A FIVE-YEAR RECORD of LIGHTNING STORMS and FOREST FIRES by H
This file was created by scanning the printed publication. Errors identified by the software have been corrected; however, some errors may remain. MONTHLY WEATHER REVIEW Editor, ALFRED J. HENRY
- ._ ~ .~ ~ ~~ ~ VOL. 59, No. 4 CLoSmD JUNE3, 1931 W. B No. 1045 APRIL, 1931 IEBIJEDJULY 11, 1931
A FIVE-YEAR RECORD OF LIGHTNING STORMS AND FOREST FIRES By H. T. GISBOKNE,SILVICULTURIST
[Northern Rocky Mountain Forest Experiment Station, hiissoula, Mont.] Acc.ording to the records compiled by the supervisors on other work cnn be nioved to locations more strategic, of the national forests in the northeiv Rocky Mountain for fire suppression, and additional men can be hired region, lightning has been responsible for a gre.ater spec,ifically for suc,h an emergency. But when a single number of fires, more burned area, more damage, and national forest of 600,000 acre.s, such as the Raniksu in more expense of suppression in this territory than all northern Idaho, is visit.ed without any warning by light- other causes of forest fires combined. Smokers, campers, ning storms, that in one da.p start over 150 forest fires, brush burners, incendiarists, lumbering operat.ions, and as occurred on July 12, 1926, no forest protective organ- railroads combined start annually an average of 379 fires izat,ion is able to expand fast enough to cope with the on these 23,000,000 acres of Federal forest land, but situation. Under such c.onditions several fires are certain lightning is credited with an annual average of 824 fires to be left without attention long enough for them to during the 10-year period, 1919 to 1928. In 1926, which become so large that they spread a.s ‘conflagrations is accepted as one of the worst seasons for lightning fires, through the crowns of the trees. One such crown fire 311,607 acres of Federal, State, and private forest land often burns over more area, destroys more timber, and, in Idaho and Montana were burned over by fires st8nrted after it drops to a ground fire and becomes approachable, by lightning. The damage on this area was evnluttted often costs more for its suppression than a hundred or at $3,572,000, while the Federal Government a,lone spent, eve.n a thousand other forest fires which are reached and approximatel $945,000 for fire suppression. extinguished before they have an opportunity to attain These condkons have long been recognized as one of such momentum. the major impediments to successful lumbering and Such occurrences make very clear the importance and forestry. As lumbering, which must depend in the future profitable application of warnings of lightning storms, upon forestry, is one of the basic industries in this see especially if the warnings could be made available two or tion of the country the economic importance of lightning three days in advance, and c,ould cover not only storm storms is obvious. It is apparent that no industry suh- occurrences but also storm fire-starting probabilities. jected to such a chance of loss as has been indicated can With suc.h warnings action could be taken that would in operate as cheaply and efficiently as it could if the danger nearly all cases reduce lightning fire damage and expense were fully understood and at least partially controlled. to nverysatisfactoryminimum. Evenat present,with the Early in 1922 the Northern Rocky Mountain Forest weathe.r predictions limit,ed to a period of 36 hours and Experiment Station corn-menced an investigation of with no indications contained in the forecasts concerning lightning storms, working in cooperation with the Forest the fire-starting characteristics of the storms, considerable Service administrative organization, which had studied progress is being made. Present investigations have the occurrence of lightning-caused forest fires ever shce contributed mate,rially toward more accurate forecasts by the national forests were created. The administratmire obtaining more detailed records of storni occurrence. study had localized the danger both in the and by area, They have also found (5) that the types of storms which but it had not attempted to investigate the storms which conmionly start many fires can be distinguished from the are the causes of these fires. The research project, types which do not start many fires. Furthermore, initiated in 1922, has been conducted for the purpose of methods have been developed for measuring forest in- assembling more and better information conce,rning t8he flaiiimnbility so that the danger of ignition and rapid occurrence and characteristics of these storms which spread can now be deternlined as a basis for administra- cause so much loss and expense. tive action (6). As Morrell has pointed out (11) the possibility of The data herewith presented add to previously pub- improving forest protection lies c.hiefly in reaching forest lished information on lightning storms in this region fires quickly and attacking them before they get large, largely through the increase in evidence on which the with an adequate and pro erly equipped crew of men. deductions are based. Some of the c,onclusionspreviously Forest fires can be attackeB with speed if t1ie.y are few in stated must now be slightly modified, but many are number, or even when numerous, if the forest protective greatly strengthened by these 14,754 reports covering a organization has a reasonably definite warning that suc.11 5-year period, as compared to the 3,500 reports for two fires are probable. If warnings are available, men engaged years used as a basis for the first report. Some new aspects of the situation are also apparent in this greater
1 The boldfsee figures in parentheses rcfer to literature citrd. volume of data. 81309-31--1 139 140 MONTHLY WEATHER REVIEW APRIL,1931
OCCURENCE OF STORMS over forested, mountainous areas, in contrast to their frequenc over low elevation, valley or plains stations One of the outstanding discoveries resulting from this such as gpokane, Ralispell, Helena, Lewiston, etc., where study is the knowledge that lightning storms are far more t,he regular Weather Bureau observations are made. frequent in the forested sections of this region than The large number of thunderstorm days reported is also previous records, largely obtained at low-elevation non- esplained by the fact that the Forest Service observers forest stations, had indicated. The present records, used in this study consisted largely of the lookout men which are most complete for July and August, less com- stationed on 270 or more high niountain tops continually plete for June and September, and fail entirely to cover scanning great areas and \vide horizons for the streamers the other eight months of each ear, give for the region as of smoke that locate forest fire,s. From these high points a whole an average of 88 thunB erstorm days each season, it is almost impossible for a thunderstorm to occur within for the five years studied, as follows: 1924, 85; 1925, 95; 30 or 40 miles of a station without being seen or heard by 1926, 83; 1927, 87; and 1928, 88 days. For this same the observer. It is probable that few, if any, storms es- region Alexander's compilation, (2) whichis quoted by niost cape detection during July and August, when these ob- meteorological authorities, records only 10 to 30 thunder- servatories are all occupied. storm days per year at individual Weather Bureau A fourth reason why the pre.sent data show more storm stations of the first order.2 days than indicated by other sourc.es of information may be that thunderstorms have been more frequent during FIG.1 the past few years. Such is the local opinion, but the MAP OF THUNDERSTORM OCCURRENCE. lookout-station records do not cover a sufiicient number of years to give either support or denial to this belief. ' When the mgional data obtained in this study are sub- divided into smaller units, such as groups of national forests, a marked reduckion in thunderstorm frequency is shown; yet even the totals surpass those based on obser- vations at low-elevation stations largely outside the forests. The frequency for these small groups of forests is shown by Figure 1 ; and Table 1 permits comparison of these figures with those resulting from carrying the sub- division down to individual national forests.
