Weather Modification

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 level have also supported cloud research projects. A group of counties in Kansas, for example, annually join 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.