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<br />Findings from the single-variable study led to a multivariate analysis. <br />Variables used in a multivariate analysis to stratify data should be <br />measures of basic physical effects and be as independent from each <br />other as possible. One variable from each of the four major categories <br />was selected. <br /> <br />The positive energy area (EPOS) represents buoyant energy (joules/gram) <br />and is a measure of the stability of the cloud, or the potential for <br />vertical motion and mixing in the cloud. It is determined from the <br />sounding data plotted on a thermodynamic diagram (specifically, a <br />skew T, log p diagram), and is the area between the level of free con- <br />vection (of a parcel initially at cloudbase) and the equilibrium level. <br />In general, the larRer the value of EPOS, the more unstable the cloud. <br />More vertical mixing of seeding material is likely during unstable <br />conditions. <br /> <br />j <br /> <br />~ <br /> <br />The saturated mIxIng ratio at cloudbase (CBWS), a measure of the <br />water available, indicates the maximum amount of water vapor (in <br />grams per kilogram of dry air) the air can hold at the temperature <br />and pressure of the cloudbase. Normally, high values of CBWS indi- <br />cate warm, usually low, cloudbases. <br /> <br />The lifted cloud top temperature with respect to ice (LCTTI) is an <br />estimate of the lowest temperature (OC) in the cloud, which is where <br />the maximum number of nucleating ice crystals is likely to occur.. It <br />is the temperature at the level in the atmosphere near cloud top where <br />the vapor pressure of the atmosphere is less than the saturation vapor <br />pressure with respect to ice. <br /> <br />The barrier trajectory index (BTI), a measure of the time available, <br />is the difference in seconds between the time required for a snow <br />crystal to travel from the average generator location to the ridge <br />crest and the time required for a crystal to fall from the cloud top <br />to the height of the ridge crest, assuming a fall velocity of 0.5 metre <br />per second. When BTI is zero, snow should fall on the ridge crest; <br />when it is positive, snow should fall upwind, and when it is negative, <br />snow should fall downwind of the crest. <br /> <br />~ <br /> <br />A multivariate analysis was made using a combined data set from six <br />of the seven projects. The Santa Barbara Project was removed because <br />a closer examination of the project indicated that its operating pro- <br />cedures were not comparable with the other projects. A total of <br />1248 cases for the crest group of precipitation gages was available <br />from the remaining six projects. The justification for combining all <br />six projects rests on the assumption that the variables selected for <br />stratification adequately describe the major physical processes that <br />determine seedability in all six projects. <br /> <br />.1 <br />~ <br />I <br />~ <br /> <br />6 <br />