Cloud seeding


Cloud seeding is a type of weather modification that aims to change the amount or type of precipitation, mitigate hail, or disperse fog. The usual objective is to increase rain or snow, either for its own sake or to prevent precipitation from occurring in days afterward.
Cloud seeding is undertaken by dispersing substances into the air that serve as cloud condensation or ice nuclei. Common agents include silver iodide, potassium iodide, and dry ice, with hygroscopic materials like table salt gaining popularity due to their ability to attract moisture. Techniques vary from static seeding, which encourages ice particle formation in supercooled clouds to increase precipitation, to dynamic seeding, designed to enhance convective cloud development through the release of latent heat.
Methods of dispersion include aircraft and ground-based generators, with newer approaches involving drones delivering electric charges to stimulate rainfall, or infrared laser pulses aimed at inducing particle formation. Despite decades of research and application, cloud seeding's effectiveness remains a subject of debate among scientists, with studies offering mixed results on its impact on precipitation enhancement.
Environmental and health impacts are considered minimal due to the low concentrations of substances used, but concerns persist over the potential accumulation of seeding agents in sensitive ecosystems. The practice has a long history, with initial experiments dating back to the 1940s, and has been used for various purposes, including agricultural benefits, water supply augmentation, and event planning. Legal frameworks primarily focus on prohibiting the military or hostile use of weather modification techniques, leaving the ownership and regulation of cloud-seeding activities to national discretion. Despite skepticism and debate over its efficacy and environmental impact, cloud seeding continues to be explored and applied in regions worldwide as a tool for weather modification.

Methods

Salts

The most common chemicals used for cloud seeding include silver iodide, potassium iodide and dry ice. Liquid propane, which expands into a gas, has also been used. It can produce ice crystals at higher temperatures than silver iodide. After promising research, the use of hygroscopic materials, such as table salt, is becoming more popular.
When cloud seeding, increased snowfall takes place when temperatures within the clouds are between −20 and −7 °C. Freezing nucleation is induced by the introduction of substances similar to silver iodide, which has a crystalline structure like ice.

Electric charges

Since 2021, the United Arab Emirates have been using drones equipped with a payload of electric-charge emission instruments and customized sensors that fly at low altitudes and deliver an electric charge to air molecules. This method may have contributed to a significant rainstorm in July 2021. For instance, in Al Ain it rained 6.9 millimeters on 20–21 July.

Infrared laser pulses

An electronic mechanism was tested in 2010, when infrared laser pulses were directed to the air above Berlin by researchers from the University of Geneva. The experimenters posited that the pulses would encourage atmospheric sulfur dioxide and nitrogen dioxide to form particles that would then act as seeds.

Effectiveness

Despite decades of research and application, cloud seeding's effectiveness remains a subject of debate among scientists, with studies offering mixed results on its impact on precipitation enhancement, according to a report issued by the US Government Accountability Office in December 2024. Whether cloud seeding is effective in producing a statistically significant increase in precipitation has been a matter of academic debate, with contrasting results depending on the study in question and contrasting opinion among experts.
A study conducted by the United States National Academy of Sciences failed to find statistically significant support for cloud seeding's effectiveness. Based on its findings, Stanford University ecologist Jerry Bradley said: "I think you can squeeze out a little more snow or rain in some places under some conditions, but that's quite different from a program claiming to reliably increase precipitation." Data similar to that of the NAS study was acquired in a separate study conducted by the Wyoming Weather Modification Pilot Project, but whereas the NAS study concluded that "it is difficult to show clearly that cloud seeding has a very large effect", the WWMPP study concluded that "seeding could augment the snowpack by a maximum of 3% over an entire season."
In 2003, the US National Research Council released a report stating, "science is unable to say with assurance which, if any, seeding techniques produce positive effects. In the 55 years following the first cloud-seeding demonstrations, substantial progress has been made in understanding the natural processes that account for our daily weather. Yet scientifically acceptable proof for significant seeding effects has not been achieved".
A 2010 Tel Aviv University study claimed that the common practice of cloud seeding to improve rainfall, with materials such as silver iodide and frozen carbon dioxide, seems to have little if any impact on the amount of precipitation. A 2011 study suggested that airplanes may produce ice particles by freezing cloud droplets that cool as they flow around the tips of propellers, over wings or over jet aircraft, and thereby unintentionally seed clouds. This could have potentially serious consequences for particular hail stone formation.
In 2016, Jeff Tilley, director of weather modification at the Desert Research Institute in Reno, claimed that new technology and research has produced reliable results that make cloud seeding a dependable and affordable water supply practice for many regions. Moreover, in 1998 the American Meteorological Society held that "precipitation from supercooled orographic clouds has been seasonally increased by about 10%."
Despite the mixed scientific results, cloud seeding was attempted during the 2008 Summer Olympics in Beijing to coax rain showers out of clouds before they reached the city in order to prevent rain during the opening and closing ceremonies. Whether this attempt was successful is a matter of dispute, with Roelof Bruintjes, who leads the National Center for Atmospheric Research's weather-modification group, remarking, "we cannot make clouds or chase clouds away".

