Published by WEATHER MODIFICATION ASSOCIATION PO BOX 26926 • FRESNO CA 93729-6926

Photographs furnished by Atmospherics Inc. unless otherwise indicated. Third edition August 1996

All rights reserved. Single copies available at $2.00 each; 50 copies at $1.50 each. Table of Contents

The Weather Modification Association...... 2 Foreword ...... 3 Natural Cloud Processes...... 4 Cloud Seeding ...... 5 Seeding Winter Storm Clouds over Mountains ...... 6 Seeding Cumuliform Clouds ...... 7 Fog Modification...... 9 WMA Capability Statement on Weather Modification...... 10 Government Involvement ...... 11 Certification...... 12 Corporate Members of the Weather Modification Association ...... 13 Answers to Questions Most Often Asked...... 14 Suggestions for Further Reading ...... 16

Lightning and rain

1 The Weather Modification Association

The Weather Modification Association * certification of members qualified to con- (WMA) was organized in 1950 under the duct weather modification projects; and name "Weather Control Research Associa- tion" to develop a better understanding of * encouraging international cooperation in weather modification among program spon- weather modification activities. sors, operators, and other scientists. The WMA now has approximately 200 members General membership is open to any indi- located on five continents. vidual interested in any aspect of weather modification. Organizations interested in Major activities of the WMA are: weather modification are invited to join as corporate members. Dues are assessed on a * promotion of responsible application of calendar-year basis. The classes of member- weather modification technology; ship and the annual dues in 1996 were:

* promotion of research and development in Corporate: $200 weather modification; Individual: 50 Student: 10 * serving as a clearinghouse and distributor of information on weather modification; Applications for membership and addi- tional information can be obtained from: * producing and disseminating policy statements on all aspects of weather Weather Modification Association modification; PO Box 26926 Fresno CA 93729-6926 * publication of the Journal of Weather U.S.A. Modification; Fax & Phone: 209-434-3486 email: [email protected]

Hailstones

2 Foreword

Nearly all modern attempts to modify the weather involve cloud seeding, which is the intentional treatment of individual clouds or cloud systems to achieve some desired effect. For a long time various materials have been known to affect the formation and growth of water droplets and ice crystals in clouds. Important discoveries in 1946 at the General Electric Research Laboratories in Schenectady, New York, led to practical methods for modifying large volumes of clouds at reasonable cost. Three key persons associated with those developments were Irving Langmuir, Vincent Schaefer, and Ber- nard Vonnegut. Building upon those early discoveries and additional scientific experiments, atmospheric scientists continue to develop cloud seeding techniques which, when properly utilized, provide results of practical importance. Clouds have been seeded in attempts to mod- ify fog, hail, wind, and lightning, but most projects have been intended to provide addi- tional precipitation in the form of either rain or snow. According to information provided by member countries to the World Meteoro- logical Organization, cloud seeding projects are now being conducted in over 40 countries. This brochure is intended to provide an educational overview of the physical basis for cloud seeding and a brief description of the most common applications.

