Colorado’s combination of high elevation, midlatitude, and continental interior geography results in a cool, dry, and invigorating climate. The average annual temperature for the state is 43.5 degrees Fahrenheit (F), which is 13.7 degrees below the global mean. The average statewide precipitation is seventeen inches, which is much lower than the global mean of thirty-eight inches. There are large seasonal swings in temperature and large day-to-night changes.
The climate of local areas is 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 over short distances. For instance, the difference in annual mean temperature between Pikes Peak and Las Animas, ninety miles to the southeast, is 35 degrees—about the same as that between southern Florida and Iceland. Different regions of Colorado have unique characteristics not shared across the entire state.
Eastern Colorado
The climate of the plains is comparatively uniform from place to place, with characteristic features of low relative humidity, abundant sunshine, infrequent rain and snow, moderate to high wind movement, and a large daily and seasonal range in temperature. Summer daily maximum temperatures are often 95°F or above. Winter extremes are generally between 0 and -15°F. The difference between the hottest and coldest officially recorded temperatures on the eastern plains is greater than 150°F.
Average annual precipitation in eastern Colorado is between ten and twenty inches. The wettest areas are on the northeastern plains near the Colorado-Kansas or Colorado-Nebraska borders. The low-elevation areas both north and south of the Palmer Divide and directly east of the Rockies are the driest. Most of this precipitation falls in the form of widespread soaking rains in April through early June or as intense bursts from thunderstorms in June through August. Year-to-year precipitation is highly variable and highly dependent on the number of large rain events in late spring and summer. Summer thunderstorms may be severe, with hail being the most common threat.
Tornadoes will occur almost every year somewhere in eastern Colorado between mid-May and early August. They are most often small, registering as EF0 or EF1 on the Enhanced Fujita Scale, a scale used to measure tornado severity that ranges from 0 to 5. The most frequent zone for tornado genesis in all of the United States is a narrow, north-to-south-oriented strip of land situated in Weld County, northeast of the Denver metropolitan area.
A number of significant changes in climate occur at the western edge of the plains and near the foothills of the mountains. 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. These winds are sometimes referred to as chinook winds when they warm and bora winds when they are associated with a strong cold frontal passage downslope off the mountains.
Mountains
Colorado is best known for its mountains. They occupy less area of the state than many realize, but they profoundly impact the climate of the entire region. The main feature of the mountainous area of central and western Colorado is the dramatic differences in climate over short distances. With elevations ranging from below 7,000 feet in the lower mountain valleys to more than 14,000 feet on the highest peaks, all aspects of the climate are affected: temperature, humidity, precipitation and, of course, wind.
In general, temperatures decrease with elevation. This change in temperature with elevation is most profound on summer afternoons, when temperatures consistently decrease by 4–5°F per 1,000 feet. Air heated at elevation quickly becomes unstable, causing it to rise and be whisked away from the land surface. This puts a low upper threshold on high temperatures at elevation. On clear and calm nights—especially with snow cover—the land surface very effectively radiates away the day’s heat, and the coldest, densest air settles into the mountain river valleys. Thus, the most extreme cold in the state of Colorado actually occurs in mountain valleys, not on mountain peaks. Under extreme conditions, temperatures have dipped as low as -60°F at Taylor Reservoir and -61°F along the Yampa valley in northwestern Colorado.
Wind patterns in the mountains are almost always controlled by topography. Mountain-valley circulations are common, with winds often blowing up the valley from lower to higher elevations during the day, then reversing and blowing down the valleys at night. The mountains form a substantial block to regional air motion, causing winds in most valleys west of the Continental Divide to be very light—especially in fall and winter—while winds along and east of the crest of the Continental Divide are much stronger and typically blow from a westerly direction for much of the cool half of the year.
Precipitation patterns are largely controlled by mountain ranges and elevation and, to a lesser extent, the direction of prevailing airflow. When weather systems move in from the west and northwest during the winter and spring months, the peaks that first intercept the air receive the most precipitation. These areas are the wettest areas in the state of Colorado. Buffalo Pass in the northern part of the state and Wolf Creek Pass in the southern part of the state both receive over fifty inches of precipitation and 350 inches of snowfall annually. Some of the high mountain valleys, which lie in the rain shadow of mountains, are the driest areas in the state and receive an average of less than ten inches of precipitation per year. Precipitation increases with elevation both in winter and summer, but the elevation effect is greatest in midwinter, when mountaintop winds are typically strongest. High peaks and mountain ranges generally receive the majority of their precipitation during the winter months. Mountain precipitation primarily falls as snow from November through mid-May. This creates seasonal snowpack at about 9,000 feet, although it is lower in some areas.
Western Colorado
Farther west in Colorado, the topography becomes slightly less extreme, with lower elevations and combinations of canyons and plateaus. Elevation and topography remain dominant controls of local climates, but precipitation gets progressively less and temperature progressively warmer approaching the Utah border. Western Colorado winter weather is colder but calmer and less variable than weather east of the mountains. Temperatures can drop below 0°F in all areas of western Colorado, but the valleys of west-central and southwest Colorado receive abundant sunshine and the winter climate is not harsh.
Precipitation west of the Continental Divide is more evenly distributed throughout the year than for the eastern plains. For most of western Colorado, the greatest monthly precipitation occurs in the winter months, while June is the driest month. Near the Utah border, late summer and early autumn can be the wettest time of year, as moisture from the Gulf of Mexico and tropical Eastern Pacific is funneled northward into the state, often falling as precipitation during afternoon thunderstorms.
Severe Weather
A variety of threatening weather events are possible in Colorado. These include extreme cold, extreme heat, blizzards, high wind events, seasonal flooding, flash flooding, droughts, forest fires, lightning, hail, and tornadoes. Even though the state is in a semiarid climate, the most damaging events from an economic standpoint are flash floods and droughts. The deadliest severe weather events in the state have historically been flash floods. The most flash-flood-prone regions of Colorado are found along the base of the lower foothills east of the mountains. Several extreme floods, such as the Fort Collins flood of 1997, the Front Range flood of 2013, and the infamous Big Thompson Canyon flood of July 31, 1976, have occurred in this vulnerable area. The Big Thompson flood is the deadliest single weather event in Colorado history, as it took the lives of 144 people.
Changing Climate
Measured temperature trends averaged across the state of Colorado are statistically significant for the last thirty, fifty, and one hundred years. The greatest warming has occurred in the southwest corner of the state, the San Luis Valley in south-central Colorado, and along the northern Front Range. The southeastern corner of the state has actually undergone a slight cooling over the last century. Temperatures have risen by 3.4°F in the spring, 2.4°F in the summer, 2.3°F in the winter, and 1.5°F in the fall. Due to the lack of reliable historic temperature data at high elevations, it is not known whether warming is disproportionately occurring at high elevations.
There are no detectable widespread trends in precipitation across the state of Colorado. Average snowpack in Colorado is lower than thirty years ago, but there is no statistically significant decreasing trend in snowpack. Climate modeling studies suggest Colorado seasonal snowpack is vulnerable to projected increases in temperatures but less so than the Cascades and Sierras of the western United States.
