" vector andthe perpendicular orieinating Warm front 2040% from the "x" vector is labeled 0 (theta). Measure Warm occluded fronts 2040% angle 0 to the nearest uegree with a protractor, Cold occluded fronts and determine the value of its sine by using 10-30% trigonometric tables or a slide rule. If' the pressure gradient is forecast to increase, 6.Let side a of the right triangle formed in decrease the component by the least percentage. step 4 represent the value of thc.eeostrophic If the pressure gradient is forecast to decrease, wind obtained in stepI and call it "Cgs. Solve decrease the component normal to the front by the triangle for side b by multiplying the sine of the highest percentage value. 0 by the value of Cgs. The resulting value of b is If the pressure gradient is forecast to remain the component of the wind normal to the front. static, decrease the component normal to the giving it its forwbrd motion. The formula is front by the middle percentage valueas listed above. b = Ces X sin 0

In a sample problem,if' the Cgs was deter- Upper Air Influence on the mined to be 25 knots and angle 0 to be 400. b is Movement of Fronts 19.1 knots. since the sine of angle 0 is 0.643. The Aerographer's Mate can see, however, A number of the rules relating the upper air that the components normal to the front should contourstothemovement of fronts were be equal on both sides of the front and that in discussedin chapter 5. You saw that a slow realityitwould matter very little where the moving cold front has parallel contours behind component is computed in advance of or to the the front. and in a fast moving cold front the rear of the front. In cold fronts the reason why contours were at an angle to the front even at the component to the rear is chosen is that this times normal to the front.

302 308 Chapter 9FORECASTING SURFACE SYSTEMS

Some additional rules are stated below, months along the eastern coasts of the American and Asian Continents. When the continentis I. During periods of strong, continued west- much cooler than the bordering ocean, fronto- erly flow aloft (high index) over North America, genesis may be progged when the air mass surface fronts move rapidly eastward. A rule of moving out onto the ocean has no distinct or a thumb that can be used with this situation is weak boundary. that the front will move eastward at a speed which is 50 percent of the 500-mb flow and 70 Frontolysis percent of the 700-mb flow. 2. Cold fronts associated with cP outbreaks Weakening or dissipation of fronts occurs are closely associated with the depth of the when: northerly winds aloft. The following relationships are evident: For el' air to push southward I. The mean isotherms become more perpen- into the Great Basin from British Columbia, dicular to the front or more widely spaced. strong northerlies must exist to at least 500 mb 2. They move out of a deep pressure trough. over the area: for cP air to push southward into 3. Either or both air masses modify. theGulf of Mexicotoprovideextensive 4. They meet with orographic barriers. northers. itis recognized that northerlies and northwesterlies must exist or be expected at 500 mb as far south as Texas: for cP air to push PROGNOSTICATING ISOBARS southward overFloridato Cuba, northerlies must exist at 500 mb as far south as the Gulf Isobars may be constructed on the surface States. prog either by computing the central pressures for the high and low centers and numerous other FORECASTING THE points on the surface prog chart and drawing the INTENSITY OF FRONTS isobars to fit the combination of highs, lows, fronts, and the computed pressures or by mov- Frontogenesis ing the isobars in accordance with the surface tendencies and indications. Fronts intensity when one of the following three conditions ora combination of them USING THICKNESS PROGS occurs: The computational procedure for construct- I. The mean isotherms ( thickness lines) being prognostic isobars using thethickness prog- come packed along the front. nosis is as follows: 2. The fronts approacv deep upper troughs. 3. Either or bothair masses move over a I. At a selected point, determine the differ- surface which strengthens their original proper- ence between the present 500-mbheight and the ties. progged 500-mb height. A progged rise at the 500-mb level is positive: a progged fall, negative. The formation of new fronts is a phenomenon 2. At the same selected point, determine the rarely observed. Air mass boundaries seem to be difference between the current and the progged perpetuallypresent onthevarious weather thickness value, a progged increase in thickness charts. Frontogenesis ot..Lurs when two adjacent being positive in value and a progged decrease in airmasses exhibit different temperatureand thickness being negative. density.andprevailingwindsbringthem 3. Subtract algebraically the difference of the together. This condition, however. is the normal progged thickness value from the progged differ- permanent condition along the polar front zone: ence of the 500-mb level. therefore, the polar front is semipermanent. 4. Convert this difference in feet to millibars Generation of a new front or strengthening of by letting 60 meters equal 7 1/2 mb, and assign anexistingfrontoccurs during the winter the proper sign.

303 309 AEROGRAPER'S MATE 1 & C

Add (or subtract if the value is negative) sure tendencies are given in tenths of a millibar, this value to the current sea level pressure. This while the unit isobars are drawn forevery 2 or 4 is the progged sea level pressure. nib. Second the normal period for measuring (I Repeat steps 1 through 5 for all the points pressure tendencies is 3 hours so that velocities sele,:ted. and sketch the isobars. derived from this formula will be expressed in terms of this unit and whatever unit is used to USINGBAROMETRIC TENDENCIES measure the distance between isobars. Finally, falling tendencies will give positive velocitiesor Me variations of the changes in thepressure movement toward higher valued isobars, and listribulion from one chart dependupon the rising tendencies will give negative velocitiesor haiometric tendencies.Havingthepressure movementtoward lowvaluedisobars.The rt,,lency at point A on the 1,000 isobar,its velocities calculated are always directed alongan displacement along an axis perpendiculai to the axis perpendicular to the isobars. ir can he readily determined. (See fig. 9-18.) The above forecast gives the instantaneous velocity of the isobar under consideration.Con- sequently, to obtain accurate results, thepres- 1008 1000 1004 sure tendency must remain constant during the forecast period. These ideal conditionsrarely existfor extended periods of time withthe result that forecasts for longer than 24hours usually become inaccurate in detail because of acceleration. TxD V3HRS = p V_ = 2.OxD 4 Extended Weather Forecasts

Long range weather forecasting dates backto AG.574 World War II, when wartime needs forextended Figure 9.18. Movement of isobars using the forecasts become necessary. What beganin an tendency formula. experimental stage has now become quiterou- tine. While most routine forecasting does Me cover a tendency at A is -2.0 mb, which means short period of time, generally 24 to 36 hours, the pressure at A was 1,002 mb 3 hoursago. the forecaster will. upon occasion, be called Sincethe pressure at A has dropped 2.0 nib upon to provide outlooks for extended periods during the last 3 hours, the 1,002 isobar will be up to 7 days or longer. displacedtowardisobars representing higher Facsimile products willbe the primary tool of pressures. Such a movement must take place, for the forecaster in providing extended forecasts. with the same drop in pressure during the next 3 At the present time the NMC providesa package hours, the barometer at A would read 998 mb. of extended forecast charts. Among theseare a Assuming the same drop would occur all along 72, 96, and 120 hour sea level prognostic chart, the 1.000 isobar, it would be moved toa point maximum and minimum temperature anomalies halfway between its present position and the for 3, 4, and 5 days, a 5 day mean temperature present position of the 1,004 isobar. anomaly, and total precipitation in classes for This example illustrates the formula shown in the period 3-7 days after the forecast day. figure 9-18 where V is the 3 hourly movement Complete information on these chartsmay be of an isobar. T is the local pressure tendency, D found in the Weather Bureau Forecasters Hand- is the distance between neighboring isobars, and book NO. I-Facsimile Products. Llp is the pressure difference between isobars. Of major importance to the forecaster in In order to use this equation correctly, the preparing an extended outlook without the following facts must be considered: First.pres- benefit of facsimile prognostic charts isthe 304 310 Chapter 9 FORECASTING SURFA( r. SYSTEMS determination of the type of flow (zonal or The foret aste! should also become familiar meridional)thatcan be expected to persist with the particular areas that tend to nigger during the outlook period. With a zonal flow a Ly Llogenesis and:or frontogenesis and utilize this steady progression of pressure systems mo ing information when preparing his outlook. Many regularly can be antiLipated, W ith the change to oftheLlimatologicalpublicationsthatare a meridional flow the introduction of told polar readily available make note of these areas. or Arctic air into the lower latitudes and mixing with the warmer air will Lreate a number of Many local area forecaster's handbooks con- problems to be considered. Among these will be taindetailedinformationon howvarious the development of new pressure systems, deep- weather situations approaching the station affect ening or tilling of present systems. and move- thestation weather wellinadvance. These ment of the systems. should be used when available.

305 311 CHAPTER 10

CLOUDINESS, PRECIPITATION, AND TEMPERATURE FORECASTING

Thick! of the most unpuitant .lenient, in a knipeiatlil1/4* of the an must be 'educed close to fulecast are LI011t1111e,s. PICLIPItat1011, aid 10; point. Condensation depends upon two perat tire.The occlillenceof4.14)LikhilL,,and vai tablesthe amount of cooling and the relative precipitationill,! be divided Into to main humidity of the Two conditions must be categoliesthat which occur, with hoots. and Iulllllyd IIthe condensation of water vapor is to that which occul, Iu an masse, not a,,,,o,iat1/4,LI occur in die atmosphere. I list, the air must be with fronts. at or near saturation; and second. hygroscopic There are many lac tins which influence the nuclei inust be present The first condition can dailyheating and cooling oftin..1t1110,1thtl, he biought ,,hoot ul Lithe' of two ways one by Willa, in turn. t the tempciattlik Soms of evapoiation of more moisture into the air, the these factors are cloudinc,s, humidity of th, MI, othci by cooling the au to its dewpoint tempera- nature of the earth's surface, wind. elevation. idle. Inst process can occur onlyif' the latitude. and the vertical Iap,c tate 01 tempera vapid pic,st,re of the air is less than the vapor lure. ('lo tic..iik..ss is quilt. obvious In its influence 1)1k:smirk: of the inoistlile source, and results in on the penetration of insulation tkthe .,nth', ( ooling is the principal condensation pro - surface during the day time and leiaidation of ducel coolingprocesses(for the net loss of heat b\tel radiation ,it c \ample. iadiation andconduction associated night. with advection) iesult principally in log, light Otherfactorswhichaltectalldoee aft diiille. dew . or host, The most effective cooling stability of the lapse Mks. Modilik..ation of an process In the atmosphere Is adiabatic lifting Of masses. distance from the moistlill: anti all.ItIs the only process capable of producing the tOPOLMIPIlltal influences, [hese elfccts ale precipitation ill appreciable amounts. It is like- much the same as theyarc on ...Innatelas wise a principal producer of clouds. fog. and discussed in an earlier chaplet), but Intit.I1nwre drizile. The meteorological processes which re- on alocal scale and over a ,limit' period of sult in veined! motion 0( air ale discussed in the time. following sections. It should be emphasized that In this chapter we are 111,111111 conceined with nom. of the cooling processes are capable of the forecasting of middle and upper cloudines,, producing condensation by themselves moisture precipitation. and local tempelatule. "I he ItqC- in the form Of water vapor must be present. eastine of convectiv.: clouds and precipitation. 9 and fog and stratus IS covered in chapter II PRECIPITATION PRODUCING PROCESSES

CONDENSATION AND PRECIPITATION Precipitation occurs when the products of PRODUCING PROCESSES condensation and/or sublimation coalesceto form hydlometeors too heavy to be supported CONDENSATION PRODUCING PROCESSES by the upward movement of the air. Although To convert sumilicant amounts of water %apt)n the process of coalescence is not fully under- into condensation or the sublimation state, the stood. it is obvious that a large and eontinuously

306 312 Chapter 10 CLOUDINISS, PRECIPITATION. AND TEMPERATURE FORECASTING replenishedsupplyofwateidroplet' or lee eneeis the typical cyclonic circulation. This crystals (or both) .1IcIleLCssaryit ICLiable combinationisa %co effeetke precipitation amounts of precipitation are to occur. producer. the examples of convergence and divergence. Adiabatic lifting of air is brought about by explained in the foregoing, are definite and clear three general methods: orographic lifting, fron- cut, associated as they are with the centers of tal lifting, and vertical stretching (or horizontal closed flow patterns. More subtle, less easily convergence). All of these mechanisms are the detected types of convergence and divergence indirect results of horizontal motion of air and are withcurved, wave-shaped, or its variation in space. associated straight flow patterns, where the airis everywhere moving in the same general direction. The Orographic Lifting qualitative variation of convergence .ind divergence is indicated in figures 10-1, 10-2, 10-3. and Orographic lifting is the most effective and 10-4 by means of the following key: intensiveof allcooling processes.1 lorizontr.1 motionisconvertedinto vertical motion in cc marked convergence, proportion to the slope of the inclined surface, e convergence. Comparatively flat terrain Lan have a slope of as o little or no convergence or di ergence. much as I milein20 miles. The greatest d divergence. extremes inrainfall records, both in amounts dd marked divergence. andintensitiesfor varying periods, occur at mountain stations. Eor this reason the fore,aster Arrows have the following. interpretotions: should be thoroughly familiar with the significant features of the terrain. wind direction and speed. movement of center. path of air current. Frontal Lifting, ti The left side of figure 10 -Iillustrates longi- Frontalliftingisthe term appliedto the process represented on a front when the inclined tudinal convergence an.1 divergence: the right surface represents the boundary between two air side illustrates lateral convergence and diverg- masses of different densities. In this case, how- ence. Many more complicated situations can be ever, the slope ranges from 1/20 to 1;100 or analyzed by separation into these components. It can be shown mathematically and verified evenless, The steeper thefront,the more adverse and intense its effects. other factors synopticallythatafairly deep layer of air being equal. These effects were discussed more moving with a marked north-south component fully in chapter t% of this training 111.111l1:11. has associated with it convergence or divergence, depending on the direction and curvature of the path followed by the stream of air. In figure Vertical Stretching 10-2the arrows indicate paths of meridional flow in the Northern Hemisphere. In general. Since itis primarily from properties of the equatorward flowis divergent unless turning horizontal wir d field that vertical stretching is cyclonieally. and poleward flow is eomergent detectable.itismore properly calledcon- unless turning a lit icyclonically. vergence. This term will be used hereafter. Convergence does not always result in upward motion of air: only if the convergence occurs through an appreciable layer bounded on its lower side by a solid surface. does convection occur. In order for subsidence to occur, there AG.575 must be a layer of divergence below. Upper Figure 10-1.Longitudinal and lateral convergence divergence associated with lower level converg- and divergence.

307 313 AEROGRAPIIER'S NIATli I & C

travel is often the factoi determining the distribution. The most common distributionfor waves moving toward the east is illustrated in figure 10-4. There is relatively little divergence /)R% at trough and ridge lines, with convergence to dd d o cc c 0 d the west and divergence to the east of the trough lines. Patterns of this type are morecommon on AG.576 upper air charts than on surt*at.e charts. Figure 10-2.Convergence and divergence in meridional flow.

The four diagrams of figure 10-3 represent the approximate distribution of convergence and divergence in Northern Ilemisphere cyclones and anticyclones. For moving centers, the greatest convergence or divergence occurs on and near the axis along which the system is moving. The diagrams of figure 10-3 show eastward movement. but they apply regardless of the direction of movement of the center. 0

AG.578 0 Figure 10-4.Convergence and divergence in waves.

Comparatively greater space has been devoted CYCLONE hereto convergence becauseitisthe least understood and most difficult to assess of all weather-producing processes.Itsareal extent 0 ranges from the extremely local convergence of STATIONARY MOVING thunderstorm cells and tornadoes to the large- scale convergence of the broad and deepcur- rents of poleward- and equatorward-moving air masses. The amount, type. am'intensity of the ANTI weather phenomena which result from any of CYCLONE the lifting processes described in this section depend on the stability or convective stability of the air being lifted.

0 STATIONARY MOVING It should not be inferred from the discussion of this section that only one of the mechanisms AG.577 is operative in any particular weathef situation. Figure 10-3.Convergence and divergence On the contrary. any combination of two,or all in lows and highs. three in conjunction, is possible and even probable.For instance, an occluded cyclone of Wave-shaped flow patterns are not quite so maritime origin moving onto a mountainous easily classified with regard to convergence and west coast of a continent could easily have divergence becausethe speed at which they associated withit warm frontal lifting, cold

308 314 Chapter10 CLOUDINESS, PRECIPITATION.AND TEMPERATURE FORECASTING frontal lifting, orographic lifting, lateial convere- developmental weather activity, usually that of eneeind convergence in southerly flow. Nearly cyclogenesis.Althoughthere are no known all fronts have some convergence associated with generalmethodsf'orforecasting cyclogenesis them. with any great success,itis well known that fallingpressure,precipitation. and expanding WEATHER DISSIPATION PROCESSES shields of middle clouds indicate that the cyclo- Each of the processes described in the preced- eenetic process is operating and, by following these indications, successful forecasts can often ing,sectionhasitscounterpart among the condensation-preventing or weather-dissipating be made f'or12 to 24 hours in advance. For processes. Downs lope motion on the lee side of longer period forecasts, the difficulties increase. orographic barriers results in adiabatic warming, Most of the winter precipitation of the lowlands and there is also heating (a nonadiabatic process) in the midlatitudes is chiefly cyclonic or frontal of an currents due to nsolation from relatively in origin though convection is involved when the cloudless skies. If the air mass above and ahead displaced air mass is conditionally or convec- of a frontal surface is moving with a relative tively unstable. Cyclones are important genera- component away from the front, downslope tors of precipitation in the Tropics as well as in motion with adiabatic warming will occur. Di- midla titudes. vergence of air from an area must be compen- Sincethe correct cloud andprecipitation sated for by bringing down air from aloft, which forecast is a combination of a number of factors. is warmed adiabatically in the process. All these consideration of these factors should be a matter mechanisms have the common effect of increas- of choice of the individual forecaster. Some of ing the temperature of the air, thus preventing the factors involved and to be considered are condensation. listed below, not necessarilyinthe order of importance: These processes likewise occur in combination with one another,buttheyalso occurin I. The location of the cyclone and/or front combination with the condensation-producing withrespecttolatitude, geography,terrain, processes. This can leadto situations which maritime influences, and the surface, over which require the most careful analysis, For instance, a the front or low will mowith the modifying current of air moving equatorward on a straight influences. or anticyclonically curved path encounters an 2. The type of low, the type and slope of the orographic barrier oriented across its path, if the front, the stability of the air masses, moisture slope issufficientlysteep or the air is suffici- content and discontinuity, temperature contrast, ently moist, precipitation will occur in spite of wind and contour pattern aloft, speed of the low the divergence and subsidence associated with or front, and future movement. theflowpattern. The dry, sometimes even Knowing normal weather patterns with fronts cloudless, cold front which moves rapidly from and lows for particular areas will also be helpful. west to east in winter is an example of upper As pointed out before, a thorough understand- level downslope motion which prevents the air ing of the physical processes by which precipita- being lifted by the front from reaching the tion develops and spreads is essential a good condensation level. precipitation forecast. Another vexing problem The precipitation process itself' opposes the to theforecasteris often the separation of mechanism which produces it, both by contrib- frozen from unfrozen types of precipitation. uting the latent heat of vaporization and by This problem is discussed later in the chapter. exhausting the supply of water vapor if the moisture source is cut off. FRONTAL CLOUDINESS AND PRECIPITATION FORECASTING FRONTAL CLOUDS AND WEATHER Cold Front The cloud and weather systems most difficult The study of constantpressure charts in toforecastarethoseassociatedwith new. conjunction with the surface synoptic situation

309 315 AEROGRAM IER'S MATE 1 C is helpful in forecasting cloudiness and precipitation associated with fronts. When the contours at 700 mb ate perpendiddar to the surface gold 57.`"FMr4C:".:%%f7' 4",41a,374(45.4*Mg2:31". front. the band of weather assoviated with the front is narrow. This is the situation that occurs ,1,1e '; with a fast moving front. If the front is a slow AO moving front.theweather and precipitation extends as far behind the front as the winds at the 700-mb level are parallelto the front. In both of these cases the flow at 700 mb also 4.40,1 indicates the slope of the frontthe region in 1: which the frontal lifting is concentrated. Since the front at 700 mb lies close to the trough line at 700 mb, itis apparent that when the wind at 700 mb is prependicur to the surface front 14 that the 700-mb trough is very nearly above the surface trough. hence the slope of the front is very steep. When the 700-mb flow is parallel to the surface front. the trough lies to the rear of thesurfacefront and beyond the regionin which the flow continues parallel to the front. Consequently. the frontal slope is more gradual. and lifting is continuing between the surface and 700 mb at some distance behind the surface frontal position. Anotherfactor whichcontributestothe distribution of cloudiness and precipitation in these two situations is the curvature of the flow aloft. Cyclonic flow is associated with horizontal convergence. and antiLyclomv. flow is associated with horizontal divergence. Ver:.little weather is associated with a cold Front if the mean isotherms are perpendicular to the front. When the mean isotherms are parallel to the front, weather will occur with the front. This principle is associated with the contrast of 20W the two air masses, hence. with the effectiveness of Ii fting. AG.579 Satellite photographsboth Advanced Vidi- Figure 10-5.An active cold front. com Camera Sub System (AVCS) and High Resolution Infrared (11R1R). provide a representative picture of the Lloud strudure of frontal clouds is comprised mainly of low level cumulus systems. /Wire Lold fronts appear as continuous and stratiformed clouds, but some cirriform may well developed cloud bands composed of low, be present.InactiveLoldfronts over water middle, and high clouds. This is caused by the occasionally will have the same appearance as upper wind flow which is parallel, or nearly active fronts over land. while over land they may parallel to the frontal zone (fig. 10-5). have few or no clouds present. Figure10-6 The perpendicular component of the upper depicts the fragmented clouds with an inactive winds associated with the inactive Lold front cold front in the lower portion, while a more cause the cloud bands to appear as narrow. active cold front cloud presentation is shown in fragmented. ordiscontinuous. The band of the upper portion.

310 316 Chapter 10 CLOUDINLSS, FION, AND INNWERNTURE FORECASTING

.1, '1

''.

-"%%% L 2.0::Y7 , . ,0*'"- .4"?..ttr icg. ..4 **L.

I ---'4.` l'ekr-.

ffirtimmiims40W

AG.580 Figure 10-6.Inactive and active cold front satellite photograph.

Warm Front When the slope of the warm front is nearly hor:,.ontal near the surface position and is steep severalhundredmilesnorth of thesurface Cloudiness and precipitation occur where the position, the area of precipitation is situated in 700-mb flow across the warm front is from the the region where the slope is stccp. There may warm to the cold side and is turning cyclonically be no precipitation just ahead of the surface or moving in a straightline.This implies frontal position. convergence associated with the cyclonic curva- A geneaal consensus appears to be that frontal ture. Warmfrontsareaccompanied by no clouds are usually layered; that the high clouds weather and few clouds if the 700-mb flow are usually detached from the middle clouds: above them isanticyclonic. Thisis dueto and that middle clouds are often well layered horizontal divergenceassociatedwithanti- and many times do not extend very high. It has cyclonic curvature. been found by flights through warm fronts, and The 700-mb ridge line ahead of a karm front Whet evalen.e, that the thick solid mass of may be considered the forward limit of prewaim clouds v.luch extends up to 7 to 8 km in a belt front cloudiness. The sharper the ridge line, the scwral hundred miles wide along and ahead of more accurate the rule. This, too, is associated the tAalm front. as depicted by the classical with divergence and convergence. The 500-mb models, is not typiciof most fronts. ridge line may be considered the forward limit Active arm fronts are at best, difficult to of the cirrus cloud shield. locate on satellite cloud pictures, while inactive

311 317 AEROGRAPIIER'S MATE 1 & C

warm fronts cannot he located. An active warm Orographic Barriers front may be associated with a well organized L loud band, but the frontal zone istelf will be In general. an orographic barrier increases the difficult to loLate. An active warm front may be extent and duration of cloudiness and precipita- plated somew here under the bulge of clouds tion on the windward side of the orographic that are associated with the peak of the warm barrier and decreases it on the lee side of the sector of a frontal system. The clouds are a orographic barrier. combination of stratiform and cumuliform beneath a cirri form covering,. (See fig. 10-7.) AIR MASS CLOUDINESS AND PRECIPITATION

It must be remembered that no one condition Surface heating or cooling and movement of represents what could be called typical condi- theairmass against orographic barriers and tionsis each front presents a different situation movement of the air mass in a cyclonic or with respect to the air masses involved. There- anticyclonic path are the impoi Lint factors in fore, each front must be treated as a separate the production of air mass weather. ,ase, using present indications, geographical lo- If an air mass moves so that it is lifted over an cation.stabilityof theair masses, moisture orographic barrier and the lifting is sufficient for content. and intensity of the front to determine the air to reach its lifting condensation level, its precipit a tion characteristics. cloudiness of the convective type occurs. if the

Sri ;.4r" .714+44,sizis r- a eicA

'ft Ir..,

i C xaa 41111111Ft'' 50W

AG.581 Figure 10-7.Warm front satellite cloud photograph.

312 318 Chapter 10 CLOUDINESS, PRECIPITATION, AND TEMPERATUREFORECASTING

air isconvectively unstable and has sufficient by a subsidence inversion near the coast. Further moisture, showers or thunderstorms occur. This downstream the vertical height of the moist is,of course, on the windward side of the layer increases, as does the height of the clouds, barrier. and the capping is caused by dry air entrain- Curvature of the flow aloft also affects the ment. In figure 10-8 the open cells behinda occurrenceofcloudinessandprecipitation. polar front over the North Atlantic indiLatt, Loki Actually the shear should also be considered, air advection and cyclonic curvature of the low but it is seldom considered, since it is difficultto level wind flow. Vertical thickness of thecum- evaluate at levels below the 300-mb level. Ina ulusat Aissmall but increases eastward cold air mass, showers and cumulus and strato- toward B. cumulus clouds are found only in those portions Figure 10-9 shows a large area of the sub- of the air mass which are moving in a cyclon- tropical high west of Peru covered withopen ically curved path. In a warm air mass moving cells. These are not associated with low level with a component from the south, cloudiness cyclonic flow or steady cold air advection. and precipitation will be very abundant undera current turning cyclonically or even moving in a Closed cellular patterns are characterized by straight line. Clear skies occur wherever acur- approximately polygonal cloud coveredareas rent of air is moving from the north in a straight bounded by clear or less cloudy walls. Atmos- line or in an anticyclonically curved path. Clear pheric conditions necessary for the formation of skies also are observed in a current of air moving closed cells are weak convective mixing in the lower levelsand from the south ifitisturning sharply anti- a cap to this mixing. The cyclonically. Elongated V-shaped troughs aloft convective mixing is the result of surface heating have cloudiness and precipitation in the south- of the air or radiational cooling of the cloud erly current in advance of the troughs, with tops. This convection is not as intense as that clearing at the trough line and behind it. These associated with open cells. The capto the rules also apply in situations where this type of instability associated with these closed cellular low is associated with frontal situations. cloud patterns isin the form of a subsidence Cellular cloud patterns (open or closed) as inversion in both polar and subtropical situa- shown by satellite cloud photographs will pro- tions. Closed cellular patterns are made up of vide the forecaster with information to identify stratocumulus elements in both the polar and regions of cold air advection, areas of cyclonic, subtropical air masses and may also be accom- anticyclonic, and divergent wind flow within air panied by trade wind cumulus in the subtropical masses, especially behind polar fronts and over high. Closedcells, when associated with the oceanic areas. subtropical highs, are located in the eastern Open cellular patterns are most commonly sections of the high pressure area. Closed cells found to the rear of cold fronts in cold unstable are associated with limited !ow level instability air.Thesepatternsare made up of many below the subsidence inversion and extensive vertical convective activity iF, not likely. Figure individualcumuliformedcells. The cellsare composed of cloudless, or less cloudy, centers 10-10 shows closed cellsinthe southeastern surrounded by cloud walls with a predominant portion of a polar high near A. ring or U-shape. In the polar air mass the open Figure 10-11 shows closed cells in the eastern cellular patterns that form in the deep cold air portion of asubtropicalhighin the South are predominately cumulus congestus and cumu- Pacific. West of A the closed cells are composed lonimbus. The opencellsthatformin the of stratocumulus with some clear walls and east subtropical high are mainly stratocumulus, cum- of the walls are composed of thinner clouds. ulus, or cumulus congestus clusters. In order for open cells to form in a polar high, there must be VERTICAL MOTION AND WEATHER moderate to intense heating(alarge air-sea temperature difference) of the air mass from Upward motion is associated with increasing below. When the cold continental air moves over cloudiness and precipitation, subsidence with the water, the moist layer is shallow and capped improving weather. There have been a number

313 319 AEROGRAPHER'S MATE IR. C

.45 ". 1r,

30w ,

Figure 10.8.Open cells on a satellite photograph. AG.582 of studies of the relationship between precipitation and vertical velocities computed by various techniques.Inallcases,the probability of precipitationisconsiderably higher inthe 6 hours following an updraft than following subsidence. Clear skiesare most likely following d ow nd rafts. v10 Vertical motion analysis charts and prognostic 4filivrit.--- charts are currently being transmitted on the National Facsimile Network. The values are computed by Numerical Weather Prediction Methods and are analyzed on the charts. Plus valuesrepresent upward motion, and minus values downward motion (subsidence). A more detailed discussion of the relationship of vertical motion to middle cloudiness precipitation, and convective activity may be found in VerticalMotionandWeatherForecasting, NWRF 30-0359-024.

90W VORTICITY AND PRECIPITATION

Vorticityhas been discussedindetailin AG.583 chapter 4 of this training manual. We have seen Figure 10-9.Open cells in the subtropical high. that relative vorticity is due to the effects of

314 320 Chapter 10- -CLOUDINESS, PRECIPITATION, AND TEMPERATURE FORECASTING

T 7`ur t 44: or & j; ; 40Ni 11110, , stiItt. Etrilir 14.lost( ; ew`.. -776.1;1, cs. at, .

4. or 4. "erati141 NI01.4 41:11ak trik, ^ Al r /I* 4.16";";I:L"'"?..4114. 30N s. 114.-411..

160w1 ,-,

AG.584 Figure 10-10.Closed cells in polar high. both curvature and shear. Studies have led to the MIDDLE CLOUDS IN RELATION followingrule:Cloudiness and precipitation TO THE JETSTREAM shouldprevailinregions where therelative Jets show as much individuality with respect vorticitydecreasesdownstreamalongthe to associated weather as do fronts. Because of streamlines. Fair weather should prevail where it the individuality of jetstreams, and also because increases along the streamlines. of theindividualityof each situation with The fact that both wind shear and curvature respect to humidity distribution and lower level must beconsidered whenrelativevorticity circulation patterns, statistically stated relation- changes are investigated results in a large number ships become somewhat vague and are of little of possible combinations on upper air charts. value in forecasting. However, the results of one When both terms arein agreement, we can study of visual cloud observations from meteor- confidently predict precipitation or fair weather, ological research probing of jetstreams found as the case may be. When the two terms are in thatthe layer types of middle clouds were conflict, computation must be made before the reported mostly to the right (or high-pressure forecast can be made. Since the former case side) of the jetstream axis. should include the most potent rain producing situations, actual computations are, in general, SHORT RANGE EXTRAPOLATION FOR unnecessary. TERMINAL FORECASTING For afull discussion of this problem, itis suggested that the AG refer to Precipitation and The purpose of this seLtion of the chapter is Vorticity, AROWA 13-0953-092. to outline several methods which are particularly

321.3'5 AFROGRAPHER'S MATE 1 & C

observations. This may be of great importance in bad flying weather. The forecasters and observers should utilize the remarks section as much as 116.".., possible to this end. A NEPHANALYSIS

Nephanalysis may be defined as any form of analysis of the field of cloud cover and/or type. The potentialities of specialized nephanalysis may seemtobeobvious, butinpractice, difficulties are encountered. The cloud observations received in synoptic codes permit onlya highly generalized and incomplete discription of the actual structure of the cloud systems; the observation stations are usually too far apart to permit a representative picture of the distribution of many features which have a high degree of spatial and time variability; somany parameters are observed that they cannot all be readily analyzed on one chart. In view of the above limitations, forecasters should approach this problem with an open AG.585 mind for further experimentation. You should Figure 10-11.Closed cells in a subtropical high. select for analysis only the particular parts of the cloud observations which are important to suited for preparing forecasts for periods of 1 to the intended application. then adjust the chart 3 hours. All the techniques presented are based scale, the degree of detail, and the mode of on a single forecasting principleextrapolation. representation to the character of the data and Extrapolation is the most powerful short range to the purpose. forecasting tool currently available to meteorologists. At present, few forecasters make full use of Extrapolation is the estimating of the future the cloud reports plotted on their charts, and value of some viriablc from observations of its often many cloud reports are omitted. Obvi- present and past values. In its simplest form, as ously, the first consideration in nephanalysis is discussed in this section. extrapolation is taken to survey what cloud information is transmitted linearly, with only very crude, if any, considera- and to make sure that everything pertinent is tiongivento accelerations. The quality and plotted on the regular charts. For very short frequency of observations do not generally range forecast, the charts at 6-, 12-, and 24-hour permitor justifyuse of more sophisticated intervals are apt to be insufficiently frequent for techniques fortreating accelerations, nor do use of the extrapolation techniques explained in short range forecasts generally require them. this chapter. Either neph charts or surface charts Even the simplest methods are sensitive to the should then be plotted at the intermediate times quality and time spacing of the observationsas from 3-hourly synoptic reports or even from well as the quality of any analysis of these hourly sequences. An integrated system of fore- observations. casting ceiling, visibility, cloud cover, and pre- The hourly sequences obviously cannot pro- cipitation should be considered simultaneously vide a complete nor even a wholly adequate as these elements are physically dependent upon picture of the weather under allconditions. the same synoptic processes. When special conditions prevail, the REMARKS With the advent of satellite cloud pictures it is section provides opportunity for simplifying the rare that hephanalysis form manually plotted

316 322' Chapter 10 CLOUDINESS, PRECIPITATION, AND TEMPERATURE FORECASTING data will be performed. Instead, the surface Precipitation Movement Using a analysisandsatellitecloud pictureswillbe Distance Versus Time Diagram utilized together. The idea of plotting observations taken at different times on a diagram which has horizon- FRONTAL PRECIPITATION tal or vertical distance in the atmosphere as one coordinate and time as the other is very old and Forshortrangeterminalforecasting,the used various forms for diverse question is often not whether there will be any has been in purposes by meteorologists for years. The time precipitation. but when will it begin or end; for cross sections (as explained in chapter 12 of this example. in cases where ithas already begun training manual) are a special case of this device, upstream or at the terminal. where successive information at only one station This problem is well suited to extrapolation is plotted. methods. The prediction of new precipitation areas, such as those associated with new wave On this diagram the horizontal coordinate is development, upper troughs, etc.. of course. an axis line passing from the terminal in ques- requires other synoptic methods. Ilowever, for tion through a series of stations in the direction very short range forecasting, the use of hourly from which the weather usually approaches, or is expected during the particular nephanalysis oftenserves to "pick up" new from which it precipitation areas forming upstream insuffi- situation. This line forms the distal.-e, or x-axis, cient time to alert a downstream area. Also, the of the diagram. The positions of the stations are thickening and lowering of middle cloud (alto- marked off onitattheir appropriate scale stratus) decks generally indicate where an out- distances. The time scale (increasing upward) is erected at right angles to the origin of the x-axis. break of precipitation may soon occur. Both coordinate axes have linear scales. Observa- tions for any time and points reasonably near Forecasting the Movement of the x-axis may be plotted on this diagram, using Precipitation Areas by Isochrones the standardplotting models for airways or synoptic reports. The resulting picture has some- The areas of continuous, intermittent, and what the character of a plotted synoptic chart showery precipitationcan be outlined on a and can be analyzed accordingly. special large -dale 3-hourly or hourly synoptic When complete synoptic models are plotted chartin a manner similar to the customary shading of precipitation areas on ordinary syn- on the x-t diagram, various factors affecting optic surface weather maps. Different types of local movement and development of systems can lines, shading, or symbols can distinguish the be evaluated subjectively in making the extra- For example, the interrelations of varioustypes of precipitotinn. lsochrones of polation. several hourly past positions of the lines of fronts, visibilities, winds, clouds, temperatures, particular interest can then be added to the pressures, and precipitation can be readily con- chart and extrapolations for several hours made sidered. Moreover, locally important details of the situation missed on the large-scale regular from them if reasonably regular past motions are synoptic weather maps can often be spotted and inevidence. A separate isochrone chart (or acetate overlay) may be easier to use. Lines for followed on this diagram. the beginning of continuous precipitation are illustrated in figure10-12. The isochrones for LOWERING OF CEILING IN showery or intermittent precipitation usually CONTINUOUS RAIN AREAS give more uncertain and irregular patterns which result in less satisfactory forecasts. When large- Frontal Situations scale section surface weather maps are regularly drawn. it may be sufficient and more convenient The lowering of ceilings with continuous rain to make allprecipitation area analyses and or snowin warm frontal and upper trough isochrones on these maps. situations is a familiar problem to the forecaster

317 323 AEROGRAPHER'S MATE I & C

t 70° 60°

1300.-1

1230 ,....., 1300 1200 1200 CH 1330 1100 1100 1000 QV0930..--0900 1000 0800 09 vW o 0700 35°x_ *1030 0630 060035a ZX 0717 1130AF 0500 0600 MPV _J 1130 0400 .------,..00700 SA 0700 e-- 0330 0600 0500 0400ALE1_/ 0315 / 0535903 0400 I 1PV0-1 044 0500 NC00436 0500 0356 0400

70° 600 1 t

AG.586 Figure 10-12.Isochrones of beginning of precipitation, an early winter situation. in many regions. In very short range forecasting, Once stratus has formed, however, extrapolation the question as to whether or not it will rain and of its displacement may be quite successful. when the rain will begin is not so often the Itis important to recognize the difference critical one. Rather, the problem is more likely between the behavior of the actual cloud base to be (assuming the rain has started) how much height and the variation of the ceiling height as will the ceiling lower inI,2, and 3 hours, or will itisdefinedinairwayreports. The ceiling the ceiling go below a certain minimum in 3 usually drops rapidly, especially during the first hours. The visibility in these situations generally few hours after the rain begins. However, if the does not reach an operational minimum as soon rainis continuous, the true base of the cloud as the ceiling, and fog usually becomes no worse layer descends gradually or steadily. The reason thanlight.Ithas been shown that, without for this is that below the precipitating frontal sufficient convergence, advection or turbulence, cloud layer there are usually shallow layers in evaporation of rain into calm lower air does not which the relative humidity is relatively high and lead to saturation and causes no more than haze which soon become saturated by the rain. The or light fog. cloud base itself has small random fluctuations At some locations with advection, upslope in height superimposed on the general trend. trajectories may bring in more moist air, where The time of lowering of the ceiling during the stratus or dense fog that forms is more or precipitation remains a local problem. Many less independent of the rain -evaporation effect. excellent sources and references are available for

318 324 Chapter 10 CLOUDINESS. PRECIPITATION, AND TEMPERATURE FORECASTING further study. The Navy publication, Synoptic during thefirsthours of rain, as new cloud Methods for Developing Emccast Techniques. layers form beneath the front. However. by NWRE 38-0563-073. lists some or the references following the ceiling trend at surrounding sta- and gives details for developing a-local objeclie tions as well. a pattern of abrupt ceiling changes technique. may be noted. These changes at nearby stations where rain started earlier may give a clue to a Time of Lowering of Ceiling likely sequence at your terminal.

Forecasting the time when the ceiling of a THE TREND CHART AS AN given heightwill be readied during rain is a EXTRAPOLATION AID separate problem. Nomograms, tables. air trajectories. time airwill become saturated. and a The trend chart can be a valuable forecasting correlation of these data can be resolved into an tool when itis used as a chronological portrayal objective technique temperedwith empirical of a group of related factors. It has the added knowledge and subjective considerations and can ad%antage of helping the forecaster to become be developed for your individual station. J. J. "current- when coining on duty. At a glance, George and Associates in their studies on Geo- the relieving duty forecaster is able to get the physical Research Directorate (GRD) Contracts pictureof whathas been occurringathis have developed several studies along these lines. terminal. Also, he is able to see the progressive effect of the synoptic situation on the weather Extrapolation of Ceiling Trend by at his terminal when the trend chart is used with Means of the x-t Diagram the current map.

The x-t diagram. as explained previously in Trend Chart Format this chapter. can he used to extrapolate the trend of the ceiling height in rain. The hourly The format of a trend chart should be a observations should be plotted for stations near function of whatisdesired fromit: conse- a line parallel to the probable movement of the quently. it may vary in form from station to general rain area. originating at your terminal station.It should, however, contain those ele- and directed toward the oncoming rain area. ments which are predictive in nature as well as Ceiling-time curves for given ceiling. heights may thequantitativevalues of parameters to be be drawn andextrapolated. There may he forecast-such as ceiling and visibility.Withthis systematic geographical differencesinceiling in mind, a suggested format is outlined below. between stations due tolocal(topographic) The trendchartismerely a method for influenceswhich causeirregularitiesinthe portraying graphically what forecasters generally curves. Such differences sometimes can be anti- attempt to store in their memory. Included in cipated from climatological studies, experience. the usual technique storehouse of most forecast- or general influence. In addition, there may be a ersisa listof keypredictor stations. The diurnal (thermal) ceiling fluctuation which will forecaster utilizes the hourly and special reports become evidentinthe curve in slow moving from these stations as aidsin making short situations. Rapid and erratic up-and-dowil fluc- forecastsforhis own terminal. Usually. the tuations also must be dealt with where the sequencesfromthesepredictor stations are ceiling is uneven due to scud or "holes" of small scanned and committed to memory. Stepwise, diameter. In this ease, a smoothing or the curves the method would be as follows: may be necessary before extrapolation can be made. A slightlyless accurate forecast may I. Determine thedirection source of the result from this process. weather-usually upstream. In view of the previous discussion of the precipitation ceiling problem itis not expected 2. Select a predictor stalion(s) upstream and that mere extrapolation can be wholly satisfac- watch for the onset of the critical factor, for tory at a station when the ceiling lowers rapidly example. rain.

319

111"-: t5K-E AEROGRAPIIER'S MATE 1 & C 3. Note the effect of this factor on ceiling and visibility at predictor station(s). 1630 2030 2230 0030 0230 0430 0630 4. Extrapolate the approach of the factor to determine its onset at your terminal. 5. Consider the effect of the factorat predictor station(s) in forecasting its effect at your terminal.

The chief weakness of this procedure is its subjectivity. The forecaster is required to mentally evaluate all of the information available on the hourly sequences. both for his own and his predictor station(s). The trend chart is a means to graphically portray the sequences and make extrapolations more objective. The question naturallyarises. How many trend charts do I need? The answer depends on the synoptic situation. There are times when , , i keeping a graphic record at all is unnecessary: rV ----4- whereas, at other times the trend for the local PRECIP a/r.,,, 1.ii.r.4-- W r terminal may suffice. There should be some VSEIY blank charts available which may be used to start recording at any time as the need arises. The trend chart format (fig. 10-13) is butene 20.000 suggested way of portraying the weather record. 15.000

Experimentationandimprovisationareen- CO 10,000 4 couraged to find the best form for any particular 7.500 location or problem. C>3 5.000 0 0 --4,, 4,000 ;.; TIME-LINER AS AN 3.000 EXTRAPOLATION AID 0 (3, 2.000 1

1.000 Inthe preceding sections of this chapter, several methods have been described for "keep- 500

ing track of the weather" on a short term basis. °630 2030 2230 0030 0230 0430 0630 Explanations of time-distance charts, isochrone devices, trend charts, etc., have been presented. It is usually not necessary to utilize all or .ven AG.587 most of these ideas simultaneously. The device Figure 10.13. Trend chart suggested format. described in this section is designed for use in .6 combination with one or several of the methods previously described. Time-liners are especially Construction of the Time-Liner useful for isochrone analysis and extrapolation therefrom. The time-liner is simply alocal area map which is covered with transparent plastic and Inasmuch as a large majority of incorrect constructed as follows: short range terminal forecasts result from poor timing of weather already occurring upstream, a I. Using a large-scale map of the local area, device such as described below may improve this construct a series of concentric circles centered timing. on your station and equally spaced from 10 to

3 3'42.06 1111111MMIN11111111111114111

Chapter 10 CLOUDINESS, PRECIPITATION, AND TEMPERATURF FORECASTING

20 miles apart. This distance from the center to use of this device is, of course, for timing or the outer circle depends on your loc ation. but in extrapolation purposes. If cannot be emphasized most cases 100 to 150 miles is sufficient. too stronglythat accurate timing isa most 2. Make small numbered or lettered station important faLtor in short range terminal fore- circles for stations located at varying distances casting. and directions from your terminal. Stations with a high predictor value, relative to your terminal. USE OF RADAR IN CLOUD AND should be selected. This may be determined by PRECIPITATION FORECASTING experience, local forecast studies, and climatology. Usually, however, the reporting network is Radarishighly useful in determining the not so dense, and most nearby stations can be various clouds that are approaching the station spotted. In addition to the station circle indica- and for estimating the probability of precipita- tors, significant topographical features such as tion reaching the local area. Refer to chapter 16 rivers, mountains, etc., may be indicated on the ofthistrainingmanual forinformation base diagram. (Aeronautical charts include these onforecastingweatherconditionsutilizing features.) weather-radar. 3. Cover and bind the map with transparent plastic. CLOUD ANALYSIS AND FORECASTING Plotting and Analysis of the Time-Liner Aerographer's Matesarefrequently called upon to make forecasts of clouds for flight and By inspection of the latest surface weather other weather briefings over areas where synop- map, sequences, and other information, you can tic observations are not readily available or over determine a section of the diagram and the other areas where clouds above the lowest layer usually parametersto be plotted. This will arefrequently obscured by the lower cloud comprise about one-third of thecircleina deck. This section is designed to acquaint the direction from which the weather is approach- AG with the principles of detection and analysis ing. Then, plot the hourly weather SPECIALS of clouds from radiosonde data. A complete for those stations of interest. Make sure to plot coverage of this problem is beyond the scope of the time of each special observation. this training manual. Further information on Overlaythe circular diagram with another this subject may be found in the following Air piece of transparent plastic and construct iso- Weather Service publications: Forecasting for chrones of the parameter being forecast; for Aerial Refueling Operations at Mid-Tropospheric example, the time of arrival of the leading or Altitudes, AWSM 105-52; and Use of thz Skew trailing edge of a cloud or precipitation shield. T, Log P Diagram in Analysis and Forecasting, The spacing between isochrones can then be AWSM 105-124. extrapolated to construct "forecast isochrones" for predicting tl,e time of arrival of occurrence of the parameter at your terminal. Refer to IMPORTANCE OF RAOBS figure 10-14 for an example. IN CLOUD ANALYSIS The time-lineris admittedly a "quick fix" tool; however, the fart that ;t can be plotted and The cloud observations regularly available to analyzed quickly isstrong point in its favor. If forecastersinsurface synoptic reports Lave convenient,the tancescalecan be con- much to be desired as a basis for cloud forecast- structed to coincide with the scale of the local ing. sectional map used and the isochrones overlaid Radiosondes which penetrate cloud systems on thelatest analyzed map. This makes a reflect to some extent (primarily in the humid- particularly effective briefing aid, since the pilot ity trace) the vertical distribution of clouds. If being briefed can see what is being told by the the humidity element were perfect, there would forecaster regarding the local area. The prime usually be no difficulties in locating cloud layers

321 AEROGRAPIIER'S MATH I & C

75°

1403 N - 40° 140 120 100 80 60 40 20 20 4060 40 100120140 ;Leq, ,120/ L/ - NBB -let\ (2105) !AL 15/i 65/ 20

ocAeltEIS42025) 9 (nns< -Nyr 55/,' 18 21 tZot51 5/ 0750 5/ a 6vr20 12019) 30/ 411*i' &Etat.' 19 -.11111111110.44101 16 18 4/ 11550 15 14 17

ROA 1213 16 /ACS tllnl 11 IS / / mss- 1413 12II 75°

AG.588 Figure 10-14.Largescale sample timeliner (isochrones show advance of precipitation field).

penetrated by the instrument. Because of the INFERRING CLOUDS FROM RAOBS shortcomings in the instrument, however, the relationship between indicated humidity and Theoretically we should be able to infer from cloudisfar from definite. and an empirical radiosonde observations of humidity the layers interpretation is neLessary. Nevertheless. radio- where the sonde penetrated cloud layers. In sondereports givevaluable evidenLe pradice. the determination that can be made when sifted with other available information, from temperature and t;ewpoint curves are often aids greatly in determining a Loherent picture at less exalt and less reliakje than desired. Never- least of stratiform and frontal doud distribu- theless. raobs give dues about cloud distribution tions. Their value in judging air mass cumulus and potential areas of cloud formation. These and cumulonimbus distribution is negligible, dues generally cannot be obtained from any

322 3Z8 Chapter I0-- CLOUDINESS, PRECIPITATION, ANDTEMPERATURE FORECASTING other source. A knowledge of the characteristics 5 of the humidity element will aid the forecaster when using data from the sounding. TF =9/10 TD 4 CHARACTERISTICS OF THE TD DEW POINT RADIOSONDE HUMIDITY ELEMENT TF * FROST POinT The carbon-impregnated plastic humidity element is presently used in U.S. radiosondes. The characteristics of this element are as MORE EXACT follow: CURVE I. The time lag is not sensitive to temperature changes. andthe polarization effectis negligible. 2. There is no washout problem, although there are some adverse effects due to precipitation actually wetting the element. 0 3. It will provide good humidity datadown -50 -40 -30 -20 -10 0 to -40°C. TD °CELSIUS 4. The sensitivityis only fair at humidities less than about 15 percent. and there is some AG.589 uncertainty in the readings near saturation. Figure 10-15.--Difference between frost point and dewpoint as a function of the dewpoint.

DEWPOINT AND FROST droplets. Minor discrepancies may occur when POINT IN CLOUDS the cloud is not in a state of equilibrium (when The data trace recorded from the humidity the cloud is dissolving or forming rapidly, or strip is calibrated in terms of relative humidity when precipitation is falling through the cloud with respect to water at negative as well as with raindrops of slightly different temperature positive temperatures. The temperature minus than the air): but these discrepancies are theoret- the dewpoint depression value. the humidity ically small. In the subfreezing part of a cloud, parameter that is transmitted over the teletype, the true temperature is between the true dew- gives the dewpoint whichis defined as the point and the true frost point, depending on the temperature to which the air must be cooled ratio between the quantities of frozen and liquid adiabatically at constant vapor pressure if satura- cloud particles. If the cloud consists entirely of tion with respect to a water surface is to be supercooled water droplets, the true tempera- reached. The FROST POINT (that is. the tem- ture and the true dewpoint will, more orless, perature to which the air has to be cooled or coincide. If the cloud consists entirely of ice, the frost heated adiabatically in order to reach saturation temperature shouldcoincidewith the with respect to ice) is higher than the dewpoint point. We cannot, therefore, look for the coinci- except at 00c. where the two coincide.In the dence of dewpoint and temperatures as a criterion for clouds at subfreezing temperatures graph showninfigure10-15the difference between dewpoint and frost point is plotted as a even if the humidity element has nosystematic function of the dewpoint itself. errors as previously discussed. At temperatures below - I2°C, the temperature is more likely to In a cloud in which the temperature is above coincide with the frost point than the uewpoint. freezing, the true dewpoint will coincide closely with the true temperature, indicating that the air The graph shown in figure 10-15 indicates between the cloud droplets is practically satu- that the difference between the dewpoint and rated with respect to the water surfaceof the frost point increases roughly 1°C for every10°C

323 I p" t.),C4U AEROGRAPIIER'S MATE 1 C

thatthe dewpoint t, hclow frecAilw. l'or e \- height reports are in pressure-altitude. The tem- ample. when the dewpoint is I0 "C.the frost perature, frost point, and dewpoint curves are point equals when the dewpoint is10T, indicated by Tt-, and Td, respectively. the frost point is18 °C; and when the dew point Infigure10-16, a marked warm frontis is -30 °C, the frost point is27°C. Thus, for a approaching from the south. Moderate continu- cirrus cloud thatisin Nuilihl ium( saturated ous rain fell 2 hours later. At 18302 an aircraft with respect to ice) ata(frost point) tempera- reported solid clouds from 1,000 to 44.000 feet ture of -40°C. the correct dew point would be (tropopause). The 15002 sounding showsan -44°C (to the nearest whole degree) increasing, dewpoint depression with height and We can state then, in eeneral. that air ina no discontinuity at the reported cloud top of cloudat temperatures below about 12°C is 15,000 feet. A definite dry layer is indicated saturated with respect to ice,ind that as the between 18,300 and 20,000 feet. The second temperatureoftheclouddecreases(with reported cloud layer is indicated by a decreasein height). the true frost point. dewpoint differ- dewpoint depression, but the humidity element ence increases. The effects of these physical characteristics serve to reinforce the effects of ATM ACT -400 the systematic faults of the humidity element (TM FT) MB itself, and they all conspire to makea radio- 23 sonde ascending through such a cloud indicate 22 increasing dewpoint depressions. Any attempt to 21 determinetheheightof cloud layers from 21,000' / humidity data of a raob is, therefore, subject to 20 errorsfromthesedefect...Itispossibleto 19 overcome some of these errors by a subjective 18 interpretation of the mobs, as discussed in the 17 following sections.

INTERPRETATION OF RAOBS WITH 15 15,000` RESPECT TO CLOUD LAYERS 14 600

The following diagrams (figs.10-16, 10-17, 12 and.10 -18) made over the United States illustrate the behavior of the radiosonde during cloud penetration. These are but three of a series 10 700 of elevendiagrams actually correlated with aircraft observations (from Project Cloud-Trail Flights) or the heights of cloud bases and tops 7 WASHINGTON, D.0 fromaircraftflyinginthevicinity of the 6 ascending radiosondes. The difference in time 4 APR 1957 15Z 5 850 - CLOUD: and space between the aircraft and sounding BASE TOP observations was usually less than 2 hours and Sy ocr 15,000' 30 miles. Some of the aircraft reported only the 3 21,000 41,000' 44 278 2 66 cloud observed above 15,000 feet; others re- do 33-ar portedallclouds.Infigures10-16 through 44 10-18, the aircraft cloud observations are en- 1000 tered in the lower left corner of each diagram under the heading cloud; the surface weather report is entered tinder the aircraft cloud report. AG.590 Where the low cloud was not reported by the Figure 10-16.Example of inferring clouds from a raob aircraft, the height of the cloud base may be with an active warm front approaching from the obtained fromthesurface reports.Aircraft south. Chapter 10 CI,OUDINESS, PRECIPITATION, AN I)-IsENIPERATURF EnRITASTING is obviously slow in tesponding.Me dew point Lase of a Lloud in the 500-nibsurface with no depression at the base ol the cloud at 21.000 preLipitation reaching the surface: the nearest -1 ennessee. The evidence from the feetis14 '(:Willat 400 nib, after about a rain was in 3-nunute chub through the cloud,itisstill sounding for placing the cloud base at 12.200 10°C. Front the sounding, clouds should have feet isstrong, yetthe baseisinexplicably been interred to he I roin about 4,500 feet ( base reported at 15,700 I eet. The reported cloud base of the rapid humidity increase) to 500 nib and a of 15.700 feet was probably not representative. second layer from 20,000 feet up. In view of the sinLe altostratus. with bases 11.000 to 14,000 feet, was reported for most stations over Ohio rapidfilling of the cloud free gap between 15,000 and 21.000 feet which followed as the and West Virginia. warm front approaLlied, the agi cementbetween I figure10-18 shows layer clouds with their reported and inferred Lond it ions is good. intermediate deallayers not showing inthe Figure 10-17 shows a middle cloud layer with no precipitation reaching thesurface. This is a

ATM ALT 400 (THSO MB 23

22 21

20 19- 500 19 18- 500 14 17-

17 16-

16 15,700' 15-

600 13-

12- 12,200' 11. 111 10- 10 .00 9- 9 8- 9.

PORTLAND, ORE MISDA 6- 3 DEC 1954 151 6 MB 5 850 CLOUD 5 -830 BASE TOP 4 4,000. 6,000' 4 6,500' 10,000' CLOUD --- 3- 3 (AT ASE T 39 PIT TSBUROu, PA 15,B/oo' 33,OP100' II ROPO ) 2- 5i 159 2 SOME LOW CLOUD / ST DEC 1955 152 31.3r -8 ' 33 I - 56242 1000 r10L6_ 2fT 0 13 °11 -1

AG.591 AG.592 Figure 10-17.Example of inferring clouds from araob Figure 10.18.Example of inferring clouds from a raob, with a middle layer and no precipitation reachingthe showing layer clouds with their intermediate clear layers not showing in the humidity trace. surface.

325

3=31 AEROGRAPHER'S MATE 1 & C humidity trace. There isgood agreement between the sounding and theaircraft report. The 100 clear layer between 6,000and 6,500 feet is not z indicated on the sounding. Le, 80 Thin clear layersas cc wellasthin cloud layers usuallycannot be recognized on the humiditytrace. z60 Comparisons of the type madein the foregoing between soundings 40 and cloudreports 4iii co provide us with the followingrules: o 20 cc a. 1. A cloud base is almostalways found ina 0 2 4 layer (indicated by thesounding) where the 6 8 10 12 dewpoint depression decreases. DEW-POINT DEPRESSION (C°) 2. The dewpoint depressionusually decreases 100 to between 0°C and 6°Cwhen a cloud is associated with the decrease.In other words, you should not always associatea cloud with a layer of dewpoint decrease,but only when the N00-850m9 decrease leads to minimumdewpoint depres- LAYER sions from 6°C' to 0°C: at cold temperatures t (below -25°C), however,dewpoint depressions 4 in clouds are often higherthan 6°C. fp) 3. The dewpointdepression in a cloud is,on a.lx "0R0(11) I 1 - the average, smaller for 4 higher temperatures. 6 8 10 12 14 Typical dewpoint depressionsare 1°C to 2°C' at DEW- POINT DEPRESSION (C °) temperatures of 0°C and above,and 4°C between - 10 °C and -20°C. 4. The base ofa cloud should be locatedat AG.593 the base of the layer of Figure 10-19.Percent probability ofexistence of cloud decreasing dewpoint layer bases for different values of depression, if the decrease is sharp. dewpoint depression (degrees Cl. Solid linesrepresent probability of 5. If a layer of decrease ofdewpoint depres- clear or scattered conditions; sion is followed by a layer of dashed lines, the proba- stronger decrease, bility of broken or overcastconditions with the cloud the cloud base should beidentified with the base layer bases between 1,000 mb and600 mb. of the strongest decrease. 6. The top of a cloud layeris usually indicated by an increase in dewpointdepression. Once a cloud base is determined, the cloud is dewpoint depression. On each graph extended up toalevel where a one curve significant showsthe probability of clearor scattered increaseindewpointdepression starts. The gradual increase of dewpoint conditions as a function of the dewpointdepres- depression with sion; the other curve shows that ofbroken or height that occurson the average in a cloud is overcast not significant. conditions. Separate graphsarein- cluded for the 1,000- to 850-mband 850- to 600-mb layers. The graphsare based on 1,027 In addition to the aboveanalysis of Project observations, which are enough to indicatethe Cloud-Trail data, another studywas made by the U.S. Air Force to see how reliable order of magnitude of the dewpointdepressions the dewpoint atthebaseof winter cloud layers. Minor depression is as an indicator ofclouds. The results of this study irregularities in the curveswere not smoothed are summarized in the two out because it is not certain that they graphs shown in figure 10-19. Each are all due graph shows to insufficient data. The graphsare applicable the percent probability of theexistence of a cloud layer in January for without reference to the synopticsituation. For different values of a given winter sounding, one can estimate from 326 Chapter 10 Cl OUDINESS, PRECIPITATION,AND TEMPERATURE FORECASTING the graph the probability of different sky cover from the cloud edge. a distanceof 10 to 15 conditions with cloud bases between 1,000 nib miles or more. into cloud free air is considerably the t 'ported and600nibforlayers of given minimum greater than the average error in dewpoint depressions. dewpoint depression.

HUMIDITY FIELD IN THE VICINITY 500-MB ANALYSISOF OF FRONTAL SYSTEMS DEWPOINT DEPRESSION Figure 10-20 shows an analysis of the 500-mb Studies of the humidity field in the vicinity of dewpoint depression field, superimposed upon frontal zones indicate there is a tongue of dry air observations) of extending downward in the vicinity of the front an analysis (based on surface areas of continuous precipitation andof areas of and tilted in the same direction as the front. One overcast middle clouds. The 500-mbdewpoint study found that such a dry tongue war more or depression isopleths were drawn independently less well developed for all frontal zones investi- of the surface data. The analysis shows that: gated. This dry tongue was best developed near warm flunts; it extended, onthe average, down I. The regions of high humidity at 500mb to 700 nib in cold fronts and to800 mb in warm coincide well with the areas of middle cloudand fronts. In about half the number of fronts, the the areas of precipitation. driest air as found within the frontal zoneitself: 2. The regions of high humidity at 500 mb on occasions it was found onboth the cold and are separated from theextensive dry regions by warm sides of the zone. Abouthalf the flights strong humidity gradients.These gradients are, through this area in connection with thisstudy in all probability, much strongerthan shown on showed a sharp transition from moist to dryair. this analysis. since this analysis has thedefect of and the change infrost point (to be explained all continuous field analyses which arebased on in laterin this chapter) on theseflights averaged discrete observations spaced widely apart: about 20°C in 35 miles. Some flights gage other words, linear interpolation betweenobser- changes of more than 20°C in 20 miles. vations smoothes out strong contrasts. As a frontal cloud deck is approached.the 3. A dewpoint depression of 4°C or less is dewpoint depression (or frost pointdepression) characteristic of the larger part of the areasof starts diminishing rapidly in theclose vicinity of continuous precipitation and also of the la.ger the cloud. Farther away then10 to 15 miles part of the area of overcast middleclouds. from the cloud, the variation was much lessand Since the 500-mb dewpoint depression analy- fact should be borne in was less systematic. This sisagrees wellwiththe surface analysis of mind when attempting to locate the edgeof a middle cloud and precipitation. thepossibility cloud deck from raob humidity data (500mb. exists of replacing or supplementing oneof these for instance). Linear extrapolation orinterpola- analyses with the other. tionof dewpoint depressions cannot be ex- The characteristics of the 500-mb dewpoint when pected to yield good results. For instance. depression analysis (outlined above) makeita one station shows adewpoint depression of valuable adjunct to the surface analysis.These 10°C and the neighboring station shows satura- analyses can be compared and. by crosschecking. tion, the frontal cloud may be anywherebe- each can be completed with greater accuracy tween them, except within about10 miles of the than if they were done independently. Arough driest station. sketch of the dewpoint depression field maybe Since frontal cloud masses at midtropospheric completed on the facsimile chart in a matterof levels are usually surrounded by relativelydry minutes. air, it is possible to locate the edgeof the cloud mass from humidity data onconstant pressure THREE-DIMENSIONAL HUMIDITY charts, at least to within the distancebetween ANALYSISTHE MOIST LAYER neighboring soundings. even if thehumidity data This is so because the A single level (for example the 500-mblevel) are not very accurate. probable typical change of dewpoint depressionin going dewpoint depression analysis to find

327 333 AEROGRAPIIER'S MATE 1 & C

7 SEP 1956 500-MB OEW -POINT DEPRESSION (C) AT mu 1.-V-Y-Y-1 OUTLINE OF 8/9MIDDLE CLOUD -y--46- SURFACE FRONTS AT 0300Z CONTINUOUS PRECIPITATION -Z.

AG.594 Figure 10-20.Surface fronts, areas of continuous precipitation, areas covered by 8/8middle clouds, and isolines of 500-mb dewpointdepression at 0300Z, 7 September 1956.

328

3=3_4 Chapter 10 CLOUDINESS, PRECIPITATION,AND TEMPERATURE FORECASTING cloud areas does not inthute clouds above or For the above reasons. experience indicates below that level. For example, if the top of a that little dependence can be placed on the usual cloud system reached only to 16,000 feet, and sounding to indicate directly the existence of there was dry air above at 500 nib, you would tall cumulus in the area. On the other hand. never suspect, from the 500-nibanalysis, the where the radiosonde samples of the environ- existence of clouds a short distance below the ment of such clouds. a stability analysis, com- 500-mb level. bined with considerations of surface weather However, an analysis of the extension of the observations,radar and aircraftreports, and moist layers in three dimensions can be obtained synoptic analysis for heating and convergence, simply by scrutinizing each individual raob. The usually provides an estimate to the extentof mobs selected should be those for the general cumulus sky coverage.. This approach uses the vicinity of, and the area 500 to 1,200 miles same principles as in thunderstormand severe upstream of. the area of interest, depending on weather forecasting. (See chapter 11of this theforecastperiod. (Most systems during a training manual.) 36-hour period will move less than 1,200 miles.) In those cases where the sounding passes up The moist layers apart from the surface layers through a cumulus, itis well to keep in mind can be determined by methodspreviously dis- that the temperature in parts of such clouds is cussed in this chapter. The heights of' the bases often colder than the environment just outside andtops(labeledinthousands of feetof the clouds. These colder regions may still have buoyancy relative to other parts of the cloud layerispresent.thisfact can be indicated, though there is little advantage in indicating a surrounding them. Also, old dissipating cumu- dry layer 2,000 to 3,000 feet (or less)thick lonimbus clouds are generally colder than their sandwiched between thicker moist layers. Usu- environment. ally, it is sufficient to indicate the entiremoist The lapse rate in cumulus and cumulonimbus layer, without bothering about any finer struc- is not necessarily saturation adiabatic due tothe ture. A survey of the cloud field is madeeasier effects of "holes," downdrafts, melting of snow by writing the heights of the bases and topsin or hail, entrainment, mixing, etc. different colors. A moist layer for the sake of simplicity may PRECIPITATION AND CLOUDS frost point be defined as a layer having a ob- depression of 3°C or less(i.e.,a dewpoint The type and intensity of precipitation depression of 4°C at -10°C; 5°C' at-20°C; 6°C served at the surface is related to the thickness particularlyto the at -30°C). of the cloud aloft, and temperatures in the upper part of the cloud.The and LIMITATIONS TO DIAGNOSIS OF processes which cause cloud particles to grow precipitate out of clouds have receivedmuch TOWERING CUMULUS AND CUMU- of LONIMBUS DISTRIBUTION FROM RAOBS attention in the last decade. Our knowledge these processes is far from complete, butmuch information of practical use to the forecaster is The cumulus and cumulonimbus of summer and tropicalair mass situations are generally available. scattered. In many, if not most cases, theydo The results of one study relating cloud-top not actually cover half of thesky. Under such temperatures to precipitation type andintensity ascents conditionstheprobabilityof aradiosonde, aregivenasfollows: From aircraft released one to four times daily at a fixedtime through stratiform cloud and correlatedsurface and place, passing up through a cloudof this observations of precipitation, it was found that type appears to be small.When a balloon does in 87 percent of the cases when drizzleoccurred, enter the base of a tall cumulus cloud,it is likely it fell from clouds whose cloud-top temperatures to pass out of the side of thecloud rather than were higher than-5°C'; the frequency of rain or the top. or it may get caughtfor a time in a snow increased markedlywhen the cloud-top downdraft, giving an ambiguous record ofverti- temperature fell below12°C. When continuous coldest cal cloud distribution. rain or snow felt, the temperature of the

329 AI ROGRAPIIFIZ'S MATE I & C' part of the cloud was below 12"C in 95 percent used to improve the accuracy of groundobserver ofthecases. Intermittentrain was mostly estimates of the height of observed cirruslayers. associatedwithcoldcloud-top temperatures. The method is published When intermittent in AWS TR 105-110. rain was reported atthe Memorandum on Estimating the Ileight of Cirro- ground, the cloud-top temperature was below stratus Clouds, and a test with commentson the 2°C in S I percent of thecases and below 20"C method was published in AWS I OA, Test of in 63 percent of the cases. From this, it appears Method for Estimating the Height ofCirrostra- that when rain or snow continuousor inter- tus Clouds. mittent -reachesthe ground from stratiform clouds, the clouds solid or layeredextend in CIRRUS FORECASTING most cases to heights where temperature is well below12°C, or even20°C. One of the senior Aerographer's Matesduties This rule cannot be reversed. When no rain or isto make forecasts otcirrus ofcirrostratus snow is observed at the ground, middle clouds coverages and heights.[his poses difficulties maywellhe presentinregions wherethe owing tothelack of proven techniques of temperature is below 12 °C or20°C. Whether universal applicability andeven more to inade- or not precipitation reaches the ground will quacies of synoptic observationsand of upper depend on the cloud thickness, heightof the air soundings. Fola complete review of the cloud base, and the dryness of the air below the available knowledle of high clouds andseveral base. forecasting methods, itis suggested you thoroughly st,:dy the Air Weather Servicepublica- INDICATIONS OF CIRRUS tion. A Compendiumon Cirrus and Cirrus CLOUDS IN RAOBS Forecasting, AWS TR 105-130. Thispublication contains a comprehensive study of thenature of True cirrus forms at temperaturesnear 40 °C cirrus, the physics of cirrus formation,cirrus in or colder and consists of ice crystals. At these relation to certain phenomena, cirrus climatol- temperatures. as soon as the air is brought to ogy.cirrustypes, and a review of several saturation with respect to water, the condensate methods of foreLasting cirrus withcomments immediately freezes. The crystals then often regarding their limitations and/or value.Also, descend slowly to levels of -30°C. andpersist refer to chapter 4, Aerographer's Mate3 & 2, fora long time if the humidity below the NavTra 10363 -I), and ,Thapter 3 of thistraining formation level is high enough. In general,cirrus manualfor additional background of cirrus isfoundin layers which have saturationor types and the physics of formation. supersaturationwithrespecttoice(atany temperature colder than 0 °C':if therelative humidity with respect to water is IOU OBSERVATION AND percent, FORMATION OF CIRRUS then the relative humidity withrespect to ice is greater than 100 percent). Cirrus (or "cirriform") clouds Present radiosonde humidity elements are convention- are far ally divided into three general fromcapable of measuring humidity values groups: cirrus satisfactorily at the temperatures of the (proper). cirrostratus, and cirrocumulus.The cirrus latteris a relatively rare cloud and levels. However. the elements will often show may be a ignoredfor practical purposes. Cirrus change inhumidity at low temperature that proper reflectthe (detached or patchy cirrus) does not usually presence of moistlayers which create a serious operational problem. Cirrostra- contain cirrus. tus and extensive cirrushaze, however, Several studies have pointed to indirect indi- a;-!. troublesome inI.zghlevel jet operations, for cations of cirrus presence whenever the dew- aerial photography, interception, rockettrack- point depression at 500, 540, and 400mb was ing, and guided missile navigational relatively low. systems. Therefore, adefinite requirementfor cirrus A study published by the U.S. Air Force has forecasting exists. No universally applicablenor shown how contrail forecastingcurves can be completely successful forecasting method has

330 6 Chapter 10CLOUDINESS, PRECIPITATION, AND TEMPERATUREFORECASTING been found, though several methods hve been Horizontal visibilities within extensive cirro- advanced which seem to have some promise of stratus over middle latitudes are generally be- success. One such method is presented in this tween 500 feet and 2 miles. But even thin cirrus chapter. haze, invisible from the ground, often reduces the visibility to 3 miles. Burton's empirical rule The initial formation of cirrus normally re- for forecasting the visibility has been successful quires that cooling take place to saturation with for the Arctic cirrus and may work elsewhere. respecttowater and to temperatures near Captain M. W. Burton of the Air Weather -40°C. Under these conditions, water droplets Service devised a rule of thumb for forecasting are first formed but most of themimmediately or estimating the visibility of one aircraftfrom freeze. The resulting crystals persist as long as anotherinthincirrusor other high cloud the environmental humidity remains near ice (temperaturesbelow -30°C).This1,deis: saturation:, a deep ice-saturated layer usually Visibility = 1/2 mile times dewpoint depression exists just below the cirrus formation level, in degrees C. For example: Temperature is which permits long cirrus streamers to descend -35°C, dewpoint is -38°C, then visibility = 1/2 (slowly) to lower levels and to persist for many X 3 = 1 1/2 miles. This rule has been used hours (or even perhaps days) before evaporating. successfully in the Arctic where poor visibilities There is some evidence that the 'speed of the inapparently cloudfreeairare often en- cooling and the kind and abundance of freezing countered. Its applicability elsewhere remains to nuclei may have an important effect on the form be tested. and occurrence of cirrus. Slow ascent (cooling) The climatology of cirrus occurrence is un- starts crystalization on specially favorable freez- satisfactory because the usual ground observa- ing nucleiathumidities substantially below tions (in humid climates) miss more than 50 saturation with respect to water; this is presum- percent of the true frequency or amount, and ably the case in extensive cirrostratus associated aircraft observations are very scarce. with warm frontaltostratus.If slow ascent Radar cloud detection sets appear to promise occurs in air having insufficient freezingnuclei, a much more complete cirrus observations than widespread haze may result which even at -30° possible from visual observers. to -40°C is predominantly of waterdrops. In A cirrus forecasting method, whether empiri- the case of more rapid cooling (ascent), there is cal or with a physical hypothesis, should exploit a tendency for the first condensation tocontain what is &ready known about cirrus in relation to a higher proportion of water drops,leading to a other forecastable parameters. A considerable "mixed cloud"(iceandwater) which will number of such parameters have been tried in convert toice or snow in time. Presumably, the course of past studies, in addition to pure dense cirrus, fine cirrus, cirrocumulus, and anvil extrapolation of the movement of cirrus cloud cirrus are of this rapid ascent type; it has been (neph) systems. Except for the use of the postulated, for example, that fine cirrus (proper) surface pressure pattern, these are all upper air is formed in shallow layers undergoing rapid parameters. Most of them were tried or chosen convection due to advection of colder air at top on the generally acceptedhypothesis that cirrus, of shear layer. like other clouds, forms where there is sufficient On the other hand, the fine cirrus and the large scale vertical motion and the initial humid- cirrostratus are so often associated, and cirro- ity is relatively high. stratus is so often reported by pilots as develop- Most of the various parameters chosen in the ing from the merging of fine cirrus streamers, more empirical procedures which have been used that there is question whether the process of or proposed for cirrus forecasting canbe given a formation in cirrus and cirrostratus is essentially rationalit erpretation as having indirect rela- different.Nevertheless,theprevailingcrystal tions to humidity or vertical motion. For ex- types in cirrus and cirrostratus seem todiffer, ample, the models of frontal, pressure, contour, though this may not be universal or may merely and wind patterns associated with cirrus are in represent different ages in a characteristic cirrus this category. Such methods are very subjective considerable experience for evolution. . and depend on

331 AEROGRAM IER'S NIATE 1 & C

success, But other hypotheses, such as a relation the fine cirrus seen ahead of (andabove) warm of the temperature.or the lapse rate, or the fronts or lows initially forms detached wind shear, to cirrus from the occurrence or height, do frontal middle cloud shield, thoughlater it may not seem to find much convincing statistical traildownward to join the altocumulus support nor a clearly defensible physical basis. and altostratus, One study reveals that withprecipi- Statistical techniques for local cirrus forecasting tation occurring in advance ofa warm front. a tried thus far have not been too encouraging. 60 percentprobabilityexiststhat but with better data and cirrusis more experience with occurring above this zone. Cirrus observedwith other techniques further trials of thissort should the cold front cloud shield either originates be worthwhile. from cumulonimbus along and behind thefront or front convergence aroundan associated upper CIRRUS FORECASTING PROBLEM trough. In many cases there isno post cold front Only a limited number of methods offore- cirrus, Probably due to marked subsidencealoft. casting cirrus type clouds havebeen developed Squall lines can also produce much anvilcirrus and published. Obviously, this fieldIs open to which spreads out in advance andpersists after further investigation. testing. andstudy. Many the cumulonimbus dissipates. forecasters have attempted to forecastcirrus or high clouds using familiar frontalor cyclone Contour or Flow Direction Aloft models. None of these methodsor models alone haveprovenoperationallyadequate, though Studies in the past relating to motions of several of thespecialmethods show some cirrus and the ensuing motions of surfacelows success or appear promising for further develop- and highs lead to many useful rulesthat have ment. There are a number of parameters,both been familiar to forecasters for generations. surface and aloft, which have beencorrelated Data on the flow direction aloftare scanty, with cirrus occurrenceor formation. A few of and direction of the wind aloneappears, by the more prominent onesare mentioned here. itself, to be a poor parameter. Therefore,direction must be considered along with otherpara- Surface Pressure Pattern meters at upper levels. When weather forecastingwas based solely on However, one rule which should be of valueas surface iosbaric analysis. many meteorologists revealed by severalstudies showed thatthe attempted to find an empirical statisticalrela- percentage of high cloud is greatest when the tionship between the directions and speeds of wind at 300 mb is from the southwestto west and least when from the northeast to cirrus in advance or depressions andthe subse- east. The quent weather. With the advent of the Nor- inverse is true for no high cloud relation.No wegian School of synoptic meteorology.frontal relation with wind speed was ev:dent. and cyclone models are developed whichem- bodied the associated idealized cloud distribu- Contour Patterns Aloft tion. In these models the cirrus thickening and lowering into altostratus is a characteristic se- One of the forecasting rules used widely quence in an advancing warm front. The cirrus in the United States ('or several decades of fair weather outside the cyclone cloud or more shields states that the ridge line at 20,000 feet (about is neither identified nor accounted for in the 500 mbs) preceding awarm front marks the Norwegian models. Itis well known that much forward edge of the cirrus cloud cirrus is observed which has no obvious relation sheet. This undoubtedly refers to the edge of the solid to any features analyzed on surface synoptic charts. cirrostratus-altostratusovt..,ast.Itprobably does not include fine cirrus, which wouldeither Fronts Aloft be higher and/or would not evaporateimmedi- ately in the lee of the ridge. Above 500 mb the usual concepts of air Commanders Wolff and Somervell of Project masses and fronts have little application. Most of AROWA devised a set of rules of thistype in

332 Chapter 10 CLOUDINESS, PRECIPITATION, AND TEMPERATURE FORECASTING which both surface and 500-mb patterns are Cirrus in Relation considered. For a typical 500-mb wave pattern, to the Jetstream they state: 1. No extensive cirrostratus will occur before A discussion of cloud types associated with the surface ridge line arrives. the jetstream is contained in chapter 4 of this 2. Extensive cirrostratus follows the passage trainingmanual.1 lowever, afew additional of the surface ridge line. observations and relations are presented here, 3. No middle clouds appear before the arrival All of the studies made in this relation agree that of the 500-mb ridge line. most of the more extensive and dense cirrus is 4. Middle clouds tend to obscure the cirrus on the high-pressure (right, or south) side of the jet axis. Much of the largely observed frequency after the 500-mb ridge line passes. of high clouds well north of the jet axis can When the 500-mb wave is rather flat the cirrus probably be accounted for as the upper reaches arrivalis delayed and the cloud is thinner. The of cold frontal systems or cold lows not directly greater the 500-mb streamline convergence from connected with the jetstream. In some parts of a trough to ridge, the more cirrus between the trough, this high cloud may tend to be dense surface and 500-mb ridge lines. and in other parts thin or scattered.

Cirrus in Relation A CIRRUS FORECASTING PROCEDURE to the Tropopause The following technique was originally devel- Experiences of pilots of high-flying aircraft oped by F. Singleton and B.G. Wales-Smith of have confirmed the earlier theory that the top, the British Meteorological Service and was pub- of most cirrus are at or below the tropopause. In lished under the title, A Note on Cirrus Fore- midlatitudes the top of most extensive and thick casting, in the April 1960 issue of The Meteor- cirrus layers is at or within several thousand feet ologicalMagazine,No.1,053, Vol.89. An of thepolar tropopauseheight, only some abbreviated method of using this procedure was patchy cirrusis found between the equivalent developed bytheAir Weather Service, 2nd polar tropopause height and the tropical (high) Weather Wing, Forecasters Bulletin C-13, and tropopause. A small percentage of cirrus cases was published in Armed Service Technical Infor- (inc:ading sometimes extensive cirrostratus) is mation Agency Bulletin, AD 252903. Credit is observed inthe lower stratosphere above the given to both sources for permission to use the pair tropopause, up to 50,000 feet, but mainly material in the following section. below the level of the jetstream core. The cirrus Itmust be realizedthatthisis only one of the equatorial zone also generally extends to among several techniques available. It was se- the tropopause. There is a general tendency for lected for its simplicity and ease of application. the mean height of the bases to increase from However,alltheother rules and empirical high to low latitudes more or less parallel to the knowledge should be applied simultaneously to mean tropopause height, ranging from 24,000 obtain the forecast. feet at 70u-80° latitude to 35,000 to 40,000 British forecasters have used as an objective feet or higher around the Equator. The thickness means of forecasting cirrus, worksheets consist- of individual cirrus layers (the clouds are often ing of 13 questions for making a 6- to 9-hour multilayered)is most frequently 800 feet in forecast, and 6 questions for making a 24- to midlatitudes, some cases, however, range up to 36-hour forecast. Although worksheets are based 10,000 feet or more with the average cirrus on work done attheBritish Meteorological zones about 6,500 feet. The mean thickness of Office, the parameters used should hawuni- cirrus affected zones tends to increase from high versal application inthat they are based on to low latitudes. In polar continental regions in generally accepted rules and theories on cirrus winter,cirrusvirtually comes down tothe forecasting. ground. In midlatitudes and in the Tropics, there A recent investigation has shown that answers is little seasonal variation. to just 3 of these questions not only provide a

333 AEROGRAPIIER'S MATE I & C

goodbasisfora "Cirrus" or "No Cirrus" forecastof thelesser amount of cirrusis forecast but also may be usedto obtain an suggested. indication of the expected high cloudcover. Step D. Answer the questions in Part C of the The technique utilizing the 3 questions isvalid worksheet. If four or more of the questionscan as far ahead as a satisfactory synoptic forecast be answeredaffirmatively,thenthe greater can be made. We will assume that it applies to a amount of cirrus is indicated. If not, thena forecast period of 24 to 36 hours. forecast of the lesser amount is suggested. Itisthe purpose of this modification to By applying the three basic questions in the combine the worksheets and thetechnique. outlined manner and by supplementing the utilizing only 3 questions intoa single forecast- resultingforecast where necessary with the ing. procedure. remaining questions of the worksheet,a cirrus The procedure is covered in the following forecasting method is obtained which is believed four steps: to have considerable merit. Step A. Answer the 3 questions in PartA of The authors note that the criteria employed the combined worksheet( figure10 -2 1). The arenot adequate for forecasting convective questions are answered from prognostic charts cirrus and that no conclusionscan be drawn as for the 24- to 36-hour forecasts and byextra- to the validity of the technique for this purpose. polating from current charts for the 6- to 9-hour forecast. For each question give a "score" of 0, PREDICTION OF SNOW VS RAIN 1/2, or1, according to whether theanswer is "No." "Uncertain." or "Yes." With thescore The problem of predicting snowvs rain is, thus obtained. go to Step B. however, important inits own right because Step B. Enter figures 10-22 and 10-23 wit:: .any decisions of great operational importance thescore obtained in Step A. Figure10-23 may hinge on the forecast. Ordinarily, an inch or shows cirrus amounts expressed in cumulative st, of precipitation in the form of rain will cause percentage us a function of the score. Figure no serious inconvenience. On the other hand, 10-23 indicates the most likelyamount of high the same amount of precipitation in the form of cloud to be forecast for a given score. Asan snow (or sleet or freezing rain) can seriously example of the use of the figures,a score of I , interfere with transportation, communications, obtained in Step A. indicates that themost and other naval operations. In suchcases the likely amount of cirrusis2 oktas while a snow-rain problem becomes afactor of the forecast range of 0-3 oktas will be correct 61 greatest operational significance. percent of the time. This and other examples are listed in table 10-1. This section is concerned chiefly with the forecasting of snowvs Examination of table10-1 shows that for rain, assuming that scores of 0-1 precipitation can be correctly predicted. The and 2 1/2 -3,fairlydefinitive intermediate elements, sleet and freezing rain, forecasts are obtained. For these scores, the which often occur in the boundary procedure stops here. zone between snow and rain, are generally grouped with But with a score of 1112 or 2 the forecast is snow in most of the treatment here. However, indeterminate. For example, a score of 2 indi- one objective technique explains a method of cates a forecast range of 0-4 oktas is likely on 51 differentiating between the several types. Speci- percent of occasions and a range of 2-7 oktas is fically this section deals primarily with forecast- likely on 48 percent of occasions. In orderto ing techniques which bear directlyupon the decide between these .alternatives, we proceedto snow-vs-rain problem in the United States. The Step C for a 24- to 36-hour forecast or to Step D relationships, however, should have a worldwide for a 6- to 9-hour forecast. application insofar as the parameters thatare considered. The values, or course, would have to Step C. Answer the questions in Part B of the be modified in accordance with your geograph- worksheet. If one or more of the questionscan ical location. This should be easily accomplished be answered affirmatively,thenthe greater through a local study of the optimum condi- amount of cirrus is indicated. If not, then a tions. The various techniques and systems given 334 340 Chapter 10 CLOUDINESS, PRECIPITATION, AND TEMPERATUREFORECASTING Table 10-1. -Optimum amounts of cirrus to forecast for "score"0-3 and examples of expected percentage of correct forecasts. Correct forecasts Optimum amount "Score" (oktas) Range in oktas Percent

C 0 0-1 66 62 1/2 1 0- 2 1 2 0-3 61 1 1/2 3 0-3 50 2 4 0-4 51 2-7 48 2 1/2 5 5-8 53 3 6-'7 5-8 . 59

PART A answer the question, Is the dewpoint depression less than or equalto10° C, at or above 500 mb?). (24-36 hour forecast period. Use prognostic charts) 2. Willthe area be ina thermal ridge as suggested by the 1,000-500 mb thickness prog? The three questions to be answered initially are: 3. Will the 300-mb wind over the area veer from the 500-nib wind by 20° or more? 1. Is the forecast area in or just to the rear of a ridge in the 200-mb (300-mb)* contour pattern? PART C 2. Is the forecast area on the anticyclonic side of a 200-mb (300-mb) jetstream and within 300 miles (6-9 hour forecast period.Extrapolate from of the jet axis? current charts) 3. Istheforecast area less than 300 miles ahead of asurface warm front or occlusion? 1. Isthedepression of dewpoint below air *QuestionsI and 2 in Part A were evaluated with temperature at 500 mb less than or equal to 10°C? 300-mb data originally but with 200-mb data in the 2. Isthedepression of dewpoint below air abbreviated method.It is a reasonable assumption temperature at 450 mb less than or equal to 10° C? that either chart may be used since a ridge position 3. Isthe depression of dewpoint below air or anticyclonic shear will normally appear at the temperature at 400 mb less than or equal to 10° C? same location on both charts. 4. Isthelapseratein the 500- to 300-mb layer greater than the wet adiabatic? PART B 5. Is the 400-mb wind between SW and NW? 6. Is three a veer of wind between 500 and 300 (24-36 hour forecast period. Useprognostic charts) mb of 20° or more? 7. Is the 1 ,000 -500 mb thermal wind greater than 1. Is the air likely to be moist?(Using the 20 KT? 500-mb progged contours and latest available actual 8. Is the forecast area in a ridge in the 1,000- chart, the air at this level should be traced back, 500 mb thickness pattern? from the area at the time for which the forecast is 9. Is there anticyclonic curvature of the 1,000- required, to the region of an available radiosonde 500 mb thickness lines? ascent.By examining the depression of dewpoint 10. Is there a deep cold pool or intense thickness below temperature at the levels L00, 450, and 400 mb, trough inthe 1,000-500 mb thickness pattern?

AG. 595 Figure 10-21.-Cirrus forecasting worksheet.

335 341 AEROGRAPI1ER'S MATE I C

GEOGRAPHICAL AND SEASONAL 100 -----71_, CONSIDERATIONS .140I 1 I I so ,.... 0 i I I The forecasting problem of snow vs rain arises .' ,0II I AI ... 0 i I .0 .. / during the colder portion of the year. However, an80 , Ii 2 , + °I 0 . I I Ii in midwinter when the problem is most serious ,IedI I II / °I ;7) , Ie , I 5 in the Northeastern and North Central States, 7 .. I 4 5 %1;2>' " 0' o' some other regions of the United States are not 0 II, o 1 ii I/ atall concerned. For example. in the Rocky 6 II / I it I' y/ I Mountain region snow is nearly the exclusive I . , fi I type of precipitation in midwinter. Along the -a5 I t4. ,-'' ./. I/ / 1,... I Gulf of Mexico and the Pacific coasts (excluding I ,,. . t /I . Io the Sierra Nevada and Cascade Ranges), snow is , .I ,I 43,' t I --1" .."9 rare. Other areas of the world experience similar i , ,I,,-- / ,:,, 0:i----- problems. , , 1 ,,.._.---- 31r / A set of figures has been reproduced in the ,I1, / ,,',,,,,, U'S. Department of Commerce's Snowvs Rain 2 1-1:'-, publication which demonstrates the geographic P'',',/ and seasonal variations of snowvsrIttil.These ; figures show the normal monthlv snowfall in inches andthe total melted precipitation in inches. as well as the ratio between snowfall and 2 3 4 5 6 7 OKTAS OF CIRRUS total precipitation. These data were obtained CUMULATIVE PERCENTAGE OF CIRRUS AMOUNTS FOR: FORECAST "SCORES" 0,1/2,1, 1-1/2,2, 2.1/2, 3 ....

AG.596 Figure 10-22.Cumulative percentage of cirrus amounts for forecast "scores" 0, 1/2, 1, 1/2, 2, 2 1/2. 3. 6 I here* often complement each other, and the alert forecaster should be aware of the application of all 5 methods. The approachused hereisa ... discussion of the general synoptic patterns, the .0 thermal relationships (that is, the Ilse of tem- 4 peratures at the surface and aloft in separating cases of rain from those of snow), and the I presentation of an objective technique to dis- 3 / tinguish the types of precipitation. The material in this section of the chapter was 2 derivedchieflyfrom two publications: The / Prediction of Snow vs Rain, U.S. Department of Commerce Forecasting Guide No. 2; and Fur- 1 ei ther Studies in the Development of Short Range or( Weather Prediction Techniques, GRD Contract 1/2 I No. AFI 9(604)-2073, Scientific Report No. 1. I-1/2 2 2 -/2 3 A more complete study of these and other FORECAST "SCORE" publications should immeasurably improve your AG.597. abilitytodifflentiate betweenfrozen and Figure 10.23. Average amounts of cirrus observed liquid forms of precipitation in your forecast. plotted against forecast "score."

336 342 Chapter 10- CLOUDINESS, PRECIPITATION, AND TEMPERATURE FORECASTING from the National Weather Service. Local Clima- The ratio charts are intended to show in a tological Data for various stations. It is assumed rough way the frequency with which the snow- for convenience that on the average I inch of rain problem occurs at various places. In spite of snow yields about 1/10 inch of melted precipita- the crudeness of the scheme, meteorologists tion, then an "all-snow" station would have a familiar with the occurrence of borderline type ratio of 10 a station with 50 percent of its precipitation have noted that the charts present precipitation as snow a ratio of 5, and a "no a reasonable picture. Figure 10-24 illustrates one snow"stationaratioof0.Itshould be of these charts for the month of December. mentioned that recent studies indicate the ratio You can readily see that by utilizing this type of snowfall to its water equivalent may average chart you would not only have an indication of as much as 12 or 13, even when temperatures the months and periods of greatest snowtall, but are notfar below freezing. The "all snow" the percentage factor would give you a good stations may have ratios in these figures which indication of the likelihood of snow. are in excess of 10. Due to the local influence of bodies of water PHYSICAL NATURE OF THE PROBLEM and the effects of altitude, interpolation between the plotted data on the charts must be The type of precipitation that reaches the done with care. ground in a borderline situation is essentially

AG.598 Figure 10-24.Ratio of normal monthly snowfall to normal total precipitation for the month of December. Large figures to the left of the station circle is the ratio, figure to the upper right is normal snowfall in inches for the period of record (1921.1950), and figure to the lower right is normal total precipitation in inches for the period of record. Regions where ratios are between 2 and 8 are shaded. 343337 AFROGRAPIIER'S MATE I C

dependent upon whethei time isa!aye' of In situations where precipitation is occurring above-freezing temperatures between the giound assoLiation with a Loki upper low, upward and the levels at which precipitation is forming. motion is acLompanied by littleif any warm and whether this layer is suffiuently deep to adveLtion. In such borderline cases. precipitation melt all of the falling snow. Thus. the corrcLt may persist ,is snow. or will tend to turn to snow prediction of rain or snow at a en en loLation if it started as rain, due to cooling resulting from depends largely upon the accuracy with which upward motion or advection. thevertical distribution ofthe temperature. especially the height of the freezing level. can be Nonadiabatic Effects predicted. On the average. it is generally satisfactory to assume that the freezing level must be at The most important of the nonadiabatie least 1.200 feet above the surface to insure that effectsisevaporational cooling which takes most of the snow will melt before reaching the Male as precipitation falls through unsaturated ground. air between the clouds and the ground. This At the present state of forecasting capabili- effect is especially pronounced when very dry ties, it is difficult to predict the entire sounding airispresentinlowlevelswith wet-bulb temperatures at or below freezing. Then, even if or even the height of thefreezing levelfor periods of more than a few hours in the future. the dry-bulb temperature is above freezing in a layer deeper than 1,200 feet in the lower levels, For periods of 12 to 36 hours, it is necessary to the precipitation may stillfall as snow. since dealwith much more general thermal param- evaporation of the snow will lower the tempera- eters. such as temperatures at a few selected tures in the layer between cloud and ground levels or mean temperatures of relatively thick until the below-freezing wet-bulb temperatures layers. Thesewillbe discussed laterin this are approached. chapter. Thefollowing isareviewof the The actual cooling which is observed during processes thatinfluencelocal temperature the period when evaporation is taking place is changes. particularly those playing a critical role often on the order of 50 to 10°F within an hour in the snow-rain problem. ortwo.Afterthelowlevelairisnearly saturated, evaporationpracticallyceases and Effects of Advection advection brings a rise in the temperature in the low levels. However. reheating often comes too Inthelower troposphere (away from the late to bring a quick change to rain since the immediate surface), horizontal advection Is usu- temperatures may have dropped several degrees allythe domimmt factor influencing the local below freezing. much snow may have already temperature change. In most preupitation fallen. and the lower levels may be kept cool particularly in the borderline situations. through the transfer of any horizontally trans- warm air advection and upward motion are both ported heat to the colder snow covered surface. occurring so that warming is generally expected to accompany precipitation. These influences Melting of Snow operate in the direction of turning the precipitation to rain. even if it starts as snw.v, however, Melting of snow to rain in its descent through this effectisfrequently offset when there is Layers which are somewhat above freezing is weak warm advection or ':wen cold adveLtion iu another process 1.vhicli may cool the air in the thecoldairmassinthe lower layers. The lower troposphere. To obtain substantial tem- maintenance of hublreezme. air in layers near the perature changes due to melting, it is necessary surface retards the effects of warmiog aloft. and to have rather heavy amounts of precipitation evenif the temperature of NigniitLant layer falling and little warm air advection. As cooling aloft (aboui 1.200 feet or more thick) rises to proceeds far enough, the temperature of the values above freezing due to advection. there entire lower sounding will read' freezing so that may be a prolonged period of sleet or freezing a heavy rainstorat can transform into it heavy rain before the precipitation turns to rain. snowstorm.

