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SUMMARY 5 ever, the cloud seeding technology itself is not without significant uncer- tainties; these must be incorporated into any risk-benefit analysis. Details appear in Section 4, The Scientific Basis. 1.5 THE CONDUCT OF CLOUD SEEDING OPERATIONS The principal elements of cloud seeding operations are selection of cloud masses that are likely to be inefficient precipitators, the production of seeding material, its delivery and dispersal within the cloud volume, the resulting cloud dynamical and microphysical transitions to stimulate additional precipitation, the fallout of precipitation, and its transfer to the watershed. The manner in which artificial nucleation can be em- ployed to increase the efficiency of the natural precipitation or to en- hance cloud development is covered in Section 5 on Cloud Seeding Modes and Instrumentation. Artificial ice-forming nuclei can be pro- duced by various methods. The most commonly used is a seeding device or "generator" that bums AgI in solution with acetone and other chemi- cals to emit very large numbers of tiny (0.01 to 1.0 J-Lm) particles com- prised of chemical complexes that include AgI as the primary agent. The generator first vaporizes the AgI and other constituents, and the vapor is then rapidly cooled in the immediate environment, condensing to form submicron smoke particles. Dropable solid-fuel flares are also available. The AgI complexes may have a hygroscopic component, for example. Different complexes nucleate clouds at different rates and by different microphysical mechanisms, so appropriate selection is neces- sary. Cloud temperature is a governing parameter. The activity of nu- cleants normally begins at about _40C and increases exponentially with decreasing cloud temperature (e.g., to 1014 ice particles per gram of AgI produced at -lSoC). A few minutes' supply of this smoke is enough to provide an average of 10 nuclei/liter in a small cumulus cloud. Nu- cleants other than the AgI complexes are available but have not yet been shown to be practical for wide use, with the exception of dry ice. A second frequently used method is to drop dry ice pellets into a cloud that contains supercooled liquid droplets. In the wake of a pellet, some of the supercooled cloud droplets are frozen simply because the air is cooled below the critical temperature of about -40oC where all cloud liquid water particles become small ice particles. However, this process only repre- sents a small fraction of the total number of ice crystals reaching 1012 or more per gram of dry ice. Although a small number of supercooled drops will be caused to freeze when they are subjected to the low temperature prevailing near the dry ice, this process is of negligible importance when compared to the spontaneous generation of ice crystals resulting from nucleation in the vapor phase. Effective delivery and dispersal of the seeding agent from the source to the recipient cloud requires well-planned operations. The type of gener- ating system employed and its mode of operation depends upon the type