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DicEMBER 1978 1775 TABLE 8, As in Table 7 except for unstable clouds. LARRY VARDIMAN AND JAMES A, MOORE TABLE 10. As in Table 7 except for highly unstable cloud. I II r Projects All less CENSARE Precipitation group Crest . Stratifications EPOS>O,OO5J g-' -36OO<BTI 3600 s CBWS>3 g kg-' -30 <LCTTI<-100C M+N=230 Seeded Nonseeded M =98 N = 132 Zeros = 7 Zeros = 26 Mean = 2,08 Mean = 1.70 Ratio (seeded/nonseeded) = 1.22 Test statistic = 2.321 P value =0.020 ~ conditions for seeding. The definitions of terms within the figures are as previously explained. Three regions of cloud stability were investigated: stable, moderately unstable and highly unstable. Stable clouds were defined as those with EPOS <.. 0,005 J g-'; unstable clouds were defined as those with EP?S . >0.005 J g-'; .the highly unstable category whIch IS > 0.05 J g-l IS a subregion of the unstable cloud region. Two major classes of precipitation trajectories were investigated. The crest trajectory class involves those cases where the snow was expected to fall on or near the crest. To achieve this condition, it was assumed that BTI had values between -3600 and +3600 s. Under . this condition, it was believed that the greatest likeli- hood of positive seeding effects would occur on the crest. The second major class of precipitation trajectories was called the blow-over trajectory. Under this condi- tion it was expected that precipitation would not fall on the crest, but probably would fall downwind of the crest area. To achieve the blow-over condition, BTI TABLE 9, As in Table 7 except for moderately unstable cloud. Projects All less CENSARE Precipitation group Crest Stratifications 0.005< EPOS<0.05 J g-' -36OO<BTI<36OO s CBWS>3 g kg-' -30 <LCTII<10oC M+N=104 Seeded Nonseeded M=41 N=63 Zeros = 3 Zeros = 14 Mean= 2.28 Mean= 1.50 Ratio (seeded/nonseeded) = 1.52 Test statistic = 2.004 P value =0.046 . ~,:\:-"~ Projects All less CENSARE Precipitation group Crest Stratifications EPOS>0.05 J g-' -36OO<BTI<3600 s CBWS>3 l~ kg-' -30 <LCTTI<10oC M+N=126 Seeded Nonseeded M=57 N=69 Zeros = 4 Zeros = 12 Mean= 1.93 Mean= 1.88 Ratio (seeded/nonseeded) = 1.03 Test statistic = 1.165 P value =0.242 was assumed to be < -1800 s. A blow-over trajectory could occasionally result in positive seeding effects on the downwind side of the barrier if sufficient moisture were present at low levels to prevent sublimation of ice crystals downwind of the mountain. The moisture variable CBWS was not investigated thoroughly, but was used mainlv to indicate the presenc~ or absence of moisture under various stability and trajectory conditions. Three ranges of cloud-top temperature were investi- gated (warm, moderate and cold). These ranges are defined as warmer than -looC, between -looC and -300C and colder than -300C, respectively. The limit at -looC was chosen based on the expected per- formance of AgI generators and from rough indications of seedability from individual project results.' The limit at -300C was based on results at Climax and model indications that sufficient nuclei should be avail- able naturally at temperature colder than about -30oC. The first combination of variables shown in Table 7 is for a stable cloud with a crest trajectory, moderate TABLE 11. Statistics for unstable clouds with a blow-over trajectory, low water content and LCTII< -300C. Projects All less CENSARE Precipitation group Crest Stratification EPOS>O.OO5 J g-' BTI<-1800 s CBWS < 5 g kg-' LCTII < -300C M+N=58 Seeded Nonseeded M=32 N=35 Zeros = 3 Zeros = 3 Mean= 1.87 Mean= 4.11 Ratio (seeded/nonseeded) =0.46 Test statistic = -2.378 P value =0.018 c..-,>i-~W~'~