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<br />"i. <br /> <br />quickly evaporate because of downward motion on the lee side of the mountain range. <br />Figure I is a generalized depiction of the primary process. <br /> <br />Cloud droplets often remain in the liquid state at temperatures lower than 32 degrees <br />Fahrenheit because of a scarcity of effective ice forming nuclei in the atmosphere. These <br />droplets are called supercooled liquid water (SL W). When effective ice forming nuclei <br />are present, ice crystals form. After formation, the crystals grow from available water <br />vapor. As they fall they collide with SLW droplets that freeze onto them, creating larger <br />crystals. The process continues as the larger crystal falls faster and grows larger into a <br />snowflake. This process is most effective in deep clouds and must occur before reaching <br />the crest of the mountain range. In situations where natural ice nuclei are too scarce to <br />efficiently convert the supercooled liquid water into snow, seeding can assist the <br />conversion process. <br /> <br />There is no doubt that the most commonly used agent, silver iodide (AgI), released into <br />sufficiently cold SL W clouds, will produce multitudes of embryonic ice particles. The <br />same result is achieved when liquid propane is expanded into even slightly supercooled <br />liquid clouds. The challenge is to create seeding-induced ice particles at such locations <br />that their subsequent trajectories will be within SL W clouds for a sufficient time <br />(distance) to permit growth to precipitation sizes (WMA, 1999). <br /> <br />Technical advances have increased the capability to augment precipitation in higher <br />temperature and shallower orographic cloud systems. Numerical modeling has improved <br />understanding of atmospheric transport mechanisms. Improvements in computer, radar, <br />satellite, and communications systems have resulted in better assessments of cloud <br />seeding potential and more effective dispersion of seeding agents from properly <br />positioned cloud nuclei generators. Sensors such as radiometers that continuously <br />monitor SL W amounts greatly improve the chances of successful seeding. <br /> <br />Figure 1 - Primary Winter Cloud and Precipitation Seeding Process <br /> <br />How Cloud Seeding Works <br /> <br />2. The silver iodide <br />particles rise inlO <br />Ihe clouds <br /> <br />S. The silver iodide causes <br />doud moIBlure 10 freeze <br />and creale ke cryslals <br /> <br />1.A mlnule amount of silver <br />Iodide Is sprayed across a <br />propane flame ~. <br />--...=:> <br />~c- ,---~.....:. <br />=~ "'-- <br /> <br />""Ice crystals grow big <br />enough 10 fall as snow. <br /> <br />, <br />~~. <br /> <br /> <br />:<(:.. <br /> <br />'., <br /> <br />-5- <br />