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LARGE SCALE EFFECTS OF CLOUD SEEDINGBACKGROUNDThis report covers the results of the third consecutive year of a fieldprogram to investigate the Large Scale Effects of Cloud Seeding.The work duringthis past year was conducted under Amendatory Agreement No.No.1 to Contract!1-06-D-6841 between the Bureau of Reclamation, Departmtnt of the Interior,and A:rometric Research Inc. (ARI). An earlier contract (No. 14-06-D-5963)- fA. i±Lg in April 1966, and subsequent amendments,enabled ARI to make a "posthoc"analysis of the effects of cloud seeding in extended areas downwind fromsix long term operational and research seeding projects in the western UnitedStates.The results of these earlier studies have been covered in previousreports (ARI 1967, 1969).The number of scientists investigating, or at least looking for, largescale seeding effects is growing each year. In addition to the earlier investig-ators(North Amt rican Weather Consultants, 1956; Braham, 1965; Brier, 1966;Adderly, 1968), more recent studies were summarized at a seminar held in SantaBarbara, California on February 15-17, 1971.The transactions of this seminar(Elliott, et al, 1971) summarized the work of a number of these people.nature.Almost all of the data on large scale seeding effects are statistical inby indirect means.The physical mechanisms controlling these effects can only be inferredTo date, no field project has been conducted to measureadequately thc necessary atmospheric parameters which might lead to an understandingof this phenomenon.The ramifications of these effects are very important.The potential forenhancing water supplies over major watersheds by minimal seeding effort appearspossible.Suggestions of synoptic scale storm modification are beginning toappear. The negative aspects must also be considered. The current concern withthe envirinm,-ntal impact of weather modification will be spread over a largerareo' and targeting problems with small areas becomes more difficult.-1--