The Area of Maximum Effect Resulting from the Lake Almanor Randomized Cloud Seeding Experiment

Abstract

A randomized cloud seeding experiment was conducted on the Lake Almanor watershed near Mt. Lassen, Calif., during five winter seasons, 1962-1957. The target area extended approximately 20 mi east-west and 15 mi north-south, and ranged in elevation from 4500-5400 ft MSL. Silver iodide was released from groundbased, acetone solution generators which were located between 5000 and 7400 ft MSL. Silver iodide releases were made for 12-hr periods, these seeding periods being subsequently divided into four weather categories, depending on wind direction and temperature. In three of these categories, which together produce approximately 85% of the total precipitation, no response to the seeding was observed in the target area. In the remaining category, characterized by westerly winds and cold temperatures, the increase peaked at approximately 57% between 5 and 11 mi downwind, and averaged 37% throughout the 21-mi distance. Both results were statistically significant at the 5% level.