THE DESERTS
Nothing about a California desert is an accident. It is downwind arithmetic.
California's deserts exist because its mountains do. Every parcel of Pacific air bound for the southeastern third of the state must first cross the Coast Ranges, then the Transverse or Peninsular ranges, or the Sierra itself — and each crossing wrings water out on the windward slope and delivers the parcel to the far side warmer and drier than it arrived. Do that arithmetic two, three, four ranges deep and the answer is Death Valley: the driest, hottest ground in North America, 84 miles from the highest peak in the contiguous United States.
There are two deserts here, and they are not the same place. The Mojave is high — mostly 2,000 to 4,000 feet — with real winter cold and occasional snow on its Joshua trees. The Colorado Desert, the state's slice of the Sonoran, is low — Salton Trough ground below sea level — with frost-free winters that grow the nation's winter vegetables and summers that are the hottest inhabited climate in the country.
How dry is it, really?
EVERY PLACE ON THE SITE, DRIEST FIRSTAnnual precipitation for all 57 places, driest first. The top of the list is a near-perfect map of the deserts and the rain shadow — and nothing labels it that way; the ordering does it. Note how far down the list the first coastal city appears, and that Blue Canyon at the bottom takes roughly 28 times what Death Valley gets.
The rain shadow, measured
FOUR CRESTS, FOUR PAIRS OF GAUGESEach pair is a windward station against a leeward station across one crest. The ratio is the range's toll, paid in water.
| Windward | Annual | Elev ft | Leeward | Annual | Elev ft | Ratio |
|---|
The only summer rain
SHARE OF THE YEAR FALLING JULY–SEPTEMBERAlmost everywhere in California, summer is the dry season — absolutely. The southeastern deserts are the exception: the North American monsoon pushes moist air up from the Gulf of California, and July thunderstorms are a real part of the water budget. The ten places below get the largest share of their year in July–September, and they are all on the desert side of the state.
What makes each desert
The Mojave: high desert
Joshua trees are a frost line you can see: the Mojave sits high enough for real winter cold, and its emblem plant needs that chill. Lancaster's January nights average right at freezing, and snow on the Joshua trees is unusual but not rare. The region page has the detail.
The Colorado Desert: low desert
The Salton Trough drops below sea level and almost never freezes, which is why El Centro and the Imperial and Coachella valleys grow winter vegetables and dates for the whole country. The price is summer: a mean July high near 107°F.
Death Valley's furnace
A basin below sea level, walled on all sides, cannot shed its heat: air that sinks into it compresses and warms, the walls re-circulate it, and the floor reheats it again. That trap — not latitude — is why Furnace Creek holds the world heat record.
The monsoon
In mid-summer, high pressure over the Four Corners turns the wind over the low deserts around to the southeast, and Gulf of California moisture rides it north. Thunderstorms build over the Peninsular and eastern ranges by afternoon. It is the one Californian rain that does not come from the Pacific in winter.
Wind through the gaps
The deserts breathe through their passes: the San Gorgonio Pass runs a near-permanent jet of coastal air into the Coachella Valley — hence the wind farms — and Tehachapi and Cajon do the same work for the Mojave. See the wind regimes.
Cold nights, wide swings
Dry air and bare ground radiate ferociously after dark, so deserts own the state's widest daily swings — Barstow routinely runs a 30°F+ diurnal range. A desert evening in January can be twenty degrees colder than the coast the same night, which surprises everyone once.
Analysis
FILED UNDER DESERTSHeading out there?
This page explains the desert climate; it carries no live observations and no forecast. For today's numbers at a point, our sister site weather.city renders the official NWS forecast for any spot you give it, and every place page on this site links its own forecast directly.
Desert heat is a genuine hazard: the National Park Service publishes summer safety guidance for Death Valley, and the National Weather Service issues Excessive Heat Warnings through weather.gov. Both are free, and always will be, everywhere on this site.