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.
The earliest scientific programs on weather modification date to the 1940s when Vincent J. Schaefer carried out experiments on cloud seeding. More than half a century later, the science of weather modification is still in its infancy with many questions surrounding the most effective way of bringing about the 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.
A number of large experiments have been conducted to determine the effectiveness of cloud seeding as a way of increasing precipitation. 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% 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 likely 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.
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.
The federal government has been interested to a greater or less extent in programs of hail and lighting 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 lighting 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 lighting 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 by the federal government between 1962 and 1983 to test theories of weather modification. The project seeded Hurricane
Cloud —A large mass of condensed moisture consisting of water droplets and/or ice crystals.
Cloud seeding —The introduction of particles of (usually) dry ice or silver iodide with the hope of increasing precipitation from the cloud.
Dry ice —Solid carbon dioxide.
Fog —A cloud whose base lies at or near Earth’s surface.
Hail —A form of precipitation consisting of relatively large masses of ice.
Precipitation —Any form of solid or liquid water that reaches the ground from the atmosphere.
Supercooled —Water than exists in a liquid state at temperatures below 32°F(0°C).
Ester 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%.
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 was that increased rainfall would make the movement of personnel and material along the Ho Chi Minh trail more difficult.
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. As an example, the county or state might have to pay more to keep roads and highways clear of the additional snow.
See also Tropical cyclone.
Ahrens, Donald C. Meteorology Today. Pacific Grove, Calif.: Brooks Cole, 2006.
Palmer, Tim and Renate Hagedorn, ed. Predictability of Weather and Climate. New York: Cambridge University Press, 2006.
David E. Newton
"Weather Modification." The Gale Encyclopedia of Science. . Encyclopedia.com. (September 21, 2017). http://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/weather-modification-0
"Weather Modification." The Gale Encyclopedia of Science. . Retrieved September 21, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/weather-modification-0
When Mark Twain said that everybody talks about the weather, but nobody does anything about it, he was wrong. In fact, the rain falling on his California roof at that moment might have been generated by secret chemicals being diffused into clouds by a hired rainmaker named Charles Hatfield. Hatfield got rich selling rain to farmers in the San Joaquin Valley until he was run out of the state of California by angry San Diegoans who accused him of triggering a flood.
Experiments in rainmaking flourished in the early 1900s in American farmlands where drought meant not only hunger but poverty. A little like snake-oil salesmen, early rainmakers sold their ability to make it rain, but it was always ambiguous: when it worked, they were paid. More often, it was hard to tell if they had performed their promised service. Lawsuits were abundant when rain intended for an arid area fell across the statelines or caused floods, or when barley growers, pressured by beer companies, paid for rain that coincidentally wiped out other crops of neighboring farmers.
Controlling weather raises myriad questions. Is there an accurate scientific way of measuring human intervention in weather? Who is legally responsible if it rains in the wrong place? Does the community feel that augmenting rainfall is good for everyone? If it rains too much, can the rainmaker be sued? How much does local intervention affect global climate ? Should regulations be in the hands of the state, the federal government, or a world organization?
For better or worse, people can modify weather both intentionally and inadvertently. Weather responds sensitively to any changes in the global atmosphere because it is a complex collection of energy systems powered by the sun. Modest rises in sea surface temperature in the Atlantic Ocean, for example, brought about by ocean circulation shifts or global warming, suppresses rainfall in the African Sahel and contributes to drought.
Sensitive climate changes are also caused by such human activities as cutting down forests or failing to let farm-fields lie fallow, which, with overgrazing by cattle, sheep, and goats, cause desertification , a condition that results when topsoil blows away and exposes bare, unplantable land. This increases albedo , the reflective quality of the surface of the earth, sends back solar radiation into space, and lowers temperature. With less heat rising, fewer clouds are formed, and rainfall is reduced.
Cities, with their clustered buildings and canyons of thoroughfares, absorb infrared heat and inadvertently modify weather because their shape alters the flow of winds. Because they are localized islands of heat, cities increase cloudiness. Aerosols, or microscopic dust particles, given off in industrial smoke , bond with water vapor and create city haze and smog . When the aerosols contain sulfur dioxide and nitrogen oxides , they cause acid rain . Increased urban traffic raises levels of carbon monoxide and carbon dioxide . In the sky, jet trails contribute to the formation of clouds.
Fossil fuels , which are ancient organic matter, release CO2 when they are burned. This collects in the greenhouse band, a protective shield that circles the earth. Naturally composed of CO2, methane , chlorofluorocarbons (CFCs), nitrous oxide , and water vapor, the greenhouse layer processes infrared heat sent back into space by earth and regulates the temperature of the earth. When it is too full to allow infrared heat from earth to pass through into space, the temperature rises on earth, affecting local, regional, and global weather.
