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<br />results of model calculations of hail embryo type in different geographical <br />areas by Nelson (1979) are consistent with the cloud base temperature rela- <br />tionship deduced by Knight. <br /> <br />Clouds with an active coalescence process appear to be associated with the <br />development of secondary ice particles, a process usually referred to as <br />SICP (secondary ice crystal production). However, an active coalescence <br />process has not been established as a prerequisite for SICP. This type of <br />SICP has been found to occur in the laboratory by two main processes, <br />freezing-splintering and rime-splintering. The freezing-splintering pro- <br />cess may generate secondary ice particles by ice splinter ejection in the <br />course of spike formation which frequently accompanies the symmetric <br />freezing of large drops (Johnson and Hallett, 1968; Hobbs and Alkezweeny, <br />1968; Pruppacher and Schlamp, 1975). The rime-splintering process, usually <br />referred to as the Hallett-Mossop mechanism, may generate secondary ice <br />particles during the growth of graupel by riming at a specific temperature <br />range (Hallett and Mossop, 1974) and with specific cloud droplet sizes and <br />concentrations (Mossop, 1976; Mossop, 1978a). Mossop (1978b) has shown <br />that cloud base temperature and cloud drop concentration are useful in <br />separating cloud conditions in which SICP by the rime-splintering process <br />takes place from those in which it does not. Considering cloud base tem- <br />perature only, the "ice multiplication boundary" appears to be at about <br />5 oC, with SICP occurring when cloud base temperatures are warmer. <br /> <br />SICP may also occur in other ways. Gagin and Nozyce (1984) found that the <br />production of secondary ice particles resulting from the freezing of 1-2 mm <br />drops in the laboratory seemed to be attributable to nucleation at relati- <br /> <br />6 <br />