Impact on environment and health

With an NFPA 704 health hazard rating of 2, silver iodide can cause temporary incapacitation or possible residual injury to humans and other mammals with intense or chronic exposure. But several detailed ecological studies have shown negligible environmental and health impacts. The toxicity of silver and silver compounds 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 about one percent of industry emissions into the atmosphere in many parts of the world, or individual exposure from tooth fillings.
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.
"In 1978, an estimated 3,000 tonnes of silver were released into the US environment. This led the US Health Services and EPA to conduct studies regarding the potential for environmental and human health hazards related to silver. These agencies and other state agencies applied the Clean Water Act of 1977 and 1987 to establish regulations on this type of pollution."
Cloud seeding over Kosciuszko National Parka biosphere reserveis problematic in that several rapid changes of environmental legislation were made to enable the trial. Environmentalists are concerned about the uptake of elemental silver in a highly sensitive environment affecting the pygmy possum, among other species, as well as recent high-level algal blooms in once pristine glacial lakes. Research 50 years ago and analysis by the former Snowy Mountains Authority led to the cessation of the cloud seeding program in the 1950s with non-definitive results. Formerly, cloud seeding was rejected in Australia on environmental grounds because of concerns about the pygmy possum. The claims of negative environmental impact are disputed by peer-reviewed research, as summarized by the International Weather Modification Association.

