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2.04.8 CLIMATOLOGICAL INFORMATION <br />1. Regional Climate <br />The Keenesburg mine site is situated in an area which has a "continental" type of <br />climate. Characteristics of the climate include low relative humidity, a large amount <br />of sunshine, light rainfall (confined largely to the warmer half of the year), moderately <br />high wind movement, a large daily range in temperature (high day temperatures in <br />summer and generally in the winter a few protracted cold spells)26. During the mid - <br />1800's early settlers referred to eastern Colorado as "The Great American Desert" <br />because of its aridness and short grass vegetation. <br />The project area represents an example of the rain shadow effect because of the <br />delicate balance of climate, soil, and vegetation. The mountains to the west cause <br />moisture -laden air to release water in the mountains. Air now low in relative <br />humidity continues to more eastward over the plains, and the water in the area is <br />rapidly taken up by the air leaving very little moisture for the vegetation. This <br />condition causes rapid cooling at night. <br />Table II -17 summarizes the statistical climatic characteristics of the project area. <br />Temperatures range from 109°F to -45°F with average seasonal temperature extremes <br />occurring in July with a high mean of 76.4°F and during January with a low mean of <br />• 24. PF. The annual average mean temperature in central Weld County is 48.4°F. <br />Severe cold waves are common on the eastern plains. The frost -free period for the <br />three -county area ranges from 139 to 160 days.27 <br />The average annual precipitation ranges from 11.12 to 16.32 inches. Less than 10 <br />inches of average annual precipitation is considered a desert climate. The maximum <br />average monthly precipitation, ranging from 2.49 to 2.94 inches, occurs in May. (See <br />Table II -17). About 70-80 percent of the annual precipitation is received between <br />April and September. The minimum average monthly precipitation occurs in January <br />and ranges from 0.27 to 0.47 inches. Generally, areas that receive less than 20 inches <br />of average annual precipitation periodically receive heavy rainfalls. The combination <br />of heavy rainfalls and impervious soils typical of and areas causes flash flooding. The <br />pattern of mean annual precipitation is shown on Map #10. <br />Relative humidity is frequently below 50 percent and will occasionally be less than 20 <br />percent. Evaporation rates are high, causing rapid cooling in the evenings at high <br />altitude (thin -air areas) within rain shadows of mountains.28 <br />]6 Climate and Man -Yearbook of Agriculture, U.S. Department of Agriculture (Washington <br />1941). <br />• 2' Climate and Man -Yearbook of Agriculture. <br />23 Climate of the States, Vol. 2, U.S. Department of Commerce (Port Washington 1974) as cited <br />in Narrows Unit. <br />65 <br />