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OJ255~ cloud seeding experiment. Socioeconomic and environmental studies that would complement the physical studies are also outlined. In addition to the design document, the contractor continued studies of predictor vari ables and covari ates as an aid in recognizing seedable events. Cloud climatologies for the three HIPLEX sites were also developed. CONTRACTOR: State of South Dakota Department of Natural Resources Development, Division of Weather Modification, Pierre, South D ak ot a CONTRACT NO. 14-06-0-7240 PRINCIPAL INVESTIGATOR: John A. Donnan PERIOD: July 1, 1974, to May 31, 1976 FUNDING: FY75 - $39,790 FY76 - $ 7,906 In connection with an operational cloud seeding project conducted by the State of South Dakota, the South Dakota Department of Natural Resources Development continued cooperative research started in FY73. Work included atmospheric research, evaluation studies, and scientific assistance for the statewide pro9ram. The purpose of the South Dakota cooperative studies was to enhance knowledge and improve program effectiveness - the key elements in a technically sound applied program of weather modification. The 90al of the State pro9ram was to produce a net economic benefit throu9h reduction of hail damage and augmentation of rainfall during the crop growing season. Analyses indicate that 6.7 percent more rainfall was received in counties where seeding was conducted during the project's four seasons than would be expected from unseeded events. I South Dakota tested the Skywater Environmental Data Network computer system to obtain up-to-the-minute weather data for the seeding projects conducted May through August 1, 1972-75. The Great Plains Cumulus Model was used to predict seedable clouds. Conceptual cloud models developed during FY73 were revised and expanded. The results were appl ied to a study of the dai ly cycle of cloud development in eastern South Dakota. The model study suggested dominant mechanisms controlling the variation of convective activity include: (1) a circulation pattern of general subsidence over the State during the day and general ascending air during the night, and (2) a low-level southerly jet that triggers convection by an associated convergence field. A decision ladder for recognition of seeding opportunities was developed to streamline the transfer of weather modification tech- nology to program operations. This ladder was adopted by the North Dakota Weather Modification Project. Its use may enhance operat ional uniformity in future projects. III-7