338 344 Chapter 10 CLOUDINESS. PRECIPITATION. AND TEMPERATURE FORECASTING

Cases of substantial lowering of the freezing considerations, such as the exact track of the level due to melting are relatively rare. This is surface disturbance. whether the windata probably due to the fact that the combination coastal station is east or northeast, the position of heavy t..in and little of no warm advection of the warm front, and the orientation of a ridge requiredfor cooling duetoinchingisan northeast of a low. The larger synoptic features infrequent occurrence. do, however, determine the approximate position of the snow-rain zone. An awareness of this Combined Effects serves as an alert co the forecaster and is a useful The combined effects of horizontal tempera- tool in forecasts extending beyond 24 hours. ture advection. vertical motion. and cooling due In the larger sense, the snow-rain zone is tied to evaporation are well summarized by observa- to the position of the polar front. The polar tions of die behavior of the bright band (melting front location is in turn closely related to the layer) on radar. R. Wexler has observed that position of the belt of strong winds inthe within the first I 1;2 hours after the onset of middle and upper troposphere. When the wester- precipitation, the blight band lowers by about lies are depressed southward. the storm trick is 500 to 1.000 feet. This is attributable primarily similarly affected and the snow-rain zone may to evaporational cooling and probably second- be as far south as the Southern United States. As arilyto melting. Since evaporational cooling the westerlies shift north of their normal posi- terminates as saturationis reached. warm .tir tion, the storm tracks develop across Canada. advection (partiallyoffset by upward motion) Concomitantwiththis northwardshift.the again becomes dominant and the bright band United States has above normal temperatures. rises back to near its original level in about 3 and the snow-rain problem may exist only along. hours after the onset of precipitation and may or north of. the Canadian border. rise a levy thousand feet more the end of 6 to 8 With afiat.fastwesterlyflow, aloft. the hours. snow-rain zone will extend in a narrow westeast Other nonadiabatic effects. such as radiation belt often well ahead of the surface perturbation and heat exchange with the surface. probably and will undergo little latitudinal displacement play a relatively smaller role in the snow-rain a:, the perturbation moves across the country. problem. However. itis likely that a difference Usuallythereisverylittlerain, and snow is inthe state of the underlying surface (snow found immediately north of the warm front. covered land vs open water) could determine Most of the stationsatwhich precipitation whether the lower layers would be above or occurs will not undergo a change from one type below freezing at a place in a particular precipi- of precipitation to another, since there is rela- tation situation. Occasionally along a seacoast in tivelylittle advection of warm or cold air with winter, heat from the %lien water keeps tempera- rapid zonal motion. tures offshore above freezing in lower levels. When the upper level wave is of large or Along the east coast of the United States. for increasing amplitude. itis difficult to generalize example. immediate coastal areas may have rain. about the characteristics of the snow-rain prob- while a few miles inland. snow predominates. lem without considering in detail the surface This is associated with low level onshore flow perturbation. whichis typical of many east coast cyclones. Up to now inthis section. the snow-rain Actually this cannot be classified as a purely pattern has been discussed in association with an nonadiabatic effect relative to the land Station at active low of the classical type. The rate of which rain rather than snow is occurring. since precipitation accumulation here is rapid and the the warmer ocean air is being advected on shore transition period of freezing rain or sleetis and over the land involved. short, usually on the order of a few hours or less. Another situationin which there isfre- GENERAL SYNOPTIC quentlyasnow-rainproblemisthatof a CONSIDERATIONS quasi-stationary frontin the Southern States The location of the snow-rain zone usually with a broad west-southwest to southwest flow depends uponrelativelysmall-seale synoptic aloft and a weak surface low. The precipitation

.)/1314) t.)9ce-I AEROGRAPIIER'S MATE I & C

area in this case tends to become elongated in ServicePublication. Catalogue ofPredictors the direction of the upper level current. The Used in LocalObjectiveForecastStudies, precipitation rate may be slow, but occursover a AWSTR I 05- I 9. longer period Often a broad area of sleet and A number of methods based on synoptic flow freezing rain exists between belts ofsnow and patterns applicable to the United States are rain, leading to a serious icing condition overan describedintheU.S. Department of Com- extensive region for a period of several hoursor merce's publication. The Prediction of Snowvs more. This pattern of precipitation changes Rain. Forecasting Guide No. 2. These methods either as an upper trough approaches from the are mostly local in application and are much too west and initiates cyclogenesis on the front, or detailed to be presented in this training manual. as the flow aloft veers and the precipitaiton dies Prognostic charts from the National Meteor- out. ological Center and other sources should be utilized whenever and wherever available, not FORECASTING TECHNIQUES only to determine the occurrence and extent of AND AIDS precipitation. butfortheprediction of the applicable thermal parameters as well. Approaches to the snow vs rain forecasting problem have generally fallen into three broad Methods Employing Local categories. The first group depends on the use of Thermal Parameters the latest observed flow patterns and parameters derived therefrom to predict the prevalent type Inthis section we will discuss methodsem- of precipitation for periods as much as 36 hours ploying surface temperature, upper level tem- in advance. The second group consists of studies peratures, thicknesses, the height of the freezing relating local parameters to the simultaneous level. and the forecast using combined param- occurrence of rain or snow at a particular station eters.All of these parameters are interdepen- car area.Inthis approach. itis assumed that dent and should be considered simultaneously. predicted values of the thermal parameter will SURFACE TEMPERATURES. -The surface be obtainable from circulation or other prog- temperature by itself is not an effective cri- noses. Naturally, this approach tends to have its terion.Itsuse in the snow-rain problem has greatest at-curacy for periods of I 2 hours or less, generally been used in combination with other since longer period temperature predi_tions tor thermal parameters. One study for the North- the boundary zone lwtween rain and snow are eastern United States found that, at 35°F, snow very difficult to make with sufficient precision. and rain occurred with equal frequency and by A third approach used here involves the use of using 35°F as the critical value (predict snow at one of the many objective techniques available. 35 °F and below, rain above 35°F). 85 percent A number of stations have developed objective of the original cases could be classified. Another techniques which arc chiefly local in application. study ba' :d on data from stations in England The method presented here is applicable to the suggested a critical temperature of 34.2°F and eastern half or the United States. Thus, the found that snow rarely occurs at temperatures general procedure in making the snow-rain fore- higher than 39°F. However, itis obvious from Last at present is to use a synoptic method (a these studies that even though surface tempera- purely subjective evaluation of the situation will ture is of sonic general use in separating rain have to suffice if an objective method has not from snow, itis an inadequate discriminator in been developed for the area in question) for crucialcases.Thus, most investigatorshave periods up to 24 to 36 hours ahead, and then to looked to upper level temperatures as a further consider expected behavior of thermal param- aid to the problem. eters over the area of intarest to obtain more UPPER LEVEL TEMPERATURES. Two precision for periods of about 12 hours or less. studies of the Northeastern United States found The third step would be to employ any local that temperatures at the 850-mb level proved to objective technique. For a listing of some of the be a good separating parameter and that includ- techniques available, consult the Air Weather ing the surface temperature did not make any

340 346 Chapter 10 CLOUDINESS, PRECIPITATION, AND TEMPERATURE FORECASTING significant contribution. The separating tempera- A more generahzed study of 1,000-500 mb tures at 850 mb were2° to -4°C inclusive. thickness as a predictor of the precipitation type Another study by J. 1'. Ifilworth found that the in the United States was made by A. J. Wagner in area outlined by the 0°C isotherm at 550 mb 1957 on an Air Force GRD Contract. More and the 32°1- i,othenn on the surfaLe chart, complete details on this study may be found in when superimposed upon the precipitation area, The Prediction of Snow vs Rain, Forecasting separated the types of precipitation in a high Guide No. 2. percentage of cases. The author's experience Wagner's study was taken from data from 40 withthis technique reveals that lower values stations over the United States for the colder should be used along coastal areas (in the -2° to months of a 2-year period. Cases were limited to -4°C category) and also behind deep cold lows. surface temperatures between 10°F and 50°F. At mountain stations some high level would The type of precipitationin each case was have to be used. considered as belonging in one of two categories, A technique that utilizes temperatures from a as follows: Frozen, which includes snow, sleet, standardisobaric surfaceis advantageous be- granular snow, and snow crystals: and unfrozen, cause these charts are usually available in the whichincludesrain,rainand snow mixed, forecast office. There is, however, the difficulty drizzle, and freezing rain and drizzle. thattemperatureinversionsare oLLasionally Equal probability or critical thickness values located near the the 850- or 700-mb levels, so were obtained from the data at each station. that the temperature of one level may not be From this studyitisclear that the critical indicative of any relatively deep layer of the thickness increases with increasing altitude. This atmosphere. This difficulty Lan be overL.ome by altitude relationship is attributable to the fact using thickness, which is a measure of the mean that a sizable part of the thickness layer is temperature or the layer. nonexistent for high-altitude stations and obvi- THICKNESSES. Thickness forecasting in re- ouslydoesnotparticipateinthemelting lation to snow-rain studies has been used for a process. In order to compensate for this, the number of years. The National Meteorological equal probability thickness must increase with Center has examined both 1,000-700 mb and stationaltitude. For higher altitude station's 1,000-500 mb thickness limits for the eastern thicknesses between 850-500 mb or 700-500 hall of the United States. The critical values mb, as appropriate, should prove to be better used are 9200-9,400 feet (2.805-2,865 meters) related to precipitation type. fortheI .000-700mbthicknessand The Wagner equal probability chart is repro- 17,600-17,800 feet (5,365-5,425 meters) for the duced in figure 10-25. 1,000-500 in'o thickness. Also it has been noted Wagner's study also indicates that the type of thatwherethe1,000-500 mb thicknessis precipitation can be specified with a certainty of 17,200 feet (5,243 meters) or less, the snow is 75 percent at plus or minus 100 feet (30 meters) more likely to be in the form of snow flurries from the equal probability value, increasing to than to be continuous snow. 90 percent certainty at plus or minus 300 (90 Two studies in the United States, one for Fort meters) feet from thisvalue. Stability is the Riley, Kansas, and the other for Mitchell Field, parameter that accounts for the variability of New York, revealed that the critical values or precipitationfor a given thicknes ata given equal probability values for the 1,000-700 mb point. This fact is taken into account in the thicknesses were 9,350 feet (2,850 meters) and following manner. For example, if the forecast 9,250 feet (2,820 meters), respectively. Another precipitationis due to a warm front which is study for Washington, D.C. suggested an apper much stronger (more stable) than usual, the line limit of 9,350 feet for snow cases. The value of separatingrainfromfrozen precipitationis 9,300 feet for the eastern hall of the United shifted toward higher thickness values. Over the States appears to be a good average value of Great Lakes, where snow occurs in unstable or equal probability for the 1,000-700 mb thick- stable conditions (frontal), the equal probability ness. thickness islower than that shown in figure

341 347 AEROGRAPIIER'S MATE I & C

176

MAP SHOWING 1000-500MB THICKNESS VALUES FOR WHICH PROBABILITY OF RAIN OR FROZEN PRECIPITATION IS EQUAL. [VALUES ( ) METERS]

AG.599 Figure 10-25.Map showing 1,000. 500-mb thickness values for which probability of rain or frozen precipitation is equal (after Wagner).

10-25 for snow showers. and higher than that It was pointed out earlier in this section of the shown in figuie 10-25 fur warm frontal snow. chapter that theoretiLal and observational evidence indicates that a freezing level averaging In using this technique over the United States. 1.200 feet or more above the ground is usually the current transmission of thickness progs can needed to insure that most of the snow will melt be utilized. Since shorter period forecasts of before reaching the ground. This figure of 1,200 thicknesses are more accurate, itis likely that feet can thus be considered as a critical or equal the results of these studies of thickness related probability value of the freezing level. in prac- to precipitation type will be of thegreatest tice, since prediction of the freezing levelis assistance in snow-rain prediction for periods of rather difficult, even for a short period of time 12 hours tfl- less. ahead, it has rather limited value as a predictor IIEIGI IT OF THE FREEZING LEVEL. The of rain vs snow. height of the freezing level above the surface Is COMBINED THERMAL PARAMETERS. one of the most Laltik.,11 thermal parameters in From the foregoing disLussion, itis concluded determining whether no.an read] the ground. that no one method, when used alone, is a good

342 3 4 OS Chapter 10 CLOUDINESS, PRECI PITA [ION. AND TEMPERATURE FORECASTING discruninatoi in the snow forecasting prob- side of the 0°C isotherm with intermediate lem. Therefore. it is suggested that the combined types falling generally within the enclosed area use of the surface temperature. height of the between these two isotherms.It was further freezinglevel. 850-mb temperature,indthe realized that in a large majoiity of situations, 1.000-700 and; or 1,000-500 mb thicknesses be ev,ipoiationand condensation wasasizable made to arrive at the forecast. There is generally factor, both at 850 mb and at surface levels in a high correlation between the 850-mb tempera- its effect upon temperature. With this in mind, ture and the 1.000-100 nib thickness and be- the wet-bulb temperature was selected for inves- tweenthe700-nibtemperatureand the tigation because of its conservative properties 1,000-500 mb thickness. Certainly an accurate with respect to evaportaion and condensation, temperature forecast lot these two levels would and also because of its ease of computation yield an approximate thickness value for dis- directlyfrom the temperature and dew point. criminating purposes. Graphs are shown in the following section. along with rules for movement of the 850-mb tem- Hilworth Method perature and 0°C wet-bulb isotherm. The sur- facechartisusedfor computations of the T; technique and the following technique 1.000-mb level since the surface chart approxi- on forecasting the area of maximum snowfall mates the1.000-mblevelfor most stations were taken from Further Studies in the De% clop- during a snow situation and therefore little error ment of Short Range Weather Prediction Tech- isintroduced. IT MUST BE REMEMBERED niques. GRD Contract No. AF19(b04)-2073 THAT ALL PREDICTIONS ARE BASED ON Scientific Report No. I by J.J. George and FORECAST VALUES. Associates. MOVEMENT OF THE 850-MB 0°C ISO- The determining factor in the type of precipi- TERM. No completely objective method of tationin this study was foundtobethe forecasting the 850-mb isotherms is available. A distribution of temperature and moisture be- reasonably good approximation can be made tween the surface and the 700-mb level at the subjectively by the use of the following rules time of beginning of precipitation. However, and by a combination of extrapolation and prediction of the sounding of this strata with advection,temperedwithsynoptic develop- any degree of accuracy was found to be quite ments. (See fig.10-26(A) and (B) for typical involved and impractical. Therefore, the median warm and cold air advection pattern at 850 mb.) level of 850 mb was studied in conjunction with the precipitation area and the 33 °F isotherm The following rules for the movement of the sketched on the surface synoptic chart. This 24-hour 850-mb temperature change areas have methodpresents anobjective.yetpractical been devised: method, by which the forecaster can make a decision on whether the precipitation in winter 1. Maximum cooling takes place between the will be rain, snow, freezing rain. sleet. or some 850-mb contour trough and the 850-mb iso- combination of these. therm ridge east of the trough. The following objective techniques can be 2. Maximum warming takes place between applied tothe land area south of 50° north the850 -nib contcmr ridgeandthe 850-mb latitude, and east of aline drawn through isotherm trough east of the contour ridge. Williston: North Dakota. RapidCity. Sc,uth 3. Changes are slight with an ill-defined iso- Dakota: Goodland. Kansas: and Amarillo. Texas. thern, andlor contour pattern. It was found that the area outlined by the 0°C 4. Usually.littlechange occurs when iso- isotherm at 850 nib and the 33 °F isotherm on ins and contours are in phase at the 850 -nib the surface chart. when superimposed upon the level. precipitation area. generally separates the ty, pes 5. The temperature falls at 850 nib tend to of precipitation. that is, most of the pure rain replace height falls at 700 nib in an average of was found onthe warm side of the 32i- 24 hours. Conversely. temperature rises replace isotherm, and most of the pure snow on the cold height rises.

343 349 AEROGRAPHER'S MATE 1 & C

10. Witheastwardmovingsystemsunder normal winter conditions (troughs at 700 mb 146 moving east aboutI I° per day), a distance of LOW 400 nautical miles to the west is a good point to locate the temperature to be expected at the 152 forecastingpoint24 hours hence. A good 850-mb temperature advection speed seems to 158 be about 75 percent of the 700-mb trough displacement. The following is a step-by-step procedure for moving the 850-mb 0°C isotherm: HIGH / I. Extrapolatefor12 and 24 hoursthe ( A ) COLD AIR ADVECTION ( 850Mb thermalridgeand trough points.If poorly defined, this step may be omitted. The amplitude of the thermal wave may be increased or LOW decreased subjectivelyif, during the past12 ocSc7itgitts hours, there has been a corresponding increase or decrease in the height of the contours at 500 +5 -- mb. 2. The thermal wave patterns will maintain 410 theapproximaterelativepositionwiththe 850-mb height troughs and ridges. Therefore, 146 HIGH the 12- and 24-hour prognostic position of the contour trough and ridges should be made and 152 158 the extrapolated positions of the thermal points cheL:ked against this contour prog. Adjustments (B) WARM AIR ADVECTION ( 850Mb) of these points should be made. 3. Select points on the 0°C isotherm that lie AG.600 between the thermal ridge and trough as fol- Figure 10- 26. Typical cold and warm air advection lows: one or two in the apparent warm advec- patterns at 850 mb. (A) Cold; (B) Warm. tion area, and one or two in the apparent cold advection area. Apply the following rules to these selected points. 6. With filling troughs or northeastward moving lows, despite northwest flow behind the a. Warm Advection Area. If the point lies trough, 850-mb isotherms are seldom displaced in a near saturated or precipitation area, it will southward, but follow the trough toward the remain practically stationary with respect to the east or northeast. contour trough. If the point lies in a nonsaturated area but one that is expected to become 7. Always predict temperature falls immedi- saturated or actually to lie in precipitation area, ately following a trough passage. then it will remain stationary or move upwind 8. Do not forecast temperature rises of more slightly to approximately the prognostic posi- than 1 or 2 degrees in areas of light or sparse tion of the 0°C wet-bulb. If the point does not precipitation in the fore-trough. If the area of fallin the above two categories, it will Advect precipitationiswidespread and moderate or with about 50 percent of the wind component heavy, forecast no temperature rise. normal to the isotherm. Note in all three cases 9. Upslope effect in the United States starts above the movement is related to the contour approximately at the 100th meridian for south- pattern. east winds. Such a pattern will result in slight b. Cold Advection Area. Advect the point 24-hour cooling where warming might otherwise with approximately 75-80 percent of the wind be indicated. component normal to it.

344 3 50 Chapter 10 CLOUDINESS, PRECIPITATION, AND TEMPERATURE FORECASTING

4. In ,!te case of a closed low at 850 mb accuracy. Known factors affecting the wet-bulb moving slowly,the 0°C isotherm will move temperature at any particular station should be eastward with respect to the closed low as cold carefully considered before entering the graph. air is adverted all the way around the low. Some of the known factors are elevation, proximity to a warm body of water, known layers of MOVEMENT OF THE 850-MB 0°C WET- BULB ISOTHERM. The wet-bulb temperature warm air above or below 850 mb, etc. Area "A" can be forecast by the above procedure and rules on the graph calls for a rain forecast, area "B" for a freezing rain forecast, and area "C" for a inageneralsense,remembering thatitis dependent upon dewpoint as well as the tem- snow forecast. Area "D" is not so clear cut. perature. The dewpoint will advect with the being an overlap portion of the graph; however, winds at nearly the full velocity, whereas the wet snow or rain and snow mixed predominate temperatureundernonsat unitedconditions in this area. Sleet occurring by itself for more than 1 or 2 hours was found to be rare and moves slower. As saturationisreached, the necessity of treatment separate from the 0°C should be forecast with caution. (See figure isotherm disappears. The following observations 10-28 for sample values and snow forecast.) with respect to the 0 ° C wet-bulb isotherm may Forecasting the Area help: of Maximum Snow I. The 0°C wet-bulb isotherm does not move far offshoreinthe Gulf andtheAtlantic, The basic intent of this section of the chapter because of convection in the cold air over warm isthe prediction of the area of maximum water. snowfall. This classification does not include 2.If the 0°C wet-bulb isotherm lies in a snowstorms which can be classified as air mass, ribbon of closely packed isotherms, movement is or purely local type storms attributableto slow. features of the terrain. In this category are those 3. Extrapolation works well on troughs and severe snowstorms which occur to the lee of the ridges. Great Lakes under more or less steady west-to- north flow of polar air. On the other hand, snow METHOD OF APPLICATION. Afterthe cases occurring in connection with active lows or forecast of the surface and 850-mb level tem- associated with areas of vertical sheer are in- perature and dewpoint values are made, you are cluded. ready to convert these values lo their respective SYNOPTIC TYPES.Four distinct types of wet-bulb temperatures. The following procedure synoptic patterns with a maximum snow area is recommended: associated with it are discussed below. BLIZZARD TYPE.The synoptic situation I.Using figure 10-27(A) and (B), compute features an occluding low pressure center. In the the wet-Lalb temperatures for the 850- and majority of cases the "wrapped around" high 1,000-mb levels, respectively. (The surface chart pressure and ridges are present. The track of the is used for the 1,000-mb level.) Admittedly, the low is north of 40° and its speed, which initially wet-bulb temperatures at just these two levels may be average or about 25 knots, decreases does not give a complete picture of the actual into the slow category during the occluding distribution of moisture and temperature, and process. In practically all cases a cold closed low erroris introduced when values are changing at 500 mb is present and captures the surface rapidly, but these are values the forecaster can low in 24-36 hours. work with and predict with reasonable accuracy. The area of maximum snowfall lies to the left 2. Enter these values on the worksheet, the of the track. At any particular synopitc position, bottom of figure 10-28. Then use the top of this the area is located from due north to west of the figure with your predicted values to obtain the low center. Within this maximum area the rate forecast. A necessary assumption for use of this of snowfall would be classified moderate (1/2 to graph is that the wet-bulb temperatures at these 1 inch per hour) in most cases. However, when two levelscan be predicted with reasonable this type occurs on the east coast with its large

345 AEROGRAPIIER'S MATE I & !MEW ti111 LAM IPA MI1141111M11PI'

COMPUTATION OF WET-BULB TEMP. AT 1000 MRS. EDoomsmmy 10 -8 MINN= _Pi -4 -7 0 -13 -16-19 -2Z -25-28 (A) DEW POINTS, DEGREES C. GRAPH I 8 EA 6OfANIAPAll IES/110.31MEM INVINIIIIIMIMN REIM-EA 111 MINN-,LPITi COMPUTATIONOF Effir a PPw" WET-M1.8TEMP. AT8 -0 MBS. r

6 lirlENE ,ifigi1.1,---

8 Mil -19 -22-25-2E 8 5 2 -I -4-7-10 -13-16 (B) DEW POINTS, DEGREES C. GRAPH IC

AG.601 Figure 10-27.Graphs for computing wet-bulb temperatures. (A) Computation of 1,000-mb wet -bulb temperature; (B) computation of 850-mb wet -bulb temperature.

346 352 Chapter 10-CLOUDINESS, PRECIPITATION, AND TEMPERATURE FORECASTING

4 cv 1 r.n U1 B FREEZINGRAIN A RAIN w 2 w

co-

co D MIXED w 2 3 TYPES

ze.,2 4 co ____ -6 SNOW 4--

1 -8 -8 -7 -6-5 -4 -3 -2 0 I 2 3 4 (A) SURFACE.WET BULB, DEGREES C. (GRAPH III)

AG.602 Figure 10-28.Graph (A) and worksheet (B) for delineating the type of precipitation using the Hilworth method. (Point A is intersection of forecast valuessnow is forecast.)

temperature contrast and high moisture avail- low o wave is east of 40° and its speed is at ability, the heaviest known snowfalls occur. The least the average of 25 knots, often falling into western edge of the maximum area is limited by the fast moving category. The upper air picture the 700-mb trough or low center, and the end of is one of fast moving troughs, generally open, all snow occurs with the passage of the 500-mb but on occasion could have a minor closed trough or low center. Therefore, it follows that center for one or two maps in the bottom of the the maximum snow area diminishes and con- trough. tracts during the capturing process. Often a new The area of maximum snowfall lies in the cold snow area forms to the east in connection with air to he left of the track of the low and usually cyclogenesis or center jump; otherwise, the snow describes a narrow belt oriented east-west or area, now confined to the vicinity of the surface northeast-southwest about 100-200 miles wide. low and considerably diminished in intensity, At any particular synoptic position the area is moves off to the north or northeast. located parallel to the warm front and north of MAJOR STORM AND NONOCCLUDING the low center. Within the maximum area, the LOWS. -The synoptic situation consists of a rate of snowfall is variable from one case to wave type low of the nonoccluding type. In another, depending upon available moisture, practically all cases, the "wrapped around" high amount of vertical shear, etc. However, it is not pressure and ridges are present. The track of the uncommon for heavy snow (I inch per hour or

347 353 AEROGIZAPFIER.'S MATE I & C

WORK SHEET FOR HILWORTH METHOD:

I I000 111'1 Wet Bulb Temp. (degrees C) A. Present

1. Present1000mb temp -990 9. PresentDewPoint -6 7u 3. PresentWetBulb Temp_ 7.8

B. Forecast

1. Fest1000 mb temp 74. D. 6') 2. FestDew Point 3. Festsfc Wet Bulb temp -/ 2° (Graph Iforthese computations).

II. 830 rub Wet Bulb Temp. (degrees C) A. Present

1. Present830temp -5" 2. Present830D.P. --7777- 3. Present830Wet B71.157-2!

B. Forecast*

1. F,.st 850 mb temp Fest 850 mb D.P. 3. Fest 830 Web Bulb --.6r.-*° (GraphII for these computa ions)

Type of Precipitations Forecast Enter Graph III wit) above forecast figures TYPE OF PRECIPITATION FORECAST -S.,/t/Oe<,) ALTERNATE

IV. OBSERVED TYPP. OF FIRST PRECIPITATION .54V al 4f-00Z * See notes in this section of chapter for techniques of forecasting isotherm F.Wet Bulb movement.

(B) 4111111

AG.603 Figure 10-28.Graph (A) and worksheet (B) for delineating the type of precipitation using the Hilworth method. (Point A is intersection of forecast valuessnow is forecast.)Continued. greater) to occur. It must be remembered tl.at inthecase of the blizzard type,itusually even though heavy snow does occur, the dura- remains in the area in excess of l0 hours. tionis short by the very nature of the storm. WARM ADVECTION TYPE. Thistype, This means thata station would lieinthe which occurred only a few times in the initial maximum snow area only 4 to 8 hours, whereas study, was separated from the other types

348 254 Chapter 10-CLOUDINESS, PRECIPITATION,AND TEMPERATURE FORECASTING because of the absence of an active low in the I.In areas of precipitation, stations reporting vicinity of the maximum snow area. A blocking snow should lie on thecold side of the 0°C high-pressure ridge or wedge is present ahead of (-3°C east coast) isotherm; stations reporting a sharp warm front. The overrunning warmair is mixed types of precipitation(e.g., rain and a steady current from thesouth to southwest. snow, sleet and snow), the0°C isotherm will lie The area of maximum snowfall is a narrow band very close to or through the station. parallel to the warm -it and moves north or 2. In areas of no precipitation, the0°C (-3°C northeast. Only under the most ideal conditions east coast) isotherm will roughlyparallel the does this area become serious: nearly stationary 32°F isotherm at the surface. In cloudy areas front, ample supply of moisture, and persistent the separation will be small, and in clear areas flow aloft. A rate of fall of moderate to heavy the separation will be larger. for a 6- toI 2-hour duration may occur. The At the 850-mb level the 0"C wet-bulb tem- usual history is a transition to freezing rain, then perature should be sketched in, particularlyin rain. the area where precipitation may be anticipated POST-COLD FRONTAL TYPE.-The synop- within the next 12-24 hours. This line ill serve tic situationconsists of a sharp cold front as the first approximation of thefuture position oriented nearly north-south in a deep trough. A of the 0°C isotherm. minor wave may form on the front and travel MOISTURE. -At the 850-mb level the -5°C rapidly north or northeast along it Strong cold dewpoint line, and at 700-mb level, the -10°C advection from the surface to 850 mb is present dewpoint line are used as the basic defining west of the front. The troughs at 700mb and lines. The area at 850 mb that lies within the 500 mb are sharp and displaced to west of the overlap of the 0°C isotherm and the-5°C surface trough 200-300 miles. Ample moisture is dewpoint line is the first approximation of the available at 850 mb and 700 mb. This typeof maximum snowfall area. All stations within this heavy snow area occurs once or twice a season. area have temperatures lessthan 0°C and spreads The area of maximum snowfall is located of 5°C or less. This area is then furtherrefined between the 850-mb and 700-mb troughs where by superimposing the sketchedI0 °C dewpoint moisture at both levels is available. The rateof line at 700 mb upon the area. Now the final area fall is moderate, although for a brief periodof is defined by the 0°C isotherm and the overan hour or less it may beheavy. The duration is lapped minimum dewpoint lines from both short, of the order 2 to 4 hours at any one levels. This final area becomes the area where station. The area as a whole generates anddies moderate or heavy snow will be reported, out in a12- to 18-hour period. The normal depending upon the particular synoptic situa- history is one of a general area of light snow tion. (See fig. 10-29.) within the first 200 miles of a strong pushof MOVEMENT. -As might be supposed, the cold air. After the cold air moves farenough area of maximum snowfallis in motion and south and the cold front becomes oriented more must be forecast. The first basicrule for moving N-S and begins moving eastward steadily,the the area is that it maintains the samerelative troughs aloft and moisture distribution reach an position to the other synoptic featuresof the ideal state and a maximum snow area appears. 850-mb level and surface chart. However, in After 12-18 hours the advection of dryair at order to forecast the expansion orcontraction -00 mb decreases the rate or fall in the area,and of the area, itis necessary to forecast the lines soon thereafter the areas as awhole dies out. that defineit. The 0°C isotherm should be LOCATING AREA OF MAXIMUM SNOW- forecast according to the rules set forthin the FALL.-TEMPERATURE. The 0°C (-3°C east section treating this particular phase.the mois- coast) isotherm at 850 mb is used as the basic ture lines may be advected withthe winds as set defining line for the snow area. This isotherm forth previously in this chapter.The 0°C iso- should be carefully analyzed, using all data at therm should also be moved with rulesstated 850 mb. It should then be checkedagainst the previously in this chapter. surface map, keeping in mind thefollowing Itwas foundthatthe area of maximum snowfall could be forecast for 12 hourswith points: 349 AEROGRAM IFR'S NlATE I & C

AG.604 Figure 10-29.Illustration of the location of the maximum snow area. The low center moved to Iowain 24 hours, and the maximum snow area spread northeast along the area 50.75 miles either side ofa line through Minneapolis to Houghton, Michigan.

considerable accuracy and for 24 hours withfair c:valuationas well. Factors to be considered accuracy. provided a reasonable amount ofcare should be the climatology ofsnow for your was exercised according to rules and subjective particular area, the type and intensity ofthe ideas mentioned above. storm producing the precipitation, its speed and QUANTITATIVE PREDICTION. TheNa- duration over your station, and the relation of tional Weather Analysis Center currentlymakes your station to the area of maximum snowfall. quantitative precipitation predictions andtrans- One rule of thumb which has been inuse for mits them on the National Weather Facsimile several years involves the use of theamount of Network. Details onthis procedure may be precipitable water from values found found in on the the U.S. Department of Commerce charttransmitted over the National Weather publication. Synoptic Meteorologyas Practiced Facsimile Network. This rule by the National Meteorological uses a ratio of I Center. The inch of precipitation equals 10 inches ofsnow. NMC Manual.l'artII. NW 50 -I P -548. latest Therefore, using this rule of thumb, themaxi- revision. These predictions are based on vertical mum amount of snowfall which couldoccur motion. precipitable water, humidity, andstabil- would bein ity. direct ratio to the amount of precipitable water overlying the station. How- For local forecasting you shouldnot only ever. you must consider moisture advection 'and consult theseforecasts but make yourown changing values. For example, ifyour station

350 Chapter 10 CLOUDINESS. PRECIPITATION,AND TEMPERATURE FORECASTING showed 1/2 inch of precipitable water at present United States) these lines will divide the snow- the with no significant change during the forecast rain areas as follows: rain will be found on period, and the precipitation forecast was snow. higher side of these lines, and snow or some 5 inches of snow would be the maximum you frozen form of precipitation on the lower side. would expect. This rule should be used with Be sure to keep in mind the rules setforth caution and in conjunction with all otheravail- previously in this chapter. (Also use theWagner able information. equal probability chart.) 2. The George method c; n also be utilized as APPLICATION TO LOCAL AREA a further check. This method was outlined previously. Use the 32°F line on the surface and the Figure 10-30 presents a composite worksheet the 0 °C' line on the 850 mb chart to divide precipitation. In some which is a suggested format for correlating the areas of solid and liquid thermal parameters and techniques presented in cases the use of the-3°C line at 850 nib is more the foregoing section. You must remember that desirable. this worksheet is based on the assumption that 3. For determination of the areas ofmaxi- the synoptic type. climatology, and otherindica- mum snowfall.usethe methods previously explained. keeping in mind the synoptic types. tions revealthat precipitation will occur and that there is a likelihood of the precipitation being in the form of snow or some other frozen TEN1KRATURE type. Also,allof thevalues usedinthese Temperature ranks among the most important techniquesare FORECAST VALUES.Local forecast elements. Temperatures are not only predictions,facsimileprognosticcharts, and other available data should be utilized to deter- important for sonic operational procedures but mine the forecast values and parameters at the also are of keen interest to all of us in everyday time of the ONSET of precipitation at your life. station or locality. The example givenisa fictitious station and merely presents an illustra- FACTORSAFFECTINGTEMPERATURES tion of the proper step-by-step procedure which can be used to take all of the parametersinto In forecasting temperatures. many factors are consideration. A worksheet of this type could be involved. These factors include air mass charac- .prepared for your local station using the opti- teristics; frontal positions, characteristics. and cloudiness: mum values. movement; amount and type of season; nature and position of pressuresystems; APPLICATION TO A LARGE AREA and local conditions. Temperature.beingsubject tomarked Frequently, flight operations reach into an changes from day to night, is not considered a conservative property of an air mass. Too, it area )utside the localsphere of influence. In it may be does not always have a uniform lapse rate from these cases. and for other reasons, This desirable to have a rough delineation of the the surface up through the atmosphere. means that the surface air temperaturewill not snow and rain areas on thesynoptic map. For a be representative because of the existence of an rough approximation of the snow-rain dividing inversion, a condition particularly prevalent at line over a relatively large area, thefollowing night. Usually the noonday surface air tempera- techniques are recommended: ture is fairly representative. I. On the thickness chart and on thethick- Letuslookatthefactors which cause temperature variations: Insolation andterrestrial ness prog,drawboththe 17,800-and 17.600-foot (5.425 and 5.365 meters) thickness radiation. lapserate, advection, vertical heat condensation lines. If an overlay acetate chart isavailable, this transport, and evaporation and can be placed in registerwith the synoptic chart In forecasting temperature, consider insolation to determine the snow-raindividing line. In the and terrestrial radiation as two very important majority of cases (over the eastern partof the factors. Low latitudes, for instance. receive more

351

')t".-t-"y AEROGRAPHER'S MATE I & C

SNOW VS RAIN

WORK SHEET Time 0900 L Precip. Onset Time 400 1. Date/S 27 C. /9 6V Forecast Time / 9.30 L

1. Synoptic Indications Type /0,fh-Ch0.- Rain Snow

Snow to Rain Freeze Rain 2. Thickness (1) 1000-500 mb 534/0/V Probability (meters) Rain 07.5% 5,370 (WAGNER) 5,430 (NAWAC) Snow7. 5 (2) 1000-700 mb .21/5-A/ Prediction: Rain 2835 & below snow 2835-2865 mixed Snow 2865 & above rain Y Mixed 3. Hilworth Method Rain Frz. Rain Snow X

Forecast g'/YeAr./ 4. 850 mb. predicted temperature- (-3 or below optimum) 5. Predicted Freezing level AreW Z7' (optimumbelow 1,200 ft) F. FORECAST: SNOW RAIN FRZ.RAIN MIXED

1. )(

2. y

7. Verification Swocti /3,F 0:9N 5- a oz. cVitirmlaspFaif6 /7/,7?$-. 8. Remarks 37/f/Utei ///r/tbz 4 7212/.1

NOTE: All above are forecast values. Remarks should include such notations as: snow beginning at 15001. changing to rainat 1800, etc. Also other comments as appropriate.

AG.605 Figure 10-30.Sample composite worksheet for thesnow versus rain forecasting problem.

352 Chapter I 0- -CLOUDINESS, PRECIPITATION. ANDTEMPERATURE FORECASTING heat duringthedaythan stationsathigh to affect the maximum on a summer day on latitudes. More daytime heat Lan be expected in which afternoon thundershowers occur. The the summer than in the winter. since inthe temperature may be affected at the surface by summer the sun's rays are mere direct and reach condensationtoasmall extent during fog the earth for a longer period. Normally, there is formation. raising the temperature a degree or so a net gain of heat during the day and a net loss because of giving off the latent heat of conden- at night. Consequently. the maximum tempera- sation to the air. Refer to an earlier section of tureisusuallyreached during the day. the this chapter for a more detailed discussion on minimum. at night. Cloudiness will affeLt insula- evaporation and condensation effects on tem- tion and terrestrial radiation. Temperature fore- perature. casts must be made only after the amount of cloudinessexpected is determined.Clouds FORECASTING MAXIMUM TEMPERATURE reduceinsolationandterrestrialradiation. BY USE OF SKEW T LOG P DIAGRAM causing daytime temperaturereadings to be relatively lower than normally expected and The Skew T Log P diagram may be used to nighttime temperatures to be relatively higher. get a good approximation of the maximum The stability of the lapse rate has a marked temperature by plotting the most recent meteor- effecton insolation and terrestrialradiation. ological sounding on the chart. If there is a With a stable lapse rate there is less vertical surface inversion on the sounding, the approxi- extent to heat: surface heating therefore takes mate maximum temperature may be found on place more rapidly, With an unstable lapse rate. the diagram by following down the dry adiabat the opposite istrue.If' thereis an inversion. from the top of the inversion to the surface. there is less cooling. since the surface tempera- This approximates the maximum temperature, ture is lower than that of the inversion layer: since any heating beyond this point will be that is, at some point the energy radiated by the spread through a large vertical section. The more is radiated by the surface balanced by that the lapse rate above the inversion approximates inversion layer. the dry adiabat, the nearer this temperature will One of the biggest factors affecting tempera- approximate the maximum temperature. tureisthe advectionof air.Advectionis particularly marked in its effect on temperature Another method of forecasting the maximum with frontal passage.If a frontal passage is temperature is by the use of the EQUAL AREA expected during the forecast period. the tem- method. On figure 10-31 if ABC represents the perature must be considered. The temperature sounding at the time of the minimum tempera- gradient with an air mass may be as important as ture on the first day and A'B'C represents the frontal passage in forecasting. Advection within sounding at the time of the maximum tempera- an air mass may also be important.This is ture. the hatched area represents the energy particularlytrue of sea and land breezes and added by heating during the day. If the air mass, mountain breezes. They affeLt the maximum wind, and cloudiness conditions remain the same and minimum temperatures and their time of from one day to the next, the same amount of occurrence. insolation should be received. Consequently, if Vertical heat transport is a temperature fac- the sounding atthetime of the minimum tor. Itis considerably affected by thespeed of temperature on the second day is used and an the wind. With strong wind there is less heating energy area equal to that of the previous dayis and cooling than with light wind or a calm since added to it, the maximum temperature for the the heat energy gained or lost is distributed day should be indicated. Normally. the actual through a deeper layer when the turbulence is meteorological sounding is not made at the time greater. of the maximum or minimum temperature; Evaporation and condensation affeLt the tem- therefore, to get the best results, the actual perature of an air mass. When cool rainfalls sounding should be modified to fit the maxi- through a warmer air mass. evaporation takes mum orMiliii3111111temperature. However, use of place, taking heat from the air. This often occurs theactualmorning soundinggivesafair

353 ''59 AEROGRAPIIER'S MATE 1 & C

FORECASTING SPECIAL SITUATIONS Cold Wave

A forecast of a cold wave gives warningof an impending severe change to muchcolder temperatures. In the United States it is definedas a net temperature drop of 20°For more in 24 hours to a prescribed minimum thatvaries with geographical location and time ofthe year. Some of the prerequisites fora cold wave over the United States are continentalpolar air with temperature below average over westcentral Canada. movement ofa low eastward from the AG.606 Continental Divide which releases thecold wave, Figure 10-31.Energy area for forecasting and large pressure tendencieson the order of 3 maximum temperature. to 4 nub occurring behind the cold front. Aloft, a ridge of highpressure develops over the western part of the United Statesor just off the approximation. One or the disadvantages of this weft coast, as a short wave troughmoves into a method is that it gives a forecast value for only long wave trough over the centralpart of the one day. United States and deepens.An increasein intensity of the southwesterly flowover the FORECASTING MINIMUM TEMPERATURE eastern Pacific frequently precedes the building BY USE OF SKEW T DIAGRAM of the ridge. Frequently,retrogression of the long wave takes place.Inany case, strong northerly to northwesterly flow is established The equal area method of forecasting may aloft through a deep layer andsets the cP air in also be employed in arriving at a forecast of the motion southward. When two polaroutbreaks minimum temperature. Instead of adding the follow each other the second outbreakusually energy area. as was the case in forecasting the moves faster and overspreads the Central States. maximum temperature. the energy area is sub- It also penetrates farther southwardthan the first tracted. The energy area to be subtractedis cold wave. in such cases, the resistance of determined by plotting the morning sounding the southerly winds ahead of the second frontis andthenplotting the afternoon maximum, shallow. At middle andupper levels, winds assuming a dry adiabatic lapse rate relative to remain west to northwest, and thelong wave the original sounding. Then plot thenext morn- trough is situated near 800 west. ing sounding. The energyarea between the Most cold waves do not persist. Temperatures modified sounding of the first day andthe trend upward after about 48 how.. Sometimes, morning sounding of the second day is the however, the upper ridge over thewestern part energy area to be subtracted. The second day of the United States and the troughover the maximum is plotted on the same chart with the easternpart of the United States are quasi- morning sounding. thus modifying itto an stationary, and a large supply ofvery cold air afternoon sounding. Then the energy area is remains in Canada. Then.we experience succes- subtracted from this modified sounding to give sive outbreaks with northwest steering thathold the forecast minimum temperature for the third temperatures well below normal foras long as 2 day. Again, this method has the disadvantage of weeks. the soundings not being normally madeat the time of the minimum temperature. Too, the Heat Waves forecast value is not available at the time of the morning forecast. since it cannot be made until In summer, heat wave forecasts furnisha the maximum temperature for the dayoccurs. warning that very unpleasant conditionsare

354 CGO Chapter I()CI OUDINFS.S. PRECIPITATION, ANDTEMPP,RATURE FORECASTING

40 J i 30

1011111111%P:riffirAilliirivirilliff11111 9ftiMIIM 8M11101111iIMMIKIIIIIIMIKAIIAMMIIIITIMPAIIRMWANNIFAMAIMEMIENIIIMINI111111 7 6INEMIPIAMWEIMOWAWAWAIWAWAIWAIIVANIMMUMINNIMIE IMIMISSIGENESIEVIVAINFARFAWAIWAINFINIRMENEIMI30 40 50 60 70 80 90 100 -80 -70-60-50-40-30-20 -10 0 10 20 1 0 TEMPERATURE (°F.)

INSTRUCTIONS, FROM THE INTERSECTION OF THE AIRTEMPERATURE AND THE WIND SPEED FOLLOW THE CURVED LINES TO THE BOTTOMAND READ THE WINDCHILL TEMPERATURE.

AG.757 Figure 10-32.Windchill temperature graph. impending. The definition of what is meant by a Bay into the northern part of the UnitedStates. the long heat wavevaries from placetoplace.For A general heat wave continues until example, in the Chicago area a heat waveis said wave train begins to move. to exist when the temperaturerises above 90°F on 3 successi%e days. Inaddition. there are many TEMPERATURE-HUMIDITY INDEX summer days which do notquite reach this requirement. but are highly unpleasant on ac- In order to accurately express the comfort or count of humidity. discomfort caused by the air at various tempera- and Heat waves develop over the midwestern turesitis necessary to take into accountthe eastern part of the UnitedStates when a long amountof moisture present. The National wave trough stagnate, o'erthe Rockies or the Weather Service uses the Temperature-Humidity Plains States and a long wave ridgelies over or Index to relate this. just oft' the east t_oast The belt ofwesterlies are The formula for computing the temperature- centered far north in Canada. At thesurface we humidity index is given below: observe a sluggish and poorlyorganized low- pressure system over theGreat Plains or Rocky T.H.I. = 0.4(td + tw ) + 15 Mountains. Pressure usually is abovenormal over the South Atlantic and .quently the Middle where tdis the dry-bulb temperatureand t, is Atlantic State,. An exception occurswhen the the wet -bulb temperature. Both are indegrees amplitude of the flow pattern aloftbecomes Fahrenheit. very great. Then. severalanticyclonic centers For example, suppose the temperatureis develop in the eastern ridge. both at upperlevels 85°F and the wet-buib temperature is68°F. and at the surface. Frequently,they are seen Substituting in the formula, we have: first at 500 mb. Between thesemeridionally of east-west arranged highs we see formation 68)÷15 shear lines situated perhaps alonglatitudes 38° TILL = 0.4(85 + ÷15 to 40°N. North of this linewinds blow from the T.I1.1. = 0.4(153) northeast and bring cool air fromthe Hudson T.H.I. = 76.2 355 AEROGRAPFIER' S MATE 1 &C

The National Weather Serviceuses the values personnel engaged in outside taskssuch as flight as follows: With a Till of 72.conditions are line or flight deck operations. slightlyuncomfortable: with The windchill a TALI. of 75. temperature reflects the cooling discomfort becomesmore power that the acute and most wind exerts on the airtemperature. people would beusingair conditioners,if available;witha T.11.1.of 79 and above. Utilizingthe wind speed and outsideair discomfort is general andair conditioningis temperature refer to figure 10-32to find the highly desirable. windchill temperature. From this temperature WINDCHILL TEMPERATURE supervisors can determine the proper type clothingto be worn or if Weather office personnel willfrequently be tasks can actually be performed. querried concernine, windchilltemperature by

356 CGZ CHAPTER 11

FORECASTING THUNDERSTORMS,FOG, TORNADOES, ICING, ANDCONTRAILS

Forecasting of severe weather conditions and under most severe conditions (incertain areas), providing timely warnings for safety ofaircraft tornadoes. and ship operations. as will aspersonnel safety, isa paramountresponsibility of the senior Turbulence (Drafts and Gusts) Aerographer's Mate. In this chapter we will discuss someof these Downdrafts and updrafts are vertical currents phenomena and methods that may beutilized to of air which are continuous over manythousand forecast their occurrence. of feet of altitude, and arecontinuous over horizontal regions as large as athunderstorm. THUNDERSTORMS Their speed is relatively constant ascontrasted to gusts, which aresmaller-scale discontinuities The thunderstorm represents one of the most or variations in thewindflow pattern extending formidable weather hazards in temperateand over shortvertical and horizontal distances. tropical zones. Though the effectsof the thun- Gusts are primarily responsible forthe bumpi- derstorm tend to be localized, theturbulence, ness (turbulence) usuallyencountered in cumu- high winds, heavy rain, and occasionally,hail liform clouds, A draft may beconsidered as a accompanying the thunderstorm are adefinite river flowing at a fairly constant rate,whereas a threat to the safety of flight and to thesecurity gust is comparable to aneddy or other type of of naval installations. R is importantthat senior random motion of water in a river. Aerographer's Mates be acquaintedwith the Studies of the structure of thethunderstorm structure of thunderstormsand the types of cell indicate that during thecumulus stage of weather associated with them, as well asbeing development the updrafts may cover ahori- able to accurately predicttheir formation and zontal area as large as 4 miles indiameter. In the movement. cumulus stage the updraft in many cellsextends Thunderstorm formation and movement were from below the cloud base to the cloudtop, a extensively covered in chaptet 7of AG 3 & 2: height greater than 25,000 feet.During the therefore in this chapter we willdiscuss, in more mature stage the updraftdisappears from the detail, the weather phenomenaassociated with lowest levels of the cloud although itcontinues thunderstorms and various forecastingmethods. in upper levels where it mayexceed a height of 60,000 feet. These drafts are ofconsiderable importance in flying because of thechange in THUNDERSTORM TURBULENCE altitude which may occur when anaircraft flies AND WEATHER through them. that the maxi- Thunderstorms are characterized byturbu- In general, it has been found updraft and down- mum number of highvelocity gusts are found at lence, moderate to extreme below the top drafts, hail, icing, lightning,precipitation. and altitudes of 5,000 to 10,000 feet 357

3 63 AEROGRAPHER'S MATE I &C of the thunderstorm cloud. while the leastsevere relationship indicates that turbulence is encountered,on the average near most rain and snow in thunderstorms is held aloft byupdrafts. the base of thestorm. This turbulencemay at times also be classified as severe. The charac- Icing teristic response ofan aircraft interceptinga series of gusts isa number of sharp accelerations or "bumps" without Where thefree-air temperaturesare at or a systematic change in below altitude. The degree of freezing,icing should be expectedin bumpiness or turbulence flights through thunderstorms. experiencedinflightisrelatedto both the In general. icing number of such abrupt is associated withtemperatures from 0° to -20°C. changes encountered ina Most severe icing given distance and thestrength of the individual occurs from 0°C to 10 °C. changes. All date in this The heaviest icingconditions usuallyoccur in section are basedon that region above the information from theThunderstorm Project in freezing level wherethe 1947. clouddroplets have notyetturnedto ice crystals. When the thunderstormis in the cumu- Hail lus stage severe icingmay occur at any point above the freezing level.However, because of Hail is regardedas one of the worst hazards of the formation of icecrystals at high levels and thunderstorm flying. It usually the removal of liquidwater by precipitation, occurs during the icing conditions mature stage of cells havingan updraft of more are usually somewhat less in the than average intensity,and is found with the mature and dissipatingstage. greatestfrequencybetween 10.000-and 15,000-ft levels. Asa rule, the larger the storm THUNDERSTORM ELECTRICITY the more likely it isto have hail. AND LIGHTNING Although encounters by aircraft with large The thunderstorm changes hailarenottoo common, hailrather than the normal electric field, in which the earthis negative with respect one-half or three-fourthsinch can damagean aircraftina to the air above it. by makingthe upper port:on very few seconds. The general of the thunderstorm conclusion regarding hailisthatmost mid- cloud positive and the latitude storms contain lower part negative. Thisnegative charge then hail sometime during induces a positive charge their life cycle withmost hail occurring during on the ground. The the mature stage. In distribution of the electriccharges in a typical subtropical and tropical thunderstormis shown in thunderstorms. hail seldomreaches the ground. figure11-1. The It is generally believed lightning first occurs betweenthe upper positive that these thunderstorms charge area and the negative containlesshailaloftthan do mid-latitude charge area immedi- storms. ately below it. Lightningdischarges are consideredto occur most frequentlyinthe area Rain bracketed roughly by the 32°Fand the 15°F temperature levels. However, thisdoes not mean that all discharges Thunderstorms contain considerable are confined to this region, quanti- foras ties of moisture whichmay or may not be falling the thunderstorm develops.lightning to the ground as rain. These discharges may occur in otherareas, and from water droplets may cloud to cloud, be suspended in.or moving with, the ovdrafts. as well as cloud to ground. Rain is encountered below Lightning can do considerabledamage to air- the freding level in craft, especially to radioequipment. almost all penetrations of fullydeveloped thunderstorms. Above the freezinglevel, however, there is a sharp decline in THUNDERSTORMS IN RELATIONTO the frequency of rain. ENVIRONMENTAL WIND FIELD There seems to beadefinitecorrelation between turbulence and precipitation. The in- During all stages ofa cell, air is being brought tensity of turbulence,inmost cases,varies directly with the intensity of into the cloud through thesides of the cloud. precipitation. This This process is knownas entrainment. A cell 358

Z.364 Chapter 11 mu(' As FINGTI1UNDERSTORMS, FOG, TORNADOES,ICING, AND CONTRAILS

POSITIVE CHARGE CENTER - 15 °F

DIRECTION NEGATIVE CHARGE OF MOVEMENT CENTER 32°F

I (1_

AREA O fF SMALL CENTER OF LI i POSITIVE CHARGE NEGATIVE RAIN / SURFACE niellm+ /4.//....,/7 Inimmt+ +

AG.607 Figure 11-1.Location of electric charges inside atypical thunderstorm cell.

sectionoftherisingair,itfallsthereafter entrains environmentalairatarate of 100 tothe doubles its mass in throughtherelativelystillair next percent per 500 mb, that is it updraft, perhaps even outside the cellboundary. an ascent of 500 mb.The factor of entrainment is within Therefore, since the drag of the falling water is important in establishing a lapse rate not imposed on the rising aircurrents within the the cloud which is greater than themoist adiabat thunderstormcell,theupdraft can continue and in maintaining the downdraft. until its source of energy is exhausted.Tilting of When there is a marked increase with height the thunderstorm explains whyhail is sometimes in the horizontal windspeed, the mature stage encountered ir, a cloudless area justahead of the of the cell may be prolonged. Inaddition, the storm. increasing speed of the wind with height pro- RADAR DETECTION duces considerable tilt to the updraftof the cell. and in fact, to the visible clouditself. Thus, the Radar, either surface or airborne,is the best falling precipitation passes through only asmall aidin detecting thunderstorms, chartingtheir

359 AEROGRAPHER'S MATE 1 C

movement, or selectinganarea for feasible THUNDERSTORM SURFACE penetration. A thunderstorm'ssize. direction of PHENOMENA movement, shape and height,as well as other The rapid change in winddirection and speed significant features,can be determined froma radar presentation. Radarscope immediately prior toa thunderstorm passage isa interpretation is significant surface hazard discussed in chapter 16 ofthis training manual. associated with thunderstorm activity. Thestrong winds whichac- company thunderstorm passagesare the result of the horizontal spreading THUNDERSTORM FLIGHTHAZARDS out of downdraft currents from within the stormas they approach Thunderstorms are virtually the surface of the earth. weather factories Figure in that the pilot flying intoone can expect to 11-2 shows the nature ofthe wind outflow and indicates how it encounter great variations in theweather; some is formed from the of them hazardous. settling dome of cold air whichaccompanies the Thunderstorms are often rain core during the accompanied by extremefluctuations in ceiling mature stage of the thun- and visibility. Everythunderstorm has turbu- derstorm, The arrival of thisoutflow results ina lence. sustained updrafts and radical and abrupt changein the wind speed and downdrafts, pre- direction. It is an important cipitation.andlightning.Icingconditions, consideration in the though quite localized,are quite common in landing and taking off of aircraftin advance of thunderstorms, andmany containhail. The the arrival of a thunderstorm. flight conditions listed beloware generally repre- sentativeof many (but notnecessarilyall) thunderstorms.

I. The chance ofsevere or extreme turbulence within thunderstormsis greatest at higher altitudes, with mostcases of severe and extreme turbulence about 8,000to 15,000 feet above terrain, The least turbulencemay be expected when flying at or justbelow the base of the main thunderstorm cloud.(The latter rule would not,be true over rough terrainor in mountainous areas where strong eddy currentsproduced by strong surface winds would extendthe turbulence up to a higher level.) 2. The heaviest turbulenceis AG.608 closely asso- Figure 11-2.Cold dome of air ciated with the areas of heaviestrain. beneath a thunderstorm 3. The strongest updrafts cell in the mature stage. Arrowsrepresent deviation are found at heights of windflow. Dashed lines indicaterainfall. of about 10,000 feetor more above the terrain; in extreme cases, updrafts in excess of 65 feet Wind speedsat per second occur. Downdraftsare less severe, the leading edge of the but downdrafts on the order thunderstorm are ordinarilyfar greater than of 20 feet per those at the trailing edge. The second are quitecommon. initial wind surge 4. The probability of lightning observed at the surface isknown as the first strikes occur- gust. The speed of the first ring is greatest nearor slightly above the freezing gust is normally the level. highest recorded during thestorm passage and may vary as much as 180 degrees indirection from the surface wind Because of the potential hazards direction which previ- of thunder- ously existed. Themass of cooled air spreads out storm flying, itis obviously nothing shortof folly for pilots to attempt from downdrafts of neighboringthunderstorms to fly in thunder- (especially in squall lines) and storms, unless operationallynecessary. often becomes organized into a small, high-pressurearea called 360 66 Chapter IIFORECASTING THUNDERSTORMS, FOG, TORNADOES, ICING,AND CONTRAILS a bubble-high or meso-high,which persists for and accurate altimeter settings are furnished to some time as an entity that cansometimes be the tower for transmissionto pilots during seen on the surface map. Thesehighs may be a thunderstorm conditions. If the data are old and mechanismforcontrollingthedirectionin inaccurate a crash could result. which new cells form. The speed of the thunderstorm wind depends THUNDERSTORM FORECASTING upon a number of factors, butlocal surface winds reaching up to 50 to 75 miles per hour for Because it can The standard method of forecasting air mass a short time are not uncommon. thunderstorms has long,consisted primarily of an extendfor severalmilesinadvance of the thunderstorm itself, the thunderstorm wind is a analysis of radiosonde data with particular emphasis on the so-called positive areas, in which highly important consideration for pilots pre- the paring to land or take off in advance of a storm's the temperature of a lifted parcel exceeds temperature ofitssurroundings and, hence, arrival.Also, many thunderstorm winds are being less dense, must rise, as a cork in water. strong enough to do considerablestructural damage and capable of overturning or otherwise Experienced forecasters are aware that this damaging even medium sized aircraft that are method, though basic, is also crude and at times wholly inaccurate. Many times conditions are parked and are not adequately secured. favorable for thunderstorm development with a The outflow of air ahead of the thunderstorm large positive energy area showing up on the sets up considerable low levelturbulence. Over relatively even ground, most of the important sounding, with no ensuing thunderstorm activity turbulence associated with the outrush ofair is whatever. At other times thunderstorms occur within a few hundred feet of the ground,but it when they should not and where they should the not be. Clearly, factors other thanthe one of extends to progressively higher levels as instability are important, and at times of over- roughness of the terrain increases. riding importance. THUNDERSTORM ALTIMETRY During recent years forecasters have come to recognize this need for better forecasting meth- During the passage of a thunderstorm, rapid ods. A number of methods have beendeveloped, and marked surface pressure variations generally but many of these are too complicated or detailedtoincludeinthistraining course. occur. These variations usually occurin a partic- of ular sequence characterized by the following. Covered in this section are the forecasting convective clouds by. the parcel method, with I. An abrupt fall in pressure as the storm conditions necessary fc . thunderstormdevelop- ment; the development of thunderstormsby approaches. brief 2. An abrupt rise in pressure associated with mechanical lifting (orographic or frontal); a discussionoftheimportanceof theslice rain showers as the storm moves overhead (often of associated with the first gust). method; and a method for the prediction 3. A gradual return to normal pressure as the these storms which enables the average fore- storm moves on and the rain ceases. caster to arrive at a fairly accurate,reasonably Such pressure changes may result insignifi- objective forecast of these storms. Credit is given this cant altitude errors on landing. to the Academic Press for permission to use Of greater concern to the pilot are pressure method from Weather Forecasting forAero- readings which are too high. If a pilot used an nautics, by J. J. George and Associates,Aca- altimeter setting given to him clOng the maxi- demic Press, 1960. deter- mum pressure and then landed a' the pressure For a more detailed discussion of the convective hadfallen,he wouldhave ,dthathis mination of instability, stability, the altimeter still read 60 feet or h. above the condensationlevel(CCL), thelevel of free true altitude after he was on the ground. convection (LFC), the lifting condensationlevel Here is where you, as a section leader, or (LCL), and the slice method, refer tochapter 3 office supervisor, can make certain thattimely of this training manual.

361 L67 AEROGRAPHER'S MATE I & C

THE PARCEL MET1101) 4. There must be sufficientmoisture in the Nearly all of the procedures lower troposphere, This has beenfound to be routinely used to themost important evaluate and analyze the singlefactorinthun- stability of the atmos- derstorm formation. phere are manipulations ofthe parcel method. The temperature of 5. Climatic and seasonal conditionsshould be a minute parcel of air is favorable. assumed to change adiabaticallyas the parcel is displaced a small distance 6. Weak inversions (ornone at all) should be vertically from its present in the lower levels. original position. If. after verticaldisplacement. the parcel has a higher vitural 7. An approximate height ofthe cloud top temperature than may be determined by assuming that the the surrounding atmosphere,the parcel is sub- top of jected to a positive buoyancy the cloud will extend beyondthe top of the force and will be positive area by a distance equal further accelerated upwards;conversely, if its to one-third of the height of the positivearea. virtual temperature hasbecome lower than that of the surrounding air. theparcel will be denser than its environment, thus subjected Formation of Clouds by to a nega- Mechanical L.fting tive buoyancy force, is retarded,and eventually returns to its initial or equilibriumposition. When using this method,itis assumed that Formation of Clouds by the type lifting will be eitherorographic or Heating From Below frontal. Ilere weare working with the LCL and the LFC. The LCL is the heightat which a parcel of air The first step is to determinethe convection temperature or the surface becomes saturated when it is lifted dryadiabati- temperature that cally. The LCL fora surface parcel is always must be reachedtostartthe formation of found at or below the CCL. TheLFC is the convection clouds by solar heating ofthe surface height at which parcel of air lifteddry adiabati- air layer. The procedure isto first determine the cally until saturated and saturationadiabatically CCL on the plotted soundingand, from the CCL thereaftcwould first becomewarmer than the point on the T curve of thesounding, proceed downward along the dry adiabat surrounding air. The parcel will thencontinue to to the surface- rise pressure freely above this level untilitbecomes isobar. The temperature readat this colder than the surrounding air. intersection is the convectiontemperature. Figure 114 shows the formationof clouds Figure11-3 shows an illustration offore- due to mechanical lifting. Thisfigure shows the casting afternoon convectivecloudiness from a formation of a stratified layer ofclouds above plotted sounding. The dewpointcurve was not the LCL to the LFC. At the LFC plotted to avoid confusion. The and above, the dashed line with clouds would be turbulent. The arrowheads indicates the path tops of the the parcel of air clouds extend beyond thetop of the positive would follow under theseconditions, You can see that the sounding was modified area due to overshooting just as in thecase of at various clouds formed due to heating. times during the day. To determine the possibility of To determine the possibility of thunderstorms thunderstorms fromthis method, thefollowing conditions by the use of this method andfrom an analysis should be met: of the sounding. the followingconditions must exist: I. The positive area must exceedthe negative I. Sufficient heating mustoccur. area;the greater the excess. thegreater the 2. The positive area must exceedthe negative possibility of thunderstorms. area. The greater the excess, the greater the 2. There must be sufficient liftingto lift the possibility of thunderstorms. parcel to the LFC. Tht frontal 3. The parcel must rise slope or the to the ice crystal Orographic barriers can be usedto determine level. Generally, this level should be 10 °C and how much lifting can be expected. below, 3. The parcel must reach theice crystal level.

362 268 ICING, AN!) CONTR \ILS Chapter II FORECASTING -ri IUNDERSTORMS, FOG, TORNADOES,

500

FORECASTING AFTERNOON CONVECTIVE CLOUDINESS

600 ARROWS SHOW PATH OF PARCEL RISING UPWARD FROM TURBULENT LAYER IN THE AFTERNOON AT TIME OF MAXIMUM H EATING

700 LEVEL OF FREE CONVECTION IF / / NEGATIVE ENERGY AREA

800

POSITIVE ENEPGY AREA 900 BASE OF CLOUDS 950 MBS L AT TIME OF MAXIMUM HEATING 'MIN LAYER FORMS AND r IMMEDIATELY DISSIPATES % jai /AFTERNOON 1000 / / MAX TEMP MORNING SOUNDING 7 / 26°

0 10 20 30 AG.609 Figure 11-3.Forecasting afternoon convectivecloudiness. 4. Even though the positive area does not it overestimates or underestimates thestability exceed the negative area, cloudiness occursafter and instability conditions. the parcel passes the LCL and precipitation may occur after the parcel passesthe ice crystal level. INSTABILITY INDICATIONS FROM One main advantage of this method is thatit THE WET-BULB CURVE can be done quickly andwith relative ease. A ispotentially major disadvantage isthat it assumes that the A layer of the atmosphere parcel does not change its environmentand that unstable if the potential wet-bulb temperature

363 e)X69 AEROGRAPHER'S MATE 1& C

500

FORMATION OF CLOUDS DUETO MECHANICAL LIFTING

600

700 ., \ TOP OF TURBULENT- - - - CLOUDS- --- \ 0 o \ o'r

800 \ . tiL : LEVEL OF FR CONVECTIO

...... **:,.....,....;:z.,... LF C x..:. 900 ...... ::.:: :,.,..,.,..:.,...... A4tvo *:::::::::. AVGMRLinJ ...... - ,...... ;...... \ \ BASE OF CLOUDS 100 0

+10 + 20

AG.610 Figure 11-4,Formation of cloudsdue to mechanical lifting.

364 370 Chapter 11FORECASTING THUNDERSTORMS, FOG, TORNADOES, ICING, AND CONTRAILS decreaseswithaltitude.Potentialinstability The two parameters usedinmaking the refers to a layer that is lifted as a whole. Some forecast, after eliminating all soundings meeting meteorologists use the term "convective" vice one or snore of the above six conditions, were "potential," however, potential causes less con- simply (I) the lapse rate between 850 and 500 figuration regarding the lifting process involved. mb (the difference in temperature between these The wet-bulb temperature may be found by two levels, for example, if the temperature at lifting each individual point on the sounding dry 850 mb was 15°C and at 500 mb it was -10°C, adiabatically to saturation and then back to its the difference would equal 25°C); and (2) the original level moist adiabatically. By connecting sum of the dewpoint depressionF at 700 and 600 the points on a sounding, a wet-bulb curve can mb in degrees C. These two figures are used as be constructed. arguments for the graph in figure 11-5. If the wet-bulb curve slopes to the right with increasing altitude, the potential wet-bulb temperature increases with height and the layer is 30 potentially stable. If it slopes to the left with 29 increasingheight,morethanthe saturation m 28 adiabats, the layerispotentially unstable. If z none of the potential curves intersect the sound- 0 ing, thunderstorms are not likely to occur. 0 27 I- A co 26 z BAILEY GRAPH METHOD 0

11,.; 25 Credit is given to the Academic Press, New York City, for permission to use this method 24 from Weather Forecasting for Aeronautics, by J. 23 J. George and Associates. It is mainly for use in C the Eastern United States. 22 The Bailey Graph method isa reasonably simple method for forecasting air mass thunder- 215 10 5 20 25 30 storms, but does not require prediction of short SUM OF DEW POINT SPREAD AT 700 AND 600 MB range changeintheverticaldistribution of temperature and moisture. This method is best AG.611 used with the 1200Z sounding. Figure 11-5.Local area thunderstorm graph. Area "A" Thefirststepisto eliminate those areas is isolated thunderstorms, with a 12 to 1 chance of at whosesoundingsdisclosedmoistureinade- least one rain gage in dense network receiving rain. quacies. This is done by means of the following Area "B" is scattered thunderstorms, with a 4 to 1 six steps: chance of reported rain. Area "C" is no rain. I. Dewpoint depression 13°C or more at any One further condition for the development of level from 850 through 700 mb. thunderstorms was found to be the absence of 2. Dewpoint depression sum of 28° or more large anticyclonic wind shear which is measured at 700- and 600-mb levels. at 850 mb. This condition is that no horizontal 3. Dry or cool advection at low levels. shear at this level may exceed 20 knots in 250 4. Surface dewpoint 60°F or less at 0730 miles measured toward low pressure from the localwithno substantialincrease expected sounding station. Figure 11-6 illustrates how this before early afternoon. measurement is made. from 850 to 5. Lapse rate 21°C or less STABILITY INDEXES AS AN 500 mb. INDICATION OF INSTABILITY 6. A freezinglevelbelow 12,000 feetin unstablecyclonicflowproduces onlylight The overall stability or instability of a sound- showers. ing is sometimes conveniently expressed in the

365 AEROGRAPIIEWS MATE I & C

AG.612 Figure 11-6.Example of anticyclonic shear and curvature at 850 mb preventing thunderstorms, with otherwise favorable air mass conditions present in the Lithe Rock area. torm of asingle numer._al value called the temperature, dewpoint, wet-bulb temperature, stability index. Such indexes havc been intro- or potential temperature in height or pressure duced mainly as aids in connection with partic- between two arbitrarily chosen surfaces. These ular forecasting techniques or studies. Most of indexes are generally useful only when com- the indexes Like the form of a difference in bined, eitherobjectively or subjectively, with 366 X72 Chapter 11 FORECASTING THUNDERSTORMS, FOG, TORNADOES, ICING, AND CONTRAILS other data and synoptic. considerations. Used 2. The chance of thunderstorms increases alone. they .ire less valuable than when plotted rapidly for index values in the range of +1° to on stability index charts and analyzed for large areas.Irathis respect they have the value of 3. Index values of3° or less are associated alertingtheforecastertothose soundings, with severe thunderstorms. routes. or areas which should be more closely 4. When the value of the index is belt :/ examined by other procedures. the forecaster should consider the possibility of There are a number of methods that may be tornado occurrence. However. the forecasting utilized for determining stability or instability; value of index categories must. in each case. ..mong these are the Showalter Index (SI), the be evaluated in the light of the moisture content Lifted Index (LI). the Fawbush-Miller Stability of the air and of -Alter synoptic conditions. Index (FM!). and the Martin Index (Ml). The National Weather Service currently em- FORECASTING MOVEMENT ploys the Lifted Index method for producing OF THUNDERSTORMS their facsimile products. All current methods are discussed in detail in NAVAIR 50-I P-5, Use of Radar can be an invaluable aid in determining the Skew T. Log P Diagram in Analysis and the speed and the direction of movement of the Forecasting. however. inthis manual we will thunderstorm. Sometimesitis desirable and discuss the Showalter Index method only. necessary to estimate the movement from winds aloft. There is no completely reliable relation- Showalter Index (SI) ship between speed of the winds or direction of the winds aloftinforecasting thunderstorm This is the most widely used of the various movement. However. one studyreveals that types of indexes. there is a marked tendency for large convective The procedure for computing the Showalter rainstorms to move to the right of the wind Stability Index is explained in the following direction in the mean cloud layer from 850 to section from the illustration in figure 11-7. 500 mb (or the 700-mb wind) with a systematic Step 1. From the 850-mb temperature (T). deviation of about 25 degrees to the right of the draw a line parallel to the dry adiabat upward flov.There appears to belittlecorrelation until it intersects the saturation mixing ratio line between wind speed and speed of movement at for the dewpoint temperature at 850 mb. This is any level, although the same study mentioned called the LCL on this diagram. A mountain above revealed that 82 percent of the storms station would have to use some higher level. move within plus or minus 10 knots of a mean Step 2. Fro:the LCL, draw a line parallel speed of 32 knots to the saturation adiabat upward to 500 mb. Let FORECASTING MAXIMUM GUSTS WITH the temperature at this intersection point at 500 NONFRONTAL THUNDERSTORMS lab be called T'. Step 3. Algebraically. subtract T' from the It should be remembered that maximum guSts 500-mbtemperature. The valueof there- associated with thunderstorms occur over a very mainder. including its algebraic sign, is the value small potion of the area in which the thunder- of the Showalter Index. In figure11-7, = storm exists and usually occur imnux lately prior 25°C and T = 22°C. the Showalter Index is to the storm's passage. Nevertheless. the possibil- therefore +3°. This Index is positive when T' lies ity of damage to aircraft and installations on the to the left of the T curve. Positive index values surface is so great tivIt every available means imply greater stability of the sounding. should be used to make the best and most For forecasting purposes in the United States. accurate forecast possible to forewarn the agen- the significance of the index values to fore- cies concerned. casting is as follows: Estimation of Gusts From Climatology 1. When the index is +3° or less. showers are and Storm Intensity probable and some thunderstorms may be ex- The Aerographer's Mate is aware that the pected in the area. season of the year and the location of the 367 V43 AEROGRAPHER'S MATE 1 R C

250o 250 yo ft, / O

300

400

= 22° 500 500

SHOWALTER INDEX T500 600 AL,A 60

ISOBARSm 700 ANR.Ativ 700 800 800

900 o 90 0 Td 1000 1000

Figure 11-7.-Computation of the Showalter Stability Index. AG.613 station have a great bearing upon t!,e maximum The storm's intensity and those reports from windstobe expected. Certain areas of the neighboring or nearby stations can give youa United States, and the world, have a history of good indication of what conditions to expcct at severe thunderstorm occurrence with ensuing your terminal. strong winds during the most favorable seasons of the year. For this reason you should have the Forecasting Peak Wind thunderstorm climatology for your station, as Gusts Using USAF Method well as for the general area, available as to time of occurrence, season of occurrence, and the There are two basic methods for forecasting associated conditions attending them. maximum windgustsof convectiveorigin 368 3 74 Chapter II FORE( .AS TIIUNDERSTORMS, FOG, TORNADOES, ICING, AND CONTRAILS outlined in AWS Technical Report 200 ( Rev). DETERMINATION OF T2. T2 is found by One invokes the use of the Di) Stabilit), Index first locating the 0°C isotherm on the wet-bulb (T1) and the other downrush temperature sub- curve. A moist adiabat through that point is tractedfrom the dry -bulb temperature (T21. followed down to the surface and the tempera- These methods will be discussed in this section. ture at that point is recorded. This temperature DETERMINATION OF is found in is subtracted from the dry-bulb temperature (or one of two ways: theforecast free air temperature) giving the I.If the sounding has an inversion. the moist value of T.). adiabat is followed from the warmest point of the in- Using the value of T2 refer to figure 11-8. version CO 600 millibars. The temperature difference This graph allows you "to arrive at the probable between the intersection of the moist adiabat at the minimum, mean, and maximum gusts. 600mb isobar and the temperature of the dry bulb at DETERMINATION OF GUST DIREC- 600mb is T1. The inversion (top) point should be with- TION. For the direction of the maximum gust. in 150 or 200mb of the surface and must not be sus- the mean wind direction in the layer between ceptible to becoming wi ped out by surface convection. 10,000 and 14,000 feet above the terrain is 2. If no inversion appears on the sounding or used. if the inversion is relatively high (more than 200 EXAMPLES OF FORECASTING METH- mb above the surface), a different method is ODS. Figure11-9 shows a typical sounding used to find Ti.The maximum temperature at plotted on the Skew T diagram for forecasting the surface is forecast in the usual manner. A maximum gust speeds. moist adiabat is projected from the maximum The following procedure may be followed: temperature to the 600 mb level. The tempera- 1. A moist adiabat isprojected from the ture difference between the intersection of the warmest point of the inversion to the 600 mb moist adiabat and the 600 mb surface and the surface, and the temperature at that intersection dry-bulb temperature at the 600 mb level is T1. is 0.8°C. After T1 has been determined refer to Table 2. The dry-bulb temperature at 600 mb is 11-1for determination of the maximum gust -7.8°C, so that Ti is about 9°C. speed. To further add to the reliability and 3. Entering Table11-1, the value of V' is accuracy of the forecast. one-third of the mean foundtobe 35 knots. To thisvalue add wind speed expected in the lower 5,000 feet one-third of the mean wind speed in the layer above the ground level should be added to the from the surface to 5,000 feet to obtain the value obtained from Table 11-1. maximum peak gust.

Table 11-1.Use of Ti for maximum wind gusts.

Maximum Gust Maximum Gust Ti Values in °C Speed (V') Ti Values in °C Speed (V')

3 17 14 47 4 20 15 49 5 23 16 51 6 26 17 53 7 29 18 55 8 32 19 57 9 35 20 58 10 37 21 60

II 39 22 61 12 41 23 63 13 45 24 64 25 65 AEROGRAPIIER'S MATE 1 & C 110 .,0- / . 100 / 1111111 90 / I/1/ 80 ///// / / e v/ /'III TO , f %e co /i t t g il/ zo / I Y , 60 If./ , ,,,i. /,, / t /1 50 Iii- fl it1 I II / I I gm 1 II I I 1 4 S 1 I 1 I I 1- 1 3O I I Fr I F .I 1 II 20 1 1 1 0 I 0 20 30 40 50 60 DEGREES CELSIUS (T2 )

AG.614 Figure 11-8.Probable maximum gusts using theT2method.

370 376 Chapter II FORECASTING TIIUNDERSTORMS, FOG, TORNADOES, ICING, AND CONTRAILS

-30 -25 -20 -15 -10 400r. .1 1 Imi ow r mrnow.v...=I 4400

0...A v 49k1 1111 c÷4'42 4.9.1,s, 0 A° 0 vA di

A16 18 20 24N 26

, 50C FREE AIR TEMPERATURE . 5 I tc)

1 ,:-7. 8° ,,,,O. 8 ° 4144A 600 , I0 1 MOIST ADIABAT ARIL 1:11 THRU WARMEST at "53AIII. 1POINT ON alA :1:2ERSION 700 WET BULB 15 CURVE

li q,... A AA800 yikra_ lair...,4 20 FREE AIR -0,AmvAterm TEMPERATURE900 VP CURVE1 1 25 11.0° ISOBARS A V41111 1 111MA 24.0°;000 WAWA% A 1 1050 5 10 I5 20 25

AG.615 Figure 11-9.Example of sounding for wind gust forecast.

371 4 7 lia

AEROGRAPIIER'S MATE I C

4. The forecast wind direction of the maxi- 2. Sini..e the surface dry-bulb temperature is mum gustis the same asthe mean wind 27°C the value of T-) is 15°C. direction from 10.000 to 14.000 feet above the 3. Enteringfigure11-8at II=15the terrain. probabiz minimum wind speed is 38 knots; the mean speed is 45 knots; and probable maximum Figure 11-10 is used to illustrate the method is 52 knots. of forecastim maximum w ind gusts using the T) 4. Wind directionis obtained in the same method. manner as the preceding method.

400

500

WET BULB TEMP CURVE CURVE 600

V700 A WET BULB 800 FREEZING/ LEVEL/ (MOIST ADIABATICALLY------....\ 900 TO SFC) 7' DOWN RUSH TEMP 1000 SURFACE PRESS 26° F 10 10 20 30

AG.616 Figure 11- 10. Example of sounding for forecasting maximum wind gusts using downrush temperature.

The fohowing steps show the correct pro- FORECASTING HAIL cedure for determining the value ofT2: Hail, like the maximum wind gusts in thun- I. A moist adiabat:s projected downward derstorms, usually takes place in a narrow shaft from the wet bulb freezing level to the surface seldom wider than a mile or two and usually less and the temperature at the intetsection is found than a mile wide. The oLuurrence ofhallin to be 12°C. thunderstorms was discussed earlierin

372 3 78 Chapter 11FORECASTING HIUNDERSTORMS. FOG. TORNADOES. ICING,AND CONTRAILS

thischapter. lloweeeri fewadditional available as to forecasting of the actualoccur- facts concerning the occurrence and frequency renLe of hail. The teLlinique presented in this of hail in flight should be discussed at this point. section of the chapter is an objective method of hail forecasting, using the parameters of the I. Sincehail isnormally associated with ratio of aloud Ltzpth below the f:eezing level and thunderstorms. the season of the maximum the height of the freezing level. The data used in occurrence of hail is coincident with the season this study were derived from 70 severe convec- of maximum occurrence of thunderstorms. tive storms (34 hail producing and 36 nonhail 2. When the stormislarge and well de- producing storms) over the Midwestern States. veloped. an assumption should be made that it Severe thunderstorms, as used in the develop- contains hail. ment of this technique. were defined as those 3. Encounters ofhailbelow10.000feet thunderstormscausingmeasurableproperty showed thehaildistribution equally divided damage due to strong winds, lightning, or heavy between the clear air alongside the thunder- rain. Severe thunderstorms with accompanying storm. inthe rain area underneath the storm, hail were defined as those thunderstorms tm.:om- and within the thunderstorm itself. panied by hail where hail was listed as the prime 4. From 10.000 to 20,000 feet, the percent- cause of property damage even though other ages ranged from 40 percent in the clear air phenomena may also have occurred. All torna- alongside the storm to 60 perLent in the storm. does w ere excluded from consideration to avoid with 82 percent of the encounters outside the confusion. storm under the overhanging cloud. METHOD OF ANALYSIS. -Plot representa- 5. Above 20,000 feet, the percentages re- tive upper air soundings (0000Z and 1200Z) on flected 80 perLent of the hail was enLountered the Skew T diagram. Then analyze the following in the storm with 20 percei.in the clear air parameters: beneath the anvil or other cloud ,..:,.tending from the storm. I. Convective condensation level (CCL). 2. Equilibrium level (EL). The EL is found at Climatology is of vital importance in pre- the top of the positive area on the sounding dicting the hail occurrence, as well as its size. where the temperature curve and the saturation Good estimations of the size of hailcan be adiabat through the CCL again intersect. This gleaned from reports of the storm passage over gives a measure of the extent of the cloud's nearby upstream stations. Here too. modifying vertical development and thus an estimate of its influences must be taken into account. top or maximum height. 3. Freezing level. This is defined as the height Hail Frequency as Related to of the zero degree isotherm. Storm Intensity and Height NOTE: ALL THREE HEIGHTS ARE EX- PRESSED IN UNITS OF MILLIBARS. Fifty percent hail occurrency may be ex- Next determine the following two param- pected in storms exceeding 46,000 feet. based eters: on radar echo height alone. With a maximum echo height of 52.000 feet. a 67 percent hail I. The ratio of the cloud depth below the frequency can be expected. and only 33 percent freezing level (distance in millibars from the at 35.000 feet. Mean echo heights are 42.000 CCL tothefreezinglevel)tothe cloud's feet for hail and 36,000 feet for rain. estimated vertical development (distance from the CCL to the EL in millibars) is defined as the A Yes-No Hail Forecasting Technique cloud depth ratio. For example, if the CCL was at 760 mb, the freezing level at 620 nib, and the Aside from thefact that had occurs with EL at 220 mb, then 'he cloud depth atio would thunderstorms both inside and outside the storm be computed as follows: and our knowledge of the relationship of hail to 140 Cloud depth ratio this type of storm, very little information is 540

373 L79 AEROGRAPIIER'S MATE I & C

580 0

.540

.500 NO HAIL .460 0 0 0 420

.380 X X X 0 000

.340 0 0 0 0 0 .300 x x .260 0 ox

0 X x .220 0 0 x

.180 HAIL 0

x 0 .140

.100 550 570 550 610 650 650 670 690 FREEZING LEVEL (mb)

AG.617 Figure 11-11.Scattered diagram showing the distribution of selected hailoccurrences at certain midwestern stations during the spring and summer of 1959 and 1960. The freezing level is plotted against the cloud depthratio. (X = hail reported. 0 = no hail reported.)

2. Height of the freezing levelin millibars Hail Size Forecasting (620 mb). With 1 as the ordinate and 2 as the abscissa, Once the forecaster has determined that the enter flume II-1 1 for occurreme or II0I1OCLUr- piob.tbility of hail exists, as previously outlined, rence of hailIn this case. hail would be forecast the next logical question will relate to the size of as the value falls in the hail forecast area. tile hail that may be anticipated. AWS Technical EVALUATION. Using the data in 70 de- Report 200 (Rev) provides atechnique for pendent cases. the percent correct for predict,on determining hail size that has been developed by of hail or no hail was 83 percent. This technique the Air Force. This method utilizes the Skew T combines the two parameters relating hailto Log P diagram. convective activity into a single predictor. Al- The first step in forecasting hail is to deter- though the data used in this study were from the mine the Conve4,tive Condensation Level (CCL). Midwest, the application need not be confined This parameter is evaluated on the adiabatic to that area. With some modification of this I'lart by finding the mean mixing ratio in the diagram, this method could serve as a basis for a moist layer ut tic lowest 150 mb, and following local forecasting tool for other areas. thissaturationmixingratiolinetoits

z?140 Chapter 11 FORECASTING THUNDERSTORMS, FOG, TORNADOES, ICING, AND CONTRAILS intersection with the sounding dry-bulb tempera- thunderstorms and the attendant weather. Fig- ture curve Next. the moist adiabat through the ure I 1-15 is a suggested format and checklist to CCL is traced up to the pressure level where the insure that all parameters have been given due dry-bulbtemperatureis-5°C. This pressure consideration. level, the dry-bulb temperature curve, and the moist adiabai through the CCL forma tdangle outlining a positive are. Figure 11-12 illustrates TORNADOES thisprocedure. The horizontal coordinate in figure 11-12 is the length of the horizontal side Tornadoes are the least understood and least of the triangle in degrees Celsius. The vertical known atmospheric phenomena. Only their rela- coordinate isthe length (in degrees) of a dry tively small size ranks them as second in the adiabat throughthetriangle. This lengthis severity of the damage they cause, with tropical measured from the pressure at the base of the cyclones ranking first. Pound for pound. torna- triangle. does are unsurpassed in the damage they cause. These computations for the horizontal length Tornadoes are violently rotating columns of air in degrees and altitude in degrees are utilized on extending downward fromacumulonimbus the graph in figure 11-13. for the forecast of cloud. They are nearly always observed as funnel hailstone diameter. clouds. They occur only in certain areas of the EXAMPLE OF TECHNIQUE. In the sound- world and are most frequentin the United ing shown in figure 11-12, the CCL is point A. States in the area bounded by the Rockies on The moist adiabat from the CCL to the pressure the west and the Appalachians on the east. They level where the temperature is5°C is the line also occur during certain preferred seasons of AB'. The isobar tram the point where the air the year in the United States with their most temperature is -5°C to its intersection with the frequent occurrence during :lay. The season of moist adiabat is the line BB'. The dry adiabat occurrencevaries with the locality. Too, 80 from the isobar BB' through the triangle to the percent of the tornadoes in the United States pressure of the CCL is the line IIH'. The base of have occurred between noon and 2100. the triangle in degrees Celsius is 6°C (from plus In the last few years the U.S. Air Force and

1 to minus 5). The length of the dry adiabat the National Weather Service have given in- through the triangle is 21°C (from minus 4 to creasing attention to the forecasting of severe plus 17). The value on the graph in figure 11-13, local storms and tornadoes. Since the establish- with a horizontal coordinate of 6 and a vertical ment of the Severe Local Storm (SELS) Fore- coordinate of 21. is a forecast of 1 inch hail. cast Center in Kansas City, Missouri. the predic- In order to more accurately forecast hail size tions of severe thunderstorms and tornadoes in conjunction with thunderstorms along the have been placed on a more systematic basis. Gulf Coastorinanyairmass where the Warnings and forecasts are issued for the United Wet-Bulb-Zero height is above i 0,500 feet, itis States from this center at periodic intervals for necessary to refer to the graph in figure 11-14. use by both the general public and the military. Thehallsize derivedfromfigure11-13is Complete details on the forecasting of these entered on the horizontal coordinate of figure phenomena are beyond the scop, of this training 11-14, and the corrected hail size read off is manual. Itis suggested that the reader consult compatible with the height of the Wet-Bulb-Zero the U.S. Department of Commerce. Forecasting temperature. Guide No. I. Forecasting Tornadoes and Severe Thunderstorms, and the many other excellent A THUNDERSTORM CHECKLIST texts andpublicationsfor a more complete understandingofthisproblem. The senior Regardless of where you are forecasting, the Aerographer's Mate should have a basic under- factorsandparametersfavorableforthun- standing of the factors leading to the formation derstorm, hail, and gust fore,:asting should be of such severe phenomena. to recognize poten- systematizedinto some sol. of checklist to tial situations. and to Ix51e to forecast such determine the likelihood and probability of phenomena.

375 AEROGRAPHER'S MATE 1 & C

S o -20° -15° -10° D -5° 0°

-20

500MB 10° S

ISOBAR WHERE TEMPERATURES- 5' 600 15°

- 10°

700 le:41Ab..20° C S D - 5° -***P-vgyrccL 4 H. 800 X X 25° / X 1' 0° WET BULB TEMPERATURE 900 S D

1000 X * 5°C S 10° D 15° S 20° D S 25°S 30°

AG.618 Figure 11-12.Example of hail size forecast sounding. Chapter I IFORECASTING TIIUNDERSTORMS, FOG, TORNADOES, ICING, AND CONTRAILS

HAILSTONE DIAMETER IN INCHES 0. q 0. 0 V' c%i If) N M / I 1 I I I I / I I I i 1 1 I N / / I in I / , I / / i I / / I / / / 1 / 0 / / /////// / I ,, / / 1 / co 0 /I / / / I / / .I. .3./ i . / i / ro / co 0r , / rei rr / // / / IR ti N r r / I .. // ____...04r I co 0.... -- / N /'. / .-/ / ..e to --...... - r/ In _ , --, / V --. I ... M ...-...- it I -- --...-, --- 1.4 / V / /I N // ./

0 0 1 0 0 r) o in o in 0 co 0 in ic) q. .3. re) PO N N -- ALTITUDE OF POSITIVE TRIANGLE

AG.619 Figure 11-13.FawbushMiller Hail Graph showing forecast hailstone diameter in inches. AEROGRAPIIER'S MATE I & C

14000 I

13500 SMALLHAILZONE APHEAVYRAIN <1/4 13000 J O 1/4 co 12500 I <1/4

o 12000 0 1/2 1/4 ....------3/4 11500 1/2 <1/4 ?4 1.5

1/4 2 1/2 3/4 1.5 10500/4 ,..-,......

1/2 3/4 1 2 3 4 5 HAIL SIZE FROM FAWBUSH MILLER GRAPH

AG.620 Figure 11-14.Hail size at surface expected from tropical air mass thunderstorms.

SURFACE THERMAL PATTERNS the surface synoptic chart. Use the following AS A FORECASTING AID technique to supplement the existing tornado forecast. This method indicates that thermal tongues The procedure to locate areas of potentially averaging 50 miles or less in width are favored severe convective activity with reference to the locations fur tornado activity within the more synoptic surface thermal pattern is as follows. general area of convective storm activity. These thermaltongues and associatedforecasts of I. Draw isotherms for every 2 degrees to tornado activity can be located quite readily on locate thermal tongues. Chapter II FORECASTING TIIUNDERSTORMIS, FOG, TORNADOES, ICING, AND CONTRAILS

WORKSHEET

SUGGESTED CHECK LIST FOR THE DETERMINATION OF AIR MASS THUNDERSTORM ACTIVITY

I. Analyze the Arowagram using the closest sounding & parcel methods

1. Determine the following

a. Convective condensat...on level (CCL) 5,000 zifoc(Sz.41F) b. Temperature necessary for convection

c. Maximum temperature forecast 3ot (e6°F)

d. Is temperature necessary for convection Y5 expected to be reached?

e. Inversions present? (Stg/Weak/Mdt/Height) NONE

f. Inversions strong enough to prevent or retard convection activity No

g. Positive energy area favorable/unfavorable FA-1/

h. Does positive encrgy area extend well above the freezing level? (Preferably above the -100 C isotherm) )/Es

i. Does moist layer extend to 10,000 ft?

j. At least 3 gr/kg moisture 12,000 ft YEs 64.5)

k. Conditionally unstable air extent to 16,000' )1s

1. Stability index (fav/unfav) I (FAY)

m. Were thunderstorms or clouds of vertical development present previous day? If so, has there been any change at lower or upper levels to retard further development today -No CN-4At6t

n. Is month and time of year favorable Clime- tologically for thunderstorm development? P.,51? go)

2. Forecast from consideration of a thru n. /1,771/

Note:The most important single predictor of daytime thunderstorms is moisture in the lower troposphere in the morning.

Determination of f. is of prime consideration. You must decide if heating and lift is sufficient to overcome the inversion.

AG.621 Figure 11.15. Suggested checklist for the determination of air mass thunderstorm activity.

,L85379 AEROGRAPIIER'S MATE I & C

II. Forecast from using the Bailey Graph SerD Ts rfrif

III. Final forecast from all of the above considerations 5cTOAcr/V kgMASSTsT115BETWEEN 14- MooLoc,AL,

IV. Gusts

1 Table 13-1 4-5 kf

2. AWS Manual Method 410 Hi

3. Estimated from reports or expected intensity of storm 3Y/er

4. Final Forecast 1.// K75

V. HailCSURFAW

1. Yes-No Method

2. Fawbush- Miller Method Note if wet bulb zero above 10,500 ft.

3. Es timated 4. Forecast e/2II

AG.622 Figure 11-15.-Suggested checklist for the determination of air mass thunderstorm activity-Continued.

2. Within the general area in which convec- TORNADOTYPES tive storms are forecast, locate the axis of all pronouncedthermal tongues orientednearly There are commonly believed to be three parallel to the gradient flow. tornado producing air structuresfor tornado 3. On this axis, locate the point with the development over the United Sto.tes. They are greatest temperature gradient within 50 to 100 the Great Plains type, the Gulf Coast type, and miles to the right, and normal to the flow. West Coast type. 4. From this reference point, a rectangle is constructed with its left side along the axis of Great Plains Type the thermal tongue by locating corner points on theaxis 25 miles upstream and125 miles Tornadoes in this type air mass favor the downstreamfromthereferencepoint. The intersection of the squall line with warm front rectangle is 150 miles long and 50 miles wide. zones. The tornado.. s will generally form on the 5. Thisistheforecastareaforpossible squallline,hencetheirpredictioninvolves tornado or funnel cloud development. timely delineation (geographical) of the squall

380 t...86 Chapter II FORECASTING THUNDERSTORMS,FOG, TORNADOES, ICING, AND CONTRAILS line formation along or in advanceof a cold FORECASTING FOG AND STRATUS front, upper cold front, or trough. Conditions must favor a downrush of air from aloft.(See Fog and stratus clouds are hazardous condi- chapter 4.) tions for both aircraft and ship operations. Senior Aerographer's Mates will frequentlybe lifting, or Gulf Coast Type called upon to forecast formation, dissipation of these phenomena. In order to In contrast to the air mass type (Great Plains assist them in providing the best information Type) tornadoes also form in an equatorial type available we will discuss the various factors that influence the formation and dissipation offog air mass that is moist to great heights. Such that may storms are most common on the coastof the and stratus, as well 'as some methods Gulf of Mexico and produce the waterspouts be utilized in forecasting these actions. oftenreportedoverFlorida. Tornadoes are triggered in this air mass primarily by lifting at EFFECT OF AIR MASS the intersection of a thunderstorm line with a STABILITY ON FOG warm front and less frequentlyby frontal and prefrontal squall lines. Fog and stratus are typical phenomena of a warm type air mass. Since a warm typeair mass that of West Coast Type is one with a temperature greater than the underlying surface, it is stable, especiallyin Tornadoes also form in relatively cold moist the lower layers. air. This air mass is the Pacific or West Coast Through theuse of upper air soundings, measurements can be made of temperatureand type.Itis responsible for waterspouts on the West Coast. Tornadoes in this type air mass are relative humidity, from which stability charac- normally in a rather extensive cloudy area with teristics can be determined. The publication,Use Fore- scatteredrain showers and isolated thunder- of the Skew T, Log P in Analysis and storms. Clouds are mostly stratocumulus.Favor- casting, NAVAIR 50-IP-5. contains complete able situations for tornado development in this information on analyzing upper air soundings air mass type include the rear of mP coldfronts; and should be consulted by the forecaster. well cooled air behind squall lines. GENERAL PROCEDURE WATERSPOUTS FOR FORECASTING FOG

Phenomena frequently encountered by naval One of the basic considerationsisthat a that personnel are waterspouts. Waterspoutsfall into potential fog or stratu' situation exists and over water andfair all of the factors are favorablefor formation. twoclasses:tornadoes of the weather waterspouts. The particular synoptic situation, time The fair weather waterspout is comparable to year, climatology of thestation, stability of the a dust devil. It may rotatein either direction, lapse rates,amountofcoolingexpected, whereas the other type of waterspout rotates strength of the wind, dewpoint-temperature cyclonically. In general. waterspouts are not as spread, and trajectory of the air overfavorable basic con- strong as tornadoes, in spiteof the large mois- types of underlying surfaces are all ture source and the reducedfriction. The water siderations to be taken into account. surface under a waterspout is eitherraised or lowered depending on whether itis affected Consideration of Geography more by the atmospheric pressurereduction or and Climatology the wind force. There is less inflow andupflow tornado. The Certain areas are more favorable climatologi- of air in a waterspozit than in a periods of waterspout does not lift anysignificant amount cally for fog formation during certain passing through the year than others.Allavailable information as of water from the surface. Ships of fog should waterspouts have mostly encounteredfresh water. to the frequency and climatology

381 AEROGRAPIIER'S MATE 1 & C be compiled for your station or operatingarea FACTORS TO BE CONSIDERED to determine the times andperiods most favor- IN able for formation. FOG AND STRATUSFORMATION Then determine the locationof the station Wind with respect to air drainageor upslope conditions. Next, determine the type of fog to which Wind velocity isan important consideration in your location would be exposed. Forexample, the formation of fog and/or inland stations would be low ceiling clouds. more likely to have When the temperature anddewpoint are close radiation fog and shorecoastal stations advec- at the surface and eddy tion fogs. A determination currents are 100 feet or should' then be made more in vertical thickness, adiabaticcooling in of air trajectories favorablefor fog formation at your station. the upward side of the eddycould give the additional cooling neededto bring about saturation. Any additional cooling wouldplace the air Frontal Fogs in a temporary supersaturatedstate. The extra moisture wilt then condenseout of theair, producing a low ceiling cloud. Forecasting of these types of Adiabatic heating fogs is asso- on the downward side of the eddy willusually ciated with the forecasting ofthe movement of dissolve the cloud particles. If all the cloud particles fronts andtheirattendantprecipitation dissolve before reaching the ground, areas. For example. fogs can form in the horizon- advance of tal visibility should be good.However, if many a warm front, in the warm air sectionbehind the particles reach the ground warm front (when the warm air before evaporation, dewpoint is the horizontal visibility will berestricted by a higher than the cold airtemperature), or behind moderate fog condition. Clouds a slow moving cold front when the air forming in eddy becomes current areas may at first be patchy andbecome saturated. identifiedas scud type clouds. If the cloud forms into a solid layer, itwill be a layer of Air Mass Types stratus. When conditionally unstableair is present in the eddy areaor if the frictional eddy currents are severe enough, stratocumulus The first step is to determine clouds a trajectory of will be identified in thearea. (See fig. 11-16.) the air at the station andestimate the changes during the night.If the air has been heated Saturation of Air normally during the day andthere was no fog the preceding morning, no marked cloud cover The saturation curve in figure11-17 shows during the day. andno trajectory over water, the amount of moisture in then no fog of marked intensity will grams per kilogram form during the air will hold at varioustemperatures. the night. However, duringfall and winter, nights are long and days The air along the curve is saturatedwith water are short, and condi- vapor and is at its dewpoint. Any further cooling tions are generally stable;and when a fog willyield water as a result ofcondensation, situation has been in existence, thesame conditions tend to remain night after hence fog or low ceiling clouds(depending upon night and the the wind velocity) will form. heating during the day is insufficientto effectively raise the temperature of the lower air. Nocturnal Cooling Also, determine if the air has hada path over extensive water bodies and whether this path Nocturnal cooling actually beginsafter the was sufficient to raise the humidityor lower the temperature sufficiently temperature reaches its maximum duringthe toform fog. Then day. The cooling will continue until construct nomograms, tables, etc; using sunrise or dew- shortly thereafter. This cooling affectsonly the point depression against time offog formation lower limits of the atmosphere, Ifnocturnal for various seasons and winds, andmodify these cooling reduces the temperature in the light of each particular to a value close synoptic situation. to the dewpoint, fog,or low ceiling, clouds will 382 88 Chapter 11FORECASTING THUNDERSTORMS, FOG, TORNADOES,ICING, AND CONTRAILS

4 0 0' TEMP60110.»TEMP66.2/b. P. 66.2 CLOUD LAYER GAF

-n

to41 O m0O TEMP. 66.9/ D.P. 66.8 TEMP 67.3 200' D.P. 66.2

-0ti DEW POINT RATE UNTIL SATURATION O O. 1° F. / 1,000' 0 CC, eb.

TEMP. 68 DEW POINT 67 0'

AG.623 Figure 11-16.How wind velocity can cause a low cloudlayer.

develop. The wind velocity and terrain rough- back to be once again absorbed by the land.This ness will control the depthof the cooled air. causes a reduction in nocturnalcooling. Noc- Calm wind will allow a patchy type of ground turnal cooling between 1530 local standard time fog or a very shallow continuous ground fog to and sunrise will vary from as little as5° to 10° form. Winds of 5 to 10 knots will usually allow with an overcast sky condition based around the fog to thicken vertically. Winds greater than 1,000 feet to 25° or 30° F with a clear sky or a 10 knots will usually cause low scud, stratus or cloud layer above 10,000 feet. Certain other stratocumulusto form. (See figs.11-18 and factors and exceptions must also be considered. 11-19 for examples of fog and stratus forma- If a front is expected to pass the station during tion.) the night, or onshore winds are expected to The amount of cooling at night is dependent occur during the night, the amountof cooling on soil composition,vegetation, cloud cover, expected would have to be modified in thelight ceiling, and other factors. Cooling may vary over of these developments. any particular area. A cloud coverbased below An estimate of the formation time of fog, and 10,000 feet has a greenhouse effect on surface possibly stratus, can be aided greatly if some asthat temperatures, absorbing someterrestrial radia- type of saturation time chart such tion and reradiating a portion of this heat energy illustrated in figure 11-20 can be constructed on

383 311 AEIZOGRAPIIER'S MATE. I & C

AG.624 Figure 11-17.Saturationcurve. which the temperature forecast versus the dew- compare the temperature ahead of the front pointforecast can be plotted. Tousethis with the dewpoint behind diagram, note the maximum thefront. If the temperature and temperature ahead of thewarm front is lower consider the general sky conditionfrom the surface map, forecasts, than the dewpoint behind thefront, air behind or sequence reports. By the front will cool toa temperature near the projecting the temperature and dewpointtem- temperature ahead of the front, causing perature, an estimated time of fog formation conden- can sation and the formation of fogor low stratus. be forecast. If smoke is observed inthe area, fog Over water areas.warm air passing over a will normally form about I hour earlierthan the relatively cold waterarea may cause enough formation line indicates on the charts because of cooling to allow condensationand the produc- the abundance of condensationnuclei. tion of low ceiling clouds. Another instance in whichtrajectory is im- Air Trajectories portantis when cold air movesover a warm water surface, marsh land,or swamp, a shallow layer of fog, called steam fog,may form. In The trajectory of the air reachingyour station addition, air passing duringthe over a wet surface will forecastperiodcanbe another evaporate a part of the surface moisture, causing important factor in fog formation. an increase in the dewpoint. Whenever there One important factor of air trajectory is a is warm moisture source present, air willevaporate a part air moving overa colder surface. This can of this moisture unless the happen after the rain area of vapor pressure of the a warm front has airisas great as or greater than thevapor moved over the station, and thestation is now in pressure of the water. The dewpoint increase the warm sector. Cooling of the air takes place may be enough to allow large eddycurrents, allowing condensation andwidespread fog or nocturnal cooling, or terrain lifting low stratus to form. To determine to complete the probabili- the saturation process and allowcondensation to tiesof condensation behinda warm front, occur.

84 ;290 Chapter II --FORECASTING THUNDERSTORMS, FOG,TORNADOES, ICING, AND CONTRAILS during the day with clear skies at night. Winds should be light, nights long, and the underlying DEW POINT TEMPERATURE surface wet.

1,0 0'

DEW POINT TEMPERATURE----- 800' 1,000' 600' 800' 400' w 600' I- 200' apo'

10° 14° 18° 22° 26° 30° TEMPERATURE °C 200' (A) 1530 LST MAXIMUM SURFACE TEMPERATURE 10° 14° 18° 22° 26° 30° DEW POINT TEMPERATUR TEMPERATURE °C 1,000' (A) 153 0 LST MAXIMUM SURFACE TEMPERATURE II 800' \41: SLOW OVERTURNING OF u_ * AIR (EDDY CURRENT) Z 600' DEWPOINT TEMPERATURE WPATCHY ° 400' GROUNiDill 1,000' 4 200 800' MPlipp-- 600' 10° 14° 18° 22° 26°30° TEMPERATURE °C 400' (8)SUNRISEMINIMUM SURFACE TEMPERATURE 200' AG.625 Figure11-18.Nocturnal cooling over alandarea producing patchy ground fog (calm winds). (A) 1530 10° 14° 18° 22° 26°30° LST maximum surface temperature; (B) sunriseminimum surface temperature. TEMPERATURE °C 03) SUNRISE- MINIMUM SURFACE TEMPERATURE CONDITIONS FAVORABLE FOR GROUND FOG AG.626 Figure11-19.Nocturnal cooling overa land area In the formation of ground fog, ideally, the producing stratus (10.15 knot wind). (A) 1530 LST air mass would be stable, moist in the lower maximum surface temperature; (B) sunriseMinimum layers and dry aloft, and under a cloud cove; surface temperature.

