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<br />Ul <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />Hygroscopic flares, which were first tested in 1997 and used for a short period in 1998, <br />appear to be useful in attempting to increase rainfall. The term "hygroscopic" itself means water- <br />attracting. Such a particle generated has an affinity for moisture. Hygroscopic flares are composed <br />of sodium chloride, potassium chloride and lithium chloride. Unlike the AgI-based seeding agents, <br />hygroscopic particles enhance rainfall in wann and cold cloud temperature environments. There <br />are still some lingering questions about the viability ofhygtoscopic seeding agents for use in hail <br />suppression as some have suggested. <br /> <br />High numbers of ice nuclei can be produced by a liquid seeding agent being vaporized by a <br />wing-tip generator. Wing generators, mounted to the tips of cloud base seeding planes, employ a <br />combustion process in which a 2% AgI liquid seeding solution produces trillions of ice nuclei per <br />gram of AgI consumed. The wing generators contain a built-in air tank which, when pressurized, <br />forces the liquid seeding solution through an aperture, becoming a fine spray. The spray then <br />flows into a combustion chamber and is vaporized by burning. As the spray burns, very pure <br />particles are formed and are exhausted out the tail of the generator into cloud base updrafts where <br />the particles are ca;ried aloft by natural action into the cloud's supercooled region. <br /> <br />From 1987 through 1995 the liquid seeding agent was the same: The oxidizers sodium <br />perchlorate and ammonium perchlorate were added to Ii AgI-ammonium iodide-acetone-water <br />solution resulting in a solution containing 2% AgI by weight. However, in 1997 the solution <br />changed to contain AgI, sodium iodide (NaCl), paradichlorobenzene (C6H.Q,) and acetone. CSU <br />Cloud chamber test results indicated the number of ice crystals produced at -10C by the new <br />solution were nearly equivalent to the old solution containing the perchlorates. Also, the new <br />particles initially acted as hygroscopic CCN insuring that the vast numbers of water droplets <br />formed would contain IN particles. The process of contact nucleation occurs when ice nuclei <br />initially are not trapped in the water droplets, but eventually are captured by other droplets <br />through random collisions within the cloud, then become ice crystals. The entire process of <br />hygroscopic condensation followed by freezing and contact nucleation, forms greater numbers of _ <br />ice crystals at relatively wanner temperatures within a cloud than by simple contact nucleation. <br /> <br />The reasons for changing to this formulation was because it burned cleanly and didn't <br />corrode the wing generator components. There is little, if any, residue produced and wing <br />generator maintenance over the years, due to corroded parts, is still much less when compared to <br />the previous seasons when we used the perchlorate additives. Nonetheless, some small degree of <br />regular, competent maintenance still is required to keep them clean-burning and dependable. <br /> <br />Within the supercooled temperature volume of clouds, there are regions in which the ice <br />crystals that do form habitually tend to grow into a characteristic shapes. This shape is based upon <br />the actual temperature of the water drop at the time of its conversion from water to ice crystal. <br />Around -10C the "habit" of water drops changing to ice crystals is that it begins forming large- <br />size ice crystals. Once formed, larger ice crystals are able to grow in mass much faster than other <br />smaller crystal shapes such as needles and columns which are formed at warmer sub-freezing <br />temperatures, thereby increasing ice crystal formation quickly. Therefore, to obtain the desired <br /> <br />6 <br />