Santa Ana winds, explained

Santa Anas are hot, dry winds that blow from the inland deserts toward the Southern California coast. High pressure builds over the Great Basin, air flows out of it toward lower pressure at the coast, and as it sinks from the high desert to sea level it heats up and its humidity collapses.

Why the wind blows backward

Most of the time Southern California's air comes off the Pacific, from the west. During a Santa Ana a big area of high pressure sits over Nevada and Utah. Air flows clockwise around a high, so on its southern side the wind runs from the east and northeast, straight from the deserts toward the ocean. It's the same reason a Santa Ana air trail on Air Story points back toward the interior instead of out to sea.

The jet stream far overhead still blows west to east. The reversal happens in the lowest few thousand feet, which is the air you feel.

Why it's so hot and dry

The desert air starts high, on plateaus several thousand feet above sea level, and pours down through passes and canyons such as the Cajon Pass and San Gorgonio Pass. Sinking air gets squeezed by rising pressure, and squeezed air warms up: about 5.5°F for every 1,000 feet it drops. Air that was cool on the plateau can arrive at the coast 20 to 30°F warmer.

None of that adds water, so as the air warms its relative humidity crashes, often into single digits. That's why Santa Anas bring the year's highest fire danger, and why the coast can be warmer than the desert while it's happening.

When they happen

Santa Anas can blow any time from fall through spring and are most frequent in the cooler months. The fall events are usually the most dangerous, because they arrive after a long dry summer when vegetation is at its driest. A single event typically lasts one to three days. Northern California has a cousin, the Diablo wind, driven by the same setup.

What it feels like

Gusty, warm wind at odd hours, startlingly clear skies, crisp distant mountains, static shocks, dry eyes and lips, and nights that barely cool off. On Air Story's first screen it shows up as a “Santa Ana winds” chip with your temperature, dew point and where the air was three days ago.