Climate of Colorado Climatography of…

                        Climate of Colorado

        Climatography of the United States No. 60 (updated 1/2003)

                                   Prepared by

         Nolan J. Doesken, Roger A. Pielke, Sr., and Odilia A.P. Bliss

         Colorado Climate Center, Atmospheric Science Department,
                 Colorado State University, Fort Collins, CO



                           TOPOGRAPHIC FEATURES


       To understand the regional and local climates of Colorado, you must begin

with a basic knowledge of Colorado's topography. Colorado lies astride the

highest mountains of the Continental Divide. Nearly rectangular, its north and

south boundaries are the 41° and 37° N. parallels, and the east and went

boundaries are the 102° and 109° W. meridians. It is eighth in size among the

50 states, with an area of over 104,000 square miles. Although known for its

mountains, nearly 40 percent of its area is taken up by the eastern high plains.


       Of particular importance to the climate are Colorado's interior continental

location in the middle latitudes, the high elevation of the entire region, and the

mountains and ranges extending north and south approximately through the

middle of the State. With an average altitude of about 6,800 feet above sea

level, Colorado is the highest contiguous State in the Union. Roughly three-

quarters of the Nation's land above 10,000 feet altitude lies within its borders.

The State has 59 mountains 14,000 feet or higher, and about 830 mountains

between 11,000 and 14,000 feet in elevation.
       Emerging gradually from the plains of Kansas and Nebraska, the high

plains of Colorado slope gently upward for a distance of some 200 miles from the

eastern border to the base of the foothills of the Rocky Mountains. The eastern

portion of the State is generally level to rolling prairie broken by occasional hills

and bluffs. Although subtle when compared to the high mountains of the

Rockies, there are also important topographic features across eastern Colorado.

Two major river valleys dissect eastern Colorado ­ the South Platte River in

northeastern Colorado and the Arkansas River to the southeast. Higher ground

extends eastward from the Rockies between the river valleys.       High ground also

extends eastward along the New Mexico border to the south and along the

Wyoming and Nebraska borders to the north. These features have an impact on

temperatures, wind patterns and storm tracks in all season of the year.


       Elevations along the eastern border of Colorado range from about 3,350

feet at the lowest point in the State where the Arkansas River crosses into

Kansas to near 4,000 feet. Elevations increase towards the west to between

5,000 and 6,500 feet where the plains meet the Front Range of the Rocky

Mountain chain. Here elevations rise abruptly to 7,000 to 9,000 feet. Backing

the foothills are the mountain ranges above 9,000 feet with the higher peaks

over 14,000 feet. The most dramatic feature is Pike's Peak near Colorado

Springs where elevations rise abruptly from less then 5,000 feet near Pueblo in

the Arkansas Valley to over 14,000 feet at the top of the mountain. During the

summer months, this topographic feature becomes a "thunderstorm machine" as

thunderstorms develop almost any day that humidity is sufficiently high.


       West of these "front ranges" are additional ranges, generally extending

north and south, but with many spurs and extensions in other directions. These
ranges enclose numerous high mountain parks and valleys. Farther westward the

mountains give way to rugged plateau country in the form of high mesas (some

more than 10,000 feet in elevation) which extends to the western border of the

State. This land is often cut by rugged canyons, the work of the many steams

fed by accumulations of winter snow.


      Colorado is a headwater state. All rivers in Colorado rise within its

borders and flow outward, with the exception of the Green River, which flows

diagonally across the extreme northwestern corner of the State. Four of the

Nation's major rivers have their source in Colorado: the Colorado, the Rio

Grande, the Arkansas, and the Platte.




                              GENERAL CLIMATE


      The combination of high elevation, mid latitude interior continent

geography results in a cool, dry but invigorating climate.   There are large

seasonal swings in temperature and large day to night changes. 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 evaporation 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. Outdoor work

and recreation can often be carried out in relative comfort year round, but

sunburn and skin cancer is a problem due to the intense high-elevation sunlight.

At night, temperatures drop quickly, and freezing temperatures are possible in

some mountain locations every month of the year.
       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 within short

distances. The difference (35°F) 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 annual snowfall at Wolf Creek Pass

(elevation 10,850 feet) in the southern mountains is averages nearly 400 inches

and sometimes exceeds 600 inches while at Manassa in the San Luis Valley just

east of Wolf Creek Pass annual snowfall is barely 40 inches. Statewide average

annual precipitation is 17 inches but ranges from only 7 inches in the middle of

the San Luis Valley in south central Colorado to over 60 inches in a few mountain

locations. 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 falling as rain or snow on the mountaintops and westward-facing

slopes. Eastern slope areas receive relatively small amounts of precipitation from

these storms, particularly in mid winter


       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 can cause 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" of the high plains and temperature rises

of 25 to 35°F 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. Due to these wind patterns, some locations in

the eastern foothills are warmer than adjacent areas on the eastern plains on

many days during the winter.


