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000107 systems have led to a much improved understanding of the chain of physical events leading to precipitation. Problems in implementing operational cloud seeding programs have been recognized and can be overcome with current instrumentation, communication, and data processing systems. While uncertainties remain, a useful tech- nology of snowfall and runoff enhancement by a well conceived and conducted CREST Program can be demonstrated. Further, continued physical investigations will allow the technology to be refined with time so that the efficiency and effectiveness of precipitation aug- mentation can be improved. The general hypothesis for seeding winter orographic clouds to increase snowpack and runoff can be summarized as follows: a. The flow of moist air over mountain barriers produces liquid water condensate at rates determined by the temperature, humidity, and upward motion of the air. The condensate, consisting of very small droplets, tends to evaporate rapidly in the lee of mountain barriers due to downward motion and associated warming of the air. b. The condensate tends to remain in the liquid state at tempera- tures colder than 0 .C due to the relative scarcity of ice nuclei in the free atmosphere, particularly at temperatures between about 0 and -20 .C. The condensate is then referred to as a "supercooled cloud." c. Ice particles present in a supercooled orographic cloud grow at the expense of the supercooled droplets, rapidly becoming much larger than the droplets and, thus, achieving far greater settling velocities. Initial ice particle growth is by diffusion due to the difference in equilibrium vapor pressure over water and ice. Further growth may also occur due to accretion of liquid cloud droplets freezing directly on the ice particles colliding with them, aggregation (collecting together of the ice particles), or both. However, total ice particle growth is often by diffusion only. If total ice particle growth and associated settling are sufficient, some particles reach the surface of the mountain barrier as snow- flakes prior to evaporation of the condensate and subsequent sub- limation of the remaining ice particles to the lee of the barrier. d. The ice particle concentration is sometimes less than required for maximum precipitation fallout. In such cases, increasing the ice particle concentration can convert more of the condensate to snow and decrease the amount of water evaporated or sublimed to the lee of the barrier. The general hypothesis stated above is the static seeding hypothesis as applied to orographic clouds. That is, the sole effect of seeding is an increase in the precipitation efficiency of the cloud systems over the barrier. The ability of clouds to respond to seeding in this way 26