385

t_. AEROGRAPIIER'S MATH I & C

TIME OF HOURLY SEQUENCEREPORTS , 4/ 4/ 4.1 0 0 0 0 ") ") ,) 0 0 (1.1 I) 0 0 65 64 63 62

61 60 59 56 57 56 55

TIME OF FOG FORMATION 0330 E POSSIBLE TIME OF SCUD, STRATUS OR STRATOCUMULUS FORMATION 0130E TO 0230 E

AG.627 Figure 11-20.Saturation time chart. (O's indicate actual temperature and dewpointobservations. Straight line is forecast temperature-dewpoint trends.)

A stationary, subsiding,high-pressure area flow from a continental polar high, furnishes the best requirements for which has light winds. moved out over the water,or by cyclogenesis off clear skies, stability, and dry air aloft. If the air the east coast is relatively coldas well as moist. in the high has been movingover a body of If this air moves inland (replacing water.or warm dry land ifitliesover ground previously air). it may be cooled to saturation moistened by an active precipitating by nocturnal front. the radiation during the long autumnnights with wet surface will cause an increase in thedew- consequent formation of fog or stratus. The fog point of the lowest layers of the air. In addition, is limited to the coastalareas, extending inland long nights versus short days in fall and winter between 150 and 250 miles, dependingon the are favorable for the formation of radiation fog. wind speed. On the east coast it islimited to the region between the Appalachian Mountainsand CONDITIONS FAVORABLE FOR the Atlantic Ocean. ADVECTIONRADIATION FOG In late fall and winter, when thecontinental latitudinal temperature gradient hasintensified Air with a high relative humidity anddew- and the land temperature becomes colder point in late summer and early fall around than the the adjacent water, the poleward movingair is western side of the Bermuda high or by a return cooled by advection over colder groundas well

386 92 Chapter 11 FORECASTING HIUNDERSTORNIS,FOG. TORNADOES, ICING, ANDCONTRAILS formation at one's own as by radiation. Ifthe air is sufficiently moist. chances of stratus fogorstratus may form. Duringdaytime, station are high. heating may dissipate the fog or stratusentirety. If not, the heating. together withthe wind, CONDITIONS FAVORABLE which is adverting the air. sets up aturbulence FOR FRONTAL FOGS inversion and stratus, or stratocumuluslayers its base. At night if the airis cooled Frontal fogs are of three types: prefrontal form at front), and again and the surtace pressuregradient is weak, a (warmfront),postfrontal(cold surface inversion may replace theturbulence frontal passage. inversion and condensation in the formof fog again occurs atthe surface. However. if the Prefrontal Fog pressure gradient is strong,the cooling beneath fogs occur in stable the inversion intensifies it.Under these condi- Prefrontal(warm-front) tions stratus or stratocumulus occursjust as in continental polar (cP) air whenprecipitating the daytime. except with a lowerbase. warmairoverridesit.therainraising the Gen- Late tall and winter advection-radiationfogs dewpoint sufficiently for fog formation. the continent that can erally. the wind speeds are slight. andthe area can occur any place; over of this type of be reached by maritime air ormodified return- most conducive to the formation Mainly, thisis over the fog is one between a nearby secondary lowand a ing continentalair. the eastern halt' of the UnitedStates. Ilowever. since low-pressure center. in the entire world high a latitude in northeastern part of the United Statesis prol tropical air does not reach as this type of winter as in summer, the frequencyof such fog ably the most prevalent region of the northern part of the fog. They are also of importance alongthe Gulf is much smaller in Midwest, and country. With a large, slow movingcontinental and Atlantic coastal plain, in the warm high coveringthe eastern half of the in the valleys of the Appalachians. country, however, the fogs mayextend all the A rule of thumb for forecastingceiling during of Mexico to Canada. prewarn-frontal fog is: If the gradient winds are way from the Gulf greater than 25 knots, theceiling will usually remain 300 feet or higher duringthe night. CONDITIONS FAVORABLE FOR UPSLOPE FOG AND STRATUS Post fron tal Fog Upsiope fog and stratus occur in thoseregions postfrontal (cold- upward and As with the prefrontal fog, in which the land slopes gradually front) fogs are caused by failingprecipitation. which are accessible to invasion byhumid stable only when a In North America the areas best Fogs of this nature are widespread air masses. cold front of an east-westorientation has be- fitting these conditions are theGreat Plains of and continental polar air and the Piedmont come quasi-stationary the United States and Canada is stable. This type of fog isfrequent in the east of the Appalachians. for the Midwest. Fog or stratiform clouds areprevalent The synoptic conditions necessary behind cold fronts formation of this type of fog or stratus arethe for a considerable distance air and a wind with an associated with stable continentalair masses in presence of humid if the cold fronts have produced upslope component. The stratusis not adverted that region over the stationas asolidsheet,Itforms general precipitation. gradually overhead. The length oftime between the formation of the first signsof stratus and a Frontal Passage Fog ceiling usually ranges from 1/2 hour to2 hours. the stratus may During the passage of a front,fog may form Also, under marginal conditions, the front A useful procedure is to temporarily if the winds accompanying not form a ceiling at all. the two air masses are near check the hourly observationsof surrounding are very light and If one saturation. Also, temporary fog mayform if the stations, especially those southeastward. moist ground with of these stations startsreporting stratus. the airis suddenly cooled over 387 C93. AROGRAPHER'S MATE I& C

the passage ofa piecipitating cold front. In low at the surface may persisteven with high winds. latitudes fog may form insummer if the surface is cooled sufficiently Maps are available, showingthe mean monthly by evaporation of rain- sea surface isotherms. Synoptic water that fell during frontalpassage provided charts show the that the moisture addition temperature and dewpoint ofthe air and its to the air and the movement, Use of these two cooling are great enoughto cause fog formation. charts in conjunction with each otherindicates whether the temperature relationship and airflows CONDITIONS FAVORABLEFOR are such as to make probable the formationor dissipation THE FORMATION OFSEA FOGS of fog or stratus. Sea fogs are advection fogswhich form in CONDITIONS FAVORABLEFOR warm moist air cooled to saturationas the air moves across cold water. The cold THE FORMATION OFICE water may (CRYSTAL) :FOGS occur as a well-defined currentor as a gradual latitudinal cooling fromsouth to north. The dewpoint and the temperature When theair temperature is belowabout of the air un- -25°F, any water dergo a continuous changeas the air moves vapor in the air condensing into droplets is quickly convertedinto ice crystals. A across colder and colder water. Thesurface air suspension of ice crystals inthe air at the surface temperature falls steadily. tendingto approach of the earth the water temperature. The is called ice fog. Ice fogoccurs dewpoint also tends mostly in the Arctic to approach the water regions, and is mainlyan temperature. but at a artificialfog produced by slower rate. If the dewpoint ofthe air is initially human activities, occurring locallyover settlements and airfields higher than the coldestwater temperature to be where hydrocarbon fuelsare burned. (Burning I crossed, and if the coolingprocess continues sufficiently pound of hydrocarbon fuelproduces 1.4 pounds long. the temperature ofthe air of water.) ultimately falls to the dew point and fog results. When the airtemperature is approximately However, if the initial dewpointis less than the -30°F or lower, ice fogfrequently forms very coldest water temperature, theformation of fog rapidly in the exhaustgases of aircraft, auto- is unlikely. Generally innorthward moving air, or in air which has previously traversed mobiles, or other types ofcombustion engines. a warm When there is littleor no wind, it is possible for ocean current, the dewpoint of the air is initially an aircraft to generate enough higher than the cold water ice fog during temperature in the landing or takeoffto cover the runway and north and fog will form,provided sufficient a fetch occurs. portion of the airfield. Dependingon the atmospheric The rate of temperature conditions,icefogs may persistfor decrease is largely periods which dependent on the speed at which vary from a few minutes to the air moves several days. across the sea surface isotherms. which inturn is There is also a flue arcticmist of ice crystals dependent both on the spacing oithe isotherms and the velocity of the air which persists asa haze over wide expanses of normal to them. the arctic basin during The removal ofa sea fog from over the winter; it may extend upward through much of the coldest water requiresa change in air mass (a troposphere-a sort of cirrus cloud reachingdown to the ground. cold front). A movement ofsea fog to a warmer landarealeadstorapiddissipation. Upon USE OF THE SKEW TDIAGRAM I heating, the fog firstlifts, forming a stratus deck; then with further heating, FORECASTING THE FORMATIONAND this overcast DISSIPATION OF FOG breaks up into a stratocumuluslayer and eventually into convective type clouds,or evaporates One of the most accepted entirely. An increase in wind velocity methods of fore- can also casting the formation and raiseasurfaceseafog into a stratus deck dissipation of log today makes use ofan upper air sounding especially if the water is not much,if any, colder plotted on the Skew T diagram.The plotting of than the air. Over very coldwater, dense sea fog an upper air sounding is useful inforecasting 388 ICING, AND CONTRAILS Chapter 11 FORECASTINGTIIUNI)EKSTORMS, FOG, TORNADOES, Determine the height of thefoe layer from the both the formation and dissipationof fog, but it value of the pressure-height curve atthis level. can be used moreobjectively in forecasting fog The dry adiabatic method is based onthe fact dissipation. is1 °C per 100 The use of an upper air sounding todetermine that the dry adiabatic lapse rate be subjet - in, or 1°C per328 ft. Using this method, follow the possibility of fog formation must intersection of the five. A study of the existing lapse rateshould be the dry adiabat from the line with the temperature made to determine the stabilityand instability average mixing ratio of the lower layers. The surface layer mustbe curve to the surface level.Find the temperature is not found to difference between the point where the dry stable before fog can form, If it point of be stable on the sounding, thecooling expected adiabat reaches the surface and the during the forecast period must beconsidered, intersection of the dry adiabat and the average 11-21 the and this modification should beapplied to the mixing ratio. For exomple, in figure is25° C. The sounding to determine if the layerwill be stable dry adiabatatthesurface temperatureattheintersection of the dry with the additional cooling. 20° C. By The difference between the temperatureand adiabat and the average mixing ratio is with a lapse the dewpoint must beconsidered. If the air applying the dry adiabatic method temperature and the dewpoint areexpected to rate of 1° C per 328 ft, wefind the height of the coincide duringtheperiod covered by the top of the fog layer asfollows: forecast, a formation of fog is,of course, very 5 likely. I = The expected wind speed mustbe considered. 328 If the wind speed is expected tobe strong, the cooling will not resultin a surface inversion x = 1,640 ft favorable for the formationof fog, but may result in an inversion above thesurface, which is Dissipation favorable for the formation of stratusclouds. To determine the surface temperatureneces- dissipation of fog, trace dry adia- DETERMINATION OF FOG HEIGHT sary for the batically on the Skew T from theintersection of the tempera- during the time the average mixing ratio line and An upper air sounding taken level. The temperature inversion. The ture curve to the surface fog is present will show a surface surface levelis the the top of the of the dry adiabat at the fog will not necessarily extend to dissipation. dewpoint have temperature necessary for complete inversion. If the temperature and This temperature is known as theCRITICAL the same value at the top ofthe inversion, it can is an ap- the top of TEMPERATURE. This temperature be assumed that the fog extends to proximation, since it assumes nochanges will the inversion. However, if theydo not have the take place in the soundingfrom the time of same value,thedepth of thefog can be mixing ratio at the observationtothe time of dissipation. This determined by averaging the modified on the basis of surface and the mixing ratio atthe top of the temperature should be onditions, (See Fig. 11-21.) inversion. The intersection ofthis average mix- loc:.. ing ratio with the temperature curveis the top GENERAL DISSIPATION RULES of the fog layer. Two methods that may be used tofind the of fog and low height of the top of the foglayer in feet are In considering the dissipation ceiling clouds, consideration shouldbe given to readingtheheight from the pressure-height the rate at which the groundtemperature can curve on the SkewT and the dry adiabatic thick fog or pressure-heightcurve increase after sunrise. Vertical!) method,Inusingthe will slow up the method, locate the point wherethe temperature multiple cloud layers in the area morning heating of the ground. Itis the heating curve and the averagemixing ratio line intersect of the ground that increasesthe dissipation of on the arowagrani.Move this point horizontally the fog overlying the ground,If advection fog is untilthe pressure-height curveisintersected. 389

irk I.-. t...* Zia AEROGRAPI1ER'S MATEI & C

Determining the Base and Top of a Stratus Layer

One of theFirststepsinforecasting the dissipation of stratus isto determine the thickness of the stratus layer. Theprocedure is as follows:

I. Determinea representative mixing ratio between the surfaceand the base of thein- version. 2. Project thismixing ratiolineupward through the sounding. 3. The intersection ofthe average mixing 0 5 or 10 15 25 ratio line with thetemperature curve gives the approximate base and A = AVERAGE MIXINGRATIO maximum top of the 8 = TEMPERATURECURVE stratus layer. Point A in figure11-22 is the base C = PRY ADIABAT of the stratus layer, and point B is themaximum top of the layer. Point A isthe initial base of the layer: but as heating AG.628 occurs during the morning, Figure 11-21.Dry adiabaticmethod of determining the base will lift. Point13 represents the maxi- fog height. mum top of the stratus layer,although in the very early morning it might lie present. the fog may be lifted closer to the base off the groundto of the inversion. However,as heating occurs a height whereitis classifiedas stratus.If during the day, the top of ground fog is present, the stratus layer will the increase in surfaceair also rise and will be temperature approximated by point B. If willcause the fog particlesto the temperature and the evaporate, dewpoint are thesame thus dissipating thefog.Further at the top of the inversion, heating may the stratus will evaporate advection fog andlow extend to this level. ceiling clouds, To determine the numerical value of thebase FORECASTING STRATUSFORMATION and the top of the stratus layer use eitherthe method previously outlinedfor the fog or the The use of the plottedsounding to forecast pressure altitude scale. the formation of stratus can be used in thesaute manner as it was for forecastingthe formation Determining Dissipation of fog. The lapse Temperatures rate as it currently existsand modifications which take place must be taken To determine thetemperature necessary for into considerationto determine if the dissipation of the a favorable stratus ..iyer to beginand type of inversion will form. for the dissipationof the L tuts layer Fog and stratus to be forecasting are so closelytied complete, the followingsteps are followed: together that many of therules and conditions previously mentioned also apply to stratus. I. From point A infigure 11-22 follow the dry adiabat to the surface FORECASTING STRATUSDISSIPATION level. The temperature of the dry adiabatat the surface levelis the temperature required A radiosonde soundingtaken during the time to be reached for the that stratus exists will stratus dissipation to begin.This is noint C. show an inversion, usually 2. From point B at a short distance abovethe surface. The base on figure 11-22 follow the and the top of this dry adiabat to the surfacelevel. The temperature inversion do not necessarily of the dry adiabat indicate the base andtop of the stratus layer. at the surface level isthe surface temperature requiredfor the dissipation 390 i ...... "/'70C ICING, AND CONTRAILS Chapter I I FORECASTINGTHUNDERSTORMS, FOG. TORNADOES,

AVG MR TURBULENT LAYER MAXIMUM STRATUS TOP

BASE OF STRATUS MORNING SOUNDING A DI ABATS

SURFACE 'Sc' TEMP TEMP DISSIPATION DISSIPATION BEGINS COMPLETE

TEMPERATURE

AG.629

showing the base and the topof stratus layers. Also note temperature Figure 11-22.Sounding dissipation is complete. at which dissipationbegins and temperature when

quite widely in the complete. This is point One rule of thumb used of the stratus layer to be dissipation of the stratus layer is toestimate the D. thickness of the layer: and if nosignificant cloud layers are present above, andnormal heating is Determining Time of Dissipation expected, forecast the dissipationof the layer with an average of 360 feet perhour of heating. After determinationofthetemperatures be made of the dissipation to begin and be In this way an estimate can necessary for stratus number of hours required todissipate the layer. completed, a forecast of thetime these temperatures will be reached mustbe made. If available, Otherwise. estimate FORECASTING ADVECTION use diurnal heating curves. OCEANS the length of time for therequired amount of FOG OVER THE heating to take place: and onthe basis of this Inthe absence of actualtemperature and estimate, the time ofdissipation may be fore- stationary high, the take into consideration the dewpoint data and with a cast. Remember to isasfollows(asoutherly flowis absence or presence of cloudlayers above the method assumed): stratus deck. In addition,the trajectory of the air over the station, iffrom a water surface, can isobar at which temperatures down fora longer than I. Pick out the point on an hold the highest sea temperatureis present (either normal period of time.

391

. Z.SP7 AEROGRAPHER'S MATE1 & C from thechartof the mean monthly sea b. Estimate the expected temperatures or the synopticchart), and assume wind velocity for that at this point the night hours. the air temperature is equalto c. Multiply that of the water andhas a dewpoint 2 degrees a by b. This will give the lower. distance the upslope wind willmove during the period of the day when 2. Find the pointon the isobar northward daylight heating cannot where the water is 2 counteract upslope cooling. degrees colder. From this d. Determine approximate point on, patches of lightfog shouldoccur. terraineleva- 3. From a saturation tion difference betweenstation and distance chart such as shown ina computed in c. Elevation previous section of thischapter find how much difference should be in thousands and tenths' ofthousands of feet. further cooling wouldhave to occur to givean excess over saturation of 0.4 gr/kg (Example, 2.5 thousand feet.) and also 2.0 e. Multiply elevation difference gr/kg. Thefirstrepresents the beginning of by dry moderate fog and the adiabatic rate of cooling.(Example, 2.5 times second represents drizzle. 5.5 = 13.75 °F of upslope 4. As the air continuesaround the northern cooling.) ridge 3. Add expectedamount of upslope cooling of the high,itwillreachitslowest temperature, and from then to expected nocturnal coolingfor net expected on will be subject to cooling. warming. The patternthen will be drizzle until the excess is reduced 4. Determine 1530 LST temperature- to 2.0 gr/kg and moderate dewpoint spread at station fog until 0.4 gr/kg isreached. under consideration. Ifactual water and If the expected cooling isgreater than the 1530 temperature data are LST spread, either fogor low ceiling clouds available, these would beused in preferenceto climatic mean data. If should be expected. Windvelocity will deter- the high is moving, the mine which of the two conditions trajectories of the airparticles will have to be will form. calculated. This method was usedwith considerable AIRCRAFT ICING success during World War IIto calculate the southern boundary of fog areas. However, the Aircrafticingis fog is usually lesswidespread than the calcula- another of the weather tions indicate and drizzle hazards to aviation. It isimportant that the pilot is less extensive. Also, be informed about icing clearing and liftingon the east side of the high is because of the serious effects it has on the aircraft'sperformance. Ice slightly faster. This methodappears to work well on the airframe decreases in the summerover the Aleutian areas where lift and increases such fog is-frequent. weight, drag, and stalling speed.In addition, the accumulation of iceon exterior movable surfaces affects the control of FORECASTING UPSLOPE FOG the aircraft. If ice begins to form on the bladesof the propeller, the propeller's efficiency isdecreased and still Terrain lifting of the air willcause adiabatic further power is demanded cooling at the dry adiabaticrate of 5 1/2°F per of the engine to 1,000 feet. maintainflight. Today, thoughmost aircraft If an adequate amount oflifting have sufficient reserve occurs, fog or low ceiling clouds will form. power to fly with a heavy This load of ice,airframeicingis is usually a nighttimephenomenon. still a serious The procedures for determining problem because it resultsin greatly increased the probabil- fuel consumption and ities of fog or low ceilingclouds during night- decreased range. Further, the possibility alwaysexists that engine-system time hours at a station havingupslope wind are as follows: icing may result in loss ofpower. 1. Forecast the amount of The total effect ofaircraft icing is loss of nocturnal cooling. aerodynamic efficiency; 2. To determine the expectedamount of up- loss of enginepower; slope cooling: loss of proper operationof control surfaces, brakes, and landinggear; loss of aircrew's out- a. Determine the approximatenumber of side vision; false flight hours between sunset andsunrise. instrument indications; and loss of radiocommunication. Chapter 11 FORECASTINGTilUNDERSTORMS, FOG, TORNADOES.ICING 'AND CONTRAILS air is not For these reasons. itis important that the Icing at 0°C will occur only if the forecaster be alert for and aware of thecondi- saturated. Thisis due to the fact that the tions conducive for ice formation andbe able to nonsaturated condition is favorable for evapora- formation of tion of part of the drop. Evaporationof the accurately forecast the possible and ice icing inflight weather briefings. This chapter drop cools the drop below freezing, covers conditions for theformation of icing, formation then can take place. types of icing, intensities of icing,and where icing is most likely to be found as well asbasic TYPES OF ICING icing forecasting techniques. There are three basic forms of ice accumula- SUPERCOOLED WATER tion on aircraft: rime ice, clear ice, andfrost. In IN RELATION TO ICING addition, mixtures of rime and clear ice are common. The type oficing, clear or rime, is Two basic conditions must be metfor ice to dependent primarily upon the droplet size. form on an airframe in significant amounts. Some of the factors which determine icing First, the aircraft-surface temperature mustbe types are: droplet size, liquid watercontent (the colder than 0°C. Second, supercooledwater amount of water in the formof droplets in a droplets, liquid water droplets atsubfreezing gien volume of air), temperature, airspeed, and temperatures. must be present.Water droplets in size and shape of the airfoil. the free air, unlike bulk water. do notfreeze at Among thosefactorsfavorablefor dense 0°C. Instead, their freezing temperaturevaries structure (like clear ice) are: High watercontent, from an upper limit near-10°C to a lower limit large dropletsize,temperature onlyslightly airfoil. near -40°C. The smallerand purer the droplets, below freezing, high air speed, and thin variables, it should the loweristheirfreezingpoint. When a Because of these numerous supercooled droplet strikes an objectsuch as the be stressed that many gradations in appearance, surface of an aircraft, the impactdestroys the type, and structure of icing may oftenbe found internal stability of the droplet andraises its on the same aircraft. freezing temperature. In general.therefore. the possibility of icing must be anticipatedin any Rime Ice flightthrough supercooledclouds or liquid precipitation at temperatures belowfreezing. in Rime ice is a rough milky, opaque iceformed addition, frost sometimes forms on anaircraft in by the instantaneous freezingof small super- clear humid air if both the aircraftand air are at cooled droplets as they strike theaircraft. The subfreezing temperatures. factthatthedroplets maintain their nearly spherical shape upon freezing and thus trapair. PROCESS OF ICE FORMATION between them gives the ice its opaque appearance and makes it porous.Rime ice occurs at ON AIRCRAFT temperatures below -8° to10 °C, although it -28°C. The first step in ice formation on anaircraft is has been observed between -2° and the surface Rime iceis most frequently encounteredin when the supercooled droplets strike ice, it of the aircraft and as the droplet, orportion of stratiform clouds. In comparison to clear fusion. Some of is relatively easy to get rid of byconventional it, freezes it liberates the heat of airfoil to a this liberated heat is taken onby the unfrozen methods, even though it distorts the portion of the drop, its temperatureis thereby larger degree than does clear ice. increased, while another portion ofthe heat is conducted away through the surfacein which it Clear Ice begins to evaporate lies. The unfrozen drop now ice due to its increase in temperature,and in the Clear ice is a glossy, clear, or translucent the heat which in turn formed by relatively slow freezingof large process it uses up some of spread cools the drop. Due to thiscooling process by supercooled droplets. The large droplets evaporation the remainder of the dropis frozen. out over the airfoil prior tocomplete freezing,

393 AEROGRAPHER'S MATE 1 & C

forming a sheet of clear ice.As a result of the Light spreading of this supercooledwater and its slow -, freezing, very few air bubbles are trapped within The rate of accumulationmay create a prob- the ice, which accounts forits clearness. Al- lem if flight though clear ice is expected mostly is prolonged in this environment with temper- (over I hour).Occasionaluse of deicing/ atures between 0° and10°C, it does occur anti-icing equipment removes/preventsaccumu- withtemperaturesascoldas -25°C. The lation.It does not present atmospheric conditions which produce a problem if the clear ice deicing/anti-icing equipment isused. are encountered most frequently in cumuliform clouds. Clear ice also forms rapidlyon aircraft Moderate while flying in zones of freezing rainor drizzle. The rate of accumulation is Frost such that even short encounters become potentiallyhazardous and use of deicing/anti-icingequipment or diver- Frost is a deposit ofa thin layer of crystalline sion is necessary. ice that forms on the exposedsurfaces of parked aircraft when the temperatureof the exposed Severe surface is below freezing (whilethe temperature of the free air may be above freezing). The The rate of accumulation is suchthat deicing/ deposit of ice forms during the nightradiational anti-icing fails to reduceor control the hazard. cooling, in a manner similarto the formation of Immediate diversion isnecessary. hoar frost on the ground. Frostmay also form onaircraftinflight when acoldaircraft ICING HAZARDS NEAR THEGROUND descends from a zone of freezingtemperatures into a warmer moist layer below.Frost can Certain icing hazards exist cover the windshield or canopy and completely on or near the ground. One hazard results whenwet snow is restrict outside vision. Duringpenetration for falling during takeoff. This landing, when visible moistureor high relative situation can exist humidity exists, frost when free air temperature at theground is near may form on the inner 0°C. The wetsnow sticks tenaciously to aircraft side of the canopy. Thiscan be a definite hazard if no preventive action is taken. components, and freezes when theaircraft encounters markedly colder temperaturesduring Frost is a deceptive form oficing. It affects the climb. the lift-drag ratio of an aircraft, andis a definite hazard If not removed before takeoff,frost, sleet, during takeoff.Allfrost should be frozen rain, and snow accumulated removed from the aircraft priorto taking off. on parked aircraft are operational hazards.Another hazard INTENSITIES OF ICING arises from the presence ofpuddles of water, slush, and/or mudon airfields. When the air temperature on the airframe is colderthan 0°C, There are four intensities of aircraft icingthat water blown by the propellers are defined meteorologically. The definition of or splashed by wheels can form ice on controlsurfaces and each was standardized by Naval WeatherService windows. Freezing mud is particularly Command Instruction 3140.4. dangerous because the dirt may clogcontrols and cloud the windshield. Trace PHYSICAL FACTORS AFFECTING Ice becomes perceptible, Rate ofaccumula- AIRCRAFT ICING tion slightly greater than the rate ofsublimation. Itisnothazardouseventhoughdeicing/ The amount and rate of icing anti-icing equipmentis depend on a not used, unless en- number of meteorological andaerodynamic countered for an extended period of time(over factors, including temperature, the 1 hour). amount of liquid water in the path of theaircraft, the 394 4.00 Chapter 11FORECASTING THUNDERSTORMS.FOG, TORNADOES, ICING AND CONTRAILS fraction of this liquid water collected by the ally greater in cumuliform clouds than other aircraft(thecollectionefficiency),andthe cloud types, it would seem logical that cumuli- amount of aerodynamic heating. form clouds should be particularly conducive to icing. However, theeffect exerted by other Temperature variables, such as the speed, shape, and size of the aircraft components, may be sufficiently Temperature has a direct effect on the frac- great for meteorological conditions leading to tion of water which freezes instantaneously on light icing for one type aircraft, to result in impact. When a large droplet strikes the aircraft moderate or severe icing for another. with temperature just below freezing, a fraction of the water is carried off the surface and a Collection Efficiency portion freezes. In the case of smaller droplets. The icing rate depends to a large degree upon or large droplets at a slightlylower temperature, the collection efficiency of the aircraft com- the entire droplet may freeze on the aircraft. ponent involved. The fraction of liquid water The temperature range in which iceis most collected by the aircraft varies directly with likely to form is generally from 0° to-20°C droplet size and aircraft speed, and inversely with the lower limit in the vicinity of-40°C. with thesize or geometry of the collecting Supercooled water droplets have been found at surface. Those components having a large radius temperatures colder than-40°C and at altitudes (canopies. thick wings, etc.) deform the airflow above 40,000 feet. more, permitting only a small portionof the Airflow around aircraft surfaces can decrease droplets to be caught on the surface. the free air temperature in some cases asmuch as2°C.Thisexplainsthefactthaticing Aerodynamic Heating sometimes takes place when the outside free air temperature gage indicates temperatureslightly This isa rise in temperatureresulting from above freezing. adiabatic compression and friction as the aircraft penetrates the air. The amount of heating varies Liquid-Water Content primarily with the speed of the aircraft and the altitude(airdensity), and may range from Under icing conditions, the liquid-water con- approximately 1 degree for very slow aircraft at tentinthecloudisthe most important low altitudesto more than 50 degrees for parameter in determining the iceaccumulation supersonic jets at low altitudes. Thus, although rate. In general, :he lower and warmer thebase itisfrequently stated that an aircraft flying of the cloud, the higher isits water content. through any supercooledliquid-water cloud Within the cloud, the average liquid-water con- must anticipate icing,it actuallyis necessary tent increases with altitude to a maximum value that the amount of supercooling exceedthe and then decreases. The maximum concentra- amount of aerodynamic heating. tion usually occurs at a lower level in stratiform than in cumuliform clouds, and the average DISTRIBUTION OF ICING liquid-water content is usually less than that of a IN THE ATMOSPHERE cumuliform cloud. The atmosphericdistribution of potential Droplet Size icing zones is mainly a function of temperature and cloud structure. These factors, in turn, vary Droplet size affects the collection efficiency withaltitude, synopticsituation, orography, and hence the icing rate. The size distribution location, and season. and median size of the droplets in a cloud are related to type, depth, and age of the cloud; to Altitude and Temperature the strength of the updrafts; to the humidityof the air mass; and to other factors. Since boththe Aircraft icing is generally limited to the layer liquid-water content and droplet size are gener- of the atmosphere lying between the freezing

395 401 AEROGRAPHER'S MATE I& C

level cm! the -40°C isotherm.However, icing clouds. Icing intensities has occasionally been reported may range from gener- at temperatures allylightinsmall colderthan -40°Cisthe supercooled cumulus to upper parts of moderate or severe in cumuluscongestus and cumulonimbus and other clouds.In general, the frequency of icing decreases rapidly cumulonimbus. The mostsevere icing occurs in with de- cumulus congestus clouds just creasing temperature, becomingrather rare at prior totheir change to cumulonimbus. Althoughicing occurs temperatures below -30°C. The normalvertical at all levels above the freezing level temperature distributionis such that icing is in a building cumulus, it is most intense inthe upper half of usually restricted to the lower30,000 feet of the the cloud. Icing is generally troposphere. The types of icing restricted to the associated with updraft regions in a maturecumulonimbus, and temperature ranges will generally be:clear, 0°C to a shallow layer near the freezing to1 0°C; a mixture of clear and level in a rime10 °C to dissipating thunderstorm. Icing inthese type - 15 °C; rime - I 5°C to -20°C, with possible clouds is usually clear or mixed. rime at lower temperatures. CIRRIFORM CLOUDS.Aircraftrarely occursin Icing in Relation to Cloud Type cirrus clouds, even thoughsome of them do contain a small proportionof water droplets. However, icing of moderate Stable air masses often produce intensity stratus type has been reportedinthe dense cirrus and clouds with extensiveareas of relatively contin- anvil-topsof cumulonimbus, whereupdrafts uous icing conditions. Unstable airmasses gener- may maintain considerable water at ratherlow ally produce cumulus cloudswith a limited temperatures. horizontal extent of icing conditions,where the pilot can expect the icing conditions, where the Icing in Frontal Zones pilot can expect the icingto become more severe at higher altitudes in the clouds. The type and About 85percent of all amount of icing will vary considerably witheach icing conditions type cloud. reportedareassociatedwithfrontalzones. Aerographer's Mates, therefore, STRATIFORM CLOUDS.Icingin middle- should have a thorough knowledge of thedanger areas in the and low-level stratiform cloudsis confined, on frontal zones so that pilots the average, to a layer between 3,000and 4,000 may be properly briefed. The basic approachto forecasting icing feet thick. However, pilots frequentlyencounter in frontal zones is to relate the situations where multiple layers ofclouds are so fronts to the close together that visual navigation cloud types in them. between Along warm fronts the stratified layers is not feasible. In suchcases the maximum cloud system depth of continuous associated with the upglide ofwarm, moist air icing conditions rarely over a warm front often reaches dangerous exceeds 6,000feet.Theintensityof icing generally ranges from light to moderate subzero temperatures, where rimeor glaze icing with the will occur. If the warm airmass is conditionally maximum values occurring in theupper portions of the cloud. Both rime and unstable, cumuliform cloudsmay form. These mixed icing are clouds are favorable to the formation observed in stratiform clouds. Themain hazard of glaze of rime, or a combination of them.Figure 11-23 lies in the great horizontal extent ofsome of shows a typical warmfront these cloud decks. High-level structure, with the stratiform clouds most probable icing zones and are composed mostly of ice crystals and give a possible flight- little icing. path to minimize icing. The cloudsystem of a warm front is extensive. A flight through such CUMULIFORM CLOUDS:- -Thezone a of system is a long one and the icingdangers are probable icing in cumuliform cloudsis smaller therefore increased. horizontally but greater vertically thanin strati- Cumuliform clouds are associatedwith cold form clouds. Icing ismore variable in cumuli- fronts. Glaze and mixed icing formclouds must therefore be because many of the factors expected in them. The cloud conducive to icing depend to systems are usually a large degree on relatively narrow as compared withthose of a thestage of development of theparticular warm front, and the time spent flying through Chapter 11FORECASTING THUNDERSTORMS. FOG. TORNADOES. ICING AND CONTRAILS

30,000

WARM AIR' COLD AIR FALLING SNOW 40 POSSIBLE FLIGHT PATH 000° C-6110 V. SNOW FALLING THROUGH WATER AND ICE CLOUD FREEZING LEVEL 3,000

FREEZING RAIN AND SLEET WARM FRONT LIMIT OF HEAVY ICE

AG.630 Figure 11-23.Warm-front icing condition.

LIMIT OF HEAVY ICE

AG.631 Figure 11-24.Icing zones along a cold front. themisshorter. However, due to the heavy less water. However, since the cloud system is precipitation. icing can be rapid and extremely extensive, accumulation can still be dangerous. dangerous. even though itis of short duration. Figure 11-24 illustrates a typical cold-front icing Orographic Influences situation. The lifting of conditionally unstable air over Occludedfrontspresentlessof anicing mountain rangesis one of the most serious hazard because precipitation has been occurring ice-producingprocessesexperiencedinthe for some time, and the cloud system contains United States. When mT air moves northward

403. 397 AEROGRAPIIER'S MATE I & C

and eastward over the Appalachian Mountains, it various kinds on them. However, these deicers is often cooled to subfreezing temperatures. An arecurative,not preventive, and the danger icing hazard exists for all air traffic thatmust remains. travel through this area. Similarly, mP air in Another serious icing condition existsnear winter, approaching the west coast of the United the airscoops, carburetor inletscreens, and other States,contains considerable moistureinits induction systems. lowerlevels.Asit is forcedaloftby the Carburetoricingistreacherous.Itoccurs successive mountain ranges encountered inits under a wide range of temperatures andcan eastward movement, severe icingzones develop. result in complete engine failure. Carburetor ice The movement of a frontacross a mountain forms during vaporization of fuel combined with brings together two important factors that aid in the expansion of air as it passes through the the formation of icing zones. A study of icing in carburetor. Temperature drop in the carburetor the Western United States has shown that almost can be as much as 40°C, but is usually 20°C or alltheice cases occurred where the air was less. The temperature at which carburetoricing blowing over a mountain slopeor up a frontal will form depends upon many factors surface or both. such as relative humidity, type of gas and its ingredients, The most severe icing zones over mountain- and the type of carburetors. tops will be above the crests and slightly to the windward side of the ridges. Usually the icing Turbojet Aircraft zones extend about 4.000 feet above the tops of the mountains. In the case of unstable air, they These high speed aircraft generally cruiseat may extend higher. altitudes well above levels wheresevere icing exists. The greatest problem will beon takeoff, OPERATIONAL ASPECTS climb, approach. 3r go-around-because of the OF AIRCRAFT ICING greater probability of encountering supercooled water droplets at low altitudes. Also, there- Duetothelargenumber of types and duced speeds result in a decrease of aerodynamic different configurations of aircraft, this discus- heating. sion is limited to the general types of aircraft Turbojet engines experience icing bothex- rather than specific models. ternally and internally. All exposed surfacesare subject to airframe ice as well as is the inlet Reciprocating-Engine Aircraft ducting and internal elements. Icingcan occur in theinletductattemperatureswellabove Because of their relatively low speed and freezing when the aircraft is operating at slow, service ceilings, this type of aircraftis more low altitude speeds or on the ground. susceptible to icing for long periods of timethan The jetaircraftusually has little concern jet aircraft. Due to the thick wings, canopies. about structural icing except possiblyon land- and other features. the collection efficiencyis ings, takeoffs, climbs and when operating at smaller than those of the trimmer and faster slow speeds at low altitudes. turbojet aircraft. However, the hazard isgreater Internal icing poses special problems to jet because the slower speeds produce lessaero- aircraft and jet engines. In flights through clouds dynamic heating and because they operate for which contain supercooled water droplets, air longer periods of time in altitudes andareas intake ducticingissimilarto wing icing. more conducive to icing. However, the ducts may ice when skiesare clear Propeller icing is a very dangerous form of andtemperaturesareabove freezing. While icingforthistypeaircraftbecause of the taxiing, and during takeoff and climb, reduced tremendous loss of power and vibration. Pro- pressure exists in the intake system which lowers peller icing varies along the blade dueto the temperatures to a point that condensation and/ differentialvelocityof the blade causing a or sublimationtakesplace,resultinginice temperatureincreasefromthehubto the formation. This temperature change variescon- propeller tip. Modern propellers have deicers of siderably with different types of engines. There-

398 404

1.: Chapter 11FORECASTING THUNDERSTORMS, FOG, TORNADOES, ICING AND CONTRAILS fore. if the free air temperature is 10°C or less 0°C. A reasonable estimate of the freezing level (especially near thefreezingpoint) and the canbe made fromthe data contained on relative humidityishigh, the possibility of freezing level charts, constant-pressure charts, induction icing definitely exists. radiosonde, reconnaissance, and AIREP observations,orbyextrapolationfromsurface Turboprop Aircraft temperatures.

The problems of aircraft icing for this type of Precipitation aircraft combine those associated with conven- Coml aircraft and turbojet aircraft. Engine icing Check surface reports and synoptic charts for problems are similar to those encountered by precipitation along the proposed flightpath, and turbojet aircraft while propeller icing is similar forecast the precipitation character and pattern to that encountered by conventional aircraft. during the flightspecial consideration should be giventothepossibility of freezing pre- Rotary-Wing Aircraft cipitation. Note that each of the following methods and When helicopters do encounter icing condi- forecast rules assumes that two `lasic conditions tions, which would either be in IFR conditions must exist for the formation of icing. These are or when flying through an area offreezing rain that the surface of the aircraft must be colder or drizzle, they are similar but morehazardous than 0°C, and that supacooled liquid-water than fixed wing aircraft. When icing forms on droplets,clouds,orprecipitationmustbe the rotor bladeswhile hovering, conditions present along the flightpath. become hazardous because the helicopteris operating near peak operational limits. Icing also ICING FORECAST affects the tail rotor, control rods and links. and air intakes and filters. Icing Intensity Forecasts From Upper Air Data PRELIMINARY CONSIDERATIONS Checkupperaircharts,pilotor recon- Thefirstphase of theprocedureinthe naissance reports, and radiosonde reports for the preparation of an aircraft icing forecast consists dewpoint spread at the flight level. A' o check of making certain preliminary determinations. the upper air charts for the type of temperature These are essential regardless of the technique advection along the route. In one study con- employed in making the forecast. sidering only the dewpoint spread aloft, it was found that there was an 84 percent probability Clouds that there would be no icing if the spread were greater than 3°C, and an 80 percent probability Determine the present and forecast future that there would be icing if thy. spread were less distribution, type, and vertical extent of clouds than 3°C. along the flightpath. Clouds can be analyzed and The type of thermal advection or the presence forecastusing the information containedin of building cumuliform clouds taken in conjunc- surface weather observations, radiosonde obser- tion with the dewpoint spread showed a definite vations, pilot reports, and surface and upper air association between the two. When the dew- charts using synoptic models, physical reasoning, point spread was 3°C or less in areas of warm and empirical studies. The influences of local air advection atflight level, there was a 67 effects such as terrain features and others should percent probability of no icing and a 33 percent not be overlooked. probability of light or moderate icing. However, Temperatures with a dewpoint spread of 3°C or less in a cold frontal zone, the probability of icing reached Determine those segments of the proposed 100 percent. There was also a 100 percent flightpath which will be in clouds colder than probabilityof icinginbuilding cumuliform 405399 AEROGRAPHER'S MATE 1 & C clouds when the dewpoint spread was 3°C or steady nonfreezing precipitation, forecast little less. With the spread greater than 3°C, light icing ornoicing.Overareaswithoutsteady was probable in about 40 percent of the region nonfreezing precipitation, particularly cumuli- of coldairadvectionwitha100 percent form clouds, forecast moderate icing. probability of no icing in regions of warm or neutral advection. However, it appears on the Icing Intensity from Clouds Due basis of further experience that a more realistic To Frontal or Orographic Lifting spread of 4°C at temperatures near -10° to -15°C should be indicative of probable clouds, Within clouds resulting from frontal or oro- and that spreads of about 2° or 3°C should be graphiclifting,neither the presence nor the indicative of probable icing. At other tempera- absence of precipitation can be used as indica- tures use the values in the following rules: tors of icing. 1.If the temperature is: a. 0° to -7°C, and the dewpoint spread is 1. Within clouds up to 300 miles ahead of the warm front surface position, forecast moderate greater than 2°C, there is an 80 percent prob- icing. ability of no icing under these conditions. . 2. Within clouds up to 100 miles behind the b. -8° to -15°C, and the dewpoint spread cold front surface position, forecast severe icing. is greater than 3°C, forecast no icing with an 80 3. Within clouds over a deep, almost vertical percent chance of success. low-pressure center, forecast severe icing. . c. -16°to -22°C, andthe dewpoint 4. Infreezing drizzle, below or in clouds, spread is greater than 1°C, a forecast of no icing forecast severe icing. would have a 90 percent chance of success. 5. In freezing rain below or in clouds, fore- d. Colder than -22°C, forecast no icing cast severe icing. regardless of what the dewpoint spread is with 90 percent probability of success. 2. If the dewpoint spread is 2°C or less at Types of Icing Forecast temperature of 0° to -7°C, or is 3°C or less at -8° to15 °C: The foregoing rules refer only to the intensity a.In zones of neutral or weak cold air of icing and not the type. The following rules advection, forecasting light icing (75 percent apply to the type of icing: probability). b. In zones of strong cold air advection, i Forecast rime icing when the temperatures forecast moderate icing (80 percent probability). atflight altitudes are colder than -15°C, or c.In areas with vigorous cumulus buildups when between1° and -15°C in stable strati- due toinsolational surfaceheating,forecast form clouds. moderate icing. 2. Forecast clear icing when temperatures are between 0° and -8°C in cumuliform clouds and Icing Intensity Forecasts freezing precipitation. From Surface Chart Data 3. Forecast mixed rime and clear icing when temperatures are between -9° and -15°C in H. upper air data and charts are not available, unstable clouds. check the surface charts for locations of the cloud shields of fronts, low-pressure centers, and Example of Forecast of Icing precipitation areas along the route. From Upper Air Diagram

Icing Intensity Forecasts It has been previously pointed out how the From Precipitation Data thickness of clouds, as well as the top of the overcast, may be estimated with accuracy from Withincloudsnotresultingfrom frontal the dewpoint depression, its behavior, and from activity or orographic lifting, over areas with changes in the lapse rate.

46% Chapter 11FORECASTING TIIUNDERSTORNIS FOG. I ORNADOES. ICING AND CONTRAILS

17,000 477 W. \ WN '''``mnvk.\ t VAA\ \ (AA\4\\\,STRATIFORMCLOUDS RIME 1 -50° \ V A -40 \\ \ .01 \ \ 1 .A W.....iz; ... ..% % A ...... -.0% "10 \ \\\\\...... t "In\ \ V \ \ A \ i \ MirTONS1 A\ \AA AAMv.i% ,i; \ \ 0°C 12,000' \ `u 1:-- %NW IIIIIIIIN Attc u MU LI FORMCLOUDSVAI \'N'.,V,...iN.LEAR IA .A, v. w ,o A--. ...v...,A\ V,Awa# 8,000' A--- `'tea': 100 / / / // R A I N / NO ICE 4,000' f.;// / // -- - 0° 3,000' / / S-./// 2,500' / IIIIM. MIIIIIIIIIIMIliiiii01111MlidlieniIIPi 1,000Ili ApIrIIMi. i Pr GROUNDLEVEL 0

AG.632 Figure 11.25. An illustration of the analysis of cloud type and icing type from an upper air diagram.

The analysis of cloud type and icing type feet. Since the dewpoint depressionis 2°C, a from an upper air diragram is illustrated in figure probability of clouds exists at this level. There is 11-25. First look at the general shape of the a rapid increase in the dewpoint depression at curve. The most prominent feature is the inver- 17,000 feet indicating that the top of the cloud sion. showing a very stable layer between 2,500 layer is at this level. We would then assume the and 3,000 feet. Note that the dewpoint depres- cloud layer existed between 8,000 and 17,000 sion is less than one degree at the base of the feet. The next step is to determine, if possible, inversion. Moisture exists in a visible form at this the type of cloudS in between these levels. If this dewpointdepression,soexpectalayer of sounding were plotted on the Skew T, you broken or overcast clouds whose base will be would be able to see that the slope of the lapse approximately 2,000 feet and will be topped by rate between 8,000 and 12,000 feet is shown to thebaseoftheinversion. The next most be unstable by comparison to the nearest moist prominentfeatureisthehighhumidity,as adiabat. The clouds will then display unstable reflected by the dewpoint depressions, at 8,000 cumuliform characteristics. Above 12,000 feet

401 Al ROGRAPIIER'S MATE 1 & C'

lirr -15.5 -17,0 w-Ail -01---Anim o0 zr,- , 8'7 3e..---A. .6 imprArAjor 4".,0:i.""AlPrAialp 0°# A

IIPAOr rdrAgl Mar -71 -6.oIMPFIEVAI VAIMPFAVAII °rmillIAPPAI MIAOWgl1P 1,6 ME AIIIIIIIIIIII MI-10 0 +10

AG,633 Figure 11-26.The 80 ice forecast metii*cl.

thelapserateis shown bythe same type not clear as to presence and possibility of icing, comparison to be stable. and the clouds there it may be determined from Skew T by the should be altostratus. following method: Now determine the freezing level. It isnoted that the lapse rate crosses the 0°C temperature I. Plot the temperature againstpressure as line at 710 mb approximately 9,500 feet, Since determined from a mob sounding. it has been determined that clouds exist at this 2. Write the temperature and dewpoint in level,thetemperatures are between 0° and degrees and tenths to the left of each plotted -8°C. and the clouds are cumuliform.forecast point. clear ice in the unstable cloud up to 12.000 feet. 3. Determine the difference (in degrees and Above 12,000 feet. the clouds .ire stratiform,so tenths) between the temperature and dewpoint rime icing should be forecast. The intensity of for each level. This difference is D. the dewpoint the icing would have to be determined by the deficit: it is always taken to be positive. considerations given in the previous section of 4. Multiply D by -8 and plot the product this chapter. (which isin degrees Celsius) opposite the cor- Forecasting Icing responding temperature point at the appropriate Using the -8D Method place. 5. Connect the points plotted by step 4 with When surface charts. upper air charts,syn- a dashed line in the manner illustrated in figure optic and airway reports, and pilot reports are 11-26.

402 408 Chapter II FORECASTING niuNDERsToRNts. FOG. TORNADOES. ICING AND coNTRAILS ,

6. The icing layer is outlined by the area Forecasting Icing Using Graphs enclosed by the temperature curve on the left and the -81) curve on the right. In this outlined Additional graphs and overlays for upper air area, supersaturation with respek.t to ire exists. diagrams hive been prepared and are included in This isthe hitched area as shown in figure the Air Weather Service Publication. Forecaster's 11-26. Guide on AircraftIcing, AWSM-105-39. The 7. The intensity of icing is indicated by the diagrams may be usedto constructlocally size of the area enclosed by the temperature prepared charts whereby icing conditions may curve and the -8D curve. In addition, the factors beforecastfrom cloudtypes, temperatures. given in the following section should be con- dewpoint spreads, and other information. 'II's! sidered when formulating the icing forecast. The methods sire much too elaborate and detailed to cloud type and the precipitation observed at the coverinthis training manual. The reader is raob time or the forecast time may be used to referred to the above publication for furiher determine whether icing is rime or glaze. information on this subject.

Modification of the Icing Forecast The factors which were mentioned inthe preceding paragraph which go into making the The final phase is to modify the icing forecast ice forecast are: as obtained by the foregoing methods. This is essentially a subjective process. The forecaster I. When the temperature and the dewpoint should consider the following items: probable coincide in the raob sounding. the -8D curve intensification or weakening orsynoptic mustfall along the 0°C isotherm. In a sub- features, such as low-pressure centers. fronts. freezing layer. the air would be saturated with and squall lines during the time interval between respect to water and supersaturated with respect thelatestdata andthe forecasttithe: local to ice.Light rime icing would occur in the influences. such as geographic location, terrain altostratus-ninbostratus in such aregion, and features. and proximity to ocean coastlines or moderate rime icing would occur in cumulonim- lake shores, radar weather observations; pilot bus virga in such a region. Severe clear ice would reports of icing: and the like. The forecaster occur in the stratocumulus virga, cumulus virga, should be cautious in either underforecasting or and stratus. overforecasting the amount and intensity of icing. An overfo-ecastresultsina reduced 2. When the temperature and dewpoint do payload for the aircraft due to increased fuel not coincide but the temperature curve lies to load. while an underforecast may result in an the left or the -8D curve in the subfreezing operational emergency. layer. the layer is supersaturated with respect to ice and probably subsaturated with respect to TURBULENCE cloud droplets. If the clouds in this layer are altostratus. altocumulus. cumulogenitus, or alto- Turbulence is or major importance to pilots of all types of aircraft and therelbre also to the cumulusvirga,onlylightrime willbe encountered. If the clouds are cirrus, cirrocumulus. Aerographer's Mate whose duty it is to recognize cirrostrat as. or cirrus nothus, only light hoar- situations where turbulence may exist and to forecast for flight operations both the areas and frostwillbe sublimated on the aircraft.In cloudless regions, there will be no supercooled intensity of the turbulence. In this chapter. the droplets, but hoarfrost will form on the aircraft causative factors, classifications, intensities. and through direct sublimation of water vapor. methods offorecastingturbulencewillbe discussed. 3. When the temperature curve lies to the right of the -8D curve in a subfreezing layer. the TURBULENCE CHARACTERISTICS layer is subsaturated with respect to both ice and water surface. No icing will occur in this Turbulence may be defined as irregular and region. instantaneous motions of air which is made up

403 409 AEROGRAPIIER'S MATE I & C' of a number of small eddies that travel in the 4. Large scale wind shear. Marked gradients general air current. Atmospheric turbulence is in wind speed and/or direLtion, dueto general caused by random fluctuations in the windflow, variatins in the temperature andpressure fields Given a smoothly analyzed wind field with both aloft,arctheprimarycause of this type streamlines and isotachs smoothly drawn, any turbulence. difference between anactual wind and this smooth field is attributed to turbulence, Two or more of the causative factors often To an aircraftin flight, the atmosphere is work together. In addition, turbulence ispro- considered turbulent when irregular whirls or duced by "manmade" phenomena, suchas in eddies of air affect the motion of the aircraft the wake of an aircraft. and a series of abrupt jolts or bumps is felt by the pilot. Although a large range of sizes of PROPERTIES OF TURBULENCE eddies exists in the atmosphere, those causing bumpiness are roughly of the same size as the Turbulence Close to the Ground aircraftdimensions andusually occur in an irregular sequence imparting sharp translation or Turbulence close to the grc and isa combina- angular motions to the aircraft. The intensity of tion of mechanical and convective turbulence. tin: disturbances of the aircraft varies not only When the lapse rate is netnialor stable, condi- with the intensity of the irregular motions of the tions are simpler, In thatLase, only mechanical atmosphere but also with aircraft characteristics turbulence is of importance. This dependson such as flight speed. weight stability, and size, the wind speed and the roughness of the ground. In the case of mechanical turbulence close to TYPES OF TURBULENCE the ground. theintensity decreases with increasing height. This is not the ease when the The atmosphereis always and everywhere mechanical turbulenceis mixed with convec- turbulent. but often the intensity of turbulence tion, as for example behind a cold front. In that is so small thatithas a negligible effect on case the convective clouds indicate rough flying aircraftoperations.Large intensities of tur- up to considerable height. bulence are to be expected whenever the shear Convection differs in two important features (horizontal or vertical) is large or wherever we from mechanical turbulence. itis characterized find instability. Turbulence, for the purpose of by larger horizontal wavelengths, and itpro- this discussion,is divided into the following duces much stronger lateral fluctuations. causative factors: The intensity of convective turbulence generally increases with height. reaching a maximum I. Thermal or convective. This type of turbu- in the upper half of convective clouds. This lence is caused by localized vertical currents due increase will be stopped by inversions. to surface heating or unstable lapse rates and by To summarize, the most intense turbulence cold air moving over warmer ground. It is more andgustiness near the ground occur under pronounced wer land than water and in the conditions of unstable lapse rates. strong winds, daytime during summer. and rough ground. 2. Mechanical. Whenever wind speeds near the groundarchigh enough, shear becomes Turbulence in Cumulus Clouds significant. causing small eddies and gusts. The degree of turbulenceisproportionalto the The appearance of cumulus clouds makes the roughness ofthe.surface and tothe wind presenLe of a great deal of turbulence apparent. velocity. The higher the wind velocity and the Ingeneral,thetallerthe cloud.the more rougher the terrain, the greater is the turbulence. turbulencethereis.Althoughtheinitiating 3. Frontal. This type turbulence results from mechanism in cumulus type turbulence is ther- the local lifting of warm air by cold air masses. mal instability. much of the roughness is caused or the abrupt wind shear (shift) associated with by smaller eddies produced by the large shears most cold fronts. between up and down drafts.

404 410 Chapter 11FORECAS-I ING THUNDERSTORMS, FOG, TORNADOES, ICING AND CONTRAILS

intermingling of downdrafts with the original updrafts results in severe turbulence, particularly HORIZONTAL at the boundary between regions of precipita- VIEW tion and regions of no precipitation. The same COLD resultisalso confirmed by radar studies of AIR MASS thunderstorms made in this country and in England: severe turbulence occurs in regions of strong echoes, and particularly in regions of sharp boundaries of echoes. I lence, as has been proved recently, radar is an excellent aid for the avoidance of extremely turbulent conditions in flights through thunderstorms. WARM The intensity of turbulence in thunderstorms AIR MASS seemstoincrease with height wellpastthe HIGH middle of the clouds. The Thunderstorm Project foundthe maximum effect of turbulence on 'A aircraftoccurredbetween15,000feetand FLIGHT 20,000 feet. The actual turbulent velocities may PATH increase to even greater elevations, but have less effect on aircraft because the air density is lower at higher levels. In the last stage of the thunderstorm, the air subsides and the storm dissipates without any indication of severe turbulence. VERTICAL CROSS-SECTIONAL The intensity of turbulence in cumulus clouds IEW depends on the difference between the tempera- FRONTAL tures inside and outside the cloud. TRANSITION ZONE Presentmethods of estimating turbulence expected in cumulus type clouds before the WARM FLIGHT clouds have formed generally make use of the AIR PATH COLD relation between temperature differences. TURBULENT MASS LEVEL AIR MASS AREA Turbulence in the _-1 Vicinity of Fronts N WIND FLOW 1SW WIND FLOW Frontal turbulence is caused by the lifting of OUT OF PAGE I INTO PAGE warm air by a frontal surface leading to instability and/or mixing or shear between the warm and cold air masses. The vertical currents in the COLD FRONT AT SURFACE warm air are strongest when the warm airis moist and unstable. The most severe cases of AG.634 frontal turbulence are generally associated with Figure 11-27.Turbulence across a typical cold front. fast moving cold fronts or squall lines. In these cases. mixing between two air masses as well as Wind Shears and Clear the differences in wind speed and/or direction Air Turbulence (windshear)addtotheintensity of the turbulence. A relatively steep gradient in wind velocity LxLluding the turbulence that would be en- along a given line or direction (either vertical or countered along the front, figure 11-27 illus- horizontal) produces churning motions (eddies) trates the wind shiftthat contributes to the whichresultinturbulence. The greater the formation of turbulence across a typical cold change of wind speed and/or direction in the front.As a general rule. the wind speedis given direction, the more severe will be the stronger in the colder air mass. turbulence. Turbulent flight conditions are often

405 411 AEROGRAPHER'S MATE 1 & C

foundin the vicinity of the jetstream where Table 11-2.Wind sheer CAT with wind speed large shearsin the horizontal and vertical are 60 knots or greater. found. Since this type of turbulence mayoccur is perfectly clear air without any visual warning in the Corm of clouds, itis often referred to as Horizontal Vertical clear air turbulence. shear CAT The term "clear air turbulence" is misleading shear intensity (naut/mi) (per 1,000 ft) because not all high level turbul, Le included in this classification occurs in clear air. However. 25k/90 9-12k Moderate. the majority75 percentis found in cloud-free atmosphere. Clear air turbulence is not neces- 25k/90 12-15k Moderate, sarily limited to the vicinity of the jetstream, at times and may occurinisolatedregions ofthe severe. atmosphere. Most frequently. the clear air turbulence is associated with the jetstream and the 25k/90 above15k Severe. mountain wave.Ilowever. it may also be associated with a closed low aloft, a sharp trough aloft, andan advancingcirrus shield.Too. narrowzone of wind shear, with its accompany- Any of the above situations can produce moder- ing turbulence, is sometimes encountered by ate to severe clear air turbulence. However, the aircrews asthey climb or descend through a combination of two or more of the conditions is temperature inversion. Moderate turbulence may almost certainto produce severe or even ex- also be encountered momentarily when passing treme CAT. A jetstream may be combined with through the wake of another aircraft. a mountainwave, or be associated with a The criteria for each type of clear air turbu- merging or splitting low. lence (CAT) are: Turbulence on the Lee I, Mountain wave CAT. Winds 25 knots or Side of Mountains greater.normaltoterrainbarriers andthe presence of significant surface pressure differ- When strong winds blow approximately per- ences across such barriers. 2. Trough CAT. That portion of a trough pendicular to a mountain range, the resulting turbulence may be quite severe. Associated areas which has horizontal vector shear across it of the of steady updraftand downdraft magnitude of 25 knots in 90 nautical miles or may extend to greater. heights from 2 to 20 times the height of the mountain peaks. Under these conditions when 3. Closedlowaloft CAT.The circulation the air is stable, large waves tend to form on the aroundthe closed low aloft may be accom- lee side of the mountains and extend up to the panied by the shears necessary to produce CAT. If the flow is merging or splitting, moderate or lower stratosphere for a distance up to 100 miles or moredownwind. severe CAT can he encountered. Also to the They are referredto as standing waves or northeast of a cutoff low aloft, significant CAT mountainwaves and may or maynotbe accompanied by turbulence. Some can be experienced. Just as with the jetstream CAT. the intensity of the turbulence is related pilots have reported that flow in these waves is to the strength of the shears. often remarkably smooth while others have reported severe turbulence. The structure 4. Wind shear CAT. Those zones in space in and characteristicsofthe which wind speeds are 60 knots or greater, and mountain wave were presented in chapter 5 of this both horizontal and vertical shear exist accord- training manual. Refer back to the diagram in that chapter for a ingtothe following criteria with intensities typical vertical illustration. indicated in table I1-2. Thewind flownormaltothemountain No provision is made for light CAT because produces a primary wave and generallyless light turbulence serves only as a flight nuisance. intenseadditionalwaves farther downwind. The

406 412 Chapter 11FORECASTING THUNDERSTORMS, FOG. TORNADOES. ICING AND CONTRAILS characteristic cloud patterns may or may not be atmospheric turbulence uniformly throughout present to identify the wave. The pilot for the the Naval Weather Service. most part is concerned with the primary wave This instruction further states that the Gust because of its more intense action and proxim0 Criteria shown in the Turbulence Criteria Table to the high mountainous terrain. Severe turbu- (table 11-3) be adopted as standard and that the lence frequently can be found out to 150 miles idealized descriptions entitled Guide to Turbu- downwind, when the winds are greater than 50 lence Classes as Typically Associated with Me- knots at the mountaintop level. Moderate turbu- teorological Conditions be adopted as a guide lence often can be experienced out to 300 miles for meteorologists forecasting turbulence. There- under the previously stated conditions. When fore, Naval Weather Service Units must adopt as winds are less than 50 knots at the mountain standard the Gust Criteria, and utilize the guide peak level, a lesser degree of turbulence may be to turbulence classes as a guide in forecasting experienced. turbulence. Some of the most dangerous features of the mountain wave are the turbulence in and below Guide to Turbulence Classes the roll cloud, the downdrafts just to the lee side of the mountain peaks and to the lee side of the As typically associated with meteorological rollclouds. The cap cloud must be always conditions, the following is a guide to classifi- avoidedinflightbecause of turbulence and cation of turbulence. concealed mountain peaks. EXTREME TURBULENCE,This rarely en- These sixules listed have been suggested for countered condition is usually confined to the flights over hmuntain ranges where waves exist. strongest forms of convection and wind shear, such as: I. The pilot should, it'possible, fly around the area when wave conditions exist. If this is not I. In mountain waves in or near the rotor feasible, he should fly at a level which is at least cloud (or rotor action) usually found at low 50percenthigher thantheheightof the level leeward of the mountain ridge when the mountain range. wind component normal to the ridge exceeds 50 2. The pilot should avoid the roll clouds since knots near the ridge level. thesearetheareaswiththe most intense 2. In severe thunderstorms where available turbulence. energy indicates the production of large haii 3: The pilot should avoid the strong down - (three-fourths inch or more), strong radar echo drafts on the ice side of the mountain. gradients or almost continuous lightning, Itis 4. He shouldalsoavoidhighlenticular more frequently encountered in organized squall clouds, particularly if their edges are ragged. lines than in isolated thunderstorms. 5. The pressure altimeter may be as much as SEVERE TURBULENCE.In addition to the 1,000 feet off, near the mountain peaks. situations where extreme turbulence is found, 6. The airspeeds recommended for the air- severe turbulence may also be found: craft should be observed in penetration of the turbulent areas. I.In mountain waves: a. When the wind component normal to the ridge exceeds 50 knots near the ridge level: CLASSIFICATION AND INTENSITY at the tropopause up to 150 miles leeward of the OF TURBULENCE ridge. (A reasonable mountain wave turbulence layer is about 5,000 feet thick.) COM NAVWEASERVCOM INSTRUCTION b. When the wind component normal to 3140.4setsfortha common set of criteria the ridge is 25-50 knots near the ridge level: up describingthemeteorologicalcharacteristics to 50 miles leeward of the ridge. from the ridge with which the respective classes of turbulence level up to several thousand feet above and at aretypicallyassociatedinthat the weather the base of relatively stable layers below the forecaster may evaluate and forecast degrees of tropopause.

407 413 AEROGRAPIIER'S MATE I & C

Table 11-3.Turbulence criteria

TRANSPORT AIRCRAFT OPERATIONALCRITERIA-2/ GUST CRITERIA

Adjectival Airfrainp Air Speed Derived gust / class Descriptive velocities (Ude)-1 limits/ fluctuation the order of

Light Not specified A turbulent condition during 5 to 15 5 to 20 fps which occupants may be knots required to use seat belts, but objects in the aircraft remain at rest.

Moderate Not specified A turbulent condition in 15 to 25 20 to 35 fps which occupants require knots seat belts and occasionally are thrown against the belt.Unsecured objects in the aircraft move about. Severe Not specified A turbulent condition in More than 35 to 50 fps which the aircraft mo- 25 knots. mentarily may be out of control.Occupants are thrown violently against the belt and back into the seat.Objects not secured in the aircraft are tossed about. Extreme a. Positive A rarely encountered tur- rapid more than 50 fps and negative bulent condition in which fluctuations gusts greater the aircraft is violently in excess of than 50 fps (Ude) tossed about, and is prac- 25 knots at Vel/between tically impossible to con- sea level and trol.May cause structural 20,000 ft for damage. transport category aircraft. b. Positive and negative gusts greater than 30 fps(Ue)3/, at all speeds up tcf VC for normal, acrobatic, and utility aircraft.

408 414 Chapter II FORECASTING THUNDERSTORMS, FOG, TORNADOES, ICING AND CONTRAILS

Table 11-3.-Turbulence criteria-Continued.

Footnotes: 1/- as. derived from the Flight Loads section CAM 4b, Airplane Airworthiness, Trans- port Categories (May 1960); and CAM 3 Airplane Airworthiness: Normal,Utility, and Acrobatic Categories (Nov. 1959) of Civil Air Regulations. 2 /AircraftTurbulence Criteria developed by NACA Subcommittee on Meteorological Problems (May 1957). 3/ Ue approximately equals 3/5 Ude. 4/ - VC is the design cruising speed.

2.Inand near mature thunderstorms and 25 knots or the atmosphere is unstable because occasionally in towering curnuliform clouds. of strong insolation or cold advection. 3. Near jetstreams within layers characterized byhorizontalwindshearsgreater than16 LIGHT TURBULENCE. -In addition to the knots/degreelatitude (40 knots/150 nautical situations where more intense classes of turbu- miles) and vertical wind shears in excess of 6 lence occur, the relatively common class of light knots/1,000 feet. When such layers exist favored turbulence may be found: locations are below and/or above the jet core and from roughly the vertical axis of the jet core I. Inmountainousareasevenwithlight to about 50 or 100 miles toward the cold side. winds. 2. In and near cumulus clouds. MODERATE TURBULENCE. In addition to 3. Near the tropophase. the situations where extreme and severe turbu- 4. At low altitudes when winds are under 15 lence are found, moderate turbulence may also knotsor where theairiscolderthanthe be found: underlying surface. FORECASTING TURBULENCE I.In mountain waves: CLOSE TO THE GROUND a. When the wind component normal to the ridge exceeds 50 knots near the ridge level: Over land at nighttime thereis very little between thesurface and about 10.000 feet turbulence close to the ground. The only ex- above the tropopause from the ridge line to as ceptionisthe case of high wind speeds over much as 300 miles leeward. rough terrain. This kind of turbulence decreases b. When the wind component normal to with increasing height. the ridgeis 25-50 knots near the ridge level. During the day turbulence close to the ground between the surface and the tropopause from depends on the radiation intensity, the lapse the ridge line to as much as 150 miles leeward. rate, and the wind speed. Turbulent intensities 2. In, near, and above thunderstorms and in tend to increase with height throughout the towering cumuliform clouds. unstable and neutral layers above the ground up 3. Near jetstreams and in upper trough, cold, to thefirst inversion or stable layer. Similar low, and front aloft situations where vertical turbulence occurs in fresh polar outbreaks over wind shears exceed 6 knots/I ,000 feet or hori- warm waters. zontal wind shears exceed 7 knots per Idegree Vertical gustiness increases with height more latitude. rapidly than horizontal gustiness. 4. At low altitude (usually below 5,000 feet Situations for particularly violent turbulence above the surface) when surface winds exceed near the ground occur shortly after cold front

409 415 AEROG IZAPI IER'S MATE 1 & C passages, expecially over rough ground. Other Table 11-4.Relation of maximum positive examples of rough air are conditions over deserts departure to thunderstorm turbulence. on hot days and thunderstorms. Peak gusts at the surface can be estimated to be essentially equal to the wind at gradient level (except in thunderstorms). AT (°C) Turbulence 0-3 Light FORECASTING TURBULENCE IN CONVECTIVE CLOUDS 4-6 Moderate 7 -9 Severe In this section. methods for determining the Above 9 Extreme type and intensity of turbulence in convective clouds after the characteristics of the upper air sounding have been determined are discussed. This method of forecasting thunderstorm Eastern Airlines Method turbulence is almost exclusively confined to the warmer months when frontal cyclonic activity is In the absence of any dynamic influences ataminimum. Duringthe cooler months, which might serve to drastically modify the frontal and cyclonic influences may cause rapid vertical temperature and moisture distribution. changes in the vertical distribution of temper- radiosondedataon hand may be usedto atures and moisture and some other methods evaluate the turbulence potential of convective have to be used. clouds or thunderstorms for periods up to 24 hours.Thefollowingisone procedure for Other Methods predicting turbulence in such clouds. The ma- terialusedinthis sectionis reproduced by Various other methods utilized in forecasting permission of the Academic Press. from the turbulence have been developed. A National book, Weather Forecasting for Aeronautics. by Weather Service method employing an overlay J. J. George and Associates. Such permission is on the plotted upper air sounding diagram is gratefully acknowledged. one. The Air Force uses a modification of the The technique is as follows: Eastern Airlines Method in which the atmos- phereisdividedinto two layerssurface to i. Determine the CCL. 9,000 feet MSL and above 9,000 feet MSL. 2. From the CCL. proceed along the moist These methods are discussed, with examples. adiabat to the 400-mb level. This is the updraft in NAVAER 50-1P-546, An Introduction to curve. Atmospheric Turbulence.Forecastersshould 3. Compare the departure of the updraft refertothispublicationfor further under- curve with the free air temperature curve and standing of them. note that value of maximum positive departure up to 400nib. For positive values, the updraft FORECASTING CLEAR curveshouldbe warmer than thefreeair AIR TURBULENCE temperature curve. The value of the maximum positive departure obtained in this step isre- Clear air turbulence (CAT) poses as one of the ferred to asT. most comm in-flight hazards encountered by the modern high altitude, high performance aircraft. Table 11-4 is based on several years relating Not all high level turbulence occurs in clear AT values to commercial pilot reports of thun- air. However, the rough cobblestone type of derstorm turbulence, and can be used to predict bumpiness does occur in clear air or the cloud- the degree of turbulence in air mass thunder- less portion of the sky, without visual warning. storms. The turbulence may beviolentenoughto

410 416 Chapter II FORECASTING THUNDERSTORMS, FOG, TORNADOES, ICING AND CONTRAILS disrupt tactical operations and possibly cause These columns are often distorted from the true serious aircraft stress. vertical by wind shear. To observe this shear Most cases of clear air turbulence at high effect clearly, the antenna must be scanning in altitudes can be associated with the jetstream or the same plane as the wind shear. To do this, morespecificallywith abrupt vertical wind adjust the azimuth until the greatest distortion shear. They are experienced most frequently from the vertical is observed. Such observations during the winter months when the jetstream give heights of shear zones and a qualitative winds are the strongest. estimate of the shear. The association of clear air turbulence with recognizable synoptic features meets only with Stability, Instability, and limited success. However, using the following as Turbulence by Radar general areas where CAT may occur, flight crews should be briefed as to the possibility of its occurrence: Radar can be useful in a qualitative evaluation of the degree of turbulence near the terminal in I. In general,in any region along the jet- those layers where precipitationis forming or stream axis where wind shear appears to be through which itis falling. Horizontally stratified strong horizontally, vertically, or both. echoesareindicativeof smoothair; 2. In the vicinity of the traveling jet maxima, whereas, vertical columns or cellular echoes particularly on the cyclonic side; most occur- indicate vertical motions which cause turbulence rences of moderate and severe clear air turbu- to aircraft.In addition, the sharpness of the lence have been reported in this area. bright band is an indication of the instability 3. In the jetstream front, below, and to the involved. Sharp wind-shear layers are alsoan south of the core. indication of associated turbulence. 4. Near 35,000 feet in cold deep troughs. It is well known that the most severe turbulence (as well as heavy icing and damaging hail) Mountains wave turbulence should be forecast is associated with actively developing thunder- if: storms extending to great heights. During this dangerous growing stage, the top of the radar I. The flow at and above mountaintop level echo risesrapidly. The growth can best be followed by using the RHI scope and scanning isapproximatelyconstantindirectionand nearly at right angles to a mountain barrier vertically on the azimuth which includes the having a steep lee slope. highest echo. Subsidence of the top of the echo indicates the end of convection and the resulting 2. The wind at mountaintop level exceeds 25 decrease of turbulence in that particular cell or knots. group of cells. 3. There is a rapid increase in wind speed with altitude in the level of the mountain range and for several thousand feet above of at least CONDENSATION TRAILS 30 knots for weak wave development, and at least 50 knots for strong wave development. (A A condensation trail (contrail) is a visible trail peak in the vertical wind profile near or someof small water droplets or ice crystals formed what above mountaintop level is a characteristic under certainconditions in the wake of an of strong waves.) aircraft. 4. An upwind inversion or stable layer exists The formation of contrails is considered to near mountaintop height. decreasesignificantlytheeffectivenessof operational aircraft under combat conditions. In Observation of Vertical daylightwhenclearweatherprevails,the Wind Shear by Radar enemy's detection problem is practically solved when the aircraft produces contrails. At night, When precipitation is showery, the cells may contrails greatlysimplifythe enemy's inter- be seen on the RHI scope as separate columns. ception problem.

411 417 AEROGRAPHER'S MATE 1 & C

To remove the detection hazard during com- varies continuously from near zero immediately bat missions. it is necessary to be able to predict behind the aircraft to an extremely large amount thealtitudesat which contrail formationis far behind it. The rate at which air is entrained probable so that flight at those altitudes may be varies with the type of aircraft, power setting, avoided. Such prediction is the function of the speed. and the density and stability of the weather office which provides the briefing for atmosphere. The ratio of entrained air to ex- the flight. haust gas, at a given distance behind tha aircraft, is greatest when the aircraft is operating at its TYPES OF CONTRAILS most efficient speed and altitude.

There arethree types of contrails:aero- Meteorological Factors dynamic, instability, and engine exhaust. The aerodynamic contrailis produced by the mo- If the temperature, pressure, and humidity are mentary reduction of pressure resulting from the suitable, the water vapor of the exhaust may flow of air past an airfoil. It is of short duration, produce supersaturation with consequent con- and for this reason is not considered an opera- trail formation. The more humid the air at a tional hazard. The instability contrail is pro- given temperature and pressure, the greater will duced by the passage of an aircraft through an be the tendency for contrails to form. otherwise undisturbed layer of unstable air with NOTE: Only in the case of jet aircraft can a a higher relative humidity. Conditions conducive definiterelationshipbeestablished between to such formations exist only rarely. The most pressure, temperature, and relative humidity. In prevalent of contrailsisthe engine exhaust propeller-driven aircraft, energy losses are vari- contrail, and it is with this type that you as an able. Since all the energy is not contributed to Aerographer's Mate will be concerned, since it is the wake, only an estimate of the limiting the only one which must be forecast. temperaturesforcontrailformation can be given. FORMATION OF CONTRAILS CONTRAIL FORECASTING When an aircraft passes through the atmosphere. the engine releases a certain quantity of Calculations based on the rates of heat dissi- water vapor and heat as a result of combustion pation and mixing with entrained air permit the processes in the engine. The exhaust mixes with determination of a maximum temperature for the air. The water vapor introduced tends to each pressure and relative humidity value above increase the relative humidity and to bring the which contrails cannot form in the wake of a jet air closer to saturation. The heat released tends aircraft. to decrease the relative humidity. The formation The Forecasting of Aircraft Condensation of a contraildepends. therefore, upon such Trails, AWSM 105-100, provides all the basic factors as the amount of heat and water pro- background for producing contrail forecasts for duced by the fuel combustion, the wake or both propeller driven and jet aircraft. entrainment characteristics of the aircraft, and The Skew T, Log P diagrams (DOD-WPC 9-16 the original temperature and humidity condition and 9-16-2) are overprinted with a set of lines of the undisturbed air. thatrepresenttheoreticalcriticalrelative- humidity values separating the categories for Entrainment forecasters to utilize in determining the probabilityof, or absence of, contrails from jet Entrained air is that which is drawn into and aircraft. The application of these curves is fully mixed with the exhaust gases of the aircraft. The explained inAirWeatherServiceManual amount of air entrained into the exhaust trail 105-100.

412 418 CHAPTER 12

TROPICAL ANALYSISAND FORECASTING

masses of air inconvergent or divergentflow in The importance of tropicalanalysis and fore- minute, his primary in recent years as relation to each other are casting has been reemphasized concern is withinternal changes in this vastbelt U. S. operations in lowlatitudes have increased. of equatorial air. Various aspects of tropicalanalysis and fore- and forecasting, founded or lacking in Success in tropical analysis casting that were poorly much like analysis in theTemperate Zones, will entirety have been brought tolight. Concerted ofthe dependlargelyon theexperience effort has been made toimprove the analysis experience. of the provide more Aerographer's Mate. The andforecasting techniques to Aerographer's Mate as ananalyst in this case informative and accurate forecasts to users. thorough knowledge of there has been a implies and includes a Also during recent years the climatology of theanalysis area. great increase in the amountof observational This has been data available to the forecaster. BASIC PRINCIPLES OF satellitesand providedbymeteorological TRCPICAL FORECASTING weather radars, as well as anincreased number of weather reporting stations. systems, tropical Tropics. the Withthe advent of satellite During any discussion of the forecasting has improved verymuch. The com- actual area underconsideration is questionable; climatology and satellite of this chapter, the bined use of extensive therefore, for the purpose willproduce the bestforecasts Tropics are defined as theregion lying between information possible in the tropical areas. the high pressure belts atthe surface of each hemisphere. andthetroposphere and lower region. Therefore, the WEATHER VARIATIONS stratosphere above this WITHIN THE TROPICS boundaries of the Tropicswill vary with space 15° andtime. They may approachwithin the tropics is oceanic. recede to as much as Most of the area within latitude of the equator or However, since this regioncontains mountainous 45° latitude from it. islands, coastal areas, andlarge portions of some Since many Navy weatherunits are located or must be given to the itis of the utmost continents, some discussion operating in the Tropics, variations in tropicalweather in each of these importance that theprinciples of analysis and situations. forecasting be understood. temperateandfrigidzones,the Inthe Island and Coastal,TropicelWeather Aerographer's Mateisrequired to acquire a and the boundaries knowledge of the air masses During the day as warm,moist air moves between them in order toeffect a valid analysis over theterrain,large forecast. To forecast in the inland andislifted and the subsequent cumuliform clouds develop.These clouds are Tropics, on the otherhand, the Aerographer's The lifting of moist air dealing with what may common in coastal areas. Mate finds that he is side of mountainousislands single air mass, the air in on the windward properly be termed a also produces toweringcumulus clouds which the equatorial zone.Since the contrasts between 413

419 AEROGRAPHER'S MATE 1 & C may frequently he seen from long distances, inthe showers. Over tropical indicating the presence ofan island ahead. Low oceans, cumu- islands also cause lonimbus are almost alwaysrestricted to areas more clouds and rain thanover affected by synoptic the water, but theclouds tendto increase or meso-scale disturbances; downwind to a maximum tops range from 30,000 to 40,000feet, except near the lee end of the in areas of intense tropical island. Cumuliform cloudsand precipitationare storms where they more abundant over island and may extend beyond 60,000 feet. coastal areas The tropical oceanic regions than over the openoceans. except in the vicinity are characterized by a mean temperaturenear 80°F throughout of certain tropicalweather circulationsystems, such as hurricanes, typhoons, the year. The mean airtemperature is generally the intertropical within a few degrees of convergence zone, and tropicalwaves. the sea-surface temperature (SST) exceptnear some coasts where there Continental Tropical Weather is a prevailing offshoreflow of cold continental air. Wind is the most important The gitatest temperatureand pressure varia- factor in tropical tions in the Tropicsare found in continental weather, even overopen ocean areas. When there is convergence of windflow, areas. The temperature on continentsundergoes there is a pilingup asignificant daily of air locally. This liftingaction in warm, moist temperature variation, be- air results in cumuliform coming very warm in theafternoon and be- clouds and precipita- coming cool during the night. tion. These cumuliformclouds often developto Cloudiness and precipitation great heights, and heavy showersare frequently are at a maxi- observed. mum for the Tropics over landareas which are exposed to airflow fromocean areas. This helps account for tropical jungle WEATHER ELEMENTS areas. IN THE TROPICS Oceanic Tropical Weather Winds Typical weatherover the tropical oceans is characterized by cumuliform Two levels that are importantfor analyzing clouds, although and forecasting synoptic-scale all forms of cloudsare observed. Although most tropical weather systems are one near the surfaceand one in the oceanic cumulus clouds havea common base of upper approximately 2,000 feet, troposphere. Fortunatelythe gradient occasionally lowering level and the 200-mb level to1,500 feet or 1,000 feet are suited to provide in precipitation, this data and they are alsolevels where there is there is great variation in theheights of the tops, an abundance of data. The gradient which depends primarily levelis on the area in which defined as the lowest levelat which predom- they form. Doldrum cumulus,which form in the inately friction free flow areas of light winds and small wind occurs. Over most of shear, have the tropics this is takento be 3,000 feet MSL; tops varying from 6,000 to 12,000feet. however, in some cases this The term "trade cumulus" may have to be applies to cumulus adjusted upward due to land clouds that develop in the elevation. broad easterly wind- The majorfeatures shown by the stream between latitudes10 and 30 degrees. mean sea-levelpressurefields(subtropicalhigh- Their chief characteristic isthe inclination of the cloud axis caused by pressure ridges and low-latitude troughs)dictate wind shear through the the variations of the windflow within the cloud layer. The heightof the trade cumulus tropical region. tops varies from about 7,000to 9,000 feet. Tall Two basic types of circulation cumulus clouds occur only patterns are at widely scattered evident in the low-latitudetrough zone. These intervals in the trade wind.Seen from the air, types are called the the low trade cumulushave a characteristic monsoon and trade wind troughs. Themonsoon troughs occur near the arrangement in bands parallelingthe windflow. large Scattered rain showers continentalareas due tothe land-sea are common from these monsoonal effects and cumulus clouds and visibilitiesare good except are characterized by a directional shear zone withwesterlies on the 414 420 Chapter 12 -TROPICAL ANALYSIS ANDFORECASTING equatorward side and easterlies on the poleward theresultof subsidenceinthe subtropical side. The trade wind troughs occur primarily anticyclones. Itis therefore dry aloft and has over the oceanic areas of theNorth Atlantic, temperature inversion in the lower levels. This Northeast and North Central Pacific and are inversion of temperatureiscalled the trade characterized by confluence of trade wind flows inversion. It is generally located below 10,000 is more pro- from the northern and southern hemispheres. feet. This temperature inversion Other types of circulation patterns also occur nounced over the eastern parts of the oceans due inparttocooling below from cold ocean in the near equatorial region in various locations. fog One of thesecirculationfeaturesis a wind currents. Over the west coasts of continents system near the equator where thetrade wind and stratus are common. Below this inversion flow from one hemisphere changes direction as the lapse rateis very steep, and above the This is inversion the lapse rate closely approximates it moves into the opposite hemisphere. low called a buffer zone:itseparates two wind that of the dry adiabatic lapse rate. This streams of opposing directions,theeasterly inversion over the eastern portions of oceans trades and the monsoon westerlies. The senseof undergoes a transition westward and completely disappears over the western portions of the therotationinthe buffer zone varies from clockwise during the northern hemisphere sum- oceans, though stable layers may appearin this mer to counterclockwiseduring the winter. region in specific weather patterns. Other prominent circulation features on the Inversions over the eastern portions of the gradient-level charts are the subtropical ridges oceans are formed by a broad-scaledescent of with their associated anticyclonic cells and the air from higher altitudes in the eastern endof heat lows over the subtropical continental areas. the subtropical highs. The eastern sideof the Detailed information on wind flow both at high may be thought of as tilting downwardand the low and upper levels is contained inAWS the western end upward. Therefore, the massof Technical Report 240, Forecasters Guide to air moving eastward on this side tends to subside Tropical Mt teorology. and to be lifted as it moves westward on the southern side of thecell. As this air aloft descends, it meets opposition from the low-level Temperature Distribution maritime air flowing equatorward. The height which the inversion forms depends on the depth 1p general. the horizontal temperaturedis- towhichthe upper air currentisableto tribution over the Tropics can be summarized as penetrate downward. The inversion isconsidered follows: The annual temperature range in the to be a mixing zone between these two masses. Tropics does not vary on the average more than The heightof thetrade inversion has a 1° to 3°C. The range is in the neighborhood of significant effect on tropical weather. When the 5° to 10°C over land and only 2° to 3°C over base of the inversion is at a high altitude, the the ocean. The diurnal range is much greater moist layer underneath has a greaterthickness is land and clouds build to greater heights, coverage than the annual range and is much greater over likely. When the than water masses. The largest variationstake more extensive and rain is more base of the trade inversion is low inaltitude, place inthe fringe areas of the Tropicswhich of mid - latitude pres- moisture is confined to lower levels, cloud cover come under the influence isless likely. sure systems. The eastern partsof oceans are and tops are lower, and rain parts duetothe Weather is better when the trade inversion is low colderthanthe western visibility due to circulation around the subtropical highswhich except for periods of reduced brings midlatitude colder air into these areas. haze and over the cold water belts near the west coasts of continents, where fog and stratus are Temperature,therefore,isone of theleast important elements on the tropicalsynoptic common. A common phenomenon of the tradewinds is surface map. One very significant feature of thevertical the presence of trade wind cumulus.This occurs temperature distribution of theTropics is the inthe absence of large-scale convectionor islandsinthe presence of very dry warmair aloft. This air is orographicliftproduced by 415

421 AEROGRAPIIER'S MATE I & C daytime, which may give rise to towering cumu- graphic effect of islands upon thetype and form lus and abundant showerytype rainfall. of cloud cover is even more pronouncedin the From one-third to one-halfof the tropical topical oceanic areas than elsewhere. The ocean area is covered by this type of clouds. higher The seasonal mean temperatures of tropicalair,withits range of temperatures aloft greater capacity to hold water vapor, results below 300 millibars is usually in in the 1') to 3 °C orographic clouds witha minimum amount of range. Cooling generally takes place below700 forced lifting. Another factor millibars. Over thewestern portions of the governing oceanic Pacific ocean the cloudiness in the Tropics is thestrength and monsoon season both winter height of the track inversion. Aswas pointed out and summer greatly affectsthe temperatures, earlier, the moisture content of the airabove the moisture content, and stabilityof the air masses. trade inversion decreases markedly, andstrong stability at the inversion effectively limits Moisture Distribution the vertical extent and development ofclouds. Too, in the Tropics. the absence ofa variety of air Humidity is almost constantthroughout the masses of different origin eliminates the associa- moist layer except at the base andlower part of tion between fronts and clouds the cloud so pronounced layer where a slight maximumis in higher latitudes and leavesus only with the found. The average depth of the moist layer is basic differences between air-massclouds over between 7,000 to 8,000 feet abovewhich it land and those over water surfaces. decreases sharply. This decrease is coincident CI RR1FORM CLOUDS. -Cirriform with the temperature inversion.Although the clouds in varying amounts are foundeverywhere in the temperature inversion is a good indication of oceanic Tropics. this moisture break. the break isevident in some Isolated maximums of high eases where this inversion is not present. cloudiness are found near the Equator,some of which may be attributed to theprevalance of Cloud Types theanviltops of cumulonimbus. An overall seasonalvariationinhighcloudinessisnot evident in the Tropics. Certain A popular conception of the clouddistribu- areas do experi- tioninthe Tropics picturestheequatorial ence seasonal changes, but they are not large regions as a tremendous factorycontinuously enough to be considered representative ofthe producing cumulonimbustype clouds which Tropics as a whole. because of the height of thetropopause rise to MIDDLE CLOUDS. Middle cloudsare likely spectacular heights. THIS IS TRUE ONLYOF to be found everywhere in the Tropics andin CERTAIN REGIONS. All types of high,middle, any season of the year: no appreciable seasonal and low clouds are present in tropicalregions. variation occurs when taking global distribution Although records indicate that cumulonimbus into consideration, and no pronouncedmini- type clouds are more common in tropicalthan mum or maximum of middle cloudinessis in polar regions. they also show thatexcept "Ar evident along any particular latitudeinthe such places as central Africa, southeastAsia, Tropics. Indonesia. the Amazon Valley, and thesouthern LOW CLOUDS.--Cumulus is the predominant United State.. cumulonimbus is the exceptional type of cloud foundinthe Tropics. Their cloud, rather than the mostcommon. In fact. development, vertical and horizontal extent, and certain equatorial regionsare known to report few, if any, cumulonimbus type persistence are controlled by severalfactors: clouds through- horizontal convergence in the wind field, depth out the year. Cumulus. however, in one form or of the moist layer, orography, andvertical another is thepredominantform of tropical stabilityof theair cloud. mass. There are many significant types of cumulus and theyusually The climatology of tropical clouds likethose range in some intermediate form between cumu- in middle and high latitudesmay be divided into lus humilis and cumulus congestus. Stratus and two parts: Climatology of cloudsover oceans stratocumulus are also found in certain regions and climatology of cloudsover land. The oro- of the Tropics.