Before seeding During seeding After seeding

3 Natural Cloud Processes Vast quantities of water in the form of In addition to oxygen, nitrogen, and trace water vapor, cloud droplets, and ice particles gases, the atmosphere contains variable are constantly on the move in the earth's amounts of water vapor. The amount of atmosphere. The atmospheric reservoir of water vapor that can exist in a given volume water vapor is much larger than the amount increases as the temperature rises. The of water which is visible as clouds. Fur- relative humidity is one measure of the con- thermore, special conditions must be met centration of water vapor; it is the percent- before any of the cloud water can reach the age of the water vapor present relative to surface of the earth as precipitation. Im- that which the air could hold. For example, portant factors controlling the initiation and if the air temperature near the ground is amount of precipitation from a cloud are (1) 25°C (77°F) and the concentration of water cloud size, in both the vertical and horizon- vapor is half of the maximum that could tal dimensions, (2) cloud lifetime, and (3) exist at that temperature, the relative humid- sizes and concentrations of the droplets and ity is 50%. If a volume of air as just de- ice particles that make up the cloud. scribed is cooled, say by lifting it to regions Precipitation forms in two distinct ways, of lower pressure, the relative humidity will which are sometimes called the "warm- increase, even though the relative concen- cloud" and "cold-cloud" processes. The trations of water vapor and dry air remain term "warm-cloud" was introduced after the same. If it is cooled to 12°C (54°F), the scientists noticed that rain often falls in the relative humidity will reach 100%; the air is tropics from clouds existing entirely at tem- then said to be saturated. If further cooling peratures above 0°C (32°F). Rain forms in takes place, the water vapor in excess of that these warm clouds as relatively large falling required to maintain saturation will con- droplets overtake and coalesce with smaller dense into cloud droplets. droplets, which have lower fall speeds. Cloud droplets form around cloud con- Coalescence of liquid (supercooled) droplets densation nuclei. Microscopic aerosol par- also takes place at temperatures below 0°C ticles are always present in the atmosphere; (32°F). The term "warm-cloud" is therefore those that are relatively large and hygro- a little misleading, but it is widely used for scopic are most apt to serve as cloud con- convenience. densation nuclei. The atmosphere usually The cold-cloud precipitation process re- has an abundance of cloud condensation quires ice particles, so it occurs only where nuclei, so most clouds consist of small temperatures are below 0°C (32°F). Tem- droplets in high concentrations. The drop- perature generally decreases with height in lets in a typical cloud are so small that it the atmosphere, so even in the tropics the takes about a million of them to make one tops of clouds more than 5,000 m (16,000 ft) raindrop. above sea level are colder than 0°C (32°F). There are also aerosol particles in the Cold clouds, in the tropics or elsewhere, are atmosphere which cause cloud droplets to often a mixture of large numbers of super- freeze or ice crystals to form directly from cooled droplets and smaller numbers of ice water vapor. These particles, which are particles, which may be single crystals or called ice nuclei, are scarce, so many cloud clumps of crystals. Ice crystals in a mixed droplets remain in the liquid state at tem- cloud grow rapidly by deposition of water peratures below 0°C (32°F), which is the vapor, while the droplets evaporate to main- freezing point of bulk water. Such droplets tain a relative humidity near 100% and are said to be supercooled. If part or all of a thereby feed the ice crystal growth. Typi- cloud is colder than 0°C (32°F), frozen par- cally, in 5 to 10 minutes the growing ice ticles (ice) may be present also, but gener- particles become big enough to have appre- ally in much lower concentrations than the ciable fall speeds. The falling particles then supercooled droplets, which have been ob- continue to grow by colliding with smaller ice served at temperatures as low as -40°C particles and cloud droplets in their path. If (-40°F). they encounter temperatures above 0°C (32°F) closer to the earth's surface, they may

4 melt completely and reach the ground as numerous but smaller condensation nuclei. raindrops. Otherwise, they fall as snowflakes Continental clouds tend to have higher con- or hailstones. centrations of small, more uniformly-sized The sizes, types, and concentrations of droplets. Therefore the warm-cloud (coal- nuclei in the atmosphere are important in escence) process for initiating rainfall is determining the efficiency by which a cloud inefficient in many continental clouds. To system produces precipitation. For instance, develop precipitation in such clouds requires large salt particles found over the oceans act activation of the cold-cloud process, which as giant cloud condensation nuclei. They means that the clouds must extend above the form large cloud droplets leading to a broad 0°C (32°F) level, and then last long enough spectrum of droplet sizes, which speeds the for the cold-cloud process to function. Fur- onset of coalescence and initiates rainfall thermore, a scarcity of ice particles some- well within the lifetimes of many maritime times limits the efficiency of the cold-cloud clouds. On the other hand, the atmosphere process in continental clouds. over the continents usually contains more Cloud Seeding Sometimes one can assist nature in the increase the efficiency of the cold-cloud formation of precipitation by seeding process. The silver iodide is usually re- clouds with appropriate types and numbers leased from devices called liquid fuel gen- of nuclei at the proper times and places. erators, or pyrotechnic flares, which can Seeding with very large condensation nu- produce as many as 1,000,000,000,000,000 clei, such as hygroscopic particles of com- particles from 1 gram of silver iodide. The mon salt or urea capsules, can accelerate nucleating ability of silver iodide increases the warm-cloud process. Seeding with ice as the temperature falls and varies with the nuclei, such as silver iodide particles, or type of device used. For many devices, the with very cold materials, such as dry ice threshold temperature below which silver pellets or liquid propane, can supply some iodide is an effective ice nucleant is around clouds with additional ice particles and thus -5°C (23°F).

