The most common chemicals used for cloud seeding include silver iodide and dry ice (frozen carbon dioxide). The expansion of liquid propane into a gas has also been used and can produce ice crystals at warmer temperatures than silver iodide. The use of hygroscopic materials, such as salt, is increasing in popularity because of some promising research results.
Seeding of clouds requires that they contain supercooled liquid water—that is, liquid water colder than zero degrees Celsius. Introduction of a substance such as silver iodide, which has a crystalline structure similar to that of ice, will induce freezing (heterogeneous nucleation). Dry ice or propane expansion cools the air to such an extent that ice crystals can nucleate spontaneously from the vapor phase. Unlike seeding with silver iodide, this spontaneous nucleation does not require any existing droplets or particles because it produces extremely high vapor supersaturations near the seeding substance. However, the existing droplets are needed for the ice crystals to grow into large enough particles to precipitate out.
In mid-latitude clouds, the usual seeding strategy has been predicated upon the fact that the equilibrium vapor pressure is lower over ice than over water. When ice particles form in supercooled clouds, this fact allows the ice particles to grow at the expense of liquid droplets. If there is sufficient growth, the particles become heavy enough to fall as snow (or, if melting occurs, rain) from clouds that otherwise would produce no precipitation. This process is known as "static" seeding.
Seeding of warm-season or tropical cumuliform (convective) clouds seeks to exploit the latent heat released by freezing. This strategy of "dynamic" seeding assumes that the additional latent heat adds buoyancy, strengthens updrafts, ensures more low-level convergence, and ultimately causes rapid growth of properly selected clouds.
Cloud seeding chemicals may be dispersed by aircraft (as in the second figure) or by dispersion devices located on the ground (generators, as in first figure, or canisters fired from anti-aircraft guns or rockets). For release by aircraft, silver iodide flares are ignited and dispersed as an aircraft flies through the inflow of a cloud. When released by devices on the ground, the fine particles are carried downwind and upwards by air currents after release.
While cloud seeding has shown to be effective in altering cloud structure and size, and converting cloud water to ice particles, it is more controversial whether cloud seeding increases the amount of precipitation at the ground. Cloud seeding may also suppress precipitation.
Part of the problem is that it is difficult to discern how much precipitation would have occurred had the cloud not been "seeded." There are no discernible 'traces' of the effectiveness of recent cloud seeding in the Snowy Mountains Australia. Nevertheless, there is hope that winter cloud seeding over mountains will produce snow. This statement arises from partial interpretation of professional societies Weather Modification Association, World Meteorological Organization, and American Meteorological Society (AMS). The AMS states that there is statistical evidence for seasonal precipitation increases of about 10% with winter seeding , however, this clearly does not apply to all cloud seeding activities. The World Meteorological Organization has indicated that cloud seeding does not produce positive results in all cases and is dependent on specificity of clouds, wind speed and direction, terrain and other factors.
The National Center for Atmospheric Research (NCAR), an institution in Boulder, Colorado, has made some statistical analysis of seeded and unseeded clouds in an attempt to understand the differences between them. They have conducted seeding research in several countries that include Mali, Saudi Arabia, Mexico, South Africa, Thailand, Italy, and Argentina.
It has also been said that in the 2008 summer Olympics in Beijing clouds were seeded so that there will be no rain during the opening ceremony. The Chinese weather modification office rarely publishes in the open scientific literature and therefore their claims of success are widely disputed
With an NFPA 704 rating of Blue 2, silver iodide can cause temporary incapacitation or possible residual injury (e.g., chloroform) with intense or continued but not chronic exposure. However, there have been several detailed ecological studies that showed negligible environmental and health impacts. . The toxicity of silver and silver compounds (from silver iodide) was shown to be of low order in some studies. These findings likely result from the minute amounts of silver generated by cloud seeding, which are 100 times less than industry emissions into the atmosphere in many parts of the world, or individual exposure from tooth fillings.
In other areas, silver toxicity arising from cloud seeding was multiple times industry levels. Accumulations in the soil, vegetation, and surface runoff have not been large enough to measure above natural background. A 1995 environmental assessment in the Sierra Nevada of California and a 2004 independent panel of experts in Australia confirmed these earlier findings. The paper does include the names of the experts, their scientific qualifications or published research papers to support the assertion that cloud seeding will have no ecotoxic impacts or affect alpine waterways. A marked increase in algal growth in alpine waters coincides with the period of cloud seeding.
