General Climate of Colorado
General Overall Summary of Eastern Colorado (east of Mountains): Colorado has a dry, semi-arid climate, with close to 300 days of sunshine during the year and relative low humidity. Because of the proximity of Colorado's lower elevation Front Range cities to the mountain ski resorts, many believe Denver and surrounding eastern communities have harsh winters. This is not correct. On the contrary, winter storms do not last long here–snow is melted away quickly with our mild winter temperatures. Summers can be hot but not uncomfortable due to the low humidity. Although we do experience a period of monsoonal moisture in July and August, this results in only brief periods of afternoon thunderstorms. Many storms consist of wind and some rain but not all day rain. Most of the rainfall from the moonson falls in the mountains. Most days during the year are sunny and pleasant. Springtime brings relatively more moist air into Colorado. During this time, short heavy thunderstoms can occur and severe weather such as hail can occur. Tornado's can occur on the far eastern plains. Colorado receives a large amount of hail during the year, especially during the springtime (more than most North American cities).
(Above Written by Gregory Truta in 2002)
The following is from the Western Regional Climate Center:
"Most of Colorado has a cool and invigorating climate that could be termed a highland or mountain climate of a continental location. During summer there are hot days in the plains, but these are often relieved by afternoon thundershowers. Mountain regions are nearly always cool. Humidity is generally quite low; this favors rapid evapotranspiration and a relatively comfortable feeling even on hot days. The thin atmosphere allows greater penetration of solar radiation and results in pleasant daytime conditions even during the winter. This is why skiers at high elevations are often pictured in very light clothing, although surrounded by heavy snow.
Elevation and Climate: The climate of local areas are profoundly affected by differences in elevation, and to a lesser degree, by the orientation of mountain ranges and valleys with respect to general air movements. Wide variations occur within short distances. The difference (35°) in annual mean temperature between Pikes Peak and Las Animas, 90 miles to the southeast, is about the same as that between southern Florida and Iceland. The average annual snowfall at Cubres in the southern mountains is nearly 300 inches; less than 30 miles away at Manassa in the San Luis Valley, snowfall is less than 25 inches. While temperature decreases, and precipitation generally increases with altitude, these patterns are modified by the orientation of mountain slopes with respect to the prevailing winds and by the effect of topographical features in creating local air movements.
As a result of the State’s distance from major sources of moisture (the Pacific Ocean and the Gulf of Mexico), precipitation is generally light in the lower elevations. Prevailing air currents reach Colorado from westerly directions. Eastward-moving storms originating in the Pacific Ocean lose much of their moisture falls as rain or snow on the mountaintops and westward-facing slopes. Eastern slope areas receive relatively small amounts of precipitation from these storms.
Storms moving from the north usually carry little moisture. The frequency of such storms increases during the fall and winter months, and decreases rapidly in the spring. The accompanying outbreaks of polar air are responsible for the sudden drops in temperature often experienced in the plains sections of the State. Occasionally these outbreaks are attended by strong northerly winds which come in contact with moist air from the south; the interaction of these air masses causes a heavy fall of snow and the most severe of all weather conditions of the high plains, the blizzard. This cold air is frequently too shallow to cross the mountains to the western portion of the State so while the plains are in the grip of a very severe storm, the weather in the mountains and western valleys may be mild.
Occasionally, when the plains are covered with a shallow layer of cold air, strong westerly winds aloft work their way to the surface. Warmed by rapid descent from higher levels, these winds bring large and sudden temperature rises. This phenomenon is the “chinook winds” of the high plains and temperature rises of 25 degrees to 35° within a short time are not uncommon. Chinook winds greatly moderate average winter temperatures in areas near enough to the mountains to experience them frequently.
Warm, moist air from the south moves into Colorado most frequently in the spring. As this air is carried northward and westward to higher elevations, the heaviest and most general rainfalls of the year occur over the eastern portions of the State. Frequent showers and thunderstorms continue well into the summer. At times during the summer, winds shift into the southwest and bring hot, dry air over the State from the hottest weather of the year over the eastern plains, but such hot spells are usually of short duration.
