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:1 2.0 THEORY OF CLOUD SEEDING FOR PRECIPITATION AUGl\:lENT A TION Two theories have evolved concerning the potential to augment precipitation. One theory postulates that a natural cloud's efficiency to make rain! snow can be increased, while the other theory postulates that seeding can enhance cloud development leading to additional rainfallJsnowfall. The fIrst theory has often been referred to as the static seeding hypothesis while the second relies upon dynamic effects of cloud growth. In many situations both processes could be operative whereby a cloud's precipitation efficiency is increased and the cloud is also made to grow larger due to the seeding. In modem-day cloud seeding programs the typical seeding agent is silver iodide. Discoveries in the mid 1940's established that minute particles of silver iodide, when injected into a cloud composed of supercooled water droplets, would cause those droplets to freeze (Vonnegut, 1947). Supercooled water droplets (minute liquid cloud droplets that are below freezing) frequently exist in clouds as evidenced by icing on aircraft or rimming of trees and shrubs. Research has demonstrated that certain natural particles, (e.g., soil particles) in the atmosphere serve as ice nuclei. These nuclei assist the freezing of supercooled water droplets. This process is referred to as nucleation. Most of the precipitation that occurs in temperate latitudes begins in the cold (< OOC) portion of clouds. Ice nuclei convert the supercooled water droplets into ice crystals which then grow at the expense of water vapor and other supercooled droplets in a cloud (because the vapor pressure over ice is less than that over water). This cold cloud precipitation process is often referred to as the Bergeron-Findeisen process. Research has indicated that naturally occurring ice nuclei that are active in the temperature range of approximately _50 to -15 oC are relatively rare. Other research has 2-1