History

In 1891, Louis Gathmann suggested shooting liquid carbon dioxide into rain clouds to cause them to rain. During the 1930s, the Bergeron–Findeisen process theorized that supercooled water droplets present, while ice crystals are released into rain clouds, would cause rain. While researching aircraft icing, General Electric 's Vincent Schaefer and Irving Langmuir confirmed the theory. Schaefer discovered the principle of cloud seeding in July 1946 through a series of serendipitous events. Following ideas he and Langmuir generated while climbing Mt. Washington in New Hampshire, Schaefer, Langmuir's research associate, created a way of experimenting with supercooled clouds using a deep freeze unit of potential agents to stimulate ice crystal growth, i.e., table salt, talcum powder, soils, dust, and various chemical agents with minor effect. Then, on July 14, 1946, he wanted to try a few experiments at GE's Schenectady 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 of his experimental chamber. To his astonishment, as soon as he breathed into the deep freezer, he noted a bluish haze, followed by an eye-popping display of millions of microscopic ice crystals, reflecting the strong light rays from the lamp illuminating a cross-section of the chamber. He instantly realized that he had discovered a way to change super-cooled water into ice crystals. The experiment was easily replicated, and he explored the temperature gradient to establish the limit for liquid water.
Within the month, Schaefer's colleague, the atmospheric scientist Bernard Vonnegut, was credited with discovering another method for "seeding" super-cooled 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. Together with Professor Henry Chessin, of SUNY Albany, a crystallographer, he co-authored a publication in Science and received a patent in 1975. Both methods were adopted for use in cloud seeding during 1946 while working for GE in the state of New York.
Schaefer's method 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 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 Mount Greylock in western Massachusetts after he dumped of dry ice into the target cloud from a plane after a easterly chase from the Schenectady County Airport.
Dry ice and silver iodide agents are effective in changing super-cooled clouds' physical chemistry, and thus useful in augmenting winter snowfall over mountains and, under certain conditions, in lightning and hail suppression. While not a new technique, hygroscopic seeding for enhancement of rainfall in warm clouds is enjoying a revival, based on positive indications from research in South Africa, Mexico, and elsewhere.
The hygroscopic material most commonly used is table salt. It is postulated that hygroscopic seeding causes the droplet size spectrum in clouds to become more maritime and less continental, stimulating rainfall through coalescence. From 1967 to 1972, the U.S. military's Operation Popeye cloud-seeded silver iodide to extend the monsoon season over North Vietnam, specifically the Ho Chi Minh Trail. The operation extended the monsoon period an average of 30 to 45 days in the targeted areas. The 54th Weather Reconnaissance Squadron carried out the operation to "make mud, not war".
An attempt by the U.S. military to modify hurricanes in the Atlantic basin using cloud seeding in the 1960s was called Project STORMFURY. Scientists tested four hurricanes across eight days and observed decreased wind speeds of 10% to 30% on four of these days. They originally attributed the lack of results to faulty execution, but the results came into question because of the lack of supercooled water in the hurricane and the inability to determine if the effects were due to human intervention or the natural processes of hurricanes.
Two federal agencies have supported various weather modification research projects, which began in the early 1960s: The United States Bureau of Reclamation and the National Oceanic and Atmospheric Administration. 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, studying both winter and summer cloud seeding. From 1962 to 1988 Reclamation developed cloud seeding applied research to augment water supplies in the western U.S. The research focused on winter orographic seeding to enhance snowfall in the Rocky Mountains and Sierra Nevada, and precipitation in coast ranges of southern California. In California Reclamation partnered with the California Department of Water Resources to sponsor the Serra Cooperative Pilot Project, based in Auburn, to conduct seeding experiments in the central Sierra. The University of Nevada and Desert Research Institute provided cloud physics, physical chemistry, and other field support. The High Plains Cooperative Pilot Project focused on convective cloud seeding to increase rainfall during the growing season in Montana, Kansas, and Texas from 1974 to 1979.
In 1979, the World Meteorological Organization and other member-states led by the Government of Spain conducted a Precipitation Enhancement Project in Spain, with inconclusive results due probably to location selection issues.
Reclamation sponsored research at several universities, including Colorado State University, the Universities of Wyoming, Washington, UCLA, Utah, Chicago, NYU, Montana, and Colorado, and research teams at Stanford, Meteorology Research Inc., and Penn State University, and the South Dakota School of Mines and Technology, North Dakota, Texas A&M, Texas Tech, and Oklahoma. Cooperative efforts with state water resources agencies in California, Colorado, Montana, Kansas, Oklahoma, Texas, and Arizona assured that the applied research met state water management needs. HIPLEX also partnered with NASA, Environment Canada, and the National Center for Atmospheric Research. From 2002 to 2006, in cooperation with six western states, Reclamation sponsored a small cooperative research program called the Weather Damage Modification Program.
In the U.S., funding for research has declined in the last two decades. But the Bureau of Reclamation sponsored a six-state research program from 2002 to 2006 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.
In Australia, the Commonwealth Scientific and Industrial Research Organisation conducted major trials between 1947 and the early-1960s:
Only the trial conducted in the Snowy Mountains produced statistically significant rainfall increases over the entire experiment.
Hydro Tasmania began experimenting with cloud-seeding over lake catchments in central Tasmania in the early 1960s in order to determine if their electricity-producing dams could be kept at high water levels through cloud seeding. Tasmania proved to be one place where cloud seeding was highly effective. Various trials were undertaken between 1964 and 2005, and again between 2009 and 2016, but none have taken place since then. Hydro Tasmania also undertook soil and water survey samples and found negligible trace elements of the materials used for cloud seeding, and determined it did not have a detrimental effect on the environment.
An Austrian study to use silver iodide seeding for hail prevention ran during 1981–2000, and the technique is still actively deployed there.