Cloud seeding is a method used to change weather conditions by altering precipitation, reducing hail, or clearing fog. The main goal is often to increase rainfall or snowfall, either to help with water needs or to prevent heavy rain later.
This process involves spreading special materials into the air that help form raindrops or ice particles. Common materials include silver iodide, potassium iodide, and dry ice. Some newer methods use hygroscopic materials, like table salt, because they attract water droplets. Techniques include static seeding, which helps ice form in very cold clouds to create more precipitation, and dynamic seeding, which uses heat released during cloud formation to encourage more rain.
Materials are spread using planes, ground machines, or balloons. Newer methods use drones to send electric charges into the air or use lasers to create particles that help rain form. Scientists have studied cloud seeding for many years, but its success is still debated, with some studies showing it works and others showing little effect.
Most research suggests the amounts of materials used are small and do not harm the environment or human health. However, some worry about how these materials might build up in areas like forests or wetlands. Cloud seeding has been used since the 1940s for purposes such as helping crops grow, increasing water supplies, and planning events. Laws usually prevent using weather control for military purposes, but rules about cloud seeding are decided by each country. Even though some people question its usefulness and safety, cloud seeding is still used in many places around the world to manage weather.
Methods
Cloud seeding commonly uses chemicals such as silver iodide, potassium iodide, and dry ice (solid carbon dioxide). Liquid propane, which turns into a gas, has also been used. It can create ice crystals at higher temperatures than silver iodide. Recent research has led to increased use of hygroscopic materials, like table salt, for cloud seeding.
Cloud seeding increases snowfall when cloud temperatures range from −20 to −7 °C (−4 °F to 20 °F). Freezing nucleation occurs when substances similar to silver iodide, which have a structure like ice crystals, are introduced into clouds.
Since 2021, the United Arab Emirates have used drones carrying tools that release an electric charge into the air and sensors to measure conditions. These drones operate at low altitudes. This method may have helped create heavy rain in July 2021. For example, in Al Ain, 6.9 millimeters (¼ inch) of rain fell on July 20–21.
In 2010, an electronic system was tested in Berlin. Researchers from the University of Geneva directed infrared laser pulses into the air above the city. They believed the pulses would cause sulfur dioxide and nitrogen dioxide in the atmosphere to form particles that could act as cloud seeds.
Effectiveness
Cloud seeding is a method used to try to increase rainfall or snowfall by adding substances like silver iodide to clouds. However, scientists still debate how well this method works. A report from the US Government Accountability Office in December 2024 said that studies have given different results about whether cloud seeding actually increases precipitation. Some scientists believe it might help a little under certain conditions, while others say there is not enough clear evidence to prove it works reliably.
A study by the United States National Academy of Sciences did not find strong evidence that cloud seeding increases precipitation. Jerry Bradley, an ecologist from Stanford University, said that cloud seeding might increase snow or rain slightly in some places, but not enough to guarantee results. A similar study by the Wyoming Weather Modification Pilot Project found that cloud seeding could increase snowpack by up to 3% over a season, but it was unclear if this effect was large enough to be meaningful.
In 2003, the US National Research Council (NRC) said that scientists could not confirm whether any cloud-seeding methods work. While scientists have learned more about weather over the past 55 years, there is still no clear scientific proof that cloud seeding significantly affects precipitation.
A study from Tel Aviv University in 2010 said that using silver iodide or frozen carbon dioxide to seed clouds has little or no effect on rainfall. A 2011 study suggested that airplanes might accidentally create ice particles in clouds when air cools around their wings or engines. This could affect how hail forms.
In 2016, Jeff Tilley, a weather modification expert, said that new technology and research have made cloud seeding a reliable and affordable way to increase water supplies in some areas. In 1998, the American Meteorological Society said that precipitation from certain clouds over mountains might increase by about 10% during specific seasons.
Despite these mixed results, cloud seeding was used during the 2008 Summer Olympics in Beijing to try to make rain fall before reaching the city, to avoid rain during the opening and closing ceremonies. Whether this effort worked is still unclear. Roelof Bruintjes, a scientist from the National Center for Atmospheric Research, said that people cannot control clouds or stop them from forming rain.
