The sound of cannon fire might one day be part of every farmer’s morning chores, as some countries are retooling anti-aircraft guns to shoot canisters of silver iodide into the air in an effort to increase rainfall to targeted areas. China, in the midst of drought conditions, participates regularly in weather modification. The organizers of the 2008 Olympics plan to force rainfall, through cloud seeding, in Beijing in the days prior to the opening ceremonies. Not only will the forced rainfall increase the likelihood of clear skies for the games, but it will help to clear the air of pollution for a few days.

Cloud seeding is not just for increasing rainfall. Evidence supports this technology as effective in fog and pollution disbursement, hail suppression and severe storm modification. However, because of the complexity of measuring rainfall, scientists are at odds as to whether cloud seeding can truly increase precipitation.

In 1946 Dr. Vincent J. Schaefer, a chemist with General Electric in New York, tried to create artificial clouds in the laboratory. He cooled the test chamber with dry ice and noticed the formation of water vapor. Rainfall works in a similar way. Ice crystals form when cold water contacts particles of dust, salt, or sand.  These ice crystals are the nuclei for gathering water droplets, which eventually become large enough to fall as rain. The freezing releases a large amount of heat, which in turn makes the cloud more buoyant and may cause it to grow larger (taller and/or wider), thereby processing water at a higher rate, more efficiently, and for a longer period of time than without seeding.

The theory behind ‘cold rain’ cloud seeding is that increasing the number of these nuclei allows the moisture in the cloud to form more raindrops. Silver iodide is similar in structure to ice crystals and can also be used as nuclei. 

The silver iodide is usually released from devices called liquid fuel generators, or pyrotechnic flares. These can be released from either ground-based devices  (including use of artillery and ground-to-air rockets) or from flares released from aircraft. Silver iodide flares are ignited and dispersed as an aircraft flies through a cloud. When released by devices on the ground, the fine particles are blown downwind and upwards by air currents after release.

Rack of 12 hygroscopic flares, one recently ignited. Courtesy of National Center for Atmospheric Research/Research Applications Laboratory.

Growing cumulus, located over the desert of the United Arab Emirates. Courtesy of National Center for Atmospheric Research/Research Applications Laboratory.

Photos courtesy of Weather Modification Association,

http://weathermodification.org

Cold rain cloud seeding is particularly effective in mountainous area, where clouds keep up to 90%  of their moisture aloft until it evaporates. These clouds, call orographic clouds, are formed when air lifts and cools as it passes across mountain ranges. Orographic clouds often do not contain enough ice particles to convert the air into precipitation.  For effective targeting in such areas, cloud seeding operators must consider the complexities of air flows over mountains.

Cumulous clouds, which produce most of the rainfall over land, form in updrafts of unstable airmasses. As the sun heats the ground, an airmass can become unstable and create a cumulous cloud. Cold fronts can also create the relevant unstable conditions for these clouds. Like the orographic clouds over mountain ranges, cumulous clouds can be seeded with silver iodide and dry ice to stimulate rainfall. When the temperature of the cloud is below -5°C (23°F), the seeding increases the number of ice particles, which grow rapidly in the humidity laden, supercooled air. As it falls, the ice particle mingles with other ice particles and droplets, increasing in size. If an ice particle encounters temperatures above 0°C (32°F) during its fall, it may melt to a raindrops.

Cloud seeding has also been used to inhibit hailstorms and to disperse fog in areas such as airports. Unlike precipitation, the effects of weather modification on fog can be clearly demonstrated and calculated. For airports, dry ice pellets released from light aircraft can produce snow within 10 to 15 minutes and effect a clearing on an area as specific as a runway.

Simply creating the artificial nuclei is not enough to guarantee successful weather modification. Factors such as cloud size, cloud lifetime, and the sizes and concentrations of the droplets and ice particles all affect the initiation and amount of precipitation. Because of these variations, some scientists believe that the positive effects of cloud seeding are simply wishful thinking. Edwards Aquifer Agency General Manager Greg Ellis said "Here, we have a very unique situation with our rain gauges, our stream flow measures and our aquifer measures to actually determine as that rain is falling how much of that is benefitting the region, how much is actually getting to the ground and into the aquifer.” Despite this seemingly unique precipitation gauge, the Edwards Aquifer budget for cloud seeding was nearly dropped in 2004, due to the National Academy of Science’s findings that cloud seeding could not be proven as an effective way to increase precipitation.

Why, when the theory is sound and laboratory trials seem to back it, can cloud seeding not be proven effective? The main problem behind evaluating the success of cloud seeding (and any weather modification) is determining two criteria. One, is the increase/decrease in precipitation due to weather modification or natural variances? Two, if there is an actual increase in precipitation, is it affecting the targeted area?

After sixty years of experimentation, most authorities such as the American and World meteorological societies agree that cloud seeding increases rainfall by an average of 10%. However, specialists warn against using the technology as a political placebo, in other words as a placation to people and farmers who must watch their land dry up in times of drought.

