The Great Plains in eastern Colorado have hot dry summers and cold dry winters. In the mountains and on the high plateaus the climate varies greatly from place to place. There, as in most highland regions, temperatures and precipitation (rainfall and snowfall) vary with elevation, exposure to sunlight, and prevailing winds.
Average January temperatures on the Great Plains range from about -4° C (about 24° F) in the north to about 1° C (about 34° F) in the south. In the mountains they are cooler, ranging from -12° to -1° C (10° to 30° F) in the lower valleys and mountain slopes and falling considerably lower at high elevations. Often the coldest spots in the state are the high mountain valleys. Extremely cold conditions occasionally occur on the plains when arctic air sweeps down from the north. On such occasions, temperatures drop to the upper -20°s C (lower -20°s F) or colder. When a warm, dry wind, known as the chinook, blows eastward across the plains in winter, temperatures rise rapidly.
Average July temperatures range from 18° to 24° C (64° to 76° F) on the plains and on the Colorado Plateau. On the lower mountain slopes and in the valleys, summer temperatures are between 10° and 16° C (50° and 60° F). Cooler conditions prevail in the higher mountains. Hot daytime spells are common on the plains and the Colorado Plateau but are rare in central Colorado. Although Colorado’s summers are hot, they are generally not uncomfortable because the relative humidity is usually low. In addition, summer nights are relatively cool.
Most of Colorado receives about 250 to 500 mm (about 10 to 20 in) of precipitation annually. The high mountains receive considerably more, while the San Luis Valley and the Colorado and Gunnison river valleys receive less than 250 mm (10 in). The eastern part of the Great Plains are generally wetter than the western part along the base of the Rockies. More than half of the annual precipitation on the plains usually falls in spring and summer. Snowfall is heavy in winter in the mountains. However, the amount of precipitation varies greatly from year to year and drought is an ever-present possibility. Severe droughts, although infrequent, can occur, as they did in the southeast during the 1930s and again during the 1950s and 1960s. The growing season, or length of time between the last killing spring frost and first killing fall frost, ranges from 120 to 200 days on the plains. Except for some small areas, the mountains have a frost-free period that is generally less than 80 days, too short a growing season for most crops. © "United States" © Emmanuel Buchot and Encarta
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