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---Cloud Seeding Frequently Asked Questions---

Cloud seeding (also known as weather modification)  is the deliberate treatment of certain clouds or cloud systems with the intent of affecting the precipitation process(es) within those clouds.  Application of this technology is increasing world-wide.  This page addresses some key questions relating to cloud seeding and its practical uses. References are provided at the end of this document keyed to numbers at the end of the highlighted topics. A brief summary of the scientific basis for cloud seeding is available here.


When did application of modern cloud seeding technology begin?

Attempts to modify the weather have been conducted for centuries.  However, modern cloud seeding dates from the late 1940's, springing from a discovery at the General Electric labs in Schenectady, New York in 1946.  The ability of dry ice shavings to convert supercooled water droplets (those existing as water at temperatures colder than freezing) to ice crystals was observed during the conduct of an unrelated experiment.  Later consideration of those observations led to a series of laboratory trials which demonstrated the nucleating properties of various materials in certain cold cloud conditions.  Trials in the atmosphere soon followed, and operational cloud seeding programs began in about 1950.

            Reference: 1, 2.


What are the most common applications of cloud seeding technology?

The most common intended effects of cloud seeding include precipitation increase (rain and/or snow), fog dispersal (visibility improvement) and hail suppression.  Of these, the majority of operational projects focus on precipitation increase.

            Reference: 1, 2.


Is cloud seeding effective?

Trials conducted by various researchers under laboratory conditions have documented the effects of cloud seeding materials.  Numerous scientific experiments have been conducted to investigate/demonstrate the effects produced by cloud seeding in the atmosphere on various cloud types in a variety of climatic regions.  Evaluations (usually statistical) have been made of many operational programs.  The results of many of these efforts have been published in the scientific literature and in industry journals. 

Summarizing the collective evidence from the various studies and operational projects, Capability Statements have been published by the Weather Modification Association, the American Meteorological Society and the World Meteorological Organization.  For precipitation augmentation, the accepted magnitude of increase to be expected from well-designed and properly conducted projects ranges from 5% to 20% for winter precipitation in continental regions and from 5% to 30% for coastal areas.   For warm season precipitation increase, single-cloud experiments have indicated increases as large as 100%.  Area wide increases over a project area vary with the frequency of occurrence and spatial coverage of suitable cloud systems, plus the ability to treat all favorable clouds.   Hail suppression effectiveness, based upon surface hail data, is estimated to be in the range of a 20-50% reduction.

            Reference: 3.


Who conducts cloud seeding activities?

The large majority of cloud seeding projects are conducted by a handful of highly specialized commercial firms, working under contract to a variety of sponsors.  Some water agencies and hydroelectric power generation companies conduct their own programs.  Researchers continue to conduct occasional trials within carefully designed and controlled experimental projects, striving to better understand the various in-cloud effects of cloud seeding, and to refine quantitative estimates of cloud seeding effectiveness. 

            Reference: 4.


Who are the most common sponsors of cloud seeding projects?

The most common sponsors of cloud seeding projects include water agencies, municipalities, operators of hydroelectric power facilities, agricultural or ranching interests, airports and recreational interests such as ski areas.  An increasing number of sponsors are incorporating cloud seeding as an integral part of their ongoing water resource management strategies.

            Reference: 5.


Why is cloud seeding so attractive to the increasing number of sponsors?

Cloud seeding is a highly portable and flexible technology.  It does not require construction of large, permanent and costly structures, such as dams or water conveyance systems.  Projects can be mobilized quickly and operations can be regulated as water needs dictate or suspended very quickly if hazardous weather conditions develop.  Comprehensive laboratory and field studies have indicated no significant environmental impacts.  Further, the benefit/cost ratios associated with most cloud seeding projects are typically very favorable, ranging as high as 25-30:1, depending, in the case of precipitation increase applications, on the value of water.

            Reference: 5, 6, 7.

 How widely used/accepted is cloud seeding technology?

Over its history of 50 years, modern cloud seeding has involved projects of various types in nearly 50 countries around the world.  Some individual projects have been in operation nearly continuously for decades, with a few operating for nearly fifty years.  As water needs increase world-wide, the demand for weather modification services will also increase.  Similar increase in demand will occur in hail-prone regions.

            Reference: 5, 8.


 How is the cloud seeding accomplished?

Cloud seeding materials are released via ground-based and/or airborne systems (see pictures).  Determination of the best suited method or combination of methods for a given project is based upon an assessment of a variety of factors.  The seeding materials are applied to the clouds (sometimes targeted very carefully into very specific portions of clouds) so that the material has adequate time to affect the precipitation process, so the effect will be focused over the intended geographic area.

