A rain shadow is the dry area on the downwind side of a mountain range, left after the range forces moist air to rise, cool and drop its rain or snow on the upwind slopes. In Southern California, Pacific storms hit the San Gabriel and San Bernardino Mountains first and lose most of their moisture on the ocean-facing slopes. The air then crosses the crest and sinks into the Antelope Valley, Mojave Desert and Coachella Valley, warming as it descends. That is why the desert an hour east of downtown can be sunny and dry while the peaks above it are soaked.
What Is a Rain Shadow?
A rain shadow is an area of reduced rainfall on the leeward, or downwind, side of a mountain range, opposite a wetter windward side that faces incoming storms. The windward slope is the one wind and moisture hit first. The leeward slope is the far side, sheltered by the terrain. Pacific storms usually reach Southern California from the west and southwest, so the Transverse Ranges' windward slopes face the ocean and their leeward slopes face the desert.
Rain shadows occur worldwide. The same process makes eastern Washington drier than the Cascades' western slopes and puts Nevada in the rain shadow of the Sierra Nevada. Locally, the effect is clearest when comparing stations on opposite sides of a mountain crest, as the table below does.
| Location | Position relative to the mountains | General climate character |
|---|---|---|
| Pasadena | Windward foothill, against the San Gabriels | Wetter than the open basin, with orographic enhancement from nearby slopes |
| Downtown Los Angeles | Basin floor, upwind of the main crest | Moderate seasonal rainfall, well short of the mountain peaks above it |
| Woodland Hills | Inland valley, partially sheltered by coastal terrain | Warmer summer extremes than the coast, with its own local rainfall variation |
| Palm Springs | Leeward desert floor, east of the San Jacinto and San Bernardino crest | One of the driest, warmest climates in the region, sheltered from most Pacific storm moisture |
How Does Rising Air Lose Moisture?
Rising air loses moisture through orographic lift. When moist Pacific air hits the San Gabriel or San Bernardino Mountains, it is forced upward.
As it climbs, it expands into lower pressure and cools. Once it cools enough, its water vapor condenses into cloud droplets, which grow and fall as rain or, higher up, as snow. Steeper, higher terrain wrings out more moisture before the air reaches the crest.
How fast rising air cools depends on its moisture, which meteorologists track with the moist and dry adiabatic lapse rates described in Weather Prediction Center forecast discussions. Saturated air cools more slowly than dry air, because condensation releases heat. Over a full climb up the San Gabriels, the air still loses a large share of its moisture. The highest, steepest terrain along a storm's path records the region's heaviest rain and snow, while lower ridges let more moisture pass over.
The rise-cool-condense sequence is covered more broadly in how LA's mountains shape its weather, which also covers heat and fog. The rain shadow comes from what happens next, when the air sinks down the far side.
Why Does Descending Air Become Warmer and Drier?
Descending air becomes warmer and drier because it compresses as it sinks into higher pressure near the ground, and compression warms it. The air already lost much of its moisture climbing the windward slope, so it reaches the desert floor warmer and much drier than when it came ashore. Relative humidity drops sharply. Some water vapor remains, but warmer air can hold more, so the same vapor is a much smaller percentage of the air's capacity.
Rain shadow air is depleted, not completely dry, and it is warm enough that its remaining moisture rarely condenses. Storms reaching the Antelope Valley or Coachella Valley usually bring high clouds, wind or a few drops, even when the same storm dropped inches on the mountains a few miles upwind. Palmdale averages 5.90 inches of rain a year and Palm Springs 4.61, compared with 32.17 on Mt. Wilson; see Southern California rainfall totals by city.
Which Ranges Shape Southern California Rain Shadows?
The San Gabriel and San Bernardino Mountains create the rain shadow over the deserts north and east of the Los Angeles Basin. Pacific moisture that crosses them is mostly gone by the time it reaches the Antelope Valley north of the San Gabriels and the western Mojave Desert beyond.
Farther south, the San Jacinto Mountains create the same effect over the Coachella Valley. Palm Springs, at the San Jacintos' eastern base, is among the driest, warmest inhabited valleys in the country, though it is barely 100 miles from the ocean. Its monthly numbers are in Palm Springs weather by month. Snow still falls on the peaks above town; see does it snow in Palm Springs.
The Sierra Nevada, far north of Los Angeles, does not shape these deserts, but it creates a larger rain shadow over the Owens Valley and the Great Basin. It is the textbook example often cited with Southern California's ranges. In every case, a range's height and steepness relative to the storm track decide how much moisture it removes before the air reaches the desert side. Death Valley, in the rain shadow of several ranges, is the most extreme example; see how hot Death Valley gets.
Local terrain adds smaller rain shadows. Woodland Hills and the rest of the San Fernando Valley sit partly sheltered by the Santa Monica Mountains and Simi Hills to the west, a smaller rain shadow on top of the larger San Gabriel pattern. That helps explain why valley rainfall and temperature extremes differ from both the coast and the basin. Any range tall enough to force steady lift can create a dry zone on its lee side, even if the zone is a single valley.
Is a Rain Shadow a Santa Ana Wind?
A rain shadow is not a Santa Ana wind. A rain shadow is a long-term rainfall pattern created by terrain blocking moisture storm after storm. A Santa Ana wind is a shorter offshore wind event driven by high pressure over the Great Basin, and it needs no Pacific storm. Both involve air warming and drying as it sinks, but a rain shadow shows up in years of rainfall records, while a Santa Ana lasts hours or days.
A rain shadow is also not a temperature inversion. An inversion is warm air layered over cooler surface air, which traps pollution in the basin; a rain shadow is a horizontal difference in rainfall across a mountain range. For inversions and smog, see temperature inversions and trapped smog. For why some neighborhoods stay hotter after dark because of pavement and buildings, see the urban heat island effect in Los Angeles.
How Can One Storm Produce Rain, Snow, and Desert Virga?
One Pacific storm crossing Southern California can bring rain, snow and desert virga along a single line of terrain, from the same air mass. Near the coast and across the basin, the storm brings ordinary rain. As the air rises up the San Gabriel or San Bernardino slopes, it cools below the freezing level, the altitude where temperature crosses 32°F. The same storm then brings snow above a few thousand feet while the basin below stays wet and cold.
Once the remaining air crosses the crest and sinks toward the desert, it may still form high clouds or rain streaks called virga. The warming, drying air below evaporates that rain before it reaches the ground.
Virga is a visible sign of a rain shadow: the storm is still producing rain, but the desert air is too warm and dry for it to reach the ground. The Weather Prediction Center's storm guidance and the snow levels and rain totals from NWS Los Angeles/Oxnard and NWS San Diego document all three outcomes: coastal rain, mountain snow and desert virga.
Heavy rain on the windward side brings its own hazards, especially on steep or recently burned slopes. The USGS Landslide Hazards Program and NWS flood safety guidance cover the debris flow and flash flood risk from the same storms.
The rain shadow is a lasting pattern, built storm after storm by the San Gabriel, San Bernardino and San Jacinto Mountains that give coastal Los Angeles its rain and the deserts to the east their sun. Before a trip from the basin into the mountains or desert, compare current conditions, snow level and forecast rain for each stop on WeatherLA instead of assuming one forecast fits the whole drive.