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<br />HAIL SUPPRESSION <br /> <br />As mentioned earlier, the basis behind hail suppression <br />seeding is reduction of the amount of cloud water avail- <br />able for hailstone growth. This is accomplished through <br />two mechanisms: <br /> <br />Glaciation - Conversion of much of the cloud water to ice <br />crystals, the silver iodide providing the necessary "ice <br />nuclei". Ice crystals are less likely to stick to the <br />surface of a hailstone than are water droplets, Increasing <br />the number of ice crystals at the expense of the water <br />droplets reduces the total material collected by the hail- <br />stones, and thus the hailstone growth rates. <br /> <br />Competition among embryos - The silver iodide crystals also <br />serve as nuclei around which new hailstone embl'Yos are <br />formed. Fro'~en raindrops are prime examples. The subse- <br />quent hailstone population explosion results in severe <br />competition among the growing hailstones for the available <br />cloud water. The consequence is more, but smaller hail- <br />stones. <br /> <br />Both glaciation and competition among embryos work together <br />to reduce the rate of growth of hailstones. The smaller <br />hailstones have a better chance of melting to l'aindrops <br />before reaching the ground.. No means is currently avail.. <br />able to make smaller the hailstones already in the cloud <br />prior to seeding. <br /> <br />From 100 to 1000 silver iodide crystals per quart of cloud <br />air are generally required for effective hail ~uppression. <br />Such concentrations can be provided through the use of hail <br />suppression flares. <br /> <br />It is frequently asked "Doesn't hail suppression seeding <br />reduce the rainfall?" There is no research evidence to <br />indicate such an effect. In fact, studies at the Institute <br />of Atmospheric Sciences show neither an increase nor a <br />decrease in precipitation from seeding clouds of depths in <br />excess of 30,000 feet. South Dakota hailers generally are <br />deeper. Such storms are naturally efficient producers of <br />precipitation. <br /> <br />41 <br />