5 Seeding Winter Storm Clouds over Mountains

Clouds often form as moist air is lifted the amount of precipitation to less than they and cooled during its passage across moun- would produce otherwise. Although there is tain ranges. Clouds formed in this manner little evidence to support this idea, weather are called orographic clouds. In winter, modification operators should be knowl- many orographic clouds form as assem- edgeable about the different types of clouds blages of super-cooled liquid droplets. Left that occur over mountain ranges during the to nature's devices, many of these clouds are winter. inefficient precipitators, keeping more than A variety of seeding agents have been 90% of their liquid moisture burden aloft applied to winter orographic clouds. Silver until the droplets evaporate as the air de- iodide, which can be released from either scends and warms to the lee of the moun- ground-based devices or from devices on tains. airplanes, is by far the most widely used. The technology of seeding winter Operators must consider the complexities of orographic clouds is fairly well developed air flows over mountains in order to prop- and understood. Some of these clouds do erly seed a designated target area. not contain enough ice particles to effi- Some projects designed to seed winter ciently convert their supercooled water into storm clouds over mountains in the western precipitation. By seeding such clouds with United States have operated continuously artificial ice-forming substances, their pre- since 1950. Statistical studies of precipita- cipitation efficiency can be increased. Other tion and streamflow data indicate that some winter orographic clouds contain abundant winter orographic projects have increased ice and adding artificial ice nuclei does not seasonal target-area precipitation from 5 to increase their precipitation efficiency. 15%, with higher localized increases in Seeding such clouds might actually reduce some storms.

6 Seeding Cumuliform Clouds

Cumulus clouds are important rain pro- more buoyant and may cause it to grow ducers in the summertime over most of the larger (taller and/or wider), thereby proc- world and are the main source of rainfall essing water at a higher rate, more effi- during all seasons in the tropics. These ciently, and for a longer period of time than billowy giants form in updrafts in airmasses without seeding. This type of seeding (for that are overturning due to instability. An dynamic effects) is often done by aircraft airmass can be made unstable by the sun dropping pyrotechnic flares containing sil- heating the ground below it or by the pas- ver iodide into the tops of promising cloud sage of an atmospheric disturbance, for ex- formations. ample, a cold front. Hail forms in large cumulonimbus clouds The modification of cumulus clouds is with very strong updrafts extending well more complex than the seeding of winter above the 0°C (32°F) level. Hailstones orographic clouds. Cumulus clouds can be grow as supercooled cloud droplets and seeded with large condensation nuclei, but raindrops freeze around ice particles. At- the amounts of material required are so large tempts to suppress hail usually involve in- that this seeding method is seldom practical. troduction of fairly large quantities of silver Silver iodide and dry ice are used to stimu- iodide into specific parts of clouds that are late rainfall when the cloud top temperature hailing or are expected to produce hail. The is below -5°C (23°F). The object of the additional ice nuclei result in higher ice seeding is to increase the number of ice particle concentrations that increase the particles in the supercooled portion of the competition for the available supercooled cloud. When surrounded by supercooled water within the clouds. As a result, the droplets, the ice particles will grow rapidly hailstones may not grow as large as they and some may become large enough to fall would if the clouds were left unseeded. If to the ground. If an ice particle encounters the hailstones are small enough, they have temperatures above 0°C (32°F) during its time to melt completely as they fall from the fall, it may melt to a raindrop. 0°C (32°F) level to the ground. Hailstorms When conditions are right, cumulus have been seeded from aircraft and from the clouds can be stimulated to grow larger and ground (including use of artillery and last longer. As noted above, the introduc- ground-to-air rockets). The great variability tion of silver iodide or dry ice into the su- of hailfalls makes the evaluation of hail percooled portion of a cloud causes some of suppression projects difficult, but success in the droplets to freeze. The freezing releases suppression of crop-damaging hail has been a large amount of heat, called the latent heat claimed by operators using all of the above of fusion. The heat release makes the cloud methods for delivering the seeding agents.