TABLE1.-Relation of number of thunderstorm days lo number of lighhing fires per 100,000 acres for individual foresta and forest groups in the northern Rocky Mountain Region
Thunderstorm days, by years 1 Firas per -100,OOO Group and forest acres 1924 1925 1926 1927 1928 Average annually ---~-I I -- Average Group I: number Beaverhead ______41 61 45 41 43 0.3 Deerlodge...... 8 Helena ...... 21 1 28 I 33 32 28 1. 2 Avmge...... 256 1.0
~ ~~ 28 2.6 AVERAGE NUMBER OF THUNDERSTORM DAYS EACH SUMMER. Group 11: fg ;; :2 19 28 .2 ~~3m~;.~~~~~~~~~/ 33 1.7 Mlssoula ...... 23 I 31 1 3 I 44 2539 FOR GROUPS Of NATIONAL FORESTS, BASED ON RECORDS 1 -- 1924-1928 INCLUSIVE FOR NORTHERN IDAHO AND WEST- Average...... '43 2.0 ERN MONTANA AS A REGION THESE DATA INDICATE AN Group 111: Blackfeet ...... 15 20 28 35 36 27 6.8 AVERAGE OF AT LEAST dd THUNDERSTORM DAYS Cabinet ...... 9 27 1 24 44 40 28 4.7 YEAR. PER Flathead ...... 26 25 33 49 54 37 5.0 Kootenai ...... 14 41 43 40 34 9.1 Lo10 ...... 30 :?I 34 42 44 36 3.5 This large increase in storm occurrence, indicated by the Pend Oreille ...... 20 W 26 37 40 28 5.5 Average...... 1...... 62 6.0 Forest Service data, is partly explained by the fact that Group IV: the region has been treated as a unit, whereas Alexander's Clearwater...... 17 35 25 28 33 28 15.1 Coeur d' Alene...... 40 37 6.9 summary considers each observation station as a unit. Kansiksu ...... 1s ;! 2; z: 37 28 15.3 St. Joe ...... 23 24 1I 20 30 I 34 26 8.8 Compilation for relatively large areas is justified, however, by the fact that fire-protection work is administered by Average 2 50 11.0 Group V: Federal and State officials for areas seldom less than a Nezperce ...... 44 3.6 hundred thousand acres and usually comprising many Selway ----- 10 7.6 millions of acres. Weather forecasts also are usually Average...... 1 ...... I ...... I...... I...... 57 6.0 worded to apply to large areas, half a State or more, and conse uently must be based upon and should be rated 1 Records for period June to September. June and September data are fragmentary. 2 Group averages are higher than the averages for the forests (as explained in the text), accorling to the records of many stations. In this report since two or more forests may or may not report storms the same day. relatively large areas are consequently treated as units. Another important reason why the present summary These data show that on areas as large as a national shows more thunderstorni days than other records is the forest, or a group of forests, thunderstorm frequency has recognized more frequent occurrence of lightning storms been double to quadruple that indicated by W. H.
~~ ~~ Alesander's isoceraunics (ibid.) in this region. This does
2 This is also true of other parts of the western third of the Unlted States.-ED. not imply any inaccuracy in the Weather Bureau reports, APRIL,1931 MONTHLY WEATHER REVIEW 14i but it emphasizes the need for detailed records from the It is apparent from this compilation that the fire forested mountain areas and for special analyses of such problem was almost negligible on 54 per cent of the data as a basis for forest-fire weather predictions. thunderstorm clays during the past fire fire seasons, when In estimating the danger resulting from lightning storms from 1 to 10 stations reported storms each day. How- one might easily be led to believe that the group of forests ever, on 45 days, when from 11 to 20 stations reported having the most frequent exposure to lightning as shown storms, there was an average of at least one report that in Figure 1 would have the greatest number of fires per fires resulted. When from 21 to 30 stations detected unit of area. On such a basis the Kootenai-Cabinet- storms there were nearly 4 reports each day stating that Flathead-Pend Oreille-Lolo group, with 62 thunderstorm fires resulted. As one report of a fire-starting storm days per.year, should show the greatest number of lipht- always means that from one to several fires were dis- ning fires per 100,000 acres; but Table 1 shows that, this covered, it is obvious that the occurrence of fires in- is not the case. creases with the number of stations reporting storms, The fact that the number of lightning fires per unit of md that the need for forecasts increases in the same way. area is not entirely dependent upon frequency of thunder- From the evidence available it appears that whenever storms, is clearly shown by this comparison of data. less than 40 stations have reported storms in this region This lack of correlation can also be shown for the region in one day no nlarlrecl regional danger resulted. This as a whole when the records for each of the past five might he accepted tentatively as an approximate measure seasons are compared. For example, an increase of 12 of the need for regional forecasts, the greatest need be- per cent in the number of storm days from 85 to 9.5, ginning whenever more than 40 stRtiona are apt to re- resulted in an 80 per cent increase in the number of port storms in one day. lightning fires, from 1924 to 1925. In 1926 the number Tnble 3 cites the exact dates, during the 5-year period of storin days decreased 2 per cent, but the number of studjed, when more than 40 stations reported the occur- fires increased 42 per cent, as compared to 1924. In 1937 rence of lightning storms. This tabulation may be of there were 2 per cent more storm days, but 28 per cent research value in the study of conditions which have more lightning fires than in 1924. In 1928 there were during this period caused greatest forest-fire danger in 3% per cent more storm days and 14>6 per cent more t& region. h-es than in 1924. Hence, it appears that regardless of TABLE3.--Occii~rance qf flnngooirs thrinilerstnrin days by number of whether the problem is considered by groups of forests, stattons ,epo?lirrg stortris or by years, thunderstorm frequency alone is not a depend- able criterion of the probability of lightning-caused Nuiiilw ol Dste of ovcurrenve of storms forest fires. stntinns re- ~ - porting This information indicates that forecasts of thunder- storms 1934 1!1?5 lY2G I!Z 192s storm days are not sufficient as a warning of lightning- fire danger. Additional information is needed for the 41 to co .... July 1.2.3. Jiine 30.. ... July 7.~..... July ?2.?1...... July 5, 8. .\up. 1.. .. July 21,?5 .... hug. S- ..... AUB. 16.20,39...... region as a whole as to the extent and the character of ...... Auy. 13.19.3L Yept. 6~...... the storms each day. The importance of extent of the ...... !3ept. 1.i.~~~-...... 61 to so .... July 4.~... Julv IO. 11.16. June 29- :... July 28 ...... June 25.28. storms is demonstrated by Table 2, which shows that as Aug. 14. .. .~..:...... July 1.14,?6. 4ug. 6.11,21 ... July 29. Sept. 5...... lug. 36 ...... Aug. 2. 14, 26. more and more stations within the region report storms SI to loo... July 5.~. July 12...... July 2 ...... June 2. in one day, the proportion of reports indicating fire- .liig,13.-~...... 4ug.33 ...... Jdy12.26,27...... Aug. 3. ?2. starting storms increases very rapidly. When there 101 to 150.. . Jllnr 511. -..~ July 14.25.2.. June 26.2i...... :liig.l...~.~~ July4..5.12. .\u~.3.4,i,S, July1,13. were few storms-only 1 to 10 stations reporting theni- IO. I7,29. usually less than one station per day, or about 5 per ...... 4ug.1. 161 to ?OK .\US. Ilj.. dt:lyL'L'.2S.~~.Julyli.~ ..... At1g.1.2.?2.2S. July17.19. cent of the reporting stations, stated that fires resulted...... IIIX.2- ...... AUR.2...... Aug. 25. 201 to 2m...... July 24 ...... July 31 ...... July 16. When there were widespread storms-from 201 to 304 ...... lug. 19 ...... Aug.1. 10.23. station reports in one day- approximately i1reports, or 361 to 304...... July30...... July18...... AUZ. 1s ...... Allg.24. about 28 per cent, stated that fires resulted......