Impact on environment and health
Silver iodide has a health hazard rating of 2 on a scale used by the NFPA to show health risks. This means it can cause short-term harm or possible long-term effects in humans and animals with strong or long-term exposure. However, many detailed studies about the environment have found very small effects on the environment and health. Some research shows that silver and its compounds, including silver iodide, are not very harmful. These results may be because cloud seeding releases only very small amounts of silver, similar to about 1% of silver released by industries worldwide or from silver in tooth fillings.
Soil, plants, and water runoff have not had enough silver to be measured above normal levels. A 1995 study in the Sierra Nevada of California and a 2004 review by experts in Australia confirmed these earlier results.
In 1978, about 3,000 tonnes of silver were released into the environment in the United States. This led the US Health Services and EPA to study whether silver could harm the environment or human health. These agencies and others used the Clean Water Act of 1977 and 1987 to create rules about this type of pollution.
Cloud seeding in Kosciuszko National Park, a protected area, has caused problems because laws changed quickly to allow the trial. Environmentalists worry that silver might harm the pygmy possum and other animals, as well as cause algal blooms in lakes that were once clean. Studies from 50 years ago and reports by the Snowy Mountains Authority led to stopping cloud seeding in the 1950s, but the results were not clear. In the past, cloud seeding in Australia was stopped because of environmental concerns about the pygmy possum. Some research, reviewed by experts, argues that the negative effects on the environment are not proven, according to 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, when ice crystals are released into rain clouds, would cause rain. While researching aircraft icing, General Electric (GE)'s Vincent Schaefer and Irving Langmuir confirmed the theory. Schaefer discovered the principle of cloud seeding in July 1946 through a series of unexpected events. Inspired by ideas he and Langmuir had while climbing Mt. Washington in New Hampshire, Schaefer, Langmuir's research associate, created a way to experiment with supercooled clouds using a deep freeze unit containing materials like table salt, talcum powder, soils, dust, and various chemical agents. On July 14, 1946, he tried experiments at GE's Schenectady Research Lab.
He was disappointed to find that the deep freezer was not cold enough to produce a "cloud" using breath air. He added a piece of dry ice to lower the temperature. To his surprise, when he breathed into the freezer, he saw a bluish haze, followed by millions of tiny ice crystals reflecting light. He realized he had found a way to turn supercooled water into ice crystals. The experiment was repeated, and he studied the temperature to find the −40 °C (−40 °F) limit for liquid water.
Within a month, Schaefer's colleague, atmospheric scientist Bernard Vonnegut, discovered another method for seeding supercooled clouds. Vonnegut found the idea in a chemistry book and tested silver and iodide chemicals to create silver iodide. With Professor Henry Chessin of SUNY Albany, a crystallographer, he wrote a paper in Science and received a patent in 1975. Both methods were used in cloud seeding starting in 1946 while working for GE in New York.
Schaefer's method changed a cloud's heat balance; Vonnegut's method changed the structure of ice crystals, a property linked to how well the crystal lattices match. (The study of ice crystals later influenced Vonnegut's brother Kurt Vonnegut's novel Cat's Cradle.) The first attempt to modify natural clouds using cloud seeding began on November 13, 1946, during a flight in upstate New York. Schaefer caused snow to fall near Mount Greylock in western Massachusetts after dropping six pounds (2.5 kg) of dry ice into a cloud from a plane after a 60-mile (100 km) chase from Schenectady County Airport.
Dry ice and silver iodide are effective in changing the physical chemistry of supercooled clouds, helping to increase winter snowfall over mountains and, under certain conditions, reduce lightning and hail. While not new, hygroscopic seeding—using materials like table salt—to increase rainfall in warm clouds is being studied again, based on research in South Africa, Mexico, and other places. Hygroscopic materials are thought to make cloud droplets larger, encouraging rainfall through merging. From 1967 to 1972, the U.S. military's Operation Popeye seeded silver iodide to extend the monsoon season over North Vietnam, specifically the Ho Chi Minh Trail. The operation increased the monsoon period by 30 to 45 days in targeted areas. The 54th Weather Reconnaissance Squadron carried out the mission to "make mud, not war."
An attempt by the U.S. military to change hurricanes in the Atlantic in the 1960s was called Project STORMFURY. Scientists tested four hurricanes over eight days and saw wind speed decreases of 10% to 30% on four days. They initially thought the results were due to poor execution, but later questions arose because hurricanes lacked supercooled water, making it hard to tell if changes were from human action or natural processes.
