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<br />to the establishment of a sound hypothesis that can be tested ~n <br />a randomized experiment. <br /> <br />After 3 years of study, a clearer understanding of the cloud types <br />that exhibit modification potential is emerging. Although <br />precipitation is produced by several types of winter clouds in the <br />Sierra, two types have been studied sufficiently to permit an <br />assessment of their potential. It appears that the deep cold-top, <br />pre-frontal, stable orographic cloud has little potential for <br />precipitation augmentation. It contains very little supercooled <br />water and low levels of turbulence, two conditions necessary for <br />the growth and diffusion of ice crystals during the early stages <br />of precipitation formation. The reverse is true, however, for <br />post-frontal convection clouds which appear to have moderate <br />amounts of supercooled water, higher levels of turbulence, and <br />a less efficient natural precipitation process. A randomized <br />experiment to augment precipitation from the post-frontal <br />convective clouds is expected to begin in the near future. The <br />seedability of convective band clouds is still being investigated. <br /> <br />Summary. The technology of modifying wintertime orographic cloud <br />systems to increase mountain snowpack has been developing over the <br />past 30 years. The accumulated evidence from a number of projects <br />indicates that seeding under favorable conditions should increase <br />seasonal precipitation 10 to 15 percent. However, there is also <br />evidence that orographic precipitation can be decreased under <br />certain conditions, emphasizing the importance of having a well- <br />defined and verified physical hypothesis, reacting to seeding <br />opportunities with a timely recognition and response system, and <br />employing seeding delivery systems that can accurately target <br />the seeding material. <br /> <br />It should be recognized that the encouraging evidence about the <br />potential for wintertime orographic snowpack augmentation is <br />largely based on statistical indications from past projects. Most <br />of these projects were evaluated by a statistical analysis of the <br />resulting precipitation and primarily through! posteriori data <br />exploration. There was, in general, only limited provision for <br />physical documentation and analysis of the experiments because of <br />inadequacy of funding and/or inadequacy of measurement capability. <br />These experiments do not, therefor~! have the necessary physical <br /> <br />support to exp~ain and quantify the suggested effects and, therefore, <br />to confidently exploit them on an operational basis. Nevertheless, <br />they do represent a considerable body of encouraging knowledge and <br />experience upon which to formulate the proposed program. <br /> <br />IV-4 <br />