The devastating fires in southern California have been whipped up by a Santa Ana wind, a rush of hot, dry air that sweeps down from the Sierra Nevada mountains.

The wind is fairly common in October, but this outbreak is certainly one of the most violent in recent times, with hurricane-force winds gusting up to 178km/h (111mph).

More worrying still, this may be the start of many more ferocious wildfires because the Santa Ana season runs until February, usually reaching a peak in December.

The Santa Ana is born from a strange mix of meteorology and geography. It begins when a big dome of high pressure air builds up over the highland deserts of Nevada and Utah. Winds always flow from high to low pressure areas and, when a low pressure system sits off the Pacific Coast of California, the desert air comes rushing through the Sierra Nevada mountains, accelerating like an avalanche, and blasts out to sea. This week, a particularly strong high-pressure system has generated strong, hot, dry winds.

The air is already dry and dusty from the high deserts, but as it descends to the coast it becomes compressed and heats up, rather like air heated up in a bicycle pump, and temperatures can soar to more than 38C (100F). The moisture in the air is also squeezed out and turns the air bone-dry, scorching leaves on plants and trees and transforming the southern California chaparral scrublands into an explosive fuel that bursts easily into flames.

To add to the problems of the fire-fighters, the Santa Ana is unpredictable and gusty. As it squeezes through mountain valleys, the winds can shed violent, erratic eddies that send flames shooting off in different directions. Already about 2,800sq km (1,080sq miles) have been burnt, nearly 2,000 buildings have been destroyed.

In one of the worst previous episodes, during October 2003, Santa Ana winds fanned wildfires over two weeks that burnt 2,920sq km (1,128sq miles), destroyed 3,640 homes and killed 14 people.

The choking wind causes breathing problems and makes people agitated, with complaints including headache, insomnia and depression.

In the worst cases, the wind has a reputation for bringing out aggression and the Los Angeles Police Department often reports a rise in violent crimes such as road rage and murder when the wind blows strong.

It was during Santa Ana winds in 1984 that Richard Rameriz, the serial killer who was also known as the The Night Stalker, went on a killing spree. As Raymond Chandler, the novelist, wrote, the Santa Ana was a time when “meek wives feel the edge of a carving knife and eye their husbands’ necks”. (Red Wind, 1938).

Blowing hot

— The Santa Ana winds can be strong enough to uproot trees and lift roofs from houses

— The winds blown out to sea stir up the waters and cause an upwelling from the ocean depths, dragging up nutrients that boost phytoplankton and fish populations. Surfers also enjoy larger waves, hence the song by the Beach Boys Santa Ana Winds

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