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III. RESULTS OF CLOUD SEEDING TO MODIFY PRECIPITATION 609 served, but high ice crystal concentrations still are observed in some cases with warm cloud tops. Even there, high ice crystal concentrations appear to be associated with the presence of large cloud droplets. In any situation the high ice crystal concentrations act to suppress the con- centration of supercooled liquid water. Despite some notable attempts, no one has yet devised a satisfactory model that combines all of the physical observations and relates them in a consistent way to the indications obtained from randomized seeding trials. A series of ex- periments at Climax, Colorado, provided evi- dence that precipitation increases were associ- ated with 500-mbar temperatures warmer than -200C and with 500-mbar winds from the south- westerly quadrant. In contrast, a randomized experiment near Lake Almanor, California, indi- cated that increases were associated with the westerly winds that followed the passage of a trough of low pressure. The difference in appar- ent results may be related to the importance of secondary ice crystal production in the warm southerly flow that precedes trough passages in California. However, results from experiments and operational programs closer to the Pacific Coast have suggested a good seeding potential in the convective bands in advance of the major troughs. Results of the various experiments and opera- tional programs suggest that a variety of out- comes is possible and that tailoring a seeding operation to a particular project area requires careful consideration of wind fields, siting of generators, diffusion and transport of seeding agents, storm types, presence or absence of sec- ondary ice formation, and other microphysical characteristics of the air masses involved. It has not yet been possible to determine whether or not dynamic effects are important in seeding winter orographic clouds. Some evi- dence has been presented indicating that con- vective bands are intensified by seeding them, but it is not clear whether the greater intensity of the seeded bands was a result of seeding or of natural variations. D. REDISTRIBUTION OF SNOWFALL Calculations of the trajectories of snowflakes and radar observations of trails of snow falling from snow-generating cells at high altitudes show that snowflakes often reach the ground 50- 100 km from where they originate. Therefore, minor changes in fall speed can lead to displace- ments of as much as 20 to 30 km in the location of snowfall. In principle, such changes can be produced by glaciogenic seeding. Investigations in the state of Washington in the 1970s considered the possibility that snow which would otherwise fall on the western slopes of the Cascade Mountains could be in- duced to fall on the eastern side, thereby provid- ing additional water for the arid regions of east- ern Washington. However, it is not necessary to deflect snowflakes as drastically as that to pro- duce an economic impact. For example, causing snowflakes to fall higher on the upwind side of a mountain barrier than they otherwise would can increase the capacity of the snowpack to gener- ate hydroelectric power without any change in the total amount of snow involved. Another suggested application of snowfall re- distribution is to deflect snow away from metro- politan areas, where its presence results in large expenditures for snow removal. During the 1970s the National Oceanic and Atmospheric Administration conducted experiments on Great Lakes snowstorms, which indicated that gla- ciogenic seeding over Lake Erie could reduce the amount of riming of snowflakes falling on the lee shore, thereby reducing fall speeds and caus- ing the snowflakes to be carried further inland before reaching the ground. The experimental findings have not yet been applied in an opera- tional mode. E. SEEDING TO INCREASE PRECIPITATION FROM CONVECTIVE CLOUDS Convective clouds are responsible for much of the precipitation over the temperate zones, particularly in the summer months, and are re- sponsible for nearly all precipitation in the Trop- ics. Numerous attempts have been made to in- crease rainfall from them. They come in a great variety of shapes, sizes, and degrees of organi- zation, and operators have used many seeding techniques on them. 1. Hygroscopic Seeding of Convective Clouds Seeding convective clouds with hygroscopic solutions or powders is sometimes called warm cloud seeding because it can produce results in clouds at temperatures entirely above OOC. Con- vective clouds have been seeded with hygro- scopic agents in many countries, notably India and Pakistan, where hygroscopic seeding has been attempted from the ground as well as from