Intentional weather modification involves taking advantage of the energy contained within weather systems and turning it toward a specific goal. To "make" rain, a scientist mimics the natural process by introducing extra water droplets or ice crystals in clouds. However, he needs the right cloud shapes with the right internal temperature and the right winds, headed in the direction of his target.
Rainmaking became a serious science in 1950 when physicist Bernard Vonnegut at General Electric devised a way to vaporize silver iodide to let it rise on heated air currents into clouds where it solidified and bonded onto water droplets to create ice crystals. Previous attempts at rainmaking involved dropping dry ice (solid CO2) onto clouds from planes, but this was expensive. Vonnegut chose silver iodide because its molecular structure most closely matches that of ice crystals.
In California, where the Southern California Edison Company regularly sends out planes to seed rain clouds over the dry San Joaquin Valley farmland, silver iodide is shot from rockets mounted on the leading edge of the wings. It is also vaporized into clouds from ground generators at higher altitudes in the Sierra Mountains. In rainmaking projects, the purpose is to avoid droughts, increase food productivity, and augment water supplies for drinking or hydroelectric plants. But gathering accurate data on successful seeding and subsequent precipitation has been difficult. Currently, most scientists agree with a longterm analysis that seasonal cloud seeding has increased precipitation by at least 10%, possibly as much as 20%. Clouds, which are ever-moving collections of water vapor, regulators of heat, and generators of tremendous internal winds, remain mysterious. Yet they are major players in earth's climate.
Other weather modification projects include dissipating cold fogs, done routinely at major airports around the world. In the former Soviet Union, damaging hailstorms were successfully broken up to protect ripening crops. However, statistics from attempts at hail suppression in the United States have been inconclusive, and research is ongoing. In the 1950s and 1960s, scientists experimented with seeding hurricanes to diminish the storms' severity and alter their path. Similarly, attempts were made to "explode" tornadoes by firing artillery into the oncoming storms. In both cases, natural energies far exceeded any attempts at control.
"We don't have the capability to turn the weather around," said Bill Blackmore of National Oceanic and Atmospheric Administration (NOAA)'s Weather Modification Reporting Program. "If we could modify the weather a hundred percent then we could predict the weather a hundred percent. What we need is a lot more understanding of its complexity."
NOAA funds the Federal State Coop Program, a six-state research group. The Atmospheric Modification Program at NOAA's Wave Propagation Lab in Boulder, Colorado, coordinates and evaluates state projects. Research there and at the Institute for Atmospheric Science at the South Dakota School of Mines and Technology involves doing remote sensing of clouds, computer modelling of clouds, and releasing tracers in convective clouds to better understand the dynamics of thunderstorms.
A new way of collecting rainwater is cloud "milking." Researchers have been collecting fog on the mountains of Chile by stringing 50 nylon mesh nets—39 ft (11.8 m) long by 13 ft (4 m) wide—at regular intervals on the mountainside. As the windblown fogs hit the net, they trap water particles. These are then collected into containers. On average, the system "milks" 2,500 gal (9,475 L) of drinking water a day.
Most rainmaking activities in the United States take place in the western states and are sponsored by water departments or districts and conducted by private and commercial companies. The mistakes made earlier in the history of altering the weather have been dealt with by regulations in each state. Internationally, the World Meteorological Organization (WMO) oversees weather modification, and the Treaty of War and Environmental Weather, signed at the Geneva Arms Limitation Talks in 1977, forbids uncontrolled military weather modification.
In 1971, the United States created Public Law 92-205, which requires states to file all weather modification activity with the NOAA's Weather Modification Reporting Program. Typically, about a dozen states file annually.
Two private organizations, the American Meteorological Society in Boston, Massachusetts, and the Weather Modification Association of Fresno, California, keep records on weather modification. The Journal of Weather Modification is an annual publication of the Institute for Atmospheric Science at the South Dakota School of Mines and Technology.
[Stephanie Ocko ]
Arnett, D. S. Weather Modification by Cloud Seeding. New York, Academic Press, 1980.
Breuer, G. Weather Modification: Prospects and Problems. New York: Cambridge University Press, 1979.
"Planned and Inadvertent Weather Modification." Bulletin of the American Meteorological Society (March 1992): 331–337.
Strauss, S. "To Catch a Cloud." Technology Review (May-June 1991): 18–19.
Blackmore, W. H. A Summary of Weather Modification Activities Reported in the United States During 1991. Silver Spring, MD: National Oceanic and Atmospheric Administration, 1991.
"Weather Modification." Environmental Encyclopedia. . Encyclopedia.com. (September 21, 2017). http://www.encyclopedia.com/environment/encyclopedias-almanacs-transcripts-and-maps/weather-modification
"Weather Modification." Environmental Encyclopedia. . Retrieved September 21, 2017 from Encyclopedia.com: http://www.encyclopedia.com/environment/encyclopedias-almanacs-transcripts-and-maps/weather-modification