416 422 Chapter 12 TROPICAL ANALYSISAND FORECASTING precipitation in the Satellite cloud pictures have been studied to The predominant form of However, when determine the tropical large-scale organization of Tropics is of the showery type. tropical disturbances are present.rain may be cloud systems in various regions of the tropics. associated alto- Three major types of cloud systems were identi- steady or intermittent from the fied and given names. These are cloud clusters. stratus deck or low stratiformclouds. monsoon clusters, and popcorncumulonimbi. GENERAL ASPECTS OF Allcloud systems are composed of cumulonimbus type clouds. More detailed informa- TROPICAL ANALYSIS tion concerning the significant features of each techniques cloud system is found in AWS Technical Report There are major differences in utilized for tropical analysis as compared to 240. those used in the temperate zones.This provides Of all forms of cloudiness in the Tropics. the the tropical analyst and forecasterwith a new of low cloud still commands the greater portion challenge. theforecaster'sattention.In many tropical The airmass conceptas appliedinthe areas. an increase ordecrease or change in the mid-latitudes does not generally hold truein the indication form of low cloudiness is often an tropics.especially onadaytoday basis. that the NORMAL weather and cloud pattern Although some writers have made adistinction will be disturbed. in the tropical air masses such as a monsoonair mass. an equatorial air mass,and a tropical air Precipitation mass, in actual practiceit is almost impossible to find any boundaries between them towhich the The general belief' that frequent andheavy criteria of a front can be applied.The whole precipitation is one of the major characteristics tropical troposphere with a few localexceptions, of the Tropics is only partly true.While itis a fulfills the standard definition of asingle air factthat the Tropics as a whole receive more mass. The Aerographer'sMate is chiefly con- rain fallthan other portions of the earth. the cerned withinternalchanges within this air climatological records also show that the annual mass. amount of rainfall within itsboundaries varies In order to observe these changes a more tremendously both in time and space.These complete method of recordingelements at variations can be as much as 300 inchesannually selected stations is necessary than thenormal between two stations 70 miles apart. 6-hour synoptic map. This evaluationshould be The zone of maximum rainfall in theTropics done by graphically recordingvarious elements include lies along the equatorial trough.The minimums hourly. The elements covered should lie along the usual position of thesubtropical pressure.temperature,dewpoint,visibility, wind. anticyclonic belt. weather, cloud amount and type, and The Tropics may be considered toconsist of Elements which ordinarily are conservativein slowly with three basic rainfall regimes: high latitudes, that is, change very time during the transformation of anair mass, I. In the vicinity of theequatorial trough. may change more rapidlyin the Tropics. More- the year. over, changes ofthese elements in the temper- characterized by rain at all seasons of to only one 2. In the trades, characterizedby summer atezone may beattributable rains and dry whit- -s. cause, while a very different cause mayoperate 3. In the subt apical anticyclonicbelt, char- in the Tropics. Thus changes in dewpoint are acterized by d1, at all seasons of the year. usually slow within a single temperate-zone air mass and are attributable to theinfluence of the These regimes give only a very broadindica- surface. over which the air mass is moving. In low tion of the zonal distributionof total rainfall. latitudes, certain rapid changes in dewpoint can Specifically, within each region, therainfall in occur within the tropical air masswhich have any year may or may notfall within the scheme little or nothing to do with the surface over which the air mass is moving. of this classification. 417 423 AEROGRAPIIER'S MATE I & C For these reasons, the Aerographer's Mate In tropical weather forecastingthe forecaster who is undertaking analysis inthe Tropics has to must be well acquainted with the localsituation take special care in evaluatingthe data, at least until he has bt.t..ome of all stations that furnishthe basis for the thoroughly familiar with analysis. As an aid, weather officesshould keep the geography of the regionand with the often a running file on all key stations, surprisinglocaleffects characteristic of this the topo- zone. graphic conditions, other localpeculiarities, the diurnal course of weather, and The analyst in the tropics the pressure and must be aware that wind in each month. For eachof the 6-hourly localeffectsoftenoutweighthesynoptic changes that are occurring, standard observation periods, thisinformation especially near the could be posted inmap report form near the surface. The local effectsare caused primarily by analysis desk for ready reference.Additional radiationalcooling andheating. andtopo- statistical data, such as frequency graphical features,as well as convective activity distribution of at pressure at a given time of day. could alsobe or near the station during observational kept on file for reference. Also, periods.Iftheseeffects obtain local area are overlooked or Forecaster's handbooks foras many stations as ignored, they may leadto errors in data inter- possible. pretation as well as analysis. Thereforeitis The main considerations ofthe weather ele- incumbent on the tropical analystto evaluate ments with reference to tropical the synoptic data as well analysis and as become thoroughly forecastingarepresentedinthefollowing familiar with topographical featuresof stations sections. to be able to separate local effects fromthe actual synoptic changes thatare occurring. Surface Data Sizable departures from the normalshould be closely checked when theyappear in reports from tropical stations. This SURFACE PRESSURE. Sea levelpressures may be attributed to and pressure changes. in conjunction observational errors or due tosome feature that with the has gone unnoticed until this time. gradient wind pattern, furnish -abasic tool of A number of analysis in low as in high latitudes. tropical meteorologists considerclimatology as Pressures are thebestmethod of forecasting within far more numerous thanupper winds or mobs: the therefore, the maximum information tropics due to the fact thatmost changes in the must be weather are not rapid. extracted from the pressure data. In any given month therange of observed DATA CONSIDERATIONS pressures will be almost wholly withina 10-mb spread in a given tropicalarea. Deviations from To insure that al! available data normal greater than 0.5 millibarare rare.It are plotted follows that the deviation of and are as accurateas pot;sible, an analyst, first a pressure from the and foremost, must use his managerial normal must be known tothenearest0.5 skill and millibar in order to be of much initiative to see that observationsare not lost value in tracing through departure patterns withany accuracy. This then ignoranceor neglect by the map is the upper limit permissible for plotters. Ata observers' and weatheroffice,theplotting other errors. personnel must be trained constantlyto be on Except in the vicinity of the alert: the analysts must be able tospot check a tropical storm, all work. 3-hourly tendencies are nearlyuseless in the Tropics and can never be exploitedas in middle An analyst must learn which stationscan be relied upon to transmit latitudes except as they deviatefrom normal. accurate weather in- There are three reasons for this: formation. A littleexperienceatatropical analysis center will show quicklythat some !. The 3-hour synopticpressure change is so stations send much betterreports than others. small (order of 0.1 millibar) The reports judged reliable should as to fall within the be plotted unavoidable range of observationerrors. and adhered to rigorously. Thisapplies to both 2. The diurnal variation (order ofI surface and upper air data. millibar) completely masks the true synopticvariations. 418 424 Chapter 12 TROPICAL ANALYSIS AND FORECASTING

oflocalcloudsystems often latitude, except winds observed in and near 3. Passage flat affects the barometer to 0.1 millibar or more. heavy showers, the daytime wind over stretches of land is most representative. Consequently, the 24-hour pressure change is Since the greatest percentage of tropical land used intropical analysis. This eliminates the stationwinds comes from areas subjectto normal diurnal change from the tendency and markeddiurnal changes. the question arises also provides for a better relation between the whether anything at all can be done with these rate of motion of disturbancesand the time winds. As inthe case of pressure, departures intervaloverwhichpressurechangesare from normal and 24-hour changes provide one measured. The 24-hour change in theTropics is possible route. comparable to the 12-hour changein middle In hurricane forecasting, one looks for weak latitudes. or westerly surface windswhere normally easter- In most areas. changes of 3 millibars are rare lies of 7 to 16 knots should be blowing.This and definitely indicate danger of a severedevel- technique is sometimes applicable. One could, in opmen t when a stormis not already in exist- fact, draw different vectors between the normal ence. Even valuesof 1.5 to 2.5 millibars warrant and observed wind. This has not been attempted careful attention, especially when thechange is on a quantitative basis.but it has been used as a at or below average pressurevalues. mentalaid.Qualitatively one may note, for Pressure gradients in tropical regionshave the instance, whether the strength of the easterliesis order of 1millibar in 100 miles, and less. This above or below normal, or if the winds at a means that most of thecharacter of a map is lost station are becoming more southerly withtime. if 4-mb isobars are drawn. Isobarsshould be Here again the time series representationis a drawn for every 2 millibars south ofthe sub- major aid; the observed time changes ofwind tropical ridge and for 1 millibar within5° of the must be correlated with those of pressureand meteorological Equator. It also followsthat the other elements. pressure must beknown within atleast 0.5 TEMPERATURE AND DEWPOINT.In trop- millibars; or else severe unrealisticdistortion of ical regions, temperature and dewpoint are not isobars will regularly occur. of any great value to an Aerographer's Mate for There are certain topographiceffects which forecast purposes. The reason for this is that the cause true pressureabnormalities. In particular temperature will fluctuate quite rapidly with the there are dynamic pressure reductions onthe lee passage of local showers overboth land and channels between side of mountain ranges and in oceanareas. Rain evaporatingduring itsfall mountainousislands. Thesereductions may causes both temperature anddewpoint to ap- thisis amount to 1 to 3millibars. Unless proach the wet-bulb temperature. Thus readings recognized and taken into account, the perma- in showers are quite unrepresentative.Even . nent cyclonic deformationsof isobars that ap- when a shower has ended, the low-level air pear in certain areas onmost charts will not be cannot immediately return to its previous state. interpreted correctly. SURFACE WIND.--In middle latitudes, sur- Exceptions occur near the boundaries of quite continents. Here theair-massdifferences face winds of 7 to 16 knots or stronger are between continental and oceanic air may be as representative, except in mountainouscountry, In the Tropics, greatas anywhere in middle latitudes.Dew- at coasts. or in thunderstorms. points lower than average also will be found in topographic and coastal features assumeeven diurnal wind regions with marked suppression of the normal greater importance in producing convection over oceans, and on the lee side of regimes. They may render thesurface wind if care is used, islands. completely unrepresentative, but be they can usually be deduced bysubtracting local At this time temperature and dewpoint can effects. The situation as encountered overlarge used mainly to estimate low cloudbases. Since areas and over ocean areasis quite these cannot be computed closer than200 to flatland 1°F at the surface do different.Ship winds arereliable; they can 400 feet, fluctuations of usually be accepted at face value.As in middle not detract from the validityof the calculation. 419 425 AEROGRAPHER'S MATE I& C

CURRENT AND PASTWEATHER. Over be placed on the most of the Tropics. thereports of current presence or absence of low, middle, or high cloudsas groups. Division into weather nearly always involvesome form of one of nine types within each precipitation.However,insome areas, group should for generally be disregarded,especially with refer- instance the Sahara andNorthern Australian deserts. dust storms ence to middle and high clouds.Although there are more important. espe- are many regional and seasonal ciallyin the dry season. Alongthe cold water variations, the following three cloudcombinations have been coasts of Africa and America,fog takes first place in importance. found to occur most frequentlyover the west Atlantic area in the hurricaneseason: Althoughitis highly desirableto separate rainfall into showery and steady precipitation, it . With disturbed conditions. frequent is not always done. Inaddition, it is not always cumulus congestus, and cumulonimbuswith layers of easy to make this observation. Astation may middle and high clouds. receive fairly steady rain fora long time; yet the rain is derived from cumuliform 2. With suppressed convection,small amounts clouds. Because of cumulus, sometimeslarger amounts of of the cell character oftropical rain, and again very shallow cumulus. almostno stratocumulus, and because of local regimes.the current weather no middle or high clouds. reportis very often rather meaningless. Many 3. Intermediate cumulusof average height. occasions are on record whenseverely disturbed isolated cumulus congestus, andcirrus in varying conditions happened to letup temporarily at the amounts. 6-hourly observation period. Herea check of hourly sequences. when available,is most help- The first two types indicatedefinite synoptic ful. Again, in some freaksituation. stations have conditions, and all other informationshould be been deluged when all aroundthem conditions considered with were quite normal. Although thisin mind. Although the not all of these middle andhighcloud types unfortunately phenomena can be spotted, itis advisable to place as much (or more) weight remain vague, thepresence or absence of such on weather in clouds is highly significant. the past 6 hours ason current weather. It is hoped that with time Concerningrepresentativeness for synoptic a place for radar purposes, ship data can be taken at face rain data will be found in thesynoptic codes. A value to the extent that the observationsare considered single radar set scansa very wide area. Such a reliable. view isvastly superior to all spotreports of weather. VISIBILITY.Visibility in the Tropicsis usually good. It is a minor elementin the analysis STATE OF SKY. -A goodpicture of the compared to middle latitudes. distribution of cloud types andamounts, in Here we need to consider only conjunction with present and haze, which past weather, is in over the oceans is mostly produced byaccumu- asensethecore of theanalysis problem. lation of salt particles absorbing Unfortunately, cloud reports often moisture. This are one of is called wet haze. Suchaccumulation goes with the least satisfactory items inthe surface report. very stable conditions and suppressed Usually clouds cannot be measured convec- but must be tion, often in conjunction withstrong surface estimated. The present codes donot permit an winds. Many salt particlesare picked up from observer to describe properly thevarious states the sea; these, however, of sky found in low latitudes. cannot penetrate the The observer's strong inversion lid present,nor are they precipi- entry nearly always entailssome arbitrary deci- tated back to theocean due to the lack of sion. Some observers will regularlytransmit the showers. Since a definite synoptic same combination of low, middle, and condition is high indicated,itis worthwhile to keeptrack of clouds. One may find thatat neighboring sta- intensity and extent of wet haze. tions entirely different typesare reported on a Dry haze occurs occasionally routine basis. especially middle and when continen- high clouds. tal air with a high dustcontent moves over the In spite of these drawbacksmuch can be ocean. This permits assigning a extracted from the cloud data. Emphe's source region for must the air. Although of not muchinterest in general 410 Chapter 12 TROPICAL ANALYSIS ANDFORECASTING Temperature and moisture distribution will vary forecasting,this may be of importance for just special purposes. Haze also isproduced at times considerablyifone instrument ascends following volcanic eruptions when a large partof outsideathunderstorm or shower, and the the erupted material is trappedunder a trade precipitation is of a local nature and is unrepre- inversion. Such haze has been known topersist sentative of the surrounding area. for more than a week, graduallyspreading over a large area. Reconnaissance Reports

Upper Air Data Regularly scheduled reconnaissance reports by weather squadron aircraftand unscheduled supplement the surface UPPER WINDS. Pilot balloon and rawindata reports by other aircraft from land stations generally are representative exceptwhen wind and upper air reports received thing to and ships. Flight procedures changefrom year to speeds are less than 5 knots. The main other pub- ascertain is the quality of upper windsoundings. year and arefullydescribedin At times. upper winds plotted ontime sections lications. CENTER FIX.itis very dangerous proce- at certain stations yield afantastic sequence. position of the Wind directions and speeds changeviolently dure to disregard a reported storm center which does not agreewith your withheightandtime.though the weather station shows preconceivedideas. There have been afew remains the same and no other reconnaissance; therefore, suchremarkablefluctuations.Whenthis erroneous reports by from the proceed cautiously when facedwith the above happens. it is best to separate the good standards of bad. Never discard a whole report.because part situation. Over the years. these find it. accuracy have been set:dead reckoning fix, of it is usually good. You just have to single line area, Aeroarapher's within 30 miles; Loran fix in a Here attain, with experience the within 15 miles; and Loran fix in agood area, Mate will learn which stations canbe relied upon within10 miles.Itis worthwhile to follow to transmit accurate wind data. them with UPPER AIR SOUNDINGS.-Certain informa- successive positions closely, correlate by wind reports, and see thatthe latest reported tion is provided with considerable accuracy capabilities of the radiosonde data. Temperatureinversions. espe- aircraft position fits with the inversions. are re- aircraft in use. This serves, also, as acheck on cially the large trade wind obtained. Radar corded very satisfactorily.The same holds true the storm center position when fixes willnormallyfallwithin the limits of for the low-level moisturedistribution. However, it is a good Comparisons between careful measurements accuracy set forth above. aircraft and hiob plan to obtain a seriesof fixes before making a made by specially equipped is not clearly data have brought out that a dryadiabatic lapse final decision. If the storm eye subcloud layer bin that defined, radar fixes have adisconcerting habit of rate is normal for the They need sonic averaging. often showsamore stable jumping around. theradiosonde When frequent fixes are given,sometimes at structure.Inthe high troposphere. observers required standard intervals as small as 20 minutes,the inherent frequently encode only the and determination of the levels and do not transmitenough significant errors in navigation shows changes exact center of an eye maylead the unwary points. As a result the lapse rate into embarrassing traps. Do some at each standardlevel, which is obviously not forecaster also affected by reasonable smoothing and keepin mind that true. Tropopause pressures are of the storm center this practice. even though manyobservers are there are minor oscillations for very about the mean track. apt toenter one significantpoint from recon- well-pronounced tropopauses ratherthan put WINDS. Surface wind reports naissance aircraft able to observethe sea are them at one of the standardlevels. unreliable in quite reliable. The error in thereported direc- Upper air soundings can be most 10°. For speed the the Tropics atcertaintimes: therefore care tion will usually be under is accepted as error will not exceed5 knots when the wind is should be used before a sounding below 60 knots, and it will beunder 10 to 20 valid.Seeifitfitsthe synoptic situation. 421 4Z7 AEROGRAPHER'S MATE I & C

knots at speeds from 60to 100 knots. Flight- values will vary radically with level winds will be about the method of as accurate in an area position determination and where Loran facilitiesare available, due to the limita- Most authorities disagree tions of instruments. The D-valueand gradient as to how accurate betweenconsecutivereports from the same the speeds are above 100 knots,but it should be aircraft are usually quite assumed theerror accurate. Wind reports will be much larger.In are averaged over a period of 20 to 30 minutes. considering theaccuracy of the surface wind, This is a point often overlookedwhen analyzing consideration must be givento the height of the the data. aircraft. The accuracy willbe less reliable it' the If you have access to the aircraftis above 10,000 feet, navigators of these and if there are flights,itispossible to conduct many clouds present, the reported wind a successful may not programfor improvementin be the highest, but will bethe highest observed. the caliber of reporting. This is an opportunitywhich should Flight-level winds obtainedwith the Doppler not be missed. equipmentareveryaccurate and compare closely with rawin reports. ANALYSIS APPROACH Other flight-level windsare averaged over a given distance and should be considered care- A rational approach to analysisin any region fully especially where Loranfacilities are poor. depends on (1) the objectives of The best way to judge these the analysis, types of reports is (2) the type and quality of theobservations, and to have several reports in a givenarea and use an (3) thespaceandtime distribution average of all the reports. of the observations. So far, we have discussedpoint The wind report should be carefullyplotted (2). With the preceding evaluationof the reports with a protractor ona large-scale map. Direction in mind, consider now point(3) and several and speed should be entered bynumber along steps which will permit an analystto come as with wind barb. The relativeaccuracy of the closely as possible to windreports a representation of the meritscarefultreatment,as space and time distribution ofpressure, temper- (1) they are used for center-positiondetermina- ature, wind, and weather. tions in the event the flight aborts and does not Both the lower and the highertroposphere penetrate far enough for a center fix, (2)they must be considered in forecasting. Thus, will enter into computations of you objective fore- need good upper air chartsas a basic step toward casting techniques. and (3) they are of value for goodprognoses.Thedata.however.are research purposes. scattered, often so widely that whole OTHER ELEM EN T S. -Surface synoptic pressure systems can be situated between stationsand reports are usually quite accurate, and heights of escape notice. Make as much of the standard upper levels are good but upper air may be in data at these scattered stationsas possible: this is error at times. A reported height can becom- accomplished with the time-section pared to a reporting rawinsonde station: if technique. there Further, utilize themore numerous and frequent isa difference, the difference is usually appli- surface observations to help with cable to all the reports from the the upper air same aircraft. analysis. Do this with the techniqueknown as The other elements suchas weather. state of differential analysis. the sea, turbulence, clouds,temperature, and the Inpreparing like are all important and should be judged charts. you must work both in downward from the upper air informationand the same manner as other similar data. upward from the surface reports, The analysis procedure is broken Commercial and Military down into four successive steps in the remainingsections of Transport Aircraft Reports this chapter. They are as follows: These reports, when available, are of value. 1. Complete utilization of theupper air data The present weather given inthe reportsis with the time section method. helpful, and the D-values are very useful in 2. Low-level wind analysis, Pilotballoons in determining the windfield. Accuracy of D- the lower troposphereare far more frequent 422 ''. VS Chapter 12 TROPICAL ANALYSISAND FORECASTING the horizontal wind componentparallel to this the wind than high-reaching soundings. Since line.Itmost commonlyreferstolines of field atthe gradient levelis usually a better cyclonic shear. Monsoon and uppertrrrospheric surface indicator of low-level conditions than troughs as well as remnants ofold coal fronts the low-leveldis- isobars.thisstepdefines are examples of shearlines. turbances and helps guidethe drawing of It has been known for many yearsthat cold isobars. deep into the 3. Surface analysis. It consists of three parts: fronts from mid-latitude penetrate tropics and occasionally move acrossthe equa- pressure.pressurechange.andcloudand tor. With the meteorologicalsatellite pictures weather analysis. relatively easy to contour available today it has become 4. Preparationofcontourand lines) well into the surfaces with the track the cold fronts (shear change charts at upper isobaric The leading edge of differential analysis method. tropics. over oceanic areas. the front is usually markedby a pronounced line APPLICATION OF SATELLITE DATA of convection and a seriesof convection lines oriented parallel to the front (and tothe wind) ifonarea There satellite datdhas been the may occur on thepoleward side of the main greatest asset to the meteorologist wereto be line. The average tops in theseshear lines are singled out, more than likelyit would be the usually not high (10,000 to15.000 feet) but the Tropical region. associated low ceilings and rainfall alongthe line The great ot..ean expanses. as well assparsity may cause poorterminal weather conditions. reports. within this region haveenabled weather especially with orographic effects. phenomena to developtogreatproportions Surface cold fronts often penetrate tolow without being discovered in normalanalysis. latitudes over tropical continental areasduring With the advent of satellite pictures on a the cold season. Even though currentsof War the meteorologist has been continuing basis, air are usually rather shallowby the time they properly, provided with a tool which. when used reach subtropical latitudes, asurface tempera- in discovery, as well as improvethe will aid ture and dewpointdiscontinuity, and a shear of the various tropical understanding line,can be maintainedto low latitudes in phenomena. continental areas because of therepeated noc- Interpretation of the satellite picturesfor the clear, d:y air discussed in that turnal radiational cooling in the tropical phenomena will be mass behind the front. portion of this chapter dealing withtropical analysis. SUBTROPICAL JET TROPICAL PHENOMENA STREAM (STJ) Within this section of the chapter wewill Subtropical jet streams are a persistentfeature discuss phenomena that arefound within the in most cases have no of the tropical general circulation.A detailed tropicalregion and subtropical jet corresponding mid-latitude phenomena.Among study of the northern hemisphere here will be the stream during winter showsitto be a simple those phenomena discussed broadscale current with very highwind speeds. shear lines, subtropical jets.tropical easterly jet, intertropical con- The subtropical jet stream iscontinuous around tropical waves. vortices. the 200 knots are and tropical cyclones. the world and speeds of 150 to vergence zone (ITCZ), a basic three wave For more in -depth discussion ofthese and other not uncommon. There is is recommended that you pattern with ridges, and maximumwind speeds tropical phenomena it of Asia and North refer to AWS Technical Report240, Forecasters are over the east coasts America and the Middle East. The meanlatitude Guide to Tropical Meteorology. is 27.5N ranging from 20 to35 degrees North. SHEAR LINE Averaged around the hemisphere. the corespeed the 200-mb A shear line is defined as aline or narrow is about 140 knots, located near zone across whichthere is an abrupt change in level. 423 AEROGRAPHER'S MATE 1 &C TROPICAL EASTERLY JET (TEJ) 150 0 150 300 450 Mills 25 The tropical easterly jet isa persistent feature over extreme southern Asia and northern Africa Law 2 0 during the northern hemisphere 20 summer. The U. tropical easterly jet extendsover the layer from O 200- to 100-mb in the latitude belt 5 to 20 15 degrees North. It is remarkablypersistent in its position, direction and intensity. 10 I- TROPICAL WAVES X 5 x 5 A tropical wave is definedas a trough or cyclonic curvature maximumin the trade wind easterlies. The wavemay reach maximumam- lel c. plitude inthe low or middle V troposphere or 80W may be the reflection from the N 70W upper tropo- 30 sphere of a cold lowor equatorward extension 30 of a mid-latitude trough. Firstextensiveinvestigationintotropical waves was begun in the 1940's. Usingsurface pressure datathey were ableto track the 20 movement of isallobaric centersacross the west- 20 ern Atlantic and Caribbean Seaarea. They found these isallobaric centerswere accompanied by westward moving, wave-likeoscillations in the basic easterly current in the lower troposphere, 10 These waves moved ataverage speeds of 10 to 10 15 knots, reached theirmaximum intensity in the layer from 700 to 500mb, and sloped eastward with height. In thetypical case where the wave moved slower thanthe basic current in lower levels, and faster thanthe basic current in AG.635 upper levels. the area west of thewave trough Figure 12-1.Model of a tropicalwave both vertical and horizontal. wascharacterizedbysubsidenceandfair weather, while areas ofconvergence and disturbed weather occurred east of the trough. The axis. The cloud patternacross the wave varied basic tropical wave model isshown in figure 12-1. The top portion of considerably, with cumulifortnclouds including the figure shows an cuinulonimbus evident west and east-westverticalcross section of the winds a nimbostratus through the wave at its latitude layer dominant east of theaxis. of maximum Another variation of cloudpattern associated development, and the bottom showsthe surface with tropical waves pressure and 10.000 to 15,000 foot was labeled the inverted-V streamline formation. This namewas adopted because the patterns. The positions of the surfaceand upper cloud bands are level troughs and the weather arrangedina herringbone distribution ac- pattern somewhat resembling companying the waveare also indicated. a nested series of upside-down V's.Figure12-2 illustrates this Later investigations revealedthat a number of type of topical wave. This waves move westwardfaster type of wave is best than the basic defined in the eastern andcentral North Atlantic current, In this case, convergence occurredwest where it shows reasonably of the wave axis andbecame strong near the good day to day continuity. These wavesmove westward at an 424 Chapter 12 'TROPICAL ANALYSIS ANDFORECASTING

To determinethis,tropicalmeteorologists need to use all available conventionaldata and have a thorough knowledge of theclimatology 50W 40W 30W of various types of vortices in specific areasand seasons. A thorough andcomprehensive discussion of this is given in AWS TechnicalReport 240, Forecasters Guide to TropicalMeteorology.

INTERTROPICAL CONVERGENCE ZONE (ITCZ)

As satellite pictures have greatly increased the amount of information providedmeteorologists to improve forecasts, theyhave also led to some changes in theories concerning variousphenom- ems. One of thephenomena soaffectedis Intertropical Convergence Zone (ITCZ), or asit isfrequently referred to now, zone of Inter- tropical Confluence (ITC). By definition the Regional Tropical Analysis Center denotes this as: "A nearlycontinuous !Thence line (usually confluent) representingthe principal asymptote of the EquatorialTrough," AG.636 In general this refers to the areawhere horizon- Figure 12.2,Inverted V tropical wave cloudpattern. tal convergence of the airflow isoccurring. cloud band which is about the Figure 12-3shows atypical average speed of 16 knots associated with the ITC. .It is withinthis cloud speed of the low-level trades. band that disturbances frequently occur.The With the increased use of satellite data aswell cloud band, at times, is narrow(2 to 3 degrees as increased amountsof meteorological data it of miles. that there are a wide latitude) and continuous for thousands has become apparent discontinuous andis inthe At othertimes,itis variety of weather producing systems characterized by a number of large cloud areas5 tropics and that the classical tropical wavedoes to 10 degrees in latitude across.On occasion, not occur as frequently asoriginally believed. vortical cloud patterns are observedwithin the ITCZ cloud band. VORTICES Generally. disturbances along the ITCZ move and Cyclonic cloud and circulation patterns occur from east to west and can move poleward disturbances frequently in the tropical region.These features develop into tropical storms. These are most frequent inthe doldrum portions of are referred to asvortices. low-level develop tropical the equatorial trough. In that area, Some of the weak vortices large areas. classification while others even- cyclonic wind shear is present over storm or higher This, together with friction, producethe forced tually dissipate with littleintensification. development of the These vortices had. for the most part,gone convergence necessary for individual cloud systems whichform the ITCZ undetected with the conventionalreporting net- the use of cloud band. Figure I 2-4 shows thecloud pattern works but became quite apparent as active doldrum troughinthe satellite pictures became widespread. typical of an satellite photo- Western Pacific. Itis still difficult, from the of graphs alone, to determine atwhat tropospheric It has been determined that the presence with the cloud surges of flow from onehemisphere to the other level the circulation associated of ITCZ clouds. most intensethelevel where the is one of the controlling factors vortex is anticyclogenesis maximum cyclonic vorticity occurs. As air moves across the equator,

4-)5

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AG.637 Figure 12-3.--Satellite photograph of cloudband associated with ITCZ.

takesplace. This results inareduction or local thunderstormsoccur. Violent turbulence clearing of clouds along one portion ofthe ITCZ may be associated within these cloud band and an intensification of storms. Cloud the cloud bases may lower to below I ,000feet. or even be band in advance of the burst ofcross equatorial flow. indistinguishable, in heavy showers.Their tops frequently exceed 40.000 feet.An extensive layer of altocumulus and Weather Along the iTCZ altostratus usually occurs due to the spreading out of theupper parts of the clouds. These cloudsvary in height The degree and severity of theweather along andthickness withthe currents of ate air the ITCZ vary considerably withthe degree of masses. At hizfier levels a broad deck ofcirro- convergence between the two air currents. The stratus spreads out on both sides of zone of disturbed weather may be as little the zone. as 20 Visibility is generally goodexcept when reduced to 30 miles in width or as muchas 300 miles. by heavy rain shower activity. Under typical conditions, frequentrainstorms, Surface wind in the vicinity of the cumulus and cumulonimbus ITCZ is type clouds and generallysqually in the heavy showerareas. 426 4;32 Chapter 12TROPICAL ANALYSIS ANDFORECASTING its maximum just before dawn with aminimum occurring late in the morning or earlyafternoon. Over land areas the reverse is true, except on 1,1 coastal areas when the wind has at",onshore component. In this case the diurnalmaximum of precipitation takes place in the early morning.

Seasonal Variation Through the use of mapped digitalsatellite data for 1967, the seasonal meridioi..; displacement of the cloud bandassociated with the 1 1 1 1 figure 12-5. 160°E 170°E ITCZ has been determined. See 140°E 150°E The cloud pictures revealed that thecloud band has somewhat differentcharacteristics in AG.638 different parts of the world. In theAtlantic, it is Figure 12-4.Typical satelliteobservedcloud cluster centered about 3N in thewinter season and patterns relative to a doldrumequatorial trough. moves to about 8N bylate summer. In the Pacific, the seasonal fluctuationof the cloud Usually these winds in the squalls do notexceed band is not readily apparent in that partof the 15 to 25 knots but winds of 40 to50 knots or band east of 150W. Seasonal pressurechanges higher have been reported. over North America maybe responsible for the the heavy cloud masses seasonal shift of the ITCZcloud band in the Ice formationin Ocean. There is some associated with the ITCZ islikely to reach eastern part of the Pacific pilots are flying at evidence of a second cloud bandassociated with serious proportions when Besides the main altitudes above 15,000 feet. (Thisis roughly the the ITCZ in the eastern Pacific. band north of the equator, a weakband appears average freezing level inequatorial regions.) at 5S in the January throughMarch average. The Theintensityof the ITCZ variesinter- lesser degree strength of the second band variesfrom year to diurnally, from day to day and to a it fails to develop at all. annually. Over ocean areas,precipitation reaches year; in some instances

20E 60E 100E 100W 60W 20W I N 140E 180 140W 100E .-;,...1, 1 -- Surface Ridgex...1t-.4"...... 0, ..17.t:Stirnc gigs..'. Ns ..., ....4* It....x...4 ...... - ...... " AtIontle.2.00MBIrL9h .4.-r 200 M8 dioClg 2 maRidge.0---.,.,iggig Pocitle 2COMB-Trouih 1 4 tvp, .."...... 72:0.70::::".."°'.c..V0t0101TI.022XL._.....is."1"11011011,6 111M111181161.'' dig

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AG.639 Hemisphere; (81 Southern Hemisphere. Figure 12-5.Mean summertime position ofthe ITCZ. (A) Northern

427

L 3. AEROGRAPHER'S MATE I & C The ITCZ cloud band inthe Indian Ocean surface wind speeds associated during with the systems the Southern Hemispheresummer is as follows: much broader than thoseof either the Atlantic or eastern Pacific Oceans. I. Tropical Depression. Maximumwinds less than 34 knots and one TROPICAL CYCLONES or more closed isobars on the surface. Normally theseare expected to intensify. The most destructive of allweather phenom- 2. Tropical Storm. Closed ena is the tropical cyclones. While surfaceisobars a tornado with maximum winds 34to 63 knots, inclusive. exceeds the severity ofa full-fledged tropical 3. Hurricane/Typhoon. Maximumwinds of cyclone in a smallerarea, it has a comparatively 64 knots or higher. shortpathandlifeduration. The tropical cyclone due to its greaterhorizontal extent and Life Cycle of the Hurricane/Typhoon longer life exceeds any other phenomenain total damage and loss of life. The energy that sustains tropical Tropical cyclones have been cyclones is given various derived from the energy that isreleased through names in different regions of the world,however the latent heat of condensation. they all have essentially the The energy same characteristics. source is furnished to the tropical stormby the Although the tropical cyclone normallycovers a warm water sources over which it developsand large area, roughly circularor elliptical in shape, it moves. The warm moist airislifted by a is usually of small size comparedwith large combination of convergence and extratropical cyclones. It instability of differs also in that the air untilit condenses. Upon condensation there are no distinct cold and warm sectors and the latent heatisliberated. However, if the no well-definedsurfaces of discontinuityor storm passes over a large land-mass. the fronts at the surface while the source cyclone is in the of energy is cut Wand the stormwill eventually Tropics. The tropical cycloneis found most dissipate. As the stormmoves from southerly frequently in summer or autumn of the hemi- latitudes to higher latitudes, themoisture and sphere in which it occurs, while the extratropical heat source are no longerpresent and the storm cyclone is most frequent in coldmonths. Tropi- will assume extratropical characteristics. cal cyclones have no moving anticyclones as The average lifespan of thistype storm is companions while in the Tropics. Inmany other about 6 days from the time theyform until they features, such as the region ofcalm or relative either move over a land surface calm called the "eye," the or recurve to east to west com- higher latitudes. Some storms lastonly a few ponent of progressive motion in its early history, hours, while some lastas long as 2 weeks. The and the distribution of rainfall,tropical cyclones evolution of the average storm from are distinctive. However. on leaving the Tropics birth to dissipation has been divided into fourstages: they take on some of thecharacteristics of extratropical cyclones. 1. Formative or Incipient Stage.This stage starts with the birth of the circulation and Classification of Tropical Cyclones ends at the time that hurricane/typhoonintensity is reached. This stage can be slow, The nomenclature of tropical requiring days cyclones varies for a weak cyclonic circulationto begin, or in considerably. At times suchterms as "tropical the case of development onan easterly wave, it cyclone," "tropical storm." "hurricanes,"and can be relatively explosive, producing "typhoons" areused almost interchangeably a well- formed eye in as littleas 12 hours. In this stage withlittleregard for differencesinsize or the minimum pressure reached is intensity. There are generally three about 1,000 recognized millibars. A good indication thata system of this categories of nonfrontal cyclones oftropical type has formed or is forming is the origin. all of which must show appearance evidence of a of westerly winds (usually 10knots or more) in closedcirculationat the surface. Theseare lowtropicallatitudes where easterly winds distinguished in terms of observedor estimated normally prevail.

428 434 Chapter I2 TROPICAL ANALYSIS AND FORECASTING

2. Immature or Intensification Stage. This Characteristics of stage lasts from the time the system reaches Hurricanes/Typhoons hurricane-typhoon intensity until the time it reaches its maximum intensity in winds and its To make the most efficient analysis of avail- lowest central pressure. The lowest central pres- able data in the vicinity of tropical cyclones. the sure often drops well below 1,000 millibarsand forecaster must be Familiar with the normal the wind system becomes organized in a tight wind.pressure,temperature,clouds,and ring aroundthe eye with aFair degree of weather patterns associated with these storms. symmetry. The cloud and precipitation fields No two tropical cyclones are exactly alike. On develop into narrow, inward spiraling bands. thecontrarythereareye"),great variations Usually the radius or the strongest winds are no betweenstorms.However,certaingeneral more than 60 miles around the center.This features appear withsufficient frequency to development may take place gradually or occur predominatein themeanpatterns.These in less than I clay. features serve as a valuable guide when recon- 3. Mature stage. This stage lasts from the structing the picture of an individual tropical time the hurricane /typhoon attains its maximum cyclone from sparse data. intensity until it weakens to below this intensity Sincethe meteorological elements are not or transforms to an extratropicalcyclone. In this distributed uniformly throughout all sections of stage, the storm may exist for several days at the storm, it is customary to describe the storms nearly the same level of intensity or decrease interms of lour quadrants or rightand left slowly. The storm grows in size, with strong semicircles. separated by the line along which winds reaching farther and farther from rate the center of the storm is moving and normal 'o center. The weather and winds usually ext.Lial thisline at the cyclone center. Actually, in farther in the right semicircle of the storm. By naturethereare no clearly definedlines of the time the storm reaches this stage it is usually demarcations between these areas. The general well advanced toward the north and west. or it Features of hurricanes/typhoons given in the has already recurved into the westerlies. The following section apply mainly to the mature typhoons of the Pacific usually last longer in the stage. mature stage and growtolarger sizes than hurricanes in the Atlantic. I. Surface Winds. The surface winds blow 4. Decaying Stage. This stage may be charac- inward in a counterclockwise direction toward terized by rapid decay as in the case of many the center with those in the left rear quadrant storms which move inland. or after recurvature havingthe greatestangle of inflow(in the thetransformationintoamiddlelatitude Northern Hemisphere). The diameter of the area cyclone. In the former case the storm steadily affected by the hurricane/typhoon force winds loses strength and character. In cases or trans- may be in excess of 100 milesin large storms or formation there is frequently a regeneration in as small as 25 to 35 miles.Gale winds sometimes the middle latitudes which results in mainte- cover an area 500 to 800miles or more. The nance or redevelopment of strongwinds and maximum extent of strong winds is usually in anti- other hurricane/typhoon characteristics. the direction of the major subtropical cyclone which is most frequently to theright of There is no set duration For the time a storm theline of movement of the storm inthe may be m any one stage. Itis entirely possible Northern Hemisphere. No accurate measurement that a storm will skip one stage or go through it of the peak wind speeds of large mature storms has been made with any reliable degreeof in such a short time that it is not distinguishable knots have been re- without the available synoptic data. Satellite accuracy. Speeds of 140 corded and it seems reasonable to assumethat picture interpretation has improved the possibil- altitudes ity of observing the various stages of tropical speeds of 200 knots may be attained at cyclones. It has led to a newer classification of several hundred feet above the surface. storms. from satellite pictures. which isdivided 2. Surface Pressure. Since the centralsurface is well below into stages and types. pressure of the mature storm 429 AEROGRAPIIER'S MATE 1 & C

average. sea level isobars furnish an excellent a well-developed eye; the sky partly clears; and tool for analysis of these storms. Isobarsmay be the wind subsides to less than 15 knots, andat nearly symmetrical or theymay assume an times there is a dead calm. Thesun or the stars elliptical shape. The pressure gradientto the become visible. In mature storms, the diameter right of the storm's motion :s strongest dueto of the eye averages about 15 miles. but itmay the forward motion of the storm againstthe :attain 40 miles inlarge typhoons. Theeye is not existingpressure gradient. Various other de- always circular, sometimes it becomes elongated formations may occur, such as the extentionof and even diffuse witli,adouble structure appear- a trough southward from the storm. Barometric ance. Radar observations indicatethat the eye is tendencies are not a particularly good indication constantly undergoing transformation and does of the movement of the storm outside ofits not stay in a steady state. sphere of influence. Usually thepressure falls 6. Precipitation. Very heavy rainfall isgener- with extreme rapidity in the 3 hours beforethe ally associated with mature cyclones. However, arrival of the storm and rises atan equal rate the methods of measurement are subject to such after passage. Central pressures of 950to 960 large errors during high winds that representative millibars arc not uncommon. figures on the normal amount and distribution 3. Surfae.. Temperature. In contrast toextra- of precipitation cannot be said to exist. Precipi- tropical cyclones the tropical cyclonemay show tation is generally concentrated in the innercore no. or very little. surface temperature reductions where the slope of the barograph trace is the toward the center of the storm. indicating that highest. Amounts of 20 inches are notun- thehorizontal adiabaticcooling dueto the common. Over the open sea, rainfallis con- pressurereductionislargelyoffsetby the sidered of operational interest primarily from addition of the heat through the releaseo, heat the standpoint ofitseffect on ceiling and to the atmosphere through the condensation visibility. Over land. orographic effects produce processes. Upper air temperatures, it has been concentrations of rainfall. which often results in found. are actually higher by 5°C or more. costlyfloods.Hurricane force winds forcing 4 Clouds. The cloudpatterns of tropical moisture-laden tropical air up a steep mountain cyclones are also at variance with those of the slope often result in phenomenal rainfall. A fall extratropicalcyclones.Inmaturecyclones of 88 inches was recorded during one storm in almostallthe cloud forms are found to be the Philippines. At the other extreme, as little as present. to be sure, but by and large the most a trace has been recorded at a station in Florida. significant clouds of these storms are the heavy which had winds up to 120 knots during the cumulus and cumulonimbus which spiral inward passage of a hurricane. towardtheedge of theeye of the storm. 7. State of the Sea. One of the first signs of becoming generally more massive and closely the tropical storm is the swell, whichcomes in a spaced as they approach the eye. These spiral series of waves with the time interval between bands,especiallytheleading ones. are also crests considerably longer than in waves usually referredto as BARS. Cirrus and cirrostratus observed in the Tropics. Winds of thestorm occupy by far the largest portion of the sky over create waves of the sea which move outward thetropicalcyclonearea.Infact,cirrus, from itscenter more rapidly than the storm becoming more dense. changing to cirrostratus progresses and thus outrun the storm and herald and lowering somewhat, arc more often than not its approach. As the wavemoves onward, its the first indicators of the approach of a distant height from crest to trough diminishes, its length storm or the development of one in the near is reduced, andit becomes a low undulating vicinity. The original appearance of the sky is wave, known as a swell. The size and speed of very similar to that of an approaching warm waves created by the winds depend upon the front. A typical cloud distribution chart fora velocity of the winds and the length ofwater tropical cyclone is found in figure 12-6. surface over which the winds blow. 5. The Eye. The eye of the storm isone of In atropical cyclone the waves and swell the oldest phenomena known in meteorology. move outward from the storm center at a rate Precipitation ceases abruptly at the boundary of whichnearlyalways exceedsthespeed of

430 4:?6_ Chapter 12TROPICAL ANALYSIS AND FORECASTING

13 0 ° E 1 4 0° I50°E

1400 LEGEND FORWARD BOUNDARY OF CI AND CS CLOUDS .5 COVER OR LESS

FORWARD BOUNDARY OF CI AND CS CLOUDS MORE THAN .5COVER

(HATCHING) FORWARD BOUNDARY OF PRECIPITATION AREA

(SHADING) FORWARD BOUNDARY OF MIPCLOUD COVER

/I A SHADING OF LOW CLOUD SYMBOLS PROPORTIONAL TO TENTHS OF COVER

AG.640 Figure 12-6.Typical cloud distribution associated with a tropical cyclone. progressive movement of the storm. The swell with the direction of the wind, the movement of waves continuetomove inastraightline the swell waves is significant. through the storm area, whereas the winds turn The period of the swell waves, that is, the to the left in the Northern Hemisphere and to time in seconds between the passage of suc- the right in the Southern Hemisphere. cessive swell wave crests,is helpful in deter- The direction of swell waves in the open sea mining theintensityof the storm.Inthe gives some indication of the location of the Caribbean Sea and the Gulf of Mexico long storm center. When considered in connection period swell waves do not commonly occur

431 4%7 AEROGRAPHER'S MATE 1 & C except in connection with a tropical storm. The semicircle causeapiling up of water along appearance of heavy swell waves with a period coastal areas as much as 10 feet above normal. of0 to 15seconds during the hurricane season Sometimes these tides are referred to as storm in those waters is an indication of the existence tides. An even greater threat is the so-called s)I.1 tropical storm in the direction from which hurricane wave. The term "hurricane wave" has the swell waves come; the longer swell wave been applied to the marked rise in the level of periods, 12 to 15 seconds, are almost certain the sea near the center of intensetropical signs of the hurricane. Figure 12-7 shows the cyclones. This rise sometimes amounts to 20 llireLtion of wind and swell around the hurricane feet or more and affects small isolated islands as center. readily as continental shores. In partially enclosed seas they may be superimposed on the hurricane and gravitational tides. The hurricane wave may occur as a series of waves, but is usually one huge wave. These waves have pro- 1, I duced many of the major hurricane disasters of i history. There is usually little warning of their \ -) approach. However, they should be anticipated / near, and to the right of, the center of intense \,,.. tropical cyclones. ,- 1 ---\%i 8. VerticalCharacterist!cs.Thevertical X/ structureof atropical cyclone alsodiffers c-. HURRICANE / considerably from the extratropical cyclone. i),V The first difference is that the tropical cyclone is

. ------4.--EC NTER ziways a warm core low. The storm may build / i, ..----';`). from the top down as well as from the surface ,f..-.... upward, and it is for this reason that only the mature model should be considered for com- / ,,, il' parison. Subsidence occurs in the eye of the i--;--- storm below about 30,000 feet extending to lze about 3,000 feet above the surface, accounting x>17 7----4- for the lack of or sparsity of low and middle /7 clouds. There is present in a mature storm, considerable horizontal and vertical mixing, extending from near the surface to between 10,000 and 20,000 feet. There is a net horizontal inflow of air atall levels to about 3,000 feet or higher, AG.641 above which there is a net horizontal outflow of Figure 12-7.Direction of wind and swell around the air. This net outflow of airisusually very hurricane center. Solid arrows show the direction of pronounced in the vicinity of the 200-mb level, the wind; dashed arrows show direction of swell. and itis for this reason that the 200-mb level Large arrow shows direction of movement of the storm. is one of the primary analysis tools in the Tropics. The outflow at the 200-mb level is manifested by an anticyclonic flow, unless the storm is Oneof the most severe effects of hurricane unusually severe and penetrates even that level, damage occurs along coastal areas by large ocean in which case the flow is cyclonic. waves. The most severe waves can be expected where land partially surrounds bodies of water Seasons and Regions of Occurrence such as ti.e Bay of Bengal and the Gulf of Tropicalcyclones may occur during any NlexiLo Strong sustained winds in the right-hand month of the yeas with a maximum occurrence

432

018 Chapter 12 TROPICAL ANALYSIS AND FORI-C ASTI ; from May to November. In general, the M1111111e1 distinbanccs hum w Lich sidini, d,.ctop hemisphere is the region of maximum occur- may be detected w itInn 5 Latitude ofk rence for that particular area. Frequency, how- theequatoibuttiicsedisturbances do not ever. varies from ocean to ocean. intensify into tc phoons,luirlicanes until they are There are eight principal regions of tropical more than 5 clegiees 01 latitudefrom the cycloneformation.Fiveregions areinthe equator. Northern Hemisphere: duce, in the Southern Additional requileineat% Law been `.tressed Hemisphere. The South Atlantic and the eastern byotherresearcher,Among these arethe South Pacific Oceans are entirelyfree from presence of a picexistiag low let el distulbanee tropicalcyclones.ThesouthwesternNorth and a regionofupperlet eldivergent evi Pacific on the other hand has by fat the great tit Outflow a1a c the suils,,,e distuibance Satan,: number of tropical cyclones developing in it. A photogiaphs Iratetierlfledthe preexiting dis- detailed breakdown of formation areas, with tuibanee foi many day, prior !o the d;celop maintracks and months of most frequent inept of tookit storm. Ilandreds of tropical occurrence, is found in table 12-1. dist in banc ( of 0rganired coin Cc Notalltropicalcyclones reachhurricane ticity) ot.eid 01.:1ihk. %cam tropical ocean, csals, intensity. Tropical cyclones that reach hurricane yeal. yet onlyaiclat;ccIytcw dccdop into intensity are more prevalent in the respective Cyclones of nopical storm in gietet intensity late summer and early fall in both the Northern Lick of suftici. at uppci an data ha, limited and Southern Hemispheres. This does not pre- study in file 1,01d11). ut titre,,uiremcw. of upper clude the formation or the intensification of level dicergenee. tropical cyclones of any intensity during the Some ,upp,,rt lli. opw,ell ,1elittP111.11 other seasons. Tropical cyclones are also much instability secotal Laid- it the less frequent than the extratropical ones. best play suit eNPUlatiOn 101 ,101:10 desl exists. -Hi istiivorystressestheintelaction SYNOPTIC FLOW PATTERN between cumulus conceetion and large scale FAVORABLE FOR DEVELOPMENT motion A ielatlnill.ttinuloninibus tell, in a tropicaldistmbanee warmstheatmosphere The necessary requirements for tropical storm slightly through the rcle.he latent heat_ !'his formation have been researched and discussed to heating lowers the wrta,e presswe ,lightl:s, thus great lengths. It is generally agreed however. that increasing the low lot:: turn there are at least 3 basic conditions necessary. as mereases the low loel ,un,erizenLe lic...vase of follows: frictional turning in the boundary layer.[his conveigeneepi,iducesmorecumulonimbus. I. Sufficiently large ocean areas with temper- t.ausine, mote Llltnt heating sr,hidt lower. the atures so high that air lifted from the lowest surface pressure even more. and so on. In order atmospheric layers and expanded moist adiabati- foi this prat_ to he clecticc. it must occur in callyremains considerably warmer thanthe a teizion.'r here the %CH Lai wind through surrounding undisturbed atmosphere. at least up thetropospheteis ,1/4.11y anallto allowilk' to a level of about 40.000 feet. releasedlatentheatto he concentrated in a 2. The value of coriolis paramenter larger fan ly small are,, This themy then stress:, small than a certain nummum value. thus excluding a vertical wind shear a,.1 primary factor innthe belt of about 5 to S degrees latitude width on development process both sides of the equator. Figure 12 S shows the average -zonal vertical 3. Weak vertical wind shearinthebasic wind sheaf between 850 nib and 200 nib over current. thetropicsfor January. April. August. and October. Due tothepredominatelyzonal The first requirement limits storm formation easterly or \Nested) mean flow in 'he tropic,. to sea or oceanic areas with a surface tempera- the mean zonalwindshear. determinedby ture of approximately 80 °F. Statistics verify the subtracting 11,e mean ca cst component at second condition in that they show that initial 850 nib from that at 200 111I)is similar to the

433 AEROGRAPIIIER'S MATE 1 & C

Table 12-1.Tropical cyclone data.

Atlantic Ocean Pacific Ocean Indian Ocean N H Areas of 1. Cape Verde Islands 1. Marshall Islands, 1. Bay of 0 E formation. and westward. Caroline Islands, Bengal. R M Western Carib- Philippines, and TI bean. South China Sea. H S 2. Gulf of Mexico. 2. Gulf of 2. Laccadive E P Tehuantepec to Islands to R H Revillagigedo Maldive N E Island. Islands. R E Main 1. Through West Indies 1. Through or near 1.Clockwise tracks. and northward to Philippines and path into U. S. or ocean area northward to- India or OR into Central ward China, Burma. America. Japan, etc. 2. West or north. 2. Northwestward 2. Clockwise to Lower Cali- path into fornia or half- India or way to Hawai- Gulf of ian Islands. Oman. Highest Aug., Sept., Oct. 1. July, Aug., 1. June to Nov., frequency Sept. , Oct. inclusive months. 2. Sept., Oct. 2. June, Oct., Nov.

S H Areas of None Coral Sea and west 1. Cocos Is- 0 E formation. of TuamotuIS- lands and U M lands. westward. TI 2. Timor Sea. H S E P Main None Counterclockwise 1. Westward, R H tracks. along northeast then coun- N E Australian coast terclock- R or toward New wise south- E Zealand. West- ward near ward to Coral Madagas- Sea. car. 2. Counter- clockwise along northwest Australian coast. Highest None Jan., Feb. , Mar. 1. July, Aug. , frequency Sept. months. 2. July, Aug., Sept.

434 440 Chapter 12 TROPICAL ANALYSIS AND FORECASTING

JANUARY

30 -30' 20 40

10 20 C, coW6A 10 20

30 40 40

I I I 120 130 ISO.

APRIL 40. -40* 30 4140TIllinim 411".40 20' 2020. JO. ( _:: Et (096T09 Os 70 - 10' .20 4 20 -40 20* Mr -30* 30 40 ar .40 40 - 40* if 30 CO' 90 120 i30 160 150 120 90 60 30

AUGUST 40. 40. 30. 30. 20' 20 IO

C4 C, 70- 20 20 -30' 30 30 .60 40- -40' 40 40" 30 120 MO. 150 120 60

OCTOSER 40. 40 -40. 40 30-

, ..------20 1 .... .-- 70 %"- 30 0--..- -i Ce l" 10. f 20 ------20 Cs (096 TOR 7'. - -20 10. 20 , 0. --___:."-- - o0 10 20 20 40 30 3° 40 40' 40 -40* 40* I I / 30 60 SO. 120 MO 130 120 90' 30 30' AG.642 Figure 12-8.Average zonal vertical wind shear between 850 mb and 200 mb over the tropics.

435 441 AEROGRAPHER'S MATE I & C

meantotalwind shear obtained fromcon- tensity. The favored area for intensification of sidering both wind componentsat these levels. these disturbances is the western North Atlantic The positive values in figure 12-8 indicatethat as the disturbances move into an area of warmer the 70nai Wind at 200 mb is stronger from the sea-surface temperatures. Early and latein the west or weaker from the east. Atlantic hurricane season. storms tend to form In general, areas of active storm formation in the southwestern Caribbean as a result of the such as the western North Pat.ific.eastern North eastward extension of the monsoon trough from Pacific. and North Atlantic (Augustmap) and the Northeast Pacific. In addition to thesyn- the South Indian and South Pacific (January optic mechanisms previously mentioned. storms man) are regions of small mean zonal wind can be initiated by downward penetration of shear The data on the chart correlates with the cold upper lows over subtropical latitudes of t!ie statistical data relating to frequency and time of central and western Pacific. formation or tropical storms. It is evident that a variety of synoptic features In summation there is generalagreement on at are associatedwith stormformationinthe least 4 of the 5 requirementswe have discussed NorthAtlanticarea.Inthe western North within this section. These are the need fora Pacific the low level monsoon troughis the preexisting disturbance. over a warm (greater primary synoptic, feature associated with trop- than 80°F) ocean area. Ics:ated more than 5 ical storm formation, 85 percent to 90 percent degrees of latitude from the equator. ina region of the storms form in that trough. the upper of small vertical wind shear through the atmo- tropospherictrough being of secondary im- sphere. portance (10 percentto 15percent of the In spite of this general agreement concerning storms). In all of the other development areas. these requirements for formation, there is still most of the storms result from the intensifica- considerabledisagreementconcerningthe tion of vortices which form in the low-level ynoptic situationinwhichtropicalstorms monsoon trough. actually form. A number of theories view the development process as progressive intensifica- Estimating the Position tion of atropical wave. With the increased of the Storm Center observingcapabilitiesprovidedbysatellite pictures during the late 60's none of the storms One of the simplest means of locating the that developed inthe Atlantic were originally storm center from surface ship synoptic reports associated with a tropical wave. This reduced is to apply the law of Buys Ballot; that is. when emphasis ontropicalstormformationfrom you turn your back to the wind in the Northern tropical waves does not mean that the circula- Hemisphere, iow pressure willlie to your left tion patterns associated with disturbances which and slightly to the front. To apply this law you intensifyintotropical storms are now com- need at least two ship reports from different pletely understoodRecent annual summaries ships or two reports at different time intervals indicate that circulation features in the middle from one ship. However. from the observations and upper troposphere and their interaction of only one ship, it is not possible to calculate with the lower levels may be more important in the exact distance of the cyclone center. this area than in the other storm development An example of locating the tropical storm regions.It has been discovered that during the center from two ship reports is given. (See fig. heart of the storm season (mid-July to mid- 12-9.) September) most Atlantic tropical storms result From the law of Buys Ballot, with your back fromintensificationoflow-level cyclonic to the wind you would expect to find the center vorticeswhichoriginateoverAfrica. These of low pressure to the left and slightly to the vortices form in the monsoon trough over Africa front (to the westor vve.,t-southwest of your near the 10.000 foot level and track westward position). Swells in the Caribbean Sea do not over the Atlantic. In their earlier stages they commonly come from the southwest. The nor- may or may not show a closed cyclonic circu- mal condition in that regionis an easterly or lation atthe surface. depending on their in- northeasterly swell, set up by the trades, hence a

436 442 Chapter 12 TROPICAL ANALYSIS AND FORECASTING

Blueficlds 0

AG.643 Figure 12-9.Center of tropical storm located by observations of two ships. Short dashed arrows show direction of waves.

,outhwestQrly swell k strongly mdiLanse of a tiOnS of wind direction is subject to certain error tropicalLy done. SinLe we cannot accurately when the circulation around the storm is not determine the location of the storm Lenter from circular. Often, the wind field around a tropical a single ship report alone. we look for another storm is elliptical as shown in figure 12-10 (A). ship report in the area to use for interpolation of From a study of the diagram, it is apparent that a fix on the stom center. application of the wind rule may result in large Obviously skip B is closer to the storm Lenter error. Occasionally very elongated wind fields than ship A. Applying Buys Ballot's law, with may contain two storm centers. During periods your back to the wind on ship B you would when tropical storm warnings are issued, it is a expecttofindthestormLenternorth or good idea to copy the fleet facsimile broadcast. possibly northeast. From these two observations the canned map analysis or actual synoptic it appears that the storm ,..enter was located at reports in order to determine the approximate approximatelyI 6N and 80'W. The next step wind field oround the storm. is to tentatively sketch m the isobars as shown in There is an indraft of wind inalltropical figure 12-9. storms. The angle of indraft varies with the The general rule for locating the direction of distance from the storm Lenter, in the outer the Lenter of a tropical storm based on obsena- limits of the wind field itis about 450, near the

437 443 AEROGRAPIIER'S MATE 1 & C

on a variety of forms. The shape and appearance of these cloud systems depends on both the type of atmospheric perturbation initiating their convection andthevertical wind shear of the storm's environment. Since the latter is closely related to aspects of the quasi-stationary plane- tarycirculation, such as the subtropical high pressureridgeor thehigh-levelmid-oceanic troughs, storms that develop in different regions of the world appear somewhat different as they evolve. As a tropical storm matures, it modifies (A) its immediate environment to the extent that storms in the later stages of their development look much the same world-wide. On any one day, numerous synoptic-scale cloud systems appear in the Tropics and subtropics. Only a limited number of these tropical disturbancesdevelop intomature tropical cyclones. A classification system has been developed for these tropical cloud formations. It first seeks to identify those areas of convection which, based on their appearance, seem most likelyto develop and, secondly, attempts to classify storms as to their intensity. Figures12-11through12-19aresatellite pictures of tropical cyclones in various stages of development. The classification ascribed to each is from an earlier method of forecasting tropical cyclone intensity from satellite pictures. The (B) National Environmental Satellite Service (NESS) employs a different system now, but this older AG.644 system may be used locally as a guide, Reference Figure 12-10.Example of errors in locating direction of to this system is contained in AWS TR 240, storm center. (A) When the wind field is elliptical and chapter 8. the wind rule is used; (B) when the wind rule is applied to fast moving storms. TROPICAL ANALYSIS center the direction of wind movement is more Tropical analysis consists of complete and nearly tangent to the isobars. detailedanalysis of the broad-scale synoptic The angle which the winds blow across the features as presented on the surface and upper isobars towards the center of a mature tropical air charts and, furthermore, of more localized storm varies widely in FAST MOVING storms. and specialized featues as they appear on time Directly ahead of the storm, the angle is the sections,spacecross sections.low-level and least. To the rear of the storm. the angle is high-level streamline charts. and weather dis- greatest. Figure 12-10 (13) shows that the appli- tribution charts. These charts and analyses are cation of the wind ruleinthe case of fast the bases for the end-product of the analysis: moving tropical storms will not be as accurate as the forecast. it is for stationary or slow moving storms. ANALYSIS OF TIME SECTIONS Satellite Appearance Thealoud systems produced by tropical Time sections should be kept regularly for all storms in their early stages of development take key stations. Their number depends on the size

438 444 Chapter 12TROPICAL ANALYSIS AND FORECASTING

15N. 140 E 150 E 11111111r2._

"- II h

14;:. Is 5N

t I

AG.645 AG.647 Figure 12-11.Stage A tropical disturbance, Figure 12-13.Stage C tropical disturbance, Western Pacific. Western Pacific.

'4135E tiw

.1 it4., 20N It

11111112...

._,A111111_

ION 1111.11-

ION

AG.646 Figure 12-12.Stage B tropical disturbance, MidPacific. AG.648 of the area to be analyzed and the number of Figure 12-14.Stage Cminus tropical disturbance, stations available. Up to 20 stations can be Western Pacific. profitablyplotted and studied on a routine The format is important for convenient and basis. During special situations, additional time rapid handling. Figure 12-20 shows a form for sections can he added for periods of a few days. time section analysis.

439 44 5, AEROGRAPHER'S MATE 1 & C

iF .n*CsEstr'''' 400E' rillf;150E E " " 170E1 4s 4

16, vr,A ii-ve.41*.C4gAt-

15N y1st

L AG.649 Figure 12-15.Stage C-plus tropical disturbance.

E AG.651 Figure 12-17.Stage X, Category 2, South Pacific.

20E

AG.650 Figure 12-16.Stage X, Category 1, Western Pacific.

The vertical coordinate of a time section may be pressure. pressure altitude. of height. It is of advantage to have both a pressure and a height scale. since upper winds are reported at fixed heights. while the signitiLant points on Nobs are given in millibars onlyrime may be plotted from left to right or vice versa. AG.652 Plotting Figure 12-18.Stage X, Category 3, Atlantic. All upper winds should be plotted on the time Sm. c it is thlfiLult to draw wind al RAC:, shouldwrite down thecodeddirectionin quik. ,.oiro.ily and sins in low latitudes small addition to drawing the vectors as shown in wind-hilts °nen are important. the observer figure 12-21.

440 446 Chapter I2-- TROPICAL ANALYSIS AND FORECASTING

or 100 feet per 24 hours or more should not be preceded or followed byrisesof the same 110W 100W 90W magnitude in the 24-hour interval centered 12 4*/* hours before or after, except when accompanied by strong winds and large wind shifts. Otherwise one or more soundings must be suspect. It has M7( r"rti alreadybeen statedthatin such casesthe IliVaiNNAN " emphasis should be placed on the nighttime data. At reliable stations these changes should be accepted as correct, except (I) when the raob is taken in heavy rain, (2) if a large change is observed, yet there is no previous indication that

woeNear a large height rise or fall center should arrive, of (3) ifthe heightsriseas much ina second 20N 24-hour interval as they fellin a first without gr, appropriate wind and weather changes. Thefirstpart of the evaluation of time sections, as shown in figure 12-22, is largely qualitative and dependent on an analyst's skill AG.653 and experience. Thisistrueinevenlarger Figure 12-19.Stage X, Category 4, Atlantic. measureforthesecondstep,sinceformal procedures for the integration of time section weather data do not exist. For most purposes,itwillsuffice to plot winds at 2.000-ft (610 meters) intervals below The following seven semiquantitative steps, 20.000 ft (6.096 meters). Higher up, the stand- however, can be carried out: ard 5.000-ft (1524 meters) interval will suffice. 1. The principal trough lines and shear lines in the wind field are marked by heavy orange Analysis lines and their direction of displacement (especially eastward or westward)is noted with an The first object of time section analysis.is to arrow. These lines give the slope of the disturb- detestarious errors and unrepresentative values ances, they will also show the bottom or top of of the reports and make the variations of wind, a disturbance, whereas quite frequently a wind pressure change. and the like as consistent as shift is mainly confined to either upper of lowei possible along the vertical and intime. The troposphere and extends only weakly into the second object is to consider surface, upper wind, other half. and raob data togther and deduce from them as The distribution of weather as given by the much as possible about the sy noptic situation. surface reports relative to the time of wind shift Of these the first is by far the easier. Not only shows whether bad weatherisconcentrated will errors usually stand out in an obvious way, mainly on the forward or rearward side, and but the Aerographer's Mate can also deduce how much weather there is. Comparison with fromthetimesequence such thingsasto time sections where the disturbance had passed whether a wind shiftin acertainlayeris previously will (I) furnish the rate of motion, transitory, lasting for only 6 hours, GI- whether it (2) show changes of intensity of winds and wind denotes a longer period change. The sequence of shifts, (3) reveal changes of weather distribution 24-hour height changes of upper pressure sur- and intensity with respect to the system. To a facesisparticularlysuitedfor time section lesser degree changes of intensity can also be analysis when overlapping 24-hour changes are deduced by variations of the amount of 24-hour computed every 12 hours.Consideringthe surface pressure changes. normal extent and rate or motion of disturb- 2.Isolines are drawn for the 24-hour height ances in low latitudes, marked upper height falls changes. The interval chosen should be at least

441 447 AEROGRAPHER'S MATE 1 & C

TAO' A44,6 St. ICA4 F b *: #';411.I.rr I.,' = . `.;+.1.,,611 .#1..# , .4:6 .I / '''*''I I***t! ....

; ......

I 1 1 1 .. I e . 4

I It* -4 t #1, ...... ; # .1

I ,; , 1 t ' 1414. .4 A # A 1,4

; I . * I " t .4-4-4A-44 44-1 ' t 1I ' rI #44441;4# ; ;1IIiill; 3s ;414 4'tte I I r r 111 Hiii_j /I ell 1, :

AG.654 Figure 12-20.Form for time section analysis.

100 feet (30 meters). and 50 feet (15 meters) insignificant at low speeds. Figure 12-23 shows below 400 millibars, since in general changes in the computation of the wind shift. its intensity the lower atmosphere are smaller than higher up. is given by the amount of the vector difference, These changes are examined closely for their irrespective of direction. relation to the wind shift lines. Trough lines, in Itis emphasized that the foregoing. while particular. must coincide by definition with the essentiallycorrectinpractice.is not always instantaneous ze,o height changes. Normally tl observed and has no necessary foundation in 24-hour zero change line should parallel the theory. Sometimes there are persistent height slope of the wind shift lines and be locateu not falls with little change of wind. This is one far from them. indication of deepening of a stationary disturb- The vertical gradient of 24-hour height change ance, and the situation should be carefully is normally correlated with the vertical variation checked for this possibility. of intensity of moving disturbance. The layer of 3. The vertical gradient of 24-hour height strongest 24-hour height changes should be the changes also indicates the areas of cooling and layer of greatest intensity. and during trough or warming, since it indicates whether the isobaric ridge passages the strongest wind shifts should surfaces have moved closer together or farther be found there. This statement applies to tine apart along the vertical. Height changes usually vectorial wind shift, not merely the directional are largestin the high troposphere both the change. since the latter can be very large yet falls ahead of a trough and the rises to its rear.

442 448 Chapter 12TROPICAL ANALYSIS AND FORECASTING

sphere to 200 to 150 millibars; higher up the ER11111r 1 EN temperaturefieldreverses andthey die out on toward the tropopause. This is the main reason mumuni ' main .-- /I-- why the 200-mb surface is the best choice for noit - + --Jai .4- high troposphere analysis in the Tropics. 1 100 NEMO A great deal about the structure of disturb- MN R + tour g i'otLi- ances can be deduced from time section data if M5T1 MI the observations are processed carefully. How- 111101111MINFIS 1 11C124111111 ever, the possibilities of analyzing the relations sigautom 1 024 4`'l between temperature, pressure, and wind field as agifirlir f f given by the time section have not been ex- 150 MI hausted.Although thevalidity of the geo- Inn + strophic wind relation in the Tropics is questionable, the vertical wind shear appears to give a 6.0 Ai VI fair idea as to the distribution of cold and warm 0 0 1 0 200--11 1 air to latitudes 10° and even to 5°. Easterlies ll decreasing with height denote colder air pole- in ward and warmer air equatorward from the --r"4 ToIt--4,--il-, 250 station. Easterlies increasing with height denote L --4-vm the reverse. Northerly winds increasing upward v. r ahead of a trough in the Northern Hemisphere KAMIlme -'In 300 Nammin indicate colder air in the trough as do southerly 0. winds increasing upward to the rear of a trough. am 350aiamn Winds usually turn counterclockwise with height in mis ahead of such troughs and clockwise to their madmil o 400:.....s.m. um rear. Thus, the pattern, though generally not the IIMISIMUNE MOM amount of geostrophic thermal advection, agrees ININSMINIMAIMILI 450 PMTIWIMMIMMEmm li with the movement of cold and warm areas. However,actualtemperaturechangesaloft 500 atilliMENNUIVRO immi.no usually are much smaller than those computed .. dmmem.uoimm.. from the vertical shear vectors, indicating subsi- 650 ...nommuconamor NMEMIN dence in areas of high-level cold advection and All.401 600 mumanammansAmami ascent in many areas of warm advection. The MXIWINWASIMPTIONE siltation is further complicated by the release of 650 MIIIMMAUMMUrwalmEMmom. CVMSKOMOMEMZemOM%ORM latent heat of condensation in ascent areas. This 700 MUMWOMMENUMUMOOMENO subject is covered more thoroughly in the upper MINIMES PLIMERMANIMEM 750 allliiiIMICOIWIIIM air analysis section of this chapter. two 111111111011111MICINHINIMMIN 4. The vertical gradient of height changes can 850 1104111b1111111111 also be related to some extent to the weather. 900 magamummulommaimilms MENMEMIONEMMOMMMINEN Heaviest activity is likely to occur where lower 950 NMMDIMMMUMMMENWARINNII rises go over into upper falls or where lower falls 1000 rAMMOSOMMOMMIMPIWAM go over into upper rises. The firstofthese 101325 IIMMUMORM6WEEMEMMENEM patterns is found usually with cold core disturb- 00 06 12 ances;itisprominent tothe rear of wave troughs in the easterlies. Although the weather AG.655 may be very intense, the situation usually does Figure 12-21.Time section of upper winds. not indicate tropical cyclone development. In contrast, lower falls changing to upper rises are This indicates that most troughs have a cold core the most dangerousofall patterns, since the rain structure and ridges have a warm core structure. area may develop into a tropical storm. The Their intensity increases upward in the tropo- Aerographer's Mate should at all times be on

443 449 AEROGRAPHER'S MATE 1 & C

.C OfOSS.S(C7.04 ... ..7.:. ,11;: . ... :: 1 ."111-

. ::.; :414;1.._...: :1 1,,,, i 1 1: 4,,st - t1014/ i, 141 ;: :; ; :A.,ii,'ill. ,.

31.1).. : :. '::, 1, to" -; Ent '-,..L.-...;41-..1-i . +10 *2 1;30 . ; ix); ; *to : 1., i , ;Zs +i0 410 ; 410 -12d'0 ., .41140 4 .7 iSt a, '0424,; .... 'I '' C %IV 1111.4, 01- I in r :tbe 4.4% ')* , ; _...,,:,:),: ix -..,j. 7.1$ 9 ,il, .,.f,... 1, f,....4, y ..... 40, 4,.1,4.6, , 41411/HUlli 11"111;r1:i. uo

1 ,.,,,:-171.11 '!,71-:1.'' 1111111 1" 011111 11'11. ,,,;,1--- ;"" ,vi-,114 ,i.,i,:11} ' 4,: 1,4, 14-,-t. it--T-r..-pi: ..,01

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06 OCT, 812 00 06 007.612 18 00 06 007 712 HI 00 18 00 06 "X'912 18

AG.656 Figure 12-22.Composite time section (analyzed). watch for this pattern and also check the passage bottom, and the inversion thickness in millibars, of a cold core disturbance at successive stations are the best measures of the inversion strength. to determine whether the disturbance is losing The lid against penetration and destruction of or gaining cold core characteristics. the inversion will be the larger, the greater the 5. The depth of the moist layer is indicated increase of potential temperature through the as given by the height of the 5 g/kg (grams per inversion and the thicker the inversion layer. kilogram) moisture surface in the rainy season The inversion analysis establishes the current and by the 3 g/kg surface in the dry season. average cloud top height and gives an indication Variations in the depth of the moist layer should to what extent an approaching high-level dis- be in accord with the observed weather and the turbance may be able to raise and weaken the deductions made from the preceding steps. Due inversion and raise the cloud tops. to the unsatisfactory status of the moisture 7. Baseandtop of equatorial anipolar element on the radiosonde instruments, this westerlies may be marked as a final step, mainly correlation will often fail when the moist layer to indicate clearly the depth of any layers with extends above the freezing level. Rather dry air westerly winds and the time changes. This is may be indicated even though the balloon is useful in determining and predicting the direc- known to pass through thick clouds. tion of motion of disturbance. These move 6. A better indication of vertical stability and eastward when a deep layer of westerliesis moisture is bottom, top, and intensity of stable present, westward in deep easterlies. A change in layers, especially the trade inversion. The poten- the thickness of a layer of easterlies or westerlies tialtemperature difference between top and may indicateareversal of the direction of

444 4k5,0 Chapter 12 -TROPICAL ANALYSIS ANDFORECASTING 5,000-ft winds. In the equatorial trough zone, the sharp shear found betweenopposing currents near the ground oftendiminish markedly with height between 2,000 and 5.000feet. On the other hand, the lower level is much more subjectto orographic influences, andin the trade region, where disturbances often damp out downward to the ground. the 2,000-ftwind fluctuations may be small and noninformative. 270° 90° Thus, the choice of level must be based onthe local and synoptic peculiarities of the area tobe analyzed.

Plotting Plotting consists in entering wind arrows and barbs, and wind direction ar.:1 speedwritten in 180° code form, in parenthesis. alongsidethe barb. Some stations may vary the last entry.The use (a) LARGE DIRECTIONAL CHANGE WITH LOW SPEEDS. of a protractor to plot informationfor key (b) SMALLER DIRECTIONAL CHANGE WITH HIGHER SPEEDS stations is recommended. A I 2-hour continuity of the streamlinecharts thedata from the off- AG.657 willusually suffice: periods are usedto supplement the ontime Figure 12.23.Illustration of a wind change. reports for continuity and when thelatter are missing. The ontime periods should bechosen for a large nearestthe mob observation times, butthis motion. if the change is representative and area. The temporary appearanceof deep wester- depends on local scheduling of observations lies at one station (luring passage of acold core may not always be feasible. low to the north cannot be interpretedin this way. Analysis

thethnesection analysis The objective of wind analysis is to construct This completes of the wind field routine asfar asitcan be described. If the a continuous representation Aerographer's Mate operates in an areawith a from the observation of the two- dimensional reasonable station network. he will, aftergoing horizontal vectors on the surface inquestion. through his time sections. have acquired afairly The complete analysis of the windfield also definite knowledge of how the charts shouldbe provides a means of obtaining othersubsidiary drawn and where the most dangerous areas are fields such as the divergence, convergence, and located. vorticity of the wind. There are two basic methods ofstreamline qualitative LOW-LEVEL STREAMLINES analysis in use: the discontinuous or method and the streamline-isotachmethod. If to be chosen for the low-level reports are sparse the discontinuous orquali- The level The most streamlineanalysis may vary from 2.000 to tative method is generally employed. feet. depending on severalfactors.In complete analysis is made with thestreamline- 5.000 the more recom- general, the wind frequency is fairly equal inthis isotach method and thisis layer. but since the cloud bases averageabout mended procedure. 2.000 feet, balloons are lost above thislevel DISCONTINUOUS OR QUALITATIVE surface wind STREAMLINE ANALYSIS.This methodin- when low cloudiness is great. Ships' tangential to can be used to supplement2,000-ft but not the volves a single set of lines drawn 445 AEROGRAPIIER'S MATE 1 & C

the wind direction and spaced inproportion to in the area. Therefore itis better to use the the wind speed. Thismeans that in areas of terms asymptotes of difluence and confluence speed convergence some lines must be dropped rather than divergence orconvergence. Typical from the field and inareas of speed divergence examples of asymptotes are shown in figure some must be added to thisfield: thus the 12 -25. streamlines are discontinuous. It is easilyseen that it is impossible to representcompletely the true wind field in this manner. Figure 12-24 illustrates this method of windanalysis.

AG.659 Figure 12.25. Streamlineasymptotes of difluence (divergence) left; of confluence(convergence) right.

2. Waves. These are perturbationsinthe AG.658 streamlines analogous to the wavelikearrange- Figure 12-24.Discontinuousor qualitative ments of troughs and ridges in isobaric patterns. streamline analysis. Waves that do not extendacross the entire width of the current in which theyare imbedded are STREAMLINE-ISOTACII ANALYSIS.This called damped waves. In thiscase the streamlines method consists of two sets of lines:streamlines on one or both sides of the current have smaller amplitude than those in which the representing wind direction and isotachsrepre- wave is more senting the wind speed, labeled inknots. The pronounced. Figure 12-26 illustratesa damped wave in the streamlines. two sets of lines give a continuousrepresenta- 3. Singular tion of the wind field from whichthe forecaster Points. These are points into can normally determine wind direction and wind which more than one streamlinecan be drawn or speed at any point on the chart. about which streamlines forma closed curve. In the streamline-isotach method ofanalysis familiarity with circulation patterns isa necessity. The following are definitions ofsome of thesecirculationpatternsthatwillbe encountered:

1. Asymptotes. These are streamlinesin the wind field away from which neighboringstream- lines diverge (positive asymptotes)or toward whichtheyconverge(negative asymptotes). Asymptotes may or may not represent lines of true horizontal mass divergence or convergence AG.660 depending upon the distribution of windspeed Figure 12-26 A damped wave in the streamlines.

446 452 Chapter 12TROPICAL ANALYSIS AND FORECASTING

The wind speed is zero at singular points and the speed immediately adjacenttothepointis relatively light. There are three classes of singular points: cusps. vortices. and neutral points. These are described as follows: a. Cusps. An intermediate patternin the transition between a wave and a vortex. They arerelativelyunimportant in synoptic windanalysis since they are short-lived and there is normally insufficient data to determine their presence. Figure 12-27 illustrates twovariations ANTICYCLONIC CYCLONIC of this class of singular points. INDRAFT OUTDRAFT

ANTICYCLONIC CYCLONIC OUTDRAFT INDRAFT AG.661 Figure 12-27.Cusps. AG.662 Figure 12-28.Vortices in the streamlines bVortices. These are cyclonic or anti- (Northern Hemisphere). cyclonic circulation centers and anticyclonic (cyclonic) outdrafts or anticyclonic (cyclonic) indrafts. Figure 12-28 illustrates four of the commonly observed vortices. At lowlevels,anticyclonic outdrafts and cyclonic indrafts arc frequently found. Data is usually too sparse at upper levels to determine the outdraft or indraft characteristics of vortices. Therefore. many upper vortices are drawn v/ Y as pure cyclones or anticyclonesfor lack of more detailed information. AG.663 c. NeutralPoints.Thesearcpointsat which two asymptotes, one of directional con- Figure 12-29.Neutral points in the streamlines. fluence (convergence) and one of directional dfluence (divergence) come together. They are of lines called isogo9s. An isogon is a line joining analogous to cols in that they represent a saddle points ina plane which have the samewind between two areas of anticyclonic flow and two direction. The isogons are drawn according to areas of cyclonic flow. Neutral pointsin the the reported wind directions with interpolation streamlines are shown in figure 12-29. beingcarriedout continuouslyasinscalar analysis. Then shortlines are ruled on each PROCEDURE.There arc two basic methods isogon corresponding to the wind direction it of performing streamline analysis: the isogon represents. Streamlines arc drawn utilizingthe method andthedirect method. The isogon original wind direction and interpolated isogons. method employs the use of an intermediate set This method, although the most accurate, is too 447 453 AEROGRAM IER'S MATE I & C

125,E i 140 35 ... 1 0 W s.: _... 1 35 '''" ., .,..i:,...... , :ye. ..../ \ ...... -...... -.., .0'. -.1;r0. ,.....,.,..., ..._,./' e...,,,e° ,,...... I ., ...... " E- -... '''...," ..., ...., ..__...... ,., --.06. r, /\ 1 . ,-1.-:'2MAX 'I: ...... ,...... :a---.--...--.----r...l,.. ---- -1,.s --' 4. L-- --...7...a ..1.. 40,1I ...".------:!IMIIMMIE1141 ..... N ...' ...... -Zs 'N. ED --"-IN. `111 \ E0 5TIME-..--.----./. DATE * I25E , , ... 130 1 1 WA' 164' ITO' AI 175'W

AG.664 Figure 12-30.Streamline-isotach analysisover a large ocean area. time consuming to be in general use. Ilowever. it the analyst should follow to achieve themost is recommended that beginnersuse this method complete and accurate product. A comprehen- toachieve familiarityand experiencewith sive listing of these as wellas detailed informa- strelline analysis. tion concerning streamline analysis is contained In the direct method of streamlineanalysis in AWS TechnicalReport 240, Forecaster's interpc!ations among the windsarc carried out Guide to Tropical Meteorology. by eye and the streamlinesare drawn directly using a trial and error approach. Theaccuracy SURFACE CHART achieved depends to a great extentupon the skill of the analyst. The low-level streamlines, especially whenat Isotach analysis is performed after theprelim- 2,000feet,determine the character of the inary streamline analysis is completed. Isotach surfaceisobarstowithin about 5° of the patterns will resemble those found in simple Equator, in the center of the equatorialzone the scalar analysis, i.e..there are centers of maxi- analyst must rely on the pressure reports alone. mum and minimum values and saddles or cols in In fixing the course of the isobars, the low-level the wind field. streamlines are thus an invaluable aid in sorting Figure 12-30 showsa completed streamline- out good and bad pressures, especially in the isotach analysis. trades. Nevertheless, since theyarc concerned There are a number of guidelinesas well as exclusively with the wind field. they donot basic rules concerning streamline analysisthat convey information about the field of mass and 448 454 Chapter12 -TROPICAL ANALYSIS AND FORECASTING its time changes, which is essential for many more are drawn at 6-hourly intervalsand then prediction problems. compared in sequence. Troughs and ridges of the Pressure change charts are considered the semidiurnal pressure wave are 900 long. apart. most important aspect of .ne pressure analysis. Supposethenormalpressure gradientina However, inorder to draw isallobars.itis tropical area is 3 millibars per 10° lat. and the isobars run straight easy -wesr at a given map time. necessaryto draw isobarsfirst and compute pressure changes in the areas where stations are If a ridge of the semidiurnal wave reaches the widely spaced. If this is not done, the isallobaric western end of a 90° long. interval 6 hours later centers will inevitably be put near the observing and a trough reaches the eastern end, the isobars stations, and their paths will follow any lines of at that time will trend equatorwardby no less stations that may exist, such as the Antilles. You than 10° lat. from eastern to western end of the difference from also need the isobars for the differential analysis map at that time, if' the pressure of the upper isobaric surfaces. Although the crest to trough is about 3 millibars.(See fig. value of the isobars to the forecaster has often 12-31.) Further, the trough area will feature been questioned. they remain a necessary tool. cyclonic isobars in a huge are -this is especially In the following discussion. consider these three noticeable in the western Pacific. As seen from steps: Surface isobaric analysis. surface24-hour figure 12-32 the 1,010-mb isobar is displaced by pressure change analysis. and weatherdistribu- approximately1,500 miles in 6 hours. Evi- be compared tion analysis from surface reports. dently.6-hourly maps cannot under such circumstances. Surface Isobaric Analysis A remedy for this difficulty has been suggested. At any point of the map the normal When sea level pressures have been plotted departure of pressure from the mean can be withthecarefulseleLtionroutine desLribed computed for the normal map times of the day. earlier, the problem of ship pressures still re- If this is done for the whole area of analysis and mains. To date. the most stio_essfulremedy has for each month of the year, the correction that been to follow the path of individual ships and should be added or subtracted from the current even plot their reports in time series.This will sealevelpressure wave can be determined. tell quickl} w !tether reliance Lan be placed on Although this wave is not quite constant from the pressures from a Lertain vessel or whether a day to day. charts drawn with such corrections constant correction should be applied. Isolated will be fairly comparable. reports from ships suddenly appearingin the It is essential to draw for 2-mb intervals in the middle of the area of dIld1Sis should receive trades andfor1-mb intervalscloseto the least %%eight. As a further aid, sea level pressures Lquator. In addition. the 1-mb interval should reported bylow-fly ine reconnaissance aircraft be used whenever the isobars are so widely should be plotted un the surface map. together spat.ed that even 2 -nib isobars do not reveal the with the winds atflightlevel.In doing so, important features of the pressure field.Itis however. the diurnal pressure variation during thus readily seen how important itis to screen theflight time must be LonsiderLd. though the pressure reports carefully. A few bad pres- synoptic pressure changes over a timeinterval sures candistortthe isobaric patternina ove3 hours before to 3 hours after map time grotesque way. All pressures of normallyreliable ma} normally be disregardedsafely. stations should be drawn to quite strictly and However. 3-hour pressure Lhanges south of disregarded or modified only when excellent I5°N over the western PaLifiL may amount to reasons exist. Too often one approaches achart as much as 2 to 3 millibars or more.This Lan be with an initial idea as to how it should look. It is taken into consideration by knowing the average much better to let the data guide the pencil. 3-hour tendencies for d particular area or sta- For application to differential analysis, the tion. Surface pressures plotted from dropsondes finished isobars must be relabeled in terms of should also be plotted in the surface chart. the height of the 1,000-mb surface. This can be The diurnal variation is particularly trouble- accomplished with the standard method of 7 1/2 some when charts extending over90° long, or millibars equal 200 feet (60 meters), starting

449 455 AEROGIZAPHER'S NIATE I & C

AG.665 Figure 12-31.Six-hour diurnalpressure change from 0000Z to 0600Z.

from 1.000 millibars: when greateraccuracy is the previous one. This. however. will oftenprove desired, use table12 2. Table 12-2 has been less satisfactory. since the isobars tendto run calculated. using the normaltemperatures pre- fairly parallel and intersect usuall}at very small vailing near the surface over the tropicaloceans. angles. The isobars are labeled directly in units ofthe I .000-mb height. The main objective of the analysis isto track rise and fall centers and note their changes in 24-Hour Pressure Changes intensity. In addition. it furnishesa check on the ocean isobaric analysis: if large regions of intense The 24-hour changes observed at all reporting falls suddenly appear without continuity.the stations are computed and written just abovethe Aerographer's Mate will do well toreexamine his pressure values. This should be a step of the work. normal plotting routine.It will help to circle In general. the isallobaric analyse, will yield these changes so that they will stand out better good continuity.It can even be extended to among the mass of information on the surface suchareas wherepressurevalues are quire map. Over the ocean area, the procedure is to irregular due to lack of precise knowledge of tabulate the pressure daily atall 5° latitude- station altitudes. longitude intersections, to compute differences. Usually. one looks for concentratedtall or and to plot theseIsallobars are then drawn at rise centers covering limitedareas only to locate intervals of 0.5 to 1.0 millibar. dependingon the intense disturbances. Wide areas of fairlyuni- magnitude of the changes. form falls are considered harmless. Alternately. the changes may be obtained by Occasionally falls of I to 2 millibars over graphical subtraction of the current chart from most if not all, of the region of analysis will 450 456 Chapter 12 -TROPICAL ANALYSIS A NIJ FORECASTING

1010 mb. at 1800 i1

AG.666 Figure 12-32.Sample normal displacement of1,010-mb isobar between standard map times.

immediate deepening even though the intensity appear suddenly on I day. to be canceled by corresponding rises within 24 to 48 hours. Such of the convection may be severe. Theforecaster pressure waves on a huge scale are notunder- should be on the lookout for rises surrounding stood at present, but for forecasting they appear an area of weak falls or even nochanges. When such wide- Especially if the rises are intensifying but not to be without significance. of the spreadfalls occur.a synopticfall area will spreading in area, sudden intensification appear as a center of stilllarger falls, but a falls is apt to follow. synoptic rise area may take the form of an area of small falls embedded in surroundings with Weather Distribution larger falls. Subsequentto the analysis, the isallobaric low-level stream- The weather distribution map is a graphical centers should be rehted to the representation of the clouds and weather over an lines and the weath -: pattern, later they should most fore- wind area of operation. In high latitudes, also be compared with the distribution of casters would be reluctant to spendthe time and temperature aloft. In particular, fall centers chart because the containing much had weather and associated necessary for analyzing such a the troposphere and anti- surface synoptic chart, with the cloudsand withwarming in weather plotted at each station togetherwith cyclonic circulation at 200 millibars must be fronts, is especially the identification of air masses and regardedas potentially dangerous. adequate for most purposes. For operationsin when thereisa trend toward an increasing association of these features. Rise areas associ- the Tropics, however, the weather distribution ated with bad weather generally do not indicate map is almost indispensablefor both forecasting 451

alz--"Ia i AEROGRAPI1ER'S MATE 1 & C

Table 12-2.Sea level pressureversus pressure patterns with parallel changes in the 1,000mb height cloud and precipitation patterns. Several methods of drawing a weather distribution map ha% e been deeloped. All involve the Sea level pressure 1, 000-mb use of the standard synoptic cloud symbols to (mb) height designate the cloud types. The deviationsare (tens offt) (meters) mostly confined to the representation and analysis of the amounts of clouds. The choice of 1,000 0 0 method can be determined locally. Thedata 02 6 18 included on the plotted charts should beas complete and extensive as possible. Besidesthe 04 12 37 regular land and ship synoptic reports, other reports entered on the map are reconnaissance, 06 17 52 off-hour ship, pirep, and aircraft in-flightre- ports, each appropriately designated as to type 08 23 70 and time of report. Past weather should also be entered on the weather distribution charts, for it 29 88 will aid in correlating the movement ofweather 12 systems, and may fill blank spots on the charts 34 104 on occasion. ANALYSIS WEATHER DISTRIBUTION.If 14 40 122 weather distribution maps form a part of the 16 46 140 standard daily analysis in the weather office, the task of drawing the map is greatly simplified by 18 51 155 the fact that the major weather systems of the Tropics have continuity from map tomap. From 20 57 174 day to day, large-scale systems moving through thearea of operation may show the same 22 63 192 characteristicintensity, asjudgedby the 24 amount. depth. and arrangement of the clouds 69 210 and the precipitation patterns. On the other hand. they may increase in 26 74 226 intensity or die away. However, during the explosive deepening 28 80 244 of some typhoons and hurricanes, the changes are sufficiently slow to be followed easily on the 30 85 259 map sequences. In some regions, systems may form aloft and remain stationary formany clays, slowly increasing in intensity from day to day. Under these circumstances. tia; wholeprocess of deterioration in the weather can be followed in and flight briefing. Ininstameswhere preuse detail and forecast by simple extrapolation. and detailed analysisof weather o%erparticulai The following discussion of weather distribu- terminalsor areas is required. itIS often neces- tion analysis is limited to times when weather sary to follow the ,ontinuity of the weather and distribution charts are not part of the normal cloud patternsinthe same fashionasthe routine of the weather office and the Aerograph- continuity of low-pressure centers and fronts.As er's Mate, as analyst, must start fromscratch. a briefing chart, the weather distribution map is perhaps the most practical means of presenting 1.If the analyst has not already become the weather toonmeteorologists. Finally, the familiar with the climatology of thearea. he art of forecasting the weather depends upon the should consult the best available informationon ability to correlate cnanges in the wind and the cloud and weather characteristics ofthe

452 458 Chapter 12 TROPICAL ANALYSIS ANDFORECASTING of the season and place. If theequatorial trough lies keep the area covered by his drawing the minimum compatible with the near or across the area. heshould be on the alert cloudto of "equa- reports and the topography: atthe same time he to discover in the reports evidence between the torialfronts" lines of cumulus congestus or should show clearly the connection cumulonimbus that simulate the cold frontsof mid cloud and the convective pillars. Inpassing, high latitudes. If he is working in atrade wind it should be emphasized that over the open sea. cumulonimbus are area. he should beprepared, before starting the lines of cumulus congestus or but this does analysis. to find characteristic distributionand often associated with alto-systems, the mid cloudis heights of cumulus and stratocumulus. Inother notnecessarily mean that likely words, his climatological knowledge shouldhelp dependent. on the contrary. the most situation. during the late stages ofdeepening of him form amental picture of the normal weather distribution map of the region.If he antipper -levelcyclone,isfor one or more finds radical departures from this pattern.he asymptotes of convergence,with accompanying to develop will know that he must pay particular attention cumulus or cumulonimbus lines, and merge to the anonialous teat tires.making every el fort under a preexisting deep alto-system to delineate them properly onthe map. with it aloft. priority 2. Examine the aircraft reports. first 4. Delineate the high clouds. Followthe same being given to any reconnaissance data.If. as is principlesaswereusedin drawing the mid customaryin some regions. transient aircrews in dis- sections of the clouds. Usually there is little difficulty have submitted pictorial cross tinguishing between dependent andindependent these weather along the air routes to the station, cirrus. Almost all independent alto - systemswill Particular should also be examined at this time. be accompanied by cirrus orcirrostratus sheets, language attention should be given to plain either separate and at a much higherlevel. or description changes of cloud form, these fusedwithitintheareas of precipitation. usually indicate the extent and orientationof than outline Usually the cirrus sheets cover a wider area major cloud systems. The analyst should the alto-system, and often cirrus inbroad bands these features lightly. After the aircraft reports either side of the alto- analyze the cover a great area on have been thoroughly examined. system. the bands beingoriented parallel to the remaining reports in approximately the same main axis of the alto-system. fashion. 3. Outline the areas of middleclouds. At The delineation of cirrostratussheets of in- tirst.allarca:, of middleclouds should he dependent formation can be of greatassistance outlined by follownw the repoi iS rathermechan- in tracing the genesisand development of an ically and making reasonable interpolationsbe- upper-level cy clonic system. At early stagesin tween reports. Then. the resultingpicture should the development of such systems. verylittle mid be examined w ith an eye to the reportsof cloud may be present. and thecirculation may cumuldorm clouds. If cumulonimbus areabsent. be evident only in the layersabove 30.000 feet any mid cloud is probablyindependent. Some (9.144 meters). On the weatherdistribution attemptshouldbe made todistinguish an map. howevei, thisearly stage of development is organized shapetothe system. Independent ol ten accompanied by extensivesheets of cirro- systems will usually he bandlike ofsickle shaped stratus. A series of maps whichclearly shows the with frayed edges of patchy altocumulus. development of the cirrostratus sheetsis often All middle clouds will not beindependent. If the best way to follow the gradualintensifica- orographic cumulus or cumulonimbus arewide- tion of the system. clouds may spread. fairly large patches of middle the distribu- their neighborhood. However. 5. By the time he has analyzed be reported in the forecaster these patches will be detachedfrom one another tion of the mid and high clouds, should have studied most of thelower cloud and will rarely form a veryextensive alto-system of cumuliform reports and should haveformed definite opin- with a definite shape. If reports Low clouds of clouds indicate that the middlecloud in any pail ions about their distribution. orographic origin should be obviousby this of the legion is dependent.the analyst should

453 459 AEROGRAPI1ER'S MATE I & C

time. The more definite frontlikelines of cumu- and specifically in the Tropics, have lus and cumulonimbus should a direct be clearly indi- relationship to streamline patterns at all levels. cated.Often,veryextensiveareas may be Complete correlation of wind and covered by more or less uniform weather distributions of requires analysis of the wind fieldat several cumulonimbus. Delineate theseareas as sharply levels.Datalimitations and as possible. time limitations usually allow a correlation for onlytwo levels: 6. Next, note allprecipitation and special thelow-levelstreamlines between 2,000 to phenomena. The analysis shouldthen be com- 5,000feet (610 to1,524 plete. meters) andthe streamlines of the 200-mb or 40,000-ft(12,191 meters) level.Followinng are some correlation Although various combinationsof cloud types rules. and amounts will occur. the following three In regions of moderate to strong divergenceat patterns tend to be most prominent: the low-level streamline level. thepredominant cloud is usually cumulus humilis with lessthan 1. A large amount of lowclouds, cumulus one-half coverage. congestus and cumulonimbus with midand high When great vertical shear accompaniesmoder- clouds and reports of currentor past showers or ate divergence. the cumulus humiliswill be rain.Thispattern, of course.istypical of drawn out and sheared off andmay be accom- disturbed areas. panied by dependent stratocumulus. 2. Few or no midor high clouds, either with North of the trade wind maximum,in the few low clouds, or withlarge amounts of Northern Hemisphere, the vertical shearin the cumulus humilis (also called fair-weathercumu- wind in the lower layers is usuallyvery great and lus) or stratocumulus. Thiscombination indi- the trade wind inversion low. The predominant cates suppressed convection. cloud is usually stratocumulus. The clouds will 3. Low clouds nearaverage. occasional cumu- exist in broken patches if the low-levelstream- lus congestus, and mid and highclouds in line pattern is divergent. Even with weakto varying amounts. In sucha transition pattern, it moderate convergence atthis level, very i.ttle is most important to study theupper clouds. cumulus forms; the result is usuallyan increase since these are independently formedin such in the amount of stratocumulus. west andsouth cases rather than derived from tall cumuliform of the subtropical anticycloniccenters. types. Thickening cloud cover aloft, especially When there is little vertical shearin the lower altostratus overcast, denotesan approaching, layers and the low-level streamlinemap shows forming disturbance. weak to moderate convergence south of the trade wind maximum. expect deepercumulus. CORRELATION OF WIND AND The cloud amount will also increaseover that WEATHER.The correlationofwindand expected in divergent flow, but notto a very weather is still in the infantstage of develop- marked extent. Even withverystrong con- ment, and rules evolved do not alwayshave vergence, the cumuliform cloud showsmany universal application. Local peculiaritiestend to breaks, the general effect of increasedconver- necessitate some modification andin many gencebeingtoincreasetheheight of the instances require rules of theirown which do convective cloud. not lend to generalization. However, thereare An asymptote of convergence in the low-level some general rules available, and, surely, future streamlinefieldwill,ifitcoincides with a experience with wind and weathercorrelation relative minimum in the speed field, be will yield many more. accompanied by a line of large cumuluscongestus or In as much as the vast majority ofclouds are cumulonimbus. formed by upward motion of airand vertical Old polar fronts sometimes penetrate into the convergence of air can be deduced from the Tropics. They rarely reach 15°Nor S lat in horizontal components of direction andspeed of oceanic regions. Such a remnant the wind field taken is often detect- over large areas, we find able as a line of towering cumulus, and thisline that many of the weatherpatterns in general, between the old air masses also marksa change 454 460 Chapter 12 --TROPICAL ANALYSIS AND FORECASTING in cloud form. The old front may, inthese cases, because many disturbances and their associated appear on the lowest streamline chart as an temperature fields occupy only the lower orthe asymptote of convergence running east to west upper troposphere; combination may remove or northeast to southwest from theneutral point the most significant map features. Thereforeit is between two middle latitude anticyclones. In logical to break the whole layer into two parts most cases. the temperature differencesbetween and carry out the upper contour analysis ;11 two the old air masses are negligible, even though the steps. The 500-mb surface is the mostconveni- cloud line may persist for some time after the ent intermediate level, though it is not anideal it air-mass contrasts have disappearedfrom the surface for tropical upper air analysis because between the soundings. is situated in the layer intermediate An asymptote of convergence inthe lower disturbances of low and high troposphere. We 1,000 to streamline fieldis not necessarily accompanied shall here operate with the layers from by a cloudline.If strong speed divergence 500 millibars and from 500 to 200 millibars. occurs along part of the line. there may be very The discussion is limited to latitudes poleward Equator is little heavy cloud or precipitation accompanying of 10°. Experience still closer to the it: furthermore. bad weather may befound at too limited to warrant treatment, some distance from the line,where speed convergence predominates. General Considerations UPPER AIR ANALYSIS Several rules guide the analysis: (1) The upper Since the upper winds of the Tropics are far contours should be drawn parallel to the upper more reliable than upper pressures acid tempera- winds when the windis10 knots or more. tures. it is natural to think of upper airanalysis (2) The vertical shear vectors throughthe two at first in terms of streamlines. Suchanalysis, in layers we are considering should indicatethe fact, has proved useful in day-to-day forecasting; orientation of the mean temperature (thickness) moreover, much of the information we now field though not necessarily the magnitudeof This applies possess about the tropicalatmosphere aloft has the mean temperature gradient. been deduced from this method.However, we especially to the layer from 500 to 200 milli- must consider the pressure and temperaturefield bars. (3) Since the mean temperature of a layer from several hundred millibars thick is nearly conserv- also. Any numerical values must come of working with the fields of pressure and tempera- ative from day to day, the thickness fields readily compar- ture.Moreover, pressurepattern(D)flying two succeeding days should be feasible over the tropical able. Regions of relatively warm and coldair appears to be as without oceans, at least to latitudes10° to 15°N and S, should be traceable from day to day difficulty and the intensity of centers should not as itis in higher latitudes, Thus, for operationof jet aircraft inthe high troposphere, a 200-mb change drastically without good reason. In par- analysis is indispensable. Since the fuel supplyof ticular, one should not find huge areas where jet aircraft is also very sensitive with respect to colder or warmer air, not pres,nt previously, tempeatureespecially during ascent and de- appears suddenly. When thishappens. analysis (4) Heights of scent - -a close estimate of the temperaturedistri- errors should always be suspected. bution is a further requirement. upper isobaric surfaces andthicknesses between The upper troughs and ridges of theTropics surfaces vary in a limited range in each area and usually reach their greatest intensity near200 season. Normals and extremes canbe established millibars, which makes this surface asuitable by plotting frequency distributions. Thiswill the levelforhightroposphericanalysisin low help the analyst to avoid values outside latitudes. Also. differential (thickness) analysis is observedranges and to realize whenhe has requisite to stabilize the contouranalysis of drawn a higher or low value with the permissible such high or low upper isobaric surfaces, range. We have seen earlier that It is impractical to make a thickness analysis values are associated with definite types of flow for the whole layer from 1,000 to 200millibars patterns.

455 461 AEROGRAPHER'S MATE I &C Analysis Procedure either aloft or throughout thetroposphere, this can only mean that latent heat of condensation The usual approach to analysiswhen the is being liberated in sucha way as to produce thickness method is used, is to firstdraw the warming aloft. The heat liberatedcan be partly upper contour charts, then compute the height converted to kinetic energy in such differences between the isobaric surfaces, cases and and there is then the possible beginningof a storm. inspect the result as to patternand change from the preceding day. Assuming the500-mb analysis isfinished, draw next theI ,000-mb chart. Estimating Wind Flow From Then obtain the thickness eithergraphically or Satellite Pictures plota chart containing thickness values and wind shears at the observation stations along Clouds over the Tropics, viewed from with thicknesses at all 5° lat/longintersections satellite over the oceans. Although the latter procedure altitude, reveal many features ofthe flow. The is distribution of widespread cloud preferable, it is more likely thatwe will use the systems has graphic approach, primarily definite relationships to major troughand ridge because of limi- positions. Wind estimates for both tations on our time. Dependingon the area and upper and season, the interval for analysis is 50 lower tropospheric levelcan be obtained from or 100 feet an analysis of cumulus and cirrus cloud forma- (15 or 30 meters). We then inspectour charts carefully and make the tions. These ca:-, be obtained byusing interpreta- necessary adjustments. tion of singular cloud pictures This process is then repeated forthe 500- to or on actual measurement of cloud motion froma series of 200-mb layer. In addition tolarger and more pictures. definitive wind shifts, the Aerographer'sMate can use here the fact that a high correlation Two of the most useful cloudpatterns used for determining upper 1?.velflow are plumes exists between the height of the 200-mbsurfaces from the tops of cumulonimbus and the thickness of the layer from 500to 200 clouds, and millibars. cirrus cloud shields associated withsubtropical jet streams. As analyst, the Aerographer's Matecan corre- latethe cloud and weather chart Vertical wind shear throughan atmospheric described layer containing clouds isone of the prime earlier with the upper flow patterns andthickness changes. In a zone where the cloud reports factors governing the shape ofcloud formation. Most of the cloudlines and bands seen in indicate definite subsidence, coolingfrom the satellite previous day cannot be observed unless pictures are oriented parallelto the a cold thermal wind through the layer from center is being advected into thearea: the latter cloud base then should be losing intensity with to cloud top. Under some circumstances,the time. A direction of shear is thesame, or nearly the deepeninguppercyclone musteitherhave cloudiness on the inflow side into the cyclone same, as the direction of motion of the air containing the cloud. This is the which is often incompatible withthe dynamics case if: (1) the wind changes speed with heightbut not direc- of the situationor colder airmust be brought tion; (2) the wind changes direction in from outside the system. Otherwisethere is with height no physical but the winds at the top ofa cumulonimbus are process except radiation which much stronger than those could achieve the central cooling which near its base. must be When these conditions present to account for 200-mb height decreases. occur, it is possible to estimate the wind direction directlyfrom the The relation between anticyclonic flowand orientation of the cumulonimbus plumes. weather is just as important. High tropospheric These conditions are common in tropical regionswhere anticyclones can increase in intensity, coupled the speed of the wind at the level of with warming in middle and lower troposphere, cumulonimbus anvils is several times greater than themean both in areas of ascent and descent. In the latter wind speed of the wind throughthe lower case we are dealing with the well-known dy- portion of the cumulonimbus cloud.In these namichigh.Ifa200-mb high strengthens, cases wind estimates based on orientation and however, when much cloudinessispresent, dimensions of cirrus blow-offare quite accurate

' 46 e Chapter 12 TROPICAL ANALYSIS AND FORECASTING of clouds (E to F) and closely approximatethe winds neart he numerous small-scale lines perpendicular cloud top. which are oriented approximately to the wind direction. These lines arebelieved to There are two conditions which can cause be considerable error in wind direction estimates be caused by horizontal shear and should not confused with cumulonimbus plumes which are derived from cumulonimbus plumes: (I)light difference oriented parallel to the vertical shear. winds at the cirrus level: (2) a large CUMULUS between the direction of the shear through the LOW LEVEL WINDS FROM CLOUD LINES.Research has shown that many convective layer and the direction of the wind at cumulus cloud lines observed between IONand the top of the layer. IOS over the Atlantic and eastern PacificOceans When the speed is light (less than 20 knots), parallel to the sur- variablecausing the are oriented approximately thewinddirectionis face wind direCion. Because the orientation of correlation between plume orientation and wind It' there is a large cloud lines relates to the shear, theselines may direction to be less reliable. wind direc- difference between the direction of the shear depart considerably from the surface must be through the convective layer and the direction at tion. Therefore, a great deal of caution will be parallel exercised when using them to definelow-level the top of the layer, the plumes rules which to the shear but not to thewind at the layer top. wind flow. There are no hard, fast where permit positive identification of theparticular This condition is common in frontal areas parallel to temperature advection is strong. but occursless cloud lines which arc approximately Ithas been observed that frequently in the Tropics. the surface wind. UPPER LEVEL FLOW FROM CIRRUS cloud lines which are very long, very narrow, :ind either wavy, zig-zag, or haveknots (wide CLOUD SHIELD. It is common to observe often pole- places along a line), are the type most huge masses of cirriform clouds extending figure 12-34, ward from the tropics. These clouds occurin parallel to the surface wind. In cloud lines are shown which areapproximately advance of 200-mb troughs and represent a wind reports poleward transport of momentum andhigh level parallei to the wind flow. Surface are entered on thepicture as white arrows. moisturefromthetropicalregion. A wind maximum is usually located on the poleward edge of these cloud formations. As with strong Upper-Level Streamline Analysis Jet streams. the direct on of the upperlevel wind All the analysis rules applicable to thehw parallelsthe cloudedge,Becausetheairis levelsalso apply to the higher parts of the moving .Tway from the equatorand accelerating. troposphere. In the upper troposphere. inthe itis crossing the contour patterntoward lower vicinity of the 20" nib level or the40,000-ft pressure. For this reason. thegeneral orientation (12,190 meters) level, you find moreoften than of the cloud shield and the striationswithin it not that your data has diminished toless than caa differ as inut,;1 as15 degrees from that of half that available for the low-levelstreamlines, the upper tropospheric contours.When this is and in view of this you find that thequalitative taken into aLLount. ciri us formationsof this streamline analysis method is more suitablefor type provide good estimatesof wind direction this level in most cases. The streamlineanalysis but less precise information as towind speed at this level is used primarilyfor the detection of Figure 12-33 shows a ty piLal cirrus cloudshield. cyclonic and anticyclonic flows, and, of course, Thedouble-shafted, white arrows (BtoC') you look most for thesigns of anticyclonic represent the 200 -rub wind111,1XiM1.1111 suggested outflowatthe 200-milevel,foritisthis by the cirrus cloud edge. Theblack, single- outflow that is the first clue to tropicalcyclone shafted arrows represent th'00-mb flow based development in the upper air. on individual cirrus&men'hat lie within the Before you final, 4e your analysis, you should largerscaleformation. vorticalcloud look over your chart objectivelyand ask your- pattern () is associatedv. ia cut-off low. self the following questions: Southeast of D. a cirriform cloud shieldextends northeast from A in advance of an upperlevel I. Do the streamlines conform to the general trough associated withthe vortex. There are flow pattern?

457 463 AEROGRAPHER'S MATE I &C

tik-----mis2vritc. " t t -r vatao

et fy Oak ° 44* mai :11.10A, orj ; r :41

,.;:.1. 111111111 ...... k.,, if lee"lc.; t n- 41- i ..A -7-- ":-. 4), . ,4

iiir457-Iro 473z . 6 .i.,k-"4.,-

AG.667 Figure 12.33. -A typical cirrus cloudshield.

2. Have I justified throwingout everywind can differentiate the minutest deviation that I could not draw to? in a mass of nearly homogeneous data, andlast, but 3.If Icannot justify discarding thewind, is not least, diligence and dedicationin theap- there any way I can draw to it? proach to the forecast. 4. Have Idrawnthe streamlinesparallel to the The types of forecasts in theTropics arethe plotted winds rather than atanangle to same as anywhere, winds? in that youencounter flight 5. forecasts, route and terminal: operationalfore- Is the chart consistent with otherlevels? casts, andgeneral or 6.Is the chart consistent with history? fleetforecasts (RATT 7. Arethe broadcast form); weather clearances:local area streamlines and isotachscon- forecasts and sistent? advices: and destructive weather forecastsand warnings. .8. Have I drawnany unnecessary lines? Your concern in this chapter is withthe local 9. Have I givenmoreweightthan necessary to light and variable winds? area forecasts, forecasting the ITCZ. tropical waves, and destructive weather forecastsand warnings for the reasons that the other TROPICAL FORECASTING forecasts will basicallydepend onthe same general tech- niquesand approach fortheir formulation as the ForecastingintheTropicsis a difficult local area forecast. The treatmentof destructive problem, necessitating a goodmeteorological weather warnings and forecastsislimitedto and physical background, vastamountsof clima- tropical storms and cyclones, becausetornadoes tologicalknowledge, a keen mind'seye which arelargelynon-existentintheTropicsand 458 464 Chapter 12 -TROPICALANALYSIS AND FORECASTING

*N.

