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CHAPTER II METHODS OF ANALYSIS .. The ~ethods used in the analysis of the attempts to in- crease precipitation and improve precipitation patterns were developed during the progress of the study. Ono method of evaluation considered was that of using scientific forecasts of what would happen if artificial nuclei had not been introduced. After a pilot atte~pt to cor~olate cOMmercial and governmental forecasts with actual occurronces was 8ado this method was abandoned as too un- reliable or indefinite to enable the investigators to detect significant changes if they occurred. The attempts to increase the total precipitation were therefore analyzed by the following methods: statistically studying precipitation on the target area and a control area; comparing snow pack on the target area and a control area; and studying the rainfall patterns over the entire western United States. ", Precipitation ~ Tar~et ~ The statistical study of precipitation on the target area consisted of computing the mean depth of preeipitation on the target area for each year from 1911 thrOU~l 1951, using the Thiessen polygon method. for the nlOnths of Ifarch. April. Hay, and the summation of tho three months. The artificial nucloati'on operations for tho target area lend themselves in general to an investigation of the cumulative effects on monthly precipitation which r:la:)' be produced by sustained cloud seeding. A plot was drawn for each of the periods, consisting of mean depth of precipitation on the target ~rea versus years, forty-year normal mean depth of precipitation on the target area. and forty- and fifteen-year secular series. A moving ton-year secular series for the threc month period was also computed and plotted. The above time series studies were made to denote trends in total monthly and seasonal precipitation. Probability curves were then plotted and the frequency of occurrence of tho precipitation which occurrod during the seeding period was computed. I .1 I f-4 ...":.. CJJ CO Precipitation 2B Target ~ Versus Control Area Comparing preCipitation on the target area and control area. see Fig. 1, consisted of computing the normal mean depth of precipitation on the target and control areas by the isohyetal method for the r,lOnth of March and the mean depth of precipitation on the areas for March, 1951. The ratios of Naroh, 1951. precipitation to normal Harch precipitation for target and control areas were compared. '-' 2