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<br />contention. He found that seeding of clouds that naturally develop hail <br />with frozen drop embryos resulted in both hail and rain decreases while <br />those with graupel embryos resulted in hail and rain increases. ThE! model <br />seeding experiments of Farley and Orville (1982), on the other hand~ <br />resulted in both hail and rain decreases from a cloud in which graupel <br />embryos were dominant. <br /> <br />Recognizing or forecasting of the above-stated seedability conditions, <br />which are rate dependent processes of clouds, has proven to be extremely <br />difficult, and it is likely to remain so. Thusfar, the approach has been <br />to identify a set of cloud characteristics which is believed, according to <br />a physically plausible conceptual or numerical model, to represent a cloud <br />that will respond positively to seeding. A cloud is considered seedable or <br />has seeding potential if it has the requisite characteristics at the time <br />of selection. <br /> <br />Early applications of this approach to the selection of seedable clouds <br />were based on observations that many convective clouds extending well above <br />the freezing level did not precipitate (Battan and Kassander, 1960). This <br />was taken as an indication of the deficiency of ice nuclei which are <br />required for precipitation formation by the Bergeron process and, there- <br />fore, such clouds were considered to be seedable. Based on this reasoning, <br />early experimenters selected clouds for seeding based on visual appearance <br />alone; if a cloud grew to at least the -5 oC level and had no visible signs <br />of glaciation it was considered to be a suitable candidate for glaciogenic <br />seeding (Smith, 1970). As time progressed other specification criteria <br />that were learned from earlier experiences were included. The principal <br /> <br />11 <br />