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OOl)90S Burning silver iodide impregnated fuels or solutions produces a smoke containing many billions of silver iodide particles, When diffused into the atmosphere, these particles act like natural nuclei around which snowflakes form. The air temperature at which some particles begin working is generally agreed to be _40 C (250 F). From a given amount of silver iodide, the number of particles becoming active increases with lower temperatures to about _150 C (50 F) or _200 C (_40 F), At tem- peratures below _200 C, natural nuclei, such as dust, become active to form snowflakes, 14/ usually in sufficient numbers so that additional silver iodide particles are not needed and under some conditions are actually detrimental. Figure 1 shows the average winter temperature-elevation criteria found for best ground seeding with silver iodide in the Park Range mountains.~j Based on soundings at Grand Junction, Colorado, the average monthly temperatures at upper altitudes and average freezing level of air masses passing over the Upper Basin were determined and are shown in Figure 2,~!!.1 141 Schaefer, Vmcent J" "Snow and Its Relationship to Experimental 1V'reteorblogy," Compendium of Meteorology, Ed., Thomas F, Malone, American Meteorology SOClety, 1901, p :t:n, 15j"Interim Report--Park Range Atmospheric Water Resources Pro- gram" E, Bollay Associates, Inc., Santa Barbara, California, September 1967. 16jSchleusener, Richard A" and Crow, Loren W., "Analysis of Pre- CTpitation Data in the Upper Colorado River Basin, " Report CER61RAS52, Colorado State University, Fort Collins, Colorado, Uctober 1i:lb1, p 4. I Iff ~ 'k{~ c~fJ'" ~{..~.~.;,..: ~~';)f! " ~.",." ~.;'Iii;~... ~*' WifJiJ @,.>l;(W~ ~~~ .~-fi&.~. ~~1 ,~:>.~~;::>. i/~f;;~~.:::: ~~~l ..;..;.---.... . :~;~~ft~ ';-.'::--::'.; 9