4.11111111,41

I ----111h

AG.668 Figure 12-34.Low level cloudsoff the coast of Africa.

prognostication. The thunderstorms are covered in chapters11 and 12 analysis and in the mental next step in the procedureis to expand and of this course. refine the mental image formed sofar. The ideal approach to a local areaforecast is LOCAL AREA FORECASTS to prognosticate the upperair analysisfirst, which (with The importance of the local areaand the usually the 500-mb chart, from been pointed out at other things in consideration, asfor instance, general area climatology has charts, Itisin the streamline analysis. weather distribution various pointsinthis chapter. chart is prognosti- preparation of the local areaforecast that this and time sections) the surface cated. This prognostic surfacechart is then used knowledge will be most beneficial. forecast, again with During the analysis of the variouscharts, most as a basis for the local area the forecast these other factors considered andcorrelated, forecasters form a mental image of climatology of the charts and perceive certainfundamental ideas of and also correlated to the anywhere in local area. the local weather (and the weather Many times this routine for various reasons the area of responsibility,for that matter) for climatology of the cannot be completed inits entirety, and the the next 24 or 48 hours. The In that case analysis and forecast area serves as aguide in the forecast must be prepared anyway. 459 465 AEROGRAPLIER'S MATE I & C

the approach is stillthe same, inthat ALL layers in the troposphere, and these levelshave available data are considered andthe best is vely different characteristics. Mostly. a steady done with what data are available. trade blows in the lower levels (surfaceto 500 mb), while a succession of large cyclonicand TROPICAL CYCLONE anticyclonic vortices are present in theupper FORECASTING troposphere in the 400- to I50-mbstrata. In some regions, however, (for instance between There is at present no one formal procedure the Mariana Islands and the South ChinaSea in for forecasting the development and movement midsummer) intense eddy activity takesplace of tropica' cyclones. Thiscan be understood also near the ground. It is not possibleto deduce when c considers the enormous complexity of from charts drawn in the lower layerswhat is the problem, the sparsity of data in the oceanic taking place above 500 millibars. Therefore, itis tropical regions compared to that available in necessary to keep track of events in both layers. the well-developed and highly populatedconti- nents, and the vanishing of ship reports from the Aside from the surface chart, the700-mb area of a tropical cyclone as soon as the first level chartis very helpful in determining low- warning is broadcast. There is also the regional level flow patterns. The 200-mb level isrepresen- tative of the upper layer, whereas the influence to consider, and this influencecannot 500-mb be slighted. A very obviousconsequence of level, often located in the transitionzone, is of regional influence can be demonstrated when much less use in tropical than in extratropical you compare the North Atlantic area with the forecasting. North Pacific area. The North Pacific has almost However, forecast considerations shouldnot twice the tropical water area and also better be limited to the two layers in the Tropics. than doublethe average number of tropical Middle latitude weather and changes also influ- cyclones per annum. ence and share in the control of weather changes Forecasting tropical cyclones evolves into the in low latitudes. The position andmovement of following problems. formation, detection,loca- troughs and ridges in the westerlies affect both tion,intensification,movement, recurvature, formation and motion of tropicalstorms. There- and decay. fore, middle latitude analysis isa requisite. Since The factorsthatenterintotheforecast forecasts generally run from Ito 3 days, this preparation are wainly dynamical (relatingto should be a hemispheric analysis. the energy or physical forces in motion), but The climatological approach to the forecast there are also important thermodynamicinflu- problem should also be taken into consideration. ences. For instance, tropical storms willnot Itis obvious that a forecaster must be familiar develop in air that is a little drier, and generally with his region and the seasonal changes:for also slightly cooler, in the surface layers thanthe instance, areas in which storms tendto form or air normally present over the western portions not form for any given month;mean storm of the tropical oceans in summer. This holds track:, and the scattering of individualtracks true if air with a trade inversion andupper dry aboutthemean. For the Atlantic area an layer are present. Surface ship temperature and elaborate set of mean tracks, percentage of dewpoint reports, along with upper air data,are frequency of motion and median speedsfor 5° extremely valuable in determining whether the latitude squares, and regions of development anti surface layers are truly tropical or whether they their subsequent effect on east coast weatherare contain old polar air not yet completelytrans- available in the U.S. Department of Commerce, formed Careful observations from groundand Forecasting Guide No. 3, Hurricane Forecasting. aircraft observers on convective activity indicate Tracks and data on Pacificstorms are also whether or not there isa lid against upward available in other publications. Thesemaps and penetration of the cumuli. charts reveal that the "climatological" approach Dynamically storms (existing and potential) gives some information, but it cannot be relied are subject to influences from the surrounding on in any area, or in the case of ally particular areas. In the Tropics. we usually encounter two storm, to the exclusion of s noptic indications

460 466 Chapter 12 TROPICAL ANALYSIS AND FORECASTING for the forecast. These probability considera- 8. Westerlies greater than average north of tions are useful mamly for long-range planning the latitude of the seasonal maximum. and when data tire missing. Therefore, we reach 9.Easterlies weaker than average in a wide the conclusion that climatological information zone. should be treated as a weighting factor to be 10. Easterlies decreasing with height. included after synoptic data. 11. Easterlies over or approaching 40.000 feet. 12. Latitude of the subtropical ridge higher Formation than normal above the 500-mb level. 13. There is evidence of a fracture of a trough A full blown typhoon/hurricane cannot be aloft (at 200 millibars). forecast as such w hen there are no indications of 14. Long waves slowly progressive. any type of irregularity on the charts and aids 15. A zone of heavy convection is present. used in tropical analysis. Only when the incip- indicating the absence of the trade inversion. ientstagefirst appears among the data can 16. Surface pressure gradient north and we begintothinkinterms of an actual 17. Disturbance has relative motion toward typhooni'hurricane, and often not even then. upper ridge at or above 400 millibars. For the most part the forecast from "tropical Other indications of development which are low" totyphoon!hurricane"isa matter of useful are sea swell and tide observations. Swell step-by-step progression. Assume the term "for- willhave a periodlessthan average and an mation" means formation ofa"potential" amplitude greaterthanaverage. The normal typhoon/hurricane. swell frequency is 8 per minute in the Atlantic Empiricalrulesorcheckshaveevolved and 14per minute inthe Gulf of Mexico. through the years which enjoy a measure of Hurricane winds set up swells with a period that reliabilityto warrant their use (of course. the can decrease to four per minute. Swellswill more signs point to formation, the more likely approach the observer approximately from the formation will be). directionin which the storm is located. The The synoptit.. Londitions favorable for devel- swellheightisan indication of the storm's opment were given in a previous section of this intensity, especially when the swells have not chapter.In the following list. no attempt is encounteredshallowwaterbeforereaching made to separate the surface from the upper air shore. indications as the two are most often occurring Abnormally high tides along broad coastlines simultaneously and are interrelated. and along shores of partially enclosed water bodies with the highest tides usually to the right I. Marked cy Llonic turning in the wind field. of the path looking downstream, are also a good 2. Shift in the low-level wind directions when indication. easterlies are normally present. Wind-speed must be 10 knots or more. 3. Greater than normal cloudiness, rainfall, Detection and pressure falls. Cloudiness should increase in vertical as well as horizontal extent. We already discussed how the meteorological 4. Sea level pressures lower than normal. The satellites have greatly aided in the detection of valueis dependent upon the region analyzed, tropical disturbances, especially in the early life however. a value greater than 3 millibars is the cycle.Itis important for meteorologists, ana- normal criterion. lysts, and forecasters to be able to effectively 5. Easterly wind speeds 25 percent and more interpret these pictures to achieve the maximum above normal in a limited area. especially when information from them. Through proper inter- the flow is cyclonic. pretation of the pictures determination of size, 6. Temperatures above normal at sea level, movement, extent of coverage, and approximate generally 26°C (79°F) or more in the lower surfacewinds can be made. Satellites have layers. become the most important method of detec- 7. Moisture above normal at all levels. tion of disturbances.

461 -467 Al ROGRAPHER'S MATE 1 & C

Aircr.ift leconnaissanta: play s.inimportant of its normal position for the season. Additional roleindetection also. This has become even indicators are as follows: more important when used in conjunction with the data from satellite pictures. Areas of suspi- 1. Movement is less than 13 knots. cious cloud structure can be imestigated by 2. The disturbance is decelerating or moving aircraft whose crews include trained meteorol- at constant speed. ogistsandobseners.Thesereconnaissance 3. The system has a northward component of flights can provide on-station data by orbiting or motion. penetration. either high- or low-loci, that would 4. A migratory anticyclone passes tothe not be available otherwise. north of the storm center. Another method of detecting tropical disturb- 5. Intensification occurs when the cyclone ances is through the use of radar. Present radar passes under an upper-level trough or cyclone ranees extend about 300 miles and can scan an provided that there is relative motion between area approximately 300.000 square milts. the two. There is some indication that intensifi- The Navy has also developed several different cation does not take place when the two remain types of floating weather stations to report superimposed. various meteoroloeical elements. These stations 6. Poleward movment of the cyclone is favor- send out measured data automatically or upon able for intensification, equatorward motion is query. not. 7. Intensification occurs only in areas where the sea surface temperature is 79°F or greater, Locati,,n with a high moisture content at all levels. (This rule has not been thoroughly tested.) The problems of formation, detection. and 8. When long waves are slowly progressive. locationareinrealityasinglethree-in-one (Applicable to genesis also.) problem. One is dependent on the other. 9. The trade inversion must be absent: con- In the case of satellite pictures. reconnais- vection deep. sanceind radar detection, the location is fairly 10. Other factors being equal, deepening will certain, barring navigational errors. If detection occur more rapidlyin higher thanin lower is made through the analysis. the exact location latitudes. (Coriolis force is stronger.) is more difficult to ascertain in the incipient 1 I. When a storm moves over land, the inten- stage of the storm, especially IA hen the analysis sity will immediately diminish. The expected is diffuse. The exact location should be decided amount of decrease in wind speeds can be .30 to upon only after the most intensive study of the data. 50 percent for storms with winds of 65 knots or The analyst should be prepared to revise his more, and 15 to 30 percent for storms with less decision in the lace of developments which are than 65 knots, if the terrain is flat; there is more more conclusive. decrease in each case if the terrain is rough. Seismic aids have also been used to some extent in detection of tropical cyclones: however, very littleispublished on the subject. Once an intense tropical cyclone has formed, Discussion of this aid cannot go beyond mention there will be further changes in its intensity in that it has been used. the course of its motion within the Tropics and during recurvature. The forecaster should con- siderthese changesinconnection with the lntensitifation predicted path of movement.

Movement Only when easterlies extend verticallyto 25,000 feet or more at the latitude of the vortex Tropical cyclones usually move with a direc- is intensification possible. This most frequently tion and a speed which closely approximate the occurs when the subtropical ridge lies poleward tropo,phericcurrentwhich surrounds them.

462 41,,65 Chapter I'_-- TROPICAL ANALYSIS AND FORECASTING

Logically. therefore. charts of the mean flow of the troposphere should be used as a basis for predicting the movement of tropical cyclones. but lack of obscrvation generally precludes this approach. N In the mean. there is a tendency for tropical cyclones to follow a hyperbolic or parabolic curve away from the Equator: however, departures from this type of track are frequent and of great variety. Tropical cyclones move toward the greatest surface pressure falls and toward the area where the surface pressure falls increase fastest with time. Calculation is necessary for this rule to be used. AG.669 Numerous theories have been advanced to Figure 12-35.Illustration of initial movement of tropi- explain the cyclone tracks of the past and to calcyclones formingina wave troughin the predict those or the future. Observational data easterlies. have never been sufficient to prove or disprove most of the;A few theoretical concepts have found limited application. b..t for the most part The motion of storms developing from the forecaster must rely upon empirical knowl- preexisting vortices can be extrapolated from edge and extrapolation when predicting the the track of these vortices. movement of tropical cyclones. In this section 3. Storms developing on the eastern edge of are discussed some of the current theories and polar troughs initially move up the trough (NW rules on forecasting these tracks: and one objec- to NE. tive method found successful in a large number 4. Pacific storms forming on equatorial shear- of cases is taken up in detail. lines usually are most active in the southern and Tropical storms move under the influence and southwestern portions atthe start and they are a result of both external and internal forces. move slowly northeast. After I to 2 days. the The external force is due to the air current that influence of the trades becomes dominant and surrounds the storms on all sides and carries the storms turn back toward a direction ranging themalong. The internalforces appear to from west to r --th. (This appears to be true produce a tendency for a northward displace- mainly for cyclones deepening near or west of ment of the storm which probably is propor- 130° E.) tional to the intensity of the storm, a westward 5. If a storm is discovered without informa- displacement that decreases as latitude increases: tion as to its past movement and the upper air and a periodic oscillation about a mean track. current. it is best to start it along the climatolog- INITIAL MOVEMENT OF INTENSE icalmeantrackandthensecure the data CYCLONES. The movement of cyclones that necessary to determine the steering current. are undergoing or have just completed intensifi- cationinitially(about 24 hours) will be as EXTRAPOLATION.--At present,the most follows: practical prognostic technique used by the tropical meteorologist consists of extrapolating the I. Storms developing inwestward moving past movement of the synopticattires on his wave troughs in the easterlies move toward the chartinto the future. The pasttrack of a west and also with a poleward component given cyclone represents the iiitegrated effects 'NI' the by an angle of approximately 20° to the right of steering forces acting upon it. Accelerations and the axis of the trough looking down-stream. (See changes in comse are the results of the changes fig. 12-35.) in these steering forces. The effect of these

463 469 AEROGRAPHER'S MATE I & C tor..-e can be examined and extrapolated di- since the observed winds represent the combined rectly from the past position of the cyclone. effects of the basic current and disturbances. As B.for.. applying the extrapolation technique, most storms extend into the high troposphere, it fore.aster must attempt to smooth out the is better to calculate a "steering layer" thana minor irregularities in the past track. The first steeringlevel,sincepresumablythewind .tep in the use of extrapolation is to determine throughout most of the troposphere influences the mean direction and speed of the cyclonic the storm movement. One writer recommends center between each two known positions. The an integration of the mean Clow between the next step is to determine the rate of change in surface and 300 millibars, over a band 8° in direction and speed between successive pairs of latitude centered over the storm. Another writer fixes. Theforecast position thus determined indicates that for moderate and intense storms fro.nextrapolation of movement should be the best hurricane steering winds would be -411°01ln:it out for minor irregularities andcom- foundinthelayer between 500 and 200 pared to the applicable climatological tracks of millibars and averaged over a ring extending e.1...lone% in the area. If large differences exist, from 2° to 6° latitude from the storm center. the forecast should be completely reexamined. Other practical applications of the steering The climatologicaltracks should receive less concept to short-range tropical cyclone motion %%eight for short-term forecasts of 6 to 12 hours use differing approaches in attempting to meas- and more weight for forecasts in exce.s of 24 ure the basic current. One is by streamline hours. analysis of successive levels to find a height at l'orshort-term forecasts (6to12 hours) which the vortical circulation diminishes to a ,:xtrapolation is as reliable as any known method point such that the winds are supposedly repre- for movement. sentativeof theundisturbedflow. Another STEERING. --The movement of atropical method is to take vector averages of reconnais- cyclone is determined to a large extent by the sance winds near the zone of strongest winds in direction and speed of the basic current in which the storm. itis embedded. This concept appears to work well as long as the cyclone remains small and 1. Use of Observed Winds Aloft for Steering. remains in a deep broad current. By the timea When sufficient data are available, it has been tropical cyclone has reached hurricane intensity, found that the use of streamline analysis of these conditions seldom exist. It then becomes successive levels usually gives valuable indications of tropical storm movement for as much as necessary to integrate the winds at alllevels through which the cyclone extends and in all 24 hoursinadvance. However, since wind quadrants of the storm to determine the effec- observations are usually scarce in the vicinity of tive steering current. Although the principle is a hurricane, their analysis is necessarily rather not fully understood and has not yet received subjective. Some forecasters claim dependable universal acceptance as a valid rule, it does have results in using this concept when data were practical applications. For example, changes in availableto high levels near the storm. The winds at one or more levels in the area surround- technique is not based on the assumption that ing the cyclone can sometimes be anticipated, windatanysinglelevelisresponsiblefor and in such easel., a qualitative estimate of the steering the storm, since the forces controlling iesulting change in the movement of the cyclone movement are active through a deep layer of the can be made. Conversely, when it appears likely atmosphere. However, as successive levels are that none of the winds in the vicinity of the analyzed, a level is found at which the dosed cyclonic circulation of the storm virtually dis- cyclonewill change appreciably during the appears. This STEERING level coincides with forecast period, no change in the direction and the top of the warm vortex and varies in height speed of movement should be anticipated. withdifferentstages andintensities of the Further,thisruleapplies to situations of storm. It may be located as low as 20,000 feet nonrecurvat tire and some cases of recurvature. It or in the case of a large mature storm as high as is difficult to determinethe steering current, 50,000 feet. Chapter 12 TROPICAL ANALYSIS AND FORECASTING

It has been found in analysis that most weight An example of Simpson's method of warm should be given to the winds in advance of the tongue steering method can be found in the U.S. storm within a radius of 200 to 300 miles in Department of Commerce, Forecasting Guide preference to those in the rear quadrants. Thc No. 3. Hurricane Forecasting. hurricane generally moves with a speed of 60 to 80 percent of the current at the steering level. Recurvature The direction of movement isnot always exactly parallel to the steering current. but has a One of the fundamental problems of forecast- component toward high pressure which varies ing the movement of tropical cyclones is that of inversely with the speed of the current, ranging recurvature.Willthe cyclone move along a from almost 0° with rapid movement to as much relatively straight line until it dissipates. or will as 20° with speeds under 20 knots. In westward itfollow atrack which curves poleward and moving storms, a component of motion toward eastward? When recurvatureis expected, the high pressure could result from the poleward forecaster must next decide where and when it acceleration arising Iron: the variation of the willtakeplace. Then, he islaced with the Coriolis parameter acrossthe width of the problem of forecasting the radius of the curved storm. This would indicate that, to the extent that track. Even afterthe cyclone has begun to thiseffectaccountsforthe component of recurve. there are great variety of paths that it motion toward high pressure. northward moving may take. At any point. it may change course storms would fit the direction of the steering sharply. current more closelythan westward moving The most common recurvature situation arises ones. The tendency for poleward drift would be when anextratropicaltroughapproaches a added to the speed of forward motion in case of storm from the west or when the storm moves a northward moving storm so thatitwould west to northwest toward a stationary or slowly approach more closely the speed of the steering movingtrough. Some of the indicators on current. Empirical evidence supports this hypo- possible recurvature are as follows: thesis. Corrections for both direction and raLe of I.if the base of the polar westerlies lowers movement should be made when this is indi- (to 15,000 to 20,000 feet) west of the storm at cated by the wincifiow downstream in the region its latitude and remains in this position, recurva- intowhichthe stormwillbe moving. For ture may then be expected to occur. prediction beyond several hours, changes in the 2. However.ifthereisthe building of a flow pattern for a considerable distance from dy mimic high or an eastward movement of this the storm must be anticipated. It should also be high to the rear of the advancing trough, and the remembered that intensification or decay of a westerlies dissipateinthe low latitudes, the storm may call for use of a higher or lower level. storm will move past the trough to its south and respectively,toestimatethefuture steering continue its westward path. current. 3. Rule 2 also holds true in cases where the 2. Thermal Steering. A number of efforts polar trough moves from the west against a have been made to correlate hurricane move- blocking high. The higher latitude portion of the mentwiththermalpatterns.ror example. trough continues to move eastward while the Simpson suggests that a storm will move along southern segment of the trough is retarded and tongues of warm air in the layer from 700 to is no longer connected with the upper portion of 500 millibars that often extend1.000 miles the trough. ahead of the storm. The orientation of the axis 4. Recurvature may be expected when a; of the tongue then D,* be regarded as a reliable anchor trough is about 500 miles west of the indicator of storm movement for the next 24 storm and whentheforwardedge of the hours. A considerable difficulty in apply ing this westerlies from 500 to 700 miles to the west. tezhnique is created by the fact that the warm Correlatedwiththis parameter R. J. Shafer tongue sometimes has more than one branch and found that a spot value of the thickness bet,...-,en itis questionable as to which is the major ax;.. 850 to 500 millibars 7.5 degrees of latitude to

465 AEROGRAM HER'S MATE I & C

the northwest of the storm was one of the most narrow subtropi. ridge separates the westerlies valuable of all parameters on the iecur% attire or from the tropical trough.In many cases the as an indication of future mu% ement of Atlantic mean trough in the westerlies may be located at hurricanes. This thickness reflects the relative the same longitude of the storm. strength of cold troughs to the west essential for recurvature. Low %attics of thickness 14.000 feet MOTION DURING RI:CURVATURE. The (approximately 4.270 meters) orless. almost following rules apply during recury ature only. always indicaterecurvature andhig'ivalues. 14.200 feet (4330 meters) or more. generally I. When the radius of recurvature of a storm indicate continued westward motion. is greater than 300 miles, it will not decelerate 5. The major trough west of the storm (in the and may even accelerate. The storm will slow westes lies) is slowly progressive. down if the radius of recurvature is less than 300 6Long waves are stationary or slowly pro- miles. In general when the radius of recurv,aure gressive. is large. it is usually very uniform. A small radius 7. There isarapidsuccession of minor will occur along a brief portion of the track. troughs aloft. (See fig. 12-36.) S. Climatological wean trackindicatesre- curvature (use with caution). 9 When the neutral point at the southern extremity of the trough in the westerlies at the 500-mb levellies ator equatorward of the (A) latitude of the cyclone. recurvature into the rough will usually occur. In this situation. the cyclone would normally be under the influence 01 southerly winds from the upper iimits to a (B) level well below the 500 -niblevel while approaching trough. 10. When the subtropical ridge at the 500-mb level is broad -nd consists of large anticyclones. recurvature usually occurs. This case represents a low index situation in which the cyclone remains under the influence of a single, large, slow-moving anticyclone for a relatively long

I I.Weal-troughs between two separate sub- AG.670 tropicalhighcells.Sometimes thetropical Figure 12-35.(A) Recurvature at a constant spend; storms move northward through very weak (B) first dz.,celerating and then accelerating. breaks in subtropical highs. 2. A largeradius of recurvatureisto be NONRECURVATURE. The followinit expected if the high northeast of the tropical patterns are associated with nonrecurvature. cyclone has a vertical axis (fig. 12-37 (A)). This I. Strong subtropit,1 anticyclone or ridg.: to will occur when long waves are stationary. the north of the storm with the mean tromp in 3. When the highopen south to southeast ilk. westerlies iocattfar to the west of GI: with height(fig.12-37 (13)). the cycloneis longitude of the storm. If this pattern develops transferred rapidly from the influence of upper strongly over the western oceans or continentsi easterlies to that of the upper westerlies and the storm will generally be thiven inland and dissi- track has a short bend. This occurs when long pate before it recurves into the western end of waves are progressive. the ridge. 2. Mat (small-amplitude waves) westerlies at The following rules refer to short term (24 latitudes near or north of the normal position. A hours or less) forecasting: Chapter 12 TROPICAL ANALYSIS AND FORECASTING

cyclone position. This rule is especially helpful when a storm is just offsuore to Determine the precise place of entry for intervals of 12 hours or less.

Changes in Intensity During Movement Out of the Tropics During movement out of the Tropics the storm comes under influences different from those inits birthplace and tropical path. The following rules and observations can be helpful in forecasting the storm's changes in intensity. I. A storm drifting slowly northward with slight east or west component will preserve its tropical characteristics. 2. Storms moving northward into a frontal area or area of strong temperature gradients usually become extratropical storms. In this case the concentrated center dies out rapidly while the area of gale winds and precipitation expands. The closed circulation aloft gives way to a wave (B) pattern and the storm accelerates to the northeast. 3.Ifthetropical cyclone recurves into a trough containing a deep slowly moving surface low,itnormally will overtake this low and combine with it. Temporary intensification may result. 4.Ifthetropical cyclone recurves into a trough containingn uppccold core low,it AG.671 appears to move into the upper low in most Figure 12-37.Surface flow pattern (dashed line) and cases. 300- to 200-mb flow pattern (solid lines).(A) Corresponds to track (A) in figure 12-37; (B) corresponds to track (B) in figure 12-37. Short -Range Prediction by Objective Techniques 1. Tropical cyclones move toward the area of greatest surface fall (12- to 24-hour pressure As in all so-called objective technique ore method will serve to produce an accurate fore- change). 2. Tropical cyclones move toward the area castforalltropical storms or extratropical where the surface pressure falls increase most systems. In the following section several techniques which have been developed inrecent rapidlywithtime. For calculation 3-hourly reports are necessaryTake, for example, the years are discussed. It should be mentioned that the objective technique shouldnot be con- 24-hou, pressure change at 1800 and subtract it sidered as the ultimate forecast and thatall from the 24-hour pressure change at 1500. This other rules, empirical relationships, and sy noptie gives the acceleration of pressure fall.If this indications should be integrated into th_ final quantity is computed for a network of stations forecast. Wholly inaceurate fore,asts may result and isolines are drawn for a suitable interval, the if the objective method were the only one negative center helps to pinpoint the expected considered in making the forecast.

467 473 AEROGRAPI1ER'S i & C

STATISTICAL MET! IOI)S. One of the most 850 and 500 millibars. The corr:latiun of these prominent and widely LIN of these ni,:thuds parameters is modified by extrapolation and the was 'dbyVeigas-oldlertopredictthe geographic locations. Nielidiunal motion IS simi- 24 -hour displacement of hurricanes based pri- lar!) predicted by sea level and thickness param- marily un the latest sea level rressure eters modified by extrapolation. The mendional Gun. Sea level pressures were used rather than and zonal computations are then combined into upper airdata due primarilytothe longer the final 30-hour forecast. availablerecordof sealeveldata and also In a test or dependent and independcot data because of the advantage of denser areas and it was round that some 85 percent or the storms time coverage and inure rapid availability of the predicted in the 31 sample cases len within 2° or data alter observation time. In addition to the the predicted position. sea level pressure field this method also incorpo- The cumplet: details or this method can be rates the past 24-hour motion of the storm and found in GRD Scientific Report No. I, Contract climatological aspects of the storm. No. ^.F19(604)-2073. Further Studie.,in the Pressure values are read from predetermined velopment of Short -Range Weather Prediction points located at intersections or latitude and Techniques. April 1958. longitude lines with values divisible by 5. Two MOVEMENT OF TYPHOONS. Griffith different sets of equations are used one set For a Wang or the Civil A.. Transport Service. Taiwan, northerlyzone (between latitudes 27.6° and developed a method For objectively predictine 40.0°N and longitudes from 65.0° to 100.0°W). the movement of typhoonsinthe western Thesoutherlyzone encompasses the same Pacific. The method istitled "A Method in longitudes as the northerly zone. but the alti- Regression Equations For Forecasting the Move- tudes are for 17.5° and 27.5°N. mentof Typhoons." The equationutilizes This method was tested by Veigas and Miller 700-mb data andis based un the Following on 125 independent cases.aboutequally criteria: distributed between the two zones, during the years 1924-27 and 1954-55. The average vector I. The 700-mb contour height and its tend- errors in 24-hour forecast position were about ency 10° lat north of the typhoon center. 150 nautical miles for the northerly zone and 95 2. The 700-mb contour height and tendency nautical miles For the southerly zone. 10° lat from the typhoon center and 90° to the Recently the University or llawaii has modi- right of its path or motion. fied this set or equations so that they will be 3. The 700-mb contour height and its tend- applicable in the Pacific area. A complete test ency 10° lat from the typhoon center and 90° has not been made and atthe time or this to the left or its path or motion. writing, the equations have not been distributed 4. The intensity and the orientation or the ror operational use. major axis or the subtropical anticyclone which Full details, with an actual example or the steers the move lent of the typhoon. procedure fur use or this method. may be round in the U.S. Department or Commerce, Forecast- ing Guide Nu. 3. Hurricane Forecasting. Percentage or frequency or direction or move- 30-110U R MOVEMENT OF CERTAIN ment and speed tables are provided for a rough ATLANTIC HURRICANES.-- R.J. Sharer has first approximation of the movement or the developed an objective method for determining typhoon. the 3( hour movement of hurricanes by use or This method,is well as all other methods se,: lc vci data over the ocean area, and upper air based on a single chart, is dependent upon a data over land area. Motion is described in two good network of reports and a good analysis. A components.zun,:iandmeridional.Westerly cull test of the value of this method has not been inutiJnis determined 5, onside-atiun of the made. but in limited dependent and independent component or the sealev,.; pre 'me gradient data casestesteditappears to have a good surrounding the hurricane and is u posed graphi- and provides anodic' useful tool in cally by the mean temperature sield between the integrated forecast.

468 474 Chapter 12 TROPICAL ANALYSIS AND FORECASTING

lull details on the procedure and application United States Weather Bureau Hurricane Fore- of this method can be found in the Bulletin of casting Service. In their study it was found that the American Meteorological Society, Vol. 41. the standard rule for steeling tropical cyclones No. 3. March 1960. (the movement of the storm from about 10° to USE OF THE GEOSTROPHIC WIND FOR 20° to the right of the curre;It flowing over the STEERING. The expansion of the aircraft re- top of the core) was only reliable prior to connaissance reports have made itpractical to recurvature and that storms alter recurvature carry out more detailed analyses of constant frequently move totheleft of the steering pressure surface over the tropical storm belt and current. Operating onthe premisethatthe make use of a forecast based on geostrophic motion of the tropical storm is not governed by components at that lcel. This technique. devel- forcesactingatany onelevel,their study oped by Riehl. Haggard. and Sanborn. and encompasses three levels, the 700-, 500-, and issued as an NA publication (Objective Predic- ;300-mb levels. They found that the 700- and tion of 2_4 -I lour Tropical Cyclone Movement) 500-mb charts were about equal in forecasting utilizesthis steering concept. The technique hurricane motion.Inthefinalanalysis, the makes use of 500-mb height averages alongside 700-mb level was .selected and combined with of a rectangular grid approximately centered on the previous 12-hour motion of the storm. From the storm. The grid is 15° long., centered at the theirstudy aslightlybetter verification of initial longitude of the storm and between 10° predictedtracks of hurricanesresultedthan and IS' lat with the southern end fixed at a from use of sea level (statistical method) or the distance of 5° kit south of the latitude of the 500-mb chart alone. storm center. The more northward extension of the grid is used for storms found to be moving The basic grid is essentially the same as that used in the 500-mb method except that gradi- more rapidly northward. The relatively small ents were computed at intervals of 2.5°lat size of the grid indicates taai tropical cyclone instead of 5°. The previousI 2-hour motion was motion for 24 hours is determined to a great also incorporated into the forecast. extent by circulation features closely bordering the storm. and that only the average features This method was tested on 23forecasts outsidethis areawillnotgreatlyaffectits during the 1958 hurricane season. The average movement within the time internal. This method error was 95 nautical miles for the 24-hour is discussed in detail later in this chapter. The forecast and ranged from 15 nautical miles to 500-mb chartisthe basic chart for computa- 170 nautical miles. tions. A further explanation of the method and its Another methodwhichuses thesteering procedure for application may be found in the concept is A Comparison of Ilurricane Steering Bulletin of the American Meteorological Soci- Levels by B. I. Miller and P. L. Moore of the ety. Vol. 41. No. 2. February 1960.

469 475 CHAPTER 13

WEATHER BRIEFING AND FLIGHT FORECASTING

As an Aerographer's Mate First Class or Chief, tive in ascertaining the requirements of his users. one of your principal duties will be to prepare Secondly, to be of maximum utility in planning and give weather briefings of all types. There are or carrying out operations, the weather infor- many variations of the types of briefings re- mation presented should be evaluated for poten- quired of y ou. They range from the individual tial accuracy. For example, weather forecasts for pilot weather briefing to the more complex and the continental United States can usually be detailed command and staff briefings. Normally, expected to verify with reasonable accuracy, due the latter type of briefing would be presented by to the excellent network of observation stations the Meteorological Officer. but in his absence or and communications facilities. The accuracy of by direction, you may be required to perform forecasts for areas where data are sparse and this type briefing. In the first section of this communications unreliable would normally be chapter, general practices, insofar as the varied expected to be somewhat less. Using agencies types of briefings you will be called upon to should be apprised of the limitations which such make, are presented for background informa- conditions sometimes impose upon the accuracy tion.Inlater sections of this chapter more of the forecasts in order that plans may be specific requirements for briefings are given. adjusted to minimize the effect of an inaccurate forecast upon such operations. Indicating the PRINCIPLES OF WEATHER BRIEFING degree of confidence ina forecast is usually permissible and should not be considered as PURPOSE OF BRIEFING "hedging." The intent of such a confidence factor is to provide the using agency with the A weather briefingisthe presentation of ,west information available. weather information ina concise, clear, and orderly form to those personnel planning, con- PRESENTATION trolling, or participating in military operations. The briefing, no matter which of the many A weather briefing may be, in many cases, the formsittakes,isconductedto insure that basic information which determines whether a planning, supervising, and operating pe,sonne: mission or a large-scale operation is executed as receiveandunderstandthebestavailable planned, postponed, altered, or canceled. The weather information, soastopermitthese manner in which the weather informationis personnel to properly evaluate and utilize the presented is considered to be of equal impor- effect of weather factors on projected opera- tance with the material contained in the brief- tions. ing. To be of maximum value to using personnel, The forecaster should understand that the the first principle of successful weather briefing information he presents will assist the using is to insure that the weatherinformation given agency in clearly determining theeffect of the tailored to meet the requirements of those usin3 weather on projectedoperations.lieisthe personnel. The forecaster should take the initia- authority on an extremely important element

470 476 J

Chapter I3WEATHER BRIEFING AND FLIGHT FORECASTING affecting the operation, and therefore he should The hints given are merely restatements of the be confident. Lonvincineind positive. He must principles of good public speaking. Of course, present the briefing so that itis clearly heard noteverybriefingsituationwillnecessarily and easily understood. employ all of these hints. However, you should Effective Weather briefing is an art. It requires remember that no matter what type briefing you alertness, poise, and judgment on the part of the aregiving,thematerial you arepresenting briefer. Whether th.: information he presents is should be heard and understood. in oral, written, or pictorial form, he must be able to think on his feet and come up with VISUAL AIDS precise factual answers to questions which may at times be difficult or even embarrassing. He Visual aids, as used in weather briefings, are should avoid verbal ambiguities. vagueness, or pictorial and graphical representations used to misplaced emphasis which could easily convey a portray the information imparted orally by the mental picture of weather completely different briefing forecaster. Just as the oral portion of from the one intended. the briefing must be heard, the visual aids must Some helpful briefing hints are listed below. be seen. The size of the pictorial representation depends upon the farthest distance the person I.Strive for force and enthusiasm. Show that receivingthebriefingisseated. Colorisa you are interested in the problem at hand and its valuableaid.Shades of theprimary colors correct solution. Remember thatall weather contrast best with shades of another primary briefings are important. color. Light shades of colors should be avoided. 2.Practice the briefing beforehand, (when You do not necessarily need artistic talent for possible), from opening statement to conclusion, producing effective briefing charts. If the chart and insure that it is an orderly presentation. neatly and accurately d picts the inforniation, 3. Avoid technical meteorological terms. For and the datum is legible, it will be satisfactory. example. use the word "high" instead of "anti- The primary purpose of the visual aidis to cyclone." enhance understanding. Accuracy should never 4. Avoid ambiguous and vague phraseology. be sacrificed for artistry. The terms "about," "probably," "I think," etc., convey no useful information and can easily Materials and Equipment create an impression of hedging. 5.* Make the entire briefing a running narra- The drawrag medium which gives the greatest tive, and interpret the weather information in brilliance and contrast between colors is pre- terms applicable to the operation. The briefing ferred. The choke of drawing media is limited forecaster does not make operational decisions, bythe material on which the illustrationis but presents an accurate and complete descrip- made. Illustrations prepared on small cards for tion of the weather to be encountered. projection on a screen require the use of colored G. Discuss only the important or essential pencils or inks of the conventional type used in details of the weather. Keep the briefing as map analysis. simple as possible. The briefer should have a basic familiarity 7.Present the briefing in the second person. with ordinary projection devices such as the The material is being presented for and to the transparentprojector,the opaque projector, audience andisnot a mere recitation of a and/or slide projectors, when appropriate. The meteorological situation or a map discussion. briefer should also know the capabilities and 8. Never apologizeforanypartof the limitations of the devices used locally. weather presentation. You have presented the best available information. If your confidence in Types of Visual Aids the information is limited, you may state so. 9. Finish forcefully, with a definite closing Facilities available to the weather office and statement. Always ask L. questions, as further the requirements preferences of the using clarification may be required. personneldetermin whichofthevarious

471 477 AEROGRAPHER'S MATE I & C methods of visualizing the weather may be used with parallel cross sections mounted vertically most advantageously. In briefing an individual onittorepresenttheweatherinnatural pilot,thestation-drawn charts, or facsimile perspective. chartsreceived by the station, are normally Whatever method is used for briefings, other adequate, for larger briefings, a number of other than for individual pilots, they should be care- methods may be employed. fullyconsideredinrelationtothe type of OPAQUE PROJECTION. Charts and cross briefing and the maximum benefit of the using sections may be drawn on suitable paper and personnel. projected by use of a projector fitted with a mirrorandlensarrangementforscreening Preparation of Charts opaque materials. For recurring missions, such paper may be overprintedwiththeterrain The information to be displayed on briefing features. Opaque projectionrequiresafully charts varies with the type of mission. This darkened room in order to show details and section should be used as a basic guide, bearing coloring satisfactorily. in mind that each situation must be resolved in TRANSPARENT PROJECTION. terms of the information desired, the materials Transparent projection may be in the form of available, and the limitations of the briefing. slides, either locally constructed or ones pre- PROGNOSTIC CHARTS. -The following pared by some other source. Transparent over- should be used as a guide for the briefer in lays with the use of the overhead projector lend preparing prognosticcharts. The area shown themselves more readilytolocallyprepared should be iarge enough to represent the general visual aids. A transparent overlay of plastic can weatherpicturepertinentto the operation. be mounted over a base map or enlarged cross Boundaries between land and water areas may section blanks. A weather picture drawn on the be shaded lightly in blue on the water side and transparent plastic with a china marking pencil in brown on the land side. Entire land and water can be erased withasoftcloth. When the areas may be shaded as desirable. All printing number of personnel briefedis small, acetate should be in black. The date and time should be with a frosted surface may be used to prevent indicated in the lower margin and should be in glare. However, marks and shadings put on with the same time zone as used in the briefing. A oilycrayons cannotbe removedfromthe solid brown line with arrowheads showing direc- frosted surface easily. For command and staff tion should be used to indicate the planned briefings where preparation time is not of the route. For convenience and clarity, the route essence, effective presentation can be made by may be divided into zones. When used, each p 'lilting on glass plates. Topography and lines zone should be numbered. Fronts and precipita- designating height and altitude can be drawn tion areas should be shaded, using the conven- permanently on the glass. If the background is tional symbols, a sufficient number of isobars co erect with blue, an illusion of depth and a should be drawn to show relative intensity and maximum contrastwith white etc uds is ob- distribution of pressure systems. They should be tained. drawn as solid blacklines. Areas of blowing ENLARGED MOUNTED CHARTS. dust, fog, haze, smoke, and similar restrictions Enlarged, expendable type charts are the most to visibility should be shaded lightly in red. desirable for briefing large groups, if time and Yellow will not ordinarily register visually. The materials are available. Advantages of this type surface flow pattern may be indicated with blue coat are that the room need not be darkened, arrows to denote the movement of cold air and charts can be of any desired size, and colors with red arrows to show the movement of warm usually contrast more sharply than on projected air. Pressure systems should be labeled in accord- or overlay type charts. ancewithconventionalmethods.Cloud THREE DIMENSIONAL CROSS SECTION. amounts, the cloud types, and the heights of For command andstaffbriefings,athree - bases and tops above MSL may be printed on dimensional mount may be prepared. This de- the chart in pertinent areas. Shading or stippling vice usually pictures the surface prognostic chart may be used to indicate cloud areas. Weather

472 4 78 Chapter 13WEATHER BRIEFING AND FLIGHTFORECASTING

hazards such as thunderstorms, showers. turbu- weather on routes commonly flown from his lence. and icing should be indicated byover- station,inorder thatthe briefing may be printing the appropriate symbol in the pertinent conducted quickly, concisely, and without ex- area. Projected movements of fronts may also be cessive delay in checking current randitions. shown. For prognostic charts used in planned Particularly for scheduled flights, this system briefings, additional details may be added and willproveofvalue.Existing andforecast some of those listed above eliminated. weather along the route and at the destination, HORIZONTAL WEATHER DEPICTION and alternates if required, should be described CHARTS.These charts are included whenever orally in order to aid the pilot in making his the Flight Weather Packet is requested. Theyare flight plan. This requires in some degree theuse discussed in detail later in the chapter. of many, if not all, of the following aids: The OPTIONAL CHARTS.Occasionally, charts latestsurface synoptic charts,latestsurface other than those listed previously arenecessary prognostic chart,latest upper air analyses at to complete the visual aids required to insurea applicable levels along with the progs at these satisfactory briefing. One such chart isa cloud levels,routecrosssectionswhicharekept distribution and visibility chart. On these charts current,teletypedataforlatesthourlyse- certain criteriafor the type of mission to be quences, terminal forecasts, pilot reports, radar briefed are selected (for example 5/10 cloud for reports, and any other information deemed of one type of bombing mission iscritical), and value. Of course, jet aircraft briefing requires these areas analyzed and indicated by colors, dataathigheraltitudesplus jetstream and stippling. shading, and outlining the areas as contrail data. Overseas stations frequentlyale desirable. Other weather elements may also be called upon to forecast "D" values. indicated. Climatological briefing chartsmay also be used as aids and guides for constructing Group Briefings similar briefing aids for other areas. Group briefings as used in this section are TYPES OF BRIEFINGS those briefings given to a number of pilots or aircrews as a group, preparatory to a flight. The Briefings may encompass a very wide range, flight may be a combat mission, a ferry flight. both as to content and scope, from briefingan transport, or training flight. For simplif kation. individualpilotforashortflightto more group briefings may be broken down into two elaborate briefings conducted for a high ranking types -cross-country flights,whichinclude navalofficer and his staff for an operation usual ferry, transport or training flights, and scheduled many weeks hence. Between the operationalbriefings,whichincludevarious extremes there are as many types of weather types of combat or combat training missions. briefings as there are varieties of militaryopera- CROSS-COUNTRY BRIEFINGS.In general, tions.Listed below are general guidelines for the content of such briefings should beas some of the briefing types. More specific infor- follows: mation is listed later in the chapter. I. Forecast Weather. A brief discussion of the Ordinary Weather Station prognostic chart should open the briefing. This Briefing for Individual Pilots discussion should be simple and brief. 2. Route Weather. After the general weather Thisis by far the most common type of discussion, weather that will be encountered by briefing provided by the average weather fore- the aircrews during the flight should be pre- caster. A duty forecaster may perform this type sented. It should include, as part of the oral of briefing from 300 to 900 times a month at briefing,takeoff data such as surface wind, the average weather station. weather,visibility,andvariationsexpected; The weather briefing should begin as soon as cloudconditionsatbase and enroute, also the pilot states his destination and route. The including cloud bases below flight altitude with forecaster should keep up to date on current data on coverage, types, bases, tops turbulence,

473 479 AEROGRAPIIER'S MATE I & C and:or king (usuallygiveninheights above expected from prognostic charts, including lows, mean sea level), freezing level and king zones, highs, fronts, general weather in each area, cloud climb wind (climb wind is the mean vector from covei, hazards, turbulence, icing and visibilities. the surface to flight altitude), weather eondi- 2. Base,route,andtargetconditionsas tions along path to include winds and tempera- shown on the IIWD chart to include cloud cover, turesatflightaltitude,air-to-air and air-to- winds, weather, hazards and visibility. groundvisibility: a descendingwind if appropriate, cloudiness at terminal, weather at In presenting this type of briefing the synop- terminal, and such landing data with variations tic chart can be used to give a review of the such as duration of local showers or thunder- present weather, the prognostic chart to present storms. Alternates should be discussed in the the forecast for an operational area at a particu- same manner as terminals. Additional data may lar time, and the IIWD chart to show weather be given, depending on the type of flight, but along a specific route. should beintegratedintothe body of the LONG-RANGE PLANNING briefing at the proper time interval. BRIEFINGS. -Briefings of this type are used in For this type of bfiefine the minimum visual planning, amphibious operations and other op- aids which should be used include an IIWD chart erations which require long-range planning. In and the latest Upper Winds chart. When re- such cases, presentation of climatic data will be quired, a flight folder may be issued to the crews required. First, a geographical description of the to include a copy of the IIWD chart. Upper area should be given followed by interpreting Winds chart, and a written forecast with addi- the frequencies and means as they appear on the tional information. charts and graphs. Information regarding the OPERATIONAL BRIEFINGS. Foropera- limitations and reliability of the climatic data tional missions, certain data are required de- presented should be given in order thatthe pending on the ty pe of mission. Any or all of planning staff may give the data proper weight the following may be necessary- winds at flight in fornmlating plans. levelandatthe surface. mean temperature, In general. visual aids for a long-range briefing visibility, both air-to-air and air-to-ground. alti- can be divided into three classifications: tudedata: Q factors:refractiveindexdata: altimeter settings. and sky cover and weather at I. Surfacecharts,suchas storm pattern the destination. charts. mean pressure and temperature charts, and windflow charts. Command and Staff Briefings 2. Graphs and cha'Is of weather elements which have as their primary purpose the presen- Briefings of this type can he divided into two tat:on of frequency a.-d means of such weather general classificationsshort range briefings in eie.ments as cloudiness, visit ility, and precipita- which weather considerations are presented as tion. they will affect operations taking place in a few 3. Special charts such as ocean currents, state hours or a few days: and long-range briefings in oh' sea, surf, etc.. where applicable. which weather conditions are presented as they willaffectoperationstakingplaceweeks, Mission Weather months, or even years later. Indoctrination Briefing SIIORT-RANGE PLANNING BRIEFINGS.- Briefings for short-range planning are directly This type of briefing is usually conducted a applicable to impending military operations, ..nd day or two prior to a scheduled operation. It is theircontentwilldepend onthety pe of purely a weathcr indoctrination, and no attempt mission. In general, the following information is made toforecast the weather which will should normally be included: prevail over the route or area involved. Typical weather should be depicted with emphasis on I. A review of the past and present synoptic theareaandfrequencyof occurrence and situation with alogical transition to weather average and extreme limits of intensity.

474 480 Chapter 13 WEATHER BRIEFING AND FLIGHT FORECASTING

Visualaidsused generally include current deliberation. Indeed. the success or fa;lure of an monthly or seasonal mean charts. such as wind- operation is often attributable ,zither entirely or flow fields, storm tracks, frontal positions. etc. inpart. to the value of weather information received by the operation commander. The fact Special Weather Briefings thatsuchdegree of preparationrequires a considerableperiod of timeisobvious and Special weather briefings are those conducted further emphasizes the need for close liaison for each of the various specialist members ofan with the operation commander inorder to aircrew or individuals involved in the oi.eration insure that sufficiently early notification of the with distinct problems. This type briefing supple- requirements for a briefing is received. ments the general briefing. Information of such specialized and technical nature which is of WEATHER SYNOPSIS primary interest to each group is presented: for examplenavigators woulddesireadetailed On many Navy forecasts a weather synopsis is discussion of climbing winds, winds and tem- made of the pressure systems and fronts. A peratures at flight levels, and descending winds. general description of the systems and fronts They may also wiel co review surface conditions with their location and expected movement is for information on possible ditching. given. Below is an example of a weather synop sis: DEBRIEFINGS The station is under the influence ora cold polar air mass. A cold front passed the station Normally, weather debriefing consists of in- early yesterday afternoon. This cold front is terrogation to determine the weather encoun- now orientednortheast-southwestalong the tered during a mission. Crews which have flown eastern coast of the United States, extending a mission arealogical source of accurate from a low-pressure area centered over south- information on actual route and target weather. eastern Connecticut. The front has become Data furnished by interrogation may not only stationary through northern Florida and extends provide the only practical means of verifying the westward along the southern coast of Alabama forecast, but may be of considerable value in and Louisiana. The warm, moist. tropical air is amending the forecast for later flights into the over-riding the cold oolar air mass over thisarea. same area. causing considerable cloudiness and light inter- The forecaster should try to impress the crew mittent rain. This condition is expected to exist member with the value of the weather informa- during the next 36 hours. A maritime air mass tion they can furnish. However, for the informa- dominates the entire western part of the United tion to be of its utmost value, the exact time, States. place, and altitude of each observation should be obtained. Weather folders, cross sections,in- SURFACE SHIP OPERATIONS flight reportforms, and checklists help the forecaster to get a complete weather picture. Weather briefings for surface ship operations will vary considerably, depending upon the type WEATHER BRIEFINGS FOR SURFACE of ship and the mission at hand. Here again, the AND SUBSURFACE OPERATIONS meteorologist should be cognizant of all phases of the operation and be able to make comments After obtaining a comprehensive understand- as to how the various weather elements will ing of the operational problems involved, the influence the unit or units. Long-range weather person who will make the weather briefing is forecasts are desirable for route planning to ready for the preparation of his discussion. Since avoid bad weather. Whileitisrealizedthat many lives and much costly equipment are forecasting beyond 36 hours isdifficult, use generally involved in an operation, preparation should be made of the latest long-range forecast cannot be haphazardinnature:it must be procedures and of climatological data for ex- exhaustive in compilation of data and detailed in tendedperiods. Aprevailingclimatological

475 481. AEROGRAPUER'S MATE I & C briefing of personnel (especially the officer in Cold weather operations are very demanding tactical command (OTC) and his staff. the com- on the meteorologist. Here he will find that manding officer, executive officer. operations of- everybit of information has value. no matter ficer. navigator, 00D's, and other department how insignificant it may seem. Sea water tem- heads)is recommended for extended cruises. peratures as well as air temperatures are very Ship routing is now being done on a large scale important. particularly when freezing tempera- with a high degree of sw:cess through Optimum tures may be encountered. An accumulation of Track Ship Routing (OTSR). ice topside can seriously affect a ship's stability One of the concerns for the c ommanding and endanger thelives of allpersonnel. A officer is the danger of encountering a typhoon coating of ice may increase the chance of visual (orhurricane). You willnormally find that detection in time of war. Ice forming on the warnings from the various weather centrals will decks of carriers is particularly hazardous. not keep you fairly well posted, but do not let this only delaying operations. but also placing the lull you into a false sense of security. Make a ship in an unready condition and adding to the special chart for keeping track of any storms. ever-present hazards to topside personnel. and use it in your briefings. You cannot give too Drift ice areas are hazards to navigation and much advance notice, remember the ship's speed may necessitate additional precautions. Slush ice is limited and advance planning is absolutely areas are also hazards. and the meteorologist necessary. should know the various ways in which the ship Sea conditions and wind speeds, as related to may be endangered. Ice may clog the intake the development of seas. are important items for strainers of theseawater chests andthus most ships. seriouslyimpairtheoperation of the ship's Visibility is also important. not only for the engineering department. Ice forming in exposed fireplugs and water breakers may cause them to safety of the ship, but to determine what speed freeze and burst. Other cold weather effects are of advance is necessary to maintain schedules. extremelyprobable: hencethe meteorologist For example. if an extensive fog bank is ex- should realize his responsibilities and use his pected en route, the commanding officer may initiative to point out how the operations of his desire to increase his speed of advance while the unit, as well as the fleet, may be affected. visibility is good in order to reduce the speed for safety considerations while proceeding through The chill factor which is covered in detail in the fog. In port, the sailing time may be altered cold weather manuals should be understood and in the event that fog is expected at the sched- included in daily briefings. Here the meteor- uled time of departure. ologistwillfind himself concerned with the Cloud coverage and ceilings arc not normally protectionoftopsidepersonnelfromthe important, however, in time of war it may be weather elements (wind and temperature) and, desirableto operate in normally undesirable ina sense, concerned with how they clothe weather which would provide cover for the themselves. ship's movementstoaidinavoiding visual Specialevolutions must take place aboard detection. The use of frontal areas and squalls ship upon the occurrence of certain weather. for cover was not uncommon in World War II. Aerographer's Mates should be familiar with Bad weather can be of use to someone. these evolutions, the officers responsible for Forecasts of conditions affecting the capabili- initiating actions, and the weather conditions ties of radar and sonar equipment are required under which theywill need to happen. An of the meteorologist. This is becoming increas- EVOLUTION CHECKLIST should be prepared, inglyimportant.andmeteorologists should listing the weather conditions and the individu- become acquainted with pertinentfacets of alsto benotified. Some examples of these oceanography and the sonar qualities of the evolutionsareasfollows:(I) Heavy rains- various ocean currents and should understand Gunnery Officer; (2) Fog -00D (5) Freezing how weather conditions affect sonar and radar temperatures, Engineering Officer. This evolu- performance. tion checklist should be as complete as possible

476 48? Chapter 13 WEATIIER BRIEFING AND FLIGIIT FORECASTING and therefore requires consultation with many duce timely and accurate weather bulletins for boy department heads. not only its air group and self but its small Radiological fallout predictions and ballistic escort vessels as well. wind forecasts should be prepared upon request Aboard carriers such as LPIrs or the newer and at any time when the situation demanding LI1A's special requirements for such information such forecasts arises. Such forecasts are of value as density altitude and pressurealtitude may to carriers as well as to the othersurface ships. exist. The many smaller ships. including Light and the ships in company not having weather Airborne N1111E-Purpose System( LAMPS) units should receive these forecasts w hen neces- equipped vessels and minesweeping forces may sary. develop more special weather requirements with increased helicopter operations. Although mine- CARRIER OPERATIONS sweepers and destroyers withLAMPS are not technically carriers. they still require considera- Each of the various ty pes of aircraft carriers tion as ship-aircraft teams and aresignificant to are subject to being affected by almost every the thoughtful forecaster. known weather element. Wind force aboard a carrier takes on added AMPHIBIOUS OPERATIONS importance in that there is such a condition as too little w Ind. All Larders need about 30 knots Amphibious operations are probably the most a,.ross the flight dell, to insuresafely conducted demanding type of operation requiring accurate operations. Greater w Ind speeds are desired for weath,:r information. Every imaginable under- jet aircraft operations. flowever, with too much taking (airborne. surface, or subsurface) found wind across theflight deck.it may become in amphibious operations issomehow affected difficult to move aircraft about the deck and by weather elements. A meteorologist forecast- conduct launchings and landings. It has been ing weather should be fullycognizant of all known for a carrier to actually back down in phases of theoperationinorder tobeas order to conduct flight operations. The flight Valuable as possible. Meteorological considera- deck wind is often different than the apparent tionsarepresentinallphases of military wind at anemometer level: for this reason. hand planning. flight anemometer wind observations on the Meteorological considerations usually show deck may be necessitated. Catapult personnel that one season. month, or other period is more may be able to take the required spot measure- favorable for the planned 1) -day than another. In deck winds withthehand ments offlight making the evaluation.the following effects anemometer. Type and duration of precipitation take on should he considered: added importance. especially at near freezing temperatures. A sheet of ice forming onthe 1.Effects of wind and pressure upon tidal deck will make it more hazardous for the flight conditions. 2. Effects of wind. sea. and visibility upon deckcrew: and withaircraft operating the hazard becomes more acute. beaching conditions. unloading conditions, sonar The catapult officer can be aided by knowing conditions. and speed of ships and craft, both in advance of low temperatures in order to during the assault and during the period that adjust his equipment to care for sluggishness due beach maintenance continues. to weather causes. A temperatureof 32°F is 3. Effects of wind. temperature. and humid- critical and important aboard ship. ity on aerosols and smoke. With the advent of the CV as attack platform 4. Effects of visibility upon navigation and both for targets above and below the sea surface gunfire. 5. Effects of extreme temperatures on per- it isimperativethatAerographer's Mates be sonnel, equipment, and planned operations. cognizantof the interplay of the air-ocean environment. Usually in the role of flagship. the 6. Effects of weather conditions upon air carrier weather office can be expected to pro- operations.

477

483 AEROGRAPHER'S MATE 1&C

The following considerationsmay influence ous operations, but in the main, the foregoing the determination of the hour of landing: are his most pressing forecasting responsibilities. I. Wind and sea. Strong winds and heavy seas SUBMARINE OPERATIONS or swell materially affect the approach of ships, particularly landing craft typesto the assault area. Heavy weather may also cause transports As far as subsurface warfare is concerned, naval oceanography has already developedfar and other ships to roll excessively,therefore slowingthe enough to play a part in both operational and operation of hoisting out and materialactivities. Knowledge of the sound loading boats with assault troops and equip- conditions often makes it possible touse sonar ment. The--e-cfeebefht-avy weatherare particu- gear more effectively and is sometimes larly intensified at night. a factor in deciding the proper tactical deploymentof 2. Surf. Winds affect not only thestate of vessels. Knowledge concerning the subsurface the sea, but also tides, currents, and surf, and distribution of temperature andsalinityim- hence, beaching conditions. Dangerouscr even proves the diving operations of submarines and impossible swell or surf may also be causedby to a considerable degree affects their choice of distant storms when local winds are light. Strong offensive and evasivemaneuvers. winds and rough seas not onlymay delay the Itis of the greatest importance in submarine timing of boat waves, but may produce seasick- operation to be able to change depth efficiently. ness among the troops and rend::: many ineffec- It often takes an appreciableamount of time to tual prior to reaching the shore. Onexposed flood or pump the required amount ofwater. beaches, the height of the surfmay reach such During offensive or defensive operationsa delay proportions as to make landings impracticable. of a few minutes caused by faulty judgmentas Other beaches may be somewhat sheltered,but to the correct ballast change may be costly. The the high surf breaking over outlying barsor noise in diving operations is also an important shoals may render them unusable. consideration when operating amongenemy 3. Visibility. Navigation during theapproach ships that may be maintaininga listening watch. may be impeded by reduced visibility produced Maintaining efficient diving operations would by fog, haze or precipitation:at nighttheir be simple if the buoyance ofsea water were effects are greatly intensified. Thismay result in everywhere uniform. Since itis not, the varia- delayof thearrival of shipsintheinitial tions in density that occur make each divea transport area, where further delaysmay be separate problem, requiring slightly different suffered in the rendezvous of boats in compli- tactics. Temperature variations havea big influ- ance with the time schedule. Landing ships and ence on the density of water. With decreasing craft have difficulty in stationkeeping: under temperature the density will increase and vice reduced visibility conditions, theymay straggle versa. The density of sea water is generally and delay their arrival at the destination and dependent on its temperature, saltcontent. and correspondingly delay the execution of attack the pressure of the surrounding water. Density plans. increases when the salinity orpressure increases, 4. Flying conditions. Suitability offlying but it decreases when the water expands with conditions for military operations is determined increasing temperature. When these properties by a combination of factors, including cloud are known, the density can be determined cover, ceiling. visibility. wind, turbulence, and readily from standard tables. icing. Air observation in support of the assault Other properties of the ocean thata meteor- waves, pre-H-hour bombing of enemy defenses ologist should brief the submarinerson are the and beaches, and airborne operationsmay be water masses they will encounter in their restricted opera- or even eliminated by unfavorable tions and the effects thesemasses will have on flying conditions. their operations. They should be briefedon ocean currents and how to make the best use of There are many other operational problems them. Thetransmissionof sound wavesis that the meteorologist is faced with in amphibi- another consideration that should be discussed.

478 4.84 Chapter 13WEATHER BRIEFING AND FLIGHT FORECASTING

HELICOPTER OPERATIONS difficult, but very interesting task. The purpose ofthis sectionistohelpto equip senior Helicopter pilots are thoroughly familiar with Aerographer's Mates with the necessary informa- the flight characteristics of their craft and they tion to adequately perform flight forecasting determine the effect of meteorological elements duties. on their craft. The elements helicopter pilots are Although pilots are responsible for reviewing interested in are listed in the following para- and being familiar with weather roaditions for graphs. the area in which the flightis contemplated, The wind speed in combination with blade weather briefingsshallbe conducted by a rpm and forward speed can be criticalwith qualified meteorological forecaster, when avail- regard to the rotor blade efficiency and flight able.Thesebriefings may be conductedin safety:therefore, an urgentneed exists for person or by telephonic, autographic, orweath- accurate wind speed forecasts. Wind direction is ervision means. important from the navigational aspects of the flight. FLIGHT WEATHER BRIEFING FORM Helicopterpilots have akeeninterestin density and pressure altitude because it controls A DD Form 175-1, such as the one described the load capacity /operating ceiling of the craft. later in this section, is completed for all flights Weather units aboard ships or stations with to be conducted in accordance with instrument helicopters on board should take great pains flight mules, when military weather services are when computing density, pressure altitudes. and available. The forecaster completes the form for upper air temperatures to facilitate thehelicop- briefings conductedin person and for auto- ter pilots flight planning. graphic briefings. It is the pilot's responsibility to Snow and ice accumulations on the helicopter complete the form for telephonic or weather - prevent takeoff completely and must be re- visionbriefings.For VFR flights usingthe moved from the craft prior to takeoff. Freezing DD -175, the following ,.ertification on the flight precipitation and frozen precipitation are ex- plan may be used in lieu of a completed DD tremely dangerous to helicopter flying. and the Form 175-I: forecasting of these elements is critical in plan- Z. ning helicopter operations. BRIEFING VOID Surface temperatures control the type of FLIGHT AS PLANNED CAN BE CON- lubrication for certain systems of the helicopter DUCTED UNDER VISUAL FLIGHT and arc an important element in flight opera- RULES. VERBAL BRIEFING GIVEN tions involving helicopters. AND HAZARDS EXPLAINED. In addition to the above, there are the normal forecast elements of ceiling,sky, and flight visibility. Under bad weather conditions these elements may well determine the feasibilityof (Signature of forecaster) helicopter operations, especially atsea, even though most military helicopters are equipped Ifthe intended VFR flightplan includes with radar equipment. airfields having VFR minima higher than the basic1,000feet ceiling and 3 statute miles FLIGHT FORECASTING visibility, itis the responsibility of the pilot to advise the weather briefer of these higher min- One of the major tasks of Aerographer's ima. Mates First Class, and Chief Aerographer's Mates A Flight Weather Packet, such as the one is forecasting the weather for flight operations. described later in this section which includes a Much of your time is spent in completing Flight Horizontal Weather Depiction (HWP) chart, may Weather Briefing Forms and briefing and de- be requested by the pilot.Itis normally re- briefing pilots on the weather aspects oftheir quested that pilots allow a minimum of 2 hours flights. Flight forecasting is a comprehensive, for preparation of the packet.

479 485 AEROGRAPHER'S MATE 1 & C

NAVY FLIGHT RULES "broken" or "overcast" within controlzones must be at least 1,000 feet at point of departure, Due to the density of air trafficover the or,if a more stringent requirementhas been world today, itis imperative that there bean established, ator above the VFR minimum organized system for controlling aircraft. There prescribed in the aerodrome remarkssection of must be specific rules and regulations set forth the DOD Flight Information Publication(IFR for the safety of aircraft operations. Supplement). NOTE: A "Controlzone" nor- The following sectionson Visual Flight Rules mally refers to a circulararea with a radius of 5 (VFR) and InstrumentFlightRules (IFR), miles of the airport, andany extensions neces- which are based on Op Nav Instruction3710.70, sary to include instrument approach and depar- acquaints the Aerographer's Mate withthe flight ture paths. In addition, the weatherat destina- rules that deal with weather. tion must be forecast to remainat least 1,000 feet, or at or above the prescribed VFR published VFR minimums for that aerodromeduring the period 1 hour before until 1 hourafter ETA. Table 13-1 l'ststhebasic VFR weather 3. Aircraft may be operatedon a visual flight minimums. rules clearance above "broken clouds"or an Additional VFR weather minimumsare listed "overcast" provided climb to and descent below: from such "on top" flight can be made inaccordance with visual flight rules. 1. Destination must have 3 milesor more 4. Because of their special flight characteris- visibility at time of departure and be forecastto tics. helicopters may be flown at lessthan the remain 3 miles or more during the period 1 hour minimums established above, subject to appli- before until 1 hour after ETA. cable FAA regulations. VFR flight minimums for 2. The distance from the surface of the clearance of helicopters are establishedas 1 mile airport to the lowest cloud layer reportedas forward flight visibility and 500 foot ceiling.

Table 13-1.Basic VFR weather minimums.

Altitude Flight visibility Distance from clouds 1,200 feet or less above the surface (regardless of MSL altitude) 500 feet below. Within controlled airspace 3 statute miles... 1,000 feet above. 2,000 feet horizontal. Outside controlled airspace 1 statute mile (except as in 91.105(B))... Clear of clouds. More than 1,200 feet above the surface but less than 10,000 feet MSL 500 feet below. Within controlled airspace 3 statute miles.. 1,000 feet above. 2,000 feet horizontal. 500 feet below. Outside controlled airspace 1 statute mile . 1,000 feet above. 2,000 feet horizontal. More than 1,200 feet above the surface and ator 1,000 feet below. above 10,000 feet MSL 5 statute miles... 1,000 feet above. 1 mile horizontal.

480 486 Chapter 13 WEATIIER BRIEFING ANDTLIGIIT FORECASTING

IFR National Weather Service WW informationis unavailable. Air Force advisories do not constitute a National Weather ServiceWeather Watch The weather criteria used for IFR flights are Bulletin. Except for operational necessity, emer- as follows: gencies, and flights involving all weather research projects or weather reconnaissance, approval 1. IFR clearance is based onthe actual authorities shall not approve flights nor shall weather at the point of departure at the time of pilotswhopossessapprovalauthorityfile clearance, and forecast weather en route and at through areas for which the National Weather both the destination and destination alternate Service has issued an aviation severe weather

1 during the period 1 hour before until hour watch bulletin (WW) unless one of thefollowing after ETA. Existing weather may be used as exceptions apply: basis for clearance when no forecast weather is available and the pilot's analysis of available data progressed indicates satisfactory conditions for the planned a. Storm development has not for theplannedroute.In such flight.No clearance canbe authorizedfor asforecast destinations at which the weather is forecast to situations. the following rules apply: be below minimums upon arrival.unless an (1) VFR clearance may be permitted if alternate airfield is available which is forecast to existing and forecast weather for the planned be equal to or better than 3.000 feet ceiling and route permits such clearance. 3 miles visibility during the period I hour before (2) IFR clearance may be permitted if aircraft radar isinstalled and operative, thus until 1 hour after the ETA. Flights must be planned to circumvent areas of forecast atmos- permitting detection and avoidance of isolated pheric icing conditions and thunderstorms when thunderstorms. practicable. (3) IFR clearance is permissiblean positive control ar( as if visual meteorological conditions can be maintained, thus enabling aircraft NOTE. The following is an exception to the to detect and avoid isolatedthunderstorms. preceding rule: When urgent military necessity b, Performance characteristics of the air- dictates. Commanding Officers of aircraft carri- craft permit an en route flight altitude above ers, Commanders ofFleet Air Detachments. existing or developing severe storms. Marine Aircraft Group Commanders. and seniors inthe operational chain of command may 3. An alternate airfield is not required when be approveflightplansfor aircraft under their the weather at the destination is forecast to 3 cognizanceinweather conditions below the equal to or better than 3,000 feet ceiling and prescribed minima as specified in Op Nav Instruc- miles visibility during the periodI hour before

1 hour after the ETA. Otherwise, an tion 3710.7(). until alternate airfield is required. 2. Aviation Severe Weather Watch Bulletin issues (WW). The National Weather Service FLIGHT WEATHER BRIEFING FORM unscheduled WW's whenever thereisa high probabilityofsevereweather development, These WW's are for a designated area and a The Flight Weather Briefing Form, DD Form 175-1, specified time period. The WW's are used by the pro..idesa comprehensive record of weather Naval Weather Service for forecasting hazardous flight planning information for pilots and flight flying conditions. The Air Force issues sched- clearance authority. This form should be com- in uled MilitaryWeather WarningAdvisories pleted with the utmost diligence and care (MWWA). These graphicaladvisoriesare an accordance with the policies set forth in Nav- estimate of the weather producingpotential of WeaServCom Instruction 3145.1( ); it should be the existing air mass. These advisories will be completed by qualified Naval WeatherService given to all pilots filing from all U.S. AirForce personnel as set forth in NavWeaServComIn- bases and will be used for flight planningwhen struction 3140.5( ).

481 487 AEROGRAPHER'S MATE I & C

BRIEFING PERSONNEL completes the weather entrieson DD Form 175-1. The completion of Hight WeatherBriefing Forms and briefings of pilots will becarried out Entries by personnel who possess written authorization to conduct such briefings. All weather entries should be made in the To qualify as a meteorological/oceanographic Airways Code form (hourly sequence)as de- forecaster a candidate must satisfyone of the scribed in the effective edition of FMH No. 1, following academic requirements- Surface Obseavations. Temperaturesshould be entered in degrees Celsius (°C). wind directionin I.I Completion of a course of instruction in tens of degrees. and wind speed in knots. All meteorology at the USN Postgraduate School. dates/times should be entered in GMT. Flight 2. Attainment of a degree in meteorology levels and heights of all meteorological phenom- from an accredited university. ena should be entered in hundreds of feet MSL 3. Completion of AG"B" School. except as noted. ALL blocks on the form should 4Qualificationasameteorologicalfore- be completed as described in NavWeaServCom caster inthe USAF or the National Weather Instruction3145.1( ).Thisinstructionalso Service. contains instructions for orderingnew supplies of forms. The candidate' must alsofulfillall of the Figure13-1 is an example of entrieson a following general requirements. completed Flight Weather Briefing Form, DD Form 175-1. 1.Demonstrate proficiency in briefing and forecasting to the satisfaction of the Command- FLIGHT WEATHER BRIEFING PACKET ing Officer. Officer in Charge.or Chief' Petty Officer in Charge of the activity. 2. Be indoctrinated in local meteorological/ The flight weather packet. which is issued upon the pilot's request. is enclosed in a knee- Oceanographic phenomena (Local Area Fore- - casters Handbook) and in local operations and board size flight forecast folder for the convenience of the pilot. The reduced size (5" X 8") of procedures. Unless exempted by the Command- thefolderallows ing Officer/Officer in Charge. all flight forecast- pilotstoclipittothe ers will participate in familarization flights in kneeboard when logging en route weather. The their respective local flying areas. folder has entered on ita variety of weather 3. Be thoroughly familiar with current Inter- information as illustrated in figure 13-2. In addition to the forecast folder. the packet nationalCivilAviation Organization( ICAO) contains the DD Form 175-1. an 11WD (highor Regulations, and Navy Directives as they pertain lowlevel). to aircraft operations of the activity and upper wind chart(s) for the proposed flight level. When appropriate,a "ditch Personnel meeting the above criteria may be heading chart" for over water flights and "predicted altimeter setting charts" forover water be authorized to conduct meteorological/ocean- flights 1.500 feet or below, or when requested. ographic briefings for which qualified and to are enclosed. prepare forecasts for aircraft flights. This author- The charts enclosed should be prepared using ization will be in writing and signed by the standard entries as described on page two of the Commanding Officer. Officer in Charge. or Chief' forecast folder. Petty Officer in Charge of the activity.

WEATHER ENTRIES ON FOLDER DESCRIPTION DD FORM 175-1 Although the folderis illustrated in figure After thepilothas completed the Flight 13-2, the following description is provided for Clearance Form (DD Form 175), the forecaster clarity:

482 LIES Chapter 13 -- WEATHER BRIEFING AND FLIGHTFORECASTING

FLIGHT WEATHER BRIEFING

I. ....MISSION OAT( ACTT/NUMBED 0EP/(10 01ST /(TA ALTN/ETA 'IMEIFINGNO. /0 -/i 7/ Cl/7-423478 /430: /790 = /70-6 2 /23 AKEOFF DA.A II. srp IND Tim, DEV PRESSURE ALT DENsITY ALT OCR RUNWAY Tim., DETIPOINT PT 10- 2 ic 943 rt. 0 / 7 i'c 6- c 21106 LOCAL .(A WARNING OA m(T WATCH ADVISORY CETUS WINOS 26 12 NONk.- RimARKS/TAXIOFFALTO 'CST

ENROUTE DATA III. al LEVEL .11107 /TEMP PIT LEVEL/00 2530 2°C OLT LEVEL OUTSIDE CLOUDS 7 _MILES.Du( TO CLOUDS AT FIT LEVEL MINIMUM v SSSSSSSSS AT J ILA!( 0 TOG 0 AM(C ON 0 NO OISTmUCTION 0 YES 0 NO gg IN ANO OUT 0SmOAE 0OUST mtoimum TAMING LIVIL LOCATION .ARIMUM CLOUDS TOPS LOCATION MINIMUM CEILiko LOCATION TT mSLfiNRTE 12 FT AGL T / K / 6.6 FTmsi. RTE 90 (.1thin ton ;in: ttwto net(within1W:)14Nin tout. ' MP;"::;::m., Pout.)(.1thin %:$111::E01 ,0 t. not saeocistsi .0th rsrMS) (within net eRsOc2820d with 1144) associatod .0th rsrms) NONE 2 NONE 0000 MO CAT ADVISORY .:4,... DORRAIN%Om14.1(1 %MCLEANNCLOUDK...... , 010 mITED CLEAN NONE I 'AREA 1 ILI (XINONE LIGHT LIGHT TRACE ISOLATED I 2% MOD 000 LIGHT . ITEIT 3.1St 000 NAVY sCATTEmE0 11.400 SVM SHOOT NUmE0O0S.M0A1 TmAN ASS EXTREME TV* FR24 2:2:::4 ..:':'22. It HA L. 100 TURD, 5(0(00IZTMO. ..,,.,. LEVELS IND PgCCIPITATIOM EXTICTED INs''''' 76-- /2r7 LOCATION AMO NW ISTMS. LOCATION LOCATION LOCATION 4.1t/RT TERMINAL FORECASTS IV. VIS/WEA SFC 7211O ALTIMETER VALID TIME °ESTIMATION CLOUD LAYERS Pik 1208083 2612 29.921.0/64c 0T. /840 0

ALTERNATE TAR 40 CD 6 t 32 /5.30.06,0s1656 . /856 z INTmED STOP INS 2 TO

INFURO STOP INS 2 TO

COMMENTS/REMARKS VI

BRIEFING RECORD VI. Li NOT AVAILASLE VOID Tim( /50 0 1011110 ON LATEST 'CR TOR 01ST AND ALTO lag YES ExT0440 TO REQUEST PIDEP AT TOMICAST TITS SIGNATUmE VIA REDDIET120 AT SE( rtiml MO 0(1 ORIMO /400 FOWAST(01.1 INITIALS STOO I 04041 CMA101 NA.' OF PINION DECtIvIND timarixa 010 'Wry TAI( NO. I CDR C. AcoNk oY :1 z 1 /V Q A PLATE HO. 16411 4.1717 5/14 0102.001.$701 v O. ors, ;,0,707.175 -1 VIOUS COITION VIE

AG.672 Figure 13-1.Example of entries on Flight WeatherBriefing Form, DD Form 175-1.

483

4 R9 AEROGRAPUER'S MATEI & C

U, S. NAVY RIGHT FORECAST OFNAv 1046 VANTS (144. 1040

( /SEO MaTHIS 44 1-2 02302 j 7 Jan 1971 AGC H O. MOORE

CDR R,P,NZLSON .4, TA

07,91409Z__ ITT . _ 10708001_

120 .4,10.0 100400 - 4..k - 12004 Si

Moderate turbulence vicinity%124

%Of, 0% WftficArAl: 044444 tiPkOit% io6,t 4443, 4,,., of stipoll,Ant 4,10w/in...I Are deigned 3$ Its, e,s1t4. or mote go eeeeee Ind All fumulontmbus

411 u,/ Amount, ate ente,e,f .n e.,hth. , snd hAt std. to 144164 Ate eeeeee to honrIreers 6/ feet tho.e raP sr, fete( e 46 0,....16,0. om4 l4s P. 1060leer 4' 1-6 lops 5000 feet

J The ter.. deg!, 1..4s ...the.. ,.entered .zero At .n..... 9, M,, WO feet 10044 (eelIwo( 1400 feet 4 0,,,4a1 £414.4 44vis44s l..r "Chl...1 to...1..0 .4 its4., 4ostutboroos

AG.372 Figure 13-2.Forecast folder, OpNavForm 3140/25.

I. Thesectionentitled "FOLDER IN- commanders who commonlyuse the AIREP CLUDES" on page one of thefolder lists all FLIGHT LOG for transoceanic forms and charts that must be flights should be included in all encouragedtocontinueto o so as these packets. Other inclusions shouldbe listed in the A1REPS provide key datafor computer analysis blank spaces provided. centers. 2. Page two lists thesymbols used on the 4. Page four is enclosed charts. a self-mailer for the folder. The issuing weatheractivity should enter its 3. Page three includes the EnRoute Flight address to the right andbelow the guide dot. Log which may be used in lieu ofthe AIREP The receiving weatheractivity should review the FLIGHT LOG, OpNav 3140-30,and should be En Route Flight Log portion,enter its address in brought to the attention of thepilots. Aircraft the return address section,remove all staples,

484 490 Chapter l3 wEATIIER BRIEFING ANDFLIGIIT FORECASTING

01110uTErootLOG N,A, 5. AA. % A h A. , A.,1/4 5 og0. h he( IA1h1 A1A1A.C. (. \5 tA:1IAZi 1 3 \ 0

A V DC

A 1

N S h %

es am ,,A U, ' IA At.

AG.373 Figure 13.2.Forecast folder, OpNav Form3140/25Continued.

Briefing Neal the open side with a gummedfastener. and I.1)1) Form 175-1. Flight Weather of receipt. the Form. This formis completed as described mail the form within 72 hours this chapter. An original and two Al REP FLIG1IT LOG isusedinstead of the En earlierin enclosed copie$ are completed. Copy twoshould be Route Flight Log portion. it should be in the weather office for 3 in the folder. retained on file months. DESCRIPTION OF INCLUDED 2.HorizontalWeather DepictionChart CHARTS AND FORMS (11W1)). The 11WD chart is basically astrip chart weather presentabrief containingoperationallysignificant The followinu paragraphs Flight Weather description of the charts and formscontained in data. This chart is required in all Packets. The basic chart or charts tobe used the Flight Weather Packet.

485 491 AEROGRAPIIER'S MATE I & C

may be prescribed by individual activities.When- with the necessary information ever possible. base charts. listed in procedures. and Section 4 of techniques to preparean adequate flight briefing the Department of Defense"Catalog of Weather for aviators. PlottingCharts," NA -SO- I G-518.shouldbe used. HORIZONTAL WEATHER DEPICTION (HWD) CHARTS The size of the chart usedfor the IIWD chart included in the Flight WeatherPacket should be The IfWD is a grog chart for no wider than 8 inches and a fixed time no longer than as near as possible to mid-time of theflight. and necessary to depict the flight path.The overall portrays all forecast hazards to flight, size of the chart should be related kept to a minimum weather patterns, and significantpressure and consistent with legibility of entriesto facilitate frontal systems. However.when prepared from ease of handling within the cockpit. Char: scales available facsimile/NEDNcharts, they %%ill verify from 1:10.000.000to 1;20.000.000 art. ideally at the 6-hrly synoptic time suited to this purpose. nearest the mid-time of the flight. Two basiccategories of HWD IIWD's are discussed in more detail inter in charts will be issued. dependingon the proposed this chapter under Flight Forecasts. flightlevel. The low -level impcovers the 3. Upper Wind chart. The Upper Wind chart. stratum surface to 400 mb; the iai4hlevel HWD valid time nearest midtime ofthe flight, at or covers the stratum 400 to 150 nab. near flightlevelshould be includedinthe packet. The proposed line offlight should aiso be entered on this chart. Low Level HWD 4. Ditch .,.eading, chart. Thischart. consisting of plotted ditch headingarrows or point values. The following steps outlinethe rroi.edure for should be included for allover water flights. preparing a low level HIM 5. Predicted Altimeter Settingchart. This chart contains a plot of pointvalues of predicted 1. Show aundaries of weatherareas of altimeter settings and should beincluded for all significant cloudiness (5;8'sor more cloudiness, over water Bights 1,500 feetor below, or when including cirriform whichwould prevent a celes- requested. tial fix) and all cumulonimbusareas by a black 6. Miscellaneous charts. Anyother operation- scalloped line. ally necessary charts forspecific flights should 2. Enter cloudamounts in OKTAS, cloud be included;e.g..ConstantPressure charts. type: and height of bases andtops in hundreds of feet above MSL. within Streainlinecharts. Surface Wind charts.Sea the appropriate areas. Surface Temperature charts.etc. Use standard abbreviations forcloud type. Enter the height of the cloudtop above the height of the base and separate witha horizontal line. FLIGHT FORECASTS When more thanone layer is forecast to occur. enter the higher layer directlyabove the entry Flight forecasts consist ofthe oral, written for the lower layer. Sonic . nple entries are: and pictorial statementsto the pilot relating the expected weather along his flightroute and at his destination. Flight forecasts 7 AS190 include horizon- -1715 tal weather depiction charts (HWD):route and terminal forecasts;pressure pattern flight (for flight 6 CB 10=-- . planning);icing:contrails; turbulence; 30 'POPS TO 230 cloudcover; ceiling; weather; visibility; and winds, all incorporated intoa flight briefing for the type of aircraft flown and 3. Outline areas of clear airturbulence (CAT) the type of or other WW with a heavy dashedblack line, operation planned. The followingsections of this chapter provide the Within these areas indicatethe degree of CAT Aerographer's Mate with appropriate symbolfollowed immediately 486 492 Chapter 13 WEATHER BRIEFING AND FLIGHTFORECASTING by the height of the base and top ofthe CAT contained on the Form DD 175-1, a section stratum in hundreds of feetand/or appropriate containing appropriate blocks for displayingthe symbol for other WW. information may be appended to the HWD. 4. Depict frontalpositions using standard Figure 13-3 is an example of a typical Hori- analysis symbols and colors and pressure centers zontal Weather Depiction (HWD) chart. with large "H" or "L" in standard colors with centralpressureinmillibars entered below. UPPER LEVEL PROGNOSIS Indicate direction and speed of movement using vectors for direction with speed in knots atthe The upper level prognoses are prepared for a end of the vector. standard isobaric surface nearest the flight level 5. Indicate the 0° C isotherm with its height with a fixed valid time as near as possible to the in increments of 5000 feet above sealevel as a mid time of the flight. This chart supplements dashed green line. the horizontal weather depiction chart. 6. Indicate areas of significant weather,ob- The following steps outline the procedures "--r structions to vision. and icing. with appropriate preparing an upper level prognosis chart: symbols followed by heights of the base and top of the phenomena in hundreds of feet. 1. Draw contours for every 60 meters up to 7. Enter the flight route as a solid brownline. the 300 mb level and for every 120 meters for 8. Place an identifying legend in thelower the 300 mb level and above in solid black lines. marein of the chart. Usedashedintermediatecontours when needed to clarify the flow. Double the interval EXAMPLE: LOW LEVEL HORIZONTAL incase of averytightgradient.Labelall WEATHER DEPICTION contours atloose ends and above or below Issued by: NWSED Memphis, Tn. closed centers with height in tens of meters. VT 0600Z 26 April 1973 Contours may also be labeled at anyother position also as required to make the chart more "VT" is the valid date and time of the chart. "Issued by" is the weather office at which the easily readable. 2. Draw and labelisotachsfor every 20 chart was prepared. knots. 3. Draw isotherms for the level in increments High Level HWD of 5° C using short dashed red lines. Enclose The following steps outline the proceduresfor label-value in a square CP preparing a high level HWD: 4. Show jet stream axis as a heavy dashed purple line with an arrowhead to show direction. I. Using the same techniques as forthe low 5. Indicate flight track as solid brown line. level HWD, depict all areas of 5/8's or more 6. Place an identifying legend in the lower cirriform cloudiness at or above flight level; all margin of the chart similar to the one used for areas of convectiveclouds extending to the the HWD chart. proposedflightlevel:allareas of icing or turbulence including CAT; all areasof severe ROUTE AND TERMINAL weather published by Aviation SevereWeather FORECASTS Warnings (WW) or other advisory; andsurface fronts and pressure centers (with directionand Of allthe sources of weather information speed of movement). available to the weather office in thecontinental 2. Enter the flight route as solidbrown line. United States, Alaska, and Hawaii, undoubtedly 3. Place an identifying legend in thelower one of the most valuable is theteletype report. margin of the chart in the same manner asthe It consists of sequence reports, regional synop- low level HWD. ses, area forecasts, windsaloft forecasts, and route and terminal forecasts. National Weather Service Weather Service Whenlocalpolicies or regulations require operational data additional to thatwhich is Forecast Offices (WSFO) issue specificforecasts 487 493 100 -1 *dr.!". s.

,-.0 of' m0 El. .6 ( ' O. NN 0 4. o fissxo4 yo.P(0.0 ISSUEDLOW LEVEL BY: HWD ...... 0 toe VT: 1700Z NAS5 GUANTANAMDAPRIL 1973 BAY Figure 13.3,Horizontal Weather V Depiction Chart. eff ' 610117. OS. AG.673 Chapter 13 WEATHER BRIEFING AND FLIGHTFORECASTING Outside the forresponsibleareasatprescribedtimes augment or alter them on occasion. throughout a 24-hour period, and these forecasts United States you will often be required to are transmitted over the teletypein code. using make your own forecast for the whole route. many of the sequence reportsymbols, abbreviations. and contractions. Terminal Forecasts For complete information regarding the for- Terminal forecasts for most airports inthe refer to the Flight mats used and times of issue, United States are issued several times dailyand Services Manual ATS 7110.10 part II. revised in the light of further developmentby Outside the continental United States, Alaska, the regional Weather ServiceForecast Offices and Hawaii, these services are provided on a (WSFO) and disseminated over the Service A much more limited basis and often are not teletype network for use in flight forecasting. available at all.In those cases. Aerographer's Mates are required to formulate their own route It. the United States, then, a flight forecaster's availabil- and terminal forecasts. workload is greatly reduced due to the ity of terminal forecasts. Terminalforecasts are COMET 11: Route Forecasts also available at r,,ilitary stations on TAF or PLATF code form is used. A pilot must know the weatherconditions Outside the United States, terminal forecasts that now exist and the forecast conditionsfor may notreadilybe availablealthoughthe his flight route in order to prepare anefficient meteorologicalservicesof mostnations do TAF code and safe flight plan. The information hereceives promulgate terminal forecasts in the from the weather office will help him tomake form. Itis aboard ship that terminal forecasts available, and it is then that an efficient flight. are least likely to be The pilot and duty forecaster shook' study the Aerographer's Mate must formulatehis own terminal various dataanddiscuss the weather. The forecast. Helpful details for formulating forecaster must be sure the pilotthoroughly forecasts may be found in chapter 10of this understands the weather which hewillen- manualandAirWeatherServiceManual counter. The forecaster and pilotshould study 105-51/1 Terminal Forecasting. and discuss the surface synoptic charts,noting Although the terminal forecasts provided by any low-pressure areas andfronts that are in the the various meteorological services are a great general area of the flight route. Possibilityof fog help to you atalltimes,itis inadvisable to andlow stratus must be considered.Hourly accept all the terminal forecasts atface value. sequence reports must bestudied. The winds Scrutinize the forecasts and make suchrevisions aloft and constant pressure charts shouldbe to them as changingsynopticfeatures may used in determining a satisfactory flightaltitude. dictate. Various levels should be taken intoconsideration, since the desired altitude may notbe granted by Air Traffic Control. Upper airsound- PRESSURE PATTERN FLIGHT ing charts should be studied to determinethe thickness of Soon after World War II, a systemof flight temperature at various altitudes. flying, was cloud layers. icing conditions, andstability of air planning, known as pressure pattern flight masses. popular as a means of effecting minimal All available forecasts for the flight routeand time for long distances. For this system,meteor- and forecast destination should be read. ological personnel had to compute The route forecasts are issued by the various D-values, and then construct D-valuecharts. is exceedingly rare for an Aerograph- regionalWeatherServiceForecastOffices Today, it (WSFO) for the various airways in theUnited er's Mate to be required to construct aD-value States. Ordinarily, these routeforecasts are of chart, but itis quite common for him to have to the highest quality: neverthelessthey may be provide D-values for specific points or areas. questioned. In the light of later information or Consequently. only determination ofD-values will be discussed in this section. developments,it may become necessaryto 489 485 AEROGRAPHER'S MATE 1 & C

Computing D-values forecastsin view of developments occurring sometime in the verifying period. Outsidethe A D-value describes the height ofa pressure continental United States, these servicesmay surface by its departure from "standard"height. not be readily available, and it is thepurpose of this section to furnish Aerographer's Mateswith D = Z Zp the knowledge necessary to successfully forecast flight level winds. Where: Z is the standard atmosphere altitude The basic approach to forecasting winds aloft above mean sea level. is to prognosticate the constantpressure surface Zr,isthe actual altitude (pressure nearest tothe desiredlevel and extrapolate altitude from a radiosonde sounding) of the upward or downward. It is permissibleto local- same point in space. ize the forecast area so long as sight isnot lost of large scale developments which might affectthe For example: The height of the 700 mb level forecast point. is 9880 feet in the Standard Atmosphere. If the Proceduresfor progging constantpressure actual height of the 700 mb level ata certain surfaces are given in chapter 8 of this manual. point is 10.200 feet. the D-value is The forecast wind can be picked offthe prog chart by simply reading the wind direction, D = 9880 10.200 = -320 feet measuring the gradient, and obtaining the speed. Isotherm-contour relationshipsare of value D-values are of significance for aircraft flying when forecasting upper winds in that theymay with the altimeter set at 29.92. In the above simplify the task a great deal under certain example, the altimeter of an aircraft flyingat circumstances. When the forecast wind direction the 700 mb level would read 9830 feet, but its is easy to obtain, the speed may be estimated by actual altitude would be 10,200 feet abovesea relating isotherms and contours. Increasing wind level. Over water, its radar altimeter would read speeds occur whenever the thermal gradient the latter value. increases, as the wind moves the isotherms closer together and decreasing wind speedscan be Forecasting D-values safely forecast when the direction and speed of the wind and orientation of the isotherms is Forecasting consists of forecasting the actual such as to spread the isotherms furtherapart as height of the given pressure surface and then illustrated in figures 13-4 and 13-5 respectively. , computing the D-value as shown above. When isotherms and contours are in phase and FLIGHT LEVEL WINDS parallel,little change in speeds and direction need be anticipated in a short-range forecast. Forecasting flight level winds or winds aloft in general is an integral part of flight forecasting. Within the continental United States, Alaska, ;.,,nd Hawaii, the task is somewhat simplified in that the National Meteorological Center regu- larlytransmitsprognostic constantpressure charts and winds aloft charts on the facsimile system. The facsimile transmissions are further supplemented by winds aloft forecasts sent out by the various regional Weather Service Forecast Offices (WSFO) ontheService C teletype system. These charts and forecasts may be used byflightbriefing personnel without further modification under most circumstances, though AG.674 itisa good idea to constantly revise these Figure 13-4.Increasing wind speed pattern.

490 4136 Chapter 13WEATHER BRIEFING AND FLIGHT FORECASTING for clearance. In briefing, definite statements must be made regarding cloud bases and tops, 92 freezinglevel, icing zones, turbulence, winds aloft, and frontal weather to be encountered, 96 and possible contingencies resulting from variation of expected trends. 98 Since each flight presents its own peculiar problems in regard to planning and weather __-10C 100 briefing, it is not possible to establish a standard procedure for briefing that will be complete and

102 adaptable to every case. However, experience 700 MEL 10C has shown thatthereisusuallyadefinite sequence of thought that should be followed AG.675 when briefing a pilot. This sequence can be Figure 13.5.Decreasing wind speed pattern. logically divided into the following five general steps: When isotherms and contours are in phase, but not parallel, or out of phase, thesituation 1. Give a brief description of the current should be studied more closely, and the prog weather conditions associated with the line of should be carried out. flightinterms of ceilings, visibilities, cloud layers, winds, turbulence, and icing, as may be FLIGHT WEATHER BRIEFINGS appropriate. A brief discussion of the synoptic situationas related to theflight should be Procedures for preparing HWD charts; for included. making route and terminal forecasts; for deter- 2. Give a brief statement of the latest se- mining pressure pattern flight routes; and for lected observations along and near the flight turbulence, and route. forecastingicing,contrails, brief statement of the forecast flightlevel winds have been discussed in the 3. Give a foregoing sections of this chapter. trends of en route weather, identifying significant changes as to expected time of occurrence, Methods for forecasting ceilings, weather, and and emphasizing areas in which severe weather visibility are discussed in chapters 10 and 11 of warnings are in effect or forecast. this manual. 4. Give a statement of the expected changes It remains now to integrate the results of the at the terminal and alternate, relating significant procedures into a flight briefing and to adapt the forecast changes to the expected time of occur- briefing to the type of operations planned. rence. 5. Point out any alternate routes, as appropriate, and discuss weather contingencies. This is CONTINENTAL FLIGHTS the time to volunteer pertinent weather information not included in the preceding discussion It is the responsibility of the flight forecaster and to answer any specific question the pilot to give the pilota complete description of may have. existing and expected weather along the route and at the terminals of interest. Upon the flight forecaster's interpretation of the weather will If the pilot requested an HWD chart, present partially depend the type of clearance filed by it at this time and discuss the "intents with the the pilot. Meticulous consideration of the latest pilot. charts and data are necessary for making a Last but not least, refer the pilot to the reliable forecast for cross-country flights. Air- various charts on display and discuss them with way reports alone are not to beused as the basis him.

491 497 AEROGRAPHER'S MATE 1 & C TRANSOCEANIC FLIGHTS clock, by making maximumuse of readyroom briefings. adequate briefingsmay be made. Most Transoceanic flights normallyoperate with aircraft carriers now have weathervision limited fuel loads, and systems a point of no return is with hookups in the readyrooms,and excellent computed by thepilot based on the wind results are being obtained through forecast. Therefore, it maximum is essential that all perti- and efficient use of thissystem. nent weather data be availableto the pilot for advance planning. Aerographer's Mates servingas flight fore- The flight packet that is prepared casters must know the type ofoperations in for trans- which his ship is participating oceanic flights is similar tothe one discussed for in order to stress continental flights except that the more important elementsin his forecast. a ditch heading Weather information for fighter chart must be included forthe transoceanic aircraft differs flight and if requesteda predicted altimeter from that desired for ASW flightsin that fighter setting chart. aircraft normally fly at higheraltitudes while The problem of obtaining low-level and sea conditionsare more important foreknowledge of to ASW operations. pending flights is important. Inasmuchas the thought and work whichgo into a transoceanic Normally a weather forecastfor a flight is for flight forecast must be thorough,flight fore- a short period of time, while the planningand casters should have a minimum of 2hours of scheduling cover a much longer period.Aerog- advance notice, althoughan 8-hour notice is rapher'sMateswillfindthattheirbiggest desirable in order to have the materialready for problem is forecasting 24to 48 hours ahead so briefing. This problem is solved byclose liaison that scheduling may be completed.A carrier can between meteorological personnel andpilots. move to an area of better weather whilea shore When the forecast is complete, it isorganized base has to sit tight and wait itout. Depending in folder form to be presentedto the pilot and upon the flexibility of the operatingarea, it may fully discussed during the briefing.The number be possible to conduct operationsalmost con- and size of charts should be heldto a minimum tinually by shifting toan area where the desired tosavespace, and the material should be flight operations are feasible. organized in a neat and readily usablemanner. Surface and upper windsare very important elements for carrier aircraft. Thisis especially truefor fighter aircraft with onlythe pilot CARRIER AIRCRAFT BRIEFINGS aboard who may find himselftoo busy to take note of the winds to check his drift. Normally, Flights of carrier-based aircraftare normally the CIC of a carrier tracks all aircraftwith radar classifiedasoperational and, as such, have and gives bearings and distancesto the aircraft flexible weather minimums whichare decided via radio. However, it is possiblefor the ship to upon by the officer in tactical command (OTC). lose contact with the aircraft. Theaircraft then The deciding factor for conducting flightsin the is on its own andmay have to rely on forecast event of unfavorable weather conditions will winds to navigate back to the ship. normally be the importance of the mission. The Visibility may be important onlyfor launch- weather briefing of the OTC and/orthe coming and landing aircraft; however in manding officer of the carrier quite the case of frequently fighters or dive bombers, ceiling andvisibility in will be of considerable importancein deciding the target area are important. Radar whether the scheduled flight operations has reduced will be the visibility requirement for navigationto some conducted. however, do not forget thepilots. degree. For visual searches, it is still Whileit important. isobviously difficultto conduct Horizontal Weather Depiction charts briefing of thepilotsfor for use allflights when in flight are not normally practicalfor carrier operationsarebeing conducted around the aircra ft.

498 492 CHAPTER 14

SEA SURFACE FORECASTING

The task of forecasting the various elements When a wind starts to blow, the sea surface concerned with the sea surface, such as sea instantaneously becomes covered with tiny rip- waves. swell waves, surf. and surface currents ples which form more or less regular arcs of long will be encountered by senior Aerographer's radii. As the wind continues to blow, the ripples Mates filling a variety of billets. increase in height and become waves. Aboard carriers sea condition forecasts for A wave is visible evidence of energy moving flight operations, refueling, or underway replen- through a medium, in this case the water, in an ishments will be provided on a routine basis. undulatingmotion.As theenergymoves Staffs of larger facilities will generally provide through the water, there is little mass motion of surf forecasts for amphibious operations, while the water in the direction of travel of the wave. atairstations supporting search and rescue This can best be illustrated by tying one end of a (SAIL) units, forecasts of sea conditions and rope to a pole or other stationary object. Wen surface currents may be required at times. the free end of the rope is whipped in an up and Itistherefore important that personnel be down motion, a series of waves move along the familiar with these elements and be able to rope toward the stationary end. There is no mass provide forecasts as necessary. motion of the rope toward the stationary end, In this chapter we will discuss these elements only the energy traveling through the medium, as well as methods that may be utilized to in this case the rope. forecast the extent of them. A SINE WAVE is a true rhythmic progression. The curve along the centerline can be inverted SEA SURFACE CHARACTERISTICS and superimposed upon the curve below the centerline. The amplitude of the crest is equal to To accurately forecastsea conditionsitis the amplitude of the trough, and the height is necessaryto understand the process whereby twice that of the amplitude. Sine waves are a waves si re developed, the action that takes place theoretical concept seldom observed in reality. as the energy moves, and to have a thorough They are used primarily in theoretical ground- understandingofthevariousproperties of work so that other properties cf sine waves may waves. be applied to other types of waves such as ocean This section of the chapter will discuss this waves. Principles of other types of waves are background information and terminology used. modified according to the extent of deviation of A complete understanding of these items is their properties from those of sine waves. necessarytoproducethemost usable and Waves which have been created by the local accurate sea condition forecast. wind are known as SEA WAVES. These waves are still under the influence of the local wind BASIC PRINCIPLES OF and are stillin the generating area. They are OCEAN WAVES composed of an infinite number of sine waves superimposed on each other, and for this reason Ocean waves are advancing crests and troughs they have a large spectrum, or range of frequen- of water propagated by the force of the wind. cies. 493 499 AEROGRAPIIER'S MATE 1 & C

Sea waves are very irregular inappearance. generating winds they become SWELL WAVES. This irregularityapplies to almostalltheir For the reason that swellwaves are no longer properties. The reason for this is twofold: First, receiving energy from the wind, theirspectrum thewindinthegenerating area(fetch)is of frequencies is necessarily smaller than thatof irregular both in direction and speed: second, sea waves. They are smoother and more regular the many different frequencies of waves gener- inappearancethansea waves. Figure14-2 atedhave different speeds. Figure 14 -Iis a illustrates typical swell wave conditions. typicalillustrationof sea waves. The waves found in this aerial photograph are irregular in PROPERTIES OF WAVES direction, wave length, and speed. As the waves leave the generating area (fetch) Allwaves have the following properties in and no longer come under the influence of the common:

449 ti f3

try

ics

t. 40 kts

AG.226 Figure 14-1.Aerial photo of sea waves.

494 500 Chapter 14 -SEA SURFACE FORECASTING

Figure 14-2.Swell waves.