CLIMATE OF THE EASTERN PLAINS – The climate of the plains is comparatively uniform from place to place, with characteristic features of low relative humidity, abundant sunshine, light rainfall, moderate to high wind movement, and a large daily range in temperature. Summer daily maximum temperatures are often 95° F or above, and 100° F temperatures have been observed at all plain stations. Such temperatures are not infrequent at altitudes below 5,000 feet; above that elevation they are comparatively rare. The highest temperatures in Colorado occur in the northeastern plains, and sometimes exceed 115° F. Because of the very low relative humidity accompanying these high temperatures, hot days cause less discomfort than in more humid areas. The usual winter extremes in the plains are from zero to 10° F or 15° F below zero.
An important feature of the precipitation in the plains is the large proportion of the annual total that falls during the growing season – 70 to 80 percent during the period from April through September. Summer precipitation in the plains is largely from thunderstorm activity and is sometimes extremely heavy. Strong winds occur frequently in winter and spring. These winds tend to dry out soils, which are not well supplied with moisture because of the low annual precipitation. During periods of drought, high winds give rise to the dust storms which are especially characteristic of the southeastern plains.
At the western edge of the plains and near the foothills of the mountains, there are a number of significant changes in climate as compared to the plains proper. Average wind movement is less, but areas very near the mountains are subject to periodic, severe turbulent winds from the effects of high westerly winds over the mountain barrier. Temperature changes from day to day are not as great; summer temperatures are lower, and winter temperatures are higher. Precipitation, which decreases gradually from the eastern border to a minimum near the mountains, increases rapidly with the increasing elevation of the foothills and increases rapidly with the increasing elevation of the foothills and proximity to higher ranges. The decrease in temperature from the eastern boundary westward to the foothills is less than might be expected with increasing altitude. This results from mountain and valley winds and greater frequency of the chinook. Below the Royal Gorge of the Arkansas, the mountain and valley winds are strong enough to modify the climate over a considerable area. Descending air currents frequently prevent the stratification of air necessary for the occurrence of excessive cold. As a consequence, the winter climate is milder than elsewhere in the State.
CLIMATE OF WESTERN COLORADO – The rugged topography of western Colorado causes large variations in climate within short distances, and few climatic generalizations apply to the whole area. At the summits of mountains, temperatures are low, averaging less than 32° F over the year. Snow-covered mountain peaks and valleys often have very cold nighttime temperatures in winter, when skies are clear and the air is still – occasionally to 50° F below zero. Summer in the mountains is a cool and refreshing season. At typical mountain stations the average July temperature is in the neighborhood of 60° F. The highest temperatures are usually in the seventies and eighties, but may reach 90° F to 95° F. Above 7,000 feet, the nights are quite cool throughout the summer, while bright sunshine makes the days comfortably warm.
The lower western valleys of the State are protected by surrounding high terrain, and have a greater uniformity of weather than the eastern plains. They experience high summer temperatures, comparable to those of the eastern plains, while average winter temperatures are somewhat lower than at similar elevations in the plains, due largely to the relative infrequency of chinook of other warming winds.
Precipitation west of the Continental Divide is more evenly distributed throughout the year than in the eastern plains. For most of western Colorado, the greatest monthly precipitation occurs in the winter months, while June is the driest month. In contrast, June is one of the wetter months in most of the eastern portions of the State.
SEVERE STORMS – Thunderstorms are quite prevalent in the eastern plains and along the eastern slopes of the mountains during the spring and summer. These often become quite severe, and the frequency of hail damage to crops in northeastern Colorado is quite high. Tornadoes almost never occur in the mountains or in the west. They are also relatively infrequent over the eastern plains, where fatality rates and mean property loss rates are lower than in States farther east. Other severe storms include the winter blizzards of the eastern high plains, but these also are less frequent and not as severe as those in States farther east and north. Heavy snows in the high mountains create the danger of avalanches, a serious problem to residents and road maintenance crews.
A spring flood potential results from the melting of the snow pack at the higher elevations. In a year of near-normal snow accumulations in the mountains and normal spring temperatures, river stages become high, but there is no general flooding. In rears when snow cover is heavy, or when there is a sudden warming in the spring at high elevations, there may be extensive flooding. Heavy thunderstorms in the eastern foothills and plains occasionally cause damaging flash floods. Although these usually affect only small areas, under extreme conditions they have caused widespread heavy damage to property and crops. Similar flash floods occur on the western slopes, but with somewhat lower frequency."