            Reference: 1, 4, 5.


What are the most commonly used seeding materials?

The materials used in cloud seeding include two primary categories, tied to the type of precipitation process involved.  One category includes those which act as glaciogenic (ice-forming) agents, such as silver iodide, dry ice and compressed liquid propane or carbon dioxide, which are appropriate in cloud systems where the precipitation process is primarily cold (colder than freezing).  Of the ice-forming materials, the most commonly used is silver iodide.  The second major category is focused on cloud systems where the warm (coalescence) process predominates.  In those environments, hygroscopic (water attracting) materials such as salt, urea and ammonium nitrate can be utilized.  Of the hygroscopic materials, the most commonly used are salts.

            Reference: 5.


How does cold cloud seeding work?

In cold cloud seeding. the introduction of an ice-forming nucleating agent, e.g., silver iodide, into the appropriate cloud regions causes supercooled liquid water droplets to freeze.  Once these droplets freeze, the initial ice embryos grow at the expense of the water droplets around them (sublimation) and through contact with these neighboring water droplets (riming).  These embryos, if they remain in favorable cloud conditions, will grow into snowflakes, falling to the surface as snow if surface temperatures are below or near freezing, or as raindrops at warmer surface temperatures.  This process mimics nature where certain airborne substances, e.g., soil particles have the ability to act as ice-forming nuclei and initiate the freezing process.  A secondary effect of this process can occur, wherein the freezing of water droplets releases latent heat of fusion into the cloud.  This addition of heat, under the right circumstances, can cause the treated clouds to grow larger and last longer than would have naturally occurred. 

The first freezing process is often referred to as a static seeding effect, increasing the efficiency of the precipitation process within the seeded cloud volume.  The second freezing process, resulting from release of additional heat into the cloud, is often called the dynamic effect, whereby the treated clouds are invigorated, thus processing more moisture.

Although one might initially develop the impression that cold cloud seeding is strictly appropriate to winter clouds, it can work equally well during the summer when the precipitation forming process is active in the upper (colder) portions of cumulus clouds and adequate amounts of supercooled water droplets are available.

            Reference: 1, 2, 5.


  How does warm cloud seeding work?

Nature can produce rainfall from clouds that are warmer than freezing.  Tiny water droplets that form during condensation and define the cloud can grow as they collide with one another within the cloud.  This process is known as collision/coalescence.  Cloud seeding of this type of cloud involves introduction of additional condensation nuclei (e.g., salt particles) which can cause additional water droplets to condense within the cloud.  Various modeling and research studies have indicated that this type of seeding is effective in continental clouds, but ineffective in maritime clouds.

            Reference: 1, 2, 5.


 Do the commonly used seeding materials pose any direct health or environmental risks?

Many detailed studies have been conducted to address these questions.  These efforts have ranged from chemistry-focused work to broad ranging environmental investigations.  The bottom line is that no significant environmental effects have been observed.  Seeding materials are applied in very small amounts relative to the size of the geographic areas being affected, so the concentrations of the seeding materials in rainwater or snow are very low.  Using silver iodide (the most common seeding material) as an example, the typical concentration of silver in rainwater or snow from seeded cloud systems is less than 0.1 micrograms per liter.  This is much below the U.S. Public Health Service=s stated acceptable concentration of 50 micrograms per liter.  As another example, the concentration of iodine in rainwater from seeded clouds is far below the concentration found in common iodized table salt. 

            Reference: 1, 2, 5, 9.

 Does cloud seeding rob Peter to pay Paul?

The answer to this commonly-posed question is no.  Of the total atmospheric moisture passing over any point, the proportion falling as natural precipitation is quite small, typically less than 10-15%.  Cloud seeding-induced increases in precipitation of the order of 5-30% still results in a small overall proportion (<20%) of the total available moisture reaching the ground.  Further, especially when cumuliform clouds are present, and over mountainous terrain where air is forced to rise, the cloud-bearing layer of the atmosphere undergoes nearly continuous moisture replenishment.  Analyses of precipitation data from areas downwind of several cloud seeding projects have indicated small percentage precipitation increases extending as far as 100 miles downwind of the intended areas of effect on projects that had indications of increases in the intended target area.

            Reference: 1, 10, 11, 12, 13.


  Are cloud seeding activities subject to regulation or control?

In many jurisdictions, government agencies are responsible for regulation of cloud seeding in the public interest.  These agencies commonly require licenses and/or permits for cloud seeding, to help assure that the projects are properly designed and that those conducting such operations are properly qualified.  In the U.S., for example, nearly two-thirds of the fifty states have developed rules and regulations specific to cloud seeding activities.  These regulatory groups generally also maintain records of cloud seeding activities.

