Climatic Regions of the United States, Desert
The most arid climate in the United States is found in the Southwest. This area comprises southern inland California, Arizona, New Mexico, and parts of Nevada and Texas. Sections of neighboring states are also included. Moisture is meager and erratic. The area receives less than 250 mm (10 in) of rainfall annually. High temperatures cause any moisture to evaporate rapidly.
Many areas with arid climates can be found on the dry side of mountain ranges. These mountains create a rain-shadow effect, with a belt of arid climate to the leeward side (the side opposite the prevailing winds) of the mountain barrier. Air that has released its moisture while passing over the mountain summit descends the leeward slopes of the range and is compressed. Cooling no longer occurs, and cloud droplets and ice crystals evaporate. The air continues to warm as it descends. By the time it reaches the base of the mountain, the air is hot and dry because it has released its moisture.
Temperatures are higher in the desert than in any other climatic region because clear desert skies allow the earth's surface to heat up rapidly during the day. In the evening, temperatures drop quickly, resulting in great variations in temperature within a 24-hour period. Winter is brief and mild. Summer is long and scorchingly hot. Temperatures during the hottest months average from 29° C to 35° C (from 85° F to 95° F), and the midday readings of 40° C to 43° C (105° F to 110° F) are common. The winter daily maximum usually averages 18° C to 24° C (65° F to 75° F). Winter nights are chilly, averaging 7° C to 13° C (45° F to 55° F).
Soils in the desert climate are classified as aridisols. They are low in organic matter and high in salts. Humus is lacking because the climate supports only a very sparse vegetation.
Vegetation has adapted to conditions in the desert. Cacti and other succulent plants store water in their thick leaves. Shrubs especially adapted to the desert, such as sagebrush and creosote bush, have special forms of roots and stems. Their waxy leaves help them limit water loss. Because there is little moisture in the soil, the ground between plants is generally barren in the desert. Some desert plants are small flowering annuals that can remain dormant for long periods and quickly germinate when water becomes available. Following a rare shower, the desert landscape is often painted with brightly colored flowers that bloom for a brief period.
Like the plants, animals that live in deserts have become adapted so that they require less water. Most desert animals have small bodies, which help them more easily dissipate heat. Some burrow into the ground when the hot sun beats down and then come out at night to feed. Small desert animals include mice, hares, rabbits, kangaroo rats, and spade foot toads. Among the larger desert animals are the coyote, and mule deer.
Historically, few people have lived in desert regions. Settlements occurred only where a natural oasis or a permanently flowing stream existed. The desert was extremely marginal land that was used for occasional grazing of cattle, sheep, and goats. However, many deserts have soils rich in the minerals that plants need to grow. When these deserts are irrigated, they grow abundant and useful crops such as vegetables, fruits, and grains.
With the ability to pump groundwater, divert streams, and build dams and reservoirs, the desert Southwest boomed. Large urban settlements, retirement communities, and irrigated agriculture all expanded. However, in many areas, groundwater is being withdrawn from aquifers (underground reserves of water) faster than the aquifers are being recharged. Consequently the future of water supplies in California and the Southwest has become a matter of serious concern for residents and businesses that depend on groundwater.