The Scientific Basis

This chapter describes the scientific basis for two seeding techniques, glaciogenic seeding and hygroscopic seeding, providing a basic understanding of how to increase precipitation. Cloud formation involves the formation of a condensate phase (i.e., liquid or ice) from the vapor phase, followed by the subsequent growth of the condensed phase to hydrometeor sizes smaller than those of precipitating hydrometeors. Glaciogenic and hygroscopic cloud seeding techniques commonly are used to augment the precipitation process. Hygroscopic seeding is becoming more popular for modifying summer (convective) clouds, while most attempts to augment wintertime precipitation focus on increasing snowfall from orographic clouds subject to the generation of supercooled liquid water via orographic lift up windward slopes. The chapter focuses on winter orographic clouds and their potential for precipitation augmentation by seeding.