Saturday, February 7, 2015

Climate of Utah

NWS Description of Utah's Climate

Utah's climate and weather are both delightful and variable. During the summer, most populated areas experience daytime temperatures that are warm but not too hot, and nighttime temperatures that are cool but pleasant. Precipitation in the summer comes primarily from thunderstorms. Winter temperatures are most often invigorating but not extremely cold, while the snow seems to fall in exactly the right places. Normally, valley floors do not have extensive accumulations of snow for long periods of time, but mountain snows build to great depths and store large amounts of water for domestic, industrial, and agricultural needs. Utah's mountain snow also provides some of the best “powder” skiing in the world. Most spring and fall months experience mild temperatures and many sunny days.

Today's climates within Utah are as varied as those encountered in the southern states of the United States, northward to the interior of Canada. In the extreme southwestern area of Utah cotton can be grown, while in the higher valleys of northern Utah only grasses and some cereal grains are effectively cultivated. In some of the higher mountains permanent snowfields can be found.

Some weather phenomena that have been reported in the state since 1847 include numerous flash floods (caused by intense summer thunderstorms), snowmelt floods (which result from rapid warming or rain falling on heavy mountain snowpack), and hailstorms, tornadoes, blizzards and droughts. They have all made an impact on the landscape and residents of the state. About the only major weather phenomenon that has not been reported within the confines of Utah is a hurricane. However, remnants of hurricanes have brought heavy rains, hail and damaging winds to parts of the state.

Prehistoric Climate of Utah

Any attempt to reconstruct climatic conditions of the prehistoric past depends on information inferred from natural climatic indicators. These indicators include plant and animal fossils found in sedimentary rock, the type and thickness of the rock itself, glacial erosion and sediments, tree ring analysis and pollen studies. From these and other sophisticated indicators scientists have arrived at the following conclusions about Utah's prehistoric climate:

In the early period of the earth's existence, temperatures were much warmer than they are today. Such conditions did not allow for the widespread vegetation types that currently exist. Land masses were repeatedly overrun by large seas, and extreme temperature and precipitation conditions occurred.

About 200 to 250 million years ago there was a marked cooling trend over the northern hemisphere which was more favorable for the growth of luxurious vegetation. During that time, Utah's coal and oil fields were in the formative stages. These important modern natural resources resulted from major changes that occurred during Utah's prehistoric climate and geology.

About 150 million years ago a widespread drought occurred which resulted in extremely large sandstorms. The dust bowl days of the 1930's in the Great Plains area of the United States were a gentle picnic in comparison to this ancient “dry spell.” Sand swirled for centuries until it was piled in dunes thousand of feet high. Later, the mountains were pushed up to heights well above Utah's present mountain peaks. Eventually, another sea covered the area and endured for as long as a hundred million years. When the sea eventually disappeared, the windblown sand was revealed as the multicolored hills and rock outcroppings seen today in Utah's famous National Parks of Arches, Bryce, Capitol Reef, Canyonlands and Zion.

During the early Cenozoic Era (about 65 million years ago) another drastic climatic change took place which had a marked influence on Utah and the Central Intermountain Region. Temperatures gradually increased, with a peak occurring about 50 million years ago. This peak was then followed by a significant drop in temperatures and an increase in precipitation.

As recently as 100,000 years ago glaciers became abundant in the high mountains and northern regions of North America. This epoch is often called the “ice age.” During this time a large inland sea developed over much of western Utah. This inland sea has been given the name “Lake Bonneville.” In Utah, the last major accumulation of ice melted away about 10,000 years ago as the temperatures began to warm. Some scientists believe that we are now living in a relatively warm interglacial period and that another cooler period will eventually overtake our area. However, in our short lifetimes we will not experience the large changes that have previously taken millions of years to complete.

It is interesting to note that there is a somewhat cyclical pattern in temperature and precipitation amounts through time. The prehistoric record shows significant increases and decreases over long periods of time. Likewise, Utah's historic record of the past 15 decades demonstrates this same type of pattern on a much smaller scale. Charts of accurately recorded yearly temperatures and precipitation amounts for various locations in Utah fluctuate up and down, over and over again.

Climate Classification of Utah

Climate differs from weather in that it is a generalization of all major weather conditions of an area over a long period of time, usually for thirty years or more. Several different climate classification systems have been developed over the years, but the one that is relatively easy to understand and apply is the Modified Koppen System, which delimits various climate types according to vegetation response to temperature and precipitation patterns.

On a large scale, the climates of Utah can be divided into four types within the Modified Koppen System. They are: Desert, Steppe (Semiarid), Humid Continental-Hot Summer, and Undifferentiated Highlands.

In the Atlas of Utah, the description of these four climates can be found, a summary of which follows:

About 33% of the state consists of true desert. Deserts occur in areas where the average annual precipitation is less than one-half of the annual potential evapotranspiration. In these areas total annual precipitation is usually about five to eight inches. Utah deserts occur in two major areas: in the Great Basin to the west and in the Canyonlands region of the southeast. The Great Basin desert is a region of hot summers and cold winters, with winter temperatures averaging below 32F. Much of the Great Basin region is dominated by sagebrush (Artemisia), a plant indicative of fairly good soils and able to thrive in cold winter regions. The Great Salt Lake Desert, which is located in the northern portion of Utah's Great Basin, is dominated by extensive areas of salt flats, many of which are devoid of plant life. Areas of more humid climates are located within the higher mountain ranges of the Great Basin.

The Colorado Plateau desert is located in Utah's Canyonlands. Meteorologically, the temperature and precipitation conditions in the northern portion of this desert are similar to the Great Basin. However, these two desert regions appear to be quite different due to the great contrast in exposed geological formations, vegetation types and topography. The southern portion of the Colorado Plateau desert maintains average winter temperatures above 32F. Three local mountain ranges and several uplands within the Canyonlands area have resulted in local regions of humid climates.

