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<br />HAILSTONE GROWTH <br /> <br />The sudden fall of large hail is indeed an impressive phen- <br />omenon. It results in an average annual 10:5s of $2 m:illion <br />to insured crops in South Dakota. <br /> <br />The conditions necessary for the formation of large, damaging <br />hail are present in many thunderstorms during the South Dakota <br />summer. They include: <br /> <br />1. Strong persistent updrafts (35 mph will do) <br /> <br />2. Adequate liquid water in the cloud (1 gallon of <br />water per 500,000 gallons of air). <br /> <br />3. A hailstone embryo (such as a large frozen rain <br />drop). <br /> <br />Assuming the above conditions exist in a cloud, two tbeories <br />of the growth of the embryo to hailstone size exist. One <br />. postulates that the strong updrafts cau,se the stone, tC) make <br />several trips up and down in the cloud - a recycling process. <br />Another more recent theory explains the hailstone growth <br />through a single traverse of the cloud. The latter is simpler <br />to understand and has been shown through computer modE~ls at <br />the Institute of Atmospheric Sciences to adequately describe <br />hailstone formation in South Dakota. This process is described <br />below. <br /> <br />A hailstone will only grow in the cold region of the c:loud. <br />It will tend to reside at that level in the cloud where the <br />updraft speed equals the fallspeed of the hailston~ (the <br />larger the hailstone the greater its fallspeed, and ccmsequent- <br />ly the greater the updraft necessary to support it). As the <br />embryo is carried upward by the updraft, it grows by collection <br />of cloud water droplets. There is one hitch, however; the <br />embryo must freeze the intercepted water in order to grow. <br />As water freezes on the hailstone surface, heat is released. <br />The rate at which this heat can be transferred from the stone <br />surface to the surrounding air establishes an upper limit on <br />the hailstone growth rate. If the hailstone intercepts too <br />much water, the excess cannot be frozen, but is rather "shed" <br />from the stone surface. In this manner, the stone can acquire <br />a water film about 1/25 inch thick on its surface. This is <br />termed "wet ,growth." A hailstone in wet growth is gro,wing <br />as fast as possible, and an increase in the amount of surround- <br />ing cloud water will not increase its rate of growth. <br /> <br />35 <br />