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<br />'" <br /> <br />. <br /> <br /> <br />1. INTRODUCTION AND HISTORlCAL OVERVIEW <br /> <br />l <br />t., <br /> <br />Atmospheric water in the form of precipitation, is one of the primary sources of <br />fresh water in the world. However, a large amount of water present in clouds never <br />gets transformed into precipitation (National Academy of Sciences, 1964, 1966, 1973j <br />Weather Modification Advisory Board, 1978). This has prompted scientists and engineers <br />to explore the possibility of augmenting water supplies by means of cloud seeding. If <br />more water could be transformed into precipitation the potential benefits appear very <br />attractive. The ability to influence and modify cloud microstructure in certain simple <br />cloud systems such as fog, thin layer clouds, simple orographic clouds, and small cumulus <br />clouds, has been demonstrated and verified in laboratory, modeling and observational <br />studies (American meteorological Society, 1985). However, evidence that precipitation, <br />hail, lightning or winds can be modified significantly is quite liIclted. The complexity of <br />atmospheric processes and specifically cloud and precipitation development. has prevented <br />significant progress in developing a cloud seeding technology that can be tested and verified <br />operationally. <br /> <br />. <br /> <br />r'-' <br />\ <br />L.. <br /> <br />\ . <br /> <br />i, <br /> <br />I <br />L. <br /> <br />The physical mechanisms of cloud and precipitation development in the atmosphere <br />are much more complex than anticipated earlier and the optimism of the 1950's has given <br />way to a more cautious approach. The earlier statistical experiments using randomized <br />seeding methods in most cases only measured precipitation at the surface. Physical studies <br />did not form an integral part of these experiments (Australian experiments, Smith et al., <br />1963 and 1979; and Climax, Mielke and Grant, 1967, and Mielke et al., 1971). The physical <br />chain of events was treated as a "black box" ma.lring it difficult to explain the results in a <br />physical manner. Hence scientists have turned to the physical approach which incorporates <br />observationa, modeling and laboratory studies to track the physical chain of events irom <br />the nucleation source to precipitation at the surface. <br /> <br />I 7 <br />r "0 <br />i <br />I.. . <br /> <br /> <br />. <br />