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<br />RAIN DROP GROWTH <br /> <br />As the air parcel rises, it cools to the point of saturation <br />and condensation begins on "condensation nuclei" such as dust <br />particles, volcanic ash, pollins, and salt particles naturally <br />in the air. Were it not for the presence of these nuclei, <br />very high degrees of super saturation (relative humidity <br />much greater than 100%) .would be necessary for condensation <br />and cloud growth to take place. <br /> <br />Growth of cloud droplets around the most favorable nuc:lei <br />proceeds rapidly at first. After a few minutes however, the <br />growth rate through condensation becomes quite slow and the <br />resulting cloud droplets are still very small - 1000 of them, <br />side by side, can form a line less than an inch long. <br /> <br />These small cloud droplets are easily suspended by the upwelling <br />air in the growing cloud. Even if they could fall earthward, <br />evaporation would quickly follow as they reached non-saturated <br />air. Since the small droplets cannot reach the ground, nor can <br />they grow significantly larger through continued condtmsation, <br />another means of raindrop growth must be found. <br /> <br />The answer lies in the collision/coolescence process ()f rain- <br />drop growth. A prerequisite is that there exist, among the <br />field of cloud droplets, a few large-droplets (called "rain- <br />drop embryos") which fall faster than the small ones. The <br />large droplets then collide with the small ones, SWeeI)ing them <br />up into larger growing drops (coolescence). <br /> <br />Where do the raindrop embryos come from? Three mechanisms <br />for their formation are: <br /> <br />1. From chance collisions of small cloud droplets.' Since <br />the droplets are initially all of nearly the same size , <br />with similar fall rates, and are separated by a relatively <br />large amount of air, the probability of any two colliding <br />is small. However, since there are so many droplets, <br />such collisions do occur. The resultant larger droplets <br />have a greater likelihood of future collisions. Eventually <br />they may reach the embryo stage. <br /> <br />2. From giant salt nuclei - As salt particles have an affinity <br />for water, large salt particles may grow by condensation <br />to embryo size. The source of such particles is mainly <br />from the spray of breaking ocean waves. They are con- <br />sequently rare over South Dakota. Salt seeding for rain <br />increase utilizes this concept. <br /> <br />3. From ice crystals - Water vapor tends to be deposited on <br />ice crystals in a manner similar to that of salt particles. <br />Unfortunately, nature is rather stingy with her supply <br /> <br />31 <br />