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Climax group. The other sensors in the Climax group are snowboards. 4. Pertinent Res'.llts from the Climax and Wolf Creek Pass Investigation A preliminary analysis of the CEmax " experimental data from 1960-65 was reported by Grant and Mielke (1967). The original sample of 283 events indicated the average precipitation on seeded days was 54% greater than on corresponding " non-seeded days for 500 mb temperatures of -20C and warmer. The average precipitation on seeded days was 12% more for 500 mb temperatures from -21C through -23C, but was 15% less when 500 mb temperatures ranged from -24C to -39C. Chappell (1967) in a further analysis of the 1960-65 Climax data found that when only those events having 500 mb wind directions between 210 degrees and 360 degrees inclusive were included (as originally defined for the experiment), the precipita- tion on seeded days was estimated to exceed by 100% the non-seeded precipitation for 500 mb temperatures of -20C and warmer. Further analysis of the 1960-65 Climax data is now completed and results from two other in independent sample.s (Climax II and Wolf Creek 1) are available for comparison. The Climax I and Climax , Climax II results are presented for identical meteoro- logical stratifications. The stratific a tions used in presenting the Wolf Creek I results are quite similar but not necessarily identical to the Climax categories. This results from partitioning the Wolf Creek data independently. The stratifications presented in the results are for meteorological variables which the model presented indicates are relevant for delineat- ing modification potential. These parameters relate to vertical motion through orographic influences, stability and baroclinicity. Other variables measure the available moisture and the potential condenE:ate in the air mass approaching the mountain barrier. The influence of the natural supply of effective ice nuclei is investigated through the 500 mb temperatures (near cloud top). Since the average number of ice nudei activated increases by a factor of about ten for every 4C decrease in temperature, the coldest region where condensation is occurring (near cloud top) produces about 9/10 of the total ice crystals in the cloud system. a. Cloud top temperatures (500 mb) The distribution of seeding effects with 500 mb temperatures is shown in Table I. The trend of the seeding cffcct with temperature is striking and duplicated by all three independent samples. Several of the :statistical tests for the warmest and coldest categories are significant at less than 5% level of con- fidence and a few at the 1% level. Even the relatively small Climax II sample shows such values of significance The results indicate that snowfall has been decreased near the mountain summit when seed- ing the very cold cloud systems. This result is to be expected from the model. Figure 3 indicates that when 500 mb temperatures are -28C and colder, there is present naturally 100 or more excess ice nuclei for the conditions estimated to be representa- tive of the Climax and Wolf Creek Pass areas. Additional ice nuclei are not needed to maximize the pr,ecipitation process and would serve only to reduce the size of the growing crystals. Seeding these very cold cloud systems may be reducing the precipitation efficiency as the ice to water ratio in' the, cloud becomes too large. Part of the obS2 rved decrease may be due to the smaller crystals being carried more easily over the mountain summit, and in a few extremely cold cases the precipitation process may be halted by complete icing of the cloud system. Snowfall increases are estimated to be from near 100% to over 200% by the various statistical methods when seeding the warmest 500 mb tempera- tures. This is consistent with the model which indicated natural ice nuclei was deficient at all 500 mb temperatures above '-21C for estimated Climax and Wolf Creek Pass conditions. It is interesting that the results in the intermediate 500 mb temperature category agree well with the model. The Climax samples show near zero change or slight decreases in snowfall whe~ seeding events having 500 mb temperatures from -21C through -25C. The model indicates optimum ice nuclei concentrations are present in the Climax area at 500 mb temperatures from -21C to -23C. The snowfall increases observed at Wolf Creek Summit when seeding 500 mb temperature from -20C through -23C are also projected by the model since optimum ice nuclei concentrations appear at 500 mb temperatures from -23C to -25C for Wolf Creek Pass conditions. The stronger orographic effect at Wolf Crcck Pass extends the range of modification potential into colder cloud top temperatures. This is quite important since it is in this intermediate temperature range (-21C to -25C) that snowfall in the Colorado Rockies has its greatest frequency. b. Moisture supply The distribution of seeding effects with respect to parameters that measure various aspects of the moisture supplied to the mountain barrier are shown in Table II and Table III. The va.i:.iation of seeding effects with the 700 mb mixing ratio is depicted in Table II. The moisture para- meter shown in Table III is computed by lifting a 700 mb parcel to 500 mb by adiabatic and pseudo- adiabatic processes (above condensation level) and dividing the resultant condensate by the thickness of the saturated layer below 500 mbs. Once again the agreement between the three independent samples is excellent. All show decre ases in snowfall when the drier cases are seeded. Substantial snowfall increases are observed in all samples when seeding the most moist events. The intermediate stratifications indicate near zero change or slight increases for all samples. The snowfall decreases observed vi'hen seeding the lowest 700 mb mixing ratio category are seen to be signifi- cant at the 50/0 level for most tests in the Climax I 15