       Warm, moist air from the south moves into Colorado infrequently, but

most often in the spring, summer and early autumn. As this air is carried

northward and westward to higher elevations, the heaviest and most general

rainfalls (and sometimes wet snows) occur over the eastern portions of the State

from April through early September. For southern and western Colorado, the

intrusions of moist air are most common from mid July into September

associated with wind patterns sometimes called the Southwest Monsoon.

Frequent showers and thunderstorms continue well into the summer. At times
during the summer, winds shift to the southwest and bring hot, dry air from the

desert Southwest over the State. 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,

infrequent rains 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, 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 Arkansas Valley and lower elevations of South Platte and

Republican Rivers. The hottest temperature ever recorded in Colorado was 114°F

at Las Animas in July 1, 1933 and at Sedgwick on July 11, 1954. 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 to -15°F but have reached extraordinarily low

readings of -30 to -40°F during some of the most extreme cold waves.


      An important feature of the precipitation in the plains is the seasonal

cycle. A very large proportion (70 to 80 percent of the annual total) falls during

the growing season from April through September.      Cool season precipitation

can be important for soil moisture recharge, but midwinter precipitation is light

and infrequent. More often, winter brings dry air and strong winds contributing

to the aridity of the area. From early March through early June, periodic
widespread storms bring soaking beneficial moisture that helps crops and

grasslands. Summer precipitation over the plains comes largely from

thunderstorm activity and is sometimes extremely heavy. Localized rains in

excess of 4" sometimes fall in just a few hours contributing to local flooding. In

late May 1935 nearly two feet of rain fell along the Republican River in eastern

Colorado causing one of the worst floods in state history. June flash floods in

1965 were also devastating. The weather station at Holly in southeast Colorado

measured 18.81" of rainfall in that extraordinarily wet month. It is more

common, however, to be too dry.     Annual average precipitation ranges from less

than 12 inches in the Arkansas Valley between Pueblo and Las Animas to almost

18 inches in extreme northeastern and southeastern corners of the state. Many

years are drier than average, and some years receive only half or less the long-

term average. The region seems almost always in or on the verge of drought.

Multi-year drought is common to the area such as the decade-long drought of

the 1930s, the severe drought of the mid 1950s and 1970s and the recent

intense widespread drought of the early 2000s.


      At the western edge of the plains and near the foothills of the mountains,

there are a number of significant changes in climate. 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 of the mountains. Temperature changes from day to day are not quite as

great; summer temperatures are lower, and winter temperatures are higher.

Not surprisingly, this milder corridor close to the mountains is where the
majority of Colorado's population now lives. Precipitation, which decreases

gradually from the eastern border to a minimum near the mountains, 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 River, the mountain and valley winds are persistent

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 near Canon City

and Penrose than anywhere else in the State.




                  CLIMATE OF MOUNTAINS OF COLORADO


       Colorado is best known for its mountains. They occupy less of the 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. Summer afternoon

temperatures consistently decrease about 4-5 degrees F per thousand feet.

Typical July afternoon temperatures are in the 70s and 80s in the lower valleys

but are only in the 50s and 60s in the higher mountains.     But elevational
temperature changes are often masked by temperature inversions especially at

night and during the winter.   Cold air is more dense than warmer air and

collects in some of the mountain valleys. On clear nights, especially during

winter when the ground is snow covered, strong temperature inversions form.

Under these circumstances, the coldest temperatures are found near the center

of these high valleys, while temperatures in the high mountains are considerably

warmer. The San Luis Valley around Alamosa, the Gunnison Valley around

Gunnison, the Eagle Valley, the Fraser valley and the Yampa Valley near

Steamboat Springs all can be very cold on clear winter nights. Subzero

Fahrenheit temperatures are commonplace in these areas and the most winters

see at least a few nights with temperatures dropping below -30°F. Even in

summer, temperatures can dip below freezing.     Under extreme conditions,

temperatures have dipped as low as -60°F at Taylor Reservoir and -61°F along

the Yampa valley in northwestern Colorado. Such cold temperatures are rare

but demonstrate the extremes that mountain weather patterns can produce.

Fortunately, these cold temperatures are nearly always accompanied by light or

calm winds.


