How powerful Santa Ana winds are supercharging the L.A. wildfires

Deadly and destructive wildfires are tearing through the Los Angeles area, and powerful winds are literally fanning the flames. 

As of Wednesday afternoon, four fast-moving blazes had killed two people, injured many and destroyed more than 1,000 structures, while threatening 28,000 more and forcing at least 70,000 people to evacuate.

Here's how winds are making the wildfires more dangerous and harder to fight.

What kinds of winds are these and why are they so strong?

The Santa Ana winds are strong winds that blow from dry areas inland toward the coast, generally in the cooler months, for a few days to a week.

The National Weather Service says Santa Ana winds topped 129 km/h in some areas around L.A. by early Wednesday, and could top 160 km/h in the mountains and foothills. 

Wind gusts of up to 95 km/h are expected through Thursday, L.A. County fire crews said Wednesday.

Those winds come from dry desert regions of Nevada, Utah, Idaho and southeast Oregon, and get even drier as they sweep down the mountains.

Peter Mullinax, a meteorologist with the weather service's office in College Park, Md., called what the region is experiencing "extremely critical fire weather conditions," due to a combination of strong wind gusts on the highest terrain and extremely low humidity.

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The Mountain Fire, which started earlier this week in California's Ventura County, has grown to more than 80 square kilometres in size and prompted evacuation orders for some 10,000 people.

How does wind affect a wildfire's spread?

The faster the winds, the faster the spread of fire. The rule of thumb used by wildland firefighters is that fires spread at 10 per cent the speed of the wind. For example, a wind speed of 25 km/h would allow a fire to spread at 2.5 km/h. 

There are a few reasons for this.

• Wind helps supply the fire with air and oxygen.
• It also flattens and bends the flames toward the the dead wood and other fuel ahead. That can preheat and blow sparks and embers into those new, dry fuel sources, causing spot fires.
• The direction of the wind is what mostly determines which direction a fire spreads, the National Weather Service says.
• Wind also dries vegetation, turning it into more combustible fuel.

Lindon Pronto, senior fire management expert with the European Fire Institute, said the Santa Ana winds' ability to dry vegetation to their lowest moisture content of the year is one of their "really dangerous effects."

"The fuel itself becomes extremely available for fire and can catch and carry fire very quickly," he told Reuters.

That compounds the increased fire risk and worse fire conditions already caused by climate change.

Sylvia Dee, an assistant professor and climate scientist at Rice University in Houston, said climate change has created hotter and drier conditions overall in the region, "and that creates a sort of tinderbox."

How is it affecting firefighting efforts?

As of mid-day Wednesday, 1,400 firefighters had been deployed on the ground, but aerial efforts were hampered by the fact that it was too windy for firefighting aircraft to fly, The Associated Press reported.

The winds can generate turbulence, especially in mountainous terrain, or even damage aircraft if they are too intense.