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<br />. Moderately unstable clouds with moderate-to-high water content, <br />cloud top temperatures between minus 10 and minus 30 oc, and a <br />crest trajectory for the precipitation. <br /> <br />Seeding appears to decrease precipitation across the entire mountain <br />barrier under the following condition: <br /> <br />. Unstable clouds with low water content, cloud top temperatures <br />less than minus 30 oc, and winds such that the precipitation par- <br />ticles would be carried beyond the mountain crest and evaporate <br />before reaching the ground. <br /> <br />r. <br />) I <br /> <br />Statistical tests applied to the data indicated that it is very <br />unlikely that the observed effects would have occurred by chance. It <br />should be emphasized that these results represent only a portion of <br />the work that will be carried out. The bounds of the seeding windows <br />need to be refined, and the expected effect must be translated into an <br />estimate of the amount of additional precipitation a given area could <br />receive during the winter season. This requires knowing the percent- <br />age change in precipitation associated with each window, the frequency <br />of occurrence of storms within each window, and the contribution of <br />each window to total seasonal precipitation. <br /> <br />~ <br />j <br /> <br />I. Data Base <br /> <br />Of the numerous winter orographic projects that could have been <br />included in an analysis of this type, seven projects were finally <br />selected. Three criteria were used in screening candidate projects; <br />(1) a broad spectrum of topographic and meteorological conditions <br />were to be included, (2) the decision to seed or not seed a particu- <br />lar event was a statistically random choice. and (3) sufficient qual- <br />ity data had to be readily available to permit a reasonable liklihood <br />that statistical significance could be reached in the analysis and <br />evaluation. <br /> <br />The seven projects were: (1) the Bridger Range Experiment in Montana, <br />(2) the Climax Experiments in Colorado, (3) the Colorado River Basin <br />Pilot Project in Colorado, (4) the Jemez Project in New Mexico. (5) the <br />Pyramid Lake Pilot Project in Nevada. (6) the Central Sierra Research <br />Experiment in California. and (7) the Santa Barbara II Project in Cali- <br />fornia. Figure 1 shows their locations and table 1 lists their sites <br />and periods of seeding operations. <br /> <br />~ <br /> <br />2 <br />