Cloud seeding over Kosciuszko National Park - a Biosphere Reserve - is problematic in that several rapid changes of environmental legislation were made to enable the 'trial'. Environmentalists are concerned about the uptake of silver in a highly sensitive environment affecting the pygmy possum amongst other species as well as recent high level algal blooms in once pristine glacial lakes. The ABC program Earthbeat on 17 July 2004 heard that not every cloud has a silver lining where concerns for the health of the pygmy possums was raised. Earlier research and analysis by the former Snowy Mountains Authority led to the cessation of the cloud seeding program in the 1950s with non-definitive results (http://cires.colorado.edu/~aslater/snowy.html). Formerly, cloud seeding was rejected in Australia on environmental grounds because of concerns about the protected species, the pygmy possum. (http://www.colongwilderness.org.au/RedIndex/NSW/Jagu99.htm)
Vincent Schaefer (1906–1993) discovered the principle of cloud seeding in July 1946 through a series of serendipitous events. Following ideas generated between himself and Nobel laureate Irving Langmuir while climbing Mt Washington in NH, Schaefer, Langmuir's Research Associate created a way of experimenting with supercooled clouds using a deep freeze unit lined with black velveteen. He tried hundreds of potential agents to stimulate ice crystal growth, i.e. salt, talcum powder, soils, dust and various chemical agents with minor effect. Then one hot and humid July day he wanted to try a few experiments at Schenectady's GE Research Lab. He was dismayed to find that the deep freezer was not cold enough to produce a cloud using breath air. He decided to move the process along by adding a chunk of dry ice just to lower the temperature. To his astonishment, as soon as he breathed into the chamber, a bluish haze was noted, followed by an eye-popping display of millions of tiny ice crystals, reflecting the strong light rays illuminating a cross-section of the chamber. He instantly realized that he had discovered a way to change supercooled water into ice crystals. The experiment was easily replicated and he explored the temperature gradient to establish the −40˚C limit for liquid water. Within the month, Schaefer's colleague, the noted atmospheric scientist Dr. Bernard Vonnegut (brother of novelist Kurt Vonnegut) is credited with discovering another method for "seeding" supercooled cloud water. Vonnegut accomplished his discovery at the desk, looking up information in a basic chemistry text and then tinkering with silver and iodide chemicals to produce silver iodide. Both methods were adopted for use in cloud seeding during 1946 while working for the General Electric Corporation in the state of New York. Schaefer's altered a cloud's heat budget, Vonnegut's altered formative crystal structure – an ingenious property related to a good match in lattice constant between the two types of crystal. (The crystallography of ice later played a role in Kurt Vonnegut's novel Cat's Cradle.) The first attempt to modify natural clouds in the field through "cloud seeding" began during a flight that began in upstate New York on 13 November 1946. Schaefer was able to cause snow to fall near Mt. Greylock in western Massachusetts, after he dumped six pounds of dry ice into the target cloud from a plane after a 60 mile easterly chase from the Schenectady County Airport.
Dry ice and silver iodide agents are effective in changing the physical chemistry of supercooled clouds, thus useful in augmentation of winter snowfall over mountains and under certain conditions, lightning and hail suppression. While not a new technique hygroscopic seeding for enhancement of rainfall in warm clouds is enjoying a revival, based on some positive indications from research in South Africa, Mexico, and elsewhere. The hygroscopic material most commonly used is salt. It is postulated that hygroscopic seeding causes the droplet size spectrum in clouds to become more maritime (bigger drops) and less continental, stimulating rainfall through coalescence.
From March 1967 until July 1972, 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. The 54th Weather Reconnaissance Squadron carried out the operation to "make mud, not war".
In 1969 at the Woodstock Festival, various people claimed to have witnessed clouds being seeded by the US military. This was said to be the cause of the rain which lasted throughout most of the festival. This remains one of the many conspiracy theories put forth by members of the Hippie movement at the time.
One private organization which offered, during the 1970s, to conduct weather modification (cloud seeding from the ground using silver iodide flares) was Irving P. Krick and Associates of Palm Springs, California. They were contracted by the Oklahoma State University in 1972 to conduct such a seeding project to increase warm cloud rainfall in the Lake Carl Blackwell watershed. That lake was, at that time (1972-73), the primary water supply for Stillwater, Oklahoma and was dangerously low. The project did not operate for a long enough time to show statistically any change from natural variations. However, at the same time, seeding operations have been ongoing in California since 1948.
An attempt by the United States military to modify hurricanes in the Atlantic basin using cloud seeding in the 1960s was called Project Stormfury. Only a few hurricanes were tested with cloud seeding because of the strict rules that were set by the scientists of the project. It was unclear whether the project was successful; hurricanes appeared to change in structure slightly, but only temporarily. The fear that cloud seeding could potentially change the course or power of hurricanes and negatively affect people in the storm's path stopped the project.
