Weather Modification

Human activities can cause intentional or accidental changes in local weather situations. Many intentional weather modification experiments have focused on creating conditions that benefit agriculture.

Deliberate Weather Modification

Inadvertent weather modification, including increases or decreases in precipitation downwind from large industrial sites and fog formation, creates problems in some locales. Scientific attempts to deliberately modify weather activity and conditions have been pursued since World War II. The most popular techniques involve cloud seeding, the injection of cloud-nucleating particles into likely clouds to alter the physics and chemistry of condensation. Proponents of this technique claim that it may enhance precipitation amounts by 5 to 20 percent. However, some scientists believe deliberate efforts to enhance precipitation often yield questionable results, even in favorable situations. In 1977, the United Nations passed a resolution prohibiting weather modification for hostile purposes because of the threat to civilians. The United States signed the resolution but has continued defense research on operational weather modification in battlefield situations, as summarized in reports like the US Air Force position paper Weather as a Force Multiplier: Owning the Weather in 2025, a report published in 1996.

Studies have field-tested various methods of weather modification; results have varied widely. Weather modification has been attempted in many countries worldwide by government agencies, agricultural cooperatives, private companies, and research consortia. Farmers are convinced that hail suppression and precipitation augmentation have been achieved in agricultural areas by weather modification. In some of these same locales, meteorologists have been unable to determine if weather modification has produced any change from what would have occurred without intervention. Attempts to duplicate weather modification efforts that have apparently been successful in one locale have often been met with questionable results. Meteorologists occasionally disagree among themselves as to whether a specific attempt at weather modification has succeeded. Reexamination of data from American studies undertaken in the past has led many scientists to conclude that the efficacy of cloud seeding has been overstated.

It should be clearly understood that it is impossible to change the climate of an entire region at will through weather modification to achieve a desired outcome. It is also impossible to end a drought by seeding clouds. This results from long-term dynamic conditions and interactions that are essentially global in nature. Cloud seeding for agricultural purposes assumes that some enhancement of regional rainfall amounts throughout the growing season will increase crop yields. Weather modification for hail suppression assumes that reducing regional crop losses over the growing season is an attainable goal.

Inadvertent Weather Modification

Pulp and paper mills produce vast quantities of large-and giant-diameter cloud condensation nuclei (CCN) in the effluent from their exhaust stacks. Downwind of these mills, precipitation appears to be enhanced about 30 percent above what was observed before the construction of the mills. It is also thought that the heat and moisture emitted by these mills may play an active role in precipitation enhancement. One specific study of a paper mill near Nelspruit in the eastern Transvaal region of South Africa has indicated that storms modified by the mill emissions lasted longer, grew taller, and rained harder than other nearby storms occurring on the same day. Radar measurements supported the theory that hygroscopic particulates released by this mill accelerated or amplified the growth of unusually large-diameter raindrops.

An egregious example of inadvertent weather modification is the formation of ice fog over Arctic cities in Siberia, Alaska, and Canada. During winter, cities such as Irkutsk, Russia, and Fairbanks, Alaska, experience drastic reductions in visibility as particles released by combustion act as nuclei for the formation of minute ice crystals. No techniques are available to modify ice fogs.

An investigation of the meteorological effects of urban St. Louis, Missouri, conducted during the 1970s found that urban summer precipitation was enhanced by 25 percent relative to the surrounding area. Most of the increased precipitation occurred in the late afternoon and evening due to convective activity. The frequency of summer thunderstorms was enhanced by 45 percent, and the frequency of summer hailstorms was higher by 31 percent over the city and adjacent eastern and northeastern suburbs. During the late 1960s, studies demonstrated that widespread burning of sugar cane fields in tropical areas released large numbers of cloud condensation nuclei. Downwind, rainfall decreases of about 25 percent were noted.

Cloud Seeding

For millennia, people attempted to influence the weather by using prayers and incantations. Sometimes, rain followed, and sometimes, no rain fell for extended periods. Scientists began attempting various techniques to modify weather during World War II. In 1946, Vincent Schaefer of the General Electric Research Laboratory observed that dry ice put into a freezer with supercooled water droplets caused ice crystals to form. On November 13, 1946, Schaefer demonstrated that dry ice pellets dropped from an aircraft into stratus clouds caused liquid water droplets to change to ice crystals and fall as snow. Bernard Vonnegut, a coworker, determined that silver iodide (AgI) particles also caused ice crystals to form. Project Cirrus involved apparently successful scientific attempts to seed clouds with ground-based AgI generators in New Mexico. These researchers then tried seeding a hurricane on October 10, 1947. The hurricane changed direction, making landfall in Georgia, and resulting in several lawsuits against General Electric.

Early cloud-seeding experiments were empirical. AgI was dropped from aircraft, shot into clouds by rockets, or dispersed from ground-based generators. Researchers could selectively seed a pattern such as an “L” into a supercooled stratus cloud and see a visible “L” appear, thus “proving” that they could achieve results. When any rain occurred near a seeded area, it was attributed to the intervention. The apparent success of cloud seeding using AgI caused the technique to be modified and adopted in France, Canada, Argentina, Israel, and the Soviet Union. Wine-growing regions such as the south of France and Mendoza, Argentina, installed ground-based AgI generators. The former Soviet Union opted for rocket-borne AgI, which was launched in agricultural areas during thunderstorms to suppress hail.

