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<br />a. Dry ice <br /> <br />Airborne seeding with crushed dry ice was used in many experiments con- <br />ducted in the decade following Schaefer's discovery of dry ice's nucleating <br />ability. It was replaced by silver iodide when logistic and cost con- <br />siderations caused experimenters to conduct seeding operations near cloud <br />base or from the ground. Dry ice came into use again in the 1970's when <br />ontop cloud seeding to produce a vertical curtain of ice crystals was <br />thought to be the most effective seeding strategy. Some scientists pre- <br />ferred using pellets of dry ice for this purpose (Holroyd et al., 1978) <br />rather than silver iodide pyrotechnics, because dry ice produces ice <br />crystals nearly instantaneously at temperatures of -2 oC and colder, and <br />its effectiveness was thought to be virtually independent of temperature. <br /> <br />Despite its extensive use as a seeding agent, several questions concerning <br />the activity and effectiveness of dry ice remain unanswered. First is its <br />mode of nucleation. It is generally agreed that the number of ice crystals <br />produced by dry ice can not be explained solely by the freezing of <br />preexisting cloud droplets or by heterogeneous nucleation of droplets <br />followed by homogeneous freezing. However, there is no consensus as to <br />whether its predominant nucleation mode is the homogeneous deposition of <br />ice, the homogeneous nucleation of droplets followed by homogeneous <br />freezing or a combination of both mechanisms (Vonnegut, 1981; Mason, 1981). <br /> <br />The second question concerns its effectiveness. Schaefer (1946, 1949) <br />estimated that dry ice produces at least 1016 crystals per gram. Braham <br />and Sievers (1956) made a theoretical study of the production and growth of <br /> <br />15 <br />