TABLE2.-Comparison of $re-starting storms with thunderstorm The occurrence and extent of storms are also shown days reported, on basis oj number of stations reporting storins, graphically in Figure 2, which reveals, better than a 1924-1 928 t,abular statement, the relation of the days with few reports to t,he clays of widespread occurrence of st80rnis. I I Reports of It is believed that such information should Serve as a ere-start- ing storms basis for the study of the weather types that result in MI thunder storins in the northern Rocky Mountain region, as storm day Alexander (1) has done for the State of Washington. Such work, however, which may prove to be estremely Number Number Per cent Number Nambrr 1 to lo--...... 239 5L 65 0. 2; difficult, as pointed out by Henry (8), is more a field 11 to 20 ...... 45 10 63 1.4 No 21 to 30...... 29 7 105 3.0 for meteorologists. attempt is made in the present 31to 40 ...... 23 6 82 3. 6 report to analyze this phase of the problem. It is evi- 41 to 60 ...... 12 3 6Y 5.8 61 to Bo ...... 10 2 86 8. R dent, howeve,r, that such analysis is basic to most ac- 61 to 70 ...... 11 3 153 13.9 71 to 80...... 10 2 125 12. 8 curate forecasting, and that the c.ollec.tior1of field data 81 to Bo ...... Y 2 182 20.2 by the Forest Service should be designed to supply all Y1 to loo^^..-...^..^...... ^..-^.-.^.^--.- 3 1 85 33.3 101 to 120...... 13 3 230 17. 7 possible information needed by the Weather Bureau. 121 to 140 ...... 7 Sn 24.0 141to 1w)...... 7 ; 185 2:. 9 CHARACTERISTICS OF STORMS 161 to 200...... 9 2 345 38.3 201 to 304 ...... 11 2 1 i76 70. G --- Although it has been shown that the more widespread Total______438 la, 1 ...... the 0ccurrenc.e of thunderstorms the greater the propor- 142 MONTHLY WEATHER REVIEW APRIL,1931 tion that are reported as starting fires, more information to the clouds. It is also obvious that if the storm brings than this must be available to the forest protective only a light rain of short duration Inore fires are apt to organization in order to deternline whether exceptional result than if the rain is heavy and of long duration. action is needed to meet the danger most efficient)ly. The present study has shown that it is possible to deter- Some of the factors involved are entirely independent of mine average values of these characteristics so that they the thunderstorms, and include the timber type (13), may be recognized and rated specifically and uniformly the prevalence of inflammable fuels, the se,asonal dryness by all observers. and inflammability of these fuels, and the characte,r of Alexander (1) has stated, basing his remarks on a the weather during the preceding days and weeks. These few reports for the State of Washington, that the per- factors are important because green trees, unless covered centage of flashes reported as having been conbed to the with lichens, do not ignite as readily as dead trees, or snags. clouds and the total number of flashes in eachstorm (the There are more chances of ignition where the volume of latter not always stated) do not offer a very reliable dead wood is great than where there is less dead and down basis for a comparison of percentages as between "safe" wood. Early in the season most of the fore,st materids (nonfire-causing) and dangerous (fire-causing) storms and are usually considerably wetter, both on the surface and can not at present be given much weight as determinants of the safety factor. He finds a similar lack of authority in reports of the duration of precipitation before and F1G.D. after the flashes. A mathematical analysis of the several thousand reports available for the northern Rocky OCCURRENCE OF THUNDERSTORMS. Mountain region, however, does not support his conten-
JUNE JULY AUG SEPT tion. The analysis shows that the general electrical and 10 PO 30 10 20 30 IO 20 30 10 W 30 rainfall characteristics of lightning storms can be observed wo. ID OI LIC*T*I*O T*"*DI.,lO"l D.., D.., by field nien and that average conditions can be specified,
85 departures from which will indicate whether the storms being observed are apt to cause more forest fires or are apt to be less troublesome, in this region. It is highly desirable that these characteristics be determined so that inexperienced lookouts and rangers will be better able to evaluate the degree of danger in 95 the storms that they are observing, often hours before t,he lightning fires send up enough smoke to be detected. These few hours of possible preparation, between studying the storm and discovering the fires, often determine the difference between full preparation to cope with many fires and lack of that preparation, with resultant 1ar e expense and damage. So long as lightning storms %o vary in their ability to start fires, and so long as the official 87 forecasts can be expected merely to mdicate that storms of some sort will occur, experience has shown that fire control can at present be improved by utilizing even the most generalized estimates of the fire-starting ability of lightning storms. The present criteria of these conditions are admittedly no better than other mathemat.ical averages based on a large volume of data. They serve, however, to evaluate 10 PO 30 IO 20 30 0 PO 30 bY 20 30 conditions which are being and must be observed. They JUNE JULY *Ut SEPT provide usable measuring sticks, crude as they may be, for the men who must use them. They represent what is believed to be the first attempt to eqolve such criteria in the interior of the piece, and consequently less inflnm- of thunderstorm danger, and like most first attempts, mable than during late August and early September. they are capable of considerable improvement. A new Likewise, if recent rains have moistened the forest fuels, form of report has recently been devised, which is expected or if recent hot spells have dried them, the results of a to constitute one small iniprovenient, and the observers certain storm or a certain number of storms may differ are continually being trained to produce more complete greatly. All of these factors are recognized by forest and accurate reports. Eventually, it is hoped to improve protective organizations, and the importance of predic- the data still more by the use of instrumental measure- tions of lightning storms is rated accordingly. ments. The storms themselves also vary so much that no two PRECIPITATlON may be expected to produce similar results, even though both might theoretically cover equal areas of similar When this investigation was comnienced in the northern timber type, fuel volume, fuel inflammability, etc. Rocky h/Iountain region in 1922, and when the field report Every esperienced forest officer recognizes this fact and form was revised in 1924, an attempt was made to obtain knows that there are certain characteristics which dis- observations of the rainfall with each storm, most of which tinguish generally safe storms from prolific fire starters. are single cloud affairs often only a few miles in diameter. For example, it is obvious that storms with more than Only about half the storm clouds pass directly over obser- the usual number of flashes and with a large percentage of vation points, even when these are but 20 miles apart, the bolts striking the ground will start more fires than and consequently rain-gage measurements could not be storms with only few flashes, and those largely confined depended upon to give data on all storms. However, APRIL,193 1 MONTHLY WEATHER REVIEW 143 lookouts at their high niount,ain-top station are frequenbly The most iniportant fact in Table 5 is the regional called upon by their supervising ranger to “size up” average for the 5-year period, showing that on the basis storms some distance away. Is it raining hard from the of 9,484 observations there is an average of 37 nlinutes of storm? How long has it been raining in De.ep Creek? rainfall which may be counted on after the lightning has What drainages are getting soaked the best? The,se are ceased to moisten the fuels and prevent the spread of typical questions asked of the lookout by his ranger. fires caused by the lightning. This rainfall following the Therefore, on t.he new report form lookouts were asked lightning probably is largely the so-called “secondary for determinations of the number of minute,s of rainfall rain” described by Humphreys (9) in his analysis of the ahead of the lightning, the intensity of the rainfall accom- structure and behavior of thunderstorms. panying the lightning, and the number of minutes of In the preliminary investigation, based on the data for precipitation after the lightning-bearing section of 6he 1924 and 1925, averages of 11 minutes’ rainfall ahead of storm had passed on, and, when the storm-cloud passes the lightning and 33 minutes’ following it were obtained. overhead, the actual measurements by rain gage. These averages were based on 2,455 reports. It is Analyses of these data have shown that 92 per cent obvious from Tables 4 and 5 that the addition of data for of the thunderstorms in t
