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CLIhtA1'E <br />1Yeld County is usually warm in summer, and hot days are <br />frequent. In winter, periods of very cold weather are caused <br />by A-•ctic air moving in from the north or northwest. These <br />cold periods alternate with frequent milder periods that occur <br />when westerly winds are warmed as they move downslope. Most <br />precipitation falls as rain during the warmer part of the year. <br />Rain is normally heaviest late in spring and early in summer. <br />Winter snowfall is frequent, but snow cover usually disappears <br />during mild periods. <br />Table 1 gives data on temperature and precipitation for <br />the survey area, as recored at Greeley and Ft. Lupton for the <br />period 1951 to 1974. fable 2 shows probable dates of the first <br />freeze in Fall and the last freeze in spring. Table 3 provides <br />data on length of the growing season. <br />In winter the average temperature is 29 degrees F, and <br />the average daily minimum temperature is 14 degrees. The lowest <br />temperature on record, which occurred at Greeley on February 1, <br />1951, is -39 degrees. In summer the average temperature is 70 <br />degrees, and the average daily maximum temperature is 87 degrees. <br />The highest recorded temperature, which occurred at Ft. Lupton <br />on June 23, 1954, is 108 degrees. <br />Growing degree days, shown in table 1, are equivalent to <br />"heat units." During the month, growing degree days accumulate <br />by the amount that the average temperature each day exceeds a <br />base temperature (40 degrees F). The normal monthly accumulation <br />is used to schedule single or successive plantings of a crop <br />between the last ,freeze in spring and the first freeze in fall. <br />Of the total annual precipitation, 9 inches, or 75 percent, <br />usually falls in April through September, which includes the <br />growing season for most crops. In 2 years out of 10, the rainfall <br />in April through September is less than 6.50 inches. The heaviest <br />1-day rainfall during the period of record was 3.20 inches at <br />Greeley on June 8, 1974. Thunderstorms number about 41 each year, <br />29 of which occur in summer. Some years summer hailstorms cause <br />severe local damage to crops. <br />Average seasonal snowfall is 40 inches. 1'he greatest snow <br />depth at any one time during the period of record was 30 inches. <br />On the average, 18 days have at least 1 inch of snow on the <br />ground, but the number of such days varies greatly from year to <br />year. Some years a Heavy blizzard with high winds and drifting <br />snow strikes the county, and snow remains on the ground for many <br />weeks. <br />1'he average relative humidity in midafternoon is about 40 <br />percent. Humidity is higher at night, and the average at dawn is <br />about 69 percent. 1'he percentage of possible sunshine is 71 in <br />summer and 70 in winter. 1'he prevailing wind is from the south. <br />Average windspeed in highest, 1U.4 miles per hour, in April. <br />