7 8 Fog Modification

Fog has been modified to improve op- Seeding supercooled fog or a low stratus erations at many airports. In particular, the deck is an application of weather modifica- airline industry has benefited from this tion technology in which the effects are weather modification technology and now clearly demonstrated. Light aircraft are employs operational systems at several ma- often dispatched to fly over the fog deck and jor airports. The early emphasis was placed drop dry ice pellets. The resulting ice crys- on dissipation of cold fog, which consists of tals grow and fall as a light snow in 10 or 15 supercooled water droplets at temperatures minutes. The release of the snow produces a below 0°C (32°F). There has been some temporary clearing, which can be targeted to progress in the modification of warm fog, affect an airport runway. Occasionally, the but there is still no inexpensive efficient clearing may spread if it permits the sun to technology for dispersing warm fog on an reach the ground and raise the local tem- operational basis. perature.

Long rectangular holes cut through a stratus cloud using crushed dry ice at rate of about 1 kg/km. Such holes develop in less than an hour and become at least 2 km wide. Project Cirrus, 1974. Photo courtesy Houghton Mifflin Company, from “Atmosphere“ Peterson Field Guide, Vincent J. Schaefer/John A. Day, authors.

9 WMA Capability Statement on Weather Modification

Adopted at 1984 Annual Meeting Winter Precipitation Augmentation

CONTINENTAL—Evaluation of both research and operational winter orographic cloud seeding programs indicate that 5-20% Introduction seasonal increases in precipitation can be achieved. Detailed analyses of research pro- It has been established that weather can be grams demonstrate that both positive and modified by man under various circum- negative effects of seeding can occur over stances. The problem is one of stating under shorter time intervals such as individual storm what conditions predictable effects may be events. Consequently, it is prudent to adopt expected. The attainment of desirable weather seeding techniques and criteria, based upon modification effects depends upon several meteorological conditions, designed to opti- factors including: the prevailing weather re- mize the positive seeding effects during these gimes of a specific area, the design of a pro- short time intervals, thereby maximizing the gram to achieve a specified goal, the execu- seasonal increases in precipitation. tion of the program, and the specification of a means of assessing the effects of the weather COASTAL—Evaluations of both research modification effort. Brief capability state- and operational wintertime programs con- ments follow stating an assessment of the ducted in more coastal environments with current state of weather modification technol- more limited topographic relief indicate the ogy for a variety of applications. A word of potential of 5% to as much as 30% increases caution is necessary concering these state- in seasonal precipitation. Meteorological ments. This caution deals with the concept of situations that appear to offer the most poten- transferability of results. Differences in cloud tial in these areas are convective in nature. It microphysics, topography, seeding agent se- again appears prudent to adopt meteorologi- lection and dosage rates, and execution could cally based seeding guidelines for real time alter these expectations. seeding decision making in order to maximize the increases in season precipitation. Fog and Stratus Dispersal Summer Precipitation Augmentation The dispersal of shallow, cold (below freezing) fog or stratus cloud decks is an es- The capability to augment summertime tablished operational technology. Dispensing precipitation in an area-wide fashion is prom- ice phase seeding agents, such as dry ice or ising. Assessments from some operational silver iodide, in these situations is effective in and some research programs are encouraging, improving visibility. Clearings established in especially when a seeding mode is employed cloud decks embedded in strong wind fields which allows selective seeding of individual fill in quickly unless seeding is done nearly clouds. continuously. Evaluations of operationally conducted The dispersal of warm (above freezing) fog summer precipitation augmentation programs or stratus decks over areas as large as airport present a difficult problem due to their non- runways is feasible operationally through the randomized nature and the normally high provision of a significant heat source. The variability (temporal and spatial) present in mixing of drier air by helicopter downwash summertime rainfall. Recognizing these can create localized clearings. Various hygro- evaluation limitations, the results of many of scopic substances have also been used to im- these evaluations have indicated a positive prove visibility in these situations, primarily area-wide seeding effect in precipitation. by the military.

