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<br />Subsequent research during twenty-three years from 1946 through 1969 <br />has shown that orographic clouds do in fact have the potential to <br />provide one of the most available and manageable cloud sources <br />for beneficial weather modification to increase water supplies. <br />The greatest effort to develop this potential has been in the <br />western United States. Commercial groups, such as power and <br />irrigation companies took the early initiative. Analyses of <br />these seeding programs served as the primary basis for the conclusion <br />of the 1957 President's Advisory Panel on Weather Modification (1957) <br />that "the most probable effect of cloud seeding operations in the <br />mountains of the west, was a 10-15% increase in precipitation". <br />Indications were that seeding in a nurnber of separate projects <br />produced an overall positive effect. For other types of clouds <br />the indicated potential was not so clear. The conclusions <br />of the Advisory Committee, were, however, sufficiently encouraging <br />to stimulate increased support for research. This increased <br />research effort, primarily supported by the Federal Government, <br />has produced a continually improving basis for realizing the potential <br />for obtaining additional water supplies from cloud seeding over <br />mountainous areas. Important progress in describing and evaluating <br />a physical basis [or a usable technology has been made. Carefully <br />controlled field tests have been conducted that show, with a high <br />degree of confidence, positive increases in precipitation for <br />meteorological situations during which a potential for weather <br />modification would be expected. <br /> <br />I <br />. I <br /> <br />The physical basis for treating coJ.d orographic clouds by seeding <br />was presented by Bergeron (1949) and discussed in more detail by <br />Lcidlam (1955). The orographic induced clouds along and windward <br />of mountain ranges over the western United States, and specifically <br />those over the headwaters of the Colorado River Basin, are fre- <br />quently composed of supercooled liquid droplets. The temperature <br />activation spectra of natural ice nuclei is such that the number <br />of effective nuclei may not meet cloud requirements for converting <br />the cloud water to ice formed at the warmer cloud temperatures <br />and higher condensation rates. In such cases snow may not develop, <br />or the precipitation process may be inefficient. If artificial <br />ice nuclei can be supplied to an incoming saturated air stream <br />far enough upwind of the mountain barrier, a more efficient conver- <br />sion of subcooled cloud water to ice crystals should result in <br />increased snowfall. Otherwise, the unconverted cloud water evaporates <br />to the lee of the mountain barrier. Weather modification potential <br />can be represented as a difference between the supply rate of <br />condensate as the air stream is lifted over the mountain barrier <br />and the growth rate of ice in the cloud system which consumes the <br />liquid condensate. It can be shown under certain cloud temperature <br />conditions that the rate of water consumption by ice crystal <br />growth is less than the rate at which condensate becomes available <br />to the cloud. Considerable losses of cloud water to evaporation on <br />the lee side of the mountain barrier results under these conditions. <br /> <br />,- <br />y <br /> <br />4 <br /> <br />_J <br />