            Reference: 5.


What does cloud seeding cost?

The cost of cloud seeding varies greatly, depending on a large number of factors, such as which seeding methods and materials are appropriate to a specific application, the frequency of seedable conditions, the size of the intended area of effect and the duration of the project.  Most cloud seeding projects carry favorable benefit/cost ratios, ranging over 20:1 in some cases.  Cost questions are best addressed via direct discussion with a well qualified cloud seeding company/consultant.


What does the future hold for the cloud seeding field and its sponsors? 

As water needs steadily increase worldwide, the demand for weather modification services will also increase.  Focused research efforts will continue to yield incremental refinements to the technology.  Sponsors will increasingly enjoy the benefits of cloud seeding at very attractive benefit/cost ratios, and a growing number of those sponsors will incorporate cloud seeding as an integral part of their ongoing water resource management strategies.




                  1.         Dennis, A.S. , 1980: Weather Modification by Cloud Seeding.  Academic Press,  New York.

2.         Hess, W.N. , 1974: Weather and Climate Modification.  John Wiley & Sons, New  York. 

3.         Weather Modification Association, 1992: Weather Modification Capability Statement.  Journal of Weather Modification, Vol.  24, No.  1. 

4.         Weather Modification Association, 1997: Some Facts About Seeding Clouds.  Fresno, California. 

5.         American Society of Civil Engineers, 1995: Guidelines for Cloud Seeding to Augment Precipitation.  ASCE Manuals and Reports on Engineering Practice No.  81..

6.         Griffith, D.A. and M.E. Solak, 1999: A Cloud Seeding Program to Enhance Hydroelectric Power Production from the El Cajon Drainage, Honduras.  Seventh   Conference on Weather Modification, Chiang Mai, Thailand, February 17-22, 1999. 

7.         Stauffer, N.E. and K.  Williams, 2000: Utah Cloud Seeding Program, Increased Runoff/Cost Analysis.  Technical Report, Utah Department of Natural Resources,  Division of Water Resources. 

8.         Weather Modification Advisory Board, 1978: The Management of Weather Resources.  Report to the Secretary of Commerce, 2 volumes. 

9.         Klien, D.A. , 1978: Environmental Impacts of Artificial Ice Nucleating Agents.  Dowden, Hutchinson & Ross, Inc. , Stronsburg, Pennsylvania. 

               10.         Brown, K.J., R.D. Elliot and M.W. Edelstein, 1978: Transactions of Total-Area Effects of Weather Modification.  Report to the National Science Foundation on a workshop held August 8-12, 1977, Fort Collins, Colorado. 

               11.         Brown, K. J., R.D. Elliott and J.R. Thompson, 1974: The Seeding of Convection Bands.  AMS Fourth Conference on Weather Modification, Ft. Lauderdale, Florida, November 18-21, 1974.

               12.         Long, A. B., 2001: Review of Persistence Effects of Silver Iodide Cloud Seeding.   Journal of Weather Modification, Vol.   33, No.  1. 

               13.         Solak, M. E., D. P. Yorty, and D. A. Griffith, 2003: Estimations of Downwind Cloud Seeding Effects in Utah.   Journal of Weather Modification, Vol.   35, No.  1. 


                  How can I obtain more specific information? 

The Weather Modification Association (WMA) has produced a basic informational booklet, Weather Modification: Some Facts About Seeding Clouds.  The WMA can be contacted at 559-434-3486 or via e-mail at 

The American Society of Civil Engineers (ASCE) has published a comprehensive manual, Guidelines for Cloud Seeding to Augment Precipitation, which contains much useful information.   The ASCE telephone number is 800-548-2723, and their web site is

For those wishing to investigate the possibility of developing and implementing a specific project,  we recommend that you contact us, North American Weather Consultants, an industry pioneer and leader since 1950, to discuss your cloud seeding needs and interests.   As the longest-standing, continuously-operating cloud seeding company in the U.S., and probably the world, we have amassed a track record of many hundreds of successful research and operational project seasons in a wide variety of climatic regions world-wide.  We would be happy to discuss your specific needs and interests, and help with all aspects of cloud seeding issues, from basic questions to feasibility studies to full service cloud seeding projects.  Our projects are never of the cookie cutter variety.  We will work closely with you to design and implement a project which is tailored specifically to your circumstances.  Contact information is provided below. 


North American Weather Consultants, Inc.

8180 South Highland Dr., Suite B-2
Sandy, Utah 84093

Telephone:   801-942-9005

Facsimile:   801-942-9007


Contacts:  Garrett Cammans, President

Copyright 2021, North American Weather Consultants Inc., Sandy, Utah, USA.

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