10 Results are mixed from research programs Hail Suppression conducted on summertime cumulus clouds. Part of the resulting uncertainty is due to the Most of what is currently known about the variety or climatological and microphysical status of hail suppression, either success or settings in which experimentation has been failure, has been acquired through study of conducted. Another important factor is seed- surface hail data in a project area during ing mode. Those projects that employed a seeding periods. Little has yet been shown broadcast mode of dispersal of a glaciogenic through careful study of the physical behavior seeding material have generally indicated no of the interior of storms from the suppression effect or even decreases in rainfall. Projects efforts. Therefore, the scientific linkages which relied upon injection of glaciogenic establishing hail suppression are not well seeding material directly into clouds that met established, although the assessment of sur- certain seeding criteria (based essentially face hail differences are generally suggestive upon the stage of development of the cloud) of successful suppression in the realm of 20- generally indicate positive seeding effects. 50% reduction. Execution of the operations is These effects were normally observed on at important. Timing and correct placement of least the seeded cloud’s rainfall and often- seeding material are especially critical to suc- times in area-wide rainfall. cessful suppression.

Government Involvement

Thirty-two states have enacted legislation • U.S. Dept. of Interior dealing with weather modification. These Bureau of Reclamation state laws vary from simple reporting re- P.O. Box 25007 quirements to more extensive regulations Denver, Colorado 80225 which involve weather modification li- censes, permits and the publication of no- tices of intent. Several states have devel- • National Science Foundation oped statutes which permit the use of state Division of Atmospheric Science funds for state/county cost sharing weather 4201 Wilson Boulevard modification operations. The Weather Arlington, Virginia 22230 Modification Association can supply the name and address of the agency in your state from which you can obtain copies of any regulatory procedures. U.S. Government agencies which have been involved in weather modification re- search and/or operations include:

• U.S. Air Force Headquarters, Air Weather Service Scott Air Force Base Belleville, Illinois 62220

• National Oceanic and Atmospheric Administration Office of Oceanic and Atmospheric Research R/PDC/USWRP, Rm. 11554 1315 East-West Highway Inadvertant Weather Modification Silver Spring, Maryland 20910 Telephone: 301-713-0460

11 Certification

One of the purposes of the Weather CATEGORY C: A Master’s Degree or a Modification Association is to encourage Doctorate in Atmospheric Science and and promote the highest standards of con- three years (36 active months) experience duct. In order to further this goal and to in weather modification research and/or protect the public interest, the WMA has operations. established a certification program for indi- viduals qualified to manage and/or operate In addition to the above requirements, weather modification field programs of a the applicants must pass written and oral research or operational nature. examinations to the satisfaction of the Cer- Certification is based on character, tification Committee. knowledge and experience. All applicants must agree to accept and abide by the cur- Weather Modification Operator rent WMA Code of Ethics and any State- ment of Standards and Practices that the CATEGORY A: Twenty months actual WMA has adopted at the time of applica- “in the field” experience in weather tion. Minimum requirements are: modification research or operations.

Weather Modification Manager CATEGORY B: A degree with at least 25 seemster hours of meteorology and CATEGORY A: Eight years (96 active eight months actual “in the field” experi- months) experience in weather modifica- ence in weather modification research or tion research and/or operations. operations.

CATEGORY B: A Bachelor’s Degree The field experience must be in a project with at least 25 semester hours of mete- designed to effect a change in the weather. orology plus five years (60 active It should involve significant exposure to months) experience in weather modifica- how seeding decisions are made and how a tion research and/or operations. project is managed.

The following lists show the managers and operators whose certifications are current in 1996.