TABLE4.-Record of rainfall ahead of the lightning in both uwt aid ._. dry storms, 1.32.$-192S 334 li I 575 8 632 Average numher of minutes of rinfdl by years Total and average ...... 1 14.6 1 4,135 I S. 7 1 2,120 number ~___ Forest group 1 of All obser- This table shows an average difierence of 5.9 iiiinutes in years rntions the duration of the scud rain ahead of the lightning is dangerous as compared to safe storms. Although this is a small absolute quantity, it is proportionately very large (GS per cent of the 8.7-minute a\-ernge) ; and it is also con- sistently true that each year the nonfire-starting storms have a greater average rain than the fire-starting storms. This being true, and considering the basis of 6,253 reports 1 These groups of national forms correspond, will1 t.iit two exwyiiou.%to th0.w mod over a 5-year period, it seems entirely safe to conclude in the flr8t report on this study BS follows: Group I Beaverhead Deerlodge (no dnto). Helena; Group 11, Lewis and &lark, Missoulii, Uill’erroot; Ciroui 111. Blackfeet. Flat- that there is an appreciable and a significant difference in head, Cabinet Kootenai. Lolo, Penti Oreille; Group IV, Cletlrw8iter. St. Joe, Coenr iI’Alenr. Kaniksu; Group V, Selwriy, Keeyerce. The exceptions nre the stopging of the amount of the rainfall ahead of the lightning in safe records lrom the Jefferson Forest which wsincluded in Group I of the flrst report and che change of the Lewis and Clark &om Group I to Group 11, where its fire records &sign and dangerous storms. It with most agreement. TABLE7.-Record of niiniber of minutes rainfall following the lightning in fire-darting as contrasted with nonfire-starting storms, TABLE6.-Record of rainfall following the lightning in both wet and 1934-1928 dru storm.^, 1994-1928 Nonflre-starting Fire-starting 1 storms storms Average number of mlnutes of rainfall by yenrs ~~i~, -. -
~~ number Forest group of Basis, reportsBasis, Averagerainfall reports 1 1924 I 1E5 I 1926 I 1927 I 1628 ~ $is$$Es Number Minutes Number 651 414 w 257 Qroup LL __._.___._...... 31 855 738 27 436 895 44 338 2,lSi 1,581 32 578 1. 581 -__73 0 23 529 4.156 30.R 2,135 144 MONTHLY WEATHER REVIEW APRIL,193 1 In Tables 7, again, for the secondary rain follo&ng In the same way the data on rainfall following the the lightning, the data show 13.2 nlinutes, or 43 per cent lightning (Table 9) show that in each of the five groups longer rainfall for storms which did not start fires than for of forests there was less rain after the lightning in fire- dangerous storms. Here, also, a large number of reports starting storms than in safe storms. covering a 5-year period are used as a basis. And again t8hedata are consistent; no average for any one year in t8he TABLE9.-Record of number of miiiiites rainfall following the fire-starting storms is greater than the general average for lightning in fire-starting as compared with nonjire-starting atorms all nonfire-starting storms, and no single average for the by forest groups, 1924-1928 safe stornis is less than the general average for the dan- Nonflre-starting Fire-stsrting gerous storms. storms storms Tables 6 and 7 seem to denionstrate rather conclusively t,hat fire-starting storms in this region are generally Forest group c,haraote,rizedby less rainfall, both ahead of and following :s# Reports ~~~~~ Reports the lightning, than safe storms whic,h arc c,harac.terizedby ---~ precipitation over longer periods. MintUes Number Mininuea Number Group I ...... 36.0 507 23.2 36 The princ.ipa1 purpose served by these avera,ge,s in Group11 ...... 34.2 548 18.4 99 Group I11 .___.._..._.___._.__...... -..-----47.0 1,882 31.8 802 Tables 4, 5, 6, and 7 is the approximate dete,rmination of Group IV.__.__...... _.__....~...~.~~~~~.~ 51.0 753 28.4 687 GroupV_.____.._._.____.__.--..------...- 42.9 35.2 a condition which varies within wide limits, so that definite --~___~688 614 averages can be stated above which departures generally Region __...... ____...... _------.44.0 14,156 I 30.8 I 12,138 indicate less t,han the usual need for fire control, and below which de,partures indicate more need for action. The averages given, however, include both dry and we.t storms. It is therefore possible to eliminate the dry storms and by considering only those which brought some rainfall, either ahead of or following tmhelightning, t)o determine bhe duration of the rainfall only for the wet storms. Naturally, bhis duration will be appreciably greate,r than the average detem1ine.d on the basis of both we.t and dry st,ornis. Of 7,093 reports of bot)li classes of stsorills only 48 per cent state no rain ahead of the lightning. The reinaiuing 52 per cent stat’e an ave,rage of 25 minutes of rain ahead DRY STORMS of t,he li htning. As would be e.spec,t,ed,this is approsi- mately f ouble the average for bot’li wet and dry storms. Althaugh most light,ning storms bring some rain with On this basis it is e.vident,t8hat the averiige storm rcporte,d the.rn, so-called “dry lightning storms ” are often credited as having rain ahe,acl of the lightning may be espect,ed wit,li starting a iarge proportion of forest firesin this region. to bring about 25 minutes of precipitation. It is entirely reasonable to believe that such storms are The records show bhat there is a snider proportion of decidedly apt t,o sbart fires if their bolts are numerous, and storms t8hatare dry following the light,ning t,han of t,liose if t>heyreach dry fuels on t,he ground. The records show, that are dry &head of the, lightning. Only 33 per ce,nt,of however, that only 6 to 10 of every 100 lightning storms a total of 6,983 reports stated that no rain fell aft,er the recorded in this investigation (14,754 reports) were dry, lightning had passed on. The otther 67 pe,r cent show an delivering no rain whatever t,o the ground beneath the avera,ge rainfall of 58 minutes’ duration. This time in- st’orm cloud. The reports (8,408) which were definite terval, w.hich can be used as an additional criterion of with regard to both the rainfall and the fire-starting degree of dange.r in this re.gion, indicates t,hat the “sec- character of the storms, showed that only 33 per cent of ondary rain ” desc.ribed by Humphreys occurs in about the dry storms were fire starters. Hence, only 2 or 3 two-thirds of all the thunderstorms in t,his region, and storms out of 100 are both dry and fire starters. lasts for about one hour. The Occurrence of these dry lightning storms in this Further examination of the data on rainfall ahead of region is interesting in two respects. First, these reports the lightning, by subdivisions of the region, shows that,, prove the occurrence of a type of thunderstorm not for each of the five groups of foresbs previously de,scribed, included by some meteorological definitions; and, second, the conclusion holds true that there is less rainfall ahead the so-called dry storm is popularly c.redited in this region of the lightning with fire-st,arting storms than with safe. as being the most dangerous fire starter. During the storms. Table 8 shows the averages and t’he nunibe,r of &year period covered by this study, the tabulation of reports on which they are based, for ea& of these groups. 1,238 reports of lightning storms with no rain before, with, or following the lightinng, out of a total of 14,754 report,s, TABLE%.