Southern California presents a complex and diverse landscape, making the simple question of whether it is a desert impossible to answer with a single word. The region is often stereotyped as entirely arid due to its proximity to large desert areas and its long, dry summers. However, the climate and ecology shift dramatically across its geography, meaning the answer depends entirely on the specific location. Understanding this variability requires examining the scientific criteria used to define a desert environment.
What Makes a Region a Desert
A region is scientifically classified as a desert primarily based on aridity, which is a measure of water scarcity. While the common metric states that a desert receives less than 10 inches (250 mm) of precipitation annually, a more accurate definition considers the relationship between precipitation and evaporation. Scientists use the concept of potential evapotranspiration (PET), the maximum amount of water that could evaporate and transpire if constantly available. In a true desert, the annual water loss through PET vastly exceeds the annual precipitation received. This severe water deficit creates the dry conditions characteristic of arid lands, defining deserts universally regardless of temperature.
The Specific Desert Areas of Southern California
A substantial portion of Southern California’s inland territory definitively meets the criteria for a desert. This arid expanse is home to two distinct biomes: the Mojave Desert and the Colorado Desert.
The Mojave Desert
The Mojave is considered a high desert, characterized by elevations typically ranging between 2,000 and 4,000 feet above sea level. Annual rainfall is extremely low, generally receiving only about 2 to 6 inches of precipitation per year, mostly during the winter. This biome is ecologically defined by the presence of the iconic Joshua Tree.
The Colorado Desert
The Colorado Desert, lying to the south, is a low desert and a western extension of the larger Sonoran Desert. It includes the Coachella and Imperial Valleys and features much lower elevations, with its lowest point at the Salton Sea being below sea level. This desert is significantly hotter than the Mojave, experiencing soaring summer temperatures and occasional rainfall from the North American Monsoon during late summer. Both deserts owe their extreme aridity to the rain shadow effect created by the mountain ranges to their west.
The Diverse Non-Desert Climates
While the southeastern interior is a true desert, the majority of Southern California’s population centers and the Pacific coastline are not.
Coastal Mediterranean Climate
The coastal areas, including cities like Los Angeles and San Diego, feature a Mediterranean climate, a rare classification found in only five regions globally. This climate is defined by mild, wet winters and warm, dry summers. The coastal strip receives sufficient annual precipitation, generally ranging between 15 to 30 inches per year, which prevents a desert classification. The dry summers result from the subtropical high-pressure system that deflects storms northward, but winter rains sustain chaparral and scrubland ecosystems.
Mountain Ranges
Moving inland from the coast, the Transverse and Peninsular Ranges form high-elevation, non-desert environments. These mountain ranges, such as the San Bernardino and San Jacinto mountains, experience cool-temperate and even alpine climates. They receive significant amounts of precipitation, often as heavy winter snows, which feed the water systems of the lower elevations. The towering presence of these ranges creates a dramatic rain shadow, where moist air from the Pacific is forced upward, dropping its moisture on the western slopes before descending dry and warm into the desert valleys to the east.