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<br />tend to remain in the liquid state without freezing at temperatures <br />lower than 0 oC (32 OF) because of the natural scarcity of foreign <br />particles that can act as ice nuclei; that is, particles that can <br />cause some of the droplets to freeze. Such liquid droplets are <br />referred to as "supercooled droplets" and the clouds they form are <br />called "supercooled clouds". If ice particles occur naturally, or <br />are created artifically in these supercooled clouds, the ice par- <br />ticles will grow at the expense of the droplets, and begin to reach <br />the ground as snow. <br /> <br />., <br /> <br />If the concentration of natural ice particles is too low, the ice <br />particle concentration can be increased through the use of seeding <br />material. The most common seeding agents are dry ice, which causes <br />ice crystals to form because of its extremely cold temperature, and <br />AgI, which has a physical structure similar to that of natural ice <br />crystals. Ice crystals formed by seeding can convert more super- <br />cooled water to snow, and thereby increase the "efficiency" of the clouds. <br /> <br />! <br /> <br />The experiments in the proposed action would be limited in extent and <br />duration. Their purpose is not to create additional snowfall on <br />Grand Mesa, but to determine the effectiveness of particular seeding <br />agents and rates of release in the clouds under various cloud con- <br />ditions. <br /> <br />1. Transport and Dispersion Studies <br /> <br />The transport and dispersion studies would include release of AgI <br />either by means of ground generators (shown in fig. 7) or from an <br />aircraft, and monitoring of the resulting plume position and par- <br />ticle concentration with a specially instrumented aircraft (see <br />fig. 8). Information from this activity is needed because of the <br />importance of proper targeting of seeding agents. Much of ,the <br />uncertainity about results of previous orographic cloud seeding <br />experiments using ground generators may be due to failure to <br />achieve proper targeting of the seeding material into the desired <br />region on the selected cloud. <br /> <br />Portable AgI ground generators would be placed on the west slope <br />of Grand Mesa below the mesa rim at about the 9000-foot level, by <br />helicopter. The aircraft that would be used for the direct detec- <br />tion studies described below would also be used for the plume <br />tracing. <br /> <br />c <br /> <br />~. <br /> <br />2. Direct Detection of Cloud Modification Due to Seeding <br /> <br />This portion of the new studies includes experiments to detect directly <br />the microphysical changes in clouds that particular modes of seeding <br />cause. Seeding would be one of three modes: (1) ground-released AgI, <br /> <br />10 <br />