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<br />pilot program. <br /> <br />7.0 Indicated Results from Winter Cloud Secding Programs in the Intcrmountain Wcst <br /> <br />Indicated results from both research and operational winter programs in the <br />Intermountain \Vest are summarizcd in the foJlo\\.'ing: <br /> <br />7.1 Rescareh Programs <br /> <br />Climax I and II <br /> <br />Researchers at Colorado Statc University conductcd two wintertimc orographic cloud <br />secding experiments during the 1960's: Climax I (1960-1965) and Climax II (1965-70). The <br />research included randomizcd seeding experiments. using ground based silver iodide (Agl) <br />generators. and parallel physical studies of cloud and seeding processes. Climax I indicated a <br />positive precipitation diffcrcnec of about 6% and in Climax lithe difference was about 18%. but <br />with a high probability that either ofthc differcnces could be due to chancc. Evidenec was found <br />for increases of approximately 25% from seeded systems \vhen wamler orographic cloud-top <br />temperatures prevailed (indexed by the 500 mb temperature being ~ _:WoC). with no difference <br />indicatcd when temperatures were colder. The analysis results were reportcd in Miclke (1971) <br />and a reanalysis by the same author (Mielke et al. 1981). Re-analyses of Climax I & II by <br />Rangno and 110bbs (1987. 1993) yiclded 100\'er, but still positivc. indications of a seeding effect. <br /> <br />Colorado Rinr Basin Pilot Proiect (CROPP) <br /> <br />A five-year randomized cloud seeding experiment was conducted by thc Rcclamation <br />oflices in Denver. Colorado during the early 1970's in the San Juan Mountains of southwestern <br />Colorado. to detennine whether thc experimcntal procedurcs applied in the earlicr Climax work <br />would be effcctive in an operational mode. Seeding was accomplished using ground-based Agl <br />generators. A formal statistical analysis based on 24 hour blocks of precipitation data from 71 <br />experimental treated days and 76 experimental control uays found no significant difference <br />betwcen precipitation on seeded and unseeded days. Howe....er. a /1()steriori analyses based on <br />shorter (6hour) data intcrvals indicated that strongly positive seeding effects may have been <br />achieved during periods of relatively warm-topped cloud occurrences. as expected from the <br />Climax experiment. The results of the a posteriori analyses suggested that a flawlcssly <br />conductcd program of selective seeding could incrcase ovcrall \vintcr precipitation by about <br />10%-12%. Thcse rcsults are presentcd in Elliott et al. (1978). Thc rcsults ofthc 24 hour block <br />analysis may have been negatively alTected by seeding material targeting dilTicultics during more <br />stable storm phases as detailed by Marwitz (1980). <br /> <br />Bridecr Ranee Experiment <br /> <br />A randomized exploratory sceding expcriment was carried out in the Bridger Range of <br />southwestern Montana during the winters of 1969-72. The seed mode was ground-based Agl <br />generators located at mid-mountain or higher locations to avoid sceding material trapping by <br />lower stable layers. Airborne plumc sampling and sih'Cf~in-snow analysis provided evidence of <br />suct:essful targeting of the seeding material. A po.~1 hoc statistical analysis using control gage <br />daw indicated about 15% more seasonal target arca precipitation than prcdicted. Snowpack data <br />