       It is quite a different story on the mountain peaks. Strong winds are

common at elevations above tree-line (approximately 11,500 feet) throughout

the winter months and can exceed 50 to 100 mph in exposed locations. Outdoor

adventurers must be prepared for the most extreme of conditions year round,

but particularly in winter and spring.


       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 elevation during the day 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 much of the cool half of the year.


       Precipitation patterns are largely controlled by mountain ranges and

elevation. Precipitation increases with elevation both winter and summer but the

elevation effect is greatest in mid winter when winds at mountain top level are

typically strongest. High peaks and mountain ranges generally receive the

majority of their precipitation during with winter months. Snow accumulates

without melting in shaded or level areas at elevations above about 8,000 feet.

When it melts in the spring, this snow is the primary source of water for much of

the population of the state and provides water for extensive irrigation.

Considerable effort is made every year to measure the accumulating snowpack

so that water providers and resource managers can plan ahead for the coming

summer. Most of the mountain snow melts during May and June when rivers

reach their peak for the year.


       In summer, mountain peaks and ranges are effective thunderstorm

generators whenever the regional air masses are sufficiently moist. Some years,

local thunderstorms form nearly every afternoon in and near the mountains. The

last half of July and much of August is particularly prone to mountain

thunderstorms while June is often a much drier month in the high country. Snow

and soft hail are possible from mountain storms even in July and August. Hikers

and participants in other outdoor activities in the mountains during the summer

months must be careful to avoid exposed ridges during stormy periods, as
lightning poses a very serious threat throughout the summer. Lightning also

triggers forest fires in drier years.




                       CLIMATE OF 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 east of the mountains. Temperatures can drop below zero 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. An area of western

Colorado near Grand Junction is particularly mild and has developed an extensive

fruit growing area. Anything from apricots and peaches to wine grapes and

sweet corn is grown in the area from Delta and Paonia to Grand Junction and

Palisade. Most of the population of western Colorado lives in this region.

Summer afternoon temperatures can exceed 100 deg F several times each

summer at elevations below 5500 feet, but it only takes a short drive to higher

elevations to find cooler air. Temperatures only rarely drop below -10 F. Rare

extreme cold, however, has had devastating effects on local orchards. Back in

January 1963. Temperatures dropped below -20 in the fruit growing area and

many trees were killed.
      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. Near the Utah border, late summer and early autumn can be the

wettest time of year. While precipitation only averages from 8 to 14 inches in

these western valleys, localized flood-producing storms are still possible.

Occasionally, moisture from decayed Pacific hurricanes have fueled widespread

heavy rains. Extensive flooding occurred in October 1970 following one of these

storm systems.




                                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.   With an average frequency of 6 or may hail days per

year, some counties of eastern Colorado are among the most hail prone areas in

the entire country.


      Tornadoes, once thought to be only a small threat to the residents of

eastern Colorado, have been found to be quite common with the improvement

in severe storm detection in recent decades. Tornadoes are relatively rare in the

mountains and western valleys but do occur. In most years, at least 40

tornadoes are confirmed. Most of these tornadoes are small and short lived,

usually classified in intensity as F0 or F1. However, occasional strong tornadoes

have been reported. The number of tornado fatalities remains very low for
Colorado, but much of this is due to the low population density of some of the

most tornado prone areas of eastern Colorado.


      Lightning has emerged as one of the greatest weather hazards in

Colorado. Each year there are typically several fatalities and injuries. Unlike

tornadoes that are most common in selected areas of the state, lightning can

and does occur everywhere. Lightning strike statistics indicate that the most

lightning prone areas of Colorado are the high ground above tree line between

Denver and Colorado Springs and the Raton Plateau south and southeast of

Trinidad near the New Mexico border.


      Fall, winter and spring blizzards on the eastern high plains are another

weather hazard deserving attention. While Colorado blizzards are less frequent

and drop less snow than in areas further east and north, they can still be

devastating. As recently as 1997 several fatalities were directly attributable to

an October blizzard which caught many travelers unprepared.


      Heavy snows in the high mountains are much more common. Each year

several lives are lost due to avalanches. Avalanches pose a serious problem to

residents, road maintenance crews and back country travelers. Considerable

effort is made each year to predict and manage avalanches.


      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 years when snow cover is heavy, or when there is

widespread lower elevation snow accumulation and a sudden warming in the

spring, there may be extensive flooding.
      The greatest threat of flooding in Colorado is not snowmelt, however. It

is flash flooding from localized intense thunderstorms. 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 infamous Big Thompson

Canyon flood of July 31, 1976 have occurred in this vulnerable area. Flash

floods occur on the western slopes as well, but with somewhat lower frequency

and intensity due to a reduced supply of low level moisture to fuel such storms.