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<br />Printed January 30, 1990 <br /> <br />3. Augmentation of Precipitation from Winter Orographic Clouds <br /> <br />Precipitation normally forms in winter orographic clouds as snowflakes, which begin as ice crystals <br />formed around ice nuclei or fragments of other ice crystals. As the activity of ice nuclei increases <br />greatly as the temperature falls, most ice crystals originate in the upper parts of cloud systems. <br />They grow by deposition of water vapor as they sift downward to where conditions favor more rapid <br />growth. Growth mechanisms may include aggregation with other crystals and riming, which is the <br />collection of supercooled cloud droplets by snowflakes with appreciable fall speeds. Total time <br />elapsed from formation of an ice crystal to its arrival at the ground as part of a snowflake may be <br />over an hour, during which time it may be carried 50 km or more by the wind. In discussing snow <br />clouds in general, Jiusto and Weickmann (1973) spoke of seeder clouds, which produce the ice <br />crystals, and feeder clouds, which support their growth into snowflakes. The seeder ;and feeder <br />clouds may be separate layers, say altostratus and stratocumulus, respectively, or they may be <br />merged into a single deep layer. <br /> <br />Winter orographic clouds have been seeded with glaciogenic (ice-forming) agents to increase <br />precipitation for over 40 years. The basic hypothesis on which such projects are based has changed <br />little during that time. Ludlam (1955) attributed the low precipitation efficiency of many orographic <br />clouds to a scarcity of ice crystals, which could grow into snowflakes at the expense of the <br />supercooled cloud droplets around them. The shortage of ice crystals was attributed in turn to a <br />lack of natural ice nuclei, which could be remedied by providing artificial ones, say silver iodide <br />crystals from generators on the windward slope. Ground generator seeding appeared <br />straightforward for mountains or ridges whose summit temperaturea were below -SOC, which is the <br />threshold activation temperature for many silver iodide generator products. <br /> <br />Orographic cloud seeding as proposed by Ludlam (1955) is an example of the static seeding <br />approach, in which an increase in precipitation is sought by microphysical effects alone without <br />altering cloud dynamics. According to the WMO, <br /> <br />Clouds are considered to be "statically seed able for increasing precipitation" if (a) <br />the collision-coalescence process between cloud drops is inefficient, and (b) the rate <br />of formation of supercooled condensate exceeds or is comparable to the rate of <br />depletion of supercooled water, and (c) there is sufficient time to grow additional <br />particles, produced by seeding, such that they can reach the ground (WMO, 1982). <br /> <br />These conditions are satisfied in many winter orographic clouds, making them promising candidates <br />for seeding. As a result, seeding of winter orographic clouds to augment precipitation is closer to <br />general scientific acceptance than any other type of weather modification activity, except for the <br />dissipation of supercooled fog. According to the AMS (1984) statement on weather modification, <br /> <br />Precipitation amounts from certain cold orographic cloud systems can be increased <br />under favorable conditions with existing technolog,'Y in the western United States. <br />Increases of the order of 10% in seasonal precipitation are indicated in some project <br />areas. However, possibilities of decreases in orographic precipitation also exist <br />under certain conditions.... <br /> <br />4 <br />