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<br />These factors may help to explain the diverse results of some of the past <br />cloud seeding experiments (Kerr, 1984; Blumenstein et al., 1984). <br /> <br />c. Transport and dispersion <br /> <br />Both dry ice and silver iodide seeding material are initially dispensed in <br />highly concentrated dosages; either as vertical lines produced by airborne <br />drops of dry ice pellets or silver iodide pyrotechnics, as lines in the <br />horizontal plane produced by airborne silver iodide-acetone generators and <br />end-burning silver iodide flares, or as point sources produced by ground- <br />based silver iodide-acetone generators. Transport and dispersion by <br />natural air motions of the seeding material and/or the ice crystals they <br />produce are then relied on to achieve the proper concentration of ice <br />crystals in the targeted cloud volume at the appropriate time in the <br />evolving cloud as required by the static mode physical hypothesis (see WMO, <br />1980 for a general revi ew of the state of knowl edge of the di spersi on of <br />cloud seeding agents). It is implicitly assumed that there is no penalty <br />from the transient high concentrations of seeding material/ice crystals <br />that occur during the seeding process. It will be shown in section 5 that <br />this assumption is frequently invalid. <br /> <br />Cloud top or in-cloud seeding provides the greatest assurance that the <br />seedi ng materi al /i ce crystal s wfl 1 be introduced at the appropri ate 1 evel s <br />in the cloud in a timely manner, but the time and vertical distance <br />available for dispersion throughout the volume to be affected are quite <br />limited. Due to the nearly instantaneous nucleation rate of dry ice <br />(Morrison et al., 1984), its seeding signature is very dramatic but relati- <br /> <br />22 <br />