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<br />Mountain range east of the Animas River. The project design <br />consisted of a 24-hour experimental unit selected on the basis of <br />a forecast that met specific criteria. Seeding was conducted with <br />ground-based silver-iodide/acetone generators located various <br />distances upwind of the target area. <br /> <br />Evaluation was conducted by Aerometric Research, Inc. 14/ and <br />employed the Mann-\Vhitney rank sum test to assess the significance <br />of the ratio of seeded to nonseeded precipitation. The <br />pre-specified analysis for the 71 seeded and 76 nonseeded days <br />showed no significant difference a~tributable to seeding. This <br />evaluation also revealed two important operational problems. <br />First, the design required that seedable conditions be forecast <br />up to 24 hours in advance. It was found, however, that accurate <br />forecasts could not be made over this timeframe. As a result, <br />unseedable clouds were seeded on some occasions, and seed~b1e <br />clouds were not seeded on some occasions. The second problem <br />arose because, under stable air mass conditions, ground-released <br />seeding material was trapped near the surface. Therefore, the <br />seeding material did not enter the clouds during the seeded <br />period, but was released later to contaminate a nonseeded period. <br />The result of these two operational problems, i. e., inefficient <br />opportunity recognition and uncontrolled targeting of ground <br />seeding material, was that positive seeding effects were balanced <br />by negative seeding effects -- yielding no significant overall <br />effect according to the ~p+j.Q:ri design. <br /> <br />An ~..PQste:rio:rianalysis of 6-hour time intervals that met the <br />seedability criteria indicated seeding had increased precipitation <br />under certain conditions and decreased it under others. It <br />reaffirmed the role of cloud-top temperature in determining <br />seeding potential and the importance of the wind speed across the <br />barrier. This analysis showed that, if seeding were restricted to <br />thick clouds with warm tops and light winds, seasonal increases of <br />lO<to 14 perc~nt could be obtained. <br /> <br />Sierra Cooperative Pilot Project <br /> <br />The Sierra Cooperative Pilot Project is an ongoing Skywater program <br />which began field studies in the winter of 1976-77. It is <br />located in the American River and 'Iahoe--Iruckee River Basins of <br />the Sierra Nevada. The program objective is to develop a better <br />understanding of physical processes of precipitation formation <br />and to identify those conditions which provide the best potential <br />for winter precipitation enhancement. The primary emphasis has <br />been to obtain physical measurements which lead, through analysis, <br />to an increased understanding of the precipitation process and <br /> <br />IV-3 <br />