1. Amplitude. The amplitude of a wave is the 3. Period (T). The period of a wave is the maximum vertical displacement of a particle of time interval between successive wave crests, and the wave from its rest position. In the case of it is measured in seconds. ocean waves, the rest position is sea level. 4. Frequency (0. The frequency of waves is 2. Wave Height(H). Wave heightisthe the number of waves passing a given point vertical distance from the top of the crest to the during I second. Itis the reciprocal of the bottom of the trough. Wave height is measured period. In general, the lower the frequency, the in feet. Three values for wave height are deter- higher the wave; the larger the frequency, the mined and forecast. They are: smaller the wave. This can be seen when you a. Have (the average heightof the waves). consider that when at a given point the fre- This average includes allthe waves from the quency is 0.05, considerably fewer waves(there- smallest ripple to the largest wave. fore larger waves) are passing the point than if b. HA; (the average height of the highest the frequency were 0.15. one-third of all waves). The significant height of waves seems to represent the waveheights better 5. Wave Length (L). The wave length is the than the other values, and it will be used most horizontaldistance between two successive often for this reason. crests or from a point on one wave to the c. H1110 (the averageheight of the highest corresponding point on the succeeding wave. one-tenth of all waves). H1110 is used to indicate Wave length is measured in feet, and itis found the extreme roughness of the sea. by the formula: L = 5.I2T2.

495 501 AEROGRAPIIER'S MATE I & C

6. Wave Speed (C). The wave speed is therate 10. Effective Duration Time. The effective with which a particular phase of motionmoves duration time isthe duration time which has along through the medium. Itis the rate at been modified to account for thewaves already which a wave crest moves through thewater. present in the fetch or to account forwaves (herearctwo speeds usedin ocean wave generated by a rapidly changing wind. forecasting: group speed and individual speed. I I. E Value. E is equal to thesum of the the group speed of waves is approximately squares of the individual amplitudes of the one-halfthatof theindividualspeed. The individual sine waves whichgo to make up the individual wave speed in knots is found by the actual waves. Since it is proportional to the total formula C = 3.03T. The group wave speed is energy accumulated in these waves, it is used to found by the formula C = 1.515T. describe the energy present in them and in several formulas involving wave energy. Definitions of Other Terms 12. Co-cumulative Spectra. The co-cumulative Other definitions with which the Aerograph- spectra are graphs in which the total accumu- ei's Mate should be familiar are as follows: lated energy is plotted against frequency fora given wind speed. The co-cumulativespectra have been devised for two situations: 1. Wave Spectrum. The wave spectrum is the a fetch limited wind and a duration time limited wind. term which describes mathematically the distribution of wave energy with frequency and 13. Upper Limit of Frequencies (fu). The direction. The spectrum consists ofa range of upper limit of frequencies represents the lowest frequencies. valued frequencies produced by a fetchor that 2. Deep Water. Water that is greater in depth are present at a forecast point. This term gets its than one-half the wave length. name from the fact that the period associated 3. Shallow Water. Water that is less in depth withthisfrequencyisthe period with the than one-half the wave length. highest value. The waves associated with this 4. Fetch (F). An area of the sea surfaceover frequency are the largest waves. which a wind with a constant direction and 14. Lower Limit of Frequencies(fL). le speed is blowing, and generating sea waves. The lower limit of frequencies represents the highest fetch length is measured in nautical miles and valued frequencies produced by a fetchor that has definite boundaries. are present at a forecast point. This term gets its 5. Duration Time (t). The duration time is name from the fact that the period associated the time during which the wind has been in .vith this frequency is the period with the lowest t.on tact with the waves within a fetch. value. The waves associated with this frequency 6. Fully Developed State of the Sea. The are the smallest waves. fully developed state cf the sea is the state the 15. Filter Area. The filter area is that area sea reaches when the wind has imparted the betweenthefetch andtheforecast point maximum energy to the waves. through which swell waves propagate. This area 7. Non-Fully Developed State of the Sea. The is so termed because itfilLis the frequencies non-fully developed state of the sea is the state and permits only certain ones to arrive at a of the sea reached when the fetch or duration forecast point at a forecast time. time has limited the amount of energy imparted 16. Significant Frequency Range. The signifi- to the waves by the wind. cant frequency range is the range of frequencies S. Steady State. The steady state of the sea is between the upper limit of frequencies and the reached when the fetch length has limited the lower limit of frequencies. The term significant growth of the waves. Once a steady state has range is used because those low-valued frequen- been reached, the frequency range produced will cies whose E values are less than 5 percent of the not change regardless of the wind. total E value and those high-valued frequencies 9. Wind Field. The wind field is a term which whose E values are less than 3 percent of the refers to the fetch dimensions, wind duration, total E value are eliminated because of insignifi- and wind speed, collectively. cance. The significant range of frequencies is

496 502 Chapter 14 -SEA SURFACE, FORECASTING used to determine the range of periods present at the forecast point. FREQUENCY RANGE 17. Propagation. Propagation, as applied to ocean waves, refers to the movement of the swell through the area between the fetch and the 05 06 .07 .08 .09 to forecast point. LOW VALUES NIGH VALUES 18. Dispersion. Dispersion is the spreading out effect caused by the different group speeds of AG.676 the spectral frequencies in the original disturb- Figure 14-3.A typical frequency range of ance at the source. Dispersion can be understood a wave spectrum. by thinking of the differentspeeds of the different frequencies. The faster wave groups 40 kt, 0.0619. Table 2.1, page 36 in H. 0. 603, will get ahead of the slower ones: the total area gives the complete table of fma, values and the covered is extended thereby. The effect applies corresponding periods for wind speeds, starting to swell only. from 10 kt, at 2-kt intervals. Notice that the 19. Angular Spreading. Angular spreading re- frequency decreases as the wind speed increases. sults from waves traveling radially outward from This suggests that the higher wind speeds pro- the generating area rather than in straight lines duce higher ocean waves. The table 2.1, men- or banks because of different wind directions in tioned above, can be graphed for each wind the fetch. Although allwaves are subject to speed. An example of such a graph can be found angular spreading, the effect of such spreading is on page 33 of H. 0. 603. compensated for only with swell waves because Itisdifficultto work with actual energy the spreading effect is negligible for sea waves valuesof thesesinewaves; forthisreason stillin the generating area. Angular spreading another value has been substituted for energy. dissipates energy. This value is the square of the wave amplitude, and it is proportional to wave energy. Wave Spectrum The square of the wave amplitude plotted against frequency, for a single value of wind Ocean waves are composed of a multitude of speed, constitutes the spectrum of waves. Thus, sine waves, each having a different frequency. a graph of the spectrum ir. needed for each wind For explanatory purposes these frequencies are speed, and the energy associated with each sine arranged in ascending order from left to right. wave can be determined from these graphs. Each ranging from the low-valued frequencies on the wind speed produces a particular spectrum, and left to the high-valued frequencies on the right. thehigherthewindspeed,thelargerthe as illustrated in figure 14-3. spectrum. A particular range of frequencies, for instance, from 0.05 to 0.10, does not, however. represent only six different frequencies of sine FORECASTING SEA WAVES waves, but rather an infinite number of sine waves whose frequencies range between 0.05 Since sea waves are in the generating area, and 0.10. Each sine wave contains a certain forecastingof them willgenerallybe most amount of energy, and the energy of all the sine important when units are operating or transiting waves added together is equal to the total energy an area in close proximity to storm centers. present in the ocean waves. The total energy Problems encountered in providing these fore- present in the ocean waves is not distributed casts will include accurately predicting the storm equally throughout the range of frequencies, track and the intensity of the winds that will instead, in every spectrum, the energy is concen- develop the sea waves, tratedaround aparticular frequency( ), In this section we will discuss the generation which corresponds to a certain wind speed. For of sea waves, how to most accurately determine instance, for a wind speed of 10 knots (kt) fn.), wind speeds, and objective methods of forecastis 0.248; for 20 kt, 0.124; for 30 kt, 0.0825; for ing sea waves.

497 503 AEROGRAPIIER'S MATE I & C

GENERATION AND GROWTH because the frequency range does not increase any more. If the wind continues to blow at the When the wind starts to blow over a relatively same speed and from the same direction for a calm stretch of water, the sea surface becomes considerable period of time, the major portion covered with tiny ripples. These ripples increase of the fetch reaches the steady state. in height and decrease in frequency value as lung as thewind continues to blow oruntila Nonfully Developed Sea maximum of energy has been imparted to the water for that particular wind speed. These tiny Whenthe windisunabletoimpartits waves are being formed over the entire length maximum energy to the waves, the sea is said to and breadth of the fetch. The waves formed near be nonfully developed. This can happen under the windward edge of the fetch move through two circumstances: (I) When the distance over the entire fetch and continue to grow in height which the wind is blowing is limited, that is, and period, so that the waves formed at the when the fetch is limited; or (2) when the wind leeward edge of the fetch are superimposed on has not been incontact with the sea for a the waves which have come from the windward sufficientlength of time; thatis, when the edge and middle of the fetch. This description duration time is limited. illustrates that at the windward edge of the fetch FETCH LIMITED SEA. -Whenthefetch the wave spectrum is small: at the leeward edge length is too short, the wind is not in contact of the fetch the spectrum is large. with the waves over a distance sufficient to These waves are generated and grow because impart the maximum energy to the waves. The of the energy transfer from the wind to the rangesof frequencies and wave heights are wave. The energy is transferred to the waves by therefore limited and the wave heights are less the pushing and dragging forces of the wind. than those of a fully developed sea. The process Sincethe speed of the generated wavesis of wave generation is cut off before the maxi- continually increasing, these waves will eventu- mum energy has been imparted to the waves and ally be traveling at nearly the speed of the wind. the fetch is in a steady state. This leads to the When this happens the energy transfer from the conclusion that for every wind speed. a mini- wind to the wave ceases. When waves begin to mum fetch distance is required for the waves to travel faster than the wind, they meet with become full developed, and that if this minimum resistance and lose energy because they are then fetch requirement is not met, the sea is fetch doing work against the wind. This then explains limited. the limitation of wave height and frequency DURATION TIME LIMITED SEA.When which a particular wind speed may create. the wind has been in contact with the waves for too short a time, it has had insufficient time to Fully Developed Sea impart the maximum energ:/ to the waves, and the growth of the frequency range and wave When the wind has imparted its maximum heights ceases before the fully developed state of energy to the waves, the sea is said to be fully the sea has commenced. Such a situationis developed. The maximum frequency range for known as a duration time limited sea. This leads that wind will have been produced by the fetch, to the conclusion that for every wind speed, a andthis maximum frequency rangewill be minimum durationtimeisrequiredfor the present at the leeward edge of the fetch. Once waves to become fully developed; and that if theseaisfully developed, no frequencyis this minimum duration time requirement is not produced with a value lower than that of the met, the sea is duration time limited. The state minimum frequency value for the wind speed in of the sea, then. is one of three: fully developed, question, no matter how long the wind blows. In fetch limited, or duration time limited. brief, the waves cannot grow any higher than the Table 14 -I shows the minimum wind duration maximum :slue for that wind speed. times and fetch lengths needed to generate a When the sea is fully developed, the area near fully developed state of the sea for various wind the windward edge is said to be in a steady state, speeds. When the actual conditions do not meet r 504498 Chapter 14SEA SURFACE FORECASTING these minimum requirements. the properties of Table 14-1.Minimum fetch length (in nautical the waves must be determined by means of miles) and minimum duration time (in hours needed to generate a fully developed sea graphs and formulas. versus wind speed (in knots). DETERMINING THE WIND FIELD

As we have discussed, wind is the cause of V F t waves. It therefore stands to :eason that in order 10 10 2.4 to accurately predict sea conditions itis necessary to determine wind properties as accurately 12 18 3. 8 as possible. Miscalculation of fetch, wind speed, or duration will only lead to inaccuracies in 14 28 5. 2 predicted wave conditions. Inthis section methods of determining the 16 40 6. 6 wind properties, as accurately as possible with data available, an presented. 18 55 8. 3

Location of Fetch 20 75 10 22 100 12 Inallcases thefirststep toward a wave forecast is locating a fetch. A fetch is an area of 130 14 the sea surface over which a wind with a 24 constant direction and speed is blowing. Figure 26 180 17 144 shows some typical fetch areas. The ideal fetch over the open ocean is rectangular, with 28 230 20 the winds constant in both speed and direction. As shown in figure 144, most fetch areas are 30 280 23 bounded by coastlines, frontal zones or a change in isobars. In cases where the curvature of the 32 340 27 isobars is large, it is a good practice to use more 34 420 than one fetch area, as shown in figure 144(B). 30 Although some semipermanent pressure sys- 36 500 34 tems have stationaryfetchareas, and some storms may move in such a manner that the 38 600 38 fetchispractically stationary, then,art.also many moving fetch areas. Figure 14-5 shows 3 40 710 42 cases where afetch AB has moved to the position CD on the next map 6 hours later. The 42 830 47 problem is determining what part of the moving fetch area to consider as the average fetch for 44 960 52 the 6 hour period. 46 1,100 57 Infigure14-5, CaseI,thefetch moves perpendicular to the wind field. The best ap- 48 1, 250 63 proximation is fetch CB. Therefore in a forecast involving this type of fetch, use only the part of 50 1, 420 69 the fetch that appears on two consecutive maps. The remaining fetch does contain waves, but 52 1, 610 75 they are lower than those in the overlap area. In figure14-5, Case 2, the fetch moves to 54 1, 800 81 leeward (inthe same direction as the wind). Since waves are moving forward through the 56 2,100 88

499 505 AEROGRA1)1ER'S NlATE I C

1004 10 00 996 996

1000

1004

1012 1016 102 0 1016 1012 r 1008 10 20 I iL v- 4

1012 10 08

10161020 1012 D

AG.677 Figure 14-4.Typical fetch areas.

fetch area, the area to be used in this case is reported values from ships. This method has the fetch CD. advantage of not requiring a correction for Case 3, figure 14-5, depicts the fetch moving gradients or stability.llowever,realistically, windward (against the wind). Since the waves more often there are only a few ship reports move toward A. the region AC will have higher available and ship reports are subject toerror in waves than the area BD. Experience has shown observation, encoding, or transm:ssion. that in this case A13 is the most accurate choke for a fetch. A second way to determine wind speed is to measure the geostrophic wind from the isobaric spacing and then correct itfor curvature and Determining Accurate Wind Speed stability. At firstit would seem this would be less desirable due to the extra time makigte, The most obvious and accurate way to deter- corrections.llowever, barometric pressureis mine wind speed over a fetch is to average the probably the most reliable of the parameters

500 ,. 506 Chapter 14 SEA SURFACE FORECASTING

WIND 1020 A C BD

,; J FETCH iili,.. WIND FETCH AC BD 111* FETCH A C BD

CASE 1 1020 CASE 2 CASE3

AG.678 Figure 14-5.Examples of moving fetches.

reported byships and areasonably accurate In the majority of cases the curvature correc- isobalic analysis be made from a minimum tion can be neglected since isobars over a fetch number of h.:ports. For these reasons the cor- area are relatively straight. The gradient wind rected geostrophic v indisconsidered to be the can always be computed if more refined compu- best measure of wind speed over the fetch. tations are desired. except of course in c,v.es 1,S, here there is a dense In order to correct for air mass stability the network of ship reports where wind direction sea-air temperature difference must be com- and speed are in good agreement. puted. This can be done from ship reports in or The reason for collection to geostrophic wind near the fetch area aided by climatic charts of isthat the isobais must be straight for a correct average monthly sea surface temperatures when measure of the wind. When the isobars curve. data is too scarce. The correction to be applied other forces enter into the computations. The is given in table 14-2. The symbol Ts stands for Wind increasesordecreasesdependingon the temperature of the sea surface, and Ta for whether the system is cyclonic or anticyclonic in the air temperature. nature. The stability correction is a measure of the turbulence in the layer above the water. Table 14-2.Air-sea temperature difference correction. Cold air over warmer wateris unstable and hitthly turbulent. making the surface wind more ("r, Ta)Algebraically Percent of nearly equaltothe geostrophic wind. Con- Subtracted Geostrophic Wind versely. warm air over colder water produces a stable air mass and results in the surface wind 0 or negative 60 being much smaller than the geostrophic wind. 0 to 10 65 Three rulesfor an approximation of the 10 to 20 curvature correction are as follows: 75 20 or above 90 .For moderately curved to straight isobars no correction is applied. Determination of Wind Duration 2. For great anticyclonic curvatureadd 10 percent to the geostrophic wind speed. Once a fetch has been determined and the 3. For great cyclonic curvature subtract 10 wind speed has been found, the next step is to percent from the geostrophic wind speed. determine the euration of that wind over the

501 507 AEROGRAPIIER'S MATE 1 & C fetchItis highlyunlikely that the wind will become proficient inits use with experience. begin and end at one of the 6 hour map times. This method will be discussed in this manual. Therefore an accuratevalue must beinter- Figure 14-6 illustrates the worksheet which polated. In most as a simple interpolation of will be used with this forecasting technique. The the stt..ehshe maps v ill be sufficient to locate step-by-step instructions which follow coincide the bounds of the wind field in space and time. with those on the worksheet. Determining how long the wind has blown is relatively simple when the wind speed has been STEP I. Determine the average wind speed constant for the entire duration. It' this does not (U) over the fetch and enter this value on the occur. arepresentative duration must be se- worksheet. Keep in mind that the basic defini- lected. tion of a fetch requires that the wind be the SLOWLY VARYING WIND. Suppose the same speed throughout and the value of wind wind has been blowingfor24 hours, with speed should be very close to the actual wind at velocities of 10 knots for 6 hours, 15 knots for any point in the fetch. 12 hours, and 20 knots for 6 hours. The durationis 24 hours but the speed value is in STEP 2. Measure the length of the fetch in question. The most consistent solution is to use nautical miles and enter on the worksheet as (F). three durations with the corresponding wind STEP 3. Determine how long the wind has speeds and work up three successive states. maintained the same speed as (U) by checking MORE RAPID VARIATIONS. Suppose the back through previous weather charts. Enter the wind blows for 12 hours and during that time number of hours as (td ) on the worksheet. increasesinvelocity from10to 20 knots. Studies and experience have shown that in cases STEP 4. Turn to sea and swell graphs la and of variable winds a single value may be assigned lb (figures 14-7 and 14-8). These are two C. C. for wind speed if the change has been relatively S. duration graphs. Graph la is for wind speeds small. The following rules can be applied under of to to 44 knots and lb is for wind speeds of these conditions: 36 to 56 knots. Use appropriate graph for the wind speed. I. Average the wind speeds when the change Enter the graph with the wind speed (U) from is gradual or increasing, then decreasing. Apply StepIand the duration of the wind (td) from Ilk average to the entire duration. Step 3. At the intersection of the wind speed 2. Use the last wind speed when the speed and duration, go horizontally to the left scale ,!..ingesinthefirstfew hours, then remains and read the energy at intersection (E1). Record constant. Apply that speed to the entire dura- this value on the worksheet. From the same tion. intersection go to the bottom scale and read the frequency at intersection (I'd. Record this value OBJECTIVE METHODS FOR on the worksheet. FORECASTING SEA WAVES STEP 5. Turn to sea and swell graphs lc and Id (figures 14-9 and 14-10). These are two C. C. There are a nuns. .!r of different methods for S. fetch graphs. Graph lc is for wind speeds of forecasting sea waves. Some of the methods are 10 to 44 knots and Id is for winds of 36 to 56 tootechnicalor time consuming to be of knots. Use the appropriate graph for wind speed practical use to Aerographer's Mates. (U) found in Step I. Enter the graph with the wind speed (U) and One of the most complete methods is the fetch length (F). At the intersection of these Pierson-Neuman-James Method inII. 0. 603. values go horizontally to the left scale and read TheOceanographicServicesSection; Naval the energy at intersection (Ed. Record this value Weather Service Facility, San Diego, has devised on the worksheet. From the same intersection a reduced version of this method, incorporating go to the bottom scale and read the frequency at theuse of graphs and worksheets. Personnel intersection(fd.Recordthisvalueon the with limited knowledge of technical theory can worksheet.

502 508 Chapter 14SEA SURFACE FORECASTING

SEA WORKSHEET

FETCH # DTG CHART DATE ********************************************************************************

Observed or Forecast Parameters

1. Wind Speed over Fetch U = kts.

2. Fetch Length F = mi.

3. Duration of Wind td = hrs. *******************************************x*****************A******************* Graphical Calculations

Step Enter Sea & With And Read No. Swell Graph

2 4* la or lb U from step 1 and td from step 3. Ei ft fi cps

2 5* lc or ld U from step 1 and F from step 2. Ei ft fi cps

6 None Choose the smaller Ei from steps 4 & 5, Ei ft2 along with its corresponding fi. fi cps

7 None If the fi chosen in step 6 lies to the fu cps left of fmax on the CCS Graph, then fu is the same as fi. If not, go on to next step. If fi chosen in step 6 lies to the right 8 2 of fmax on the CCS Graph, enter Graph 2 fu cps with fi and read fu

9 3 Ei from step 6. .03Ei ft2 1 10 a,b,c, or d U from step 1 and .03Ei from step 9. fL cps

11 4 fu from step 7 or 8. Repeat for fL Tu sec from step 10. TL sec

12 5 U from step 1 and fi from step 6. Tavg sec

Havg ft 13 6a or 6b Ei from step 6. H1/3 ft H1/10 ft

*If no intersection of U and td or U and F occurs with the values found in steps 1, 2, and 3, then the sea is fully developed.Go to the fully developed sea table and read Tu, TL, Taw, HaV2' HV3, and H1/10 directly.

AG.679 Figure 14-6.Sea worksheet.

503 509 AEROGRAPIIER'S MATE I & C

58 1. t )49,..52.0 400 -24/421.02, 56

52 3,6 ,140 .6 50

44 i 2'8 2 OW; -1. ,.. 46 I 201. 47.2. 250 2a %now 44 7 .

42 7 - 3zo,,, li.,,,, 401- ZOO .. 34v. 111111 'N It7eeets -J6.1(22,21 NW% -5 34 3 4044'. 76 gners \1111111A bs.4 36 '2142.1.401110100\ 300 32 I

5: 340. 3 = 34 r--.16.0ort 15.01 41110 /7 e..s, s allialkitkeN 05 s 3Pal 32 Ira..Ai...0 , 1 1 2 02 /04.06/1 6IIIIRSL 't ..... 30 - 000%035f., 0 ,.a.". .4434/06.11 - 111RIMANNIIInallMINIMMIS:.::bl 28 4 125 15 i6 11202.2 232425 30 40 50 7 6 5 3 20mr. 26 04,11.1 2740 60J2 Knoll 11111\\ 0Z-2 24 /0 ct 22 60 .0.367+.75.0 214.. 7.304.002

20 50 411..120.1 ik te210021 16 IM. k 0 26 7 1..0, ,_ . 30 ow26 %nen .s M Immtnim,.._----"ranmilsortivalleal Ili 20pY420;-"'"" a4MNIM 12 E 02 5 1240 10 .0222 %nos 11111.11111111414: 77 7.0040 ?C)goers =InnalaaNGM* t61..6 5- 14%n6/' IIMINtit-rrk\ . 4753 /6 N0ers 11.11111111111111011.1111111MME5/-411°mtill [.1301. 260 Ls oh-- I -414 %nes --r-..,2,-... 2 05 - 0 25 -

O .J1(1111111111'11.1J1111111111111111111 11,1 :1'3'11' 03 05 05 07 0 01 0 0 7 171 03 I6 .024 75 3 314050 0 71410640Cr o+) 1 1 1 1 2 1 I I f 1 1 L-- 1 I 1 300 250240 220 200 190ISO 170 160150 14 130 120 110 100 10$070 6C 504030 20 10 011 set / F AG.680 Figure 14-7.Sea and swell graph la. Distorted C. C. S. (duration graph-wind speeds10-44 knots).

504 Chapter 14 SEA SURFACEFORECASTING

7 40 £111.1 104 t 56 xnvs .1 y , 04 3.5.0 30 41070 10 103 1300 74 E 70 2.7 100 98 1200 25

96 22 94 1100 21 92 20 0

66 900 84

82 80 800

78 r_ SO goers 76 E 755 700 74

72 650 70 600 66 550 64 500 62 40 50 58 400 9.54 aJ 7 X 54 390 6 C52

50 5 300 C 46 E. 6 50 3 644 42 40 200

36 150 34

32 30 28

26 SO 24 70 22 et7 20 50 Is 40

16 30 14 20 12

10 10

6 4 2 0 .05 1 1 7 3 1 I I.04 L reCOUESCY 11.1) 50116 42 31I 34 30 21126 24 22 20 :9 17 16 IS 14 113 12 11 10 9 6 r(w)

AG.681 Figure- 14-8.Sea and swell graph lb. Distorted C. C. S. (durationgraph-wind speeds 36.56 knots).

505 511 AEROGRAPHER'S MATE I & C

111 2,14..

Ton*. P.04

510 12

7.2Knots t`I

0 49 12 ,N4 1(4o0; 40 0 2. ai

4 ;

relenols 2 34 -.2490 2

36 " 1 X 32 0.5 O 0.23 x 0.10

28 -so 0715212 3 1 26 (.5

LL_ 23

22 60 JO Ilnols Z1154, °..280 20 50 Ca 40-28 g""t74,! 00.° 16 50 .--26 Knot/ 2,422,1150

20 Kn 12 24 020.1.50

10 10 8

1 2 05 025 I 0 I op 222(00(03011il 04 05 04 07 08 .01 JO J1 )2 J3 )44524 G. 024.2522240050 03' I I 1 1 I 1 1 1 1 1 1 1 1 l 1 I I 17.0 160 150 14013D 120 1/3 1009DAn 7nAn 'ft 411 10 2D 1.0 OTII.0 300 254240 224' 200 190 160

AG.682 Figure 14.9. Sea and swell graph /c. Distorted C. C. S. (fetch graph-wind speeds 10-44 knots).

506 51? Chapter 14 SEA SURFACE FORECASTING

1, 0 9 6 5 4 I .! 4 ° 1 3 f .0 3 14 ,546 1' 65 49 1,0 q 24 262050 .55 .40 50 40 70 113 '41 4..". 56 le'ssses Z6 4a.,rs ,7 e $}7r.,zo, .'.5.. u 14 ' - .52 Anolt 1 6,201 -...-- --..segAos .1

94 6 1.41 t 7 L 54160, ,...... c.,,,Z, r6.1.1. 21

97 .... 10 V° 10 ,x.69 69

52 .A0Is vPlc16106.5

05 603- 40,1

SJKAOIS 36 ,t,1420Nr.Th

.34

13 *50 10 48 KnOfs 600 6.6..01250.1 65 40 66 550

64 500 46 Knols 47 11.0 ....6006

60 45 5* 400 6. 41 Knols 3944,140. 54 350 57 50 Knols 40 " 4.963,00306 u

4.6 °L 016/33 1, 74 Fr 0 6 47

0 ZOO

100 44 u

34 -

50 400 9 16

24 15

12 40

10- 50 Is`

16 so 44 Zu

10 40

0 . 01 05 04 A5, 3 ratOUCMC9 11 1 111 1 50 44 42 30 36 30 I ft, 3114 )+ 'I44 <

AG.683 Fig ;:re 14-10.Sea and swell graph 1d. Distorted C. C. S. (fetch graphwind speeds 36.56 knots).

507 513 AEROGRAPHER'S MATE I & C

STEP 6. Examine the E, values found in Step STEP 9. Turn to sea and swell graph 3, figure 4 and Step 5. Chose the smaller value of E, and 14-13. Enter the left side of the graph with the its associated r,. Record these values for Step 6 energy at intersection (E,) found in Step 6. Go onthe worksheet. and usethesevalues as horizontally to the right to the bent diagonal required for all further steps. line. From this point move vertically downward to the bottom scale of the graph and read the NOTE; The durationline on the C. C. S. value of 3 percent of E. Record this value in the duration graph may not be long enough to space marked .03E, on the worksheet. intersectthe wind speed line. DO NOT EX- TEND OR EXTRAPOLATE THE DURATION STEP 10. Return to the C. C. S. graph having LINE IN ANY WAY. The fact that the duration the intersection found in Step 6. (This will be line does not reach all the way to the wind speed one of the sea and swell graphs, figure14-7. line means that the sea state is not duration 14-8, 14-9, or 14-10.) Enter the left side of the limited. In this case there is no intersection and graph at the E scale. with the value of .03E, no value of C, orf,can be recorded on the found in Step 9. Go horizoptally to the wind worksheet. speed line (U) found in Step 1. From that point move vertically downward to the f scale at the The same situation can occur with the fetch bottom of the graph and read the value at the length line on the fetch graphs. If so. it means lower frequency (II). Record this value on the that the sea state is not fetch limited. worksheet. If only one of the graphs has an intersection usethatvalue ofE,andf,for further steps STEP 11. Turn to sea and swell graph 4, without concern for a smaller E,. figure 14-14. Enter the left side of the graph If neither of the graphs has an intersection. with the value of upper frequency (1) found in step 8. Go horizontally across to the curved line, the sea isfullydeveloped and no further computations are required. Go directly to table then vertically downward to the bottom scale. 14-3 for fully developed sea and read the values Read the value of the upper period (T) and for period (T) and wave height (11). record it on the worksheet.

STEP 7. If the intersection chosen in Step 6 STEP 12. Turn to sea and swell graph 5, lies to the left of the dashed line labeled f, figure 14-15. Enter the graph with the values of then the upper frequency (0 will be the same as wind speed (U) found in StepI and frequency the value of r, found in Step 6. Record the value at intersection (Id found in Step 6. Instructions of f, as f on the worksheet. If the intersection for the use of this graph are found in the lower lies to the right of the fm,,, line, then go on to right corner. Follow these directions and read Step 8 to determine r. the value of the average period (Tayg). Record this value on the worksheet. STEP 8. If the intersection chosen in Step 6 liesto the right of the line labeled f,, then STEP 13. Turn to sea and swell graphs 6a and turn to sea and swell graph 2. figure 14-11. 6b. figures 14-16 and 14-17. These graphs are the same except that 6b is a blow-up version of On the skeletonized C. C. S. graph depicted in 6a, to give a more accurate result when working figure 14-12, the intersection lies to the right of with small values. Enter the graph with the the 1, line. energy at intersection (Ed found in Step 6. If Enter the left side of sea and swell graph 2 the E, value is less than 10 10 use graph 6b (fig. with the frequency at intersection (I.) and go 14-17). If greater than 10 ft'- use graph 6a (fig. across horizontally to the diagonal line. From 14-16). Follow the instructions printed at the thispoint move vertically downward to the top of the graph to get characteristic wave bottom of the graph and read the value of the heights. This will have to be repeated three upper frequency (f). Record this value on the times; once to get Han: once to get II; and once worksheet. to get H. Record these values on the worksheet.

508 514 Chapter 14 -SEA SURFACE FORECASTING

Table 14.3.-Wind and sea scale for fully arisen sea.

WIND AND SEA SCALE FR FULLY ARISEN SEA

0 0 O Sea like a mirror. U Calm 0.4 is Man Ripp le with the appearance of scales Light W to 0.5 10 la. S !genie are formed. but without foam Aire l-S 0.050.01 0.101.2 4c 2. 7 Light Small wavelets, still siinrt hod more 1.4 6.7R 39 .1,3 pronounced: crests have a glassy Breese 4 . 6 S 0,18 0. 290.370.4-2.11 L 0 appearance, but do not break. Castle Large wavelets. create bests to break 3.4 2.4 20 9.1 1.7 hr Perhaps a 7 10 I. S 0.6 .0 1.20.8-5.0 roam of glassy appearance. 10 2.4 scattered white bona. 10 0.18 1.4 1.81.0.6.0 2.9 07 1.4 2. 2. 1.0.7.04. 8 3.4 40 18 3.1 13.5 1. 8 1.4-7. 65.4 3,9 52 24 4,8 4 11.16 waves. Decorum' larger; fairly Moderat] 14 2.0 3.3 4.21,5.7.85.6 4,0 59 28 5.2 frequent white booms. Rime 16 L 9 4.6 5 2.0.8.86.3 4.6 7: 40 6.6 S5 8.3 Moderate waves. taking a more pro- I8 3 6.1 7 2 5-10.07. 2 5. 1 90 4 2. 8-10.67.7 5.4 99 65 9. 2 nounced long form; many white horses S Fresh 17.21 19 4.3 6.9 8.7 3.0.11.1 11. I S.7III 75 10 are formsd.lChancof some sprv). Dramas 20 5.0 8.0 0 4 10 3.4-12.2fl. 9 6.3134 100 12 14 Large * begin to form; the white 6 Strong 22-27 24 .9 lb 3.7.13.59.7 6.8 160 130 foam c SSSSS are more exteastvli every. a 24.5 a.2 17 3, 8-13.69.9 7.0 164 140 15 180 /7 when. (Probably porno spray). 06 9.6 15 204.0.14.510.5 74 188 6 7.9 212 230 20 Sea beeps up and white loam from 08 18 234.3-15.511.3 28 12.1 8,6250 280 23 7 Moderate28.33 30 2 4. 7-:6.7 ,--brktog waves Isgins to be blown to 290 24 30.5 14 23 29 4.8-17.0 12.48.7258 its along the direction of the wind. Cal. 340 27 32 16 26 33 5.0-17.5 12.99.1 285 SSpindrift begins to be 420 30 34 19 30 31 5.5-18.5 13.69.7 20 7 Moderately Mei wavs of g 63 34 36 21 35 44 5.1 -19.7 14.510. 3 500 length.edges of break into spin- Fresh 37 37 23 37 46.76. 20.5 14.910.5 376 530 drift. The foam Is Mown to well Cale 34-40 31 38 25 40 SO 6.2 -20.8 15.410.7 '392 600 marked SSSSS ks along the direction of 42 40 a 45 51 6.5-21.7 16.1 11.4 444 710 the wind. Spray effects visit:Alm, 47 42 31 SZ 64 7.23 17.0IL 0 492 830 High waves, Doins streaks et .along 7-24.2 17.7 12.5 534 960 52 Strung 41.47 44 36 the direction of the wind. Sea toolgtas to 46 40 7-25 18.6 13.1 590 1110 57 Visibility stferted_ Cale 44 .44 7.5.26 19.4 13.6 650 1250 63 Very high wave: with long over/tinging 7.5-27 00.214.3 700 14 20 69 . Tb resulting loam is in great 10 not. 411-55 50 49 9 51.5 52 83 106 11-28. 20.814.7 736 1560 73 patches and is blown is denim white Cale 75 50. 54 87 110 8.28.5 21.0 14.1 750 1610 streaks along the dirctlon of the clod. 8-29.5 21.1 15.4 810 1800 al On the bol the selfact of the sea takes 54 59 95 121 whits eppearaoce. The rolling of the sea boricome heavy sad shock-Uke. Visibility Is affected. 22.6 31-410 2100 as Esc eptionally high waves (Small and 56 be to) 130 8-5.31 meilium.eised ships might forlong 10.32 24 17.0 9$S 2500 101 time be lost to vitrw behind the waves.) 11 stn.!, 56.63 59.5 73 116 148 Tke sea Ie coonplemily ciererad with long white patchs of foam lying along the dirctio of tb. wind. Everywhere the advise( the wave creep are blows Into from.Visibility affected. Air filled with foam and .pray.Sea completely white with driving spray; 12HsrriCane64.71 >64 13110 >121 164 101(15) (26) visibility very seriously affeetwL

The objective technique of forecasting sea methods of forecasting them. With sea wave wavesiscompleted with determining of sea forecasting we are considering the point for height computations in the preceding manner. which we areforecasting to be within the The manner of presentation of the wave generating area, with the wind stillblowing. information to the user can be determined on This, however. will not be the problem in the the local level to the satisfaction of the organiza- majority of the forecasts that will be required. tions involved. Normally the point for which the forecast is prepared willbe outside of the fetch area: FORECASTING SWELL WAVES therefore it will be necessary to determine what In the preceding portion of this chapter we effect the distance traveled is going to have on have discussed the principles of sea wavesand the waves. In this section we will discussthe 509 515 S,11:111(1V11001I1V :UN% I "S' D

11 E II II 1 f 1 11 N IN 1111 NMI I I IN RR II EN N RE II II II V II II II In unnsmonnurnmonsionnninnimmumnsonno US R US ROM n m n ansnonnonannnmennnunnionmonnmul simian s r in in on m n 9' n R 1 n Nr. SM V R FA MO unninunEMEOR V EMMUMMUNI EM OIM I ER . um= n 4 sono o nonminnimonnioonn iononn nrmn IRIRONLIMEMIXERROMMERVIMMENWAISWIZALMUJILIRUMMUURRUMURSEIUMMIERS NISMOMMENWAIInum s6) nninnpnnonsmaronommonnonnonnonnorrnnninTmErmilmnranurmnorninnonnoniummonmnms (nmsnimmunnonnwoninnomonnEonnumnsnunnonnunnnnmarronsonnonninnonnnonnonnonnn( RIMESIMERMUMWEENWIE. MM RE IO A ME R 0WIESERUMEROMONUMIRIMEMORM ri 111111 1111111 NI! II I i 1M1R11111111R1111M1 11n m n Ii1n11C111111111111n nlMnnlllnllI I n111 s I 4 nA n mmmnnnmnrununnonnoronmnoMnnnnnonnoonnoinononn nonnossnnonsmonnonr.nommunnnnsrronnonnornonximennon nn n nnnisrmon rno n e ionm nn no me om n on n mmonn A innomnnnonnm nimennennninnnnonnT ll XI ' 11 1 1111 IIMIN 1111 IMEMMENRONVOI t OUNSURNMEMIXIMUMAgnminnnronsnornmannnnionnnonnnmennomnonnmnnurm onus 1111 nronninnuronenronnun 1:1111 m min nronnunnwornnionnnonnwn annnonnimmomnsonnimannnnsinnirAn mnonmonnnonnononsunnonnoninmunnommininnonnonnonnonnuninunnnnunnonnmonnomnn ROMENSEENEMMOMEMNIUMMEWRIKOMWARRUMPORMIRMIUMUMUMSUSIMMINVIRMOOMMIEUSIME onnonnonnommonnonnonninionnnonnntnnnisnennomnnisnmsonnonnonsinninnummonnoninnonnm2n IMMEMMOMMISIMEMIMMInnEMEMINIMNIAMENERMINNEREMIRMINIUMOMMIEMERNENRINMEMMENcr ennnsnommmonninonnonisnrronmennunimennonannsnannannnunnnonnnnunnonnunnnnnsunn nnnununnonsnmonnnonniarnmerrarinurnrnnmrsimonniannunnonnonmunnonnomm oun n nnunnunnonmarnsninnnunmonnnnonnonwmninnannmonnnionnonnonnnnnionnsonnonnninn mannannononnornnonnionnnrumnunnurrirdisnrnnonsmunnnonannnownnnanninnunnonsn onnumonnnionnnensinnonnnonnnonnornmurAmnriermonnnonnnonnninnomminnnennunonnuninn nonnumnennnnnonnunnninnnursonnonnnonnrannonnonnmrsommunisnommonnonnsmonnon minnoninnnsionsmonnnnsnumnnonnsennurnnnurannormnonnnmsonnwonnnunnnomminninnonn RIONVARNENNWILLOWERMUMMENIMMOREMMEMEMMvmnsnanminimmonreninnnnonrinnnonnnungnnnummennunnonnunimmonnomnnonsnonnons ImmonamminnimninnonninnsunnEnnomniumnEnnmv IIMWOROMMOURIOUUMMUMMUMEMMERREM nonnummunnnuninnionnpAnronmennanninnnonninnmonnonnomnnommmonnnnunnmormnigglin mn IlliiiiiiiiiiiiiiiiiiIIIIIIIIIIIIIIIIIIIIMIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII nonmennumnunnonnonnnxnrismonnnninnionnonnnonninnronnonm IIMEXIIMERMEEMERMIMMR MUMMOIRWRIRMUMUMIMIIIMMUSSOUSUUSIMMUUMUUMMUMURSURSURIOURNUMUMMIUMMINORMSMEN mmonnonnonsinnonnsnonmnsnumsnrnonnunnronnorminrAnninnonnmonnnnumnrunnonnonnn II nnonnonnonnonionnnnonmonnmunnumnrsumunnnonnwArnin n ,nnionntinnonngonnnumnnrAinnonnonnonnnsmonnonnonennt OM ME 111111 RELIII11111111111111111111111111111NM ..211 onsmonnnum on 111111111111111f nomnnurnsu nn nnompAnnnnonnunnonnnnnonnn 11111l NOMMIMMMUNINIWMIXN EX

11111:11:11111:111:1:1111:11111:"R 1 114 11 n III 11 111111111111 /J'A IA""111OMENEMMEMEMENSVENJWIRIXON1ONmES I t' r mu nnonnnonnunonnnnonnannonnonznwan norm nunnornonnunni man

031N161d NI 11 S 'V

V89'DV antld L as. e Pue Slams 4dei6 7

oic . 9i Chapter I4SEA SURFACE FORECASTING

Tu cu fu t-- I Tmax EL 1 f w I max _J I 1

< 1 c..) 1 t vii % 1 % 1 I >-- 1 % co % cr 1 I w 1 i z \ w % t \ \ I 34 KNOTS I \ I I \ 1 1 \ 1, % 1-: I % 28 HOURS u. \ \ MIS. \-- .... INTERSECTION OF DURATION OR FETCH z \ LINE WITH WIND SPEED LINE

u., >a 3 z a U it z in

CPS FREQUENCY (f ) SCALE

1 I 1 I I PERIOD (T) SCALE SEC.

AG.685 lines. Figure 14-12.Skeletonized C. C. S. graph showing major

511 57 I

S I p I I

S I S a -

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Length of 10:646hr Distance traveleddisturbance FrequencyPeriod41 nautical miles in nautical miles .050 20 1960 300 225 15 sec .067 15 1470 1180 180 .083 12 I2sec 980 150 .100 10 lOsec 90 .167 6 590 \.%

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AG.691 Figure 14-18.Angular spreading. may arrive continue on. however, the faster runnersmove wind in the fetch. Thus, swell waves at a forecast point eventhough it may lie to one ahead and the slower runnersbegin to fall direction of the wind. behind. Thus the field of runnersbegins to string side of the main line of This process of angular spreadingis depicted in out along the directionof travel. The wave trains leaving a fetch do the samething. The stringing figure 14-18. of waves is called The problem in swell forecastingis to detcf- out of the various groups will reach the dispersion. mine how much of the swell In a swell forecast problemit is necessary to forecast point after the waveshave spread out at determine which wave trains havealready passed angles. This is accomplishedby measuring the the forecast point and which oneshave not yet angles from the leeward edgeof the fetch to the be measured as arrived.After this has beendetermined, the forecast point. These angles must left are the ones that are accurately as possible and aredetermined by the wave trains which are illustrates the at the forecast point atthe time of observation. followingrules.Figure14-19 procedure. Angular Spreading the fetch, they may As the wave trains leave I. Draw the rectangular fetch. leave at an angle to themain direction of the 517 523 AEROGRAPHER'S MATE I& C

FORECAST POINT A This conversion is made byentering sea and swell graph 7, figure 14-20, withthe positive or 03 = MINUS IT DEGREES negative angles and reading thecorresponding percentage,, directly. Thepercentages are then subtracted ignoring the plusor minus to find the angular spreading.

04 = MINUS 44 DEGREES OBJECTIVE METHOD FOR FORECASTING SWELL WAVES

A number of terms usedin dealing with 03 = PLUS 18 DEGREES forecasting sea waves willbe used again in this process; however, a number ofnew terms will be FORECAST POINT introduced. Table 14-4 listsmost of these terms with their associated symboland definition. As with objective forecastingof sea waves a 04 = MINUS 20 DEGREES worksheet will be utilized in thisprocedure. The worksheet is shown in figure 14-21. The following specificsteps for forecasting swell waves coincides withthe steps on the 03 = PLUS 32 DEGREES worksheet.

STEP 1, Determine theaverage wind speed over thefetch and enter thisvalue on the worksheet as (U). 04 = PLUS 5 DEGREES STEP 2. Determine howlong the wind has FORECAST POINT C maintained the same speedas U by checking back through previousweather charts. Enter the AG.692 number of hours (td )on the worksheet. Figure 14-19.Measurements ofangles for STEP 3. Measure thegreat circle distance angular spreading. from the forecast pointto the near edge of the fetch. Enter this distance inmiles as the decay distance (D) on the worksheet. 2. Extend the top andbottom edge of the STEP 4 Measure the lengthof the fetch in fetch outward parallel to themain direction of nauticalmiles and enterthis value on the the wind. This is shownas dashed lines in figure worksheet as (F). The 14-19. measurement must be made parallel to the winddirection over the 3. Draw lines from thetop and bottom edges fetch. of the fetch to the forecastpoint. STEP 5. From the 4. The angles to the forecast sea worksheet determine point are desig- the upper limit of the period inthe fetch (Ti,). nated Theta 3 (03) and Theta 4 (04).Theta 3 is Record this valueon the swell worksheet. measured from the top edge of the fetch and STEP 6. Using the procedureexplained previ- Theta 4 from the bottom edge. ously, measure the angles Theta 5. Any angle which lies above 3 and 4. Enter the dashed line sea and swell graph 7 (fig. 14-20), withangles in is negative while any angle thatlies below the degrees and read their dashed line is positive. corresponding percentage values. Record thesepercentage values on the worksheet. STEP 7.Subtractthe smaller percentage After the angles Theta 3 and 4have been found in Step 6 from thelarger. The result is the measured thay are convertedto percentages of angular spreading factor (As). Recordthis value the swell which will reach theforecast point. on the worksheet.

518 524 Chapter 14- SEA SURFACEFORECASTING

sin 28] =100[12+ 800+ Sit 2vr 95 ( 0 in DEGREES)

AG.693 Figure 14-20.Sea and swell graph 7. hours for the graph 9, figure STEP 9. Add the number of STEP 8. Turn to sea and swell DTG of the weather 14-22. Enter the left side of thegraph with the first wave to arrive to the chart used to draw the fetchand the result will upper period (To)found in Step 5. Enter the decay distance (D) be the forecast time ofarrival of the first wave. bottom of the graph with Record this value on the worksheet. found in Step 3. Move to theright from To and the intersection read the STEP 10.Fillin the decay table on upward from D and at the worksheet. The following preliminarydiscussion travel time for the first wave(to) in hours on the forecaster with a better time it takes for the is given to provide the forecast sheet. This is the understanding of the use ofthe decay table. first swell wave to arrive atthe forecast point.

519 5.'eZ AEROGRAPIIER'S NIATE

SWILL WUKKSHttl FETCH # DTG CHART DATE *******************************************************************************

Observed or Forecast Parameter,

1. Wind Speed over Fetch: U = kts 2. Wind Duration td hrs 3. Decay Distance D mi 4. Fetch Length F mi 5. Upper Limit of Periodsin Fetch Tu sec ****************************************************************************** Step Enter Sea & With No. Swell Graph And Read -03 6. 7 83 and 84 measured from Z map 04

7. None Subtract the small percentage Angular Sprc-ding Factor found in step 6 from the larger As % 8. 9 Tu from step 5 and D from step 3 Travel time for first wave to hrs 9. None Add to from step 8 to theDTG of ETA / Z the chart for time ofarrival of or first waves. / local

10. Fill in Decay Table:

1 f2 T2 E2 toB-td tOB fl T1 E1 AE E0 Hun H1/3H1/10

to k

to + 6

1 t, + 12 A Co + 18

to + 24

to + 30

to + 36

t o+ 42

to + 48

AG.694 Figure 14-21.Swell worksheet.

510

,e, Chapter 14 SEA SURFACE FORECASTING

Table 14.4.Sea wave terminology.

Symbol and Definition Name Dimension

Decay distance (D) Miles Distance from point of forecast to the leeward (downwind) edge of the fetch.

Decay index (1-I0/1-11;) Ratio of deep water wave height to fetch wave height.

Deep water wave height (Ho) Feet Height of waves after leaving the generating fetch but before reaching shallow water to become surf. (Swell height).

travel Travel time (to) flours Length of time necessary for waves to decay distance (D).

Exact instructions for filling in the decay table left at the forecast point if there is no angular follow the discussion. spreading. To account for angular spreading, the When swell waves reach a forecast point some angular spreading factor As is multiplied bythe distance away from the generating area,the remaining energy at the forecast point andfrom waves do not arrive allat once. Instead they this the wave heights are computed for thegiven gradually build up to a maximum. height over a time. period of hours, then slowly decrease.Because of this,a swellforecast should actually be Steps for filling in the decay table are given several forecasts, one for every few hoursshow- for each column as follows: ing the increasing heights and then thedecreas- ing heights as the swell lowers. The decaytable I.tot, Column. Write the value of thetravel is designed to do this. It is done byforecasting time of the first wave (to) (found in Step8 on the swell height at the time of arrivalof the first worksheet)asthefirst number in the tob the wave, then the swell heightsfor every 6 hours column. Each succeeding number down thereafter for the next 48 hours. The timeof column shouldbe 6 hours larger than the each forecast is in the tob column of thedecay number before it. table. The actual number in the tobcolumn is 2. f, Column. Turn to Sea and SwellGraph the number of hours after the DTGof the map 10, figure 14-23. Enter the left side of thegraph tob. Move showing the generating area (fetch). with theadjacentvalueof the In order to forecast the height of theswell at horizontally to the right and intersect a slanted is necessary to know how line of value equal to the decaydistance (D), a given tob time, it From that much of the wave energy has alreadypassed the found in Step 3 on the worksheet. forecast point and how much has notarrived. intersection move downward to thebottom of That energy which is left is the energyat the the graph and read the value of f2.Repeat this heights process for each value of tob. forecast point and from this the wave graph 4, can be known. Thedecay table does this by 3. T, Column. Turn to sea and swell computing the frequency, period, and energy figure 14-14. Enter the left sideof the graph point (f1 , Ti , with the adjacent value f2. Movehorizontally values just passing the forecast vertically down- E1): computing thefrequency, period, and right to the curved line, then arriving at the forecast point ward to the bottom scale. Read thevalue of T2. energy values just Repeat this process for each value inthe f, (f,,T,, E,): and subtracting the arriving energy from the leaving energy. The resultis the energy column.

521 5:47 1 I 6

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column without subtracting td. By completing all the presentedsteps the 6. ftColumn. Turn again tosea and swell forecaster now has all the informationnecessary graph 10 (fig. 14-23). Enter the left sideof the to provide an accurate swell forecast. graph with the number found inthe adjacent tob-td column. Move horizontally to theright FORECASTING SURF and intersect the slanted line markedwith the decay distance (D) in miles found inStep 3. Thus far we have discussed the generation From that intersection move downward of to the sea waves, their transformation to swell waves, bottom of the graph and read the value f. some of the charges that occur as theymove, Repeat this process for each value in thetob-td andobjectivemethods of forecasting both column. waves. EXCEPTION: If the sea in the fetch is fully The Navy is also greatly involved inamphibi- developed or if the fetch has been moving nearly ous operations which require the forecasting of as fast as the wind and then suddenly stops then another sea surface phenomena, surf. add the fetch length (F) to the decay Senior distance Aerographer's Mates will, upon occasion,pro- (0) before entering the graph with D. vide forecasts for this type ofoperation. Ac- 7. Ti Column. Turn again tosea and swell curate and timely forecasts can greatlydecrease the chances of personnel injuries graph 4 (fig. 14-14). Enter the leftside of the or equipment damage.Itis graph with the adjacentvalue of f1. Move therefore important that fore- horizontallyrighttothecurvedline, then casters have a thorough understanding of the characteristics vertically downward to the bottom scale.Read of surf anda knowledge of forecasting techniques. the value of T1. Repeat this procedure foreach value in the f1 column. GENERATION OF SURF 8. Et Column. Turn to any of the C. C.S. graphs used in the computations forthe E2 The breaking of waves, in either singleor column, having the correct wind speed. Enter multiple lines along the beachor over some the I scale at the bottom with the adjacent f1 submerged bank or reef is referred toas surf. value. Go vertically upward to the wind speed The energy whichisbeing expanded in line (U). From this pointmove horizontally to producing this phenomenon is the the left and read E1 from the E scale. remainder of Repeat that energy that was impartedto the sea surface this process for each value of ft. when the wind developed thesea waves. It, of

524 530 Chapter 14-- SEA SURFACE FORECASTING course, has been depleted as the swell waves Refraction transited from the fetch area to the area of occurrence of the surf. When waves arrive from a direction that is The surf zone is the extent from the water perpendicular to a straight beach, the wave up-rush onthe shoretothe most seaward crests will parallel the beach. If the waves are breaker,Itwill be within this area that the arriving from a direction other than perpendicu- forecast will be prepared. lar or the beach is not straight, the waves will When waves enter an area where the depth of bend, trying to conform to the bottom con- the bottom reaches half their wave length, the tours. This bending of the waves is known as waves are said to "feet bottom." This means refraction and results from the inshore portion that the wave is no longer traveling through the of the wave having a slower speed than the water unaltered, butis entering intermediate portion still in deep water. This refraction will water where changesin wave length, speed, cause a change in both height anddirection in direction, and energy will occur. There will be shallow water. no change in period. These changes are known as shoaling andrefraction. Shoaling affects the Surf Development height of the waves, but not direction, while refraction effects both, both effects result from When a wave enters water which is shallower a change in wave speed in shallow water. than half its wave length, the motion of the water near the bottom is retarded by friction. Shoaling This causes the bottom of the wave to slow down. As the water becomes more shallow the The shoaling effect is caused by two :tors. wave speed decreases, the wave length becomes The first is a result of the shortening of the wave shorter, and the wave crest increases in height. length; as the wave slows down the crests move This continues untilthe crest of the wave closer together. Since the energy between crests becomes too high and is moving too fast. At this remains constant the wave height must increase point the crest of the wave becomes unstable if this energy is to be carried in a shorter length andcrashes down intothe preceding wave of water surface. Thus, waves can become higher trough: when this happens the wave is said to be near shore than they were in deep water.This is breaking. The type of breaker, that is, whether particularly true with swell since it has a long spilling, plunging, or surging, is determined by wave length in deep water and travelsfast. As the steepness of the wave in deep water and the the swell speed decreases when approaching a slope of the beach. shore, the wave length shortens, and a long swell A spilling breaker breaks gradually over a which was barely perceptible in deep water may distance, white water forming at the crest and reach a height of several feet in shallow water. expanding down the face of the breaker. The The second factor in shoaling has an opposite wave energy is expanded gradually and breaking effect and is due to the slowing down of the is mild. wave velocity until it reaches the groupvelocity. In a plunging breaker the wave crest advances As the group velocity represents the speed with faster than the base of the wave, causing a wave which the energy of the waves is moving, the crest to curl over and break with a crash. The height of the individual waves will decrease with resulting white water appears almost instantly its decreasing speed until the wave and group over the complete front face. velocities are equal. The second factor predomi- Illustrations of both of these type breakers nates when the wave first feels bottom, decreas- are found in the Rate Training Manual. Aerog- ing the wave height to about 90 percent of its rapher's Mate 3 & 2, NAVTRA 10363-D deep water height by the time the depth is A surging breaker is one that peaks up, but one-sixth of the wave length. Beyond that point, instead of plunging or spilling, surges up on the the effect of the decreased distances between beach. crests dominates so that the wave heightin- When a wave train strikes a beach at an angle creases to quite large values close to shore. refraction occurs, but at the same time another

525 AEROGRAPHER'S MATE I & C

result is the mass transport of water parallelto STEPI.Determine the deep water wave the beach in the same direction as themovement height approaching the beach from either ob- of the wave train. This masstransport is called servations or a swell forecast. Enter this valueas the longshore current or Littoral Current. It is of IIa on the worksheet. vital importance to amph/bious operations. STEP 2. Determine the deep waterwave Definition of Terms period approaching the beach from either observations or a swell forecast. Enter this valueas T. on the worksheet. The following are some terms that will be used extensively in surf discussions and that STEP 3. From the weather chart determine should be understood by the forecaster: the direction from which the swellwaves will approachthe beach. Assuming that the line I. Breaker height the vertical distance in along the wave crests will be perpendicular to be feet between the crest of the breaker and the direction of wave travel, the angle between the level of the trough ahead of the breaker. beach contours and the deep waterwave crests can be measured. Enter this value as as on the 2. Breaker wavelength - the horizontal distance in feet between successive breakers. worksheet. 3. Breaker period the time in seconds STEP 4. Turn to surf graph 1, figure 14-25. between successive breakers. This is always the Enter the left side with H. found in step 1, and same as the deepwater wave period. the bottom with T. from step 2.At the 4. Depth of breaking the depth of the intersection of these values read 11./T02 from water in feet at the point of breaking. the curved solid lines of the graph. Enter this 5. Surf zone the horizontal eistance :a value on the worksheet. yards between the outermost breakers and the STEP S. Turn to surf graph 2, figure 14-26. limit of wave uprush on the beach. Enter the bottom with Ho/T.2 found in step 4 6. Number of lines of surf the number of and go up vertically to the curved line marked lines of breakers in the surf zone. with the beach slope of the beach for which the 7. Deep water wave angle the angle be- forecast is being made. From that pointgo to tween the bottom contours and the deep water thelefthorizontally and read. the value of swell wave crests. Hb/H. at the left side of the graph. Record this 8. Breaker angle the angle between the value on the worksheet. beach and the lines of breakers. It is always less STEP 6. Turn to surf graph 3, figure 14-27. than the deep water wave angle. Enter the left side with H. found in step I and the bottom with Hb/Ho from step 5. At the OBJECTIVE TECHNIQUE point of intersection read the value of Hb from FOR FORECASTING SURF the curved lines on the graph. Enter this value on the worksheet. The objectivetechnique presentedinthis STEP 7. Turn to surf graph 4, figure 14-28. manual is the same method contained in NWRF Enter the bottom with Ho/T. found in step 4 36-1264-099, Surf Forecasting. Some minor and go up vertically to the line marked with the innovations in the graphs used were made by slope of the beach for which the forecastis personnel of the Oceanographic Department of being made. Read the breaker type written at Naval Weather Service Facility, San Diego. that point and enter the type on the worksheet. Through the use of the procedure as pre- STEP 8. Turn to surf graph 5, figure 14-29. sented in this chapter the forecaster can provide Enter the bottom with H0 /T.2 found in step 4. an accurate forecast of surf conditions when Go up vertically to the curved line on the graph required. and from that point of intersection goacross Figure 14-24 provides an example of the surf horizontally to the left side. At the left side of worksheetthat may be utilized inthis surf the graph read the value of db /H. and record it forecasting procedure. The steps in the method on the worksheet. If Ho/T.2 is less than .01 go conform to steps on the worksheet. directly to step 9.

526

I. 532 Chapter 14 SEA SURFACEFORECASTING

SURF FORECAST WORKSHEET

FTA SURF BEACH NAME BEACH SLOPE ****************************************************************************************

FROM OBSERVED OR FORECAST SWELL

H 1. Deep water wave height: o ft.

T = sec. 2. Deep water wave period: o ao a 3. Angle between deep water waves and depth contours: deg. ************************************************************************************** SURF CALCULATIONS

Enter SURF And Read Step GRAPH With

from step 1 and To. from step 2 H o/T o2

5 2 Ho/T02 from step 4 Hb/H0

ft. 6 3 H from step 1 and Hb/H0 from step 5 Hb Ho/ 02 from step 4 and BeachSlope from Breaker Type heading

8 5 Ho /To2 from step 4. If Ho/T02 <.01 go to next step

9 6 Ho from step 1 and db/Ho from step 8 or use db .. 1.3 Ho ifHo/T02 <.01 db ft.

db from step 9 and Beach Slope from Width of Surf Zone yds, 10 7 heading

Lb ft. 11 8 db from step 9 and To from step 2

No. Lines of Surf 12 9 Lb from step 11 and Width of Surf Zone from step_ 10. 13 10 db from step 9 and To from step 2. db/Lo

ab deg, Kd 14 11 ao from step 3 and db/Lo from seep13

corrected for refrac- 15 12 Hb from step 6 and Kd from step 14 H. tion cor Hb ft.

ab rom step 14 and Beach Slope from Longshore Current kts. 16 13 heading. lib from step 15 and To from step 2 AG.697 Figure 14.24. Sample surf worksheet.

527 513 044o Wolof Wovf Hotont Ho (Ff.)

20 Lloes of 001401 To2 19 18 IIIIIIIIIIIIIIMMIIIIIMIIIIIIMIIMUMMINIMMINIIIIIIIIMMIIIIIMIr 17 16 MIEMINIUMENIIIIINWANIWAINNMINMENINOP:miINNUMMENEWAIIIIIINIMMEMEr 15 MUM IV AMINIMIIIMI 14 ....rAumw...... m.....- mum...... mnr....a.m...... -50 30 20 13 EN 1.0 KinnMme.. .05 .101.04 12 IIIIMI 1111111 WallW11111111111§111111111%1111111111MINIMMINI 11 IMIIIIIIMINIA11111111=1" 10 MUM WARM= 101111/M12111111111/211111MIMIIIIMIMEN 9 INIMIVANIMUNIAM IIPAIIIMMIIIIIMMIIIIMIIIIIIIMMIN 8 MIIIMISMIAIIIIMMIIIIIIIMIIIM1111111111M1111141111111MINNII 7 11/A111/4111EINIVAIIIIMINILMIIIIIIIEWIMMOMMINNIff' 6 MIIIIIMINIVMMININIEMIIIIMINIIIPIIMMINIMAIIIIIIMM11. S MIIIMMWAIIIMERWIIIMIMINIMMIIIIIMINNItAINIAIIII%llrAOMIIIIIMIIIIPMIIPE:rillMdlIllMIMIIIIIIIPMIIIIII 311111111101MMIIIIIIMIIMIIIIMMITVIIMMIIIIIIIIIIII OM=MI= 2 AIVAIMIMMINIEWENIP%Mil- IMF-.0.0111M11110- =MN .11115.21MPITO-WililiwWilliN11"..M11111111111_Own 005 .003 min 5 6 7 8 9 10 11 12 13 la 15 16 17 18 19 WAVE PERIOD T (See )

AG.698 Figure 14.25. Surf graph 1.

STIT 9. Turn to surf graph0. figure 14-30. STEP 12. Turn to surf graph 9, figure 14-33. Enter the side of the ,raph with 11 foundin Enter the left side with the width of thesurf stepI and t he bottom with db I found instep tone found in step 10 and enter the bottom 8: \t the intersection read db from the curved ith1.1, found in step II. At the intersection lines on the graph and record this valueon the read the number of lines of surf. Ifa whole worksheet If 11/T2 is less than .01 in step 8, number of lines is not found, record the valuein do not use surf graph 6. Insteaduse the forint:la terms ofI or 2 lines," etc. STEP 13. Turn to surf graph 10, figure 14-34. (lb=1.311b Enter the left side with db found instep 9 and the bottom with To found in step 2.At the STEP 10. Turn to surf graph 7. figure 14-31. intersection read the value of db/L0 fromthe Enter the left side with db found instep 9 and Lurved lines. Record this valueon the work- go across horizontally to the line marked with sheet. the beach slope. From this pointgo down to the STEP 14. Turn to surf graph 11. figure 14-35. bottom of the graph and read the width ofthe 1,nter the left side with a found instep 3 and surf zone Record this value on the worksheet. the bottom with db/1.. found in step 13.At the STEP II. Turn to surf graph 8, figure 14-32. intersection read ab from the curved dashed Enter the left side with db found in step 9 and lines of the graph and readKdfrom the curved go across horizontally to the line marked with solid lines. Record the values ofab andKdon the value of To found in step 2. From this point the worksheet. go down to the bottom of the graph and read STEP 15. Turn to surf graph 12, figure 14-36. thevale: ofLb.Recordthis value on the Enter the left side withlib worksheet. found in step 6 and the bottom with Kd found in step 14. Atthe

528 5"34 Chapter 14 SEA SURFACE FORECASTING

2.80

2.60 BEACH SLOPE-1:10

2 40

2.20 0 ..,x i°2.00 X Li,o z 1.80 xH c.o 1.60

1.20

1.00

0.80 0.004 0.006 0 01 0.02 0.04 0.06 0 1 0 2 04 0 6 0.8 DEEP WATER WAVE STEEPNESS INDEXHo/T02

AG.699 Figure 14-26.Surf graph 2.

529 3r- ir"43 AEROGRAPER'S MATE 1 & C

BREAKER HEIGHT (Hb) AS A FUNCTION OF DEEP WATERWAVE (H0) FT. HEIGHT (Ho) AND BREAKER HEIGHT INDEX (Hb/Ho) 20 19 18 17 1611111112111101111 MI 15 MINIM M11011111111331174.. 14 MI 13WANNIMIMM1111111:1- MIMI 12ONII1011111M1. MieMINICIO1-10111111111.M(ER HEIGm. MI itON211611111MINIMIlMil 10 -- 9 --...., 20 8.11110.1.1.11.1.1MCMMI1M.1 mourn weemi 18 7MMEfia":...111M.IIIIIIII MEW! 1-1111..116 16 6 111.11.11 14 511.1=11110M-1.11 111111.12 _Wm. 12 4 ll1M-11_111.1111.11 Mawr Rime. 3...... Timmpem 1... 2imilmi...... -....IITAZZ.= I.. 1.0 1 2 14 I6 I 8 2.0 2.2 2.4 2.6 2.6 BREAKER HEIGHT INDEX (Hb/H0)

AG.700 Figure 14-27.Surf graph 3. intersection read a new value for 1-lb Lorrected All of the necessary information to provide a for refraction. Record this new value on the complete surf forecastis noAr available. The worksheet. presentation to the user can be made in any STEP 16. Turn to surf graph 13. figure 14-37. manner thatis agreed upon; however. figure This it, a nomogram to determine the longshore 14-38 illustrates one. of the most commonly current. The instructions for its use are written used methods that is employed. on it. Record this value on the worksheet.

530 5 7,6 Chapter 14SEA SURFACE FORECASTING

I Bench Slope 1:10

I SURGING\ PLUNGING SPILLING

Beach Slope 1:20

PLUNGING SPILLING

Beach Slope 1:50

PLUNGING SPILLING

0.01 0.02 0.04 0.06 0I 0.2 0.4 0 60.8 DEEP WATER WAVE STEEPNESS INDEX 1-10/T02

AG.701 Figure 14-28.Surf graph 4.

531 AEROGRAPIIEP'S MATE 1 & C

3.6

3.4

3.2 3.0\ 2.8

2.6

2.4

2.2

2.0 N

1.8

1.6

1.4

1.2

1.0 .01 .02 .09 .06 . .2 I.4 .6 .8 DEEP WATER WAVE STEEPNESS INDEX 110/1

AG.702 Figure 14-29.Surf graph 5.

532 5 38 Chapter 14 SEA SURFACE FORECASTING

20 19 18 17 16 15 14 20 u 13 012 18

11 .11M1 16 DEPTH OF BREAKING 0= 10 (db) FT. 9 1111:1111\1 1=MINNSIOIJIIARE-M1111. 8IIMMEML11IMII-Mi 6IMM 10 NIMMAIIIM -"ml 18 5 14 4 11.7.4.111111Mi'm- 10 3 mom 11111/11ratElonliM 6 2MEEKmum 1.0 1.2 1.4 1.6 1.8 2 0 2 2 2 4 2.6 2.8 3.0 3 2 3.4 3.6

BREAKER DEPTH INDEX (dh /Ho )

AG.703 Figure 14-30.Surf graph 6.

533 ta AEROGRAPHER'S MATE 1 & C

20 16 ir

4 VA F

0 100 200 300 400. WIDTH OF SURF ZONE (YDS.)

AG.704 Figure 14-31.Surf graph 7.

534 540 Chapter 14--SEA SURFACE FORECASTING

1111111111ENO 111111111=e1IME MOILMIEWE

8 IMINIMMEINEIN o 9 ct 10 w

0 I- L% 15

25 11111111111111k111%

30 IN EMI MI111 11d111111 50 70 100 200 300 400 500 BREAKER WAVE LENGTH, ( Lb ) FT.

AG.705 Figure 14-32.Surf graph 8.

535 541 AEROGRAPHER'S MATE 1 & C

NO. LINES OF SURF 5 4 500

400

300 5///4 3 200

100

100 200 300 400 500 BREAKER WAVE LENGTH (Lb) FT.

AG.706 Figure 14-33.Surf graph 9.

536 54Z Chapter 14 SEA SURFACE FORECASTING

Depth of Breaking (d b) FT Lines of equal Id /L0 ) 20 19 IN/11111/111/11111W IIIMINEMINIIIIIMMIAMMEMININIIIIII 18 AINIIIIMMIMNIVMMIIIIMPAIN=IIIIPME AMMINEMMINIIMMININWAIIMINIMIMIIMI 16 Nielliff VAIIIIMIIIAMININOWII=IUMMI 15 AMU/ MONIVAININIMMINIUM111111.1. 14 wags=411MEMINIIIIIMIFAMENEVAII1111111111E 13 611/1---1 ZAN IIIIIIIMINIIMMIMIWIIIIM111111% 12 21111112111111WilaiiigkilMIKANNIMPfdl=1 111111111111IMIIMI 1 11 WM AMNIA' INEWAIMMIMMIMIPS11111 1:NM IMINEWAIII ANOVA IMMII112121MINIEWIIIMMIPIMI 8 7liall1iAIllAW'aillillArAill. daILIMIllaial 190/ananalmiaialirla 6MU I' ANIAMWWAMIPVIIIIIMIIMMININICIIIIMMOIWZMIMIE 5 BLIMMTAWAINVA11111/ IIMILMINELMNIMMOMMIP1111111M1111 4 SIMIWAIIIMILMIPIAIIIM1 MR%IMPP.J.11111122MMIIIIII! 3 AllI2 A11/2111MIEWIEN11111/1011MOSIEMIKO=IIIIIIMMNII 2WAWA lips-,diPP-1...... -al- mr.r..a.- 1- Imi-imeowAraiza-aillimmem wmammt....P1--.....--_...... i=--=-.-- wiii...ismumwr.....a.,-,--.0.

8 9 10 11 12 13 14 15 16 17 18 19 20

Wove Period ( ) Sec

AG.707 Figure 14-34.Surf graph 10.

537 543 'S.1 S. 'S. S. ,. S - S I S. S I.0. Chapter 14 SEA SURFACE FORECASTING

Lines of Equal Breaker Height (Hb), Ft. MICM11111111TMCorrected for Refraction IIMMINMINMENIE=11INE.IMMINNMMIN111111 MICM11111AE1111W1111l'INIEMMIIIIIEBI..- IMMIIIIMIIM1111IMMMENIMENII11- woj11="11KLMINEMIIMpat. U_ MMI .o 111111 MINNAMEM 5 11.1111MINI CD 111111MMII Man

.50 .60 .70 aD . 80 .90 1.0 COEFFICIENT OF REFRACTION (Kd)

AG.709 Figure 14-36.Surf graph 12.

539 AEROGRAPIIER'S MATE I & C

Example (I.) Enter graph with T and 116. Locate intersection of line between To and 116 and the left diagonal. ° To 8.! (2.) Construct a line from beach slope through intersection, located above, to breaker 24 H 5.75 height line andmurkthis intersection. STopet. 1:60 (3.) Construct a line from ab to horizontal breaker height lineat top of figure and mark -"-110 ab 24° intersection on right diagonal. (4.) Construct line front mark located in (2) and intersection located in (3). Continue 22 line toet correct s eed: V3.5 knots. 15 10 6 44 2 0 t1. ) t 20 BREAKER HE1GHT,(Nb) FT.

16 00 la6 J-1:15 o Cl) / cl.:, O 14 R. tr' w . a. / c7 0 h 4/ 1 0As ,:4' 12 v/ e w o a -12o3< 4. 04. ex w .0 a3 -: -P .4 w J-1.25 8 c(... cc ...... a -P a) ...... - .... t) -Pt.,1 1:30 'PG ../ al x OA l?' 6)_1.35 wra / Nc 7-1'50/ a w 4'4.60 cc 12 d-1:70 / rN 2 /

4 30 25

BREAKER ANGLE, (ab) DEG.

AG.710 Figure 14.37. Surf graph 13.

540 546 Chapter 14 SEA SURFACE FORECASTING

sumsT (Beach) (Time) ORRLIE ALPHA BRAVO

FOXTROT DELTA EQ10

110T111

ALPHA = Sipificant Breaker Height(ft.)

BRAVO = Maximum Breaker Height (ft.)

CHARLIE = Period of Breakers (sec.)

DELTA . Type of Breakers

EC110 . Angle Breakers Make withBeach (deg.) pomor= Longshore Current(kts.)

GOLF = Number of Linesof Surf, Width of Surf Zone (yd.)

HOTEL = Remarks

AG.71 I Figure 14.38.Example of final forecast form.

541 5.-t7 AEROGRAPHER'S MATE I & C

FORECASTING SURFACE which are those associated with density differ- CURRENTS ences in water masses; and temporary currents, such as wind-driven currents which are devel- Although the forecasting of surface currents oped from meteorological conditions Tidal cur- has been performed by weather service person- rents are usually significantin shallow water nel for a number of years, the prominence of only, where they often become the strong or such forecasting became more evident whena dominant flow. number of incidents involving large sea-going oil The system of currents in the oceans of the tankers occurred. Collisions angroundings in- world keeps the water continually circulating. volving tankers caused great amounts of pollutant, The positions shift only slightly with theseasons oil mainly, to be spilled on the water except in the Southeast Asia area where mon- surface. The movement, both direction and soonal effects actually reverse the direction of speed, of such contaminants isdirectly con- flow from summer to winter. Currents appearon 'dolled by the surface currents in the affected most charts as well behaved continuous streams area. More concerned emphasis has now been placed on the ability of forecasters to predict defined by clear boundaries and with gradually changing directions. These presentations usually the movement of such contaminated areas. are smoothed patterns which were derived from In the past, weather service units havepro- averages of many observations. videdforecaststoassistinthe location of personnel or boats adrift in the open sea as well The speed of a current is known as its drift. as forecasts utilized in estimating ice flow. Drift is normally measured in knots. The term With the continued growing concern about velocity is often interchanged with the term pollution and contamination of ocean waters. It speed in dealing with currents although there is a is anticipated that more requests for current and difference in actual meaning. Set, the direction drift forecasts will be directed to weather service in which the current acts or proceeds, is meas- units. ured according to compass points or degrees. Inthis section we will discuss the general Observations of currents are made directly by characteristics of currents, how they form, and mechanical devices that record speed and direc- different types of currents. There are presently tion, or indirectly by water density computa- no hard and fast rules or techniques that are tions, drift bottles, or visually using slicks and universallyfollowed. Most weather units in- water color differences. volved in providing such forecasts have their Ocean currents are usually strongestnear the own innovations and methods. surface and sometimes attain considerable speed, such as 5 knots or more reached by the Florida CURRENTS Current. In the middle latitudes, however, the strongest surface currents rarely reach speeds Aerographer's Mates have a knowledge of the above 2 knots. major ocean currents and the meteorological Eddies, which vary in size from a few milesor results of the interaction of sea and air. Oceanic moreindiameter to 75 miles or more in circulation (currents) plays a major role in the diameter, branch from the major currents. Large production of and distribution of weather phe- eddies are common on both'des of the Gulf nomena. Principle surface current information Stream from Cape Hatteras to tne Grand Banks. such as direction, speed, and temperature distri- How long such eddies persist and retain their bution is relatively well known. characteristicsnearthesurfaceisnot well Currents in the sea are generally produced by known but large eddies near the Gulf Streamare wind, tide, differences in density between water known to persist longer than a month. The masses, sea level differences, or runoff from the surface speeds of currents within these eddies, land. They may be roughly classed as tidal or when first formed, may reach 2 knots. Smaller nontidal currents, Nontidal currents include the eddies have much less momentum andsoon die permanent currents in the general circulatory down orlosetheirsurfacecharacteristics systems of the oceans; geopotential currents, through wind stirring.

542 .J. 8 Chapter 14- SEA SURFACE FORECASTING

Wind Driven Currents Tidal currents, a factor of little importance in general deepwater circulation. are of great in- Wind driven currents are. as the name implies, fluenceincoastalwaters. The tides furnish currents that are created by the force of the energythroughtidalcurrents, which keep wind exerting stress on the sea surface. This coastal waters relatively well stirred. Tidal cur- stress causes the surface water to move and this rents are most pronounced in the entrances to movement istransmittedtothe underlying large tidal basins that 1,-ve restricted openings to water to a depth which is dependent mainly on the sea. This fact often accounts for steerage the strength and persistance of the wind. Most problems experienced by vessels. ocean currents are the result of winds that tend to blow in a given direction over considerable WIND DRIVEN amounts of time. Likewise, local currents. those CURRENT PREDICTION peculiar to an area in which they are found, will arise when the wind blows in one direction for Attempts at current prediction in the past some time. In many cases the strength of the have only been moderately successful. There has wind may be used as a rule of thumb for been a tendency to consider ocean currents in determining the speed of the local current, the much the same manner as wind currents in the speed is figured as 2 percent of the wind's force. atmosphere, when in actualityit appears that Therefore, if a wind blows 3 or 4 days in a given ocean currents are. affected by an even greater direction at about 20 knots, it may be expected number of factors. It therefore requires different that a local current of nearly 0.4 knot is being techniques to be utilized. experienced. In order to predict current information it A wind-drivencurrentdoesnotflow in must be understood that currents are typically exactly the same direction as the wind, but is unsteady in direction and speed. This has been deflected by the earth's rotation. The deflecting well documented uy a number of studies that force (Coriolis force) is greater at high latitudes have been conducted. The reasons forthis and more effective in deep water. Itis to the variabilityhas been attributedtothe other right of the wind direction in the Northern forces b sides wind and tides which affect the Hemisphere and to the leftinthe Southern currents. Hemisphere. At latitudes between ION and IOS Climatological surface charts have been con- the current usually sets downwind. In general structed for nearly all the oceans of the world the angular difference in direction between the using data from ship's drifts. However, this data wind and the surface current varies from about has been shown to have limitations and should 10 degrees in shallow coastal areas to as much as be used as a rough estimate only. 45 degrees in some open ocean areas. The angle Synoptic Analysis and Forecasting of Surface increases with the depth of the current and at Currents, NWRF 36-0667-127, provides a com- certain depths the current may flow in the posite method of arriving at current forecasts, opposite direction to that of the surface. This method utilizes portions of other methods Some major wind-driven currents are the West that have been used. Forecasters should =Ire WindDriftin the Antarctic, theNorth and themselves aware of the information contained South Equatorial Currents that lie in the trade in this publication. windbelts of the ocean, andthe seasonal monsoon currents of the WesternPacific. COASTAL AND TIDAL CURRENT PREDICTION Coastal and Tidal Currents Prediction of tidal currents must be based on Coastal currents are caused mainly by river specific information for the locality in question. discharge, tide, and wind. Ilowever, they may in Such information is contained in various forms part be produced by the circulation in the open in many navigational publications. ocean areas. Because of tides orlocal topog- Tidal Current Tables, issued annually,list raphy, coastal currents are generally irregular. daily predictions of the times and strengths of

543 J' 9 AEROGRAPHER'S MATE I & C

flood and ebb currents and the time of interven- art,alliiedin the preparation of prognostic ing slacks. Due to lack of observationaldata, charts which are disseminated. coverage is considerably more limited than for Sea surface temperature charts,wave height, tides The Tidal Current Tables do include and wind drift currents are examples of thetype supplemental data by which tidal currentscan of charts available on the fleet facsimilebroad- be determined for many places in additionto cast. Up to date schedules should reflect the those for which daily predictions are given. charts currently available as this is subjectto change. Tailored forecasts for sea conditionsare available on a request basis from wother facil- NUMERICAL SEA SURFACE ities and centrals. Oceanographic departments ANALYSES AND FORECASTS within these organizationsare prepared to provide this service. Analyses of a number of sea surface elements Ra tt-graphic types ofmessages are also util- is carried out on a routine basis. Althoughthese ized to disseminate sea condition information charts are not readily availableas analyses they over the fleet teletype broadcast.

544 550 t CHAPTER 15

OCEAN THERMAL STRUCTURE FORECASTINGAND ASWEPS

Itisgenerallyagreedthatpresently the mended that personnel review this chapter for submarine poses the greatest military threat to applicationtothe data as presented inthis the security of the United States. They may be manual. employed to tire missiles at inland targets as well as disrupt the merchant shipping which is so THERMAL STRUCTURE critical to our nation's survival. Since World War FORECASTING 11 the Navy has directed a concerted effort to the improvement of methods and equipment to Bathythermograms show that the ocean is detect these undersea weapons. more or less stratified. Two points separated by The most effectual means of reducing their several hundred yards but at the same depth will effectiveness is by detecting them within their have practically the same temperature. If the own environment, under the oceansurface. ocean were in equilibrium, thisstratification Soundisemployed indifferent manners to would be complete; the warm lighter water accomplish this the active and passive detection being at the surface, the lower strata consisting systems being examples. Active systems put of cooler, heavier water, and the boundaries sound into the water with the echo returning between strata being horizontal surfaces. This from the submarine, being sensed by the detec- equilibrium is disturbed by three processes: tor. The passive system utilizes sound sensing advection, the heat budget, and mixing. equipment to detect the noise emitted by the submarine. Surface ships and aircraft, including OCEAN THERMAL EFFECTS helicopters, employ these systems or variations ON ECHO RANGING of them. Most people are aware that sound is affected The direction that a sound wave will travel in to a great extent by the changes in air tempera- the ocean is largely dependent upon the speed of ture. The temperature of the water alsoplays the individual sound wave or ray within the games withthe sound waves as they pass beam. The speed of sound in sea water depends through it. on the properties of the water. In generalthese Aerographer's Mates work in close association properties vary both horizontally and vertically, with units involved in ASW operations. Itis but only temperature is of any great signifi- important that they have a thorough under- cance. Changes invelocity with depth. even standing of the thermal structure of the ocean, slight ones due to warming of the surface water as well as what numerical products areavailable. on a bright calm day, deflect the soundbeam their application, and what type of services and from the desired straight path and may cause it assistance are provided them by support activi- to overshoot or undershoot an object. ties. These areas will be discussed in this chapter. The velocity of sound is directly proportional In chapter 22 of Aerographer's Mate 3 & 2, to the temperature of the medium. The speedof NavTra 10363-D, illustrations and definitions of sound in air is 331.5 meters per second at 0 the basic terminology is presented. It is recom- degrees Celsius and increases 0.6 meters per

545

r-tI.t.)4..) AEROGRAPER'S MATE 1 & C second for each degree increasein temperature. In sound transmission In the vertical velocity sea water with salinity of 35parts per gradient thousand at 0 degrees Celsius is more important than thevelocity the speed of sound itself, sinceit is is the change in velocity with 1449.1 meters per secondor 4.4 times the depth that determines how much speed of sound in air. Therate at which speed refraction will takeplace. The velocity gradientisreadily increases with an increase intemperature is not determined from the gradients of uniform: itis greater at lower temperature temperatures. and salinity. The salinity gradientis defined as Figure 15-1 illustrates the effectof temperature the rate of change of salinity with on the speed of sound in water. depth in parts per thousand per meter. The temperature The pressure and salinity affect gradi- the speed of ent is the rate of change oftemperature with sound also but to a lesser degreethan tempera- depth in degrees Celsius ture. As the depth or pressure increases per meter. These gradi- so does ents are therefore called positive ifthe quantity the sound velocity. An increasein salinity will also increase speed. in question increases with depth,and negative if it decreases. In the majorityof cases, tempera- In echo ranging work, inwhich only the ture gradients in the sea upper few hundred feet are involved. are zero or negative. tempera- Moreover, exceptincertain tureisgenerallythe most important factor localized areas, temperature gradients control the soundvelocity causing variations in sound velocity.Salinity is gradients. relatively uniform in theopen ocean and therefore of minor importance when With a zero gradient intemperature (mixed dealing with layer) echo rangesare long because the sound sound velocity. Furthermore. inlayers where rays are very nearly straight, having onlya slight vertical salinity gradients exist,there is nearly upward curvature due to the always a vertical temperature gradient. pressure effect. On the other hand, with a strong negativegradient

1 I I

5 024 -SALINITY 35 PARTS PER THOUSAND

v ta co I 4930 tz.

4835

4740 30 40 50 60 70 80 90 TEMPERATURE (°F)

AG.712 Figure 15-1.Effect oftemperature on the speed of sound in water.

546 5.5 Chapter 15 OCEAN TIIERMAL STRUCTURE FORECASTING ANDASWEPS nearthesurface. echo ranges will be short Permanent Currents because the sound beam is refracted sharply downward. In the ocean it is cone: on to find a The redistribution of density resulti g from mixed layer (isovelocity) overlying a negative the wind-driven current is what maintains the gradient. In such cases the echo range on an permanent current. Under the influence of the object in the mixed layer will be long, but the steady wind systems, such as the trade winds, in part of the sound beam that enters thenegative the lower latitudes and the westerlies in the gradient will be refracted downward, resulting in higher latitudes, these permanent currents form a reduction of range, and a splitbeam pattern. the large scale current systems of the oceans. With a strolls negative gradient from the They are partly the indirect result of geographic surface downward, each ray of sound beam differences in the heating and cooling of the curves down in a great arc, and that areabeyond water and partly the result of wind action. The the horizontal limits of the beam is a so- called character of the currents is also influenced by shadow zone into which no sound penetrates the configuration of the oceans, but in general other than by scattering. An echo ranging vessel thereareclockwisegyralsinthe northern will not be able to detect an object in the hemisphere and counterclockwise gyrals in the shadow zone, but as soon as the object comes southern hemisphere. Smaller currents exist near within the direct beam, the echoes will come in the continents. A countercurrent flows eastward loud and clear. between two westward flowing equatorial cur- Withslighternegativegradients, generally rents. with any gradient underlying a mixed layer, the The permanent currents have several effects shadow zone is not very clearly defined. on the temperature conditions.Currents with An object in the negative gradient beneath a poleward flow tend to carry warm water into mixed layer may be within the direct beam but cooler regions; conversely, equatorward flowing still be undetectable because the echoes are too currents bring cooler waters into warmer re- weak to be heard against the background or gions. Within the currents themselves the distri- reverberation and ship's noise. This is known as bution of density produces a temperature gradi- the layer effect. ent such that. in the northern hemisphere, the water on the left side of a current has a lower average temperature than water onthe right FORECASTING ADVECTION side. This may be reflected by a thinner mixed layer or even by lower surface temperatures. In The effect of the addition or removal of water the southern hemisphere the structure isre- by currents which result in the changing of the versed. thermal structure of the water at a specific point is referred to as advection. In most cases this Divergence and Convergence mass transport of the water masseswill be of Surface Currents accomplished by the ocean currents, of which there are varying types. Divergence of surface currents may occur under the influence of the wind. Examples of Wind-Driven Currents this are found along the western coasts of the continents and in the vicinity of the equator in Thefrictionaldrag of the wind sets up the eastern parts of the Atlantic and Pacific. In wind-driven currents which flow at less than 3 these areas upwelling brings water toward the percent of the wind velocity. Thesewind-driven surface from moderate depths and the thermo- currents do not flow with the wind but are cline may be shallow or,in extreme cases, deflected 45 degrees to the right in the northern absent. The opposite effect, convergence occurs hemisphere and 45 degrees to the left in the in the center of the subtropical gyrals in the southern hemisphere. Thisiscaused by the northern and southern hemispheres. In these earth'srotation andis closely related toits regions the surface water accumulates and conse- influence on the depth of mixing. quently the thermocline may be very deep.

547 553 AEROGRAPHER'S MATE I & C

Tidal Currents

65 °F Tidal currents in partially isolatedshallow areas have a marked effect on thetemperature 70°F conditions because they alsocause turbulent mixing. In the areas of strong tidalcurrents such as the English Channel, the watermay remain 75 °F virtually mixed throughout theyear. although there is. of course. heating and coolingof the / 80°F water column as a whole.

Internal Vaves

Internal waves also affect the 65°F temperature 80°F distribution. The effect of thesewaves is re- flectedin a periodicriseandfallof the 70°F thermocline. Periods as longas 12 or 24 hours

are known.to exist and studies have shown that 75 °F waves of only a few minutes period mayoccur. Whether thereisa continuous spectrum of frequencies is not known. 80°F

8° F Methods of Fore- casting Advection

Advection can be computed from knowledge Figure 15.2.Typical advection. AG.713 of the temperature and ocean currentfield. The logical approach would beto assume the Unfortunately the former is only moderately I3T trace at point B represents thewater that well defined in terms of the detail required for will be at the forecast point in 24hours, and advection computations, and the velocityfield is apply the heat budget and modificationsto this not only difficult to predict. but is not directly trace instead. This is still not completely valid, observed There are two ways to regard ticket:- since the entire water column from thesurface tion- t l ) by computing the change oftempera- to the bottom of the BT trace does notmove at ture with time at the forecast point: or (2) by the same speed. The speed of thecurrent finding the source of the water expected at the generally decreases with depth, and inthe case forecast point and using the thermalstructure of of wind drift, currentsmay be zero at the the source as the forecast. thermocline depth. This problem also arisesin f he difference is illustrated in figure 15-2. the first approach since advecting thewhole where a forecast is desired for point ,\,and it is thermal structure at the same speed would bein assumed that the 24 hour advection is from error. point B to point A. Recent 13T observations are The publication. Ocean ThermalStructure available for both points. One approachcom- Forecasting, SP-I05, Volume 5,outlines four monly used is to make all the heat budget and conditions which cover most forecastingprob- mixing modifications to the BT at point A. and lems that will be encountered. Proceduresand as alaststep the changes resulting froma necessary graphs for predicting advectionare different water structure being adveLtedfrom contained within this publication. Theproce- point B are considered. This is not themost dures given are too lengthy to becontained realistic approach as the water at point A will within this manual: however, it isrecommended not be present in :4 hours. The water coming that direct reference be made to theSP-I05 from point B represents the water that willbe publication for detailed informationon fore- there instead. casting advection.

548 554 Chapter 15 OCEAN THERMAL STRUCTUREFORECASTING AND ASWEPS

FORECASTING THE Effective Back HEAT BUDGET Radiation The temperature structure of' the ocean is Effective back radiation is the term used for determined primarily by its heat content. which the excess of infrared emitted by the sea surface This balances isa constantly varying quantity.There is a over that received from the air. continuous exchange of heat at thesurface of somewhat less than one-half of the incoming the ocean. The ocean receives heat by absorp- solar radiation. on the average. It decreases with tion of the sun's radiation and by thecondensa- increasing humidity and increasing cloud cover, tion of water vapor in the air, when the wateris and may increase or decrease withincreasing colder than the air. The ocean loses heat by water temperature. The latter isdependent on radiation to the atmosphere. by evaporation of how much vaporization affects the water vapor water vapor when the water is warmer thanthe content of the overlying air.With heavy. low air, and possibly by conduction. of the received lying clouds present, the effective back radiation heat. by far the largest quantity is due tothe drops to less than 25 percent of that on a clear incoming solar radiation. Over the ocean as a day, largely because the clouds themselves are whole itis balanced by thecooling resulting sources of infrared andradiate heat into the from reradiation and evaporation. ocean on their own account.Clouds prevent direct solar radiation from reaching the sea Incoming Radiation surface. Heat losses from back radiation occurin the uppermost fraction of an inch inthe water The incoming radiation includes the invisible and are transmitted to greater depths by convec- infrared and ultraviolet as well as visible light. tive overturn and wind mixing. Since itis received from the sun throughthe atmosphere. it obviously varies withlatitude. Evaporation season, time of day, and theatmospheric conditions. particularlythe cloud cover. The total Evaporation depends primarily upon the tem- energy received during the yeardecreases with perature of the water and the air,the humidity, increasing latitude and in the lower latitudesof and the wind strength. Evaporation canbest be the tropical regions the seasonalvariationis understood by considering the process as oneof small, but with increasing latitude thedifference transfer of water vapor away from thesurface. betweentheamountsreceivedduringthe The greater the water vapor gradient the more summer and winter becomes verygreat. The rapid the evaporation and hence the greaterhe effect of clouds is very pronounced; a heavy heat loss. Cold. dry air overlying warm water cloud cover may reduce the incomingradiation therefore favors rapid evaporation.High winds toless than 25 percent of that received on a increase evaporation byremoving the water clear day. vapor. Direct heating of the water by the sunis limited to relatively shallow depths. Onlyabout Forecasting Method below 300 3 percent of the radiation penetrates by the feet and over 50 percent (all theinfrarred) is fhe heat budget will be represented absorbed in the first few inches.If there were no following equation: compensating heat losses and no mixingfantasti- cally high surface temperatures andextremely Q = Q, Qb Qh sharp negative gradients just belowthe surface would occur. The penetration of lightvaries where Q =Net gain or loss of heat somewhat from place to place depending upon QC=Insolation the amount of suspended debris andorganic = feat gain by condensation pigments in the water. This applies to open Qh Effective back radiation ocean, since near shoreand in areas of heavy =Heat loss owing to evaporation plant growth the water is practically opaque to Reflected radiation Heat conduction across interface all wavelengths. Q1, = 549 555 AEROG RAMER'S MATE I & C

To determinethevalue of the individual components, itis necessary to compute values NORTH LATITUDE SOUTH LATITUDE 50 40 from graphs andnomograms. The publication. 30 20 10 0 10 20 30 40 50 175 Ocean Thermal StructureForecasting. SP-105. contains these values alone withthe necessary 150 graphs and nomograms. It is recommended that 125 forecasters utilize this publicationwhen determining the gain or loss of heatto the ocean 100 surfaces. 75

50 FORECASTING MIXING PRECIPITATION

To forecast the mixing that SURFACE SALINITY will occur in a 3E00 portion of the ocean itisaccessary to first ;i discuss some of the mixingprocesses that take 3500 place. 3400 Convective Overturn

When surface water cools. itsdensity increases AG.714 andit sinks.causing convectiveoverturn. Figure 15-3.Variation ofaverage evaporation, precipi- Equally importantis the increasein salinity tation, and salinity with latitude. Shadedareas show resulting from evaporation. Theincreased den- regions where precipitation exceedsevaporation. sity, arising from thiscause. contributes greatly to overturn and the development ofisothermal Mechanical Mixing surfacelayers. Thus, cooling by evaporation increases density in two way s and is less likely to Mechanical mixing is caused by wind-Ind does be accompanied by positivetemperature gradi- not necessarily involve any gainor loss of heat; ents than is cooling by radiation alone. neverthelessitmuy modify the temperature Conditions that tend to lessen thesalinity of distribution. The effect of winddep' ads not the surface layer would have theopposite effect. only uponitsstrength. butalsouponits and would tend to favor thedevelopment of duration and on the distanceover which it has positive gradients. Sucha condition might result blown It is quite obvious that the firsteffect of from precipitation. For theocean as a whole the wind will be confinedto the immediate however, evaporation exceeds precipitation.This surface, but that the turbulencewill extend to is shown in figure 15-3. Itw ill be noted in the greater Cepths after the wind has beenblowing figure that regions ofexcess evaporation in low for some time. The original densitydistribution and mid-latitudes correspondto regions of rela- of the surface layer will affectthe rate at which tively high surface salinity and deepthermo- the turbulence penetrates thelayer. A very clines. Just north of the equator and inlatitudes stable layer will be less easily mixed. above 40 degrees, where precipitationexceeds evaporation, the surface salinity is low, Rotation of the Earth The deficit in the water content of the ocean It is a remarkable fact that thedaily rotation that is caused by the generalexcess of evapora- of the earth about its axis also affects tion over precipitation the depth is made up by runoff towhichthewind mixing penetrates. The from land. Near land, and especially near the theories concerning the rotation effectare too mouths of rivers, surface salinitiesare lower than involved to be included in this manual, in the open oceans or at depths. This however, favors the allagreethat a wind of given development of positive temperature gradients. force will ultimately produce a deeper mixedlayer in low since it increases their stability. latitudes than in high.

550 r556 Chapter 15 OCEAN THERMAL STRUCTUREFORECASTING AND ASWEPS

Forecasting Methods Charts dealing with ocean surface conditions are prepared and disseminated also.Although Forecasting procedures for predicting mixing sea conditionis frequently ignored or lightly action over the ()Lean involves the computation touched by forecasters, it is of great importance of a number of variables. The procedures are tothe ASW operator, asitcan reduce the discussed, with examples, inthe publication. effectivenessof equipmenttoaminimum. Ocean Thermal Structure Forecasting, SP -I05. Combined seaheight datais computed and Personnel should refer to this publication for disseminated in both facsimile chart and message procedures and formulasfor preparing such form via the Fleet Broadcast. Data on sea state forecasts. and sea waves are also prepared but not as widely disseminated. This information is available upon request. APPLICATION OF NUMERICAL PRODUCTS A chart of selected BT traces is also received TO SUBSURFACE FORECASTS by some units. These are actual BT traces for selected points. Location may be given, keyed to In preparing forecasts for ASW operations operational orders, or the traces may be for forecasters will rely to a great extent on charts predetermined points. The usage of the BT that are received via facsimile. Most of these will traces is somewhat iimited unless they are in be numerically produced and they will include close proximityto the point for which the the Sea Surface Temperature Analysis (SST). the forecast is being prepared. They will, however, Sonic or Mixed Layer Depth Chart (SLD or for their position, provide excellent data for MLD), the Gradient Below tin. Surface (GRD), estimating sonar conditions, by allowing the user Selected Bathy thermograph Traces, Sea Height to evaluate the temperature gradient in each Prog Charts, and others whose availability may layer of water. vary. Two of the most widely used numerical Sea Surface Temperature charts are con- products used in the preparation of the ASW structed to provide a representative view of the forecast are the ASRAPS and SHARPS which main sea surface temperature patterns. Only the are received via message. Bothof these products features of the SST field are shown. Diurnal and will be discussed in the next section of this other short-term temperature changes are not normally detected. SST charts provide funda- chapter. mental information for the conversion of en- vironmentaldataintooperationaldataof concern to ASW forces. This information may THE ANTISUBMARINE WARFARE be readily utilized in forecasting of sonar ranges. ENVIRONMENTAL PREDICTION Sonic Layer or Mixed Layer Depth charts are SERVICE based on BT reports received. MLD/SLD charts SST charts to are used in conjunction with The Antisubmarine Warfare Environmental determine the depth at which submarines may Prediction Service, commonly referred to as or may not be readily detected.Areas in which ASWEPS, was established during the early part sonar ranging may alsobe affectedcan be of 1959 for the purpose of developing an determined. integrated system of predicting and displaying The Gradient Below the Layer depicts the parametersfor antisubmarine warfare opera- vertical temperature gradient in terms of change, is designed to provide oceanographic plus or minus, that is taking place over a given tions.It distance, which in this case is depth. The chart analyses and forecasts utilizing up-to-tbe-minute delineates areas of change in degrees per 100 data collected from various observation points. feet below the some layer. Temperature gradi- Oceanographic data is collected by the Re- ents provide a measure of the verticalsound gional Oceanographic Net and the Mobile Ocean- velocity which may be utilized to determine the ographic Net. The raw data is then processed ray curvature and other variables. into usable ASW oceanographic data and re-

551 557 AEROGRAPHER'S MATE 1 & C

turnedtothefleetusers intheform of ceived in message form, interpretedand pre- environmentalforecasts,regionalcharts, and sented to the user by Naval Weather detailed area charts. Service personnel, except in thecase of smaller ships where this is done by sonaroperators. OCEANOGRAPHIC SERVICES ASRAP provides standardenvironmental and A variety of oceanographic services tactical predictions for patrol (VP) ASWforces are avail- for both active and passive able to fleet users. These include:( I) general systems. AS RAP oceanographic analysis and forecasts; displays are standard with thepassive ranges (2)tai- being shown using lored products such as analysisand forecasts agraph-likechartwith prepared for environmental effects decibels and range innauticalmiles as the upon specific vertical and horizontal axis, ASW sensor systems; and (3)oceanographic respectively. Infor- outlooks for specific areas. mation is provided for both shallowand deep Fleetmeteorological targets using hydrophones. The active ASRAP unitsand NWSED's display presents forecasted modify and tailor data receivedfrom central ranges for active sys- computing points, then presentthem to the tems with varying combinations of Target/SUS and Hydrophone depths. Theforecasts pro- local users, primarily in the formof predicted vided sensor ranges. arefor predetermined points oflati- tude and longitude. These points The various charts, both analysisand prognos- are considered tic, are utilized by the weather unit to be representative for the entirecorresponding along with area surrounding them. message type forecasts to preparea standard ASW environmental briefing folder. The SHARPS provides routine dailyrange This folder calculations based upon propagation provides each flight crew with all thesynoptic loss data and forecasted oceanographic data for fleet sonar systems. The SHARPSsystem is as well as area oriented, an advantage to both operational forecasted sensor ranges. It will usuallybe issued planners and totheflight crews atthe preflight weather tacticians. SHARPS providesa briefing. spectrum of ranges expected in anarea. Bottom reverberationlosscalculations,convergence zone, and bottom bounce ranges ASRAP and SHARPS are provided for shipboard systems, while passiveranges for helicopter systems are also included. Thesystem The Acoustic Sensor Range PredictionSystem isflexible in thatitis applicable to all active (ASRAP) and Ship Helicopter AcousticRange sonar systems in a variety of operating situ- Prediction System (SHARPS) have beendevel- ations. oped to provide a standard environmental and Both systems are under continuousevaluation tactical acoustic range predictionsystem applic- and are subject to change to able to both active and passive cover additional systems. This systems, take advantage of new data,or present range predictioninformationis normally re- a better display to the user.

