The United States contains significant areas classified as desert climate zones, primarily concentrated in the western and southwestern regions of the country. The widely used Köppen system categorizes these arid regions as BW, or true desert. This classification is based not only on low rainfall but also on the severe imbalance between precipitation and potential evapotranspiration. Understanding which states contain these zones requires looking at the specific geographic distribution of North America’s major deserts.
Characteristics of Desert Climates
A desert climate is characterized by a severe lack of moisture, where evaporation significantly exceeds the amount of precipitation received. True deserts typically receive less than 10 inches of annual precipitation, which is insufficient to support dense vegetation. The resulting scarcity of plant life contributes to the dry, often rocky or sandy surfaces that are quickly heated and cooled.
The Köppen system divides these areas into two main subtypes: hot deserts (BWh) and cold deserts (BWk). Hot deserts, like those in the Southwest, feature high average temperatures and mild winters. Cold deserts, found at higher latitudes or elevations, have lower average temperatures and experience cold, sometimes freezing, winters, with much of their limited moisture falling as snow.
Primary U.S. Desert States
The states most extensively dominated by arid conditions and desert landscapes are Arizona and Nevada. Nevada is largely covered by the Great Basin Desert, the largest desert in the United States and a prime example of a cold desert. This high-elevation desert is characterized by sagebrush steppe and experiences cold winters with snowfall.
Arizona is home to the extensive Sonoran Desert, classified as a hot desert (BWh) due to its subtropical location and high temperatures. The Sonoran is known for its biodiversity, including the iconic Saguaro cactus, and extends across much of the southern portion of the state. The Mojave Desert, while primarily in California, extends into southern Nevada and northwestern Arizona, solidifying these two states as the core of the U.S. desert zone.
States Containing Partial Desert Zones
Several other states contain large desert pockets, though they are not predominantly arid like Arizona and Nevada. California is a major desert state, hosting the majority of the Mojave Desert and the lower-elevation Colorado Desert, a sub-region of the Sonoran Desert. These deserts occupy the southeastern portion of the state, contrasting sharply with its temperate coastal regions.
New Mexico and Texas share the Chihuahuan Desert, the largest hot desert in North America, which extends significantly into Mexico. This desert covers southern New Mexico and western Texas, including the area around Big Bend National Park. Utah also contains substantial desert areas, including the western half of the state, which is part of the Great Basin Desert, and a small part of the Mojave Desert in its southwestern corner. States further north, such as Oregon, Idaho, and Washington, contain smaller, isolated cold desert pockets linked to the Great Basin and the Columbia Basin.
The Geographic Reasons for U.S. Deserts
The primary mechanism responsible for aridity in the western United States is the rain shadow effect. This phenomenon occurs when moist air from the Pacific Ocean is forced to rise over high mountain ranges like the Sierra Nevada and the Cascade Mountains. As the air rises, it cools and releases moisture as precipitation on the windward, or western, side of the mountains.
By the time the air descends on the leeward, or eastern, side, it has lost most of its water content, resulting in a dry, warming air mass. This effect is directly responsible for the formation of the Great Basin and Mojave Deserts, which lie in the shadow of the Sierra Nevada and Cascade ranges. Hot deserts like the Sonoran and Chihuahuan are also influenced by persistent, subtropical high-pressure systems, which suppress cloud formation and precipitation.