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<br />/ <br /> <br />Following the passage of an upper level trough there is a marked lowering <br />of cloud top heights and most of the cloudiness is below the 500 mb level. <br />Since cloud tops are more clearly determinable on adiabatic diagrams (~85~ RH) <br />in posttrough situations, the actual .fJoud~QJ2 tel!lPerature is used. Again, the <br />range in temperature that has been determined to be favorable is from -12oC to <br />-27oC. <br /> <br />In both pretrough and posttrough situations, convective cells embedded in <br />orographic cloud systems provide seeding opportunities with cloud top temperatures <br />as low as -300C. The reason for this is that the updrafts in the convective <br />cells protect the condensed cloud water from the ice crystals which would other- <br />wise be falling from higher levels to provide natural seeding. <br />Current 500 mb and cloud top temperatures over the target areas arc estimated <br />from the Grand Junction and Denver rawinsonde data. Forecast changes in the <br />500 mb temperature, the timing of upper-level trough passages, and posttrough <br />cloud top temperatures are derived by evaluating the current NWS standard upper <br />air analyses (850, 700, 500 and 300 mb charts), the previous 12- and 24-hour <br />charts for these levels, and all available prognostic charts from the NWS that <br />are based on computer models. All available information is evaluated in light <br />of our own experience and used to develop the operational forecast. The 500 mb <br />temperatures, time of trough passage and posttrough cloud top temperatures are <br />entered on the sketch of expected cloud conditions mentioned in Section 1. <br /> <br />3. MIXING AVAILABLE <br /> <br />Good mlxlng from convective currents and upslope flow aids significantly <br />in the transport of AgI nuclei from the ground-based generators into the <br />orographic cloud system. Standard plume dispersion calculations would require <br />up to 120 minutes for the nuclei to reach cloud base; however, mixing reduces <br />this time to 15 to 40 minutes. Parameters useful in estimating the available <br />mixing are surface winds, temperature inversions and atmospheric stability. <br />The most favorable condition is definite upslope flow from the ground- <br />based generator locations toward the target areas. This flow is determined by <br />evaluating the pressure gradients on the surface and 850 mb analyses, the actual <br />wind reports from NWS reporting stations near the target areas, and current <br />observations from ski areas in the target areas. <br /> <br />-3- <br /> <br />I <br />~ <br />