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<br />III. Results <br /> <br />Comparisons betw~en average 6-hour precipitation amounts from seeded <br />events and those\ from nonseeded events for the total sample showed <br />higher seed/no-seed ratios than was expected, suggesting the possi- <br />bility of a strong positive seeding effect not previously observed. <br />When the Central Sierra Experiment was removed, the ratios decreased <br />to values consistent with results from the individual projects. How- <br />ever, the determ~nation of whether the high values represent a real <br />seeding effect has not yet been made. <br /> <br />The results of t~e analysis of precipitation at the crest group of <br />gages, excluding! the Central Sierra Experiment, indicate that when <br />at least moderat~ cloudbase moisture was available, the cloud top <br />temperatures were between minus 10 and minus 30 oc, and precipita- <br />tion had a trajectory on or near the crest, increases in crest <br />precipitation of: about 20 percent were associated with seeding, <br />irrespective of the stability of the clouds. For the same moisture, <br />temperature, and! trajectory conditions, increases near 50 percent <br />occurred for mod~rately unstable clouds. Decreases of more than <br />50 percent occur~ed at the crest with unstable clouds, low cloudbase <br />moisture, low clpud top temperatures, and trajectories carrying <br />precipitation downwind of the crest. When cases with more cloudbase <br />moisture and traoectories nearer the crest were included in the cold <br />cloud top window!, the results were about the same, suggesting that <br />the presence of the cold cloud top may override otherwise favorable <br />ranges of the other variables. Figures 2 through 5 present the results <br />(the P-value is a statistical term indicating the probability that the <br />observed effect ~ould have occurred by chance; e.g., a P-value of <br />0.01 means one chance in 100 that the effect is a chance occurrence, <br />or 99 chances ou~ of 100 that the effect is real). <br /> <br />The results apply primarily to the Rocky Mountain region, although <br />the Pyramid Lake Pilot Project results are included in these con- <br />clusions. Increases may be greater in the Sierra Nevada because of <br />different meteorological and topographic conditions, but the reason <br />for this differepce, if it truly exists, has not yet been resolved. <br /> <br />The results pres~nted thus far apply to the crest group of precipi- <br />tation gages, bu~ it is important to consider the effects outside <br />the crest group.' Three gage groups were used in this portion of <br />the study: upwind, crest, and downwind. Because the Climax Experi- <br />ment had data on~y for the crest, it was removed from the analysis. <br />I <br />I <br />, <br />Results of the t~ansbarrier analysis suggest that seeding stable clouds <br />with moderate-tothigh cloud water, cloud top temperatures between minus <br />10 and minus 30 ~C, and a crest trajectory led to increased precipitation <br />for the crest and downwind areas and had no effect for the upwind area. <br /> <br />7 <br />