Two Federal agencies have supported various weather modification research projects, which began in the early 1960s: The United States Bureau of Reclamation (Reclamation; Department of the Interior) and the National Oceanic and Atmospheric Administration (NOAA; Department of Commerce). Reclamation sponsored several cloud seeding research projects under the umbrella of Project Skywater from 1964 to 1988, and NOAA conducted the Atmospheric Modification Program from 1979 to 1993. The sponsored projects were carried out in several states and two countries (Thailand and Morocco), studying both winter and summer cloud seeding. More recently, Reclamation sponsored a small cooperative research program with six Western states called the Weather Damage Modification Program , from 2002–2006.
Funding for research in the United States has declined in the last two decades. The Bureau of Reclamation sponsored a six-state research program from 2002–2006, however, called the Weather Damage Modification Program A 2003 study by the United States National Academy of Sciences urges a national research program to clear up remaining questions about weather modification's efficacy and practice.
Only the trial conducted in the Snowy Mountains produced statistically significant rainfall increases over the entire experiment.
An Austrian study to use silver iodine seeding for hail prevention ran during 1981–2000, and the technique is still actively deployed there.
The largest cloud seeding system in the world is that of the People's Republic of China, which believes that it increases the amount of rain over several increasingly arid regions, including its capital city, Beijing, by firing silver iodide rockets into the sky where rain is desired. There is even political strife caused by neighboring regions which accuse each other of "stealing rain" using cloud seeding About 24 countries currently practice weather modification operationally. China has used cloud seeding in Beijing just before the 2008 Olympic Games in order to clear the air of pollution. Again, see above for disputes regarding the Chinese claims.
In the United States, cloud seeding is used to increase precipitation in areas experiencing drought, to reduce the size of hailstones that form in thunderstorms, and to reduce the amount of fog in and around airports. Cloud seeding is also occasionally used by major ski resorts to induce snowfall. Eleven western states and one Canadian province (Alberta) have ongoing weather modification operational programs In January 2006, an $8.8 million cloud seeding project began in Wyoming to examine the effects of cloud seeding on snowfall over Wyoming's Medicine Bow, Sierra Madre, and Wind River mountain ranges.
A number of commercial companies, such as Aero Systems Incorporated , Atmospherics Incorporated , North American Weather Consultants , Weather Modification Incorporated , Weather Enhancement Technologies International , Seeding Operations and Atmospheric Research (SOAR) , offer weather modification services centered on cloud seeding. The USAF proposed its use on the battlefield in 1996, although the U.S. signed an international treaty in 1978 banning the use of weather modification for hostile purposes.
In Australia, CSIRO’s activities in Tasmania in the 1960s were successful. Seeding over the Hydro-Electricity Commission catchment area on the Central Plateau achieved rainfall increases as high as 30% in autumn. The Tasmanian experiments were so successful that the Commission has regularly undertaken seeding ever since in mountainous parts of the State.
Beginning in Winter 2004, Snowy Hydro Limited is conducting a six-year research project of winter cloud seeding to assess the feasibility of increasing snow precipitation in the Snowy Mountains in Australia. The NSW Natural Resources Commission, responsible for supervising the cloud seeding operations, believes that the trial may have difficulty establishing statistically whether cloud seeding operations are increasing snowfall. This project was discussed at a summit in Narrabri, NSW on 1 December 2006. The summit met with the intention of outlining a proposal for a 5 year trial, focussing on Northern NSW.
The various implications of such a widespread trial were discussed, drawing on the combined knowledge of several worldwide experts, including representatives from the Tasmanian Hydro Cloud Seeding Project however does not make reference to former cloud seeding experiments by the then Snowy Mountains Authority which rejected weather modification. The trial required changes to NSW environmental legislation in order to facilitate placement of the cloud seeding apparatus. The modern experiment is not supported for the Australian Alps.
At the July 2006 G8 Summit, President Putin commented that air force jets had been deployed to seed incoming clouds so they rained over Finland. Rain drenched the summit anyway.
In December 2006, the Queensland government of Australia announced AUD$7.6 million in funding for "warm cloud" seeding research to be conducted jointly by the Australian Bureau of Meteorology and the United States National Center for Atmospheric Research. Outcomes of the study are hoped to ease continuing drought conditions in the states South East region.
In India, Cloud seeding operations were conducted during the years 2003 and 2004 through U.S. based Weather Modification Inc. in state of Maharashtra In 2008, there are plans for 12 districts of state of Andhra Pradesh