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<br />".... <br /> <br />- ..... J "'!. <br />...:' ... <br /> <br />~ <br /> <br />These two changes in the characteristics are as <br />would be expected if seeding were affecting the <br />cloud over Wolf Creek Pass and are highly unusual <br />if not caused by the seeding. The preponderance <br />of irregular crystals rather than more plate-like <br />crystals is somewhat puzzling although, since the <br />crystals are all smaller, the type and degree of <br />rime are particularly difficult to classify. <br /> <br />Sufficient data have not been collect- <br />ed to make firm statements regarding the effect <br />of seeding on ice crystal characteristics, but <br />the February 19, 1974 storm lends strong support <br />to changes in crystal size and concentration by <br />seeding. The photographic replicator is expected <br />to be operated again this next winter at Wolf <br />Creek Pass with a maximum effort expended on <br />observing seeded events. <br /> <br />5. <br /> <br />CONCLUSIONS <br /> <br />Natural winter orographic storms <br />over Wolf Creek Pass precipitate ice crystals which <br />are 50-85 percent irregular with an average size <br />near 500 microns. Riming is fairly heavy and <br />frequently includes graupel. Concentrations of <br />crystals normally agree within a factor of seven <br />of expected concentrations knowing cloud-top <br />temperature and ice nucleus concentrations. <br /> <br />The one good seeded storm analyzed <br />so far on February 19, 1974 shows a reduction in <br />average crystal size by half and an increase in <br />crystal concentration to 45 times that expected <br />naturally. <br /> <br />The degree of rime and crystal types <br />observed in the winter of 1973-74 suggest that <br />the cloud structure may have been different for the <br />two years. The crystals in 1972-73 were more <br />heavily rimed than 1973-74 suggesting wetter <br />clouds in 1972-73. <br /> <br />Additional data are needed from <br />seeded storms to allow firm conclusions to be <br />drawn regarding the effects of seeding on winter <br />orographic clouds over Wolf Creek Pass. <br /> <br />6. <br /> <br />ACKNOWLEDGMENTS <br /> <br />This study was sponsored by the <br />United States Department of the Interior, Bureau <br />of Reclamation, Division of Atmospheric ~later <br />Resources Management under contract number <br />14-06-D-6644. <br /> <br />7. <br /> <br />REFERENCES <br /> <br />Brown, S.R. 1970: Terminal velocities <br />of ice crystals. Atmospheric Science <br />Paper No. 170, Colorado State University, <br />Ft. Collins, Colorado. <br /> <br />Zikmunda, J. and G. Va1i, 1972: Fall patterns <br />and fall velocities of rimed ice crystals. <br />J. Atmos. Sci., 29, 1334. <br /> <br />472 <br />