What are Santa Ana winds and how are they fuelling LA wildfires?

Santa Ana winds can bring disruptive impacts to daily life in Los Angeles and parts of southern California, but when coupled with wildfires they can escalate the situation rapidly, causing the fires to take hold and spread at great speed.

These strong and gusty east or north-easterly winds blow from inland towards the coast. They can occur numerous times throughout the year and have been known to happen on over 20 occasions during some years.

A Santa Ana event usually occurs during the cooler months, from late September to May and generally lasts just a couple of days, but on rare occasions can continue for up to a week.

The weather set-up is key to the Santa Ana winds blowing. They occur when a large area of high pressure becomes established over the interior of western US, centred around the Great Basin, which includes much of Nevada and parts of Utah, Idaho and south-east Oregon. These are generally dry, desert areas, which means the winds that arrive in California are lacking in moisture.

With winds flowing clockwise around high pressure systems, they move south and west into southern California, where they descend over the inland Sierra mountains. As the air descends, it dries further and accelerates in speed. Wind speeds can increase further as they funnel through the valleys and gorges in the area, much like water increases its speed as it passes through a hose.

Santa Ana winds not only cause ripe conditions for wildfire development, but they can also be responsible for the scale of destruction that follows.

The low humidity of the air is key to the initial fire taking hold. The air is initially dry due to the desert locations where it originated from, but it dries out even further as it rolls down off the mountains.

Like a hair dryer, it then strips the vegetation of a lot of its moisture, meaning that fire can catch quicker and the vegetation burn more readily.

The strength of the wind is what helps to spread fires rapidly. Speeds of 60 to 80mph (95-130km/h) are common, but gusts of up to 100mph (160km/h) can occur during the worst Santa Ana events.

When these sort of strengths continue for days on end, it can prove incredibly difficult for the emergency services to contain the fire. The gusty nature of the winds add to the unpredictability of the spread.

No one is completely sure where the name originates from. It's most commonly thought that it stems from the Santa Ana canyon in Southern California's Orange County, however the winds affect more places than just one canyon.

Other names for the Santa Ana winds include "devil wind" or "red wind".