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Methods of weather modification
1.The first scientific attempt at coaxing moisture from a cloud was in 1946, when scientist Vincent Schaefer dropped 3 pounds of dry ice from an airplane into a cloud and, to his delight, produced snow. The success of the experiment was modest, but it spawned optimism among farmers and ranchers around the country. It seemed to them that science had finally triumphed over weather.
2.Unfortunately it didn´t work out that way. Although there were many cloud-seeding operations during the late 1940s and the 1950s, no one could say t whether they had any effect on precipitation. Cloud seeding, or weather modification as it came to be called, was clearly more complicated than had been thought. It was not until the early 1970s that enough experiments had been done to understand the processes involved. What these studies indicated was that only certain types of clouds are amenable to seeding. One of the most responsive is the winter orographic cloud, formed when air currents encounter a mountain slope and rise. If the temperature in such a cloud is right, seeding can increase snow yield by 10 to 20 per cent.
3.There are two major methods of weather modification. In one method, silver iodide* is burned in propane-fired ground generators. The smoke rises into the clouds where the tiny silver-iodide particles act as nuclei for the formation of ice crystals. The alternate system uses airplanes to deliver dry-ice pellets. Dry ice does not provide ice-forming nuclei. Instead, it lowers the temperature near the water droplets in the clouds so that they freeze instantly - a process called spontaneous nucleation. Seeding from aircraft is more efficient but also more expensive.
4.About-75 per cent of all weather modification in the United States takes place, in the Western states. With the population of the West growing rapidly, few regions of the world require more water. About 85 per cent of the waters in the rivers of the West comes from melted snow. As one expert put it, the water problems of the future may make the energy problems of the 70s seem like child´s play to solve. That´s why the U.S. Bureau of Reclamation, along with state governments, municipal water districts, and private interests such as ski areas and agricultural cooperatives, is putting increased effort into cloud-seeding efforts. Without consistent and heavy snowfalls in the Rockies and Sierras, j West would literally dry up. The most intensive efforts to produce precipitation was during the West´s disastrous snow drought 1976-77. It is impossible to judge the efficiency of weather modification based on one crash program, but most experts think that such hurry-up programs are not very effective.