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<br />CONCLUSIONS <br /> <br />The principal conclusions arrived at on the basis of the overall evalua- <br />tion are: <br /> <br />1. The a priori statistical analysis found no conclusive difference <br />between the net target area precipitation on the 71 seeded and 76 unseeded <br />experimental days. Apparently increases from seeding warm-top clouds were <br />countered by decreases from mistakenly seeding cold-top clouds. <br /> <br />2. During 52% of the time in the 147 experimental days the clouds were <br />classified as stable, i.e. the air mass was stable, or neutral, or contained <br />only shallow layers of instability. This accounted for 33 percent of the <br />experimental day precipitation covered by the diagnostic analysis. During 37 <br />percent of the time, with about ten percent of the diagnostic analysis experi- <br />mental day precipitation, warm-top stable clouds occurred. When these were <br />seeded, substantial precipitation increases occurred in the target area and on <br />the downwind flank. During 15 percent of the time, with about 23 percent of <br />the experimental day precipitation, stable orographic clouds with tops colder <br />than -290C occurred. When these were seeded, decreases in precipitation <br />occurred. However, when cold top clouds were seeded under light wind condi- <br />tions, there were increases in precipitation. <br /> <br />3. During approximately 48 percent of the time, and accounting for <br />about 67 percent of the experimental day precipitation, the air mass had <br />sufficient thermodynamic instability to be considered outside of the stable <br />orographic category, i.e., deep convection was present. Seeding under these <br />conditions may have resulted in an increase in precipitation on downwind <br />slopes, particularly when the normal wind component was low. It probably <br />resulted in decreases on the upwind slopes. However, these results do not <br />stand out clearly above the background variability in precipitation. <br /> <br />4. During a considerable number of hours on experimental days, block- <br />ing type flow prevented nucleant from ascending over the target area. <br />Effective entrainment of nuclei was blocked, in whole or in part, in 15.2 <br />percent of the three-hour blocks. Under these conditions the nucleant was <br />carried well to the west or northwest of the target area. A not-seeded <br /> <br />-18- <br />