Santa Ana winds occur from spring to late fall or even early winter, when a high-pressure system forms over the deserts of the Great Basin. The high pressure circulates clockwise, pushing air westward toward the lower-pressure areas of the coast.

NEV.

CALIF.

GREAT

BASIN

Sacramento

Carson

City

San

Francisco

Los Angeles

San Diego

NEV.

CALIF.

GREAT

BASIN

Sacramento

Carson

City

San

Francisco

Los Angeles

San Diego

CALIFORNIA

GREAT

BASIN

Carson City

Sacramento

NEVADA

San Francisco

San Jose

Los Angeles

Santa Ana

San Diego

Air gets hotter and drier

As the wind flows over the Sierra Nevada and Santa Ana mountains, it drops from high elevation to sea level. The sinking air becomes compressed and heats up, and its relative humidity drops.

Cold

air

Warm, dry

air

Mountains

Valley

Coast

Cold

air

Warm, dry

air

Mountains

Valley

Coast

Cold air

Mountains

Warm, dry

air

Valley

Coast

Cold air

Mountains

Warm, dry

air

Valley

Coast

Gaps in the mountains form wind tunnels

The wind speeds up as it pours over the mountains, particularly where it is forced through passes and canyons like water through a funnel. Gusts of 40 to 60 mph or even stronger are common.

Suddenly dry, warm — or downright hot — air is whooshing toward the coast.

Mountains

Warm, dry air

Fast-moving,

hot, dry air

Valley

Coast

Warm, dry air

Mountains

Fast-moving,

hot, dry air

Valley

Coast

Warm, dry air

Mountains

Fast-moving,

hot, dry air

Valley

Coast

The result is like a giant hair dryer

The wind careens over low-lying Southern California, parching vegetation in its path that can easily become fuel for a fire. The wet winter made for a particularly leafy spring and summer in California in 2019.

Embers and

debris

Flames

Gusty

wind

Dry

vegetation

Bigger

fires

Embers and

debris

Flames

Gusty

wind

Dry

vegetation

Bigger

fires

Gusty

wind

Flames

Dry

vegetation

Embers and

debris

Bigger

fires

Gusty

wind

Flames

Dry

vegetation

Embers and

debris

Bigger

fires

Once a fire starts, gusty winds speed it along and carry burning debris to new areas.

Angela Fritz, Tim Meko and Denise Lu contributed to this report.

Sources

National Weather Service. Maps4News/HERE. Fire perimeters from GEOMAC Wildland Fire Support. Wind map imagery from ESA Sentinel, Dec. 7. Wind directions generalized from the Global Forecast System (GFS) spectral model accessed via Global Forest Watch Fires. Percent containment of fires from CAL FIRE.

Originally published Dec. 8, 2017.