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<br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br /> <br />2.5 Atmospheric Stability <br /> <br />Temperatures in the free atmosphere typically decrease with increasing heights. <br />An atmospheric inversion occurs when the temperatures above the earth's surface at a <br />given point actually increases in temperature. These inversions lead to trapping of <br />materials below the inversion. Inversions that persist over cities can often lead to the <br />formation of smog beneath the inversion (i.e. Los Angeles). Atmospheric stability refers <br />to the rate at which the temperature decreases with height. A standard rate (normally <br />referred to as a lapse rate) of decrease is approximately 2 oC per 1000'. In cloud seeding <br />applications, we are interested in knowing if atmospheric inversions or stable lapse rates <br />(rates less than the standard lapse rate) are present in the area of interest during storm <br />periods. If so, they will likely restrict the vertical rise of the seeding material which may <br />render seeding under these circumstances ineffective if the silver iodide particles do not <br />reach the -5 Oc level in the atmosphere. <br /> <br />The following lower-level stability analysis, which utilized the balloon sounding <br />information from Grand Junction, has some limitations since the soundings from Grand <br />Junction are in an area of different topography from that of the project area. Stability <br />below about 600 mb (-14,000 feet) was examined in these soundings, and the results <br />summarized in Table 2-3. The atmospheric temperature profile was well-mixed in 72 <br />cases, or about 43% of the time; there were minor stable layers or slight static stability in <br />49 cases, or about 30% of the time; and a more definitely stable atmosphere was observed <br />in 45 cases, or 27% of the time. The atmosphere was most stable during the month of <br />January, with half of the soundings quite stable and over three-quarters having some <br />stability. March and April soundings had the least stability, with over half being well- <br />mixed. Figure 2.10 provides a plot of this stability information by month. <br /> <br />A stability analysis comparing wind direction sectors does not seem to show a <br /> <br />clear relationship between stability and 700-mb wind direction, although the atmosphere <br /> <br />was perhaps most well-mixed when 700-mb winds were westerly. Of the few cases with <br /> <br />an easterly component, most had at least some stability below 600 mb. <br /> <br />18 <br />