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<br />complexes, and silver iodide-silver chloride complexes appear to be more <br />active yet (DeMott et al., 1983). <br /> <br />The real effectiveness of silver iodide as a static mode seeding agent, <br />however, depends on the number of ice crystals it produces at the cloud <br />temperatures specified by the seeding hypothesis. The ice crystal yield of <br />the seeding material, as distinguished from its cloud chamber calibration <br />of effectivity (potential effectiveness), is mainly determined by its <br />nucleation rate and residence time in the portion of the cloud of interest. <br />The nucleation rate is, in turn, determined by the particular silver iodide <br />agent's mode of nucleation and the cloud environment factors which affect <br />it. Seeding agents such as silver iodide-silver chloride and silver <br />iodide-ammonium iodide which appear to act by the contact-freezing <br />nucleation mode have long time constants of nucleation, their nucleation <br />rate being dependent on Brownian coagulation which is a function of tem- <br />perature and the size and concentration of both the silver iodide aerosols <br />and the cloud droplets. Using chemical kinetic theory and experimental <br />methodology, DeMott et al. (1983) showed that at temperatures warmer than <br />-16 oC it takes from about 15 min. to 1 h for 90 percent of the silver <br />i odide-sil ver chloride aerosol s to nucl eate ice crystal s by the <br />contact-freezing mode, the exact times being dependent on the specific che- <br />mical composition and cloud chamber conditions used. At temperatures <br />colder than -16 oC, DeMott et al. concluded that the deposition nucleation <br />mode was dominant with even longer nucleation time constants. These fin- <br />dings are generally consistent wHh field observations (Dye et al., 1976; <br />Strapp et al., 1979; Marwitz and Stewart, 1981; English and Marwitz, 1981) <br /> <br />20 <br />