-Record of number of ni.inutes’ rainfall ahead of lightning in fire-starting a8 coni pared with .tLo?ifire-starting storms, by forcst should remove all doubt as to dry storm occurrences. And QrOILPS, 1924-1028 such condit>ionsdepart decidedly from Clayton’s descrip- tions (4) and disagree with Moore’s definition of a thun- 1 Nonfirc-starting Fire-starting storms 1 st.nriun derstorm (10). This definition reads: “The thunder- storin, so familiar to e,veryone, may be defined as a local Reports rain awonipanied by lightning, thunder, gusts of wind, and frequently hail.” Humphrey’s definition (Op. cit.), Number 33 “A thunderstorm, as its name implies, is a storm charac- 9i 803 terized by thunder and lightning . . .,” appears to apply 687 more conclusively in t.his region. ___ 500 Of the reports of absolutely dry storms, stating defi- 12,120 nitely whether or not fires resulted, 68 per cent showed that no fires resulted. Of the definite reports of wet storms, APRIL, 193 1 MONTHLY WEATHER REVIEW 145 on the other hand, 66 per cent stated that no fires resulted. A sorting of these data by groups of forests, rather than This close similarity refutes the popular conception that by years, also shows uniformly that the safe storms have dry storms as a class are more dangerous than wet storms. more of their lightning confined to the clouds than do the It also raises a question concerning other data, previously dangerous storms. Whereas the first report on this presented, which show for the majorit,y of lightning investigation, which covered only t'he data for 1924 and storms that with less rain there are more fires, and with 1925, indicated that there might be a slightly significant more rain less fires. difference between groups of forests in the percentage of The reason for this apparent anomaly undoubtedly lies lightning confined to the clouds, the present and more in the nature of dry thunderstorms, in which the flashes comprehensive data show rather uniform percentages. are generally few, and nearly all confined to the clouds. As is evident from Table 11, no group average varies The meteorological remns for these peculiarities in more than 9 per cent from the average of any other group, activity may lie in the well-supported theories (9) that and none departs more than 6 per cent from the regional violent, turbulent action of large masses of drops of water averages. Apparently, the criteria of degree of danger forming the cloud are most favorable to the generation of according to the amount of lightning confined to the lightning, and that practically continuous sheets of water clouds, as established on the basis of the regional data, (rain drops) and streaks of highly ionized air form favor- are applicable in each of the subdivisions. able paths for the bolts. Hence, clouds too small to pre- cipitate moisture enough to reach the earth (as in dry TABLE11.-Percentage of lightning flashes confined to the clouds storms) may often be large enough and active enough to in safe and dangerous storms generate a few flashes which will be almost entirely con- Non6i-e-starting fined to the clouds. Large, active, and rain-producing . I storms clouds, on the other hand, may be expected to produce a Forest group greater number of flashes, with a greater number of bolts Flashas, Basis, striking the ground. conflned number to the of report LIGHTNING clouds
That the proportion of the lightning flashes confined to Aeerage the clouds has an appreciable effect upon the starting of ptr cent Group I. .. __._..____._____~-._____.____._. 72 568 fires is shown b the analysis of the records bearing on Group I1 ____._._._.___._...______76 726
Group 111...... _....___~ ..._._...___..._____ 77 2, 236 this feature. ?yable 10 shows this for the region as a Group IV__...__...___._...__..-----.-.---.. 78 975 whole by years. The safe storins consistentJy show a Group V...... --.-.. .-..._____....___. 72 983 higher percentage of lightning confined to the clouds than Average and total ._._..._._._._.__...._ 76 5,487 56 I 2,731 do the dangerous storms. The significance of data concerning percentage of lightning confined to the clouds, and striking the ground, OTHER CHARACTERISTICS obviously would be increased if the total number of flashes In addition to t'he.rainfall, or lack of it, and in addition per unit of time, or the total number as a storm passed t,o t,he c,harac,ter of the. lightning, there are other features over a certain spot, were known. It has been found of lighbning storms which affect successful forest-fire difficult, however, to obtain such counts as accurately as control. These include the time of day when the storms desired. Consequently, this aspect of the problem can are first seen, the prevalence of storms after dark, the not be examined at present. Work is being done, never- passage directly over the lookout stations, and the com- theless, to obtain this information for later use. mon direction of storm movement. When lightning storms are recognized as such during TABLE10.-Percentage of lightning jlashes confined to the clouds the forenoon hours, it is generally possible for a ranger or his assistant to get in touch by telephone with his various Nonfire-sbarting Fire-st,vting storms storms e,mployees, including trail crews, before the resultant fires have begun to be discovered by the lookouts. This Year is, therefore, one possibilit,y of obtaining greater speed in fire control. Table 12 shows the percentage of reports by years and by groups of forests, stating that the storms were first I -4veragr I I Average I seen during the forenoon hours. TABL,E13.-Storms jirst seen in the inorning, by forest groups
Averageand total ____._...._._.____._..I 76 I 5,487 I 56 I 3,731 I-' I-' l- I- I+-[- Pfr cent Pcr cent Per cent Per cent Per cent Per cent This summary for the region, based on S,21S observa- 1024_._._...___..__._.__. 24 13 18 19 35 23 1925___.___.___.___...__. 17 25 n 25 34, tions, shows 20 per cent more lightning confined to the 1926 ___.___..__.._____._.25 31l3 31 33 44 34 clouds in safe storms than in dangerous ones. 1927 __.._._____.__._.._.16 17 23 27 22 From these 1938 ...... 22 20 232o 28 30 25 data it is possible to establish indexes of the degree of p______p Average _..___.__.20 20 24 26 31 a5 danger as influenced by this factor. It is apparent that I as a general rule if less than half of the lightning is con- _. fined to the clouds the storm is of the fire-starting type, The outstanding indication from this compilation, based whereas if more than three-fourths of the lightning stays on over 14,000 observations, is that about one storm out in the clouds the storm is of the safe type. Intermediate of four is first seen before noon, and t,hereby permits fire- amounts probably should be considered 8s more dangerous control action early in the day if the lookout immediately than safe. reports storms to his ranger. 146 MONTHLY WEATHER REVIEW APRIL,1931 The consistently high percentages shown by Table 12 danger as Ward (14) has described them on the authority for all groups of forests during 1926 may be of meteoro- of E’. G. Plumnier (12). These zones are speciiically logical significance, and it is hoped that the study of such defined as being areas of lightning danger, but it is im- phases will appeal to some meteorologists. It should be plied that high mountain tops may be above the clouds mentioned here that all of the reports, for the entire day, and so in a zone of safety. To investigate the occurrence have been specially sorted and tabulated on an hourly of lightning-fire zones the locations of several thousand basis at the request of the Spokane office of the United lightning fires in the northern Rocky Mountain region States Weather Bureau, so that the development of have been plotted on maps by H. R. Flint, in charge of storms throughout this region may be studied intensively fire control of the Forest Service in this region. Although in relation to the twice daily synoptic weather maps. these maps show conditions for more than 10 years, there From this study it is hoped to derive considerable infor- is not any consistent evidence that lightning lires occur in mation ’of value for increasing the accuracy of future zones limited altitudinally in any way, except by the forecasts and for localizing them. absence of inflammable material, and by the wetness of The prevalence of storms at midnight has been deter- these fuels. There is, instead, a most baffling scattering mined merely as an approximation of the number of of these fires which renders the problem of successful con- storms occurring during the middle of the night. The trol all the more difEcult. It is recognized that these data show the following percentages of the total number maps show merely the occurrence of lightning-caused fires of observations, which stated that the lightning storms and, therefore, not all points struck by lightning, but this were occurring at that hour: 1924, 5 per cent; 1925, 3.5 nppears to be the best information available. per cent; 1926, 7 per cent; 1927,4 per cent; and 1928,4.5 Flint is also making a study of the occurrence of per cent; average for the 5-year*peiiod, 4 per cent. The lightning fires in relation to mineral-bearing areas in an importance of storms occurring during the hours of dark- effort to determine whether or not geological formations ness lies in the inability of the lookouts to detect the re- have any appreciable effect upon lightning strokes and sultant fires, unless the flames are so located as to be fires. The Forest Service fire records and the geologic directly visible. In the meantime, and before men can maps for the Coeur d’hlene region are being used for this reach the spot and commence suppression, the fire may work. have several hours in which to spread. Furthermore, Another characteristic of lightning storms that may since it is very dacult at night for the lookouts to nialie have meteorological significance which will be of value in azimuth readings of lightning bolts striking, the effects of prediction is the common direction of storm movement in storms at night can not be accounted for so accurately. different parts of the northern Rocky Mountain region. With some 4 per cent of the storins occurring at mid- There are 14,595 reports which serve for determining this night, it is obvious that an appreciable number of storms characteristic of storms for this region. Tables 13 and 14 are in action during the night when close observation is show the results of the analysis by years and by groups extremely difficult or impossible. Compilations of future of forests. data will be made to develop more specific information on TABLE13.