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<br />Winter is usually rather mild because of the abundance of sunshine <br />and the protection afforded by the nearby mountains. Cold air from <br />the north does invade the survey area, and blizzards occasionally <br />occur in Pueblo County. These cold spells usually last 3 to 5 days, <br />when they are often ended by "chinook" winds. Chinook (or Foehn) <br />is the name given to the warm winds that blow down the eastern <br />slopes of the Rocky Mountains. The rise in temperature is the <br />result of three factors acting together. The replacement of a cold <br />air mass by the advection of warmer air, dynamic heating by <br />subsidence, and the destruction of the normal night ground <br />inversion. These "chinook" winds may cause sudden temperatures <br />rises of more than 40 degrees F. As a rule, less precipitation is <br />received in winter than in fall. Strong winds occur frequently in <br />winter and spring. After an unusually dry winter, duststorms may <br />occur, particularly in dryfarming areas. The prevailing wind is <br />from the west at about 8 miles per hour. The relative humidity at <br />noon is about 45 percent. <br />The average growing season for Pueblo County is 167 days. The last <br />freeze in spring generally occurs on April 29 and the first in the <br />fall on October 13. Table 10 shows the probabilities of freezing <br />temperatures. <br />Near the mountains, winds are more moderate and day-to-day <br />temperature changes are not so great as in areas away from the <br />mountains. Temperatures in the foothills are cooler in summer and <br />warmer in winter. Above an elevation of 7,000 feet, the average <br />July temperature is about 60 degrees F. and the average January <br />temperature is about 20 degrees. The growing season is <br />considerably shorter at this elevation. Also, precipitation <br />generally increases rapidly with increasing elevation. <br />