The Nature of Winter Clouds and Precipitation in the Cascade Mountains and their Modification by Artificial Seeding. Part III: Case Studies of the Effects of Seeding

Abstract Three case studies are described in which detailed airborne and ground observations were made in order to evaluate the effects of artificial seeding from the air on cloud structure and snowfall on the ground in a small predetermined target area in the Cascade Mountains. On 31 January 1972, stratocumulus clouds just west of the Cascade crest were heavily seeded with silver iodide for 72 min. Airborne observations showed that the clouds were glaciated by the artificial seeding, new crystal types appeared, and there was an increase in convective activity. Particle trajectory analysis showed that snowfall in the target area should have been affected by the seeding. On the ground in the target area near the Cascade crest new snow crystal types appeared, riming decreased, freezing nuclei in the snowfall increased, and the snowfall rate decreased during the predicted period-of-effect (PPE) of seeding. Further east in the target area, new crystal types appeared, freezing nuclei in the snowfall increased,...