-Dircclion of moi*enrenl of storms, by years, toward fhc this phase of the problem. tlirr 1.1 ions giwtL The passage of lightning storms directly over a lookout ~ ~~ station is important in several ways. Frequently the \Iovement 01 storm 1V24 reports 1925 reports 19-16 reports resultant fires will be close to the station and can be I 1 Per I I per I I P#r attacked immediately. On the other hand it often happens that a storm cloud whose path includes the mountain top lookout station will envelop the station and South-- - ____ ~.__...... --... 36 3 70 3 89 4 reduce visibility from that point to such a degree that the Southwest.... ______...._._____ 36 364 3 91 4 M est ___..._.._._...__._...... ~~~..31 3 52 2 73 4 direction of lightning flashes can not be discerned and Northwest...-...... ~ _.__. ____.-.... 40 3 si 3 89 4 resultant fires only a half mile away are invisible until Stationary or revolving _____.._.._._18 1 7 0 34 2 the cloud has moved by. Totd...... __.__.___.______~ 1.264 100 2,431 100 2.164 loo The reports by the lookouts show the following percent- ages of cases in which the storms passed directly over one Prr Per Per or more of the lookout stations: 1924, 48 per cent; 1925, Nirniher cent Number ctnl Number ccnl 47 per cent; 1926, 47 per cent; 1937, 42 per cent; 1928, North ...... 461 11 451 10 1,708 12 Northeast ..__...._._._...__...-.-..1.207 29 1.233 28 4.458 31 39 per cent. This means that in about 4 or 5 of 10 obser- E<&t 1.3% 32 1,403 32 4,550 31 vations a storm will pass directly over some lookout sta- Southeast ___._..._._...... -- J‘2.5 12 714 11; 1,731 South _.._..._..._.______...... ~~~3-19 6 259 ti 702 ’: tion and consequently that many reports on storms will Southwest ..._....._.______...... 140 3 134 3 465 be, and frequently must be, made by stations at some distance from the storm. Hence, no method of ohserva- tion should be used which depends upon the storm passing directly over the observation station. Probably the best method of determining whether or Table 13 clearly shows the marked tendency of light- not storms are apt to start fires near lookout stations, ning storms to travel toward the northeast and east in and the likelihood of such clouds reducing visibility from t,his region. Only 12 per cent of the reports show a move- that point, will be to examine the data separately for each ment toward the north, and the same proportion toward lookout. It is believed, however, that there is not yet a the sout,heast, all other directions being credited with sufficient volume of reports for individual stations to only 5 per cent or less. The changes from year to year warrant such a detailed sorting and tabulation. Such an during the period studied do not appear to be significant. analysis is planned for some future date. Table 14 indicates that there are no marked differ- One feature of storm occurrence upon which we have no e,nces in direction of storm movement between the various dependable data is the altitudinal range of the clouds, groups of forests. While Group I, in the northeastern which might influence or even control zones of lightning pa.rt of the region and largely east of the Continental Di- APRIL, 1931 MONTHLY WEATHER REVIEW 147 vide, shows the highest percentage of movement toward When storms are accompanied by more rain than the the east, Group V, in the southwestern part of the region, average and when they have more than the usual amount shows the greatest movement toward the north and of lightning confined to the clouds, a smaller proportion northeast. The significance, if any, of t'hese differences of them cause fires. When t,hey are accompanied by less is a problem for the meteorologists. Perhaps these data rain than the average, however, and have most of the serve as an index of the probability of summer rain, lightning striking the earth a larger proportion is dan- from each cardinal direction, but the practical applica- gerous. Though the seasonal wetness or dryness of the tion of such information in fire control is doubtful. forest fuels undoubtedly influences the probability that a storm of certain character may start fires, the two condi- TABLE14.-Direction of movement of storms, by orest groups tions, occurrence of storms (as indicated by number of (movement toward the directions given{ thunderstorm days) and the characteristics of storms (as shown by percentage of storms starting fires) are essen- Movement of storm tially indicative of the lightning fire hazard per unit of area in a given timber type in this region. Vumber Per cent lVumbtr Per cm h'umbcr Per cent The relationship between storm occurrence, character- North ...... 55 7 146 11 69< 11 Northelvlt...... 281 34 396 31 1.51-1 25 istics, and the number of resultant fires can be expressed East ...... 339 "2 503 38 1, !I1 rj 31 as a simple formula which, when applied to the data for Southest-...... Si I15 9 !l7,5 16 Bouth...... "3 i 41 3 3Y1 6 each of the five regional subdivisions, serves to compute Southwast...... li 2 26 2 23 I 4 West-- ...... 14 2 37 3 1-19 L the average number of lightning fires per unit of area Northwst...... 217 4 with a maximum error of one and one-half fires per 100,- Stationary or revolving..-...... 5 i 1 -__ 10b Total ...... 6.14.5 :iM 000 acres. The formula used was -=c. The data are .~ - a, shown in Table 15. (froup IV re- Group '.' re- hIovement of storm ports port,s TABLE15.-Th iirrdersform occiirreti.~~,dangcr and $res
VU 712 be Per rest Number Pu cell Nictnbi~rPer cent North...... 424 12 3bV 13 1, 708 I? Northeast...... 1.003 ?Y 1,264 43 4.45s 31 East ...... 1,006 29 904 1s 31 1.i, :in dol Southest ...... 3sa 11 1en 12 Forest South--...... IYS 6 so 2 4,.-70:! Southwest...... 137 4 54 "J < west-...... 102 3 41 1 3-12 Northwest...... 121 79 3 .Ii4 3 (a) (111 ic) : ,.- I Stationary or revolving ...... -57 -8 1 11.5 1 --~ I- Total ...... 3,436 Num bcr Per cent Num bcr Nuin her Group I ...... 5R 6 1 1 Group 11...... 43 15 a 3% Group I11 ...... 62 31 li One other feature of the movement of lightning storms rhoup TV...... SO 47 11 2 $5 Groul, v ...... !< 7 41 has a possible bearing on fire control. This concerns the Regiu!i...... h: 31 :I ; possible tendency of storms to follow more or less regular ~~~ ~~ - . . ~ . ~ ~ ~~~~ ~~ paths, perhaps according to the topography or the local presence or absence of thunderstorm-breeding condit,ions, as Brooks (3) and Hallenbeck (7) have described them. Naturally, the deternhation of localities most fre- quently exposed to lightning storms is of great importance in deciding upon the forest protection facilities that should be provided for different areas. Field observers in this region are strongly of the opinion that certain topographic features and localities are common centers of formation of storms and that there are certain paths of movement which are followed much more frequently than other paths. The present records covering all subdivisions of the region have not been exhaustively examined, but detailed analyses have been made of several restricted areas, comprising up to about a million acres, and €or some of the most dangerous days thunderstorm reports for the entire region have been plotted on maps. These minor studies have failed to find any marked topographic paths along which thunderstorms may commonly be ex- pected to travel. It is planned to investigate these con- ditions more intensively a few years hence, after the vol- ume of records from each station has increased materially.
