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The upper limit of water content - where hygroscopic seeding will not produce addi- tional precipitation over that which would have occurred naturally - is currently be- ing defined. This limit involves a complex interaction between updrafts, tempera- tures, ice crystal concentrations, drop sizes, and liquid water content. I I ~, ! Hygroscopic seeding has its most pro- nounced effect on small- to medium-sized cumulus clouds, with the. most dramatic reactions in small clouds. Some smaller clouds, including many which normally may not precipitate at all, can be expected to yield up to a 500 percent increase in moisture. Generally, the precipitation mechanism in the deeper clouds already is efficient and probably cannot be im- proved through hygroscopic seeding. Temperatures, cloud depth, water content, updrafts, droplet size, the number of ice particles present, and wind velocity and direction each form their own window. The point where these mini-windows overlap offers the greatest potential for increased precipitation in summer cumulus clouds. OROGRAj iPHIC $EEiJl1WG WINDOWS I I I I WINTER OROGRAPHIC SYSTEMS also have windows, somewhat simila.r to the ice-phase criteria of summer cumulus. As winter clouds rise over mountains, the ability of the air mass to hold water in vapor decreases as it moves into colder regions at higher elevations. The moisture takes the form of snow, but this process sometimes is not too efficient. It can be enhanced considerably by the introduction of artificial ice particles, such as silver .., .. ," a.......II........._ _ \...... ,-- ~ ,,:,-._~ t,' ':~~:! ~ /D,M,E, Transponder and Antennae Instrumented research aircraft used to obtain data in clouds (MRI Photo). 12