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<br />7 <br /> <br />Unfavorable conditions are calm or very light winds and a strong temperature <br />inversion near the surface. These conditions would tend to trap the AgI nuclei <br />in "pools" near the surface and it would be difficult for the nuclei to reach <br />the target area in a timely manner, if at all. <br />An adiabatic chart is used to determine the buoyant enel"gy in the atmosphere, <br />this is important in evaluating the potential for the vertical mixing of seeding <br />materials. Initial moisture and temperature conditions, and the forecast changes <br />in these parameters over the target area, are plotted on an adiabatic chart (the <br />determination of the moisture and temperature forecasts were discussed in Section <br />1 and 2 respectively). A moderately unstable atmosphere from the surface to above <br />cloud base is the most favorable situation; however, stable conditions are <br />seedable. <br /> <br />4. TIME AVAILABLE <br /> <br />The weather situation must allow sufficient time for the AgI nuclei to be <br />transported from the ground-based generator site into the cloud system, for the <br />cloud water to condense on the nuclei resulting in snow, and for the snow to <br />fall from the cloud system into the mountainous target area. Wind flow (direction <br />and speed) from the surface to 2,000 ft. above the average height of the mountain <br />crest need to be evaluated for making targeting and time available decisions. <br />In determining the targeting wind direction, it must be understood that low- <br />level winds can be retarded by the mountain barrier so the Coriolis force cannot <br />balance the pressure gradient force; a drift in the direction of lower pressure <br />results. Moderate low-level winds nearly paralleling the barrier have occa- <br />sionally been observed. However, when favorable mixing is available, the nuclei <br />plume would only travel a few miles from the generator site before rising above <br />the influences of the surface winds. Therefore, with only a slight error, the <br />use of the average wind direction from 4,000 feet below the mountain crest to <br />2,000 feet above the mountain crest will fairly accurately represent a <br />targeting wind direction. Obviously, a favorable direction is one which <br />directs the plume over the target area. <br />The wind speed normal to the mountain barrier at 2,000 feet above the <br />average crest height is critical. If the wind speed is very light, it is <br />questionable if the nuclei will be exposed to sufficient mechanical turbulance <br />and at the same time travel far enough downstream to grow and fall into the <br /> <br />-4- <br />