LIGHTNING-CAUSED FIRES The number of lightning fires per unit of area is a basic consideration for foFest-protective agencies. This figure is not the same for different parts of the United States, nor even for different parts of the northern Rocky Mountain region, being governed largely, as has been seen, by the number and characteristics of storms. 148 MONTHLY WEATHER REVIEW APRIL,1931 opportunity for the lookout to inform his ranger of the able for presuppression action after the storms are path of the storm, the probability of fires, and even the sighted. Four hours can be allowed for equal prepara- probable location of firgs, according to his observations of tion for second fires, five hours for third fires, and over where lightning is striking. Likewise, a long period offe,rs six hours are available for the majority of the fourth fires. the ranger more time to request an air patrol of his district, The importance of these determinations lies in the fact and more time to communicate with his fire guards, that regardless of whether or not lightning storms have smoke chasers, patrolmen, and road, trail, and telephone been previously predicted by the Weather Bureau, there construction or maintenance crews, so that they may move is, after the storm actually appears, a very appreciable to more strategic locat,ions if the ranger desires. All this period of time available to mobilize men and otherwise will assist .greatly in catching fires while small-the first prepare for the probable fires. Obviously, in regions like essentia.1 in effe,ctive fire control. Prompt and well- northern Idaho, where 4 or 5 storms out of 10 usually advised action is therefore required, especially when not start fires, such an opportunity for preparation is de- enough men are regularly em loyed to attack an escep- cidedly desirable. t,ional number of fires efficient y. Speed and dependable Another important fact indicated in Table 16 is the information are gven more importantY in the region under considerable number of discoveries of fire niade only study than e,lsewhere, because here are several national after a lapse of 48 hours. For the region as a whole, for forests with an average of only 16 to 18 miles of road and the 5-year period, 333 reports show this so-called “hang- trail per t,ownship of 23,040 acres, and measured trave.1off over” condition. These reports comprise 8 per cent of the trail is at the seemingly low average rate of only 1 the reports studied, and it is worth mentioning here that mile per hour. Under swh conditions it is exhmely in 160 instances in these reports these hang-overs were desirable to grasp dl opportunities for moving men as first discoveries. Consequently, it is obvious that even early as possible to locations from which a minimum of though a lightning storm passes by and no fires are dis- travel will be required to reach the fires. covered within the following two days, there is still an As an indes of hhe t’ime available for reporting on the appreciable chance that fires, were started and will important features of lightning storms before the fires are show up later on. detected, 4,149 reports state definitely the time whe,n the storm was first seen and the hour and minute when the TABLE17.-Percentage of $res discovered 48 hours or more after fire.s were discovered. Of the,se 2,335 re.ports show t,he Ihe storm was $rst seen time between first sighting the storm and the discove.ry of Second Thud Fourth t,he first fire, 959 reports state the time before discovering Tear I E I fire I fire 1 fire the second fire, 538 reports cover a third fire., and 314 give. dah for a fourth. The results have, been arrnnged Per cent Per cent Per cent Pcr ccnt 192.1...... 8 3 1 4 to repmsent conditions for an ave,rage lookout, stmat8ion 1925 ...... 10 is 20.5 23 1328...... 8.5 12 15 a0 and show the proba.bility of a first disc,overy of a fire 1927...... 6 5 8 15 within definite priods of time afte,r sighting the storm, 192s ...... -~___--5 4 4 6 or if two fires are discovered the probable. e,lapsed t,ime ..rQrsgp ...... 7 8 10 14 for the second discovery and similarly for third and fourth fires. The records show that occasionally a single Table 17 shows that since 1925 t,here has been a consist- lookout has reported 10 or 12 fires caused by one lightning ent reduction in the percentage of discoveries made more storm, but the majoritmyof t,he reports show far less than than 48 hours after the storm appeared. Whether this this. has been brought about by the establishment of more When the cumulative perce,ntage of discoveries were lookout st.ations directly surveying more area, the removal plotted by hourly periods (hours following the first sight- of old stat,ions to new and better locations, or by better ing of the storm) and the data we.re curved, the va1ue.s trained and more conscientious personnel can not be given in Table 16 were obtained. determined from the.se reports. A marked and very desir- able improvement in fire control is evident, nevertheless. TABLE16.-Percentages of storm reports showing varioiis intervals between sigh,ting of storm and discovery of subsequent $res When the speed of detection is compared for the five subdivisions of this region the most outstanding fact is Second Third Fourth that the Beaverhead-Helena group of forests, which has Hours after flrst sightlng storm First flre 8re be I flre very few lookout stations at high elevations, shows a con- siste.ntly longer period of time between sighting the storm Pcr cent Per cent Per ccnt Per cent and making the first discovery of a fire. For example, 0 to 1 ...... 20 ie 12 8 It0 2-----.------.-.------.-.------16 12 12 13 four hours after the storm appears only 50 per cent of the 2tO 3 ...... 11 11 10 8 3 to 4...... 8 9 0 6 first discoveries have been made on the Beaverhead- 4 to 6...... 6 6 5 4 Helena group, while 60 per ce,nt have been made on the 6b6...... 4 4 6 4 6 to 12...... 12 14 15 16 north Idaho forests comprising Group IV. Twelve hours 12 to 18 ...... 7 8 8 11 18 to aP ...... 5 e 6 7 after the storm is first seen the easte.rn Montana forests 24 to 48...... 5 8 8 9 7 8 10 I4 have made only 66 per cent of their first discoveries, while Over 48 ...... -~Group IV has made 77 per cent and Group V 79 per cent. Tot.81...... 100 100 100 1 100 At the 24-hour inark the Beaverhead-Helena have only 74 pe.r c.eiit to their wedit, while Groups IV and V have 89 Table 16 shows that the period of time between first per cent and 90 .pe.r mnt, respectively. sight,ing the storm and the discovery of fire lengthens In this re.gion there are two other observations which very appreciably with each subsequent fire. For ex- have also been found of immediate value and usefulness. ample, about half the first fires are discovered within The first is the measurement of azimuths or compass three hours after the lookout first sees the storm. In bearings on points struck by lightning and the recording other words, to be amply pwpared to reach and hold as of these measurements so that these spots can be watched many as half of the first fires, only three hours is avail- very carefully for at least 45 hours to come. (A special APRIL,1931 MONTHLY WEATHER REVIEW 149 form is provided for noting such observations.) As a out oi 100. Of all these dry storms only one-third result of such action many fires may be discove.red as soon st.art8edfires, whereas one-third of the wet storms were as the first wisp of snioke rises from the spot and hours also dnngerous. The, reasons for this anomaly are be- before the usual smoke column begins to be clearly visible. lieved to be in the le.sser number of lightning flashes in The second observation is whether the spot,s struck by dry storms toge.ther with a lesser proportion of these lightning are well soaked, lightly sprinkled, or entirely flashes re,aching t8he ground. Naturally, no fires can unmoistened by the storm. Experience has shown that be started by a dry storm if all of the lightning is con even with storms bringing heavy rainfall lightning ma.y fined to bhe clouds. strike one or more places near the ed e or even outside the * * * * * area wetted. Naturally, if the fueB types are similar, a From t,he,se fa&, developed by the analysis of the bolt falling outside the rain area will have a better c.hance report,s on lightning storms, it is obvious that the,re a.re of starting a fire than one at a spot whic.h is thoroughly four obse,rvat,ions whkh forest-fire lookouts can make soaked by rainfall. But of even more importance is t,he, which are, of immediate value t80t)he forest ranger directly fact that if fires result in both cases the one outside the in charge of fire control: (1) The occurrence of storms; rain area is much more in need of immediate attent>ioii (2) the4r pat,hs: (3) the acc,ompanying precipitation, if t'han the one that is surrounded by wetter fuels, and the any; and (4) the perc.entages of lightnmg confined to former should be attac.ked first if there must be a choice. t,he c.louds and striking the ground. Always, the flash Usually the lookout is the only member of the forest pro- of lightning or the rumble of thunder will be the best tective organization who is in a position to make t)liis assurance of thunderstorm occurrenc,e, but usually the important fact known to the ranger, c.entra1 dispat'cher, formatmionof cumulus clouds of the thunderstorm type or whomsoever may be responsible for sending n1e.n to and size is sufficiently definite, especially if storms have these