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SECTION IV. PRESENT STATUS OF TECHNOLOGY . Since the late 1940's, numerous experimental and operational weather modification programs have been conducted to increase precipitation in the mountains of the western United States. Although they were conducted over a wide range of topographical and meteorological conditions, they all traced their beginnings to the studies of Bergeron 10/ and Ludlam 11/. They were designed either to produce additional precipitation or-to investigate the means by which addi- tional precipitation could be produced. All of these projects were based on the physical concept of making the natural precipitation process more efficient by ice-phase seeding. Project Back~round Of the numerous projects conducted in the western United States, four have had the most impact and are of most interest in the planning of this Colorado River Basin Program. They are the Climax experiment, the Wolf Creek Pass experiment, the Colorado River Basin Pilot Project, and the Sierra Cooperative Pilot Project. These projects were conducted under different criteria and in different locations. A brief description of each program and a summary of the initial and reanalyzed results are presented to provide an historical perspective for the proposed program. Climax Experiment The Climax experiment consisted of two consecutive 5-year phases (Climax 1,.1960...1965, and Climax II, 1965-1970) conducted by Colorado State University to investigate mountain clouds and the modification of their precipitation processes. The experimental area was located in the central Colorado Rockies near the town of Leadville. The experimental design consisted of randomized blocks of. seeded and.non,seeded24-:hour experimental units. The suitability of any experimental unit was based upon a forecast made by the National Weather Service of precipitation at Leadville. Seeding was conducted with ground-based silver-iodide/acetone generators located varying distances upwind of the Fremont Pass target area. The evaluation was based on precipitation data from 65 project- operated sites in the target area and eight National Weather Service control stations outside the target area. Three statistical methods -- a two-sample Wilcoxon test, a two-sample sum of squared ranks, and a parametric test described by Tham 12/ -- were used to evaluate the difference in precipitation between seeded and nonseeded periods. The two phases of the experiment were evaluated IV-l