The Mojave Desert's boundaries are generally defined by the presence of Joshua Trees — they are considered an indicator species for the desert. The topographical boundaries include the Tehachapi together with the San Gabriel and San Bernardino mountain ranges. The mountain boundaries are quite distinct since they are outlined by the two largest faults in California: the San Andreas and the Garlock. The Great Basin shrub steppe lies to the north; the warmer Sonoran Desert (the Low Desert) lies to the south and east. The desert is believed to support between 1,750 and 2,000 species of plants.

Climate

The Mojave is a desert of temperature extremes and four distinct seasons. Winter months bring temperatures dipping to below 20 °F (-7 °C) on valley floors, and below 0 °F (-18 °C) at higher elevations. Storms moving from the Pacific Northwest can bring rain and snow across the region — more often, the rain shadow created by the Sierra Nevada as well as mountain ranges within the desert such as the Spring Mountains bring only clouds and wind. In longer periods between storm systems, winter temperatures in valleys can approach 80 °F (27 °C).

Spring weather continues to be influenced by Pacific storms, but rainfall is more widespread and occurs less frequently after April. By early June, it is rare for another Pacific storm to have a significant impact on the region's weather, and temperatures after mid-May are normally above 90 °F (32 °C) and frequently above 100 °F (38 °C).

Summer weather is dominated by heat — temperatures on valley floors can soar above 120 °F (49 °C) and above 130 °F (54 °C) at the lowest elevations — and the presence of the North American monsoon. Low humidity, high temperatures and low pressure draw in moisture from the Gulf of Mexico, creating thunderstorms across the desert southwest. While the Mojave does not get nearly the amount of rainfall that the Sonoran desert to the east receives, monsoonal moisture will create thunderstorms as far west as California's Central Valley from mid-June through early September.

Autumns are generally pleasant, with one to two Pacific storm systems creating regional rain events. October is one of the driest and sunniest months in the Mojave, and temperatures usually remain between 70 °F (21 °C) and 90 °F (32 °C) on the valley floors.