The term weather modification refers to any deliberate effort on the part of humans to influence weather patterns for some desirable purpose. Probably the most familiar example of weather modification is the seeding of clouds, most often done in order to increase the amount of precipitation during periods of drought.

Many societies in history held beliefs regarding a causal relationship between human intervention and Page 4768  |  Top of Articlerainfall. Agricultural towns in northern Europe used to fire cannons without ammunition, believing that the sound prevents hail; there had been reports of rainfall after every large battle on an open field. In the late nineteenth century, the US Department of War purchased $9,000 of explosives (about $253,000 in 2020 dollars) to detonate them in Texas to test this theory. Although some rainfall was observed after a few barrels of gunpowder had been set off, the experiment was inconclusive, as the increase in rainfall was not statistically significant.

The earliest scientific programs on weather modification date to the 1940s, when American meteorologist Vincent J. Schaefer (1906–1993) carried out experiments on cloud seeding. Almost 80 years later, there are still many questions surrounding the most effective way of bringing about the weather changes desired in any particular setting. The major types of weather modification that are currently in use or under study include cloud seeding, frost prevention, fog and cloud dispersal, hurricane modification, hail suppression, and lighting suppression.

A cloud is a large mass of water droplets and ice crystals. Precipitation normally occurs in a cloud only when ice crystals grow large enough to fall to Earth as rain, snow, hail, or some other form of precipitation. When conditions do not favor the growth of ice crystals, moisture remains suspended in the clouds, and precipitation does not occur.

The general goal of cloud seeding is to find some way of converting the supercooled droplets of liquid water in a cloud to ice crystals. Supercooled water is water that remains in a liquid state even below its freezing point. The two substances most commonly used to transform water droplets to ice crystals are dry ice (solid carbon dioxide) and silver iodide.

The ability of dry ice to trigger the condensation of supercooled water droplets was discovered accidentally in 1946 by Schaefer. He had planned to use a block of dry ice to cool a container of moist air, but discovered that the dry ice actually initiated the formation of ice crystals in the container. Shortly after Schaefer's research, the ability of silver iodide to produce similar results was also discovered.

Any technique of cloud seeding depends on the release of millions of tiny particles of dry ice or silver iodide into a cloud. One way of accomplishing that goal is to ignite solid silver iodide in burners on the ground. The smoke thus formed consists of many tiny particles of the compound which are then carried upward into a cloud.

A more efficient way of seeding a cloud is to drop the seeding agent from an airplane onto the top of the cloud. If silver iodide is used, it can be released from flares attached to the wing tips of the aircraft. If dry ice is used, it is first pulverized into a fine powder and then sprayed onto the cloud.

The government of Thailand established an official rainmaking research and development project in 1971 to address the drought conditions in many parts of the country. The United Arab Emirates have also been experimenting with artificial rainstorms since 2010.

A number of large experiments have been conducted to determine the effectiveness of cloud seeding as a way of increasing precipitation. In the United States, the Atmospheric Water Resources Program of the Bureau of Reclamation, for example, has supported about a dozen research projects on cloud seeding. In one of these projects, the Colorado River Basin Project, cloud seeding was thought to have produced an increase of about 30 percent in the amount of snow falling in Colorado's San Juan Mountains. Experts estimated the value of this additional water for farmers and other consumers at about $100 million.

Other agencies at federal, state, and county levels have also supported cloud research projects. A group of counties in Kansas, for example, annually joins together to support cloud seeding experiments during periods of unusually low precipitation.

A great deal has been learned in the last 50 years about the conditions under which cloud seeding is most Page 4769  |  Top of Articlelikely to be effective. Scientists have discovered, for example, that the optimal rate for seeding with dry ice is 0.17 oz (5 g) for each 0.62 mi (1 km) of cloud surface and 0.89 oz (25 g) per 0.62 mi (1 km) for silver iodide. Still, precise evaluations of the effectiveness of various forms of cloud seeding are often difficult because of the inherent uncertainty about most weather patterns such as cloud formation and dissipation and precipitation rates.

