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4-17.3 <br />• 4.9 METEOROLOGY <br />The climate of west-central Colorado is characterized <br />by dry air masses, which are modified Pacific air masses <br />that move eastward across the Rocky Mountains. Winter <br />snows and summer showers or thunderstorms result in un- <br />usually even distribution of precipitation throughout <br />the year. Prevailing winds vary greatly throughout the <br />Upper Colorado River Basin, and are markedly affected by <br />differences in elevation and by the orientation of moun- <br />tain ranges and valleys with respect to general air move- <br />ments. <br />• Five years of upper air. observations at Grand Junction <br />show that surface based inversions occur on 84 percent <br />of the mornings. During the afternoons they are not as <br />common, occurring 11 percent of the time in winter but <br />less than 3 percent of the time in other seasons. The <br />area is subject to a relatively high frequency of stag- <br />nation situations, mostly in winter. <br />National Weather Service Records at Fruita indicate an <br />annual average precipitation of 8.8 inches. Vegetation <br />on the tract suggests an annual precipitation of approxi- <br />mately 11 inches at the bottom of East Salt Creek Valley <br />-~- <br />and perhaps 15 inches on the higher ridges above 7,000 <br />• feet. Evaporation is estimated to be about 45 inches an- <br />nually. <br />