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<br /> <br /> <br />stable conditions) and the LFC was now at <br />600 mb (compared to the earlier 640 mb). <br />Precipitable water amounts ofO. 70 in (1. 79 <br />em) in the lowest 150 mb layer and of 1.14 <br />in (2.90 em) in the surface-to-500 mb layer <br />had also decreased slightly from the 1920 <br />GMT values. <br />A plot of equivalent potential temper- <br />ature ((Je) vs. height for the three Sterling <br />soundings (Fig. 17) demonstrates the dif- <br />ferences in moisture and stability charac- <br />teristics of the air masses ahead of, im- <br />mediatelv behind, and well behind the <br />trailing front. Of most interest are the <br />changes that occurred within the lowest 2 <br />km. The 1340 GMT sounding showed a <br />layer of high (Je values very near the surface <br />with a rapid decrease to a minimum just <br />above 2 km. Although Sterling was well <br />within the cool air mass behind the leading <br /> <br />100 <br /> <br />150 <br /> <br />200 <br /> <br />150 <br /> <br />600 <br /> <br />700 <br /> <br />800 <br /> <br />900 <br />1000 <br /> <br />front, the moist layer was actually very <br />shallow. The 1920 GMT sounding indi- <br />cated a dramatic increase in (Je throughout a <br />layer extending from the surface to almost 4 <br />km. Values of (Je at the surface had in- <br />creased from 343 K to 353 K, and (Je at 1 km <br />AGL had increased from 334 K to 345 K. <br />During the same period the surface tem- <br />perature had increased only 5. 9aC, while the <br />temperature at 1 km AGL had actually de- <br />creased 0.9aG. The large changes in (Je <br />were therefore primarily due to the arrival <br />of the more moist air behind the trailing <br />front. The zone characterized by high (Je <br />and a deep moist layer was approximately <br />100 km in width. The 2202 GMT sounding, <br />taken when the trailing front was about 120 <br />km west of Sterling, showed a decrease of <br />4.5 K in mean (Je for the lowest kilometer. <br /> <br />Figure 13. SkewT/Log P plot 01 Sterling, Colorado, upper-air sounding taken at 1920GMT, 31 July <br />1976. <br /> <br />17 <br />