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<br />volume within a cloud that is important in creating rainfall and hail. <br />Technically, supercooled water can remain unfrozen to as low as -40C <br />before spontaneously turning into ice. <br /> <br />When some ice crystals do develop within the supercooled volume <br />of t.he c loud, thE."~Y I;;JI"'OlAl ra.pidly from watel"' vapo\"' SI.AI"'1"'oundin1."J t.hem. <br />Inside growing Cl01Jds the continlJOUS movements of water droplets and <br />ice crystals insure inevitable collisions aml~ng them. These <br />collisions produce a sort of chain reaction as water droplets will <br />Sl~ontaneously freeze and shatter, repeatedly. EV811tlJally, the ice <br />particles fall OlJt of suspensiorl in the cloud, warm up and melt while <br />fal1i119 to the gl"'olJnd and usually seen as rain on the ground. <br />However, if the melting isn't complete, hail, graupel or snow is seen. <br /> <br />The theory under which hail suppression usually operates is that <br />hailstones are able to grow to large sizes because there are often too <br />few numbers of ice crystals formed naturally during vigorous cloud <br />development, thereby allowing abundant supercooled water to collect on <br />the few ice crystals which in turn grow to a size too large to melt <br />before reaching the ground when falling from tt,e cloud. <br /> <br />The obvious problem, then, is to find out how best to increase <br />ice crystal concentrations within those clouds naturally deficient in <br />them so that the large amounts of supercooled water cannot create <br />hailstones large enough to reach the ground to damage cr'ops and <br />pl"Op~.7!l"' t..y. <br /> <br />. Earlier it wa~ mentioned that dry ice and silver iodide remain <br />the primary materials used in treating clouds. The WKWM program uses <br />both kinds of materials and delivers them to the clolJds by air.cl~aft <br />f]vl.~r t.he yea.\"'~.;, WKl.JM has obse\.'v~~d t.he ~::ystern.atic dl?vE~lopments in cloud <br />seeding technology and has applied ~esearctl results to its own <br />operational program thereby maintaining a "state of the art'! level <br />within the field of weather ml~dification. <br /> <br />In producing ice nuclei fl~om silver iodide it is necessary to <br />first vaporize it. This is done on the WKWM program through employing <br />a combustion process which produces trillions of ice nuclei per gram <br />of silver iodide consumed. Carley-type acetone-silver iodide <br />generators are placed in the position of wing-tip tanks on each cloud <br />base seeding aircraft. tJtlilizing air from a pressurized tarlk! an <br />acetone-silver iodide liquid solution is forced through a nozzle into <br />a combustion chamber where the solution is ignited and silver iodide <br />par'ticles are formed. The particles are then expelled into the <br />atmosphere much as is the exhaust from an automobile. <br /> <br />Also used this past season were silver iodide flares. Due to <br />budget considerations, only one plarle was able to be outfitted with <br />them. Special racks mounted on the trailing edges of each wing held <br />12 such devices on each side of the plane, 24 in all. The flares were <br />electrically conl1ected to a control box inside tt1e cockpit so that the <br />pilot could select and fire allY flare desired. There were two <br />varieties of flares used this past season---a unit burning 70 grams of <br />silver iodide and one burning 20 grams. <br /> <br />6 <br />