551 558 CHAPTER 16

SPECIAL OBSERVATIONS ANDFORECASTS

This chapter is intended to provide informa- application of this data as part of their environ- tion related to the observation and forecasting mental support services. The NationalWeather of those environmental parameters which are Service has established Environmental Meteoro- difficult to categorize with the commonly recog- logical Support Units (EMSU) at major cities in nized types of meteorological and/or oceano- the United States to assistin providing the efficient graphic data. necessary data required to maintain an pollution forecasting program. Naval Weather Service personnel, as practicable, should estab- AIR POLLUTION POTENTIAL lish a close working relationship with theEMSU Meteorologist. Air Pollution Potential (APP) is definable as a The problem of air pollution is caused basi- measure of the inabilityof the atmosphere to cally by a source emitting pollutants within a adequately dilute and disperse pollutants emit- space which is unable toadequately dilute or ted into it based on values of specific meteoro- dispersethese pollutants. Both the pollutant logical parameters of the macroscalefeatures. source and the dispersion ordilutant capabili- The Development Division of the NationalMete- tiesof thearea must be consideredwhen orological Center (NMC) and the Divisionof determining air pollution potential. Some under- Meteorology of the National Air PollutionCon- standing of the nature of pollutants is therefore trol Administration (NAPCA) havedeveloped required. criteria and procedures to delineate areas onthe macroscale in which high APP has the greatest POLLUTANTS possibility of occurring. Asa result, air stagnation guidance data prepared by NMC is disseminated Pollutants are particles, gases, or liquid aero- via facsimile and teletypewriter. sols in the atmosphere which haveundesirable The Naval Weather ServiceCommand is re- effects on man or his surroundings. Themagni- sponsible for developing and maintainingup-to- tude of the concentrations of thesepollutants dateproceduresrelatedto interpreting and ordinarily determines their undesirablecharac- tailoring National Weather Service airpollution teristics. If, for example, air reaching aninstalla- potential (APP) forecasts for interestedU.S. Navy tion is pollution free, then theinstallation might sea and shore activities.This section will present be able to emit pollutants withoutexceeding information relative to the use of APPforecasts undesirable concentrations evcri underrestricted and the procedures fo.iloring them for local dispersion conditions. On the otherhand, if the application. air reaching tit.installation is already saturated dispersion The routine applicb of meteorology to with pollutants, then even under good local air pollution coati.,, is relatively newby conditions, pollution emissions might notbe predict- comparison to other aspects of forecasting.It is advisable. Such cases can be resolved by forecasters to ing pollution concentrations through acompari- incumbent upon the individual properties of the apply their experience and judgementin the son of existingdiffusion 553 559 AEROGRAPHER'S MATE I & C

atmosphere with the known operatingcharacter- FAVORABLE CONDITIONS isticsof theplant,factory, ship, or other FOR POLLUTION pollution source. The three principal meteorological conditions Dispersion Concepts which have been found to bemost favorable for the formation of a pollutant stagnationarea are Atmospheric dispersion conditionsare classi- as follows: fied as good. moderate,or poor, the latter being acondition of highair pollutionpotential I. A slow moving anticyclone witha small (HAPP). horizontal pressure gradient. The idea of the atmosphere dispersing,dilut- 2. Light surface winds not exceeding ing, or ventilating pollutants is easily seven visualized. knots and winds aloft not exceeding 25knots. With an unstable lapse rate througha deep layer 3. Subsidenceinthe lower layers of the of the atmosphere and a strong wind, pollutants atmosphere. This phenomenon with itsattend- may be spread through an extensive volume of ant warning and drying effect produces stabili- the atmosphere and diluted to minimal concen- zatic n and the formation of inversionswhich trations. On the other hand, a low inversionand limit vertical mixing. a light wind may confine emissions to a shallow atmospheric layer and pollutant concentration3 The greatest variations of air pollutionpoten- become larger. If the latter conditions persist tialare those of short duration due to the and emissions continue, pollutant concentra- systematic variation of wind and stabilitybe- tions may become unacceptable. tween night and day. As stated earlier in this chapter, atmospheric APP Terminology dispersion of pollutants is describedas good, moderate, or poor in intensity. The intensity Discussing pollutant dispersion quantitatively assigned depends on the mixing depth (MXDP) requires the use of a number of descriptiveterms and the transport windspeed (TW). Whether with which the forecaster must become familiar. these conditions will persist is governed bythe These terms are defined in the followingpara- presence or absence of a stagnation graphs. area. A synoptic situation with a deep unstablelayer AIR POLLUTION POTENTIAL (APP).A and a strong wind is describedas having a large measure of the inability of the atmosphere to MXDP and a large TW; whereas,a synoptic adequately dilute and disperse pollutants emit- situation with a low inversion and low wind ted into it. based on values of specificmeteoro- speed is characterized as havinga low MXDP and logical parameters of the macroscale features. low TW. The product of MXDP and TWis MIXING HEIGHT.The surface-based layer termed ventilation andis in a measure of the whichrelativelyvigorous mixing occurs atmosphere's capability to diluteor to disperse (niters). pollutants. The use of the term "stagnation TRANSPORT WIND SPEED.Ameasure of area" provides an objective delineationof a the average rate of the horizontal transport of geographical area where the atmosphere will air within the mixing layer (metersper second). undergo little synoptic change and where pollut- VENTILATION. The product of the mixing ants will accumulate. It is an important forecast height and the transport wind speed. Ameasure parameter and will be discussed inmore detail of the volume rate of horizontal transport of air later in this chapter. withinthe mixing layer, per unitdistance, normal to the wind (meters2 per second). Parameters and Critical Values for STAGNATION AREA.A combination of Delineating Stagnation Areas stable stratification, weak horizontal wind speed components, and little, if any, significant preci- Computations for delineation of stagnation pitation. Itis usually associated with a warm- areas are accomplished at NMC primarily by core type anticyclone. computers. Furthermore, asaresult of the 554 560 Chapter I 6SPECIAL OBSERVATIONS ANDFORECASTS relative newness of APP forecasting, parameters thus, most usaful in describing dispersioncondi- and critical values are continually being reevalu- tion areas within or downwind of urban areas. ated and are subject to change. In view of these Ilowever, by tailoring the forecasts in a manner facts. itis not feasible to list specificitems in similar to that described later inthis section this publication.Itis sufficient to state that they may be applied to other locations. wind speed, stability criteria, and precipitation In the preceding paragraphs we have discussed are major considerations. dispersion of pollutants in general terms. In the following paragraphs information is presented Calculation of Mixing Height relatingto obtaining Air PollutionPotential and Transport Windspeed products over the facsimile and teletype circuits and the application of this data toforecasting, Once the stagnation areas have been determined, the next step is to calculate the mixing Facsimile height andthetransport wind speed. These computations as well as those previously men- Facsimile charts depicting APP information tioned for stagnation areas are normally accom- are transmitted daily overthe NMC FOFAX plished objectively by computers at NMC. circuitsatvarious times depending on the circuit. A detailed description of these charts NMC AIR POLLUTION may be found in TechnicalProcedures Bulletin POTENTIAL P' )DUCTS No. 69 (or its revision) available from U.S. Department of Commerce, NOAA, NWS, Silver National Meteorological Center APP products Spring, Maryland 20910. transmitted over facsimile and/or teletype cir- The data entered on the four panel facsimile cuits and the dispersion criteria derived from chart will be similar to that illustrated infigure them identify meteorological conditions associ- 6-1 . Again, the newness of the program makes ated with the large-scale buildup and dispersion thedatasubjecttochange. The datawill of pollutants over or downwind of an urban primarily be used to assistin making HAPP area. Such areas emit pollutantsfrom ground- advisories. To a limited extent, the data may level sources as well as from elevated sources, also be used in making local forecastsof good such as stacks, etc. Downwind of the sources and moderate dispersion conditions. and over the areas where pollutantsfrom all concentrations APPLICATION.--The APP charts have two sources mix together, pollution applications. Primarily, it alerts the forecaster to correspond well with observed APP conditions. poor dispersion conditionsand to the need for a Keep in mind how..:ver that current and forecast HAPP advisory. Secondarily, it can be used to a meteorological conditions must be considered issuing if a limited extent as a data source for when utilizing these products. For example, To exists below stack advisories of good and moderate dispersion. very stable ground inversion of a local HAPP advisory, itis very use the chart as a basis heightoutside of an urban area, determine if probable that stack emissions woulddrift over the forecaster examines the chart to effect on his ship or station will be in an area conduciveto the area above the inversion with little local ground pollution concentrations. Pollution con- the accumulation of pollutants. To check the TW or MXDP 'values, the forecaster can consult centrations would probably remain low and FKUS-1, pollution-free despite the detailed mesoscale information of area might be considered the local Environmental MeteorologicalSupr,rt an APP forecastcallingfor poor dispersion radiosona, conditions. On the other hand, in an urban area Unit (EMSU) sounding, or a nearby from Assuming HAPP criteria are met, an advisory where ground-level sources of pollution authorities. high volume vehicular traffic, numeroushome issued to the appropriate control and commercial heating installations, etc.,exist, All Naval Weather Service units receivevia these factors would make up for thereduction Comet III circuitry the FKUS 1-AirPollution in pollution from the elevated sourcesand the PotentialData and FKUS-2 Air Stagnation is APP forecast would verify. Forecastsof APP are. Narrative. Detailed information on FKUS 1 555 561 START TRANStriSSION HERE .1, ' ';'. , kj,..:,.,=.11ft .... t .r. t ' .13'0.."T7."Is ,...:', p. ---.., ' " .....14e4...... ,4,4 ' '.**. ' ,,, 1-, `mil . -.---- . ...-...._...._.,":, r ....1-.., -... / 10, nb...e -7,. r --- 4 '.' 4;...-...... ,_//IFS' . . /,- -- - ...1 x .....1 \. -a __,-- -.--, ../f AO' . rj, ,..._ i l l' I,:-.-1 \ . ,! ...: F .__ .. " S.>, 4 4

The NM(' APP products can be tailored for Figure 16-2 illustrates the possible format for local application. Some factors to be .onsidered anairpollutionforecast worksheetThis k are includedas merely a suggested format and should be altered as necessary to meet local I. Emission Source. APP criteria are most requirements. accurate in predicting dispersion conditions over Itmust be emphasizedthatforecasts are and downwind of a large urban pollution source. prepared for pollution potential and notfor The closer the atmospheric characteristic, of the specific pollutant concentrations Procedures for ship or station resembles an urban area,the doss initiating pollution forecasts should be de- more likely is the APP forecast tobe accurate velopedincoordmation withbase pollution over all dispersion condition,APP forecasts of control authorities.

557 AEROGRAPIJER'S MATE 1 & C

SUGGESTED AIR PouurioN FORECAST WORKSHEET

1. Obtain appropriate WAX chart, teletypemessages FKUS-1 and FKUS-2, and local 1:36U and radiosondereports for the day.

2. Determine weighting factor (iF) for morning conditions:

a. Tiff = h T (WO =

b. im, = & ,NDP (WF) =

3. Determine weighting factor for afternoon conditions.

= t TW G'.T)

b . LX1)P = t MXDP (ItiF)

c. Vent = MPOti = & Vent (WF)

4. Calculate morning APP Index.

TW (WF) + :EDP (W1)

5. Calculate afternoon APP Index.

TW (hT) + Vent (WF)

6. If APP Index is between:

a. +1 t-1, then forecast moderate dispersion.

b. -1 & -4, then forecast good dispersion conditions.

7. Determine if base to be in a stagnation area for 36 hours by examining FOFAX chart or FfUS. If the base is so affected, and morning and afternoon APP index > +1, then forecast HAPP.

3. Determine end of ilPP conditions by noting when ventilation exceeds 3000 m2/sec or TW exceeds 4.5 mps.

AG.716 Figure 16-2.--Suggested air pollution forecast worksheet.

HIGH AIR POLLUTION POTENTIAL public. The latter is especially true ifa nearby (HAPP) FORECASTS cityis under a pollution alert asa result of a HAPP forecast. A HAPP advisory is the most important of the pollution forecasts. Conceivably, it could affect One systeminuse for determining if an aircraft or ships operations and, certainly, it is advisory should be issued relatesan APP index the one to generate the widest interest with the to dispersion as follows:

558 564 Chapter 16SPECIAL OBSERVATIONS AND FORECASTS

API' Index Dispersion pollution potential. The weighting !actor riteria are provided in Table 16-1. The morning API' +3 Poor. i.e., HAPP index is equal to the Weighting Factor for TW +2 plustheWeighting Factor for MXDP. The +1 Moderate to Poor afternoon APP index is equal to the Weighting 0 Moderate Factor for TW plus the Weightir.1-actor for 1 Moderate to Good Ventilation (Vent): Vent is determined by multiplying MXDP by TW. 3 Good 4 The dispersion criteria provided in table 16-1 are included for use as a guide only and are for For measurable precipitation, subtract 1 from locations away fromanurbanarea. These the APP index if the morning or afternoon APP criteria normally require moditkation to meet is greater than zero. individual requirements. In the absence of urban The APP index is determined by using MXDP pollution, dispersion Londitions defined as good and TW to determine weighting factors which bythesecriteriashouldverify while those are then added separatelyfor morning and defined as bad may not be quite as bad as a city afternoon conditions to obtain a measure of air area under similar meteorological conditions.

Table 16-1.Pollution dispersion criteria.

Minimum dispersion period (morning)

Weighting Factor for Weighting Factor for Mixing Depth (MXDP) Transport Wind (TW)

MXDP < 250 m = +1 TW < 2 mps = 250 m < MXDP < 500 m =0 2 mps < TW < 4 mps = +1 500 m < MXDP < 700 m = -1 4 mps < TW < 6 !ups =0 MXDP > 700 in = 6 mps < TW < 8 mps = -1 TW > 8 nips =

Maximum dispersion period (normally afternoon)

Weighting Factor for Weighting Factor for Ventilation (Vent) Transport Wind (TW)

Vent < 4000m2 = +1 TW < 2.5 mps = +70 Sec m2 nr2 4000 Vent < 6000 = +0 2.5 mps < TW < 4.0 nips = +1 sec sec in- ni2 6000 Vent < 8000 = 4.0 mps < TW < 5.0 mps =0 sec sec ni2 8000 < Vent = -2 5.0 mps < TW < 6.0 mps = -1 sec TW > 6 inps

559 565 AEROGRAPHER'S MATE I& c

Criteria for Issuing close proximity to a city under a 1APP advisory a HAPP Advisory and meteorological conditions at the shipor station are similar to those of the nearby city. Suggestedcriterialbr issuingalocal area 11APP advisory are as follows: Typical Use of HAPP Advisory by Control Agency I. Detachment information indicates the following: The use of a HAPP advisory ina pollution a. Local HAPP currently exists. alert system is illustrated in figure 16-3. b. It is expected to persist for at leastan additional 36 hours. Termination of HAP? Advisory c. Objective threshold HAPP criteriaas follows: A 11APP advisory will normally be terminated ( II Morning APP will be greater than whenever ventilation exceeds 8,000 m2/secor the transport wind exceeds 4.5 mps. (2) Afternoon APP will be greater than RADARINTERPRETATION (3) Stagnation is forecast for 36or more The application of radar as an aid to observing hours. and forecasting weather has provided the fore- d The ship or station is adjacent toor in caster with information of inestimable value in

HIGH AIR POLLUTION ALERT WARNING SYSTEM CITY OF NEW YORK

STATEMENT EXTENDS. STATEMENT EXTENDED HAPP STATEMENT ANOTHER 12 HRS. AND 12 HRS. AND AIR BY EMSU AIR QULITY STATEMENT EXTENDED -QUALITY DETERIORATES DETERIORATES FURTHER

GENERATES GENERATES GENERATES GENERATES

1 1 STAGE I-FORECAST STAGE 2-ALERT STAGE 3- WARNING STAGE 4-EMERGENCY

ACTIONS: ACTIONS: ACTIONS: ACTIONS: (I) BURN RATES REDUCED (I) CONTINUE STAGE (I) CONTINUE STAGE 2 (I) CONTINUE STAGE 3 MEASURES MEASUR ES MEASURES (2) PREPARE TO CHANGE (2)CLOSE SOME (2) (2) LARGE EMITTERS FUEL PAT FERNS CLOSE MORE INCINERATORS INCINERATORS PREPARE FOR (3) LARGE BOILERS USE SHUTDOWN (3) REDUCE OPERATION NATURAL GAS OR (3) ALL CITY INCINERATION OF LARGE EMITTERS I% SULFUR FUELS ELIMINATED (4) ETC. (4) ETC.

AG.717 Figure 16.3.- Example of utilization of HAPP advisory bya controlling agency.

560 566 Chapter 16 SPECIAL OBSERVATIONS AND FORECASTS many instances. Its use in thunderstorm, tor- Radar Manual, Nav Air 50-IP-2,isusedto nado. and hurricane detection and warning Lave determine the characteristic of an echo area. materially reduced the destruction and loss of Figure 164 illustrates the appearance of a solid lifedue to these phenomena. Nleteorokwical line of echoes on a PPI scope. radar is discussed in chapter IS of this manual. Other types of shipboard radar, although not specifically designed to observe weather, can be useful to the forecaster during adverse weather if its current operation is not of a higher priority. The value of this equipment, as with all other meteorological equipment, is to a large degree dependent upon the experience of the operators and forecasters involved. The intent of this section is to present basic information related to the interpretation of radar echoes, thereby providing a basic foundation upon which the forecaster may build with increasing knowledge and experience.

IDENTIFICATION OF WEATHER ECHOES USING A PPI-SCOPE

Although a synoptic map gives definite indication of an approaching front or hurricane, or of the presence of thunderstorms in a specific area. minute-to-minute tracking of these weather phenomena is not possible. From the usual source of weather information,the exact time of advent of adverse weather cannot be forecast, although under favorable conditions it can be AG.718 approximated. The reflection of radar pulses Figure 164.Line Echo Wave Pattern (LEWP) in a from clouds associated with precipitation per- squall line as it appears on a PPI scope. mits the continuous tracking of the position of such clouds with resp,:t to the location of the Scattered echoes may be related to air mass station.Thus, a degree of accuracyinthe weather; lines of echoes may be related to squall forecasting of the approacn of unfavorable lines or cold fronts; certain characteristic curves weather, not possible by standard methods. can in echo lines may be related to frontal waves; be achieved using radar methods. and widespreadrelatively homogeneous echo It cannot be overemphasized that all available patterns may be related to warm frontal precipi- scopes should be usedin studying weather tation under stable conditions (banded struc- phenomena. Refer to chapter 18 of this manual tures under less stable conditions). for a description of the various meteorological radar scopes. This section is written, however, as Thunderstorms though the PPI were the only available scope. ThePlanPositionIndicator(PPI) scope Consider, for the moment. the typic.1 physi- should be used to determine the character of the cal appearance and behavior of I thunderstorm. echoes which are classified as isolated, widely Usually, the onset of rain is quite sharp and he scattered area, scattered area broken area, solid precipitation is heavy. As quickly as a thunder- area, line of widely scattered echoes, line of storm passes, the rain stops. It is not surprising, scattered echoes, broken line of echoes, solid ,!terefore, that the echo which is returned train line of echoes. spiral band area, stratified ele- a thunderstorm is almost always bright (good vated echo, and fine line. Table 5 -I in Weather raindrop targets) and reasonably sharp cottY!

561 567 AEROGRAPHER'S MATE I & C

(yen.littlelightlam. which givesmarl.nnal Cold Fronts and Squall Lines echoe, )The brightness and the sharpness of a thunderstorm echo distinguishitfrom almost In considering the appearance of cold fronts any other type of echo and squall lines on a PM-scope, start with a concept of how these phenomena appear in The developm,:nt of the thunderstorm can be nature. A cold front with nothing in it contains followedbytheAerographer's Mate quite no precipitation particles and does not show up clearly on the PPI. The horizontal extent can be on radar. A well-defined active cold front and determin:d by watching how nun!, degrees of particularlyasqualllinecontain convective ainnuth are covered by the cello. If there is onl. clouds and are marked by a band of active one thunderstorm in the area. sector scan is precipitation. On the PPI. all areas of precipita- desirable. since the time taken by the antenna in tion will show up. The typical. unmistakable rotating 36O' need not he lost. appearance of a front or squall line is that of a narrow band of discrete echoes oriented in a The location of the active cells within the line. moving across the scope as a unit. (See thunderstorm can be found by using the gain figure 16-4.) control. The higher the gain setting. the higher is the power of the set. The highest setting shows Experience seems to indicate that this band of up everything that the radar is capable of seeing. echoesisclosely associated with the frontal As the gain is reduced, the less intense parts of position as drawn on a weaiher map. Italso the echo drop out. Since the cells are the most indicates, however, that the frontal position and active part of the thunderstorm. they are the last the cloud position do not necessarily coincide. to disappear as the gain is reduced. The front on the map. drawn by skilled analysts, may be in advance of or behind the cloud band. Sometimes, the two lines coincide. During the passage of a front across the radar station, When Rain or Snow tracking is effected from one side of the scope to the other, itis apparent that even for the Sinec water droplets NL um- about tine times single front, the position of the clouds relative asmuchliere.N as correspouding snow crystals. to the front varies with time. the returi. front snow tends to be weaker, and the differences of intensity within a snowstorm are generally much less than in a rainstorm. Both Warm Fronts texture and behavior help in distinguishing rain echnefrom those of snow. A typical PM The classic picture of a warm frontal region is preen ation of snow is a uniform hazy or coarse that of a wide cloud shield containing layered, echo with very diffused edges. The texture of precipitating clouds with perhaps a few thunderstorms penetrating the layers. On the PPI. the snow echoesisoftendescribedas softin contrast to the sharp or hard echoes produced warm frontal picture is substantially the same. A by rain. large part of the scope may be covered by soft echoes(indicatingcontinuousratherthan AnotherInc « to what kind of echo is showery precipitation).Usually, atfullgain involved may be found byval.!, ins the gain setting. thunderstorms even if present do not s%:t1,1111:,. There seems to be much more variation show up. They re hidden in the continuum of in the intensit, of rain than of snow. A rein thewarm-frontechoes.Withreduced gain gout P. redULcd. does not disappear setting, the overall echo tends to disappear and as a unit. Certain poi dons fade out much more only the thunderstorm cells remain. quickly than others. With snow, there is muzh greaterlikelihoodthattheentire echo will Hurricanes and Typhoons disappear at once. This characteristic may be used to good advantage in identifying the nature Each time another tropical storm is tracked of the echo-producing, substance. by radar, a new characteristic of such storms is

562 56f3 Chapter 16 SPECIAL oBsERvATioNs AND FORECASTS observed. Any attempt to give a valid descrip- tionortheappearance of ahurricaneor typhoon on radar would undoubtedly be in error. On rat' Ir. as in every other way. each tropical storm seems to be an individual. None- theless. a generalized description can be given with the understanding that any specific storm may be an oddity. Assume that a hurricane is approaching the radar set, passes close to the station, and then moves off. Until the rain shield associated with the storm is within approximately 250 miles or the radar. regardless or the power or the set. the PPI will not show any evidence of the storm. This isthe result of the fact that the earth's surface is curved so that even when a radar beam leaves the set in a horLontal path. it exceeds the height of the hurricane clouds beyond about 250 miles. As the storm continues to approach. echoes begin to appear on the scope. These look almost ident;cal with those produced by a warm front and. if knowledge of the presence of a hurricane were not available from other sources, they would be mistakenfor warm frontal AG.719 clouds. As the hurricane continues to come Figure 16-5.Hurricane showing echofree eye closer, the echoes begin to develop in distinctive on PPI scope. patterns. They acquire structure and appear as a series of concentric bands. (Sec figure 16-5). The position of the eye of the storm may be istoo shortfor spotting and identification. approximated by finding the center of curvature Motion pictures of the PPI with active thunder- of the bands. Actually. the bands seems to spiral storms do show the lightning strokes. When about the eye: an1 as more and incr.:,1 the seen. there can be noiistake in their identifica- hurricane is seen on the scope, the spiral pattern tion. On the scope. they resemble exactly their becomes more evident. The eye of the storm is a appearance in the sky. blank area on the scope. No precipitation occurs within the eye. The relationship between the Tornadoes close-in spiral hands and the eye is not clear. It appears that the eye shifts. forms and re-forms. One of the most controver...,t1 issues in rada or develops asymmetrically with respect to the meteorology isthe identification of tornadoes nearest precipitation bands. As the storm re- on a PPi. In general. tne contention is that the cedes, thetrailinghail of atropical!.Zorn tight tart.adation associated with a tornado shows picture is far less clear, since the trailing half of a up as a hook. V. or eye on an intense thunder- tropical storm usually contains less precipitation storm echo. There are many instances in which a than the leading half. PPI picture was taken of such a manife3tation while a tornado was in progress. The location of the hook or V coincided exactly with the known Ltghtning location of the tornado. Unfortunately, hooks, V's and eyes are also seen on intense thunder- It has been observed that lightning strorses storms when tornahes are not present. In a still show up on the PPI. Usually, the strokes cannot picture. those associatedwill;flie destructive be identified by the naked .:ye. Their persistence storms arc act different :win those which are

563 AEROC1RAPIINR'S MATE I & C' not. Only when the mannci of hook develop- slow ly'educe the receiver gain while the an- ment is watehed over a pet iod of time can an tenna is scanning in elevation. Also, short range exper'enced obsuvem distinguish between tor- will give the best mesults in this type of observa- nado hooks and meaningless hooks and even tion. then. there may be more or less doubt. A bright band is the result of the simultaneous existence of snow, melting snow, and rain in PLOTTING MOVEMENT a vertical cross section of the atmosphere. At OF WEATHER ECHOES upper levels, the precipitating particles are snow crystals. Snow is a relatively poor reflector of a Weather echoes may be plottedinseveral radar beam. In the higher levels of the atmos- ways. A suggested method is to use acetate phere. therefore, where the snow predominates. overlay. To do this. first, cut to site an acetate the echo rt turn is poor. Rain droplets. on the sheet that will lie flat over the PPI-scope. Next. other hand, make relatively good reflectors and mark the four directional reference points tN, L. sienal return is high. A well-defined and thin S. and W). and then a center reference point to horizontalbright bandisindicative of very lay over the center oldie scope. stable air. Under extremely unstable conditions. When severe weather is approaehine the sta- however, the layer represented by the bright tion. lay the acetate overlay over the scope and band becomes :1 deep and mixed tip that there at designated tmine intervals. Llependine on the islittle or no effect noticeable on the radar- speed of movement of the weather, place refer- scope. ence marks alonethe weather echoes. The The Acrographer's Mate should note from the dis- movements can then he extrapolated to deter- cussion above that the bright hand is not the position mine w Inch ,tortes. if aims . are likelyto affect of the 0° isotherm. When the relative rate of fall of the tmninal. Obviously. attention should be snow and rain is superimposed on the differential focused on potentially entical upstream areas. reflectivay of the two, the bright band may be ex- When extrapolatingthe movement overthe plained. Snow falls at a slow rate (about 0.5 mos); terminal. allowance should be made for modifi- rain falls as a fast rate (about .1 to 8 mps). During cation by significant local terrain et facts and for the time in which a snow particle melts to form a rain tendencies of the a:ea to change in site and droplet, it has the approximate rate of fall of snow, intensity as indicated by successive radar obser- bat the reflectivity' of rain. Due to the slow rate of vations. fall, there is hign concentration of particles of poor reflectivity in the snow region. This results in a IDENTIFICATION OF WEATHER relatively small signal return. ECHOES USING AN RU I -SCOPE While the particlesfall through the 0° isotherm and melting starts, there is a high concen- The scope which shows range height indica- tration of particles for a limited zone (as in the tion can provide vet y valuable meteorological snow) with good reflectivity (as in rain). This information which is not available in any other means a high signal return. way. The presentation of the RIll is a vertical Below the melting layer, thereisa lower cross section of the atinm.,spherc. When the RI II concentration of particles due to the high rate of isused. the antenn.mis11...edwith respect to fall of the rain. Even though the rain has high azimuth and permitted to scan in elevation only. reflectivity.the lower concentration of rain results in a reduced signal return. The region of Bright Band highest signal return corresponds to the melting layer. It is the bright band. Observersusingan RI II-scopefrequent'' The meltingprocess whichproduces the speak of the bright h mild which appears on their bright band only starts at the 0° isotherm and scopes and relate the phenomenon to the 0' the bright band maximum must be below the 0° isotherm. The propel procedure for observing level. The usual position of Cie bright band is this phenomenon istor the radar operator to approximately 1.500 feet below.

564 590 Chapter 16 SPECIAL OBSERVATIONS AND FORECASTS

Atmospheric Stability Thunderstorms and Turbulence

Careful examination of the bright band may Since the Rill gives considerable information yield some information about the stability of concerning thunderstorms, and since a great deal thatportionof the atmosphere around the of know Lige is available relating thunderstorms inciting level. With very stable air, the band is and turbulence,itis apparent thatthe RHl clearly defined.Itis narrow verticallybut of portraying a thunderstorm gives a fairly com- widehorizontalextent. As the atmosphere pletepicture of associatedturbulence (and. becomes less stable. the distribution of precipita- inLidentally, icing and damaging hal!). tion becomes cellular and the band broadens In an actively developing thunderstorm, tur- vertically.Withstrong convection extending bulence is most severe. During the time that the through the 0° level, the melting and freezing top of the radar echo rises rapidly, the thunder- processes become mixed through so deep a layer storm is most active. The higher the top of the that nothing resembling a bright band can be storm, the more violentis the storm and the detected. greater is the attendant turbulence. A decaying storm can be seen as decreasing in altitude with time. This subsidence of the top of Cloud Types the storm shows thatconvective activityis decreasing and turbulence is decreasing as well. In general, itis rather simple to identify the The brightband develops as turbulence de- various cloud types which produce echoes on creases. the RH1-scope. Instead of discussing these types with descriptive terms. it is probably desirable to Upper-Level Winds reproduce RI-II picture:. of some of the clouds which may be seen on A number of these A careful study of the RHI will frequently echo patterns are illustrated in figure 16-6. provide valuable information concerning winds In figure 16-6 (A), for example. the broken aloft. When precipitation shows any shower pattern shows convective ty pc showers. The fact charcteristics. the cells can be observed in the that the echo does not extend above ' 1,000 feet Rill scope as separated columns of falling rain indicates that the showers .ire weak. or snow. These columns often slant out of the Figure16-6 (13) shows shower cells in the vertical because of wind shear. lower levels. These showers seem to originate in Examine figure 16-7 closely. This is a typical a great cloud layer extending from about 12.000 Rillpicture from which some indication of to 26.000 feet. wind structure can be obtained. Note that the picture shows a curving streak which begins at Figure16-6 (C) shows a definite series of high altitude and extends downward. stratified layers between 8.000 and 18.000 feet. Consider a cloud moving with the wind at The relative intensity of the layers could be cloud level. (See fig. 16-8.) As the cloud moves determined by decreasing the gain progressively. along, the rain or snow particles drop from it Only the main layer would remain at the lower and fall progressively to lower levels. Track these gain settings. individual particles. Label each particle with a A thick layer directly above the stationis letter starting with "a" and each time interval shown by the Rill in figure 16-6 (D). A short with an Arat,ic numeral starting with time "I." distance away. theprecipitationreaches the Tnus, in the notation c.4, the dot between the ground. The advance of the precipitation with "c" and the "4" represents the particle, Mine time can be followed by watching the blank area i.he letter "c" shows that it is the third particle near the station. The streaky nature of the echo to be dropped. The "4" indicates that four units i, the upperlevels indicates shower activity. of time havt' elapsed since the start of the In figure 16-0 (E), three layers are pictured observation. Label with Roman numeral:: the with precipitation falling through them. Again, a layers of the atmosphere through which ttiL cellular structure seems indicated particles drop.

57565.1, (A) (8) (C)

( D) y (,E) Figure 16-6.R HI scope precipitation patterns. AG.720 Chapter 16SPECIAL OBSERVATIONS AND FORECASTS

The dashedlines show thetrajectories of some of the individual particles. Consider particle "a." It falls from the cloud at time ''1." As it calls through atmospheric level VIII, it is under the influence of the mean wind in that lay er. It is displaced horizontally to the left so that it reaches the bottom of layer VIII at dim' "2." The fall through layer VII results in continued displacement toward the left (although a smaller one) since the wind in layer VII is less than that in VIII.Itis now displaced to the position it occupies at time "3." It will continue to be displaced dependent on the wind speeds the particle encounters on its descent. Particle "b" is generated later than particle "a." Let us say that it is rdeased from the cloud at time "2." Thus. it starts its fall through layer VIII just as particle "a" leaves that layer. The trajectory of particle "b" is identical with that ofparticle"a" exceptthattheparticleis delayed. Particle "c" is further delayed. as is AG.721 each new particle falling from this cell. Figure 16-7.RHI picture with some indication of wind structure. RADAR WAVE PROPAGATION AND REFRACTIVE INDEX Figure 16-8 shows the generation of precipitation patterns in a wind field. The wind field Propagationmeansthetravelof electro- assumed is indicated at the right side of the magnetic waves, as sent from the radar antenna, figure by arrows. The wind direction is in the through a medium. Dissemination of radar waves plane of the paper (page) going from right to is another way of expressing it. left. The length of the arrow indicates the wind Radar waves,likelightwaves, must pass speed. The double arrow at the top shows the through th., :It:no.,pnere to travel from one place speed of the generating cloud. to another. The characteristics of the medium

j .10 i. 9 h. 8 ...g. 7 f. 6 e. 5 d. 4 c. 3 b. 2 - -a. 1,.. E VIII i.10 h.9 g .8" f.7 e. 6 d. 5 c. 4 b. 3 a.2" -4 VII h.10 g.-9.-- f. 8 e.7 d.6 c.5 b.4 a . 3 . -"`" VI g.-10 f.9 e.8 d. 7 c. 6 b. 5 a,. 4 . -*-- V f. 10 e.9 d . 8 c . 7 b.6 a,. 5. . +.1.---IV e.10 d.9 c.8 b. 7 a. 6 . -0--III d.10 c.9 b.8 V7 / -41---II c.10 b.9 a :8 // b. 10 a.9

AG.722 Figure 16.8.Generation of precipitation patterns in a.,o field.

567 573 AEROGRAPHER'S MATE 1 & C through which the waves pass affect the manner of their transmission. Thus, although it is sometimes assumed that both light and radar waves follow perfectly straight paths, the properties of the atmosphere are such that the waves are made to follow curved paths.

REFRACTION

(A) STANDARD PROPAGATION The bending of radio or radar waves due to a change in the density or the medium through which they are passing is termed refraction. The measure of the bending that occurs is indicated by the index of refraction from one substance to another. The density of the atmosphere normally changes at a gradual and continuous rate. Therefore. under these conditions the index of refractionwouldchange graduallywithin- creased height. However, under certain condi- (B) SUBSTANDARD PROPAGATION tions, the temperature may first increase with height then begin to decrease (a temperature inversion). More important, the moisture con- AG.723 tent may decrease more rapidly with height just Figure 16-9.Normal propagation and surface duct. above the sea. This effect is called a moisture lapse. Either a temperature inversion or a mois- Analysis of many meteorological soundings turelapse,aloneorincombination, may and their associated refraction curves indicates produce a great Lhange in the index of refraction the possibility of classifying these curves into of the lowest few hundred feet of the atmos- severaltypes. When objects are observed at phere, resulting in greater bending of the radar many times greater than tne normal visual range waves. This may greatly extend or reduce the due tothe unusual distribution of pressure, radar horizon, depending on the direction in temperature, and humidity, we have what is which the waves are bent. The radio energy may known as superrefractive conditions. When radar be so highly concentrated in a limited region of waves are reduced, we have subrefractive condi- space that it appears to be following a duct. (See tions (quite rare). fig. 16-9.) The height of the radar antenna with respect to the duct formed by this occurrence is The normal N gradient for a standard atmos- of much importancein producing an effect phereat60 percentrelative humidityisa known as trapping. decrease of 12 units per 1,000 feet. Negative In order to locate tht..,e trapping layers. the gradients larger than this are defined as superrefractive. However, since radiosonde data is not meteorologist uses a refraction index which is exact, gradients of N cannot be established called the N curve. The refraction index N is precisely, and it is more logical to think in terms defined as the ratio of the wavelength of an of zones. Such zones may be defined as follows: electromagnetic wave in a vacuum to that in the actual atmosphere. The atmospheric pressure, temperature. and moisture content are the pa- I. Normal zone gradients between 0 and rameters used to find the value of N. The bending -24 per 1,000 feet. of radio-radar waves is Laused by gradients in the 2. Subrefractive N increases with height. refractive index and not by the value of N itself. 3. Superrefractive -between -24 and -48 per In other words, when large changes of pressure, 1.000 feet. temperature, and moisture content occur with 4. Trapping gradientslarger than -48 per altitude, refractivity must also,occur. 1,000 feet.

568 574 -0=

Chapter 16 -SPECIAL OBSERVATIONS AND FORECASTS

The value of N at sea level ranges fluin 250 to generally disappear early in the morning. Radia- 450 N units. tion inversions rarely occur more than I or 2 miles offshore; and when oue exists away from CONDITIONS CAUSING DUCTS land, the duct will be weakened and the base willgraduallyriseabove the surface of the What types of atmospheric conditions give water. rise to the trapping refraction curves and result in extraordinary ranges on VHF and UHF radio Stratum of Cool Air equipment? Important meteorologicalfactors Blowing Over Warmer Sea that cause changes in the distribution of temperature and moisture throughout the atmos- Coolair blowing over a warmer sea may phere are the flow of warm dry air from a land produce surface ducts. With this partkular phe- mass out over cooler sea; nocturnal cooling (over nomenou thereis no associated temperature land); flow of cool air over warmer sea; and inversion. and the entire effect is caused appar- low-level subsidence. ently by the evaporation of water into;the Perhaps the best method by which to consider lower levels of the atmosphere. Ducts usually thequestionistosubdividethesignificant accompany winds thathave blown for long meteorological conditionsintosevenclassifi- distances over the open seas, their height and cations, discussing their relative effects on radio size increasing with the wind speed. At higher propagation: wind speeds typical of the Pacific trade belt (10 to 20 knots), ducts from 50 to 60 feet high 1. Surface (radiation) inversions. occur quite generally. These ducts arc particu- 2. Stratum of cool air blowing over warmer larly important because they extend for such sea. long distances and because they seem to occur 3. Inversions produced by warm continental so frequently. air blowing over colder sea water. 4. Subsidenceinversionsinwarmhigh- Inversions Produced by Warm pressure areas. Continental Air Blowing 5. Coastal stratus type inversions. Over Cooler Water 6. Frontal inversions. 7. Postfrontal subsidence inversions in cold When warm air blows over cooler water, a high-pressure areas. ductisformed by the temperature inversion which is caused by the warmer air aloft. The Surface (Radiation) Inversions base of this type inversion is at sea level with the ducting layer extending to several hundred feet. The surface inversions result from nocturnal The height of the trapping layer is usually from cooling, that is. loss of heat from the ground by 200 to 600 feet. and it tends to remain as far as radiation. A temperature inversion is established, WO to 200 miles out to sea. Near the roast, the andalow-level ductresults. This nocturnal trappingisthe strongest just after noon and cooling is greatest when there is no overcast and. weakest just after dawn. Fog is generally not as a result, trapping rarely occurs over land when associated this type inversion, but w hen the sky is cloudy. Radiation ducts will be found surface fog appears 5 to 10 miles out to sea, it chiefly over large flat land areas in the middle indicates the presence of a duct. and higher latitudes. They will be stronger in the higher latitudes and on the leeward shores of Subsidence Inversion in a large islands and land masses. One should expect Warm High-Pressured Area them chiefly at night and in the early morning, when there are clear skies, little or no wind. and Subsidenceinversionsarepredominantin low moisture content in the air. When the wind high pressure systems. Subsidence results in a is strong, the skies are clear, and there is little temperature inversion which spreads out later- moistureintheatmosphere.theductwill ally and creates comitions favorable for a dust.

569 6"15 AEROGRAPIIER'S MATE I & C

Such a duct b usually relatively high and may propagationissimilartothat produced by become intenst. to a height of few thousand subsidenceinversionsin warm high-pressuie feet. This type of duct will not materially affect systems. surface propagation. The height of such inversion may be estimated from observations of WAYS OF DETECTING A DUCT clouds since stratified clouds or thin altocumu- Meteorologists are at present studying care- lusclouds usuallyformatthe base of the fully the various ways in which weather affects inversion during the night or early morning. This high frequency radio propagation. It is hoped type of duct is often found in the Tropics and thatsome day theywillhave developed a off the western coasts of continents in the trade programof forecasting duct conditions by wind areas. It is more prevalent in summer than means of which complete and accurate radio in winter andontheeastern side of the coverage may be predicted. Since this goal has subtropical highs. not yet been attained, one must use the informa- tionathis disposal.Radio propagationfor Coastal. Stratus Type Inversions certainfrequenciesisdetermined by atmospheric conditions and closely associated with the This situation occurs when warm continental weather.- Consider several rules by which the airis underrun by cool sea air. Strong surface presence of a duct may be detected. Trapping ducts aretheresult.Theseducts may be should be expected when the following condi- expected most often on the western shores of tions occur: continents in the lower and midlatitudes and where the higher atmosphere is blowing off- 1. A wind is blowing from land. shore, with the lower cooler portions stationary 2. There is a stratum of quiet air. or possibly wen blowing onshore. They vary 3. There are clear skies, little wind, and high with the season, being present almost entirely in barometric conditions. late spring. summer, and earlyfall. They' are ..In open ocean when acool breeze is most pionounced during night and early morn- blowing over warm ocean, especiallyinthe ing. tropical areas and in the trade wind belt. 5. Smoke, haze, or dustfails torise but Frontal Inversions spreads out horizontally. 6. There are skips in the ground clutter on Th. temperature int.rease with altitude in the the scope; for example, when nearby islands are frontal inversionisa gradual increase and not not received on the se.ope. abrupt,butthemoil, .irecontentincreases 7. There is strong fading of the pip. A rapidly sharpiythroughoutthetransition zone. The fading pip may indicate a duct with ranges on a increase or moisture within the inversion layer higher swc,.p. This rule, however, is not Iniver- leads to M11)110111141 rather than trapping condi- sally true. The duct may be well formed and tions. The region in whit.h they are prevalent steady; or when there are standard conditions, varies with the season. .ranges in the aspect of the target may cause rapid scintillations of the signal. Postfrontal Subsidence Inversions 8. The moisture content of the atmosphere at bridge level is considerably less than just above This type of inversion is usually not strong the sea surface, enough to produce more than a slightly less than 9, Therean offshore wind and the tempera- normal &crease in N with height. The base of ture at bridge levelis1° to 2°F greater than the inversion is generally 4,000 to 8,000 feet, that of the sea. This pc inversion is most frevently observed 10. In open ocean, the temperature at bridge in middle and high latitudes and never in the level is definitely less than that of the sea. doldrums. They are stronger in winter than in 11, In trade wind areas, generally for frequen- summer, andcanincreaseinintensity and cies of 3,000 me and higher, with low-level decreaseinheight at night. Their effect on antennas.

570 r 576 Chapter 16 SPECIAL OBSERVATIONS AND FORECASTS

12 The potential temperature at a level of most shipboard radar because they are usually about 2.000 feet exceeds the surface value by shallow (75 feet is about average). In order for 10 °F or more while the mixing ratio for this such a duct to be effective, the radar must be level is at least 5 giu;kg below the surface value. within50 feet of theseasurface, and its wavelength must be shorter than 30 cm. The NOTE: Keep in mind that potential tempera- normalpractice of installing radars on the tureis determined by moving the parcel dry highest point on the ship places most radars adiabatically from the 2.000 ft level (in this above radar ducts which may be present. For case) to the surface level. low-sitedradars, however. the duct is often useful in extending the normal limit of coverage. COMPUTATION OF Inactualpractice one can determine from REFRACTIVE INDICES simple meteorological data whether such a radar Direct measurements of the propagation of at a particular wavelength will have extended electromagnetic energy in a particular medium coverage. can be made and. when compared with the For these purposes a Ductogram was designed speed of light. give a direct measure of the to facilitate the prediction of extended radar refractive index or refractivity. Because of the coverages with an oceanic duct. For input data, complexity of the equipment needed, this ap- the Ductogram utilizes surface observations of proaL.h is impractical for operational use. There- air, dewpoint, and sea water temperatures taken fore. the refractive index must be computed on boardship. The quantity of N'isthe from available radiosonde data. Numerous meth- approximate difference between the refractivity ods have been devised for computing N. These N, at the elevation of the shipboard observation take the form of tables, slide rums. computers, (bridge height), and the refractivity Ns at the and nomograms. water surface. Complete details and illustrations The publication entitled "The Analysis and of the Ductogram may be found in N A 50-1P-1. ForecastingofAtmosphericRadarRefrac- tivity." NA 50-IP-I. is issued as an aid to Naval LOCAL FORECASTS USING Weather Service Command Units in sa'isfying an MICROANALYSIS increasing requirement of the operating forces for radar refractiondata.Itdeals with the A generalization of the expected weather Arowagram. conditions, which is too inconclusive for many NAVAIR 50 -I P -7, Computation of Atmos- military operations, is the usual result of the pheric Refractivity of the USAF Skew T. Log P large scale analysis that precedes most forecasts. diagram deals with the Skew T as indicated by Quiteoften, andespeciallyduring wartime its title. operations, the need arises for a forecast of the One version of the Skew T diagram (DOD exact conditions at a particular locality during a WPC 9-16-2) has been printed with a gild for specifictime. This kind of forecast requires determining refractivity already printed on it. microanalysis, which should determine whether Refractivity can be read directly from the traces a particular spot will have cloud coverage at a of temperature and dewpoint entered on the specific time of the day or night. Local and diagram, without the use of an external nomo- diurnal effects have to be carefully weighed. gram. The accuracy and range of computations For example, operations might require send- should Lover most of the applications of weather ing aircraft to their maximum range, making it un its.Instructions for its use are covered in imperative that they encounter no delays in NAVAIR 50-IP-7. landing due to weather conditions at arrival time. In this instance the important question is OCEAN DUCTS AND not what type of weather can be expected on a THEIR DETERMINATION particular day but what type of weather will be expected at a specific time. Evaporation of "oceanic" ducts are not par- To achieve maximum accuracy in this type of ticularly cffcdive in extending the coverage of forecast, it is necessary to utilize more individual

571 577 Ro(;R AplIF R'S \IA

station data than is toured On the synoptic chart is set at 2') ()2 inches of in. rc ur,v.Consequently. Use of small scale weather dish charts Is thealtimeterissetat29.92 inches. the one method thatnu!.b..used Ilereclose indicated altitude of an airc rail will he -1.780 scrutinizationis made or reportediouds (), fee tatthe.S50 -nib Irvel. 9.SS0 feetatthe otherweatherplienom,na. \nigherusctul 700 -tab tete!. etcSince the actual atmosphere method is the plotting 01 infoimation Rom an does not conhnin to standard conditions, the individual station On a tiequeat sontinuing actual (geometric) neights of the pressure sur- basis.\gain byfollow ing the .hangcs as they lac., ma). depot t considerab1y from that of the oc4cur a more .iceuratc prediction of ewected plessme heights. lor many flight operations this changes can he made. departure of the geometric height of a pressure 13oth of Illese methods are ,*Xlf,111C1y useful in sill lac, from the pressure height for that surface tropical areas where the local and diurnal effects is of extreme onportance. are so great!y pronounced. Ihe dist, :m bon of the elements mentioned in the preceding puagraphs may he presented in plz EssuR -lit: !Gin- FORLC.\STS inanN combinations on various thermodynamic charts. Each combiation of these meteorologi- Iti- important for many practical purposes to cal variables and their presentation on a Nrtieu- he able to determine the altitudeat lar thermodnamic diagram niav be suited to certain sired.' of ir nia\ exist in tht_ atmosphere. ...Amin specializedt.v, pc!, of computation. but no c.g.. in% 1/4*rsions,, isothermal Lw cr,. moist 1.13..:1. one diagramis convenientfor all Forecasting etc he ii.1on 01 pressure altimeters also uses. lio.vever, one diagiant which is considered requires the computation of pr,sure height to be especially convenient lot problems related data. topressure-heightcomputationsiscalleda l'ASTAGRAM. Detailed information pertaining l'I.CsslIrC-11C1:4111 or pressure-altitud refers to to this diagram may be found in the booklet the height of a glen pressure stirt:,,e under entitled seof thePastattraminPressure- standard atmospheric condmons. ( (Imputations Height( (imputations. NA 50-IP-SOL This used in determining --standard atmospheric on- bookletalsocontains tables for determining ditions within the United Stale, ale based on altitudes based on the U.S. Standard Atmos- whatis commoni'vreferredto asthe U S. phere In)in pressure values. The Past:1gram has Standard Atmosphere. The figures Inch repre- imam, meteorologicaluses. among which are senttheU.S,Standard \ tinospherc al, as included piesstue-height determinations. check- 10110W1 ing or extrapolating heights in the troposphere. and conversion of value reports to pressure Nlean sea level pressure .013,2: tilt values. 2. Nlean se., level temperature =I 5 C. 3 I apse rate = 1165 C per IOU toI ().769 AIR DENSITY AND M135.332 tor 234 nib), 4. Lapse rate above the tropopause = 0.(I01) WATER VAPOR COMPUTATIONS with a L(nstant temperature 0155 '(' fabote 35) 32 tt or 234 mb). Air density and the water vapor content of the air have an important effect upon engine l'he actual heights for selected pressure !CVOs performance and takeoff liaracteristies of air- based on this standard atmosphere willvary craftInthis section of the chapter we will fromthatIlla standard atmospheie as the discuss methods for computing these elements environmental conditions and elements involved fronta meteorologicalstandpoint. The four are changed froth those of thc standard at IlMs- most .ommon elements on which the Aerog- phere. raphers Mate is iequested to furnish information are pressure altitude. density altitude. vapor ,\ pressure altimetei Is calibrated to read the pressuic. and specific humid*. All of these may heights of points in the1' S. Standard Annus be detelmined horn the Density Altitude Com- phere when the piessure ,.ale on the instrument m! t el contained inNWRI;publication 578 Chapter 16 SPECIAL OBSERVATIONS AND FORECASTS

37-0862-066. Complete instructions for the use Pressure Altitude of this computer may be found in the narrative The pressure altitude is defined as the altitude section of (his pUblild 1011 with two examples of a given atmospheric pressure in the standard for computation of deasityaltitude on the atmosphere. The pressure altitude of a given reverseside oftip.compute'.Density and pressure is usually a fictitious altitude. It is equal pressure altituvic are given in feet. vapor pressure to the true altitude only in the rare case when commonly in millibars oi inches of mercury. and atmospheric conditions between sea level and specific humidity gams pc' gram or pounds the altimeter in the aircraft correspond to those per pound. of the standard atmosphere. Aircraft altimeters are constructed For the pressure-heightrelation- DENSITY AND DENSITY ALTITUDE shipthatexistsinthe standard atmosphere. Therefore, when the altimeter is set to standard The density of the atmosphere's a factor in mean sea level pressure (29.92 in.). it indicates many of the important problemsin meteorol- pressure altitude and not true altitude. ogy. as wellas in those related sciencesand engineering. The performance of an aircraft or Density Altitude missile depends on the density of the air in The density altitude is defined as the altitude which itis flying. Itis more difficult to take off at which a given density is found in the standard from high-altitude airports than from airports at atmosphere. If.for example. the pressure at low altitude, and itIs more difficult to take off Cheyenne. Wyoming (elevation 6.140 it) is equal on a hot afternoon than on acool morning. to the standard atmosphere pressurefor 6.140 feet but the temperature at that station is1010F. The Aerographer-s Mate in the field may be the density there is the same as that found at called upon to furnish air density information 10,000 feet in the standard atmosphere. and an for operational purposes or to give lectures on aircraft on takeoff would perform asif the this subject. In either case. he is confronted with altitude of the runway were at 10.000 feet in a problem that requires him to spendconsider- the standard atmosphere instead of 6.140 feet. able time reviewing available literature. This is The effects of air density on the pitot tube true since synoptic nksteorologists do not com- and the calibration of the airspeed indicator of monly use air density as a parameter. and thus an aircraft are such that indicated airspeedand are not always familiar withthe application of trueairspeedareequalonly when density density considerations to aircraft Operations. altitude Is zero: that is. when the air density is Since both temperature and pressure decrease thatof dryairatstandard mean sea level with altitude. it might appear that the density of pressure(29.92in.)and standard sealevel the atmosphere would remain constant with temperature (1r C). True airspeed exceeds in- increased altitude. This is not true. for pressure dicated airspeed when the density altitude in- (hors more rapidly with increased altitude than creases. does temperature. Since standard pressures and No instrument is available to measure density temperatures have been associated witheach altitude directly. It must be computed from the altitude. the density the air would have at those pressure (for takeoff. station pressure)and the standard temperatures and pressures must be virtualtemperature atthe particular altitude considered standard. thus. there is associated under consideration. This may be accomplished witheachaltitude a particular atmospheric by using the Density Altitude Computer or by density. Remember density altitude is not neces- using achartin Manual of Barometry. NW sarily true altitude. l'or example. on a day when 50-ID-510. Remember. virtual temperature is the atmospheric pressure is higher than standard. used in the computation of density altitude and the temperature is lower than standard. the density which is standard at 10.000 feet might WATER CONTENT OF THE AIR occur at 12.000 feet. Inthis case, at an actual altitude of 12,000 feet. we have air which has The capabilities of an aircraft will be affected the same density as standard air at 10,000feet. significantly by the water content of theair.

573 679 AEROGRAM HER'S NIATli 1 & C

Water content is frequentlydescribed as fog or inches and tenths of in the extreme, precipitation. mercury. This value may The forecasting of also be found on the DensityAltitude Co- fog and precipitationwere discussed quite exten- puter. sively in chapters I0 and II ofthis manual. Two other frequent requests concerningwater con- Specific Humidity tent are for vapor pressure and specifichumidity. The mass of watervapor present in a unit Vapor Pressure mass of air is known as specific humidity.The mass of the unit of air taken is consideredto be a unit mass of moist air. Since the Vapor pressureis mass of a unit that portion of the at- of dry air differs but little fromthe mass of a mospheric pressure that is exertedby the water unit of moist air, the mixing vapor inthe atmosphere and is expressed ratio and the in specific humidity are nearlynumerically equiva- inches and in tenths ofan inch of mercury (11,2). lent. The dewpoint for a given conditiondepends on the amount of watervapor present, so a direct In tropical countries, wheretemperatures are relationship exists betweenvapor pressure and high and rainfall is excessive,the water vapor the dewpoint. Vaporpressure may significantly content of the air reaches high proportions.All affect engine power. engine test data are calculatedon dry air. Since water vapor is incombustible, it isa total loss as Figure 16-10 shows a chari fordetermining vapor pressure from the dewpoint far as the engine is concerned. Specifichumidity temperature. can be determined from the Density Altitude You enter the bottom of thechart with the Computer or from the two followingmethods. dewpoint temperature andgo up vertically until The values given in these you intersectthe curve and read the examples show the vapor amount of water vapor in poundsper pound of pressure at the left side of the chart in tenthsof dry air:

VAPOR PRESSURE DIAGRAM (30" Hg)

X1.6 GIVEN: DEW POINT 70° F CI) FIND: VAPOR PRESSURE INS. Hg I.4 POINT "A" INTERSECTS (70°F DP ) INSTRUCTIONS:

I1.2 VAPOR PRESSURE CURVE w I. DETERMINE DEW POINT TEMPERATURE 2 GO UP FROM DEW POINT CI) READ .74 IN Hg HERE 11_I' TO INTERSECTION OF VAPOR a. -8 PRESSURE CURVE cc 3. READ VAPOR PRESSURE 0 AT LEFT OF PAGE

.4 FROM: TABLE 95, SMITHSONIAN .2 METEOROLOGICAL TABLES, NA50-113-521

0 10 20 30 40 50 60 70 80 90 DEW POINT TEMPERATURE °F

AG.724 Figure 16.10. Vapor pressure chart.

574 580 Chapter 16- SPECIAL OBSERVATIONS AND FORECASTS

1. If the dewpoint is known, use figure 16-11 areanothavingaweather station.Recent (A) and interpolate for the value of specific implementation of the COMET System requires humidity. In this case the dewpoint temperature that designated naval air stations prepare and is (36°F and the speolic humiduty byinterpola- disseminate Plain Language Terminal Forecasts tion is 0.0135. in the PLATES Code. A forecast of the lowest 2. Ifthewetbulb and temperature are altimetersettingfortheforecast periodis known and not the dewpoint use figure 16-11 required. For these reasons it is important that (B). In this example the dry-bulb temperature is forecaster personnel have a basic understanding 72 °F and the wet-bulb temperature is 67°F. of the importance of correct altimeter settings With a straightedge. enter the dry-bulb scale at and a knowledge of a procedure for frrecasting 72°F (Point A), locate 67°F wet-bulb on the altimeter settings. Refer to EM1-1 No. I Chapter wet-bulb scale (point B), and project a straight A8 for computation of altimeter settings. line through these two points to the specific humidity scale. It reads approximately 0.0133. BASIC CONSIDERATIONS

FORECASTING ALTIMETER SETTINGS An altimeter is primarily an aneroid barometer calibrated to indicate altitude in feet instead Under certain conditions it may be necessary of units of pressure. An altimeter reads accu- to forecast or develop an altimeter setting Err a rately only in a standard atmosphere and when station or alocationfor which an altimeter thecorrect altimetersettingisused. Since setting is not received. There is also a possibility standard conditions seldom (if ever) exist. the that an altimeter setting may be required fo- an altimeter reading usually requires correction. It

120 .-.--.03 90 .030 .025 80 90 .020 100 oe 70 .015 A - -- C >- >-oe oe 80 >- n >- 60 o- 0 80 .02 .010 Ei A 3 co :24-Ce 4.1 ai70_ 50 = 0 B Ex 2 ct- 60 LL E C uQ 40 '360 .005 2- -0 oC o- W < < 30 40 ta50

40 co 20 30 20 10 20 -- 10 .001 0 0 0 0

IF DEW POINT TEMPERATURE IS IF ONLY WET AND DRY TEMPERATURES KNOWN, READ SPECIFIC HUMIDITY ARE KNOWN, DETERMINE SPECIFIC FROM CONVERSION SCALE ABOVE HUMIDITY FROM NOMOGRAM ABOVE (A) (8)

AG.725 Figura 1611.Determination of specific humidity from notiogram.(A) When dewpoint is known; (B) When temperature and wet bulb are known.

575 581 AEROGRAPIIER'S MATE I & C

will indicate 10,000 feet when thepressure is Assume that an aircraft takes off from Miamito 697 millibars, whether or not thealtitude is fly to New Orleans at an altitude of 500 feet.A actually 10,000 feet. decrease in du- mean sea levelpressure of 10 The altimeter is generally correctedto read millibars from Miami to New Orleans would zero at sea level. A procedure used in aircrafton cause the aircraft to gradually lose altitude; and the ground is to set the altimeter readingto the although the altimeter indicates 500 feet, the elevation of the airfield. Tne altimeterthen aircraft would be actually flyingat approxi- readsthealtitudeabove sealeveland the mately 200 feet over New Orleans. The correct Kollsman window indicates thecurrent altimeter altitude can be determined by obtaining the setting. correct altimeter setting from New Orleans, resetting the altimeter to agree with the destina- Altimeter Errors Due to tion adjustment. Change in Surface Pressure NOTE: The following relationships generally hold true up to approximately 15,000 feet: 34 The atmospheric pressure frequently differs at millibars = Iinch (Hg) = 1,000 feet elevation. thepoint of landing from that of takeoff: Since Imillibar is equal to about 30 feet below therefore. an altimeter correctly set at takeoff 10.000 feet altitude, a change of 10 millibars may be considerably in error at the time of would result in an approximateerror of 300 landing. Altimeter settings are obtained in flight feet. byradiofrom navigationalaids with voice facilities. Otherwise, the expected altimeter setting for landing should be obtained by the pilot Altimeter Errors Due to Variation before takeoff. From Standard Temperature To illustrate this point. figure 16-12 shows the pattern of isobars in a cross section of the Another type of altimeter error is dueto atmosphere from New Orleans. Louisiana.to nonstandardtemperatures. Even though the Miami. Florida. The pressure at Miami is 1,019 altimeter is properly set for surface conditions, millibars and the pressure at New Orleans is it will often be incorrect at higher levels. Ifthe 1,009 millibars, a difference of 10 millibars. air is warner than the standard for the flight

500 FEETFEET

PRESSURE. ALTIMETER READS "500 FEET"

200 FEE

NEW ORLEANS MIAMI 1009 MB 1019 MB

AG.726 Figure 16-12.Altimeter errors due to change in surfacepressure.

576

58,Z Chapter 16 -SPECIAL OBSERVATIONS AND FORECASTS altitude, the aircraftwill be higher than the pressure in millibars must be converted into altimeter indicates,ifthe ouris colder than inches of mercury before it can be used for an standard for flight altitude, the aircraft will be altimeter setting. lower thanthealtimeter indicates. (See fig. The computations for forecasting in altimeter 16-13.) setting are illustrated as follows:

FORECASTING PROCEDURE CASE 1 The first step in the forecasting of altimeter Forecast sea level pressure 1004.0 mb settings is to forecast the sea level pressure for -2.0 mb the valid time of the desired altimeter reading. Diurnal change This may be done by using the recommended Corrected forecast 1002.0 rah procedures of prognosispresentedinearlier chapters of this training manual. Facsimile prog For final forecast, charts can also be used. 1002.0 mb converts to 29.95 in. After the value for the expected sea level pressure has been obtained. it must be modified to reflect the diurnal pressure change at the CASE 2 location in question. Pressure tendency charts, locally prepared diurnal curves, and other avail- Forecast sea level pressure 996.0 mb able information can be used to obtain represen- Diurnal change +1.0 mb tative diurnal changes. The final result of the first two steps will Corrected forecast 997.0 mb normally be expressed in millibars since itis conventional to work in these units on related For final forecast, charts.If this is the case, then this resultant 997.0 mb converts to 29.44 in.

ALTIMETER READS ALTIMETER READS CORRECTLY /0, READS ^78 HIGH ALTITUDE ,

lirI I I1:111;

WARM AIR

1023 MB SAME SEA LEVEL PRESSURE

AG.727 Figure 16-13.Altimeter errors due to nonstandard air temperatures.

577 S83 AEROGRAPHER'S MATE 1 & C FLIGHT LEVELS VERSUS TRUE ALTITUDE

A flight level is definedas a level of constant atmospheric pressurerelatedto areference datum of 29.92 inches ofmercury. In other words, the altimeter is set to 29.92 regardlessof thetruealtimetersetting. andlevelsflown thereafter are based on this indicationof altitude. For example. flight level 250 isequivalent to an altimeter indication of 25,000 feet when the altimeter is set to 29.92. AG.728 In the continental United Statesand Alaska, Figure 16-14.--Example of surface-tosurfacetrajectory. FAA regulations require allaircraft to use a standard setting of 29.92 inchesat and above a gun. since it may reduce the gun's range by 18.000 feet. Also. this standardsetting of 29.92 more than half of what it would be if the inches is used over theoceans at all altitudes atmosphere were absent. when an aircraft is a distance of 100miles or Recent developments in new types of more from a defined area. naval gt,n-fired projectiles. suchas the rocket assisted Over Europe, the standard altimetersetting of projectile (RAP) and improvedconventional 29.92 inches is used at and above 3,000feet. projectiles have revived interest in meteorologi- Pilots must be briefedon low-pressure areas cal ballistics on the part of the operatingforces. along the flight route. Otherwise,a standard The ,development of simplified proceduresfor altimeter setting of 29.92 inches couldgive a computing and applying ballistic wind aridden- false indication that would indicatean altitude sity corrections has becomean urgent require- significantly higher than the actualaltitude. In iti for fleet operations and for training. mountainous terrain, this couldprove disastrous unless the pilotis alerted to the presence of these areas of low pressure along theroute. BALLISTIC FORECASTS The effects of air density and wind METEOROLOGICAL BALLISTIC FORECASTS upon the trajectory of a projectile cannot beaccounted The still (no wind) atmosphere for without considering the meteorologicalef- acts on a fects. Corrections for wind and densitymust be projectile as a retarding force, whosemagnitude applied isadirect in the form of "ballistic" windand function of the density of the density corrections, for which additional atmosphere. The problem of calculating compu- the tations, requiring some knowledge ofmeteorol- extent to which a projectile will be slowed down ogy. are necessary. These computations demand thus involves measuring the atmospheric density measurements. equipment andskillsusually along the projectile's trajectory. This trajectory available only to Naval Weather Service may be surface-to-surface as illustrated in figure Com- mand units, and instructions for makingthem 16-14 or it may be surface-to-air, air-to-air,or are written primarily for use by meteorological air-to-ground. personnel. The dissemination of ballisticwind Although the Aerographer's Mate will most and density forecasts for Fleet operatingunits is frequently be called upon to furnish corrections the responsibility of the Naval Weather Service for surface-to-surface and surface-to-airtrajec- Command. tories.he mightalso be askedto provide The areas for which ballistic forecastsare air-to-ground (bomb) trajectory corrections. The issued include most of the northern hemisphere latter are referred to as Q-factors. The magni- and a significant portion of the southernhemis- tude of the retarding effect due to wind and air phere. Requests for ballistic wind anddensity density is quite important in directing the aim of forecasts should specify the location andtime 578 584 Chapter 16 -SPECIAL OBSERVATIONS AND FORECASTS period of interest and should be forwarded to theactual atmospheric densityis5 percent the cognizant Fleet Weather Central in accord- greater than the standard ballistic densityfor ance with NavWeaServComInst 3140.1(Series). that height. U.S. Navy range tables (with the exception of RAP) and fire control computer cams are based NATO STANDARD BALLISTIC onthe Navy Ballistic Standard Atmosphere METEOROLOGICAL MESSAGE which is described as a density of 1.2034 kglm3 at a pressure of 1,000 nib with a temperatureof The United States, by ratification of NATO 15°C and a relative humidity of 78 percent. Standing Agreements (STANAGS). has adopted This data differs from the NATO atmosphere in the standard atmosphere and the standard ballis- that the Navy atmosphere is lower up to 7,000 tic meteorological message formats approved for meters. At approximately 8,500 meters the use by forces of the North AtlanticTreaty values of density in the two atmospheres are Organization (NATO). All ballistic wind and equal, but the Navy atmosphere rapidly becomes density forecasts are transmitted in the NATO the more above that level. The physical proce- format unless specifically requested otherwise. dure for computing ballistic density is contained in NavOrd OP 3784, mentioned earlier in this The NATO Standard Ballistic Meteorological Message is applicable to 3-inch through 8-inch section. standard projectiles.Itconsists of two parts: ( I ) a mandatory preamble, followed by (2) six Ballistic Wind figure groups of ballistic data. in pairs. which, Ballistic winds for conventional gunfire are for naval gunfire, may number up to ten or more obtained in much the same manner as ballistic pairs. The format and content are contained in densities. The same weighting factors are used the technical manual Ballistic Wind and Density for both range and crosswind components. For for Naval Gunfire. NavOrd OP 3784. rocket - assistedprojectiles,however,wind weighting factors are not the same for range wind as for crosswind. Thus, a correct determi- DETERMINATION OF nation of the range wind component depends BALLISTIC ELEMENTS upon aballistic wind based on range wind weighting factors, and determination of the With the introduction and advancement of crosswind component depends upon a ballistic computertechnology many of theballistic wind based on crosswind weighting factors. Both corrections required for inis Iles and naval gun- components must be used to obtain theballistic fireare applied automatically. However, raw wind. Ballistic wind computational procedures data must still be determined and it is necessary are contained in NavOrdOP 3784. for the AG tofamiliarizehimself with the computational procedures involved. Q-FACTORS

Ballistic Density The effects of wind are as important to the fall of a bomb as they are to the trajectory of a Ballistic density is obtained by computing the projectile. In the case of bombing, however, the weighted average of the relative values in the effects of wind are treated slightly differently various zones, the relative valuesin each zone than the effect of wind on a projectile, primarily being obtained byfirst computing the zone because the wind corrections must be of such a density from the mean virtual zone temperature nature that they are readily adaptable for use as and the pressure at the mid-point of the zone, an input to the bombsight. The term"Q-factor" andthenexpressing thiszone densityas a is subsequently used and is a modification of the percentage of the standard density for the zone. ballistic wind concept. The procedure for com- For example,iftheballisticdensityfor a puting Q-factors may also be found in NavOrd particular zone is expressed as 105 percent, then OP 3784.

579 585 AEROGRAM IER'S MATE I & C'

SOLAR RADIATION-HIGH ALTITUDE and lasting anywhere froma few minutes to (SOLRAD-HI) OBSERVATIONS several hours after the flareceases to exist. Solarflares The arefoundtohave adirect SOLRAD -IIIprogram which is currently relationship with disturbances under development involves occurring in the the launching of a earth's ionosphere. These series of satellites for the ionospheric disturb- measurement of high ances occur coincident with andsome hours altitude solar radiation emittedby the sun. following flares and solargeomagnetic storms. indicating that flares emit TECHNICAL FACTORS ultraviolet radiation, x-rays. and electrically charged particles.The more intense magnetic storms Militaryactivities concerned with are strongly corre- satellite lated with solar flares. Ilowever,it must be kept surveillance, communications.and research and in mind that although development ionospheric disturbances arebecoming increasinglycon- may be correlated with solar flare cerned with solar-geophysical activity, all activity. Since the solar flares do not necessarilycreate ionospheric Aerographer's Mate is directlyinvolved in satel- disturbances. lite surveillance for the receipt of meteorological During the disturbances ofthe ionosphere data and involved either directlyor indirectly in attributed to solar flare activity, communications. he should keep long distance current on radio communicationsmay deteriorate or be certain factors related to thesefields. In the case completely blacked out for severalhours. Dis- of satellite surveillance, solargeophysical activ- turbances may create changes inthe strength of ity has been determinedto affect the amount of the geomagnetic field resultingin interruptions dragon some spacesatellites:cause some in land line currents sufficient changes to affect wire relatedtoorbitalparameters: and communications. produce various tracking difficulties. In the case SOLARPROMINENCES.-Solar prominences of communications. the effectsare primarily in appear in various configurations. Amongthese the area of the quality oftransmission and are those designated as "loop" prominences.The reception of data. Thereare many technical loop prominence is indicative ofa highly erup- factors involved in the field ofsolar geophysical tive solar activity center in its study which are beyond the most active phase. scope of this Rate When loop prominencesare observed on the Training Manual. However. the AGshould have eastern side of activity centers, solarflares such a basic understanding of some of the characteris- as those mentioned in the preceding tics related to this paragraph fieldbecause. due to his may be expected during the following 2 involvement in receiving satellite weeks. data. he may Solar prominences are discussedin more detail be called upon atalater date to perform in chapter 3 of this manual. functions relatedto the acquisition of solar geophysical data. SOLAR OBSERVATIONS Some of the physical features ofthe solar disk were presented in chapter 3 of this manual. Solar observations for thepurpose of studying solar emissions are currently performed EFFECTS OFSOLAR FLARES in a variety of ways. among whichare included the following: Outbursts of radio noise oftenaccompany solar flares. particularly the larger ones. This !. Optical observations. Optical noise may take the form of observatories, a sudden onset of through the use of visualtelescopes, provide radio noise in a narrow bandat high frequency, detailedobservations for analysis,prediction, drifting downward in frequencyto below 100 and detection of solar activity. MHz over a period of from 2 They also pro- to 5 minutes or in vide world-wide, 24-hourcoverage of the sun's some cases over a period of a few seconds. Flare physical appearance. Although associated radio disturbance much valuable may occur as an data is obtained in thismanner. the limitations onset of noise over a band of several hundred placed on this means of observationare readily MHz commencing during visiblephases of a flare apparent.

580 586 Chapter 16 SPECIAL OBSERVATIONSAND FORECASTS

2. Radioobservation. Theseobservatories Greater utilization of Arctic and Antarctic need for Aerogra- supplementthe optical observatories through areas has brought about a the utilization of radio telescopes, These tele- pher's Mates to develop an understandino, of sea distribu- scopesinactuality are large antenna-receiver ice, the terminology used, its general combinations. They monitor solar radioemis- tion, and the Ice Observation Program. The need from attack has sions at discrete (isolated)frequencies set aside to protect our northern borders for this purpose. They provide solar mapsin madeitnecessary to establish a networkof selected wavelengths for detailed analysis. listening posts such as radar sites, throughout the Arctic, These weather stations andradar 3. Satelliteobservations. This method of sites must be supplied. Usually these supplies are solar observation currently provides observations brought by ships. These ships must be awareof of solar x-ray radiation, electromagnetic parti- where the ice hes in relation to their position. cles, and solar wind characteristics.Satellites Only in summer does the icepack is treatfrom also provide early warning of changes inthe our northern coastlines.Therefore, thisis the interplanetary or near-earth space environments. season 'hen supply routes mustbe traversed. They also provide increased solar coverage since For these and other reasons personnel arc they may be positioned to detect solar events all aspects of ice which occur on some portions of the sun not given thorough training in observing priorto assignment to operational visible from the earth. duties. These duties may include assignment to aerial ice observing units or to various arctic or ARCTIC AND ANTARCTIC ICE antarctic bases. OBSERVATIONS The study or the air-ocean environment. ICE IN THE SEA especially in polar meteorology, would be in- the presentation of some Ice in the sea consists, for the most part,of completewithout of top information related to the characteristics of sea either sea ice formed by the freezing layers of the ocean, or icebergsoriginating from ice, glaciers or continental ice sheets. Sea ice ac- The U.S. Naval Oceanographic Office(NAV- of the area ice fleet counts for around 95 Percent OCEANO) was engagedinsupporting encountered, but bergs are important becauseof operations within the arctic and antarctic seas the manner in which they drift fromtheir point through the implementation of an ice observa- of origin, constituting a navigation hazard.A tion and forecasting program. Thisresponsibility certain amount of ice encountered at seaorigi- has now been assumed by the NavalWe!.ther fresh water ice: Weather nates in rivers or estuaries as Service Command and assigned to Fleet however, as itis already in a state of deteriora- Facility, Suit land. Included in this programis reaches the open sea, its of compi- tion by the time it the issuance of bulletins which consist importance is local. lation of data obtained from shipboard,shore. and aircraft observers who forwardtheirice observations to FLEWEAFAC, Suit land.These ICE OF LAND ORIGIN observations may be received as special reports often or they may be appended toroutine weather Ice of land origin in the sea. though reports. For this program to be fullyeffective, it spectacular,is of minor importance in Arctic operating operations, except in localized areas.Icebergs is essential that all vessels and aircraft detached from the fronts in ice areas cooperate with the NAVWEASERV- are large masses of ice of information adds to the of glaciers, from glacier ice tongues, orfrom the COM. Eachbit of the Antarc ic.Smaller masses, steadilyincreasing knowledge of theseleast shelfice add to termed growlers and beigy bits, mayoriginate traveled and remote seas. Such data also from the the accuracy and usefulness of theglobal sea ice from glaciers.or may be formed forecasts produced and distributedby FLE- disintegration of icebergs and other masses of WEAFAC, Suit land. land formed ice.

581 AEROGRAPIIR'S MATE I& C FORMATION OF SEA ICE The density ofse: ice is dependentupon its When sea water with content of brine and air bubbles.The density of a salinity that is higher pure ice at 0°C is 0.9168. but than 25.0 o/oo iscooled at the surface. the the density of sea density of the surface ice may be either aboveor below that of pure layer inewases, causing ice, depending convective movements onitsbrine and air bubble that continue until the content. surface water is cooledto the freezing point and ice begins to form. Elongated crystals of pure ice GENERAL TERMINOLOGY are produced first. Since thesalinity of the surface water increases with cooling, theconvec- To make an intelligent tive currents aremaintained. With continued approach toward an understanding of ice,itis freezing, the icecrystals form a matrix in which first necessary to certain amounts of know some of themore or less standardterms sea water become trapped. by which iceis observed and The more rapid thefreezing. the greater the reported. This amount of sea water that terminology is a new language.It is very broad will be enclosed in the andis used almost exclusively ice. When thetemperature of the ice is further by those who lowered some of the work with ice. For furtherinformation on terms trapped sea water freezes. used for reporting ice. makingthecellscontaining consult Ice Observation thesaltbrine 11.0.Pub.No. 606-d. Someof the most smaller, resulting in thesalt concentration in the com- enclosed brine becoming monly used termsare listed in the following greater. Thus. sea ice is sections. made up of crystals ofpure ice separating small cells of brine whose concentration of salt de- Sea Ice pends on die temperatureof the ice. If the ice is cooled to low enough temperatures. solid salts Sea ice is that ice formedby the freezing of will crystallize out. sea water at temperatures in With a rise of the the neighborhood temperature of sea ice, the of -2°C, subject tocertain conditions.as stated ice which encloses thebrine-filled cells melts and in the previous section the salt crystals begin on the formation of sea to dissolve. As melting ice. The first sign thatthe sea surface is freezing progresses. the brine cells becomelarger; as the is an oily opaqueappearanct of water. This temperature approaches 0°C.the cells join, appearance is caused by theformation of spic- permitting the trappedsea water to flow down- ules, minute ice needles, ward. Where the and thin plates of ice sea ice has become hummocked known asfrazilcrystals. which increasein (ridge of ice), most ofthe salt brine will flow number until thesea surface attains a thick. downward leaving only pure ice, This ice can be soupy consistency. This is knownas grease ice. used as a source of potablewater. Sea ice which Upon further freezing, and has not been hummocked depending on wind will become soggy and exposure, waves, and salinity, thegrease ice disintegrate. develops into nilas,an elastic crust with a matte Salinity of ice dependson how fast it freezes. surface, or into ice rind,a brittle shiny crust. Except in wind-shelter its age, and on thetemperature changes to which areas, the slush, as it thickens, breaks up into it has been subjected.When sea ice freezesvery separate masses, and is rapidly, brine and salt oftenaccumulate on ice frequently characterized bya pancake form. The surfaces, making the surfacewet at temperatures raised edges and roundedshapes result from collisions of the cakes. With as low as -30° to -40°C. Thisaccumulation the continuation of greatly increases frictionon sled runners and low temperatures. the cakesfreeze into a contin- skis, uous sheet.

Sea ice properties differgreatly from those of Age fresh water ice. Theproperties of sea iceare dependent upon theamount of enclosed brine and the number of air The age of the ice isdefined as thestage in bubbles left in the ice if theice cycle from inception all or part of the brine hastrickled down. to dissolution. There are basically threestages in the formation 582 588 Chapter I6- SPECIAL OBSERVATIONS AND FORECASTS of sea ice. YOUNG ICE is newly formed level PUDDLE isa depression on sea ice thatis ice. Its thickness is from 10cm to 30cm (4 in. to usuallyfilledwith melted water caused by 12 in.). Young ice is further classified as gray ice insolation when they have burnt completely and gray-white ice. FIRST-YEAR ICE is more or through the ice. They are then called THAW less unbroken level of ice of not more than one holes. POLYNYA is any sizable sea water area winter's growth, originating from young ice. Its that is enclosed by sea ice-simply expressed, a thickness is from 30 cm to 2 m (12 in. to 6.6 ft). large hole in the ice. First-yearicemay be subdividedinto thin first-year ice, medium first-year ice, and thick Size first-year ice; the latter is more than 120 cm (4 ft) thick. OLD ICE is extremely heavy sea ice Generally, sea ice is categorized into seven which has survived at least one summer's melt. differentsizes. The sizesare giveninthe Old ice may be subdivided into second-year ice following section.Refer to figure16-15 for and multiyear ice. relative sizes and a comparison to other fe-f-ires.

Topography ICEBERGS Topography, or the configuration of the sea Icebergsarelargemasses of floating(or surface, deals with the degree of surface rough- stranded) ice derived from the fronts of glaciers, ness of sea ice from flat to extremelyrough. The from glacier tongues, or from the shelf ice of the terms most frequently used to describe the Arctic/Antarctic. They are products of land, and topography of the ice, sometimes used in combi- not of the sea. Their structure, and to some extenttheir appearance, depends upon the nations are listed below. Arctic bergs RAFTED ICE.-Raftingisassociated with source from which they are derived. young ice and young first-yearice.Rafting originate mainly in the glaciers of Greenland, occurs when icecracks or floes are forced which has 90 percent of the land ice of the north together due to the pressure of wind and is polar region. Arctic bergs are irregular in form formed by one cake overriding another. Rafted and take many varied shapes. Most common are the irregular cone-shaped bergs, produced by icehas well-definedcontours and when observed, may be regarded as a relatively recent glaciers that have plowed across the uneven foreland on their way to tidewater. They diffa occurrence. entirely from the flat-topped straight-sided bergs RIDGED ICE. -Ridgingisassociated with first year ice. It is another form of deformed ice (tabular bergs) originating where the ice sheet in the form of a ridge or many ridges. Itis itself is thrust directly out to sea. (See fig. usually much rougher than rafted ice. 16-16.) HUMMOCKED ICE.-Huminocking is associ- Irregular icebergs known as glacier bergs are ated with oldice.Itis defined as ice piled often accompanied by rams which are a protru- haphazardly into mounds or hillocks. At the sion from the submerged portion of the iceberg. time of formation, hummocked ice is similar to These rams can be a great hazard to vessels that rafted ice, except that the former requires a might pass close to an iceberg. greater degree of pressure and heapingthan the The ratio of mass of the submerged portion of latter. a berg to its total mass is equal to the ratioof the specific gravity of the berg to the water in Water Features which it is floating. On account of the origin of glacial ice in compacted snow, berg ice contains There are a great variety of water features up to 10 percent trapped air andis, therefore, Itis associatedwith seaice. Some of the most somewhat less dense than ordinary ice. common features are as follows.A FRACTURE often erroneously assumed that a berg that is is any break through the ice. LEAD is along, one-eighth above water and seven-eighths sub- narrow, but navigable fracture or passagewayin merged should be floating with a draft seven seaice. The lead could be open or refrozen. times its height above water. These ratios hold 583 589 I GIANT FLOE ( OVER 10KM)

,....-

,14,I7.>"=4! 41M)/18 Ee VAST FLOE (2-10KM) SMALL CITY

BIG FLOE (500-2000M) GOLF COURSE

41,*

51111V-4-4 4441041, MEDIUM FLOE (100-500m) him*

SMALL FLOE (20-100M)

ICE CAKES (2-20M) ) VOLLEY BALL COURT

^1. SMALL ICE CAKES (LESS THAN 2M) POOL TABLE TOP

AG.729 Figure 16-15.Sizes ofsea ice.

584 590 Chapter l6' SPECIAL OBSERVATIONS ANDFORECASTS

GLACIER BERG

- .....-

...... i ,...

11.::::::::"...4,11..i-.44.:...... :.: ....y,a -7-'72-1-- .,. ,...... ;..::::::"-:... ---4:.::.....,..:_,-.....::,:e.:7:::-:,:.:...,s.:::si-t..ig::::-.::::-;:ri:::::;. ..:.::::.: ... "...... - _41.,-,c:,:;:i:::::1-g ... . _- .i, .. :7i:::3--- ,sf:....-'' ::::...... :.. .,.,..: ......

TABULAR BERG

AG.730 Figure 16.16. Types of icebergs. usually of pod only for mass, and notfor linear dimen- of ice awash, smaller than a bergy bit, glacial origin, and generally greenish incolor. It ions. Actual measurementsof Arctic bergs show hat the draft is seldom more thanfive times the is about the size of a grand piano. :xposed height for the blockiest bergsand may )e as low as one or twotimes the height for the MOVEMENT OF ICE pinnacled and irregular types. of ice, BERGY BITS and GROWLERS,like icebergs, Sea ice, other than fast ice (all types formed from the either broken or unbroken, attached tothe originate from glaciers or are shoal water, or disintegration of icebergs and other massesof shore, beached, strandedin attachedto the bottom of shoalareas), in landice. A BERGY BIT isa medium sized the size of a sheltered bays or along the coast iscontinually fragment of glacier ice and is about of wind, tide, small cottage. A GROWLER is asmall fragment in motion as a result of the effects 585 591 AEROGRAPHER'S MATE 1 & C

and current. Although this motion may be the Pack ice drifts with the and tide usually same for a time over a considerablearea, there to theleft of the true windwind in the Southern area number of factors tending to produce Hemisphere and to the right in theNorthern differential motion of adjacent masses. Cakes, Hemisphere. The speed of driftmay not depend for example. vary inarea and thickness, so that entirely upon the strength of the wind, theeffect since it is of wind and current differson influenced greatly by thepresence or absence of different masses of ice. Wind and current are open water in the direction of the drift.even also subject to continual local variationswind though the open water is somewhat distant. from the usual meteorologicalcauses, and current from tidal effects. Neglecting the resistance of the ice, Ekman's theory of wind drift calls for the iceto drift 45° The swinging or turning of floes is due to the from the wind direction. Observationsshow that tendency of each cake to trim itselfto the wind the actual driftis about 30° from the wind when the pack is sufficiently open to permit direction on the average, orvery nearly parallel freedom of movement. In closepack (any large to the isobars on a weathermap. In winter, area of floating ice driven closely together),this when the ice is more closely packed and tendency may be produced by offers pressure from more resistance, its drift deviates less from the another floe: but since floes continually hinder wind direction than in summer, and tidalinflu- each other, and the windmay not be constant in ences become more important. direction, even greater forces result. Thus,wind The speed of drift of pack icecan be fairly produces rotation as well as translation. This closely determined. from the wind speed.Ob- screwing or shearing effect results inexcessive servedaverage speeds of drift of ice in pressure at the jutting corners of floes, and the Northern Hemisphere ranges from 1.4percent of forms a hummock of looseiceblocks. Ice the wind speed in April to 2.4percent of the undergoing such movements :s calledpressure wind speed in September. ice and is extremely dangerous to vessels. The general circulation of the iceof the In its motion. the ice opens and shutslike an Arctic Ocean is determined by the direction of accordion; a number of leads must bepresent, the ocean currents, whichare the result of two otherwise the ice could notmove. In summer chief factors: the circulation of the atmosphere these leads remain open, except invery high abovethepolar basin andthe. latitudes, but in winter they surrounding are soon frozen adjacent seas, and the influx intothe polar over with young ice. Swell also tends to break basin of water of oceanic and river origin, witha up ice,'as well as the vertical movement of the compensatory outflow of the water from the tide in narrow or shallow waters. Asa result of polar basin. these factors, the ice is alternately being broken up, even throughout the winter, and subjected BREAKUP AND MELTING OF ICE to pressure. The onset of pressureor release of pressure may happen at any time ofyear, even Heating agents during the lowest midwinter temperatures. Ice and snow are evaporated andmelted by DRIFT OF ICE direct absorption of radiation and byconduc- tion of heat from the surroundingair, rocks, or While the general direction of the drift of water. The ultimate source of heatenergy is the icebergs over a long period of time is known, it sun in either case, but the relative importance of may not be possible to predict the drift of an radiation and conduction in melting ice willvary individual berg at a given place and time, for with climatic conditions in differentlocalities. bergs lying close together have been observedto In the case of the longerwave lengths in the move in different directions. They move under infrared portion of the spectrum, which the influence of the prevailing current makes at the up slightly over hall the total radiantenergy depth to which they are submerged, which often received from the sun, the proportionsreflected may be in opposition to the existing wind and by a snow surface are only 15 to 25percent. sea or surface drift. The proportions reflected by ice andwater are

586 592 Chapter 16 --SPECIAL OBSERVATIONS AND FORECASTS correspondingly less, and since water is opaque Another factor tending to accelerate the rate to infrared radiation, the heat absorption by of ice melting from solar radiation, once it has water in this region of the spectrum is concen- started, is the increased stability of the surface trated in the surface layers where itis of most layers of the sea brought about by the freshen- significance to the melting of ice. It is obvious, ing effect of the melt water. Mixing between the therefore, that a surface interrupted with areas surface and deeper layers, already diminished by of water, either leads between floes or pools of thewave-damping action of floatingice.is melted water accumulating on top, will absorb further decreased by the formation of a surface much more radiant heat than a continuous ice or stratum of relatively low density, In regions snow surface. Once disintegration of anice sheet where the spring melting of ice is brought abibut has proceeded to the point where free water chiefly by atmospheric transfer of heat from surfaces appear, the rate of further disintegra- lower altitudes. and where local fogs restrict the tion is accelerated. solar radiation reaching the ice and sea surface. the fresh surface layers of sea water may become greatlychilled,andtherate of meltingis Evaporation reduced. !fere, vertical exchange of water caused by wind. sea. current and tides contribute heat The absorption of heat by ice or snow results to the upper layers and expedite clearing of ice. in either evaporation or melting. Melting takes place as soon as the temperature of any superfi- cial layer of the ice surface is raised above the Stages of Disintegration freezing point, but evaporation may occur at any temperature. Under still airconditions, the In spring, as the duration of daylight begins to layer of air nearest the ice will soon become increase and the mean air temperature at the sea saturated with water vapor. Evaporation there- surface rises, the snow cover of the sea ice and fore depends on the diffusion of water vapor the top layers of the ice begin to thaw. Under from the surface and generally proceeds at a conditions of low humidity, most loss on the relatively slow rate. Air currents tend to increase upper surface of the ice takes place through the rate of evaporation by inducing turbulent evaporation imperceptible to the ordinary ob- mixing of the layers of air near the ice and by server. Where humidity is higher, pools of dew bringing new unsaturated air masses from drier and melt water form on the surface. This fresh regions. Under conditions of low relative humid- water, running down through cracks and holes in ity, brisk wind may therefore result in ablation the ice, freezes again on contact with the cold (removal for cause) of large quantities of ice and sea water, thus sealing the openings.However, snow even though the air temperature never when cracks extend onlypart of the way reaches the melting point and no inciting occurs. throughtheice,they are widened by the expansion of this water freezing in them; and even though plugged at the top, they will now Melting extend through to the water. On further rising of theair temperatures and melting of the Melting of icetakes place mostly atthe surface, these cracks open up again, and fresh expense of the heat of the surrounding water. water in a layer as much as to 2 to 3 feet thick This heat may have been absorbed from solar flows under the ice. radiation in the vicinity or provided by currents originatingin warmer latitudes. Melting also Decay of the pack is accelerated by mechani- results from direct absorption of radiation by cal attrition from the swell. The physical erosion the ice and from contact with warm air. Ice will of the floes produces scaling, resulting in the condense dew from warm, moist air on its formation of a quantity of small blocks and surface, and each increment of moisture so brash. The scaling process enables the sea to condensed will melt several times its weight of witch more extensive areas of ice where the ice in the ratio of the latent heat of evaporation reduction to finer and smaller particles takes to the heat of fusion. place.

587 593 AEROGRAPIIER'S MATEI & C

The final stages of melting vary with the type plans and preparations. Youare primarily con- of ice. Ice of one winter's growthmelts readily cerned with furnishing information in low latitudes, if brine is as to the stillpresent. The structure dissipation. movement, and breakup of internal melting due to variationinthe salt the ice. content produces a honeycombedappearance with a much greater surfacearea. Since the rate Before operationsareundertakeninthe of heat absorption through conductionis pro- Arctic. elaborate planning suchas in any naval portional to the area exposed, therotten ice so operation. is clone in great detail. In theNaviga- formed quickly disappears. Fresherand hum- tion and Meteorology Annexto a standard Arctic opei-ation mocky iceislonger lived. The old floesare plan, pertinent information heavily undercut at the waterline,but honey- should be outlined on the navigablesea ice areas combing is rare, owing to the absenceof salt. as wellasinstructionsfor determining and The years-old hummocks of theArctic pack, forecasting movements on thearea of operation. having a homogeneous structure of nearlysalt- From your knowledge gained free through the ice. and having a minimum ofexposed study of sea ice in this chapter. andthrough surface in proportion to their bulk. survive the further study of available publications,you can longest in warmer waters. readily see how the meteorological officecan be of invaluable assistance in planning Breakup on rivers usually occurs 3or 4 weeks a successful after the mean air temperature has risen operation into the Arctic regions. To accomplish above this task you must be 32°F. Ice on lakes breaksup 2 or 3 weeks later. aware of and alert for sea and sea ice may break conditions which could favorablyor adversely up at about the same affect the operation. time. The breakup on the rivers is botha rapid andviolentevent. The force of the water flooding down the channelstears up the ice and OPTIMUM TRACK SHIP ROUTING (OTSR) drivesit seaward at a rate of around 4 knots. Bends or constrictions in the channel cause The international steamer tracks inuse today temporary piling up of the ice with the result are modifications of routes laid out by LT that the water and the blocks flood thevalleys Matthew Fontaine Maury, USN (1850). Among until pressure breaks the ice barriers. Ina week other things.theseroutes were designed to or less. the entire river rids itself of ice. River optimize steaming distance and reduce theprob- levels risetremendously during the breakup. ability of encountering delayingor damaging reaching heights of 70 feetor more over the storm conditions. winter levels. In earlier years, the climatologicalor statisti- EFFECT OF ICE ON cal approach was the best method availableto NAVAL OPERATIONS marinersfor prevoyage route determination. Although this. method avoidsareas of known This discussion is tailored to therequirements high frequency of storms, blindadherence fre- for Arctic operations.since we are primarily quently results in low efficiency. Conditionsfar concerned with the effect ice hason resupply of from average may exist alonga seasonal route, Arctic bases and the feasibility ofsubsurface or portion thereof, such that an out-of-season operations in the Arctic track is often a much betterroute. As has been pointed out previously in this Today a thoroughly tested ship routingserv- chapter. bases have been established andthey ice.basedupon considerations of extended must be resupplied. This occurs mostly during weather forecasts, is now available. Todate, the the summer months of June, July, andAugust. Navy Oceanographic Office, which Operationsinto developed the Arctic regions must be theprogram,theFleet Weather Centralat thoroughly planned and prepared in advance to Norfolk, Virginia. and the Fleet WeatherCentral insure their success. A knowledge ofsome of the at Alameda. California, have successfully under- requirements will aid you in assisting inthese taken hundreds of routings,

588 594 Chapter 16- SPECIAL OBSERVATIONS AND FORECASTS

BASIC PRINCIPLES one of the most Important aspectsof the AND OBJECTIVES program. Each day the routed ship sends abrief message which includes the position, course, Optimum Track Ship Routing (OTSR) is an speed, wind, and sea conditions. When weather advisory service which provides recommended deviations from the originalforecast are de- ship tracks to minimize time enroute and to tected. the ship's route can be modified to avoid reduce storm damage to ships and cargo while at undesirable weather conditions. Diversions to the same time considering any special require- the original route are not the rule but do occur, ments of the ship. The procedure can be tailored particularlyinwinter,especiallyinregions to assistin accomplishing any mission with a where changes in pressure patterns are apt to be higher degree of efficiency on an average. sudden and of considerable magnitude. The number of recommended diversions is surpris- In essence the program utilizes an effective ingly small. To date. the averageis one per system of longrangeforecasting to predict the voyage with a maximum of three. fastest and safest port-to-port route for transoceanic voyages. This system does notpromise Upon completion of avoyage, avoyage smooth seas, sunny skies, and light winds, but summary memorandum is sent to the weather rather the high probability of sailing the most service which provided the routing service. This report indicates position of ship. sea, swell, and rapidobtainableroutewithsafetyfeatures unobtainable on nonrouted voyages. wind conditions: and ship's heading and speed at specifictimes. This summary provides a means The OTSR service is normally of significant for determining theeffect of wind and sea benefitonly for voyages where the distance conditions on the sailing and for continuing from port-to-port exceeds 1500 miles and there development of ship rout:ng techniques and is a choice of routes. A route in which geog- evaluation of the program. raphy dictates the track would not lend itself to OTS R. NAVAL WEATHER SERVICE The program can be broken down into three RESPONSIBILITIES phases: route selection, surveillance, and summary. Optimum Track Ship Routing services are available for all U.S. Navy ships in accordance with established criteria. The Fleet Numerical Route Selection WeatherFacilityatMontereyprovides this When a Naval Weather Service unit having service for the entire Pacific Ocean while the Fleet Weather Central provides service for the responsibilityfor providing OTSR service receives a request for a route, sophisticated tech- Atlantic Ocean. Additional information is avail- NAVWEASERVCOMINST 3140.1 niques are applied to all available data.Weather ablein prognoses, sea condition prognoses, oceancur- (Series). rent data. ship and cargo characteristics.etc., are allutilizedto determinethe recommended BENEFICIAL RESULTS route. When the route has been selected and tested, The chief benefits of this service are to allow itissent by dispatch directly to theship, for a minimum time en route which affords a arriving from 24 to 48 hours before departure. savings of fuel oil, increased passengercomfort. the reduction of damage to the ship or cargo, Surveillance and the avoidance of rough seas and severe storm areas consistent with time requirements. From this point on, a continuous watch is You can readily see that a program of this type kept on changing conditions which might affect would also be beneficial to commercial vessels. the recommended route. The individual atten- Time on commercial ocean vessels is money, in tion rendered to each ship while underwayis term of salaries, fuel maintenance, etc,

589 595 CHAPTER 17

MAINTENANCE OF METEOROLOGICAL EQUIPMENT

Accurate environmental observationsare the LOCAL MAINTENANCE PROGRAM basis for accurate forecasts. Withoutproperly functioning observational equipment forgather- The maintenance support of meteorological ing environmental data theaccuracy of the equipments is normally the responsibility of the observation is invariably going to becomeques- host or local command. Separate NavAir instruc- tionable. This means that the instrumentsin- tions delineate the maintenance responsibility volved must be maintained in the best possible for meteorological equipment installed in air- operating condition at all times. craft. Operator's preventive maintenance is the Aerographer's Mates first class and chiefare responsibility of the Aerographer's Mate. expected to perform operator's tests, calibra- Preventive maintenance is the systematiccare tions and adjustments to meteorologicalequip- and inspection of all meteorological material for ment excluding electronic components. Theyare the purpose of retaining it in serviceable condi- also expected to inspect workareas for potent- tion. By detecting and correcting minor incipi- ially hazardous conditions and practices:and entfailures before they develop into major interpret directives and instructionson safety defects or malfunctions, the operationalservice precautions applicable to workareas and equip- life of any piece of equipment ment within their area of responsibility. can be extended considerably. Preventive maintenance This chapter will be confined to information on elec- pertaining to the above stated qualifications. tronic equipment must be accomplished by permanently assigned qualified technical Information pertaining to description,purpose, personnel who are thoroughly familiar with theopera- and operating instructions for meteorological tional equipments may be found in AG 3 & 2, NavTra characteristics of the equipment. The 10363D or the appropriate instrument techni- procedure for adequate preventive maintenance cal manuals. checks is described in the Maintenanceinstruc- tion Manual for each of the complex equipment. Strict adherence to this procedure isnecessary MAINTENANCE PROCEDURES for the peak performance of theequipment. Figure 17-1 illustrates a sample dailypreventive At the present time a few meteorological maintenance schedule. equipmentsaremaintained under the Navy A modification of this schedule is presented Maintenance and Material Management System in chapter 18. Similar schedules forweekly, (3M System). Information pertainingto the 3M semi-monthly, monthly, quarterly and annual System may be found in the latest revisionto checks should be preparedon each piece of PO 3 & 2 and PO I & C training manualsor by appropriate equipment. reference to the Standard Navy Maintenance and A maintenance log should be maintainedfor Material Mapagement System Manual, OpNav each piece of meteorological equipment, especi- 43P2. ally items of complex equipment. Time anddate 590 596 Chapter 17MAINTENANCE OF METEOROLOGICAL EQUIPMENT

DAILY PREVENTIVE MAINTENANCE SCHEDULE

*1 2 34 5 67*8910II 121314151617 18192021 -2232425262728193031

MERCURIAL BAROMETER

I CLEAN CASE AND 1, k, t_ tz. *4'0!itt\ ''t #.i EXTERNAL SURFACES 1"',, * '.1 RADAR SET I CLEAN ALL SURFACES 4,i( AixA<< 1 , ,, c, ty ,, OF SET 4,cti` siN Kt, t, (:'. q,,', ," Y'lC,1 tftfkl0/t ik$ 2 CLEAN ALL SCOPES Ix\)X 1/4 ))c )x, ))( )' 4\ )x C4 PkPOet,t!,,Pgtii' 6kti '''e.. WITH WINDEX th J'ik, t) c;\ kf.4,t,k g AN/UM0- 5

I RECORDER CHART 04,.) o cl OPERATION k,,,,,,14,4,',,k.,%.,L \csA\ N'i'',,&'''';,i'';'',A."$ 2INKING SYSTEM #00'l000$h1)000'010.'9,P#'$'3fi"il' IN, )), \N )X 1/4 )X rsn N\ 'I", 3. VANE ALINEMENT /4 1/4 :Is4,*'k ii R,.(-,,';ct.,,4-4,,,.;,:z,,,, 4 AGREEMENT OF ALL A,.1!\.04, ,itr,? rt 1!,,,64,i, I'4, ,'4 RECORDEAS a INDICATOR 4. i0( 4444 4 tit *CHECK WEEKLY MAINTENANCE SCHEDULE

AG.731 Figure 17-1.Sample preventive maintenance schedule. of allemergency, routine, and/or preventive assistance required in connection with meteow- maintenance should be logged along with the logical equipment. parts replaced. Cause of outages and the remedy The administration, organizati .,u areas of therefore should be noted. responsibilityare outlined inthe U.S. Navy Meteorological and Oceanographic Support Man- ual, NavWeaServComInst 3140.1 ( ). METEOROLOGICAL AND OCEANOGRAPHIC All ships, stations. and Marine Corps Aviation EQUIPMENT PROGRAM (MOEP) Field Activities concerned with the installation. operation, maintenance and rework of assigned meteorologicaland oceti:lographic equipment The Meteorological and Oceanographic Equip- are expected to utilize the services of MOEP ment Program (MOEP) was established to insure personnel. The service provided will be in the that meteorological and oceanographic equip- form of assistance in solving meteorological and ment i, installed, operated, and maintained in oceanographic equipment problems beyond the the most effective, reliable, accurate, and eco- capability of local maintenance personnel. nomical manner. The program is staffed by specially trained ,experienced officers designated The ship or station should arrange for local as Meteorologkal and Oceanographic Equipment maintenance personnel to be present for km-the Technical Liaison Officers (MOETLO's). Addi- job training and to assist MOEP personnel during tional personnel include a select group of civilian their visits. Whenever possible they should uti- engineers and military technicians. These person- lizetheinformalshop-classroomtechnician nel,military and civilian,providefleet and training offered at MOEP activities as listed in shore-basedLommandswiththespecialized thesupportmanual, NavWcaSen,CumInst 31 10.1(

591 AEROGRAPHER'S MATE I & C

The ship or station will also be requiredto aeronautic manuals distributed by Nav Sup and maintain maintenance records on assignedme- stocked for issue as of the date of publication. teorological and oceanographic equipment. All manuals are listed by both code number and title. Requests for Assistance Thesepublicationsstocklistsare supplemented bimonthly. Each supplement containsa Commanding officers are requested to indi- listing of publications distributedor canceled cate their requirements for planning, installa- since the issue date of the stock lists. tion. and!or technical assistance whenever the In Nav Air 00-500A, all maintenance manuals needarisestotheappropriateactivity. AU for meteorological equipmentare listed in alpha- requests must include the following informa- numerical order under the heading "Meteor- tion: ology Instruments,"

I. Name and exact location of the activityto which theengineeristo report(for ships. Publications Numbering System inclusive dates of availability at a specific location). Coded numbers are assigned to publications 2 Type and model of equipmenton which issued by Nav Air to orderly divide this material assistance is required. into proper subject categories. Knowledge of 3 Nature of technical difficulty and availabil- these subheadings will more readily enable the ity of spare parts if required. user to determine the desired information in the minimum amount of rime. A brief explanation Spare Parts of the publications numbering system is givenin the following paragraphs. The Nav Sup Manual charges supply officers For manual type publications the number with the responsibility for stockingspare parts consists of a prefix and a series of three parts. listedasallowanceitemsintheNav Air The prefix consists of Jeffers which identify 00-35QL-40/50/60 Initial Outfitting List series. aeronautic publications under the cognizance of Parts of a consumable nature listed in Nav Air Nav Air. Allowance List 00-35QL-22 are still issued di- The prefix may consist ofany of the fol- rectly to meteorological units, upon receipt, by lowing: the supply officer. Meteorological units afloat and ashore are urged to avail themselves of this I. NavAir -This prefixis assigned to those concept by initiating action to have spare parts technical publications originated by the Naval stocked in local supply to the extent allowed by Air Systems Command. In the stock list, it is the allowance list. shortened to NA. 2. NavAer-This is the prefix assignedto the MAINTENANCE MANUALS old Bureau of Aeronautics nr.,nuals thatare still in effect. To conserve space in stock lists it has Throughout the next three chapters, reference been shortened to NA. is made to consult the appropriate technical 3. NavWepsThis prefix is assigned to techni- manual for more complete details on operation, cal publications originated by the now abolished service, and maintenance of the equipment Bureau of Naval Weapons. It is abbreviated NW, mentioned. Such manuals will be listed in one of 4. AN This prefix was previously assignedto two placesthe Navy Stock List of Forms and manuals used by the Navy and the Air Force Publications.Cognizance SymbolI.Section when such were prepared to coordinated mili- VIII. Part C. Nav Sup Publication 2002. or in tary specifications. It is no longer used on new Nav Air 00-500A. Naval Aeronautic Publications publications, but the ones in issue remain in Index, Equipment Applicability List. effect. Nav Sup 2002, SEction VIII, Part C, contains 5. TOThis is the prefix assigned toan Air a complete numerical listing of all available naval Force originated technical publication.