In 1962, the US Navy and Weather Bureau began an ambitious cooperative plan to modify hurricanes called Project Stormfury. Only a few hurricanes were seeded in attempts to reduce the intensity of the storms. Stormfury proponents suggested that seeding Hurricane Debbie in 1969 caused a reduction of 30 percent in wind speed on one day. No seeding was done the following day, followed by another seeding attempt. The second seeding was thought to have caused a 15 percent reduction in wind speeds. Proponents believed that 10 to 15 percent reductions in wind speeds might result in a 20 to 60 percent reduction in storm damage if similar results could be achieved by seeding other hurricanes. Stormfury was terminated in the late 1970s with no definitive results.

During winters between 1960 and 1970, the Climax I and Climax II randomized cloud seeding studies were conducted in the Colorado Rockies. Although it was initially thought that precipitation enhancements on the order of 10 percent may have resulted, a more recent examination of the results indicates cloud seeding had no statistically discernible effect on precipitation. During the Vietnam War, the US military attempted to increase precipitation along the Ho Chi Minh Trail to impede enemy forces. In the United States during the 1970s, some entrepreneurs deployed ground-based AgI generators in selected agricultural regions, billing farmers for their services. Aircraft delivery of AgI became increasingly popular. By the late 1990s, several private companies were delivering airborne cloud seeding services in various areas worldwide.

Cloud physicists have explored why cloud seeding might be effective. The evidence suggests that seeding increases the size of droplets or ice crystals, allowing them to fall as precipitation. Two concepts have emerged: a static mode theory, which assumes that natural clouds are deficient in ice nuclei, and a dynamic mode theory, which assumes enhancing the vertical movement in clouds increases precipitation. The static mode assumes that a “window of opportunity” exists for seeding cold continental clouds, during which clouds must be within a particular temperature range and contain a certain amount of supercooled liquid water.

Fog Dissipation and Hail Suppression

During World War II, when improvements in visibility were crucial for military operations, efforts were made to dissipate fog. Fog may be dissipated by reducing the number of droplets, decreasing the radius of droplets, or both. Decreasing droplet radius by a factor of three through evaporation can provide a ninefold increase in visibility. Possible methods of fog removal include using dry ice pellets or hygroscopic materials, heating the air, and mixing the foggy air with drier air. Airports plagued by supercooled fog in winter, such as Denver and Salt Lake City, can dissipate the fog by dropping dry ice pellets. Dry ice causes some liquid water droplets to freeze and grow, evaporating the remaining liquid droplets and allowing the larger frozen ice crystals to fall. One way of clearing fog at military airports when there is a shallow radiation fog close to the ground is to use helicopters to provide mixing. Entrenched jet engines can also be used to heat the air over runways by directing their hot exhaust gases. This is an expensive technique that has been used operationally in France, but also poses significant risks to smaller aircraft during landings and take-offs.

Farmers and vintners worldwide fear damaging hailstorms that can devastate crops. There are three approaches to suppressing hail damage: converting all liquid water droplets to snow to prevent hail formation, seeding to promote growth of many small hailstones instead of larger damaging hail, and introducing large condensation nuclei to reduce the average hailstone size. Most weather modification proponents believe that seeding with lead iodide or AgI to cause many small hailstones to form can substantially reduce hailstone size. Because small hailstones are less damaging than large ones, this technique could potentially lessen (but not eliminate) crop losses. It has been claimed that rocket-borne lead iodide seeding in Bulgaria reduced crop losses from hail by 50 to 60 percent. Similar seeding operations in the former Soviet Union were said to have reduced crop damage by 50 to 95 percent. A randomized study in North Dakota over four summers claimed that seeding helped reduce hail severity.

Although weather modification continues to be explored in the twenty-first century, most studies work to refine and improve existing technologies. The United Arab Emirates has been at the forefront of research into cloud seeding due to its arid climate. It has developed a nanomaterial coated with titanium oxide for use as a seeding agent that has increased the effectiveness of the technology. As global climate change becomes an increasingly dire issue and drought more commonplace, cloud seeding technology has become increasingly important. Novel technologies, including solar geoengineering and carbon sequestration, are also being explored. With new technologies in weather modification comes ethical implications and the necessity for international cooperation among nations. 

Principal Terms

cloud seeding: the injection of nucleating particles into clouds to enhance precipitation formation

dynamic mode theory: a theory proposing that enhancement of vertical movement in clouds increases precipitation

fog dissipation: removal of fog by artificial means

hail suppression: a technique aimed at lessening crop damage from hailstorms by converting water droplets to snow to prevent hail formation or, alternatively, by reducing hailstone size

hygroscopic particulates: minute particles that readily take up and retain moisture

static mode theory: a theory assuming that natural clouds are deficient in ice nuclei, whereby clouds must be within a particular temperature range and contain a certain amount of supercooled water for cloud seeding to be successful

supercooled: a liquid cooled below its normal freezing point without crystallizing or becoming solid, typically referring to water

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