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<br />1. INTRODUCTION <br /> <br />1.1 General Background <br /> <br />An investigation of ice crystals which precipitate from winter orographic <br /> <br />clouds near Wolf Creek Pass was conducted by Western Scientific Services, <br /> <br />Inc. (WSSI) to examine the effect of seeding on ice crystal character- <br />I <br />istics in the Colorado River Basin Pilot Project. The Colorado River <br /> <br />, <br />Basin Pilot Project was a major cloud seeding research effort of the <br /> <br />Bureau of Reclamation's Atmospheric Water Resources Program. The target <br /> <br />area for this research project embraced 3,367 km2(l,300 mi.2) in the San <br /> <br />Juan Mountain Range of southwestern Colorado. Wolf Creek Pass is centrally <br /> <br />located in the target area. <br /> <br />The rationale behind the investigation was that seeding to increase <br /> <br />snowfall in orographic clouds causes changes in the characteristics of <br /> <br />the ice crystaI:s in seeded clouds compared to natural clouds. Numerous <br /> <br />investigators have shown that artificial seeding of cold clouds causes <br /> <br />changes in ice crystal characteristics (e.g., Hindman, 1967; Heymsfield, <br /> <br />1970; Holroyd and Juisto, 1971; Reinking, 1973; Vardiman, et. al., 1974; <br /> <br />Hobbs, 1975). Generally, results indicate that if seeding is effective in a <br /> <br />cloud, the concentration of crystals should be increased. Since additional <br /> <br />crystals are competing for the available moisture in the cloud, crystal size may <br /> <br />be diminished, the degree of rime should be less, and changes in crystal <br /> <br />type may occur. <br />