1999-04-26_PERMIT FILE - M1999051 (2)

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Evaporative demand is high at all elevations within the Basin and probably exceeds precipitation throughout most of the year. Average evapotraspi ration is estimated at 17 inches. High pressure systems move in from the west coast and often persist for several days. These conditions result in clear skies with abundant sunshine and low precipitation. The great number of clear sky days promotes rapid heating of the land during the day and also rapid cooling at night resulting in warm days and cool nights during the summer. The additional radiative cooling when a snow cover exists produces greater diurnal temperature changes in the winter. The frost-free period is about 60 days. Mean maximum temperature is about 90 degrees F and occurs in July, while the mean minimum temperature is about 4 degrees F in January. Temperature inversions occur during about 75 percent of the nights throughout the year. Inversions are conditions where air temperature increases with increasing altitude, holding surface air down along with its pollutants. Table K-1 illustrates some of the climatic conditions around the Lease. The differences in temperature are largely due to elevation and whether a station is located in a valley or plateau. Winds are predominantly from the southwest and average 7 miles per hour. At night, air cooling at the surface tends to flow downhill following the local drainage patterns. Channeling effects caused by the terrain result in persistent night-time winds coming from the south, down the Piceance Creek Basin. Winds in the White River Valley are strongly west-east oriented. Highest wind speed ordinarily occur in the late morning, and lowest speeds occur during early morning hours. Average visibility is between 70 and 75 miles, with a maximum visibility of 150 miles. Maximum visibility occurs more frequently during the winter months when the particilate loading of the atmosphere is the lowest because of the snow cover and generally frozen conditions. Sites located in mountain valleys are subject to small scale circulations or wind patterns induced by the valleys themselves. Two of these wind patterns that occur in the Lease tract are called mountain-valley winds and upslope-downslope winds. Depending on the orientation of the valley axis to the upper level winds, these two wind patterns may enhance or oppose the general prevailing wind. G:%LMM78545.002tReclamation%ReGamexh.DOC K-2