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<br />r,O[)74' <br /> <br />Burning silver iodide impregnated fuels or solutions produces a smoke <br /> <br />containing many billions of silver iodide particles. When diffused into <br /> <br />the atmosphere. these particles act like natural nuclei around which <br /> <br />snowflakes form. The air temperature at which some particles begin <br />wo"king is generally agreed to be -4' C (25' F). From a given amount <br />of silver iodide, the number of particles becoming active increases with <br />lower temperatures to about _15' C (5' F) or _20' C (_4' F). At tem- <br />peratures below _200 C, natural nuclei, such as dust, become active to <br />form snowflakes,1.1/ usually in sufficient numbers so that additional silver <br />iodide particles are not needed and tmder some conditions are actually <br /> <br />I <br /> <br />detrimental. Figure 1 shows the average winter temperature-elevation <br /> <br />criteria found for best ground seeding with silver iodide in the Park <br />Range mountains.~/ <br /> <br />Based on soundings at Grand Junction, Colorado. the average monthly <br /> <br />temperatures at upper altitudes and average freezing level of air <br /> <br />masses passing over the Upper Basin were determined and are shown <br />in Figure 2..!.!!J <br /> <br />H/Schaefer, Vmcent J., "Snow and Its Relationship to Experimental <br />1Weteorology, tI Compendium of l\leteorology. Ed., Thomas F. I\lalone, <br />American Meteorology SOCIety, 1951. p 22l. <br />15/"1nterim Report--Park Range Atmospheric \\!ater Resources Pro- <br />gram" E. Bollay Associates. Inc., Santa Barbara, California, <br />September 1967. <br />16/Schleusener. Richard A., and Crow, Loren W.. "Analysis of Pre- <br />Clpitation Data in the Upper Colorado River Basin." Report CER61RAS52, <br />Colorado State University. Fort Collins, Colorado, October 1951, p -t. <br /> <br />9 <br />