7. The Dry Interior West

Most of the country’s western interior—an area that extends from the desert Southwest and eastward from the Sierra Nevada and Cascades to approximately the 100th meridian—receives scant moisture. Both high and low temperature extremes are much greater here than in the eastern part of the country. In this region, actual temperature conditions vary depending on lati­tude and elevation. Most of the area receives less than 20 inches (50 centimeters) of precipitation annually. The desert Southwest, including much of eastern California, Nevada, portions of Utah, the southern half of Arizona and New Mexico, and western Texas, receives less than 10 inches (25 centimeters) of precipitation per year. Nevada is the nation’s driest state, with an annual average of 9 inches (23 centimeters) of precipitation, and Las Vegas is the driest city, receiving about 4 inches (10 centimeters) of moisture each year. The distinction of being the nation’s driest spot goes to the parched desert floor of Death Valley, California, which receives a scant annual average of 1.4 inches (3.6 centimeters) of rainfall. Despite its extreme aridity, during the very wet spring of 2005, a large lake formed on the valley’s floor! Another record goes to Tucson, Arizona. That city holds the distinction of having recorded the world’s lowest relative humidity—a bone-dry 0.8 percent (statistically 1 percent). Summer is the wettest season, when most rain falls in torrential thunderstorms.

The region’s aridity is the result of two primary influences. As prevailing winds blow from west to east across the high mountains, most of their moisture is released on the windward (western) side. As they descend down the leeward (downwind) side of the Sierra Nevada and Cascades, they warm, creating a “rain shadow” effect. East of the western mountains, only scat­tered mountain peaks catch enough moisture to support forest growth. In the desert Southwest, a second control influences aridity. The region is overlain by a semipermanent high pressure system. If you have seen the face of a barometer, you know that rising (higher) pressure is associated with stable weather conditions. This condition persists throughout most of the year in these drier areas of the country.

Because of its continental location or as a result of its being far removed from the moderating influence of the ocean, the interior West commonly experiences weather extremes. Summers can be fiercely hot and winters frigidly cold. Temperatures have reached a sizzling 135°F (57°C) in California’s Death Valley, only one degree below the world’s record high registered in the Sahara Desert. Throughout the region, but particularly in the desert Southwest, afternoon temperatures can soar higher than 100°F (40°C) for weeks on end. Low temperatures are a function of latitude and elevation. Throughout the winter months, the nation’s low temperature is often recorded in spots such as Wisdom, Montana; Stanley, Idaho; or even Bellemont, Arizona. Each of these communities is located at a very high elevation. The lowest temperature ever recorded in the 48 coterminous states was a frigid -70°F (-57°C) at Rogers Pass, Montana.

Before the arrival of Europeans, much of the central interior was covered with a seemingly endless sea of grass. From west to east, shorter steppe grasslands gradually gave way to taller prairie grasses as precipitation amounts increased. This region was home to the American bison (buffalo), an estimated 60 million of which roamed the grassland-covered plains. Within a century, their numbers plummeted to about 1,100 in one of the greatest mass slaughters in history. Other large animals that roam the plains include deer, elk, and antelope. Where water is available for irrigation, soils tend to be quite fertile. Because of the aridity, nutrients have not been leached (washed out).