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13 <br />Coo <br />3 METEOROLOGY <br />3.1 Regional climatology <br />The climate in the vicinity of the Colowyo Mine is semiarid and cool. Due to the low <br />humidity and elevation, the temperature contrast between day and night is considerable. <br />Humidity is generally quite low, which favors rapid evaporation and evapotranspiration. <br />As a result of Colorado's distance from major sources of moisture (the Pacific Ocean <br />and the Gulf of Mexico), precipitation is generally light in the plateau country of <br />northwestern Colorado. Prevailing air currents reach Colorado from westerly directions. <br />Eastward- moving storms originating in the Pacific Ocean lose much of their moisture as <br />rain or snow on the mountaintops and westward- facing slopes. Warm, moist air from the <br />south moves into Colorado infrequently, most often in the spring, summer and early <br />i autumn. As this air is carried northward and westward to higher elevations, heavy <br />rainfalls (and sometimes wet snows) occur over the eastern portions of the state. For <br />southern and western Colorado, the intrusions of moist air are most common from <br />mid -July into September, and are associated with wind patterns sometimes called the <br />Southwest Monsoon. <br />For most of western Colorado, the greatest monthly precipitation occurs in the winter <br />months, while June is the driest month. Near the Utah border, late summer and early <br />autumn can be the wettest time of year. While precipitation only averages from 8 to <br />14 inches in these western valleys, localized flood - producing storms are still possible. <br />Occasionally, moisture from decayed Pacific hurricanes fuels widespread heavy rains <br />(Colorado Climate Center, 2005). <br />3.2 Site data sources <br />Weather conditions have been recorded at ten stations located in the vicinity of the <br />Collom study area and the surrounding region at the locations shown in Figure 3.1. The <br />rainfall- recording stations include four weather stations at the Colowyo Mine, which are <br />designated as SCN 16, SCN 24, WSTPT and GOSSARD. All the stations are on bluffs <br />above the Colowyo pit, except for GOSSARD, .which is located in the valley by the <br />Gossard loadout (Figure. 3.1) . The Colowyo Mine is located about 4 miles to the east of <br />the Collom study area. Data recorded from the Colowyo stations included average daily <br />temperature and precipitation. Precipitation data from the Colowyo stations are <br />summarized in Appendix 3.A. <br />2572 -R2 <br />Colowyo Coal Company <br />Water Management Consultants <br />