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8 , " "HIPLEX results have tended to confirm earlier findings concerning the initiation of natural precipitation in cumulus clouds. In the southern plains, where cloud-base temperature is often +10 to +150C, precipitation is usual- ly initiated by coalescence; however, major storms always involve the Bergeron Process and, possibly, hail formation in their later stages as the cloud tops rise far above the OOC isotherm (Tex. Dep. Water Resour., 1982). In the northern plains, where cloud base temperatures are typically 0 to + 100 C, precipitation generally forms through the Bergeron Process (Hobbs et al., 1980). These findings agree with earlier observations. As noted above, the design of HIPLEX-1 provided a mechanism for moni- toring links in the physical hypothesis linking dry ice seeding to precipitation in supercooled cumulus clouds. A first look at the results of the 20 test cases indicates that dry ice seed- ing produced additional ice crystals (Table IV). Fig. 1, taken from Cooper et al. (1981), demonstrates clearly the marked difference in CIC2 that ex- isted between seeded and unseeded clouds. Table V shows P-values obtained from the MRPP analysis of the HIPLEX- 1 primary response variables tabulated to date (P. Mielke, private communi- cation, 1982). A P-value is simply the probability that the difference be- tween the seed and no-seed cases for the variable and sets of cases in ques- tion arose entirely by chance. The P-values confirm that dry ice seeding pro- duced changes in the concentrations of ice crystals at 2 and 5 min after treatment time in both type Al and type B clouds. The pooled data also show a significant difference in the concentration of rimed crystals 5 min after treatment time. TABLE IV Response variable Average values of HIPLEX-l primary response variables Average value Unseeded Seeded eIe 2 (I-I) eIe 5 (I-I) eCR5 (1"') PIC 8 (1-1) MVD 8 (mm) Awe 8 (gm-3) TFPI (s) TFE (s) TIPA (s) TIPR (s) AER (mm km') RERC (mm km') 0.2 1.1 0.5 0.02 0.97 0.03 790a 1 750a 2110a 2210a 0.37 0.0015 15.8 20.8 45.2 0.05 1.13 0.04 650 1 440a 1 930a 2270a 1.23 0.32 a Average reflects inclusion in calculations of default values for non-precipitating clouds.