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Plateau top. Therefore, the plume top temperature can likely be estimated to within 1 or 2 oC even if the aircraft cannot descend into the plume. Each type of generator site will be tested under as wide a variety of storm conditions as nature makes available during the 2-mo field program. The observed T &D of AgI seeding material and SF 6 tracer gas will be related to the prevailing meteorological situation. Post-season simulations with a sophisticated numerical model will be very important in understanding the complex processes involved. For planning purposes it is anticipated that about 15 in-cloud aircraft missions will be attempted over the 2- mo field program. Some may be only partially successful because of instrumentation problems, heavy aircraft icing, severe turbulence, etc. It will be attempted to partition the aircraft mission attempts as listed below, realizing that the actual "mix" achieved may be somewhat different depending upon weather opportunities and other factors. 1. 5 missions will examine simultaneous valley releases of AgI and canyon-mouth releases of SF6. 2. 5 missions will examine co-released propane and SF 6 plumes from the high altitude sites. 3. 5 missions will examine co-released AgI and SF6 plumes from the high altitude sites. Consideration was given to simultaneous valley or canyon mouth releases of AgI during high altitude releases of propane and SF6. However, the risk of confusing interpretation of results was considered too great. The rejected approach has the possibility of simultaneously affecting the same target region with two seeding agents and/or contaminating potential control regions. Further studies of the effectiveness of seeding from the 3 different types of sites will be conducted during periods of darkness, or other times when low-level aircraft sampling is not safe. For example, nighttime releases of AgI from the mouth of Birch Creek Canyon should be readily detectable at the RRS, provided the AgI is transported over the Plateau. 4.3 Microphysical and Snowfall Responses to Seeding A second major objective of the 1994 field effort will be to document microphysical and snowfall changes caused by AgI and propane seeding. Such information is limited in the scientific literature although a few recent articles have provided encouraging results. However, several additional direct detection seeding experiments are needed to document the key physical processes following seeding. These experiments need to be conducted under the typical range of storm conditions that affect Utah's mountains. It is especially important to monitor snowfall changes at the surface under the plume of seeding material. The credibility of winter orographic cloud seeding will be limited until the results of such cause-and-effect experiments are documented. The direct detection experiments will use seeding material releases from the HAS and AHS high-altitude sites. In 1991, the plume from the HAS site crossed the upwind highway on each occasion that seeding was attempted. A plume from one of the two sites is expected to pass over the target (TAR) for most wind directions associated with SLW. During the 1991 field program about 75 percent of hours with SLW greater than 0.06 mm were associated with Plateau top wind directions between 210-270 degrees. However, HAS winds had a more southerly component so the resultant plume was found along the northern half of the upwind highway. High-altitude seeding usually will be with AgI if Plateau top temperatures are colder than -4 oC because plume tops should be near -8 oC in such cases. According to cloud simulation laboratory studies, AgI should produce significant ice crystal concentrations over the Plateau in SLW cloud colder than about -8 oC. 10