Joshua Tree National Park is located in southern California on the eastern end of the broad mountainous belt called the Transverse Ranges, which stretch from Point Arguello, 50 miles west of Santa Barbara, eastward for nearly 300 miles to the Eagle Mountains in the Mojave Desert. Unlike most mountain ranges in North America that run north-south, the Transverse Ranges lie on an east-west axis.

The area of southeastern California is a rain shadow desert. The rain shadow effect is produced by the high mountains on the west, which block the movement of wet winter storms. Coastal storms moving east collide with Mount San Jacinto (10,804 ft.) and Mount San Gorgonio (11,502 ft.) dropping most of their moisture on the west sides of these mountains. Land on the east side receives much less rain, which results in a desert environment.

During late August or September occasional tropical storms move into southern California from the south. These storms end up on the east side of the Peninsular Ranges and can dump a considerable amount of water in a short time. Some five to 10 inches of rain may fall in a few hours, representing a large portion of Joshua Tree’s annual precipitation.