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<br />1 <br /> <br />I <br /> <br />To produce high number of ice nuclei from silver iodide, it <br />must first be vaporized. On the WKWM Program we employ a <br />combustion process which produces trillions of ice nuclei per gram <br />of silver iodide consumed.' Carley-type acetone-silver iodide <br />generators are mounted in the position of wing-tip tanks on each <br />Cloud base seeding aircraft. The generators are pressurized by air <br />contained in an air tank built into tbe generators. The pressurized <br />air forces the liquid seeding solution through an aperture and <br />produces a fine spray that flows into a combustion chamber where it <br />is vaporized by burning. Upon burning, very pure silver iodide <br />particles are formed and exhaust freely from the combustion chamber <br />into the cloud base updrafts which carry them aloft into the cloud <br />by its own natural acti~n, <br /> <br />1 <br /> <br />In 1990 we used the same seeding agent formulation used since <br />1987. ~he formulation contains quantities of the oxidizers sodium <br />perchlorate and ammonium perchlorate added to a standard 2% silver <br />iodide-ammonium iodide-acetone-water solution, A chemical ratio of <br />4 moles sodium to a 1 mole silver results in particles acting <br />initially as strongly hygroscopic condensation nuclei that help <br />insure the formation of vast numbers of water droplets containing <br />silver iodide nuclei in the lower parts of clouds. Silver iodide <br /> <br />I <br /> <br />ice nuclei not trapped in the water droplets hygroscopically, near <br />cloud base, will be collected in other droplets by tlcontact <br />nucleationtl. Contact nucleation occurs when ice nuclei rise inside <br />moist cloud updrafts, make random collisions with water droplets <br />and are captured in those droplets. The combined hygroscopic <br />condensation-contact nucleation process produces greater numbers of <br />ice crystals formed at relatively warmer temperatures within a <br />cloud than when simply using the older formulation depending solely <br />upon contact nucleation. Contact nucleation allows silver iodide <br />activity to linger within a treated cloud after seeding ends and <br />helps clouds precipitate more over a longer time; usually, silver <br />iodide is the preferred seeding agent for rainfall stimulation. <br /> <br />I <br /> <br />Finally, in order to obtain the desired result from cloud <br />seeding with silver iodide, each cloud must be treated within a <br />proper time interval---a "window of opportunity!'---not only to <br />produce the optimum ice crystal concentrations in clouds naturally <br />deficient in ice nuclei! but also to be able to get the proper <br />concentrations into that volume of the cloud where they can become <br />effective. In the case of using silver iodide! a cloud which grows <br />to maturity and collapses must be treated in time far the ice <br />nuclei to be able to be lifted by natural claud action into the <br />appropriate temperature and moisture regime and kept. there for <br />sufficient time to react with the supercooled water in that volume <br />or the cloud may collapse prematurely and the effort will be <br />wasted. This "residence" time of silver iodide in the supercooled <br /> <br />8 <br />