CERTIFIED MANAGERS CERTIFIED OPERATORS

Keith J. Brown Thomas J. Henderson Port Ludlow, WA Atmospherics Inc. Thomas J. Henderson Donald W. Duckering Atmospherics Incorporated Fresno, CA Robert D. Elliott Don A. Griffith Montecito, CA North American Weather Consultants Don A. Griffith H. Robert Swart North American Weather Consultants State of Utah, Division of Air Quality Edward E. Hindman John R. Thompson City College, New York City North American Weather Consultants Mark E. Solak North American Weather Consultants James A. Heimbach, Jr. University of North Carolina, NC Bruce A. Boe North Dakota Atmospheric Resource Board

12 CERTIFIED OPERATORS continued John Girdzus North American Weather Consultants Albert Schnell AIRAO Incorporated Ray Pat Jones Colorado River Municipal Water District David L. Newsom Atmospherics Incorporated William J. Hauze North American Weather Consultants Mark E. Solak North American Weather Consultants Patrick H. Sweeney Weather Modification Incorporated Danny A. Risch North American Weather Consultants Peter E. Murphy Melbourne Water, Australia Einar L. Hovind Santa Barbara, CA John Barry-Murphy Melbourne Water, Australia

Corporate Members of the Weather Modification Association

Aero Systems, Inc. Oklahoma Water Res. Board Attn: Sharon Carley Attn: Mr. Michael E. Mathis 2580 So. Main St. 3800 North Classen Boulevard Erie, CO 80516 Oklahoma City, OK 73118

Aerotech Corporation Sacramento Municipal Util. Dist. Attn: Mr. Don Wells Attn: Mr. Al Ortega P O Box 334 P.O. Box 15830 Rosney Park, Tasmania Sacramento, CA 95813 7018 AUSTRALIA Southern Calif. Edison Co. Atmospherics Incorporated Attn: Mr. Brian M. McGurty, Rm. 162 Attn: Mr. Thomas J. Henderson P.O. Box 800 5652 East Dayton Ave. Rosemead, CA 91770 Fresno, CA 93727 Travers & Associates Colorado International Corp. Attn: Glen Travers Attn: Dr. Larry G. Davis P.O. Box 26651 P.O. Box 3007 St. Louis, MO 63122 Boulder, CO 80307 Utah Div. of Water Resources Desert Research Institute Attn: Dr. Norman Stauffer, Jr. Attn: Dr. Arlen Huggins 1594 W. No. Temple - 3rd Floor P.O. Box 60220 P.O. Box 146201 Reno, NV 89506 Salt Lake City, UT 84114

Enterprise Electronics Corp. The Weather Modification Group Attn: Mr. Larry Collins Attn: Mr. R.P. Seller 128 S. Industrial Blvd. PO Bo x 576 Enterprise, AL 36330 Okotoks, Alberta TOL 1TO Canada Los Angeles Dept. Water & Power Attn: Mr. Jim Perrault Weather Modification, Inc. P.O. Box 111 - Rm. 1466 Attn: Mr. Patrick H. Sweeney Los Angeles, CA 90051 3802 20th Street North Fargo, ND 58102 North American Weather Conslt. Attn: Mr. Don Griffith Western Kansas Grndwater Dist 1 1293 West 2200 South Attn: Mr. Keith Lebbin Salt Lake City, UT 84119-1471 Box 604 Scott City, KS 67871 North Dakota Atmospheric Res.Brd. Attn: Mr. Bruce Boe, Director 900 East Blvd. Ave. Bismarck, ND 58505

