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<br />tures vary from a high of _6oC in the southeast to a low of -lloC in <br />the west-central High Plains. The mean top temperatures for pre- <br />cipitating systems are 1.4oC colder than those of nonprecipitating <br />systems and h~ye an axis of cold top temperatures oriented north- <br />south lying along the western edge of the High Plains. This relation- <br />ship between cloud top temperature and precipitation is logical <br />because cold temperatures would lead to larger numbers of active <br />freezing nuclei and would generally indicate thicker cloud layers. <br />Both conditions would be favorable for producing precipitation. <br />Results of several cloud seeding experiments in the mountains of <br /> <br />the Western U. S. have indicated that seeding can be effective on cold <br />orographic clouds with top temperatures warmer than -24oC (Chappell t <br />1972). Figure 26 shows how changing the model 's top temperature limit <br />from -15oC to -23oC would increase the number of seedab1e upslope con- <br /> <br />ditions by 45 percent in a 10 year period at Denver, Colorado. An <br />extension of the temperature range criteria from -15 to -23oC would <br />thus increase the number of seedable episodes at Denvert Colorado, <br />from 13.7 per year (fig. 5) to 19.2 per year. <br /> <br />MOIST LAYER TOP TEMPERATURES <br />.Denver. Colorado <br /> <br />Number 01 <br />Occurrences <br />25 <br /> <br />. Period 01 Record. Jan. 1961 to Oec. 1970 <br /> <br />Figure 26: Cold season frequency <br />distribution of cloud top tempera- <br />tures for seedable upslope cloud sys- <br />tems at Denver, Colorado, for the <br />10 years from January 1961 to <br />December 1970. An estimate of the <br />frequency distribution for cloud top <br />temperatures be 1au..' -17oCis in- <br />dicated on the figure. <br /> <br />20 <br /> <br />15 <br /> <br />10 <br /> <br />;Esllmate <br /> <br /> <br />31 <br />