fires. be.en predic.ted by the Weather Bureau. As the storm SUMMARY develops and the lookout is able to estimate its probable pat,h he ma.y be able to determine whether the particular Because lightning is the most important single cause ranger dist8ric,t8which includes the lookout station is apt of forest fires in the northern Rocky Mountain region a d. If it is, then some prominent topographic special study has been made by the Forest Service of the fe,at,urewill serve to time the duration of rainfall and to occurrence and characteristics of lightning storms in that est,iiiiate the character of the ,lightning as the storm region. This study utilized the data obtained from fire passes over t8hatpoint. Such information, obtained by lookouts stationed on approsimahely 200 mountain tops, a careful obse,rver, is now mcognized as an excellent so distributed that very few storms could occur during basis for fire cont'rol action in bhe northern Rocky Moun- the siimmer months without being reported by the look- t,nin region. outs. A special form was used for obtaining the desired Some ot,lier characterist,ic.s of storms determined by inforniat8ion, which- is now suiiimanzed for t,he 5-year t,his st8udy include the finding t,hat 35 per cent of the period 1924 to 1928, inclusive. storms are, first see,n be,fore noon; about 4 per cent of the The results show that about 34 storms out of 100 have st,ornis are active during trhe middle of the night; only caused forest fires -and that there are from two to four d or 5 observations out of 10 report the storm as passing times more thunderstorm days per year in this region than direct,ly over the observation station, thewby indicating had been previously estimated. It is found, however, that the local character of lightning storms in this region; 62 the danger of forest fires caused by lightning is not in pro- per cent of all t,hunderstorms in the northern Rocky portion to the number of thunderstorm days but varies Mountain region move toward the northeast and east, more with the characteristics of the storms. The greatest, while 86 per ccnt are found to travel in a direction be- number of fires and the greatest proportion of reports of tween nort,h aad southe.ast; no common topographic fire-starting storms were found on 20 days, which consti- pat,hs of trave,l have as yet been disceimible. tute only 4 per cent of the total number of thunderstorm This st,udy is to be cont'inued, using an improved re- days. Re.cords are included so that these so-called easy port form to obt,ain additional detail of information as and bad days may be studied in relation to the daily well as to record if possible any increase or decrease in weather maps. the occ.urrence,of lightning storms over a longer period. An analysis of the 14,754 lookout storm reports avail- able shows that there are recognizable differences between LITERATURE CITED the types of storm that usually start fires and the type,s (1) ALEXANDER,G. W. that are generally safe. These differences are found t)o 1927. Lightning Storms and Forest Fires in the State of lie in the duration of the rainfall ahead of and following Washington. U. S. Mo. Weather Rev. 55: the lightnin , together with the electrical ac.tivit,yof t'he 133-129. (2) ALEXANDER,WY. H. storm and tE e pe.rc.entage of lightning flashes confine,d t)o 1924. The Distribution of Thrindemtorms in the United the clouds or striking to the earth. Average duration of St.ates, U. S. Mo. Weather Rev. 52: 343-348. rainfall, and average percentage of light,ning flashw (.3) BROOKS,CHARLES F. striking to the ground, have been determined separstely 1922. The Local or Heat Thunderstorm. U. S. Mo. Weather Rev. 50: 281-254. for the safe and for the dangerous stornis. It is pointed (4) CLAYTON, H. H. out that by observing these c.haracteristics it is often 1923. World Weather., illus. New York. possible for the looliout,s to classify a storm as eit'lit'r (5). GIBBORNE,H. T. generally safe or generally dangerous hours before any 1926. Lightning and Forest Fires in the Northern Rocky Mountain Region. U. 8. Mo. Weather Rev. 54: of the resultant fires produce enough smoke to permit, 281-2%. discovery. Suc.h advance information permits the forest protective agency to move men into or near the danger (6) 1928. Meaburing Forest Fire Dan er in Northern Idaho. area so that the fires can be reached more quic.kly and U. 8. Dept. Agr., Forest iervice. Misc. Pub. 29, 63 p., illus. be extinguished more cheaply while yet small. (7) HALLENBECK,CLEVE It is found that t.he so-called dv storm (having no 1932. The Topographic Thunderstorm. U. S. Mo. Weather rainfall reaching the ground) occurs in only 6 to 10 cases Rev. 50: 284-287. 150. MONTHLY WEATHER REVIEW APRIL,1931 HENRY,A. J. PLUMMER,F. G. 1927. Can Thunderstorms be Classified? U. S. Mo. 1912. Lightnin in Relation to Forest fires. U. 5. Dept,. Weather Rev. 55: 118. Agr., forest Serv. Bul. 111, 39 p., illus. HUMPHREYR,W. J. SHOW,S. B., and KOTOK,E. I. 1920. Physics of the Air. 665 p., illus. Philadelphia. 1929. Cover Type and Fire Control in the National MOORE,W. L. Forests of Northern California. U. 8. Dept. 1910. Descriptive Meteorologv. 344 p., illus. New York. Agr., Forest Serv. Bul. 1495, 35 p., illus. MORRELL,FRED WARD,R. DEC. 1929. Forest Protect,ion Needs in the Inland Empire. Jour. 1925. The Climates of the United States. 518 p., illus. Forestry 27: 143-147. New York.
THE CALENDAR YEAR AS A TIME UNIT IN DROUGHT STATISTICS’ By ALFREDJ. HENRY [Weather Bureau, Washington, March 17, 19311 As almost everyone knows, the year .is generally con- for each of the 42 districts organized into what were sidered as being too long a unit for use in compiling formerly known as State weather services, now known as drought statistics. While admitting the general sound- cliinatological sections, of which as a rule there is one in ness of that view, it is believed that the disadvantages of operation for each State or conibination of States, except, the calendar year have been somewhat esaggernted. that the six New England States are organized under the The case of Arkansas in 1930, when the percentage of the name “New England,” and Delaware and the District of normal precipitation received was 96 per cent, will natu- Columbia are conibined with Maryland. The list of rally come to mind. In this case two of the mont’hs, sections with the term of years covered by each section January and May, had 223 and 300 per cent of the is given in Tables 1 and 2. normal, respectively, and the real lack of rain that In some of the sections the record goes back to t,he. caused the failure of the c.otton crop was confined to the early eight,ies and in others it does not begin until about months of June and July, with 22 and 19 pe.r cent,, 1900, so t,hat the early part of the period is not as fully respectively. Most persons fail to consider that, the covered as that part subsequent to 1900. The size of very great rainfall of January and May was in itself t)he respe.ctive sec.tions varies grest81y-say, from about quite abnormal and not likely to again happen in the 15,000 square miles in the smallest to 265,896 in the next 50 years. 1a.rgest. The network of climatological stations is sonic- The reason why a shorter period than the year has not what closer east of the Mississippi than to the westward, heretofore been used in compiling drought statistics is especially in Ro&y Mountain and Plateau St,ates. most likely becauseof the overlapping of drought periods The plan followed was t80take out for each State the from one month to the next and the fact that its ending least annual precipitation that had been recorded during rarely occurs at the end of a mont.h; thus it would be the forty-odd years during the life of the record, then the necessary to make a special compilation in order to fis nest, lowest annual amount, and so on, on an ascending t,he definite limits of the duration of droughh. This has wale until the tenth year on that’ scale had been reached. not been done, and to do it now for previous droughts is Thus it has been practicable to c.onstruct for each State prohibitive on acc,ount of t’he labor involved. a diagram hginning at the low point and increasing to, The object of the present c,onipilat,ionwas therefore to say, about 90 per cent of the annual average precipita- ascertain to what ext,ent the calendar-year rec,ord of pre- t,ion. In like manner the year of greatest annual precipi- cipitation would serve to ac,curately fix the tiims and tation has been taken out and the nine subsequent years places of drought in the United States. In the begin- when the next greatest annual amount was received, and ning of the study the individual records of stations so on until the tenth year, on a decreasing scale, had been within a State using both the nionthlyand annual amounts reached. were used. As the work progressed the difficulties of Tables 1 and 2 include the tabulation above described, distinguishing the beginning and the ending of drought the annual amounts of precipitation being expressed as a even from the mont8hlytotals of prec.ipitat,ion le,d to its perce.nt,age of the mean annual or, in other words, the abandonment and the substitution of a shorter method normal for the State. based on the averages of precipitation for each ye,ar In Table 1 the scale is an increasing one and in Table 3 a demeasing one. 1 The substance of this article was presented before the May, 1931, meeting of the American Meteorological Soclety in W’ashmgtoo, D. C.