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<br />Partly because of this, most physical studies and experiments designed to <br />investigate or test the static seeding hypothesis have been conducted on <br />single, isolated convective clouds and cloud clusters in which dynamic <br />effects, from seeding are expected to be negligible. <br /> <br />Since the goal of static seeding is to improve the efficiency of a cloud's <br />natural precipitation process, a brief review of our understanding of <br />natural precipitation processes in convective clouds is appropriate before <br />proceeding. Results of cloud physics research have shown that all impor- <br />tant convective rain from mixed-phase clouds involves graupel as a dominant <br />precipitation type (Mason, 1957; Braham, 1981). It has also been shown <br />that graupel in these clouds develops through two main processes (Braham, <br />1968): ice crystal growth by vapor diffusion followed by riming into <br />graupel (hereafter referred to as the IRG mechanism) and coalescence grown <br />drizzle drops followed by freezing and subsequent riming into graupel <br />(hereafter referred to as the eRG mechanism). Both processes may be opera- <br />tive in some clouds but one is usually dominant. For nearly 2 decades <br />after Bergeron (1933) presented his precipitation initiation concept to the <br />scientific community many scientists believed that the IRG mechanism was <br />responsible for all rains of consequence, especially those producing large <br />rain drops. In the early 1960' s the University of Chicago Project W'hitetop <br />group (Koenig, 1963; Braham, 1964) documented the existence of the CRG <br />mechanism in Missouri summer cumuli through in situ measurements from <br />aircraft and, thereby, established it as an important rain-producing pro- <br />cess in mixed-phase clouds. These results confirmed the coalescence deve- <br />lopment of precipitation embryos and, therefore, the probable existence of <br /> <br />4 <br />