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SUMMARY REPORT GENERALIZED CRITERIA FOR SEEDING WINTER OROGRAPHIC CLOUDS .- The Bureau of ~eclamation is carrying out an analysis of data from past weather modification projects to identify criteria that define conditions when cloud seeding will increase winter snowfall in moun- tainous terrain and when it will have no effect or decrease precipi- tation. The s~udy is seeking generalized seedability criteria based on physically ~eaningful and easily measurable variables. Seven proj- ects that operated during the 1960ls and 70's in the Rocky Mountains, the Sierra Nev~da, and Californials southern Coast Range are being used in this s~udy, representing a wide range of meteorological and topographic co~ditions. , I Four variables :calculated from the mountain topography and the ver- tical distribution of temperature, moisture, and wind were selected for detailed arlalyses of the effects of seeding. These variables are (1) the st~bility of the cloud, which is a measure of the like- lihood that seeding material will reach a level in the cloud where it can affect the precipitation process; (2) the saturation mixing ratio at cloudQase, a measure of the amount of water available for conversion to precipitation; (3) the calculated cloud top temperature, a measure of tne number of natural ice nuclei available to start the precipitation Rrocess; and (4) the calculated trajectory index, a measure of the :time available for precipitation particles to form, grow, and fall to the ground. Previous research suggested that certain ranges of the individual variables are ~ssociated with positive seeding effects, while other ranges are associated with decreases or no effect. Many combinations of ranges are possible, such as stable clouds, low water content, cold cloud tops, and downwind trajectory, but attention was first given to those combinaqons, or "windows," most likely to show the effects of seeding. Six-hour precipitation totals for both seeded and nonseeded events in each :of the windows were determined. Ratios of the average seeded precipi~ation to the average nonseeded precipitation were com- puted to determine the effects of seeding for each window and to com- pare seeding effects among windows. I Results of the study thus far suggest that seeding can increase precip- itation at and ;near the mountain crest under the following conditions: . Stable clou4s with moderate water content, cloud top temperatures between min~s 10 and minus 3Q oe, and winds such that the precipita- tion particles would be expected to fall at or near the crest of the mountain barrier.