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<br />a seeding agent does not appear to be in any better shape. Most of what is <br />known on the nucleation characteristics of silver iodide agents has been <br />learned from theoretical (see e.g., Fletcher, 1962) and isothennal cloud <br />chamber studies (see e.g., Blair et al., 1973; Garvey, 1975; DeMott et al., <br />1983) but the applicability of these results to real cloud conditions has <br />not been established. One of the major difficulties in making this <br />transfer is the fact that ice nucleation on silver iodide particles can <br />occur by four different mechanisms, that is deposition, condensation- <br />freezing, contact-freezing, and immersion-freezing, and existing ice nuclei <br />counters are not able to simulate or distinguish among them. While ice <br />nucleus counters are useful in confirming the existence of a silver iodide <br />plume in the atmosphere, they cannot indicate with reasonable accuracy how <br />many ice particles will be nucleated in a cloud. <br /> <br />Soon after his discovery of silver iodide's ice nucleating ability, <br />Vonnegut (1949) found that the number of ice crystals fanned in a cloud <br />chamber by a given quantity of s.ilver iodide increased with decreasing tem- <br />perature and that the nucleation rate decreased with increasing temperature <br />and decreasing aerosol size. In the years that followed theoretical and <br />laboratory studies were conducted to explain the nucleating behavior of <br />silver iodide with activity focused on establishing the relative importance <br />of such factors as aerosol size (Fletcher, 1958a; Gerber, 1972 and 1976), <br />surface active sites (Edwards and Evans, 1968; Fletcher, 1969), mode of <br />nucleation (Fletcher, 1959a; Weickman et al., 1970; Sax and Goldsmith, <br />1972; Demott et al., 1983; Blumenstein et al., 1983), chemical complexing <br />(Burkardt et al., 1970; Sax et al., 1979a; Finnegan et al., 1984), activa- <br /> <br />18 <br />