The deserts of the southwestern United States stretch in an irregular pattern from Oregon southward to beyond the Mexican border and from California to West Texas. Although they vary widely in terrain and temperature, these lands share one trait: extreme dryness. Dryness in the southwest U.S. can be traced to two sources: a high-pressure system called the North Pacific High, and a phenomenon called the rain-shadowing effect of mountains.
The North Pacific High is part of a relatively stable high pressure system in the Pacific Ocean that influences the weather from northern Mexico, to the far-west and Southwest United States. In a high pressure system, air from up above tends to move downward to ground level. As the air descends, it grows warmer and its relative humidity drops. Clear skies and dry weather result.
Rain shadow is the result of the prevailing westerly winds losing most of their moisture to the mountain ranges on the western edge of the desert lands. The western Sierra Nevada, for example, receive ten times as much precipitation as areas immediately to the east.
Combined, the North Pacific High and rain-shadowing create an extremely dry climate. Parts of the Sonoran and Mojave Deserts receive only one and a half inches of rainfall a year. When rain does come, it is often in short cloudbursts that dump water on the dry land more quickly than it can be absorbed, resulting in flash floods. During "false" thunderstorms, common during the hot summer months, rain evaporates before reaching the ground.