Two federal agencies supported weather modification research starting 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 funded cloud-seeding projects under Project Skywater from 1964 to 1988, and NOAA ran the Atmospheric Modification Program from 1979 to 1993. Projects were conducted in several states and countries, studying winter and summer cloud seeding. From 1962 to 1988, Reclamation focused on winter cloud seeding to boost snowfall in the Rocky Mountains and Sierra Nevada and rainfall in southern California. In California, Reclamation partnered with the California Department of Water Resources to run the Serra Cooperative Pilot Project in Auburn. The University of Nevada and Desert Research Institute provided support. The High Plains Cooperative Pilot Project (HIPLEX) studied summer cloud seeding to increase rainfall in Montana, Kansas, and Texas from 1974 to 1979.
In 1979, the World Meteorological Organization and Spain led a Precipitation Enhancement Project (PEP) in Spain, but results were unclear, likely due to poor location choices.
Reclamation funded research at universities like Colorado State, Wyoming, Washington, UCLA, Utah, Chicago, NYU, Montana, and Colorado, as well as research teams at Stanford, Meteorology Research Inc., and Penn State University, and schools like South Dakota School of Mines and Technology, North Dakota, Texas A&M, Texas Tech, and Oklahoma. Partnerships with state water agencies ensured research met local needs. HIPLEX also worked with NASA, Environment Canada, and the National Center for Atmospheric Research (NCAR). From 2002 to 2006, Reclamation supported the Weather Damage Modification Program with six western states.
In the U.S., research funding has decreased in the last 20 years. However, the Bureau of Reclamation funded the "Weather Damage Modification Program" from 2002 to 2006. A 2003 study by the U.S. National Academy of Sciences urged a national research program to address remaining questions about weather modification.
In Australia, the Commonwealth Scientific and Industrial Research Organisation (CSIRO) tested cloud seeding from 1947 to the early 1960s. Only the trial in the Snowy Mountains showed statistically significant rainfall increases.
Hydro Tasmania (then called the Hydro Electric Commission) began testing cloud seeding over lakes in central Tasmania in the early 1960s to see if their dams could be kept full.
Legal frameworks and implications
The Convention on the Prohibition of Military or Any Other Hostile Use of Environmental Modification Techniques (ENMOD) is the only international agreement that regulates the use of weather and climate modification technologies. It was created after cloud-seeding operations were used during the Vietnam War and the Cold War. The agreement only applies to the military or hostile use of weather modification technologies. Using these technologies for peaceful purposes is not banned by the treaty. ENMOD has faced criticism because some parts of the agreement are unclear, making it hard to interpret in certain situations.
As weather modification programs become more popular, the legal rules provided by ENMOD may not be enough. One issue is that the agreement does not explain who owns the water produced by these programs. A 1948 article in the Stanford Law Review said that assigning legal ownership to a cloud is not practical because clouds are constantly changing in shape, location, and form. Brooks argued that private ownership of clouds is not possible because control over a cloud is only temporary, as it moves over land. Quilleré-Majzoub (2004) said that the idea of owning clouds is not valid because clouds are like air, running water, or wild animals—they are not owned by anyone. She suggested that clouds should be treated as common property, like resources that belong to everyone. This idea would mean that international laws should focus on managing these shared resources, rather than treating them as unowned. While water is often considered unowned in international law, there is growing support for treating it as a shared resource. However, the legal status of cloud moisture is still unclear. Quilleré-Majzoub proposed that international law should create rules that consider the unique nature of clouds and the effects of new technologies.
Brooks (1948) said that the situation changes when artificial rainmaking technologies make cloud moisture usable. Naturally occurring rain is usually owned by the first person to collect it, typically the landowner, as long as no other rights are broken. However, this rule does not apply to rain created by human actions. In California, laws treat water from cloud seeding as part of existing water rights and groundwater rules, calling it "natural supply." However, courts might decide that induced rain should be considered "additional precipitation," allowing the group that seeded the clouds to claim some of the water. This approach would be difficult to apply because it is hard to measure how much extra water was created by cloud seeding.
Conspiracy theories
Cloud seeding has led to many theories that suggest governments may control weather to influence conditions such as global warming, population management, military testing, public health, and flooding. These ideas have been supported by government actions, including programs like Operation Popeye.
In 2016, a classified ad in the Pasadena Star News by Los Angeles County’s Department of Public Works caused people to believe that large-scale weather changes were happening. The department later explained that the ad only described cloud seeding, a method used to help reduce drought. This practice has been used sometimes for over 50 years in Los Angeles.