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16 Meteorology <br />l <br />Table.3.3 Storm precipitation amounts for the Collom Pit area <br />Return period <br />Precipitation <br />(years) <br />(inches) <br />2 <br />1.2 <br />5 <br />1.5 <br />10 <br />1.75 <br />25 <br />2.1 <br />50 <br />2.3 <br />100 <br />2.6 <br />3.4 Temperature <br />Temperatures at the Colowyo Mine are cold in the winter, with average January <br />temperatures in the low 20s ( °F), and warm in the summer, with July having the highest <br />mean temperature in the upper 60s ( °F). Dry air characteristic of northwest Colorado <br />generates large daily fluctuations in temperatures. In Meeker, the average maximum <br />temperature is in the upper 80s ( °F), and the average minimum temperature is <br />approximately 10 °F (WRCC, 2005a). Graphs of extreme maximum, average maximum, <br />average minimum, and extreme minimum are provided in Appendix 3.A for the WRCC <br />stations near the Collom study area. <br />C <br />3.5 Wind <br />Wind data were obtained from the airports at Craig, Hayden and Meeker. At Hayden <br />and Meeker, the annual prevailing wind directions are east - southeast and northeast, <br />respectively, at average velocities of 7.5 and 5.7 mph, respectively. In Craig, the <br />prevailing wind direction is westerly, at an average velocity of 5.5 mph (WRCC, 2005b). <br />Occasionally, winds at these locations reach speeds of 20 to 25 mph. <br />3.6 Evaporation <br />Site - specific pan evaporation or evapotranspiration data are not available for the Collom <br />study area. However, estimates of these parameters are available for the general area. <br />It is important to note that two different types of evapotranspiration values are available - <br />potential evapotranspiration, which is the sum of evaporation and plant transpiration <br />assuming an unlimited supply of water, and actual evapotranspiration. <br />Potential evapotranspiration was estimated as a function of elevation and vegetation <br />during the Colowyo Mine feasibility investigation (VTN Colorado, 1976). The estimated <br />values (at 7,400 ft amsl) ranged from less than 1 inch per month in December to a <br />maximum of 7.5 inches per month in July. The annual total was estimated at 40 inches <br />per year. The potential evaporation rate for the northwest corner of Colorado is <br />estimated at 40 inches per year (NOAH, 1982). Actual evapotranspiration rates for the <br />area range from 10 to 21.5 inches (WATEC, 1981). <br />2572 -R2 Colowyo Coal Company <br />Water Management Consultants <br />