Reanalysis of the Colorado River Basin Pilot Proiect data set indicates the -5 °C height, chosen to represent the height of the threshold activation temperature of AgI nuclei, to be a significant criterion for anticipating seeding effects from ground-based seeding systems. In association with criteria of cloud top temperature and wind speed for analyzing the stable or neutral stages of winter orographic storm systems, it separates the sample into cases with and without apparent seeding effect.
A wintertime operational cloud seeding program was conducted in portions of the Bear Lake watershed of Utah, Wyoming, and Idaho for a 15 year period. Seeding operations were curtailed in 1970 but initiated again in 1979. This sequence of events provides the opportunity to examine possible seedinq effects over the 18 year period as well as studying the natural precipitation regime that fell between the two seeded periods. Results of this analysis indicate approximately an 11 percent increase in April I snowpack water content based upon a target/control evaluation. A double mass plot of the target/control April 1 snowpack water content indicates breaks in slope coincident with the years of project initiation, termination, and re-initiation. This double mass plot lends considerable credence to the interpretation that the indicated 11 percent difference in snowpack can be attributed to the cloud seeding activities.
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