Other concerns for cloud seeding is its effect on the environment. At every turn, we are told that spraying chemicals into the atmosphere is harmful. However, the types of chemicals used in cloud seeding and their quantities have shown no impact on land, water or air pollution. The silver concentration in rainwater from a seeded storm is well below the acceptable concentration of 50 micrograms per litre as set by the U. S. Public Health Service.  Many regions have much higher concentrations of silver in the soil than are found in precipitation from seeded clouds. The concentration of iodine in iodized salt used on food is far above the concentration found in rainwater from a seeded storm.  No significant environmental effects have been noted around operational projects, including projects of 30 to 40 years duration.

Another weather modification fallacy is the idea of rain “stealing” or the idea that rainfall increases in one area must be offset by decreases elsewhere. Precipitation data from a number of cloud seeding projects have been examined in detail for evidence of "extra-area" effects.  In some cases, there have been weak indications of increased precipitation at distances of 150 km (90 miles) or more downwind from the target areas.  There are no significant indications of rainfall decreases downwind from any long term cloud seeding projects.

Does natural weather exist any more?

“Natural” weather is a misnomer. Every living creature affects weather to a certain extent. Every particle in the atmosphere may impact positively or negatively, on the formation of clouds and precipitation. Since the first humans learned the skills to make fire, man has been seeding the atmosphere. This impact on weather and environment has been drastically increased since the Industrial Revolution. With or without silver iodide flares, humans affect weather. The difference with cloud seeding is that the modifications are relatively harmless to the environment. They are temporary, quantifiable (with the proper research techniques and conditions) and limited to controlled areas.

Cloud Seeding References

1967 to 972, the US military Operation Popeye cloud seeded silver iodide to extend the monsoon season over North Vietnam, specifically the Ho Chi Minh Trail. The operation resulted in the targeted areas seeing an extension of the monsoon period an average of 30 to 45 days.

1971 - Colorado River Municipal Water district shows long term effects of cloud seeding to be an increase of 34% in precipitation.

1978 - The U.S. signed an international treaty banning the use of weather modification for hostile purposes, though the USAF proposed cloud seeding on the battlefield in 1996.

1986 to 1994 Oklahoma and Texas joint project precipitation in cloud seeded areas increased by 43% as compared to non-seeded areas.

1986 - Russian military pilots seeded clouds over Belarus after the Chernobyl Disaster to remove radioactive particles from clouds heading toward Moscow.

2001 Edward Aquifer began a study to determine the efficiency of cloud seeding. 2005 EAA eliminated all but $153 520 from its budget for cloud seeding, after finding little evidence of its success.

2003 - National Academy of Sciences concluded that there is no scientific evidence to show that cloud seeding actually increases precipitation.

1995 to 2003 China spent 266 million dollars on rain-making technology in 23 provinces and regions and now boasts some 35,000 people who work in the field.

2006 - The government of Australia announced AUD$7.6 million in funding for "warm cloud" seeding research to be conducted jointly by the Australian Bureau of Meterology and the United States National Center for Atmospheric Research.

2007 Nevada State Cloud Seeding program hopes to Augment snowfall in selected mountainous regions of Nevada to increase the snowpack, the resultant spring runoff and the water supplies of municipalities, agricultural regions, recreational lakes, and environmentally threatened terminal lakes (Pyramid and Walker).

2007 -American Meteorological Society’s official position is that statistical evidence shows 10% increase in precipitation, but the actual cause and effect can not be proven.

U.S. Government agencies which have been involved in weather modification research and/or operations include: U.S. Air Force, National Oceanic and Atmospheric Administration, U.S. Dept. of Interior Bureau of Reclamation, National Science Foundation (Division of Atmospheric Science)

A number of commercial companies, such as Aero Systems Incorporated, Atmospherics Incorporated, North American Weather Consultants, Weather Modification Incorporated, Weather Enhancement Technologies International offer weather modification services centered on cloud seeding.

The Edwards Aquifer Website. http://www.edwardsaquifer.net/cloudseeding.html

Rainmaking in China by Global Directions Incorporatedhttp://www.thingsasian.com/stories-photos/2987

The Nevada State Cloud Seeding Program. http://cloudseeding.dri.edu/Program/Program.html

Battan, Louis J., 1985:  Weather.  Prentice-Hall, Inc., Englewood Cliffs, NJ  07632.  135 pp. 

Dennis, A. S., 1980:  Weather Modification by Cloud Seeding   [International Geophysics Series No. 24].  Academic Press, New York.

Klein, D. A., ed., 1978:  Environmental Impacts of Artificial Ice Nucleating Agents.  Academic Press, New York.  272 pp.

Schaefer, V. J., and J. A. Day, 1981:  A Field Guide to the Atmosphere.  Houghton Miflin Company, Boston, MA.

American Society of Civil Engineers, 1995: Guidelines for Cloud Seeding to Augment Precipitation. ASCE: 1801 Alexander Bell Drive, Reston, VA 20191-4400, 1-800-548-ASCE.

Weather Modification Association, Journal of Weather Modification, http://weathermodification.org/