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<br />the CRG mechanism in cold-top summer cumuli that radar meteorologists had <br />inferred from data on the height of first echo development (Battan, 1953 <br />and 1963; Clark, 1960). <br /> <br />Cloud base temperature appears to be a good indicator of whether iCE! <br />crystals (IRG) or large drops (CRG) will likely be the dominant prec:ipita- <br />ti on embryos in a cloud. Warmer cloud base temperatures generally f'avor <br />the CRG mechanism. Other factors such as liquid water content, cloud <br />droplet concentration, cloud depth and updraft speed are also important in <br />this determination, but they are generally related to, influenced by or <br />correlated with cloud base temperature. Consideration of factors other <br />than cloud base temperature is most important in this determination at <br />cloud base temperatures near the value which appears to be the transition <br />between the IRG and CRG precipitation growth processes. Maccready et al. <br />(1957a) developed a precipitation initiation model and calculated that the <br />cloud base temperature separating these two processes was 14 oC. Based on <br />theoretical calculations and an examination of observations of cloud base <br />temperature in different geographical areas, Johnson (1982) showed that the <br />separation cloud base temperature is about 10 oC. Knight (1981) examined <br />hail embryo types in different geographical areas and found that the fre- <br />quency of occurrence of graupel and frozen drop embryos correl ated rather <br />well with average cloud base temperatures calculated from rawinsonde soun- <br />dings on hail days. Knight showed that the percentage of frozen drop <br />embryos increases and the percentage of graupel embryos decreases as the <br />average cloud base temperature increases, with the 50 percent bvalue for <br />both occurring at an average cloud base temperature of about 9-10 oC. The <br /> <br />5 <br />