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<br />\\~~ <br /> <br />. <br /> <br />. <br /> <br />3. What cloud seeding can and cannot do today <br /> <br />In general, precipitation requires three factors: (1) A <br />supply of moist air, (2) lift or cooling, and (3) nucleating <br />material. Virtually all weather modification activities <br />operate on one or more of these factors. Cloud seeding deals <br />primarily with supplying nucleating material, although there <br />is controversy over just how this material induces precipi- <br />tation. The accepted theory for many years has been that <br />the nucleating material initiates the Bergeron-Findeisen <br />process. Recently, however, considerable evidence bas begun <br />to accumulate suggesting that the seeding agents trigger <br />dynamic effects, principally buoyant forces. The seeding agent <br />induces sublimation or condensation and freezing. If this <br />taKes place in a short time period, the latent heat released <br />will csuse convective currents which significantly alter the <br />cloud circulation. Thus, it is not so much the ice crystal <br />which is important (as in the Bergeron-Findeisen theory) but <br />rather the latent heat released, leading to greater lift. <br />Nature undoubtedly makes use of both actions. <br /> <br />Cloud seeding has been scientifically investigated over a <br />period of 20 years. Much knowledge has been accumulated <br />during this time; much more information remains undiscovered. <br />We know that under certain conditions precipitation can be <br />increased by cloud seeding. At present, orographic clouds <br />offer the greatest likelihood of consistently meaningful <br />increases. Orographic clouds are extensive cloud systems <br />which form when moist air is forced to rise over mountain <br />barriers. The liftiIlg results in s cooling of the air and the <br />formation of clouds. If the air moving up the slopes con- <br />tinues to be moist these clouds will persist for many hours <br />or days at a time. If a seeding agent (usually dry ice or <br />silver iodide) is introduced into the cloud, the precipitation <br />will frequently be increased. Notice the use of the word <br />"increased." Experience suggests that seeding is more effective <br />on orographic cloud systems which are already precipitating; <br />the seeding increases the total amount of precipitation. <br /> <br />A cap cloud, Which may be considered a special type of oro- <br />graphic cloud, forms on or over an isolated mountain peak. <br />These stratifonn clouds rarely precipitate naturally. University <br />of Wyoming scientists have clearly demonstrated that seeding <br />these clouds with silver iodide does cause snow to fall. <br /> <br />The effects from seeding shower-producing cumulus clouds are <br />difficult to evaluate. The problem involves measuring the <br />rain which falls and trying to detennine how much would have <br />fallen anyway. The individual clouds are relatively small <br /> <br />.f <br />