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<br />If the seeding material is dispensed further from the intended zone of <br />effect, by either cloud base or ground-based silver iodide delivery <br />systems, more time is available for volume dispersion and dilution. In <br />addition, much of this dispersion can take place prior to activation of the <br />silver iodide. However, targeting and timing of the seeding effect become <br />more difficult and uncertain. Holroyd and Super (1977) and Hobbs et al. <br />(1980) seeded small cumulus congestus clouds and convective complexes with <br />silver iodide from cloud base and reported difficulty in detecting seeding <br />effects at the targeted levels in the clouds. Dye et al. (1976), on the <br />other hand, reported success in intercepting the silver iodide plume from <br />cloud base seeding of small cumuli in most cases. Maximum concentrations <br />of ice crystals in the plume of about 400 to 800 1-1 produced by cloud base <br />silver iodide seeding have been observed (Dye et al., 1976; Hobbs and <br />Politovich, 1980). Linkletter and Warburton (1977) and Warburton et al. <br />(1982) used silver content of the preciptation as an index of the targeting <br />effectiveness of the cloud base seeding techniques used in NHRE (National <br />Hail Research Experiment) and found uneven distributions both temporally <br />and spatially such that only about 10 percent of the samples had observed <br />concentrations comparable to those expected by theoretical calculations and <br />those requi red by the physi cal hypothesi s. Foote et al. (1979) est irnated <br />that the seeding coverage in NHRE was only about 50 percent on the average <br />which was attributed to finite aircraft reaction time and unpredictable <br />stonn evolution. They concluded "that seeding convective clouds using <br />aircraft fiying near cloud base is more difficult than is widely <br />acknowledged." <br /> <br />24 <br />