The southwestern region of the state is a desert region also having winters averaging above 32F. It is in this area that the highest mean annual temperatures in Utah are recorded, with Zion National Park and St. George averaging 61 to 62F. This is a marginal extension of the Mojave Desert, as suggested by the presence of joshua trees in the vicinity of St. George.

Steppelands occur between the desert margins and the higher mountain regions. The average annual precipitation of the steppelands is less than the potential evapotranspiration, yet more than half of these regions average between eight and 14 inches of precipitation annually, creating a semiarid climate sufficient for the growth of short and medium grasses, sagebrush and other woody plants. Much of this grassland region forms the basis for Utah's livestock ranching industry. Most of the state's steppeland area experiences winters averaging below 32F., with only the southern margin enjoying less severe conditions. Approximately 40% of the state is steppeland—the most extensive climatic zone in Utah.

Located along the Wasatch Front from the Idaho border southward almost to Nephi is a narrow belt of humid climate where the total annual precipitation is in excess of the potential evapotranspiration. Within this more humid climate type the winters are fairly cold, again with temperatures averaging below 32F. The summers, however, are quite hot, with mean July temperatures being about 77F. This great range in average mean monthly temperature is due in part to the state's mid-latitude position in the interior of a large continent. The climate is therefore referred to as humid continental-hot summer. One of the major factors which influences precipitation amounts for this region is the Wasatch Mountain Range, which acts as a barrier to moisture-laden winds approaching from the west. As the approaching air rises to clear the mountain crest, temperatures drop, creating conditions that cause the air to give up its moisture. The extent and effect of this phenomenon is reflected in the concentration of population, industries and intensive agriculture within this humid area, though it occupies only 3% of the state's land area.

Approximately 24% of the state consists of mountainous regions having an undifferentiated highland climate. Mid-latitude highland climates are generally considered as humid regions with severely cold winters and cool to cold summers. The treeless summits of many of these mountain ranges have a tundra climate, where the temperatures are too cold to permit the growth of trees. Mean monthly summer temperatures in Utah's highland regions are usually below 72F. Within the highland climate zone there can actually be a great variety of temperature and precipitation conditions ranging from the cool summers of the Wasatch Range's “back” valleys to the alpine tundra conditions of the higher Uinta peaks.

Controlling Factors in Utah's Weather

Utah's weather and climate can be explained by a variety of factors or controls. They include altitude above sea level and latitudinal position about midway between the North Pole and the Equator which is in the path of the prevailing Westerly Winds. Of equal importance are the Polar and Subtropical Jet Stream winds and air masses, the state's position on the edge of a semipermanent high pressure system, local and surrounding mountains, and the Sierra Nevada’s to the west and the Rocky Mountains to the east.

If the average altitude above sea level were calculated for the state it would be approximately 6,100 feet. Although this is an average value, it does not mean that most of the population resides at that elevation. In fact, a majority of the people live at approximately the 4,500 foot level. As altitude increases, air temperature normally decreases, thus the relatively high altitude of Utah is partially responsible for the state's cool temperatures.

Utah's latitude also plays an important role in determining the type of weather the state receives. Utah is situated between 37and 42 north latitude. This mid-latitude position means that the effectiveness of the sun is not nearly as great here as it would be closer to the equator, but much more energy is received than farther to the north. This position, associated with the earth's axis, also accounts for the state's distinct seasons. The latitude of Utah is also in the zone of the prevailing Westerly Winds, where there is constant interaction between polar and tropical air masses and associated frontal activity. When low level tropical moisture moves into Utah from locations off the southern California coasts to the Gulf of Mexico, summertime thunderstorms increase in intensity and winter and spring storms often produce heavy, wet snowfalls.

The existence of a Subtropical High Pressure Cell to the southwest of California also has significant impact on the state's weather and climate. This pressure cell which increases and decreases in intensity with time and also moves northward and southward with the seasons, often acts as a barrier to moist air that would otherwise invade the state.

All mountains have some influence on air flow movement, but the large ones have a significant impact. The main chain of the Rocky Mountains often protect Utah from the cold arctic weather of the northern Plains, while the Sierra Nevada and Cascade Mountains often prevent low level moisture associated with Pacific storms from reaching the state. Therefore, by the time Pacific storms reach Utah, they have usually lost some of their low level moisture. Because of this and the Great Basin's low topography, Utah is the second driest state in the nation. (Nevada is the driest.) The local mountains are also effective in causing increased rainfall and snow with increased altitude. When storms move into Utah from the west, they impact against the high Wasatch and south central mountains of Utah, causing “orographic lifting” (or the forced lifting of air) which “squeezes” out moisture that otherwise might pass over the state. They likewise act as barriers to air mass flow and are often responsible for areas being rather dry. The Uinta Basin is a good example of an area surrounded by mountains that hinders the movement of moist air into the region.

The Sierra Nevada and Cascade Mountains also play an important part in determining the type of snow that falls in the highlands of Utah. Because of the long distances that storms have to travel and the mountains they have to pass over to reach Utah, the snow that falls is often light and fluffy, and is known as “powder.” This “powder” snow, when deep enough, gives skiers the feeling they are “floating on air” as they ski down a slope. This is one of the main reasons that many skiers call Utah's snow “the greatest snow on earth.”

Much of the information for this section originally appeared in the copyrighted book Utah's Weather and Climate, edited by Dan Pope and Clayton Brough, in 1996. UCCW Directors have received permission from the copyright owners of this book to reproduce such information on its website and to revise and updated it where appropriate.

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