Other than the United States, numerous countries with drought conditions have been using cloud seeding techniques for increased precipitation. These include China, India, Russia, and Saudi Arabia, with different scales of artificial rainfall production in each case.

The techniques of cloud seeding can also be used for a second purpose—the removal of clouds and fog. This goal is desirable, as an example, in regions around an airport where prolonged fog can bring air travel to a halt at great economic cost. The use of dry ice as a seeding agent can cause water droplets in fog to condense on ice crystals, after which they precipitate out of the air. In the process, fog banks and clouds may disappear.

Some dramatic results have been achieved with commercial fog dispersion at airports. In some cases, areas of a few square kilometers have been cleared in a matter of hours by seeding with dry ice.

In China, there have been suspicions that the government is leveraging cloud seeding techniques to improve the conditions in substantially polluted areas, such as Beijing, by effectively washing away the smog.

The US federal government has been interested in programs of hail and lightning suppression for more than two decades. Hail suppression is of interest because of the devastating effects that this form of precipitation can have on crops, while lightning suppression is of importance because of the many forest fires it causes.

The principle underlying most hail suppression research is that hail can be prevented if the atmosphere is flooded with nuclei on which moisture can condense and freeze. The more nuclei present, the argument goes, the less likely large pieces of ice (hail) are to form. Results of research on hail suppression thus far have not been encouraging.

Research on lightning suppression, while of considerable economic value, has so far not received a great deal of attention. One suggestion has been to seed thunderheads with very small aluminum fibers in an attempt to dissipate electrical charges in a cloud. Early research in the late 1960s and early 1970s produced some promising results, but relatively little work is now being done in the field.

The control of hurricanes would be another area with very significant economic and human benefit. Some researchers have suggested that cloud seeding techniques might be a way of dissipating the energy stored in a hurricane. Proposals have been made for the seeding of both the hurricane center (its eye) and the high velocity winds that surround it.

Project Stormfury was conducted between 1962 and 1983 to test theories of weather modification. The project seeded Hurricane Esther in 1961, Hurricane Beulah in 1963, and Hurricane Debbie in 1969, among others. At its most effective, the seeding process appears to have reduced hurricane winds by as much as 30 percent, but overall the project was sunset as inconclusive. Teams of academic researchers from the Massachusetts Institute of Technology (MIT) and the University of California, Berkeley, as well as private companies, have been working on several proposals for novel approaches to hurricane modification. These include modifications on the ocean surface, such as environmentally friendly oils that could prevent the formation of droplets and mitigate hurricanes, and special polymers dropped into clouds to absorb excess moisture. None of these solutions has had practical applications so far, but there is active research and development of methods to reduce the detrimental effects of hurricanes.

The most promising field of weather modification may well be in the prevention of frost. Frost is such a devastating event for grape growers, citrus farmers, and other fields of agriculture that extensive efforts have been made to develop fog prevention systems. The two general principles that underlie most of these systems have been to increase the temperature of air near the ground, where frost forms, and to reduce the amount of heat lost at night, when frost formation usually occurs.

Some of the specific techniques used to accomplish these goals include heaters (to warm air), wind machines (to insure mixing of air), sprinkling systems (to provide water which will release heat when it freezes), and smudge pots (to release heat).

Some defense experts have suggested the use of weather modification techniques as a weapon. The claim has been made, for example, that the United States used cloud seeding during the Vietnam War. The expectation Page 4770  |  Top of Articlewas that increased rainfall would make the movement of personnel and material along the Ho Chi Minh Trail more difficult. This highly classified operation run by the US military from 1967 to 1972 is believed to be the first successful wartime application of weather modification, although such tactics were actively discouraged for future wars.

The use of weather modification techniques is often surrounded by controversy. An increase of precipitation over an area might be of benefit to some individuals in the area but a disadvantage to others. For example, suppose that the owner of a private ski resort wants to have clouds seeded in order to increase snowfall over his or her property. If that effort is successful, the ski area benefits economically. But other individuals and businesses in the area might suffer from this change in the weather. The county or state, for example, might have to pay more to keep roads and highways clear of the additional snow.