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SUMMARY REPORT GENERALIZED CRITERIA FOR SEEDING WINTER OROGRAPHIC CLOUDS ~ I The Bureau of Reclamation 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 study is seeking generalized seedability criteria based on physically meaningful and E~asily measurable variables. Seven proj- ects that operated during the 1960ls and 70's in the Rocky Mountains, the Sierra Nevada, and California's southern Coast Range are being used in this study, representing a wide range of meteorological and topographic conditions. Four variables calculated from the mountain topography and the ver- tical distribution of temperature, moisture, and wind were selected for detailed analyses of the effects of seeding. These variables are (1) the stability 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 cloudbase, a measure of the amount of water available for conversion to precipitation; (3) the calculated cloud top temperature, a measure of the number of natural ice nuclei available to start the precipitation process; 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 associated 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 combinations, 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 precipitation 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. Results of the study thus far suggest that seeding can increase precip- itation at and near the mountain crest under the following conditions: . Stable clouds with moderate water content, cloud top temperatures between minus 10 and minus 3Q oC, and winds such that the precipita- tion particles would be expected to fall at or near the crest of the mountain barrier.