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<br />\.\.Q~ <br /> <br />. <br /> <br />. <br /> <br />Numerous other substances were investigated in the early 50's <br />as possible nuclei. Very few are effective at temperatures <br />as warm as -5" C. In recent years still IIlOre substances have <br />been tested. Two or three of these are effective at warmer <br />temperatures, but other factors prevent their beiDg used <br />extensively right now. Studies are under way to overcane <br />these problems. At present, only two substances, dry ice and <br />sil ver iodide, are being used operationally as artificial <br />seediDg agents. <br /> <br />Some field experiments have been conducted in which water <br />droplets or large hygroscopic particles were introduced into <br />"warm clouds" in an attempt to initiate the coalescence process. <br />The results were inconclusive, but research continues. <br /> <br />The search for better seeding agents has revealed the need <br />for greater knowledge of the microstructure of clouds, ice <br />crystals, and raindrops. Thus, the bistory of rainmakiDg <br />expresses the development of cloud physics, beginning with <br />comparatively crude ideas of condensation processes and pro- <br />gressiDg to rather sophisticated studies of small scale cloud <br />characteristics. <br /> <br />I <br /> <br />2. How cloud seeding can belp <br /> <br />At present, the most effective means of assuring adequate <br />water supplies is to have plenty of water in storage. If <br />the precipitation is incressed in wet years and stored in <br />man-made reservoirs or the storage facilities nature provides, <br />a supply of water for dry years will be available. There <br />are several ways to do this, one being to increase the winter <br />anowpack in the high mountains where the melting snow recharges <br />the streams which feed the storage facilities. This water <br />can then be used for irrigation, industrial uses, and domestic <br />supplies. <br /> <br />SUIIIlIler precipitation can also be increased and the runoff <br />stored in reservoirs. However, sane of the rain goes into the <br />soil and is subsequently used in the evapotranspiration process. <br />How much of the total storm rainfall is lost this way depends <br />upon such factors as precipitation intensity and duration; <br />vegetation type, extent and density; slope of the ground; <br />moisture already in the soil; etc. While a smaller percentage <br />of the precipitation flows directly into reservoirs or perco- <br />lates to subsurface storage areB!, compared witb snowmelt, the <br />rain on the ground is immediately available for use by plant <br />life. That portion wbicb falls, for example, on crops decreases <br />the amount of irrigation necessary, whicb indirectly increases <br />reservoir storage. <br /> <br />b <br />