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<br />EXECUTIVE SUMMARY <br /> <br />The main goal of this feasibility study was to examine the suitability of the Sevier River Basin for <br />application of winter cloud seeding intended to enhance the high altitude snowpack. In large measure this <br />meant examination of the meteorological conditions during stonns with particular emphasis on the <br />availability of supercooled (below 0 oc) liquid cloud water. Additional goals were to develop an <br />experimental design to validate the effects of winter cloud seeding in the basin, and to estimate probable <br />benefit-cost ratios for an operational seeding program. <br /> <br />A brief discussion is given of the fundamentals of winter mountain clouds and cloud seeding. Basically, <br />for seeding to be effective, clouds must exist that are at least partially composed of tiny SL W (supercooled <br />liquid water) droplets that are not naturally converted to snowflakes. Seeding involves conversion of some <br />of the cloud droplets to ice crystals that can grow to snowflake sizes and settle to the surface. Ice crystals <br />rapidly grow in a supercooled liquid droplet environment because of the difference in saturation vapor <br />pressure over water and over ice. Forced uplift of moist air over a mountain barrier can produce abundant <br />SL W over the windward slope and crestline but the SL W nonnally evaporates to the lee of the mountain <br />as the air descends and wanns. Thus, cloud seeding is a "race" between fonnation of ice crystals and their <br />growth and fallout to the surface before the cloud droplets descend and evaporate downwind of the barrier. <br /> <br />The most common seeding agent is silver iodide (AgI) which has a crystalline structure similar to ice.. <br />It can be released from the ground to produce significant concentrations of ice crystals in liquid clouds <br />colder than about -8 oC. Methods of abruptly chilling the air; for example, by the use of dry ice or by <br />the expansion of propane or other gases, can be used to create ice crystals in liquid clouds colder than <br />o oC. <br /> <br />Statistical evidence is reviewed from both operational seeding intended to increase snowfall, and <br />experimental seeding intended to increase knowledge. The statistical evidence from Utah is inconclusive <br />but suggests seasonal snowfall increases of about 10 percent may have been achieved. Similar suggestions <br />have resulted from some other projects in the West but scientific evidence is still lacking. <br /> <br />A considerable body of physical evidence has been collected in Utah and some neighboring states in recent <br />years concerning the availability of SL W, a necessary ingredient for cloud seeding to be effective (it is <br />recognized that SL W availability does not in itself guarantee seeding potential). Review of this evidence <br />shows a considerable amolll1t of SL W passes over mountain barriers during the course of each winter <br />without being converted to snowfall. The amount of excess SL W is roughly of the same magnitude as <br />the annual streamflow from the mountain watersheds that have been studied in Arizona and Utah. Thus, <br />the "raw material" needed for cloud seeding exists in abundance. Most of the seasonal flux of SL W is <br />concentrated in a few large stonns that produce significant snowfall during their more efficient stages. <br />The challenge is to optimize the conversion of excess SL W into snowfall during inefficient stonn stages. <br /> <br />The major uncertainty with winter orographic cloud seeding concerns delivery of appropriate <br />concentrations of seeding material to the proper cloud regions, Many winter orographic programs use AgI <br />ground generators but limited documentation exists concerning the T&D (transport and dispersion) of the <br />AgI into the clouds. Does the AgI reach the desired cloud region when and where SL W is present and, <br />if so, are AgI concentrations adequate to create significant numbers of ice crystals? There is considerable <br />evidence to suggest that valley-released AgI often is trapped in a shallow layer due to the stability of the <br />lower atmosphere. On the other hand, frontal passages may tninsport this seeding material into the clouds. <br />Moreover, embedded convection was shown to transport valley-released AgI well up into the mountain <br />clouds during the early 1991 field program in central Utah. A number of studies have shown that high <br /> <br />Hi <br />