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<br />If the surrounding cloud water is decreased enough, the rate <br />of collection may be reduced to the point that all the inter- <br />ceptedwater can be frozen. This is termed "dry growth" as no <br />water film develops on the stone surface. Further reduction <br />in the water supply does reduce the hailstone growth rate. <br />Thus it is seen that-rKere exists an optimum amount of cloud <br />water for hailstone growth (dependent upon air temperature, <br />pressure, and hailstone size). More than the optimum supply <br />will not affect the stone growth rate, but less than the <br />optimum supply will reduce the rate of growth. The basic <br />concepts of hail suppression are predicated upon reducing the <br />amount of cloud water available for hailstone growth. <br /> <br />Before the hail can fall to the ground, its fallspeed must <br />exceed the upward speed of the surrounding air. This can <br />occur in one of several ways: <br /> <br />1. The stone gets so big it can no longer be supported <br />by the updraft. <br /> <br />2. The updraft gets weaker allowing the stone to fall <br />through it. <br /> <br />3. The stone is ejected from the main updraft through <br />the side or top of the cloud. <br /> <br />The size of hailstones reaching ~he ground depends heavily <br />on the strength of the updrafts in the cold cloud region. <br />The greater the updrafts, the larger the hailstones which <br />can be supported. Additional factors are cloud water con- <br />tents, temperatures, cloud lifetime, and melting or evap- <br />oration of the hail as it falls below the cloud. <br /> <br />r~v <br /> <br />36 <br />