592 598 Chapter 17- MAINTENANCE OF MEI EOROLOGICAL EQUIPMENT

6. CO- This wit; previously used to designate designed to enable maintenance peisonncl to a technicalpublicationwith aConfidential identify and order replacement parts I'mthe security-classification. Itis no longer assigned to model of equipmeat Lowred by the publication. new material, but remains in effect for existing It contains a GROUP ASSEMBLY PARTS LIST publications until superseded. and a NUMERICAL PARTS LIST. 7. TM -This is the prefix assigned to an Army To find a part when the name of the part is originated technical manual. known. tune to the table of contents, find the name of the part. and turn to the page indicated. The three parts which make up the remaining portion of' the number are utilized to indicate Meteorological Equipment the following: Maintenance Manuals

PartI. This consists of numbers to identify These manuals are currently being written by the general subject series with the basic subject the Naval Aviation Engineering Service Unit to which they pertain. (NAESU) for certain meteorological electronic PartII.This consists of numbers and/or and electro-mechanical equipments in Use 01 numbers and letters to designate the specific under procurement bythe. Naval Air Systems class, group, type, or model and manufacturer of Command. They are written foithe A1','LI2 the equipment. The subject breakdowns for this level and Love' intermediate type maintenance part are found listed io the beginning of each procedures. (1ntermeuiate type maintenance is separate major division within the stock list. component repair maintenance.) Several!LILL, Part III. This consists of numbers to designate been written and issued atthe that of this a specific publication. For airframes and engines writing. Simple operator maintenanLe prole this part designates a specific type of publica- dures for AG personnel are also included. tion. For other types of equipment this part is assignedinnumerical sequence and has no TEMPERATURE INSTRUMENTS reference to the type of publication. Thermometers are not repairable items. but Meteorological publications are found in the require pertain maintenaike srrviLes to maintain 50 series and meteorological instruments gener- LLuracy and ease of reading. The most effeLtive ally in the 50-30 series, with some in the 16-30, maintenance is careful handling. Thermometers 16-35, and 16-45 series. and psychrometers are delicate. precision instruments and must be protected from shocks and Operation and Service jars. Instructions Manual VISUAL CHECKS This manual gives the operation and service instructions for the particular item covered. Also Visual checks are made to note any defects in included are instructions for installation, service, the thermometer or inthe metal back upon inspection, maintenance, and lubrication. which itis mounted. Be especially careful to note at the time of each observation, whether Overhaul Instructions Manual any dirt, moisture. or any other foreign matter is present on the thermometer bulb or tube: and This manual contains instructions for com- whether any separationsexistinthefluid plete overhaul of the instrument or equipment column. as performed by the major repair facilities. REPLACEMENT Illustrated Parts Breakdown If a thermometer is found to be inaccurate, it This is actually a parts catalog which lists all must be replaced with one that is known to be of the parts of the complete equipment. Itis accurate.

593 599 AEROGRAPHER'S MATE 1 & C

For care and maintenance of the thermome- INSPECTION ters. see NA 50-30FR-518. Operation and Main- tenance Instructions. Inspect for cracks or breaks in the plastic parts.Inspect threaded holes and screws for TOWNSEND SUPPORT (ML-54) wear. Inspect the fan for damage. Inspect the thermometers for cracks and separated mercury Lubrication columns. Inspect the carrying case for damage. Inspectthe motor shaftfor smoothness of Once each month. place one or two drops of rotation. For troubles, probablecauses. and MIL-0-6085 oil in the oilhole in the maximum remedies. refer to the technical manual. thermometer holder. Do not overlubricate. Repairs or Replacement HUMIDITY INSTRUMENTS All plastic parts that are broken, cracked. or The humidity instruments discussed in this have missing or damaged threaded inserts should seLtion are the sling and electric psychrometers. be replaced. Damaged screws should be replaced. They aredelicate. precision instruments and If the motor Shaft does not turn freely or if the should be treated as such. fan is damaged. the motor and fan assembly should bereplaced.Ifthe contacts on the CARE OF PSYCHROMETERS contact block do not contact the motor terminals correctly. they should be carefully bent so The wick onthe psychrometer wet bulb that good electrical contact is insured. If one should be replaced at least once a month, and thermometer isbroken, boththermometers more often when local atmospheric conditions must be replaced with a new matched set (two causea rapid collection of dirt and foreign thermometers whichreadthe same without matter on the wick. Psychrometers used on wicking attached). A carrying case that is dam- board ship collect salt very rapidly, and daily aged beyond use should be replaced. replacement of the wick may be necessary to obtain accurate readings. TESTS The metal back upon which the thermometers To testthe instrument. are mounted should be cleaned at least once firstinstallfresh battery cells. Then turn the control knob on the every- 3 months, or more often if necessary. rheostat-switch ina clockwise direction. The Excess lubricant, dirt, and other foreign matter motor should run. causing the fan to draw air should be removed from the psychrometer sling into the sliding air intake and force it out of the assembly at least once a month. exhaust ports.If this does not happen. the Once each month, place one or two drops of battery cells are improperly installed. Proper MIL-0-6085oilontheslingpsychrometer installation of the battery cells should correct swivel link. Do not overlubricate. the trouble. As the control knob is turned in the clockwise ELECTRIC PSYCHROMETER direction. illumination should increase. If it does ML-450A/UM not. checktherheostat-switch and lamp.If either is defective, replace it. Although the electric psychrometer is constructed primarily of noncorrodible materials, prolonged exposure to weathering, salt air, stack PRESSURE INSTRUMENTS gases. and other corrosive elements will shorten the useful life of the instrument. The instrument MARINE BAROGRAPH should. therefore, be sheltered when not in Maintenance actual use. Whenever the electric psychrometer is to be stored or used infrequently the batteries Use a damp cloth to clean the plastic sheet should be removed to prevent corrosion. window in the case. Do not use a solvent cleaner

594 600 Chapter 17 MAINTENANCE OF METEOROLOGICAL EQUIPMENT or a dry cloth. as either Lan damage the plastic surfaces of the barometer regularly with a dean. pane. Very little other maintenance is required. lint-freeLloth. lightly dampened with watel. Under normal operating conditions this instr.1- Occasionally. the scales may be wiped dean and went should be cleaned well once a year. The a thin coat of high grade Llock oil applied. element cover should be removed only to clean out any bulky dirt. cobwebs, etc. Do not wipe out this mechanism. Check the pen for wear or Preparation for Reshipment roughness and replace as necessary. The chart drive mechanism should be cleaned When a Fortin barometer is to be moved over and oiled annually. This oiling should not be a considerable distance. itis necessary to trans- attempted by personnel unless they are properly port the instrument in an inclined or inverted instructed in doing it. position. cistern uppermost. For other troubles and remedies with this piece of equipment. consult the appropriate An adjusting screw is provided beneath the NavAir technical manual. barometer cistern for the purpose of raising the mercury during the process of inverting the barometer for shipment. The procedure to be FORTIN BAROMETER followedduringtheinverting processis of critical importance and must be followed ex- Maintenance actly to avoid damage to either the leather bng in the cistern or the glass tube. Refer to the Only minor repairs should ever be attempted technical manual for specific instructions and by meteorological personnel. Mercury leakage, precautions to be followed. After the barometer broken tubes. damaged instruments, or other has been inverted, with cistern uppermost. the faults that affect the accuracy of the readings cistern screw should be loosened about one or are cause for immediate survey and replacement one and one-half turns in order that there may or the instrument. be sufficientfree space for expansion or the mercury in the event of an increase of tempera- The following minor repairs may be made ture. The barometer may be safely transported locally as long as no damage has occurred that or carried by hand in an inverted or inclined will otherwise affect the accuracy or the instru- position. longascareisusedtoavoid ment. subjecting it to rough handling. BROKEN THERMOMETER. Removethe The inside or the barometer case should be plate that holds the attached thermometer. Take wellpacked with excelsior or other packing out the broken stem by removing the two small material and the cover securely closed. The case screws inthe brack.et at the top or the stem. is then wrapped well with heavy wrapping paper. Insert a new thermometer with the bulb project- inginsidethe barometer ease.Replace the A special wooden packing case is used for screws and the plate. shipping the inverted barometer and its case. The packing case must be carefully packed with I3ROK EN CYLINDRICAL GUARD excelsior or other packing material. insuring that GLASS. The guard glass may be replaced unless the box is completely filled. The cover is then the .,cales have been displaced or injured. Re- securelyfastenedwithscrews. Cross braces move the four screws holding the top fitting. should be attached at the bottom, projecting at Remove brokenglass andcarefullyreplace. least a foot from the sides to insure shipment taking care not to disturb scale settings. Any withthe barometer remaininginavertical slight jar might displace the scale settings and position. The case should be labeled plainly in introduce a constant error in readings. Replace red paint or red printed letters: HANDLE WITI I the top fitting. making sure the cork support for CARE, DELICATE INSTRUMENTS, GLASS, the top of the tube is correctly placed. VERY FRAGILE, and THIS END UP. Labels CLEANING. The cleaning of this instrument should be in large letters on all four sides of the isrestrictedto wiping the case and external box.

595 601 AEROGRAPIIER'S MATE I C

Tests least 2 inches of cushioning material, and then placedin a snug-fitting corrugated cardboard The at.i.uracy of the meri.unal barometer is 5(..x. For further protection in shipment, the tested by Lomparison with a pressure staisdard pak.kagedbarometer shouldbe placedina of greater ai.i...uraL.yProi.edu:e foi this %.ompan- wooden shipping Lrate and protected with addi- son may be found in the Manual of Barometry tional cushioning material. (W13AN), NW 50-11)-510.

PRECISION ANEROID Tests BAROMETER (NI L-448/UM ) 'Ile barometer should be checked by care- ThePrecisionAneroidBarometer. fully raising and lowering it through a uistance of 8 to 10 feet without jarring. If the pointer ML-4-18. UM. isusedinthe Semi-Automatic does not Lalicate a preeeptible change due to MeteorologicalStation AN/GNIQ-1-1( Iand aboard ship. this change in height, the baroisieter requires servicing by an instrument shop and should be Checks and Adjustments returned for repairs. The accuracy of the instrument is determined Prior to its initial use in a weather office, a during the comparison noted inthe section comparison check with an approved standard entitled "Checks and Adjustment." Failure of liarometer is required in accordance with the thebarometertotrackwiththe approved standard barometer indicates a lack of accuracy procedure presentedin FMI INo. I Surface Observations, After a correction value has been and the need for recalibration or repairs in an obtained, periodic cheeks are made to ascertain overhaul shop. the instrument's reliability. NOTP ; The only authorized adjustment to be RECONNAISSANCE ANEROID made inthefield on this instrumentisthe BAROMETER (FA-112) current pressure adjustment. The Reconnaissance Aneroid Barometer Maintenance WA-112) is briefly described in chapter 8 of the latest revision to the AG 3 & 2 Rate Training The exterior of the Lase should be dusted Manual. A more detailed description may be whenever required and Oa dial window should obtained from the instruction booklet provided be wiped with a clean damp cloth if necessary. by the manufacturer. Inspect the eeneral physical appearance of the instrument. A crackeddialwindow, dents. Maintenance bends, and other external physical damage probably indkate a need for overhaul of the in- Since the dial of each barometer is individu- struments sini.e impak.t suffikaent to Lathe ex- ally calibrated for its particular mechanism, very ternal damage is usually sullkient to render the little maintenance Lan be done in the field. Scale instrument inoperative or of suspek.t adjustmentisnecessary when the barometer No repairorparts replaLementisto be must be reset to agree with a known standard of att.:mpted at the ubseBer's maintenance ak.i.urai.y. A small bladed screw driver inserted in No attempt to lubrkatc the instrument Is to be the slot of the adjusting disk, which is flush with made at this maintenance level. the dial,is used for this prupose. A counterclockwise rotation of the disk causes a like Preparation for Reshipment movement of the pointer. The adjusting disk is accessible thru the screw plug in the transpare6t If necessary to package the precision aneroid dial cover of the instrument. for shipment. itshould be ,Nrapped in heavy Glass dial covers may be cleaned by wining paper, closed with tape. overwrapped with at with a damp cloth.

596 602 Chapter 17 MAINTENANCE OF METEOROLOGICAL EQUIPMENT

The barometer has afilter plug which con- replacement of paitsilthough periodic adjust- tains a stainless steel filter screen. This screen ments may be required. should be cleaned occasionally to remove any To dean the telescope lenses, first use a clean, foreign material. The plug may be removed with soft-haired brush to lightly brush off the lens a screwdriver. It' the orifice which admits outside surfaces. Then wipe clean with a special tissue pressure to the case is found to be plugged, it provided for the purpose. If special tissue is not should be opened by the use of a small diameter available. use clean. dry chamois or soft toilet wire. The screen should then be cleaned with tissue. Itis very easy to ruin a lens surface by gasoline. after whichitshould be moistened scratching. theiefore: be especially careful when with oil and drained before replacing in the wiping it. Do not wipe hard. barometer case. Since these instruments are calibrated individ- When inspectingtheobjectlens, do not ually.fieldrepairs are not recommended. In attempttoremove the objectlen\ from its emergencies. certain operations which are out- mounting. If' the retaining screw has loosened so lined in the instrument handbook are possible. that the lens is not tight in its mount. the screw These include removal of the dial cover. pointer. may be tightened by means of the Spanner and the instrument from the case. and readjust- wrench used for this purpose. Handle the object ment of the rack for tooth engagement. lens and barrel very carefully and inspect the The FA-1 12 barometer has been constructed tine screw threads to see that they were not just as ruggedly as the service for which itis damaged by being crossed when the assembly intended will permit. However. in an instrument was installed. assensitive and accurateasthis barometer. Do not attempt to disassemble the eyepiece delicateparts must be used. This barometer assembly. as any disassembly or attempt at must be handled with care. disassembly may disarrange the lenses used to make up the eyepiece and render the assembly Lubrication useless unless it is properly adjusted with precise optical equipment. Examine the assembly care- THE MECHANISM DOES NOT REQUIRE fullyfor damage and see thatthe eyepiece OIL. Oilwill only interfere with proper func- moves freely in thepiral groove in the eyepiece tioning and introduce serious errors in readings. adapter. The silver surfaces of the vertical and horizon- talcircles may become tarnishedfrom too THEODOLITES frequent contact with the hands. Touching the scale surfaces leaves a deposit of moisture and oil that tends to oxidize the surface. The surface SHORE TYPE can be brightened to some extent by rubbing with boneblack or by applying a few drops of The shore type theodoliteisillustrated in MIL-L 7870 oil and leavingiton the scale figure17-2 for ease in identifying its various overnight. Then wipe the scale clean with a component parts. clean. soft cloth. Leave a thin flim of oil on the surface to help keep it bright. The horizontal circlescalesurfaceis covered and does not Maintenance require particular attention. Never use prepared commercial polishes in any form on the gradu- There areveryfew wearing parts on the ated scale surfaces of the theodolite. theodolite. Ifthe instrument is properly When cleaning the tangent screw mechanisms. handled, carefully packed inits carrying case particular care must be taken to remove all dust when not in use (fig.17-3), covered with the and grit from the worm gears that move the canvas hood for protection when out of the horizontal and vertical circles. 1'se a soft cloth case. and kept wiped clean of lint and dust. or a toothbrush moistened with solvent, such as there should be very little need for overhaul or alcohol, for this purpose.

597 603 AEROGRAPHER'S MATE I & C

7- tit)N I ELEVATION SIGHT DUST CAP ON SCALE OBJECT LENS I TELESCOPE GUARD FINDER TELESCOPE

CROSSHAIRS RETICLE SCREWS

BATTERY EYEPIECE BOX '".--...,...... ,,

BUBBLE LEVELS BUMPER - -... ELEVATION SCALE"' TANGENT SCREW RHEOSTAT

AZIMUTH SCALE AZIMUTH SCALE SLOW - MOTION TANGENT SCREW SCREW

WING SCREW SUNSHADE AND SCREWDRIVER

/ LOWER/ CL AMP

LEVELING LEVELING LEVELING BLOCK CONTAINING PL ATE SCREWS HEAD CAPSTAN WRENCHES, OIL, a SPARE BULBS BASEBOARD

AG.732 Figure 17-2.Shore type theodolite and baseboard assembly.

Tests and Adjustments ment, adjustment of horizontal axis, adjustment of vertical circle fiducial mark, and adjustment The theodolite is carefully tested and adjusted of tangent screw verniers. Most of these are before issue. However, rough handling or usage precision adjustments and require skill, patience, may resultin the instrument getting out of and experience on the part of the person making adjustment. All theodolites should be given a the adjustment. Extreme care must be taken not "General Test" as outlined in FMH No. 5, Winds to damagethetheodolite, and adjustments Aloft Observations, upon receipt and quarterly should be made by a qualified instrument man thereafter. only. Consult the appropriate technical manual Other tests and adjustments include the spirit for a more complete discussion of the tests, level check and adjustment, collimation adjust- checks, and adjustments.

598 604 Chapter I 7MAINTENANCE OFMETEOROLOGICAL EQUIPMENT

TELESCOPE TURNED OVER (SIGHTS DOWN) BOTH TANGENT SCREWS OBJECT END POINTED TO UPPER LEFT REAR DISENGAGED EYEPIECE TO RIGHT REAR

TWO LEVELING SHIFTING PLATE PUSHED SCREWS LOOSENED TOWARD REAR

AG.733 Figure 17-3.Theodolite in case.

TRIPODS clean softcloth.If the cover glass on the clinometer is fingerprinted or dirty, wipe the Maintenance outside with a damp cloth and polish withz soft cloth or tissue. The tripods require very little maintenance. The corrosion on the metal legs of the shipboard TESTS tripod can be removed with silver polish. Periodically apply a small amount of oilto Check the accuracy of the clinometer every the movable joints on the tripods. Removeany month as follows: With the sighting tube in a excess oil to prevent dust and dirt from collect- horizontal position, rest the front edge of the ing on these joints. quadrant scale plate on a level surface and check the instrument. The pendant should indicate 0°. CLINOMETERS If the instrument does not register accurately, return it to the repair depot for adjustment. ML -1 19 (SHORE TYPE) ML-591/U (SHIPBOARD TYPE) Maintenance Maintenance When the clinometer is not isuse, keep it in its case to protect the instrument from dust and Maintenance of the Clinometer ML-59I/U dirt. Keep the pendant clamped to prevent it which is designed for shipboard use consists from being damaged by sudden movementsor primarily of checking the accuracy and perform- jolts. Remove dust from the instrument witha ance periodically on a 30 day cycle. There are

599 695 AEROGRAPHER'S MATE I & C

,pceitie steps to be followed accomplish- must be taken to avoid improper connections, ing the pci tormance test.I he proper procedure since errors introduced to the indicators/record- is described in the technical manual. ers as a result of improper connections are not I'heM L-59 I/U should also be checked period - always readily apparent. icallyfoi damage, ::ear, and corrosion. Mainte- DETECTOR. There will be occasions when nanccislimitedtocleaning. minorrepair, the detector direction syncliro will need zeroing; replacementof damaged or worn parts, and thatis, when the vane points to north, the adjustment. Instructions for major repairs and indicators and recorders should read north. The disaNseinbl>arecontainedinthetechnical procedure for zeroing the components of the manual, UMQ-5( ) may be obtained from the instrument technical manual. WIND EQUIPMENT Repainting byAerographer's Matesisre- stricted to minor touchup of scratches on the WIND MEASURING SET AN/UMQ-5( detector vane and housing and complete refin- ishing as necessary on the support. Care must be ncici normal conditions Wind Measuring Set taken not to apply too much paint anywhere on L'N1Q-5( %%All require very little testing or the detector vane sinceit may unbalance the adjustinglu this section, only the ni,antenanee instrument and cause erroneous indications. that Aelogiaphei's Mates First Class and Chief INDICATORS. The only service inspection normally perform is Jiscussed. required for the indicators is an observation of their operation to detect erroneous readings, or Maintenance failure of lamps. No special procedures are required for repair Maintenenceislimited to service, replace- and replacement of parts in the indicators. ment, adjustment, and minor repair which can RECORDER. -Serviceinspectionsonthe he pcif °ruled without disassembly or with par- wind direction and speed recorder consist of tial disassembly not requiring the use of over- scheduled checks and inspections. Table 17-1 haul facilities. Instructions for major repair are lists the scheduled times and the nature of the contained in the overhaul instruction manual. inspection. Such work is to be performed in overhaul shops For a further discussion on recorder mainte- only. nance, see the Handbook of Operation and COMPLETE. WIND MEASURING SET AN/ Maintenance Instructions, NA 50-30FR-525. UMQ-5( 1,Service inspection of the complete setconsistsof adailyobservationof the detector and indicator behavior and a monthly Component Tests check on cables. connections. and mechanical security. Most of theelectricaltests on the Wind The daily inspection consists of a check to see Measuring Set AN/UMQ-5( ) are performed by that the vane alines the detector in the apparent the ship or station electronics personnel. Tests Ind and that the impeller turns freely. Com- which can normally be performed by Aerogra- pare the indications of all recorders and indica- pher's Mates are presented in this section. tors on the detector circuit. They should be in COMPLETE SET.For the wind direction agreement, circuittest, position the wind direction-speed The monthly check consists of checking the detectors so that the vane is pointing the nose cable, for physical damage or deterioration. directlytoward north. This may be accom- Clle%.1the mechanical security of component plishedbyclampingastraightedgetothe mountings. machined side of the connector housing and When damage to cabling is discovered during aliningthevane withthestraightedge.All inspection or indicated during trouble- indicator direction pointers and recorder direc- Nhootuig.thecable shouldbe checked and tion pens in the same circuit with the detector repaired or replaced as necessary. Particular care should indicate north.

600 606 Chapter 17MAINTENANCE OF METEOROLOGICALEQUIPMENT

Table 17-1.Recorder inspection schedule.

Item Nature of inspection Time

Recorder chart Inspect for clear, legible record trace, proper Daily. time setting, sufficient chart reserve, takeup without binding, and agreement of recorded values with indicator readings.

Pens Check for evidence of clogged ink, fuzzy line Daily. on record, recording on sudden swings.

Ink tanks Check to see that sufficient ink is in tanks. Weekly.

Complete instru- Check for evidence of dust, dirt, or cor- Monthly. ment rosion. Check operation in accordance with the section Quarterly. entitled "Component Tests." Check lubrication in accordance with the Quarterly. applicable i3uWeps technical manual.

DETECTOR.Only tests ofa mechanical na- To check the vane for excessive ture which are performed by Aerographer's friction, Mates attach a test disk (the weight of thisdisk is equal are discussed in this section. As stated earlier, to a 50-cent coin) to the flat side of the tail electrical tests are normally performedby the surface nearest the trailing edge. ship or station electronics personnel. Hold the vane approximately in line with the benchtop; then When checking the wind direction-speedindi- release. The weight of the disk should cator for excess friction. remove the be detector sufficientto carry thetailsurface down if from the connector housing. Attach theprotect- excessive friction is not present. If the weight ing cover to the top of the of connector housing to the disk is insufficient tocarry the vane down- prevent foreign matter from entering the female ward itis possible that the synchro, ball bear- connector. Carry the detector inside, away from ings, or brushes need replacing. Before any effect of the wind. making the above vane check, make certainthat the Attach a test disk (the weight of this diskis vane is in static balance. equal to the weight of a 1-cent coin)at the tip INDICATORS.Tests on the indicators of one of the impeller blades with are of a piece of anelectricalnature.Sincethesetestsare scotch tape. Turn the blade so that itis 45° normally performed by maintenancepersonnel, from the top center. Thenrelease the impeller. they are not discussed. The weight of the test diskshould be sufficient RECORDER.For the to carry the blade to the tests on the wind bottom if excessive direction and speed recorder, onlythe chart friction is not present. If theweight of the disk drive mechanism is discussed. The is insufficient to chart drive carry the blade to the bottom, mechanism istested during operation. If the itispossiblethat either the magneto needs chart feeds smoothly, takes replacing, or the impeller holder up without binding, may be rubbing and does not gain or lose too muchtime, it may on another surface. be considered to be operating properly.

601

UL.:IV', -/ t AEROGRAPHER'S MATE 1 & C

WIND MEASURING SET AN/PMQ-3( ) WIND SPEED DETECTOR.To replace the wind speed detector (fig.17-4), grasp the de- Inspection tector and give it a slight twist in a counterclockwise direction (looking down on the instrument) The onlyhe ry ice inspection requiredisto andpullitoff straight. Remove the spare make Lei lain that the turbine and vane are free detector from the case in a like manner. Install to rotate and thatth-!reis pointer movement the spare detector by reversing the above proce- when the turbineisrotated. A method to check dure, making certain that the unit is securely for Freenessisto hold the instrument inits locked in position. operating position and walk at a moderate rate in an area where there is no air movement. if the vane assumes the correct position and there is a speed indication on both scales, itis probable that the instrument isin a satisfactory condition.

Maintenance CAGE The CallSeN or trouble encountered on the TURBINE PMQ-3( )can normally be identified through observance of the characteristics of the wind speedpointerindications. The indicator will presenterratic or abnormal wind indications dependent upon which component is defective. COVER The various troubles, the probable causes, and recommended remedies are describedinthe instrument technical manual. A trouble can generally be isolated by replacing a component with another known to be in good condition. Table 17-2 includes the components of Wind Measuring Set AN/PMQ-3( ) that the Acrographer's Mates are authorized to install.

Table 17.2. Replaceable parts listAN/PMQ-3( I.

AG.734 Figure 17-4.Wind Speed Detector (AN/PMQ-3( )). Units per Rirt %nue Assembly 1.1: sixed &lector WIND VANEMinor defects and dents are wd not cause for replacement. If twisted parts affect I lit the accuracy, try to straighten them. If the vane obi-Pew is not repairable, the spare wind vane from the case should be installed and another wind vane I r And ,u itch requisitioned from stock spares. (f' 710,1 WIND SPEED INDICATOR.If physical damage is not visible, and as detective indication may be caused by another component, do not (.0,( discard the unit untilits condition has been proved unsatisfactory by a replacement. Cha )ter 17-MAINTENANCE OF METEOROLOGICAL EQUIPMENT

TRIGGER AND SWITCH ASSEMBLY. -If recommended for cleaning the cover glass and physical damage is notisible, and as a defective the reflectors. Avoid scratching or otherwise indication may be caused by another com- damaging the reflectors. Replace the cover glass ponent, do not discard the unit until its condi- door gasket if water is leaking into the housing. tion has been proved unsatisfactory by a replacement. MAINTENANCE The anemometer-wind vane requires no lubrication or cleaning by the operator. The maintenance that is performed by Aerog- Inallcases of replacement or adjustment, rapher's Mates will consist of replacing lamps, consult the current technical manual for com- focusing the lamp, and beam alinement. plete instructions. WARNING: HIGH VOLTAGE is used in the operation of this and somt other meteorological METEOROLOGICAL MEASURING SETS equipment mentioned in other sections of this (AN/PMQ-5( ) AND AN/PMQ-7) Rate Training Manual. DEATH ON CONTACT may result if personnel fail to observe safety For the operating principles and service in- precautions.Learn the areas containing high structions refer to the Handbook of Operation voltage in each peice of equipment. Be careful and ServiceInstructions,AN/PMQ-5, NA not to contact high-voltage connections when 50-30FR-513, and Handbook of Operation and calibrating,adjusting or testingthese equip- Service Instructions, Meteorological Measuring ments. Before working inside the equipment, Set, NavWeps 50-30 PMQ-7-1. turn power off and ground points of high Aerographer's Mates do not normally come in potential before touching them. contact with these equipments; th::-efore they are not covered in any more detail here. Replacing lamps

CEILING LIGHT PROJECTOR ML-121 Loosen the wing nuts, replace the defective lamps, and secure the access door in accordance INSPECTION AND CLEANING with instructions in the instrument technical manual. Once a week, and more often if necessary to Replace defective lamps promptly whenever insure full beam intensity, clean the cover glass the lamp has begun to blacken or the filament on theprojector housing and the reflecting sags to a noticeable extent. Lamp life will vary surface of the reflectors. Inspect the drainage considerably duetolocalconditions and is holes in the mirror and housing, and clean them highly critical with respect to overvoltage or as oftenasisnecessaryto insure adequate excess voltage fluctuations. The optimum volt- drainage and ventilation of the enclosure. age is11.8 volts and should never exceed 12 When the sunis shining brightly into the volts. Lower voltages decrease the intensity of. projector, the intensity of heat and light concen- the spot and the minimum voltage for satisfac- trated in the area above the parabolic reflector, tory operation is 11.3 volts. The voltage across especially in the area near its focal point, may the lamp should be checked and adjusted if be sufficient to burn the skin or seriously injure short lamp life or reduced lamp intensity is the eyes. Dark glasses must always be worn experienced. when looking into the reflector or at the lighted For instructions on focusing the lamp and Filament of the lamp. beam alinement consult the Handbook of Opera- Liquidglasscleaners or other nonabrasive tionandMaintenanceInstructions, NA glass cleaners used with soft clean cloths are 50-30FR-521.

603 609 CHAPTER 18

MAINTENANCE OF AUGMENTING METEOROLOGICAL AND OCEANOGRAPHIC EQUIPMENT

As the scientific fields of meteorology and adjustments describedinthe following para- oceanography continue to advance. so does the graphs should be accomplished by the Aerog- importance of rapid and accurate meteorological rapher's Mate. and oceanographicenvironmental measurements.Electricalandelectronicequipments CHECKS AND ADJUSTMENTS currently in use continue to be improved as well DURING NORMAL OPERATION as augmented by newer and more accurate equipment. I. BACKGROUND CHECK. Thischeck As senior Aerographers Mates. y op must not measures the effect of background illumination only be responsible for the Lorre Lt operation of on the transmission reading. Turn the back- these equipments but you must also insure that ground switch to TEST and hold it there. With they are kept operating to their fullest extent. this switch in the TEST position. a relay in the An importantpartof thistaskentailsthe projector power supply is energized. thus turn- periodic performance of operators tests. calibra- ing off the lamp. Then the reading of the meter tions. and adjustments. The maintenance of and recorder is that due to background. electronic equipments are not performed by 2. ZERO ADJUSTMENT. If. after stabiliza- Aerographer's Mates. This equipment will be tion. the meter does not read zero when the zero maintained by the ship or station's electronics switch is thrown to TEST position, adjust the personnel. zero adjustment potentiometeruntilzerois Many of the tests, calibrations. and adjust- indicated. ments ometeorologicaland oceanographic equipment are discussed in this manual. How- Turn the range switch to the HIGH position ever, the intent is to proide a general knowl- and note the meter leading. If it no longer reads edge and notto substitute for the technical zero, be sure to zero the meter for the range to manual. Wherever possibl, reference should be be used. Switching the range switch should also made totheapplicable technical manual to actuate the recorder range pen near the right- insure correct maintenance of the equipment. hand edge of the chart. AG 3 & 2. Nav Tra I0363-D should be reviewed where necessary for general information relating PERIODIC CHECKS AND to description. theory. or principles of opera- INSPECTIONS tion. Careful routine checks of the equipment by TRANSM[SSOMETER SET Aerographer's Mates very often prevent failure AN/GMQ-I 0() under conditions when maintenance personnel are not available. Use of checklists will prevent If accurate measurements of visibility are to the omission of pertinentchecks. Entry of be obtained through utilization of the Trans- pertinent data and remarks in their proper places missometer Set AN;GMQ-10( ). the checks and on the checklists will assist in the early detection

604 61D Chapter I8 MAINTENANCE OF AUGMENTING METEOROLOGICALAND OCEANOGRAPHIC EQUIPMENT of malfunctioning equipment and improper ad- Biweekly Checks and Inspections justments or operations. All adjustments should be recorded. Operating personnel are responsible In addition to the weekiy checks and mainte- for daily checks, some of the weekly checks, and nance. the following checks and maintenance are a few others that are limited to amechanical performed biweekly by AG personnel: nature. All other checks or corrective maintenance should be performed by maintenance 1. RECORDER. Change thechart,taking personnel. Checks should be made at least as care to date both the old and new charts. Be frequentlyasindicated and more frequently sure that the chart is properly positionedand when local conditions require them. feeding properly. Ciean the recorder by blowing out the paper fragments and debris from the recorder case. Daily Checks and Inspections 2. INDICATOR. RECEIVER, AND PROJEC- TOR. Clean interior and exterior of cabinets. The daily check of the transmissometer is made at the indicator, using the past record as a Monthly Checks and Inspections basis for checking the field units. It is desirable to make the daily check at approximatelythe The monthly checks rld inspections should same time every day. A time when t.ievisibility be made at 4-week intervals, and should be made is good should be chosen. A form such as that to coincide with a weekly or biweekly check. illustrated in figure 18 -I may be used for such The checks made on the indicator, projector. inspections, checks and adjustments. It should and receiver are of an electronics nature and are be pointed out here that forms readily available performed bymaintenancepersonnelrather through normal supply systeris may be modified than Aerographer's Mates. locally to meet the requirements for maintaining The checks on the recorder will be confined accurate logs and records. The entry ofinitials to the writing system. Remove the inkwells and indicates that the item or un:tis functioning the pen elements from the recorder and wash properly. Enter in the remarks section of the them with warm water. Pass a stream of water form any remarks regarding maintenance, adjust- through the pen elements until a clear stream is ments performed. or weather conditions atthe obtained.Fillthe inkwells with the specified time of the check which may be of value to type of ink,replacetheinkwells, and pen maintenancepersonnelLonductingfurther elements, and draw sufficient ink through the maintenance, or to station personnel in evaluat- pen elements to remove all airbubbles. Check ing the operation of the transmissometer. the balance of the pen elements. Check to see that the pen elements do not rub the scale plate or the inkwell at any positionthroughout its arc. Weekly Checks and Inspections Quarterly Checks, Inspections, Certain weekly checks. inspections, and ad- and Preventive Maintenance justments. if necessary. are performed by AG personnel. As pointed out previously, some of The quarterly checks are of an electronics these may be made by the maintenance person- nature and are performed by electronics personnel.Figure18-2illustrates a sample weekly nel rather than Aerographer's Mates. preventive maintenance checkoff listfor the transmissometer. Although the inspections and Semiannual and Annual Maintenance checks are made by AG personnel, the maintenance technicians perform theindicated correc- Semiannual and annual maintenance consists tive maintenance in cases in which malfunction- of lubricating the transmissometer set and carrying exists. These checks should be made on the ing out anticorrosion measures such as cleaning same day and at as nearly the sametime as rust from surfaces. touch up, andrepainting as possible each week. necessary.

605

! 611 AEROGRAPHER'S MATE 1& C

DAILY INSPECTION CHECKOFF LIST

(TRANSMISSOMETER INDICATOR/RECORDER AN/GMQ-10)

Week of 23 March 19-- Ref: NW 50-30 GMQ10-2

Accomplished by ITEM MTWT F REMARKS OMMIIMMIIMMISMOSIMME 1. Check for proper zero adjustment I^A - I 4I n? k 1 I 2. Check for proper low- l r 0 range calibration / / . p 4 Aa it Al L 11T 3. Check recorder for past I i 24-hr trouble symptoms Su\ 1WENT 4. Clear legible trace 1 0 4 'I t 1 i&I 5. Proper time setting p Ilri ffIIIIMIT/I 'IMO /Pc% ?S- Lit:Maid 6 E 4 6. Sufficient chart reserve ! 1 iFiFictr'Orr 7. Takeup without binding lo( p.1gg .1 a1.tAlarWI 8. Agreement recorded , IT41frillFP,. 4 DA 11641* % ATull 9. Evidence of clogged ink d 1 1 f 4 (fuzzy line on record) .b R% PE4 4ME, 10. Recording on sudden swings i S 1 ./ .4 . 0 IT .#\ ilITPIT i ' Additional remarks: P A 41=1.0111111 6lie TN

NOTE: For detailed instructions on correctivemeasures consult the referenced BuWeps technical manual. Bring any major discrepancies to the attentionof the division chief. Turn in this sheet weekly to the administrative sectionof the meteorological office.

Figure 18-1.DaiN inspection checkoff AG.735 list for Transmissometer SetAN/GMQ-10

606 61.2 Chapter 18MAINTENANCE OF AUGMENTING METEOROLOGICAL AND OCEANOGRAPHIC EQUIPMENT

WEEKLY PREVENTIVE MAINTENANCE CHECKOFF LIST

(TRANSMISSOMETER SET AN/GHQ-13)

NW 50-30GMQ10-2 Date 30 March 19 Ref:

ITEM INITIALS REMARKS

INDICATOR 1. Check high range calibration (adjust if necessary) i.(1,

2. Check effect of small volume and bias adjustment on recorder i- 21 3. Check pen balance in recorder .-

4. Adjust sechanical zero of recorder if necessary 8. L

5. Wind recorder at 0800LST every Monday Morning g

6. Check chart to be sure that hourly cutoff is working

. 7. Check for noticeable shifts in chart record after hourly cutoffs -x2

PROJECTOR 1. Examine chart for past week to determine if projector lamp has .ri:2... become defective (notify technician if Damp is defective)

Clean projector lamp face 2. g-&

RECEIVER 1. Feel Receiver lens compound (not heater housing) near heater to determine if heater is operating (notify technician if not operating) I

g,_ 2. Clean large Receiver lens if necessary time as possible.Note NOTE: Perform checks on same day of week at near same any corrective action taken inremarks. Notify the division chief of any major administrative section when trouble. Turn in this sheet to the meteorological complete for necessary action.

AG.736 Figure 18.2. Weekly preventivemaintenancecheckoff list for Transmissometer Set AN/GMQ-10(

607613 AEROGRAPHER'S MATE 1& C

The semiannual and annual lubricationsched- The preceding checks ules are contained in the Periodic may be performed by Inspection, the operator. However. thereare other checks Maintenance, and Lubrication sectionof Techni- for isolating problems described cal Manual of Operation and in the technical Service Instruc- manual, which are performed by qualifiedtech- tions.TransmissometerSetAN/GMQ-I 0( ) nicians.In any event, if ,...pairs NA50-30GMQ10-2. are deemed necessary, they should be accomplished bythe To insure that proper maintenanceof this technicians. equipment is performed at scheduledintervals. a The checks mentioned in the job description card, such preceding para- as the one illustrated graphsindicate onlythatthe equipmentis in figure 18-3 should be preparedand kept in a operating. To determine if it is operating tickler file. at the minimum acceptable standards.other checks must be made. TROUBLESHOOTING AND REPAIR MINIMUM PERFORMANCE TESTS Troubleshooting and repair of Tiansmissom- eter Set AN/GMQ-I 0( ) is limited to those items As already stated, thereare many tests which previously mentioned, and toa few additional may be performed by qualified technicians. itemsofamechanical nature. A complete However, there are two preliminarytests which troubleshooting chart may be foundin the do not take much time andmay be performed technical manual for this piece of equipment. bytheoperator.Thesetestsaretermed accelerated tests and are referredto individually CONVERTER INDICATOR GROUP as the "systems test" and the "display test." 0A-7900/GMQ-10( ) Systems Test If the Converter Indicator GroupOA-7900/ GMQ-I 0( )isusedto obtain runway visual Insure that the converter indicator range (RVR). Aerographer's Mates should insure group (fig. 18-4) is properly connectedas described in the that minimum acceptable performancestandards technical manual. The transmissometer of operation are adhered to. The receiver theory of may be disconnected if desired. Set thecon- operation and operating proceduresforthis verter switches in the various combinations and equipment as usedin conjunction with the notice if displayed data is in accordancewith Transmissometer Set AN/GMQ-10( )are pre- table 18-1. sented in chapter 10. AG 3 & 2, NT 10363-D, and the equipment technical manual. Allow 2 minutes for each display ofdata and The following prclure should be followed several repetitions to follow in orderto observe to determine if the..., stem contains a defective consistent conversion and display. Thistest is module or circuit: sufficienttoinsurethat the equipment will operate properly and should be performedat I. With the power switch turnedon, the fan least once a day. should become operative; the pilot lightshould light; and when the cover is removedon the Display Test encoder. you should be able to observe the encoder disk rotating twice per minute. The display test is performed in conjunction 2. With the power switchon, the display with the converter as part of thesystem test. In screenofDigitalDisplayID-1348/GMQ-10 this test items to be checked shown mounted on top of the are: the readout converter in units, switching relay, and display lamps.As the figure 184 should be fully illuminated whenthe system is being tested, operation of the readout brightness knob is turned fully clockwise. units can be checked by listeningto the clicking 3. When the "runway light setting" is moved of the relays and slide plates. andobserving the from the "normal"position, the redlight screen illumination. The clicking, as wellas indicator on the display panel should light. change of readings, should take placeonce a 608 614 Chapter 18 MAINTENANCE OF AUGMENTING METEOROLOGICAL AND OCEANOGRAPHIC EQUIPMENT

JOB: LUBRICATION OF TRANSMISSOMETER SET AN/GMQ-10 JDC: 1

EQUIPMENT: Indicator and Recorder used with Transmissometer Set AN/GMQ-10

DESCRIPTION: Lubricant is applied to hinge pins TYPE: Lubrication of indicator cabinet door; bearings of interior of recorder INTERVAL: Semiannually

TOOLS ANDPetroleum jelly REF: NA 50-30GMQ10-2 MATERIAL: Light machine oil (SAE-10) Smell screw driver

IMPORTANT: Be sure to read the instructions manual and obtain necessary tools and materials before starting job. USE ONLY CLOCK OIL AND WIPE OFF EXCESS OIL. DO NOT USE ORDINARY LUBRICATING OIL.

PROCEDURE: PERFORM THE FOLLOWING OPERATIONS

Apply a few drops of light machine oil to the 1. Indicator cabinet. hinge pins of the cabinet door.

assembly 2. Recorder. First remove the roll chart and then the chart drive following this procedure. (1) Turn the Recorder switch to OFF position. the recorder (2) Lift up the scale plate and remove the transmission pen, pen ink reservoir, and the range penassembly. mounting screws (3) Using the screw driver remove the four chart drive located one in each corner of the recorder housing. straight (4) With both hands, carefully lift the chart drive assembly forward and out of the recorder housing. 3. Remove the chart drive change gears.Apply one or two drops of oil to the bearings of each gear shaft and chart drive roller. Clean each gear with solvent and apply a thin film of oil to each gear. Reinstall the gears. bearings in the reroll 4. Apply one or two drops of oil to each of the four gear case and on the opposite bearingsin the right side of the tear case. of the chart buttons and 5. Apply one or two drops of oil to the bearings the rerollbracket. of oil to each of 6. Lift the cover of the escapement and apply one drop the bearings and one or two drops of oil to theteeth of each gear. Close cover. 7. Apply one nr two drops of oil to each bearing of thewinding arbor. Apply a small amount of petroleum jelly to the worm gearof the winding arbor. 8. Reinstall the chart drive assembly. 9. Reinstall the roll chart.

AG.737 Figure 18-3.Job description for semiannual lubrication of Transmissometer Set AN/GMQ-10( ).

609 -201011111171111111111Mil.iI

AEROGRAPIIER'S MATE I & C

AG.139 Figure 184.Converter Indicator Group CIA-7900/GMQ-10().

minute. In case the display lampsare not lighted PREVENTIVE MAINTENANCE with the switch in the ON position. flipthe INSPECTIONS AND CHECKS switch to LAMP TEST position. and turnthe BRIGHTNESS knob clockwise f they still do To insure that required maintenance isper- not function, notify the te.hnician. formed on schedule. a preventive maintenance checkoff listshould be maintained. This list should follow a similar format to that used for CLOUD HEIGHT SET ANIGNIQ131 ) the AN/GMQ-10 as illustrated in figures 184 and 18-2 of the chapter. The followingpara- The AG's responsibility for maintenance.tests graphs indicatethe checks.inspections, and andcalibration of theCloudHeightSet preventive maintenance which should beper- AN/GMQ-13( )is very limited. It consists pri- formed weekly by the AG's. For detailson the marilyof performingtherequiredweekly .iccomplishment of these functions. see chapter checks. This includes insuring that scheduled 10. AG 3 & 2. NT 10363-D.or the Operation checksinspectionsandrequiredpreventive and Service Instructions Manual. Cloud Height maintenance have been performed. Set AN/GMQ-13C, NA 50-30GMQ13-3. 610 6 j-6 Chapter 18 MAINTENANCE OF AUGMENTINGMETEOROLOGICAL AND OCEANOGRANIIC EQUIPMENT

Table 18-1. Switch setting and 3. ('heck calibration weekly to position 2 to test data for Converter Indicator check 18-degree markers. Group 0A-7900/CW-10( ). Detector Switches RVR Value I. Clean the glass cover inside awl outusing RVR Light Displayed Day-Night Mode clean cloth and alcohol, Setting 2. Clean the reflector with a .swab of cloth and alcohol. Using clean cloths each time, repeat LS3 Day T1 08 the process until no dirt remains on theswabs, LS3 Day T2 18 3. Enter condition of reflector on checkoff 13 62 LS3 Day list. LS4 Day '1' 08 4. Stop and start to check for smoothness of LS4 Day 18 operation. LS4 Day T3 62 5. Turn on drive motor and note motion of LS5 Day TI 08 rotary mount. Check for noise. LS5 Day T2 24 LS5 Day 13 62 LS3 Night TI 08 Recorder LS3 Night T2 38 LS3 Night 13 61 Taking care of the recorder (fig. 18-5) takes LS4 Night T1 08 verylittletime. Like any piece of precision LS4 Night T2 46 equipment, continued highlevel of operating simple LS4 Night T3 62 results can be assured by following a few LS5 Night TI 08 procedures. LS5 Night 54 Aerographer's Mates normally perform the LS5 Night T3 62 following functions:

NOTES I. This table is to be used for the .The helix and helix strip should be cleaned accelerated test only. with each change of paper or after any extended 2. RVR valuesdisplayedmay be idle period within +0, -1 of the values indicated. 2. The paper supply tray should be cleaned 3. Switches do not have to be set in before call new roll of paper is to be installed. the order shown above. 3. The drum and cradle assembly shouldbe cleaned after every 1,500 to 2,000 hoursof operation. Projector 4.Install new rolls of paper in accordance Make sure I. Clean the reflectors and donie coverwith a with the basic manual of instructions. that the recorder cover is securely closedafter clean soft cloth dampened with ethylalcohol. dry discoloration, installing the roll of paper so that it will not 2. Check the lamps for blisters, recorder, be replacing, out. After installing the paper in the and replace if marred. If !amps need is flat. smooth, and moist. This will always replace both and insure1/8 -inch clear- sure that it assure the best markingcondition. anLe of bulb and shutter;1/16-inch is minimal. and 5.If the degree marks are hazy, wider than 3. Check for oil leakage near the gearbox usual. or fading out, open the coverof the drive motor. recorder and make sure the blade is moving.If the blade is not moving, or is roughin certain Indicator areas, check the bladedrive unit and replace the blade. In order to maintain clear and accurate I. Clean the air intake. the blade is against its 2. Cleanthe top plastic with water and a marking, be sure that clean cloth. stops.

611 617 AEROGRAPHER'S MATE 1 & C

TAKEUP ROLL FEED ROLL TOP PLATE

TOP PLATE (CRADLE)

VERNIER CONTROLLEVER

RIGHT BLADE STOP

ALFAX PAPER

PAPER ROLL IN PROPER POSITION

HELIX TIMING BELT DRIVE ASSEMBLY LUCITE WINDOW

RIGHT SCREW COVER BLADE CAM LOCK (ENDLESS LOOP ELECTRODE)

AG.738 Figure 18.5. Open view ceilometer recorder. 6. Replacing the helix and the blade (endless AG personnel are performed byship, station, or loopelectrode) should be accomplishedas contract needed in accordance with electronicpersonnel.Detailsfor instructions in the accomplishing these functionsare found in the technical manual for this piece ofequipment. Operation and Service Instructions Manual. 7. The only motor within the unitwhich will need lubrication is thepaper feed drive motor. This motor should be lubricated(with the oil LUBRICATION supplied) at the two oil holesat either end of the motor shaft. It should belubricated every 6 The trunnion shaft bearingsand the drive months. pulley bearings of the projectorrequire semiannual lubrication. (See fig.18-6.) No lubrica- TROUBLESHOOTING AND REPAIR tion is required for the motors.or for any of the detectors components. Details for Troubleshooting and repair other than performing that this lubrication are found in theOperation and listed previously under the responsibilitiesfor Service Instructions Manual. 612 618 Chapter 18MAINTENANCE OF AUGMENTING METEOROLOGICAL AND OCEANOGRAPHIC EQUIPMENT

GREASE FITTING GREASE FITTING

POINTER TRUNNION SHAFT

ACCESS HOLE IN DRIVE PULLEY

TRUNNION SUPPORT TRUNNION SUPPORT (INDEX END) (DRIVE END)

AG.739 Figure 18-6.Projector lubrication diagram.

SEMIAUTOMATIC METEOROLOGICAL To calibrate the dewpoint transmitter, use the STATION AN /GMQ -14() following procedure: 1. Prepare two containers of water. Insert an The extensive use of the AN/GMQ-14 in accurate thermometer in each one; keep one measuring and recording surface environmental bath at 55°F and the other at 170°F, approxi- itsoperating condition directly data makes mately. relatedto the accuracy of the data derived 2. Remove the dewcel from its protector, and through itsuse.Itis therefore essential that remove the temperature bulb from theperfo- Aerographer's Mates be familiar with the use and measuring rated guard. care of this important meteorological 3. Immerse the bulb in the lower temperature and recording system. bath; stir well for about I minute. The tempera- tureindicated by the thermometer may be CALIBRATION assumed to be the temperature of the dewcel. Convert this reading to dewpoint temperature Calibration of the AN/GMQ-14( )isper- using the chart shown in figure 18-8. formed by the Aerographer's Mate as described 4. Adjust the connecting link by means of it the following paragraphs. Prior to commenc- the adjusting screw (fig. 18-7) so that the scale ingcalibration,firstwash and moisten the reading is the same as the dewcel reading. dewcel element as described in chapter 9. AG 3 5. Immerse the dewcel bulb in the higher & 2. NT 10363 -D. temperature bath (170°F); stirfor about 1 minute. Convert the thermometer to dewpoint Dew Point Transmitter reading and compare with reading now indicated on the data scale. A partial cut-away view of the dew point 6. If scale reading is low, turn the calibration transmitter and dewcel assembly are illustrated thumbscrew clockwise to adjust. If the scale in figure 18-7, reading is high, turn the calibration thumbscrew

613 619 AEROGRAPHER'S MATE 1 & C

DEWCEL PROTECTOR DEWCEL PERFORATED GUARD CALIBRATION THUMBSCREW CONNECTING LINK

DATA SCALE

FACTORY SET THUMBSCREW ( DO NOT MOVE )

ADJUSTING SCREW

DATA ARM

AG.740 Figure 18-7.Dewpoint transmitter and dewcel assembly.

in the opposite direction. Adjust the reading of two baths alternately until theerror is elimi- the data arm by means of the adjustingscrew as nated from the scale reading. mentioned in step 4. Air Temperature Trahsmitter CAUTION: Do not alter the position of the The calibration of the air temperature trans- "factory set" thumbscrew; this isa compensa- mitterisdoneinasimilar manner as the tion adjustment made at the factory and should calibration of the dewpoint transmitter, except not be moved. that the temperature of the baths should be 10°F and 80°F, approximately. andthere is no 7. Repeat step 6 using the first container need for temperature conversion. The thermom- (55°F) again. Continue thisprocess using the eter readings may be used directly. 614 t 6,f40 IMOMMEMMEMMININEMINIUMMEMMME ME ME MMEMEMEMENEMMENNIONWOMMUMMEMMOOM ORMEMOMMINEMMINIMMOUNIOMM 11 MMEMMEMEMMEMENIMMINIMMIUMMSUNOMMM11MMEME OMMEM - IIIIMMININEMSSINIUOM EMIUM INIMMEM . mu. o MENIMMUM '411111111ONOMMIUiiimunnmm IMMUMMEMMINIENEMEM .. 11110====.- '4111i 11110======.1 ' . NIMEMEMMO 'MU' MINIMMIIMEMEMEM - UMMINIMMEMOMME. 'MU 11111111111MIMMOM Ilib. 11=====1011 . MI UM NUMEEMMUIll MUM MINIUMMMIUM MOUNHIMU. OINIMM MMUUMBOU0======.1111.MMENIMINEMMEM . N IIMMININIRSUMUNNUHUOIMMUNMEMMIRM MEW11111101MMIMI1111110111 MUUUOUNNUOMMUMINM NEOMMINIE, INIREMO MMUHUUNIUNNUO - INIMMOM MIIIMMUMUUHUHUOM EMMENENEMME-OMMEME MMININNENNENWEEMMUM EMI IIIMMERN MEMMEMMEMMINOMMIRMEM AEROGRAPHER'S MATE 1 &C Air Temperature Receiver The camera assembly withits component partsismounted rigidly Calibration of the air to the top of the temperature receiver is cabinet over the light table.A covered housing similar to that of the dewpointreceiver, except that the air temperature receiver protects the lens zoom and focusmotors. indicator is The recorder is located justabove the video compared with the airtemperature transmitter indicator. screen in the left section of the console.It is a single channel. 24 hour. magnetictape recording and reproducing unit designed MAINTENANCE to record from either a microphone,a telephone, or a direct line.It will record continuouslyfor 24 hours The maintenance that Aerographer'sMates (plus a 15 minute overtime arerequiredto allowance) without perform on Semiautomatic tape change. Cranksare provided on the reel Meteorological Station AN/GMQ- I 4()isdis- spindles for manually advancing cussed in chapter 9 of Aerographer's or rewinding Mate 3 & 2. tapes. The tape motor is driven inthe forward NavTra 10363-D. direction only and must be The tests that are performedon the electronic rewound manually. A portable tape demagnetizeris provided with components of this equipment are made by the unit for degaussing of the qualified technical personnel only. magnetic recording tape. The unit will degaussea 2-inch roll of magnetic recording tape in 5to 10 seconds. WEATHER TELEVISION SYSTEMS The audio system permitstwo way voice The consoles of the Weather communications between the centralweather Television Sys- station and the remote stations.An audio and tem AN/GMQ-I 9()currently in use and its video selectivity system is provided replacement. Weather Television to permit System AN/ briefing any remote station inprivate, or the GMQ-27( ) are illustrated in figuresI 8-9(A) and entire system simultaneouslyat the briefer's (B). option. There are a number of modificationswhich Control panels on the console have been incorporated into contain all the the newer AN/ necessary switches and controls for operationof GMQ-27. However. thesechanges are primarily the system. improvements in solid state circuitry.camera The television monitors used design. etc. Maintenance procedures at remote loca- performed tions consist of a 21-inchtelevision monitor by contract or station electronicpersonnel have mounted ina metal case assembly. Normal changed to some degree. The minor preventive television controls pertaining tocontrast, bright- main tenance required of the Aerographer'sMate ness. etc.. are provided on the front panel. remains relatively unchanged andwill be discussed later in this section. Thesystem arrange- CHECKING OPERATION ment and related functions have remainedrela- tively stable and are brieflypresented in the The weathertelevision following paragraphs. system should be checked daily for proper operation.Both the video and audio portions should beactivated, SYSTEM ARRANGEMENT with checks of receptionat remote stations being made. If faulty operationis noted during All components of the central weather station the daily check, proper maintenancepersonnel are mounted within or on the metal cabinet should be notified. assembly. Two rear access doorsare provided which are hinged and will lift off. Thelight table MAINTENANCE on the right side of the console contains six fluorescent lamps beneathaglass plate and Contract employeesor station diffusion glass cover. Two 150 electronics watt floodlights personnel perform the requiredelectrical and are provided for opaque illumination when not electronic maintenance using the light table for transparent illumination. on weathervision systems. However. there are a number ofminor 616 62 Chapter 18MAINTENANCE OFAUGMENTING METEOROLOGICALAND OCEANOGRAPHIC EQUIPMENT

CAMERA HOOD ASSEMBLY SPEAKER AND CLOCK CAMERA MOUNT ASSEMBLY PLANEL

CAMERA rTAPE RECORDER

POWER CONTROL im.../.PANEL 746.1 MICROPHONE ter=se , CONSOLE CAMERA la ,--RECEIVER MICROPHONES CONTROL ...... ,,, 77:1;11--: SELECTIVITY PANEL PANEL ...... 4 POWER PANEL , Og...17772i rer--rf;

CONTROL PANEL

RECEIVER SELECTION PANEL REMOTE CONTROL PANEL SYNC GENERATOR AND PROC AMP (A) ( B)

AG.742 Figure 18.9.(A) Weather Television SystemAN/GMQ-19( )console; (B) Weather Television SystemAN/GMQ-27( ) console. compound preventivemaintenance tasks and inspections To clean painted panels use cleaning which may be performed by theAerographer's Military Specification MIL-C-18687, and warm, Mate. These arelimitedtotheinspections. clean water. Rinse with clean water. checking of switches. and cleaning describedin CAUTION: If solvent is used be sure the area the following paragraphs. is well ventilated. Do not inhalesolvent vapors in contact with the Chassis Assembly Components or allow solvent to come skin. Keep solvent away from openflame. The components of allchassis assemblies should be inspected for dirt. rust. orcorrosion Television Viewers on a monthly basisunder normal conditions. If exceptionally The glass on the front of thetelevision the equipment is located under normal hot, humid, or windy and dustyconditions, or viewers should be cleaned weekly under in the proximity of salt water,these inspections conditions;if other than normal conditions exist. they should be cleaned daily. should be conducted weekly. soaked If cleaning is required, remove greaseand oil If cleaning is required use a soft cloth using cleaning in aliphaticnaphthaFederalSpecification from the chassis and cabinets TT-N-95A. Polish with a clean dry cloth. solvent, Federal SpecificationP-S-661, or equal.

617

.64 237 AEROGRAPHER'S MATE I & C

Console Light Table METEOROLOGICAL RADAR The plastic cover on the consolelight table Radar and weather are very closely should be cleaned weekly unless allied. The required more use of radar has provided meteorologists witha often due to windy and dustyconditions. tool which permits the collection of atmospheric Cleaning should be accomplished by washing data under conditions when themore orthodox withasoft cloth and warm, clean water and methods fail. On the other hand,a knowledge of then polishing with a soft dry cloth. atmospheric conditions provides the radar The lamps in the console light table opera- should be tor with information vital to theaccuracy of his checked daily to insure that theyare operating results. properly. The word radar was derived fromthe phrase WEATHERVISION PRESENTATION "radio detection and ranging." Fundamentally, all radar dependupon the emission ofa short Weathervision systems haveproven to be a sharp pulse of e.4ctromagneticenergy in a given valuable tool in the weather briefingsituation at direction. This pulse on interceptinga target is stations where they have been installed.Figure scatteredinall directions. That part of the 18-10(A) and (B) showssome of the types of energy which is scattered in the direction of the weather data that can be presentedon weather- radar is picked up by the radarantenna, produc- vision equipment. ing an echo or "blip" on the receiverscope. The time taken for the pulse to cover the pathto the target :Ind return is a measure of therange to the target. Azimuth and elevation of thetarget are determined from the direction in whichthe pulse is emitted and returned. Since the electromagnetic pulse travelswith the speed of light, range is determined bya timing procedure. At the instant the pulseleaves the antenna, a timing device starts counting. When the return echo reaches theantenna, the counting is stopped. The distance to thetarget is equal to one-half the distance traveledby the pulse. (A)SURFACE WEATHER MAP WITHOUT STATION MODELS RADAR INDICATORS The purpose of the indicator (cathoderay tubes) in a radar is to display informationto the Aerographer's Mates about therange, bearing, and elevation of surrounding targets. Ingeneral. information is presented piecemeal, anda sin3le viewing of an indicator gives onlya small part of the picture. Different types of radarscans are employed to display wanted informationfrom returning echoes on the different indicators. ulasmumIIll A-Scan Presentation (B)RADAR SUMMARY CHART The simplest type of display is theA-scan presentation, which is illustrated in figure 18-11. AG.743 The display is one which gives return signal Figure 18-10.Typical weather vision transmissions. intensity against range. This display shows only (A) Surface weather map; (B) radarsummary chart. the range to a target.

618

£Z4. Chapter 18MAINTENANCE OF AUGMENTING METEOROLOGICALAND OCEANOGRAPHIC EQUIPMENT :. displays the total range starting at 0 range oh the left of the scope and ending at any die of TRANSMITTER ECHO several ranges selected; the R-scan presentation PULSE PULSES. isolates a portion of the range scale and expands itacross the entire face of the tube. Thus. perhaps only the range between 20 and 40 miles is displayed, or any other range interval may be selected.

PPI-Scope Another type of display commonly used in radar is termed plan position indication (PPI). This display makes it possible to read range and bearing information simultaneously. A diagram of a basic PPI-scope is shown in figure 18-12.

PRECIPITATION 'CHOES

SWEEP LINE \

FACE OF SCALE OF MILES TUBE ENGRAVED ON TRANS PARENT OVERLAY

AG.744 Figure 18-11.General appearance of A-type scan presentation as it would appear on the range indicator CRT.

If the target is a single, discrete object such as an aircraft, the bearing of the target canbe found by moving the antenna horizontally to RANGE AZIMUTH MARKS the position of maximum signal intensity (high- INDICATOR est pip). Changing the angle of elevationof the SCALE antenna to get maximum signal intensity provides information concerning the elevationof AG.745 the target. The horizontal and vertical limitsof a Figure 18-12.Diagram of PPI-scope, rainstorm can be found in much the same way by noting the positions of the antenna at which In essence, with the PPI-scope, the sweep the return drops below a detectable level. starts at the center of the tube instead of at one :dge as it does on the A-scope. As the antenna R-Scan Presentation rotates around a 360° circle, the sweep rotates simultaneously. The intensityof the sweep The R-type scan is almost identical to the display is modified by the presence of a return A-type. The signal intensity is given as afunt.:- signal. Thus, the position of a target is indicated tion of range. Essentially, the differencebe- at the correct azimuth and range by abright tween the two is that the A-scanpresentation spot on the display tube. A maplike pictureis

619 6,Z5 AEROGRAPHER'S MATE 1 & C

thus produced on the tubeasthe antenna Purpose of Equipment (AN/FPS-81) rotates in the azimuth plane. Meteorological Radar Set AN/FPS-81 isa RHI-Scope ground-based radar system used to establish the geographic locations of storm centers relativeto The range height indicator (RHI) isanother a fixed base reference site. The equipment is scope occasionally found on radar equipment. capable of detecting storm centers withina Its function is not unlike that of the PPIexcept radius of 200 nautical miles. Radar echosignals that when this scope is used, the azimuth of the from concentrations of high moisturecontent antenna is kept fixed while the antennamoves are presented on cathode-ray tube (CRT) indi- from viewing at 0° elevation toa predetermined catorsinsucha manner as to convey the elevation. The sweep always starts at thelower positions of storms in terms of azimuth.slant left-hand side of the cathode tube andmove. range, and elevation (height). The radar echo with the antenna as it oscillates vertically. signals may be displayed with iso-echocontour- ing, if desired, to provide maximum clarification Range Markers of storm configurations. Limitations Each of the conventional scopes provides for some direct indication of range by display of The antenna may be made toscan manually range markers. The range markers may be turned in both planes (azim uth and elevation) on or off and may be varied. When the scope is simultaneously, orit set for 10- or 25-mile display, range markers at will scan automatically inone I- or 5-mile intervals are convenient. At 100, plane while manual search is conducted in the other. The antenna will notscan automatically 200, or 400 miles. 25- or 100-mile markersare frequently used. in both planes simultaneously. Radar targets will not normally be visible at less thana1-mile range. RADAR EQUIPMENT Component Parts Meteorological radar provides a uniquemeans of obtaining meteorological data foruse by the Meteorological Radar Set AN/FPS-8I consists forecaster when issuing warnings andother of an antenna assembly.a receiver-transmitter- environmentalforecasts. As weather require- modulator (RTM) assembly, an indicator ments continue con- to expand and change, new sole. and a remote indicator assembly. (Secfig. designs andmofificationstometeorological 18-13.) The antenna may be separated fromthe radar will continue to appear in the effortto RTM assembly by a maximum distance of 100 improve and keep pace. feet; the indicator console may be locatedup to The Meteorological Radar Set AN /FPS -I06 is 2,600 feet from the RTM assembly; andthe the newest model of radar designed formeteoro- remote indicator assembly may be operated logical 'use. A limited number of these have been 5,300 feetfrom the indicator console. The manufactured and put into operation at this major portion of the system circuitry iscon- time. The AN/FPS-81 meteorological radar is tained in modular units of the plug-intype to the most common set in use in the Navy today. simplify and expedite maintenance procedures. There are however. older models. FPS-41, and ANTENNA ASSEMBLY. The antennagroup FPS-68, which are still in operation andmay be (fig.18-14(D)) consists of a spun aluminum encountered. Since there are many similarities parabolical dish, measuring 8 feet in diameter;a between the FPS-41, FPS-6S, and FPS-8 I, and horn-shapedreflectorfeed; and an azimuth the FPS-106 is in such limiteduse, only the pedestal. Radar illumination of targetareas up FPS-81willbe discussedatlengthinthis to 200 nautical miles away is provided by the manual; brief mention will be made of the older antenna which directs and concentrates the models where notable differences exist. high-level RF (radiofrequency)energy into a 620 Chapter 18 MAINTENANCE OF AUGMENTING METEOROLOGICAL AND OCEANOGRAPHIC EQUIPMENT

AG.746 Figure 18-13.Meteorological Radar Set AN/FPS-81. (A) IndicatorGroup 0A3871/FPS81; (B) Indicator Group 0A-3872/FPS-81; (C) Receiver-Transmitter, Radar RT658/FPS-81; (D) AntennaGroup 0A-3870/FPS-81,

621 ir." ). ;_ AEROGRAPHER'S MATE I & C

narrow beam. The radiated beam is a burst of energy generated in the frequency band of 5.450 to 5,650 megahertz and pulsed 324 timesper second for a duration of 2 microseconds. During the impulsed portion of the pulse repetition interval, radar echoes are returned from target areas to be detected by the antenna and fed to amplifying.timing.andindicatingcircuits throughout the system. Control of the antenna may be accomplished manuallyorautomatically.Each mode of antenna scanning permits separate control of azimuth motion. In automatic azimuth scanning. the antenna scans 360° at a r:onstant rate of 5 rpm. In automatic elevation scanning. the antenna nods continuously from +60° to -2° R EC E IVER-TRANSMITTER-MODULATOR (RTM) ASSEMBLY. The receiver-transmitter assembly (fig.I 8-I4(C)) contains the electronic components required to transmit and receive the RF signals. Controls are provided for the adjustment of output power and transmitter quency. The magnetron oscillator in the transmitter-modulator section of the RTM assembly generates RF energy bursts which are radiated by the antenna system. INDICATOR CONSOLE. The indicator console.from which the entire system may be AG.747 controlled and monitored, houses the range/ 1. Range Indicator IP-644/ 6.Desk height indicator (RHI), the plan position indica- FPS81 tor (PPI), and the Range Indicator as illustrated 7.Cabinet 2. Range Height Indicator 8.Elapsed-time Meter in figures I8-13(A) and 18-14. IP643/FPS-81 M7201 A power panel on the rear of the indicator 3. Right-hand control 9. Amplifier-Power Sup- console, mounted between the PPI and range panel ply AM-3306/FPS-81 indicator assemblies. accepts 115-volt. 60-cycle. 4. Azimuth Range Indica- 10. Power Circuit Breaker a-c power for distribution to the entire system. tor IP-642/FPS-81 CB7201 PLAN POSITION INDICATOR (PPI). The 5. Left-hand control panel 11.Phone jack PPI.locatedinthecenterportion of the indicator console, presents range and azimuth Figure 18-14.Indicator console, AN /FPS -81, information in a plan view (as seen from zenith assembly locations. directly over the radar site). RANGE/EIGHT INDICATOR (RHI). The target from inputs of slant range and antenna RHI indicator occupies the left side of the elevationangle. The height video signal dis- indicator console and presents range and height played on the RI!! is automatically corrected for information.Range informationisdisplayed eaith curvature at all ranges within the capabil- along the horizontal axis of the RHI display, ities of theradarset. Antenna scanning in while height data appears along the vertical axis. elevation is controlled from the RIII. Selection may be made of two height scales, RANGE INDICATOR. -The range indicator, 0-40,000 feet, or 0-80,000 feet. housed in the upper right-hand section of the The RIII assembly contains the circuits which indicator console, features a standard A-scan resolvetheheight component of a selected 5-inch cathode-ray tube, which also containsa 622 6 3 Chapter I8-- MAINTENANCE OF AUGMENTING METEOROLOGICAL AND OCEANOGRAPHIC EQUIPMENT ranging function. Range information (A-scan) is transmit-receive units are in the same assembly displayed along the horizontal axis of the CRT with the reflector. display. The R-presentation along the horizontal Otherimportantdifferencesarethatthe axis consists of a continuously variable range remote indicator can only be mounted a maxi- strobe which may be superimposed manually mum distance of 500 feetfromthe main over the target echo pulse to provide precise console, the range is from 250 nautical miles to ranging data. The range strobe, generated within 1;2 mile. the indicator console is divided into six the range indicator assembly, represents a range panels instead of three on the AN/FPS-8I. and interval 5 miles in depth. By actuating a switch the controls are divided among the upper and to select the R-scan. this range interval may be lower panels. whereas on the AN/FPS-81 the expanded to fill the range indicator screen, per- controls are located on the panel with the three mitting a more detailed analysis or photography scopes. of the echo trace selected by the range strobe setting. The selected range strobe also drives a METEOROLOGICAL RADAR decimal digital device on the front panel which SET AN/FPS-68 presentstheactualrangeinnauticalmiles. Sweep timing circuitspermitselection of The AN/FPS-68 meets basicallythe same sweep ranges of 30. 60. 120. and 200 nautical contract specifications as the AN/FPS-81, but is miles. Range markers may be displayed along not as compact and modern in design. the horizontal display baseline at intervals representing 5-.10-. 25-, and 50-mile range incre- SAFETY PRECAUTIONS ments. Therangeindicatorincludesself- checking circuitry which enables the operator to High voltage is used in the operation of all check the accuracy of the range marker signals radar. Extremely dangerous voltages exist in the for all ranges simply and quickly. receiver. transmitter. modulators. main console, REMOTE INDICATOR. The remote indica- and remote indicator. Death on contact may tor assembly. which may be iocated up to 5.300 resultifoperating personnelfailto observe feetfrom the indicator console, provides a safety precautions. Only authorized operation of repeat display of the information presented by equipment should be carried out by the oper- thelocalindicatorattheindicator console ator. All operators must know how to secure Video and synchronizing signals for the remote main power. both remote and locally. to th set indicatoraresupplied bytheconsolelocal in use. All operators must know how to apply indicator assembly. The sweep trace on the artificial respiration and how to contact im- remote indicator rotates in synchronism with mediate medical aid. the antenna scanning motion in azimuth. FACTORS AFFECTING PREVENTIVE MAINTENANCE RADAR PERFORMANCE Radar equipment is complex electronic equip- There are many factors. or elements. that ment: therefore, servicing and maintaining of affect efficient radar performance, not all of thisequipment istheresponsibility of the which are completely understood. Many of the electronics technicians. Acrographer's Mates' re- limitations of the equipment are a result of radar sponsibilities only include keeping the exterior design. If you recognize these limitations and of the equipment clean and promptly reporting their causes. you will be better able to determine internaldifficultiestotheresponsibletech- the type of performance to be expected from nicians. your radar. One important factor is the radar operator's METEOROLOGICAL RADAR knowledge of this equipment. He must know the SET AN/FPS-41 maximum and minimum ranges at which he can The components sire essentially the same as expect to pick up the weather echoes and the thoseforthe AN;FPS-81, exceptthatthe range and bearing accuracy of the gear. These

623 .6 9 AEROGRAPHER'S MATE I & C characteristks he deteimined for your par- pulse length. All the reflected energy from the ticular set from the applicable techniLal manual. closest echo must have returned to the antenna In this se,.tion of the chapter only those factors before the reflected energy from the second which generally affeLt all radar equipmentare echo reaches the antenna, if the two are to discussed. appear as separate echoes.

Frequency Pulse Repetition Frequency(PRF)

Frequency/wavelength determinestheeffi- The PRF determines the maximum measur- ciency with which radar energy is scattered bya able range (Mtn) of the radar. Ample time targetThe closer the wavelength of the rat:ar must be allowed between pulses for an echo to energyis to the size of the target. the more returnfromanytargetlocatedwithinthe efficiently thetargetscatterstheradiation. maximum workable range of the system. Other- Targets not only scatter radar energy. they also wise, returning echoes from the more distant absorb itThis is why sonic wavelengths do not targetswillbeblocked by succeeding trans- penetrate sonic tare0s. Loss of radar energy as a mitted pulses. This necessary time interval fixes result of two factors. scattering and absorption. the highest PRF that can beused. Thelower the isknown as attenuation. For meteorological PRF. the greater the range. However.the PRF phenomena. the targets are the water droplets or must be high enough se that sufficient pulses hit ice particles in the clouds and areas of precipita- the targe and enough are returned. tion. A 20-cm radar would detectthelarge raindrops of a thunderstorm, but would not detect the small raindrops and cloud droplets. Beam Width On the other hand. a 3-cm radar would detect the small droplets. but the energy would be so I3eam width is determined by the size and rapidly dissipated by absorption and scattered shape of the antenna. With a wide beam there is so effectively by the small droplets on the near more chance of detecting a target. but there is side of the Lloud that no energy would penetrate less chance of locating it accurately in azimuth to the back side. and elevation. The more concentrated the beam, the greater the range capabilities for agiven Pulse Length amountof transmitted power.

The pulse length is a factor in determining the RADAR FACSIMILE RECORDER maximum and minimum range at which a target AN/GMH-6( ) can he detected and the power of resolution (degree to w high details .an be distinguished). The Radar Facsimile Recorder AN/GMH-6( ) I. Minimum range. If a targetis so close to is illustrated in figure 18-15. the 'attar site that the reflected energy returns to This recorder isa remote picture printing the antenna before the trailing edge of the pulse device used to record weather data transmitted has left the transmitter. it will not be detected from the radar/transmitter site where the trans- beLause itwill ict urn while the receiver is not mitting device ho-izontally scans a plan position listening. indicator (PPI) radar scope. The data is then 2, Maximum range The reflected radar signal transmitted via telephone line to the recorder. must be above the minimum energy level to Tiler recorder provides a hard copy printout of whichtheradarwillrespond. Long pulses the weather pictures.including "data insert contain moie energy than short pulses. there- information" on a continuous roll of electrolytic fore. distant targets and weak targets are more paper. The data insert information consists of likely to be detected. automatic printing of a time-date code, indi- 3. Resolution. Whether two targets located cating the day of the year and the time of the on Cie same azimuth from the radar site will be day that the particular picture was received and detected as one echo or as two depends on the printed.

624 610 C

Chapter 18MAINTENANCE OF AUGMENTING METEOROLOGICAL AND OCEANOGRAPHIC EQUIPMENT

of a consecutive group to the beginning of the first picture in the following consecutive group. Controls are also provided for the adjustment of printing quality, such as degree of contrast and whiteness. Other controls set the time and datainformationprintedon each weather picture. As illustrated in figure 18-15, the facsimile recorder consists of the electronic chassis. and

RECORDS? HEAD the recorder head (printing display area and ASSEMBLf paper takeup) which are mounted in and on, respectively, one metal console. The console rests on four swivel casters, to permit mobility. The two front casters can be locked to fix the console in a stationary position. The electronic chassis is a slide-mounted pullout drawer which provides for easy removal, servicing, cleaning, and testing of the chassis. The electronic Chassis contains all operating controls. fuses; and indica- ELECTRONIC tors for the facsimile recorder. except the paper CHASSIS take-upswitchandfuse,the paper supply ASSEMBLY indicating light. and the paper fast feed switch, * ,*t/e't which are located on the Recorder Head. 400'4 OPERATION

CONSOLE Prior to operation of the Radar, Facsimile ASSEMBLY Recorder. personnel should refer to the publication NAVAIR 50-30GMH6-1. Complete operating instructions arecontainedinthis publication.

MAINTENANCE Servicing and maintaining of this equipment arethe responsibilities';of trained electronics personnel. As with other complex electronic equipment the maintenance duties of the Aer- ographer's Mate arelimitedto keeping the exterior of the equipment clean and promptly reporting any internal difficulties to the respon- AG.748 sible parties. Figure 18-15.Radar Facsimile Recorder AN/GMH6( ). OCEANOGRAPHIC INSTRUMENTS

The recorder has controls that determine the BATHYTHERMOGRAPH SYSTEMS frequency at which consecutive groups of pictures are printed. A group may consist of 1, 2, The operation of bathythermograph systems or 3 pictures. The period between the groups such as the AN/SSQ-61 illustratedin figure may be 5, 10, 20, 30, 60, or an infinite number 18-16 was described briefly in AG 3 & 2. NT of minutes. A period in this case refers to the 10363-D. Additional details and modifications length of time from the end of the last picture are discussed in the following paragraphs.

625 631 AEROGRAPHER'S MATE I & C

RECORDER, BATHERMOGRAPH DATA RO 326 B/SSQ-56.UNIT1

LAUNCHER, BATHYTHERMOGRAPH SHIP'S MX-8577/SSQ6I.UNIT2 HULL

BATHYTHERMOGRAPH (PROBE) OC I4/SSQ-56 UNIT 3 (CFM) (EXPENDED PROBE WITH CANISTER IN LAUNCHER)

AG.749 Figure 18-16.Bathythermograph Set AN/SSQ-61, relationship of units.

This bathythermograph(BT) systemwill affecting installation, operation, and mainte- automatically obtain and record ocean tempera- nance may be readily determined by reference turedatainvirtually any sea state without to the applicable manuals. altering the ship's normal operation.Itwill obtain a complete temperature profile of the sea Operating Malfunctions to a depth of 1,500 feet in 90 seconds. The AN/SSQ-61 is the most current in the series of As with all equipment. the ability to detect BT systems.Itspredecessors were the AN/ and correct potential problems by the operator SSQ56, AN/SSQ-56A. andthe AN/SSQ-60. may save major expense and equipment outage. Each system incorporateselectrical and me- Table 18-3 lists some of the potential problems chanical improvements over the older model. and correction procedures on the AN/SSQ-6 I Differences between the various sets lie only in which may be performed by the operator. the recorder and the launcher; all sets use the same XBT probe (OC -14 /SSQ -56). Table 18-2 Maintenance lists the set components and major differences The maintenance which may be performed on along with the applicable technical manuals. the BT system AN/SSQ-61 by the operator is limited to that presented in the following paragraphs. Any combination of recorder and launcher Although many similarities exist between systems may be utilized with the XBT. Minor differences as stated previously, the applicable technical

626 632 Chapter 18MAINTENANCE OF AUGMENTING METEOROLOGICAL AND OCEANOGRAPHIC EQUIPMENT

Table 18 -2. Equipment Similarities

Differences Differences from from Technical Set Recorder Launcher previous previous manual recorder launcher

AN/SSQ-56 RO-326/ Earliest model. Chart MX-7594/ Earliest model, NAVSHIPS SSQ-56 paper moves from SSQ-56 deck 0967-225 - top to bottom. mounted. 6010 Bridge, trigger logic and power supply on two circuit boards, hard- wired to harness.

AN/SSQ-56A RO-326A/Major mechanical and MX-7594A/ Minor mechan- NAVSHIPS SSQ-56 electrical changes. SSQ-56 ical changes 0967-305 Recorder instru- in stanchion, 6010 ment assembly in- breech verted for improved adapter and chart display. Bridge, launch tube. trigger logic and power supply com- MX-84I6/ Major mechan- bined on single SSQ-60 ical changes. connector-mounted Launcher in- circuit board. Test stalled panel and switches through hull AN/SSQ-60 added. rather than on deck. Uses heavy muzzle protector.

AN/SSQ-61 RO -326B/Minor mechanical and MX-8577/ Major mechan- NAVSHIPS SSQ-56 electrical changes. SSQ-61 ical changes 0967-333 - Added power switch in launch 6010 and servo muting. tube and mounting. Stanchion and muzzle protector eliminated. Launch tube changed to urethane material.

627 633 AEROGRAP!IER'S MATE I & C

Table 18-3.Malfunction Chart (Operator Level)

This table describes the simplest malfunctions, their symptoms, causes and corrective actions. These are generally considered on-site procedures and are within the capability of the operator.

SYMPTOM CAUSE CORRECTION

During measurement run stylus Leak from probe wire to the Launch new XBT. makes erratic excursions to salt water. the right end of the chart paper (high temperature end)

During measurement run stylus Complete wire break. Launch new XBT. makes excursion all the way to left (low temperature end) of chart paper and remains there.

During measurement run sty lus Contamination between pin Breech pins should be wiped makes erratic excursions to B of breech and canister. thoroughly and new XBT the left (low temperature inserted for another end) of the chart paper. launching.

Excessive salt water in breech Wipe breech and receiver or receiver assembly causing thoroughly and insert new conductive path across con- XBT for another tact end of canister. launching.

Recorder completely inactive, POWER switch off or fuse Replace blown fuse. Loca- indicators not illuminated. blown, tions are on top rear of Recorder int,:.rior. If fuse blows again notify the maintenance technician.

Calibration line marked during Recorder out of Calibration. Calibrate following the pro- Check/Run Mode full out- cedure described in the side 61.8° - 62.2° band. equipment technical manual. manual should be LheLked prior to LommenLc- weekly for nicks or burrs which might damage ment of maintenance procedures. the fine XBT wire during launching. The tube LAUNCHER. The operator at the launLher must be smooth over its entire interior surface must insure that no scran wire from the launch- and around the opening. ing remains in the launch tube when the ball CALIBRATION CHECK. Before a series of valve is closed. and that the breech and tube are XBT launchings, or at weekly intervals when keptcleanandfree of contaminants. Con- XBT's are launched daily, a system calibration taminants may be removed with fresh water and check should be made by the operator. The cloth. The launchtubemust be inspected calibration check is performed with the use of

628 67e4 Chapter 18MAINTENANCE OF AUGMENTINGMETEOROLOGICAL AND OCEANOGRAPHIC EQUIPMENT the test canister provided with the system and deteriorationisevident,inspect the window following theprocedure as outlinedinthe surface for evidence of a haze of dust: if present, technical manual. remove with a clean, optical-gradecamel's hair lif RECORDER. Remove, install, and aline brush, using a flicking motion. Do not scrub, chart paper as described in the applicable tech- and do not use lens tissues or cleaners. If further nical manual. The procedure to be followed cleaning is required, the instrument should be involvesa number of steps which mustbe returned to an authorized repair facility. followed precisely and since the technical man- ualshould accompany the equipment,itis LUBRICATION considered unnecessarily repetitious to include these steps here. All moving parts in the Portable Radiation BATHYTHERMOGRAPH PROBE. No pre- Thermometer PRT-4( ) are sealed And require ventive maintenance is required for the probe no lubrication. and canister. NEAR SURFACE REFERENCE PORTABLE RADIATION TEMPERATURE DEVICE (NSRT) THERMOMETER PRT-4( ) The NSRT system consists of a meter as- The PRT-4( ) system is a noncontact passive seiably and a thermistor probe used to obtain instrumentthat measures the radiation from remote water surfaces, clouds, backgrounds.and other targetsfilling thefield of view of its opticalsystem.Itisparticularlysuitedfor airborne use, where its fast response and high TAPE resolution power permit the mapping of thermal RECORDER TAPE contours and surface currents. Theoperation of RECORDER the PRT-4( )is presented in AG 3 & 2, NT AMPLIFIER

10363-ft POWER CONTROL MAINTENANCE PANEL

Other than occasional visual checks and clean- ANTENNA ing of the lens surface, no routine maintenance CONTROL of the system isnecessary. The visual checks UNIT should be conducted annually, or at intervals as FACSIMILE required by the service environment and extent RECORDER Of use. During these checks, particular attention should be given to cables and connectors, front panel components (switches and indicators),and mounting hardware. RECEIVER FACSIMILE Cleaning the Optical Lens CONTROLS

The front window may require cleaning from FACSIMILE time to time. (This is most often indicated by DEMODULATOR deterioration in system performance.) However, excessive cleaningisundesirable because the window may become permanently scratched or marred. Therefore, before undertaking toclean AG.750 the window, checkthe system performance Figure 18-17.Receivar Recorder Set, under a known set of operating conditions.If Meteorological Data, AN/SMQ6(V). f ,, ....629 ow.-.a AEROGRAPIIER'S MATE 1 & C instantaneous measurements ofsea surface tem- SATELLITE GROUND RECEIVING peratures. A description of this instrument along EQUIPMENT with a discussion of operatingprocedures may be found in AG 3 & 2, NT 10363-1).

AUTOMATIC PICTURETRANSMISSION (APT) RECEIVING ANDRECORDING MAINTENANCE EQUIPMENT The NSRT system requires littleor no maintenance other than occasionally replacingthe Automatic picture transmission via flashlight battery. weather If maintenance is required, satellite has become acommon means of obtain- the IC electrician does the required work. ing environmental data relatedto meteorological and oceanographic forecasting. Thesenior AG must insure that the receiving and CALIBRATION recording equipmentiskeptin an operational status insofarashis maintenance responsibility To calibrate the indicator, adjust ex- the needle tends. Electronics maintenancepersonnel will be to the red line on the face of themeter (replace responsiblefor most of the the battery if the needle cannot necessary tests, be adjusted to calibration, and maintenance; however,the AG the red line). The instrumentshould be cali- may perform certain maintenance related tasks brated daily following this procedure. as described in the following paragraphs.

(A) (B)

AG.183 Figure 18-18.--(A) Antenna, Dual Array Helical AS2192/SMQ6(V);(B) Antenna, Fixes Omnidirectional, Conical AS2191/SMQ6(V).

630

i 6 .6 Chapter 18MAINTENANCE OF AUGMENTING METEOROLOGICAL AND OCEANOGRAPHIC EQUIPMENT

MAINTENANCE For further information pertaining to other equipment, refertothe applicable technical manual. There are a number of APT receiver recorders Figure18-18illustratesthe two types of in use in the field as discussed in the AG 3 & 2 antennas which may be utilized. If the fixed Rate Training Manual. Only the maintenance omnidirectional antenna isused, the antenna taskspertainingtoReceiverRecorderSet control panel on the receiver recorder is replaced AN/SMQ-6(V) are presented here due to the by a blank panel since it is not required. similarity of the maintenance requirements of The maintenance which will be performed on the various types of equipment. Figure 18-17 this equipment by the Aerographer's Mate oper- illustrates Receiver Recorder Set AN/SMQ-6(V). ator is contained in NAVAIR 50-30GMH6-1.

631 CHAPTER 19

RADIOSONDE AND RAWINSONDEEQUIPMENT MAINTENANCE AND CALIBRATION

The upper-air observationprogram is one of Many delicate pieces of electronics equipment the more important of the variousprograms require that an adequate preventive maintenance required by the environmental services. Quality program be implementedtoprevent unwar- upper-air data is vitally important to the fore- ranted breakdowns and interruptionsto the caster in the preparation of required forecasts. operational programfor which they are de- Theprerequisiteforcollectionof accurate signed. The Radiosonde Receptor isno excep- upper-air data is the use of properly maintained, tion to this rule. This equipment must also be tested,andcalibratedradiosonde/rawinsonde calibrated monthly and whenever a technician equipments. replaces electronic parts or makes any mechani- The maintenance, test. and calibrationpro- cal adjustments to the pen drive system to insure cedures presented in this chapter are at the AG 1 thatitisoperating within certainlimits of & C level. For details on the component parts acceptable accuracy. and operation of the equipments covered. refer to the applicable equipment manual. INSTALLATION RADIOSONDE RECEPTOR AN/SMQ-l( ) The Radiosonde Receptor AN/SMQ-1( ) isa radio !ovationandthemounting of the receiving and recording device operating in the AN/SMQ-I are discussed in the technical manual frequency range of 390 to 410 mHz. It is used for the equipment. by the Aerographer's Mate to receive and graph- The antenna normally used with this equip- icallyrecordmeteorological data transmitted mentisthefolded ground plane type. The from aballoon-borne radiosonde transmitter applicable instructions manual should be fol- (AN/AMT-11( )). by means of an RF carrier lowed when installing the antenna assembly. pulse modulated at audio rates which correspond to the ambient temperature and humidity of the air through which the radiosondepasses. MAINTENANCE The receptor is designed for maximum reliability andlifeunder adverse conditions of Maintenance Requirements Cards (MRC's) shock. vibration, inclination. temperature. and have been prepared for use by maintenance humidity encountered in shipboard use. Toa great degree. it personnel when performing maintenance tasks is self-compensating for wear, on the AN/SMQ-1( ). (See fig. 19-2.) aging, and fluctuations inline voltage. When The operator's maintenance on the AN/SMQ- properly mounted, installed, and alined in ac- 1() is normally limited to routine cleaning and cordance with the NavAir technical manual, it minor mechanical maintenance. All other main- accurately records transmitted data using operat- tenance should be performed by a qualified ing controls only. (See fig. 19-1.) electronics technician.

632

6-, CILipter 19 RADIOSONDE AND RAWINSONDE MAINTENANCE ANDCALIBRATION

MAIN TUNING CONTROL

SPEAKER ANTENNA SELECTOR FOCUS

SIGNAL SELECTOR SWITCH INTENSITY

OSCILLOSCOPE LOCKING CONTROL ANTENNA TRIMMER

F R M CONTROL

R F GAIN CONTROL SPEAKER VOLUME ---'61111.1P-

CHART r. CHANT DRIVE SWITCH ILLUMINATION

SWITCH 11

CHART TABLE SUPPORT pp---- CHART SPEED PLUNGER PINS (2)

CHART TABLE SIDE REST

i CHART TABLE SIDE REST RECORDER DRAWER ...,

LOCKING SCREW (3)Allillik.n n

DRAWER LOCK MAIN POWER SWITCH

0 0 x411 ;" 0 0 HEATER SWITCH

FUSE INDICATORS

AG.751 Figure 19.1. Radiosonde Receptor AN/SMQ-1( ).

633 6 -.9 SISIEM COMPONENT andcomnunioatiors Ontrol As.tswQ1cadiusonde Receptor MR NUMSER ' -114 ,U-I Tools. Parts,6.S. 4.teriata, Teat Lcsipee,t tsont'O)Rocket with warm water and detergent rocedure nt 'di SUBSISIEMRadio Communications RELATED MR RATES 1.1Mf11 9,8,7. CleanInkWI.1" bristle 41L4-6085 rags brush P type ) Retort re order tn cat inet . nearundersideApply faone 1 den;tearing of of)of ;enon seonf.s,arr.ase. se .1, It *tett I pen carriage, and :v. dr p en shaft MR DESCRIPTION W..1, 0-2 PUS IOTA( 11.10. Syringeshorting probe 'One( (2)(1) ShortLCOselthe across theeasinet /as all le.00 tocapntit,ra the .11,4 full pullto electrical true riser ArCnrui asPOW (n.' FP-)612A2sw,-1, stops. 1. receptor.Clean. inspect. and Ithrhate radiosonde ELAPSED 1.11 1 Mht TIME Procedure (Cont'd) (7) (I)()) easilyteeWipewith broth alla accessible..hotting accessibleto remove probe. dustsurfaces an' Girtwith it4 Lleart areas rag. nnt SATETY PIECAUtIONS 2.1. ShortObserte across standard all capaelttrssafety precautions. t, electrical ground with a b. Recorder 120-71/SMQ-1. (8) Returngrease.Cleancleaning oldreceiver grease solvent to from cabinet. and drawer apply slide thin trackscoat wt rest, fresh Wipe away excess grease. (5)(S) or leakingInspectclearer.Remove remaining capacitors,interior Gustef dtae.loredeqvipment and dirt wit),ex s-,renc) inn) I oculn bulged 1000,PARTS.dATERIAIS.TESTEOU/MENI1, Cleaningshorting probe.solvent, P.D-Sbn (2)(I) withShortcabinetLoosen a acrossshorting theto thepan sit fulllocksprobe. capacitors out and stops. pull to theelectrical recorder ground from the (I) Clean ,11 grease from drawer slide tr. tubeconnections.resistors, shields. cracked loose ortube., frayed and ins,latt n. loose ststIns

CABLE TENSION ADJUSTING SCREW

CABLE CLAMP IN BACK OF PEN CARRIAGE

BEARING SET SCREW

PEN CARRIAGE SLIDE. BALL POINT POTENTIOMETER PEN CABLE

PULLEY

REVERSIBLE SERVOMOTOR CABLE DRIVE

CHECK CABLE CLAMP TENSION HERE SPUR GEAR

GEAR TRAIN

AG.753 Figure 19-3.Pen drive system, AN/SMQ-1( ).

Cleaning DAILY MAINTENANCE.Perform the following tests and inspections daily, and adjust- The operator should clean the exterior and ments as required: inside work area daily with a clean, lint-free cloth. Check the legibility of the ball point pen I.Inspect the pen drive system cable for tracedaily.Ifthetraceisnotclear and fraying (fig. 19-3). continuous, wipe the penpoint with a clean, 2. Test the cable tension and adjust if neces- lint-free cloth.If the discrepancy is not cor- sary. Cable tension is tested on theleft-hand side rected by this procedure, replace the ballpoint of the recorder drawer between the two pulleys pen. (fig.19-3). The cable should deflect 1/16 to 3/16 inch under slight finger pressure. The pen Mechanical Maintenance cable adjustment isa screwdriver adjustment found on a sliding block to which is attached an Mecham Lal maintenance of the Radiosonde idler pulley located on the right side of the Receptor AN/SMQ-1( ) consists of certain in- recorder drawer. A setscrew which holds the spections, tests, checks, and adjustments which sliding block may have to be loosened to relax can be performed on an operator'slevel. the cable tension, but simply turning the screw

635 6.1 AEROGRAPI1ER'S MATE 1 & C

CLUTCH ADJUSTMENT NUT