13 Answers to Questions Most Often Asked

Q. Does cloud seeding really work? Q. Is such a concentration harmful to people or the environment? A. Yes! Fifty years of research and opera- tions in 43 countries have demonstrated that A. No. The silver concentration in rainwater properly designed programs operated by com- from a seeded storm is well below the acceptable petent persons can dissipate supercooled fog, concentration of 50 micrograms per litre as set beneficially increase seasonal rainfall or snow- by the U. S. Public Health Service. Many re- fall, and decrease seasonal hail damage. gions have much higher concentrations of silver in the soil than are found in precipitation from Q. Does seeding clouds to suppress hail seeded clouds. The concentration of iodine in reduce rainfall? iodized salt used on food is far above the con- centration found in rainwater from a seeded A. Seeding for hail suppression is intended storm. No significant environmental effects to increase the number of precipitation particles, have been noted around operational projects, which tends to increase the efficiency with including projects of 30 to 40 years duration. which the clouds convert cloud droplets into rain. The available evidence suggests that Q. What about downwind effects? Do you seeding for hail suppression, if anything, in- sometimes "rob Peter to pay Paul?" creases rather than reduces rainfall from the seeded clouds. Nothing indicates that seeding A. No. The idea that rainfall increases in clouds to suppress hail actually breaks up the one area must be offset by decreases elsewhere clouds. is a misconception. Precipitation data from a number of cloud seeding projects have been Q. Is there a large amount of silver iodide examined in detail for evidence of "extra-area" or other seeding material in the precipitation effects. In some cases, there have been weak which falls from seeded clouds? indications of increased precipitation at dis- tances of 150 km (90 miles) or more downwind A. No. The amounts are very small. The from the target areas. There are no significant typical concentration of silver in rainwater or indications of rainfall decreases downwind snow from a seeded cloud is less than 0.1 mi- from any long term cloud seeding projects. crograms per litre (one part in 10,000,000,000).

14 Q. If microscopic particles Q. In that case, does natural in the atmosphere have such a weather exist any more? strong influence on Nature's ability to produce clouds and A. If "natural weather" means precipitation, don't all of the weather completely unaffected by particles from homes, industry, human beings, there has been no automobiles, and even campfires such thing since human beings in wilderness areas influence learned to use fire. However, the weather? impacts of human activities on weather and climate were much A. Yes. Every particle in smaller before the Industrial the atmosphere, regardless of Revolution than they are today. how it originates, may influence the formation and growth of clouds, including their ability to produce precipitation and to modify the radiative properties of the atmosphere.

Recent Weather Modification Activities in the United States Number, Purpose, and Total Target Area of Weather Modification Activities by State for 1994 (Source: Golden, NOAA) No. of State Activities Purpose Target Area

California 15 Increase precipitation 15,934 Utah 3 Increase precipitation/Fog displ. 10,360 Kansas 1 Hail suppression* 12,000 North Dakota 2 Hail suppression* 9,203 Idaho 3 Increase precipitation 3,941 Nevada 3 Increase precipitation 4,923 Colorado 4 Increase precipitation 550 Texas 1 Increase precipitation 5,500 Wyoming 1 Increase precipitation 939 Washington 1 Cold fog dispersal 50 Oregon 1 Cold fog dispersal 10 Georgia 1 Cold fog dispersal 1

12 states 36 Total 63,592

* The purpose of the hail suppression projects was also to increase rainfall. Note: Target areas for 3 activities overlapped state boundaries (2 for CA/NV; 1 for ID/WY).

15 Suggestions for Further Reading

Battan, Louis J., 1985: Schaefer, V. J., and J. A. Day, Weather. Prentice-Hall, Inc., 1981: A Field Guide to the Englewood Cliffs, NJ 07632. Atmosphere. Houghton Miflin 135 pp. Company, Boston, MA.

Dennis, A. S., 1980: Weather American Society of Civil Engi- Modification by Cloud Seeding neers, 1995: Guidelines for [International Geophysics Se- Cloud Seeding to Augment Pre- ries No. 24]. Academic Press, cipitation. ASCE: 1801 Alexan- New York. der Bell Drive, Reston, VA 20191-4400, 1-800-548-ASCE. Klein, D. A., ed., 1978: Envi- ronmental Impacts of Artificial Weather Modification Associa- Ice Nucleating Agents. Aca- tion, Journal of Weather Modi- demic Press, New York. 272 pp. fication, Volumes 1-28, 1969- 1996. P.O. Box 26926, Fresno, CA 93729-6926.

16 Water for the future

17 Weather Modification Association An international organization promoting research, development and application of weather modification. POST OFFICE BOX 26926 FRESNO CALIFORNIA 93729-6926 USA Fax & Phone: 209-434-3486 email: [email protected]

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