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<br />~\ <br />-I <br />, <br /> <br />I <br />I <br />I <br />\ <br />I <br />I <br />I <br />I <br />I <br />I <br />, <br />, <br /> <br />2.0 THEORY OF CLOUD SEEDING FOR PRECIPITATION' <br />AUGMENTATION <br /> <br />Two theories have evolved concerning the potential to augment precipitation. One <br />theory postulates that a natural cloud's efficiency to make rainlsnow can be inCre;lsed, <br />while the other theory postulates that seeding can enhance cloud development, leading <br />to additional rainfa11l snowfall. The fIrst theory has often been referred to as the static <br />seeding hypothesis, while the second relies upon dynamic effects of cloud growth. In <br />many situations both processes could be operative, whereby a cloud's precipitation <br />efficiency is increased and the cloud is also made to grow larger due to the seeding. <br /> <br />In modem-day cloud seeding programs the typical seeding agent is silver iodide. <br />Discoveries in the mid 1940's established that minute particles of silver iodide, when <br />injected into a cloud composed of supercooled water droplets, would cause those droplets <br />to freeze (Vonnegut, 1947). Supercooled water droplets (minute liquid cloud droplets <br />that are below freezing) frequently exist in large quantities in clouds, as evidenced by <br />icing on aircraft or riming of trees and other vegetation. <br /> <br />Research has demonstrated that certain natural particles, (e.g., soil particles) in <br />the atmosphere serve as ice nuclei. These nuclei assist the freezing of supercooled water <br />droplets. This process is referred to as nucleation. <br /> <br />I <br />! <br />I <br />i <br />I <br />I <br />I <br /> <br />Most of the precipitation that occurs in temperate latitudes begins in the cold <br />(<OOC) portion of clouds. Ice nuclei convert the supercooled water droplets into ice <br />crystals which then grow at the expense of water vapor and other supercooled droplets <br />in a cloud (because the vapor pressure over ice is less than over water). This cold cloud <br />precipitation process is often referred to as the Bergeron- Findeisen process. <br /> <br />Research has indicated that naturally occurring ice nuclei that are active in the <br />temperature range of approximately _50 to -15 oC are relatively rare. Other research has <br /> <br />2-1 <br />