Los Angeles has experienced one of its worst disasters as powerful Santa Ana winds, sometimes at hurricane strength, swept down the surrounding mountains and pushed wildfires into several neighborhoods starting Tuesday.

Five blazes were sparked Tuesday and Wednesday: the Palisades, Eaton, Kenneth, Hurst and Lidia fires. The latter three were under 1,000 acres as of Friday. The Lidia fire had the most containment at 60%.

With the winds so strong, there was little firefighters could do to control the flames.

Jon Keeley, a research ecologist with the U.S. Geological Survey and adjunct professor at UCLA, explained via UCLA’s The Conversation what causes extreme winds like this in Southern California, and why they create such fire risk.

What causes Santa Ana Winds?

“These winds occur when there is high pressure to the east, in the Great Basin, and a low-pressure system off the coast. Air masses move from high pressure to low pressure, and the more extreme the difference in the pressure, the faster the winds blow.”

Topography also plays a role

“As the winds rush downslope from the top of the San Gabriel Mountains, they become drier and hotter. That’s a function of the physics of air masses. By the time the winds get to the point where the Eaton fire broke out in Altadena on Jan. 7, it’s not uncommon for them to have less than 5% relative humidity, meaning essentially no moisture at all. These strong, dry winds are often around 30 to 40 mph. But they can be stronger. The winds in early January were reported to have reached 60 to 70 mph.

“These strong, dry winds are often around 30 to 40 mph. But they can be stronger. The winds in early January were reported to have reached 60 to 70 mph."

Have the Santa Ana winds changed over time?

“My colleagues and I recently published a paper comparing 71 years of Santa Ana wind events, starting in 1948. We found about the same amount of overall Santa Ana wind activity, but the timing is shifting from fewer events in September and more in December and January.

Due to well-documented trends in climate change, it is tempting to ascribe this to global warming, but as yet there is no substantial evidence of this.

“California is seeing more destructive fires than we saw in the past. That’s driven not just by changes in the climate and the winds, but also by population growth.

“More people now live in and at the edges of wildland areas, and the power grid has expanded with them. That creates more opportunities for fires to start. In extreme weather, power lines face a higher risk of falling or being hit by tree branches and sparking a fire.”

The fires were made worse by a phenomenon called mountain waves. Mountain waves are oscillations in airflow that can happen when wind blows across a mountain or hill.

If a mountain wave occurs and a fire is present, it may become unexpectedly severe as the strong downslope wind fans the flames. A few areas in Los Angeles County had gusts over 100 mph, sharply limiting suppression efforts by land and air.

A red flag alert is usually issued when a combination of high winds, low humidity and high temperatures is expected.

END OF THE WIND?

From 1950 to 2020, there were 3,219 Santa Ana wind days, and fires ignited on 12% of them. In this period there were 22,704 nonwind event days, and 12% of those days had ignitions. Winds contributed to much larger burns. The number of Santa Ana wind days will decrease but continue through February. A marginal Santa Ana wind event is expected through Wednesday.

Winds are created by flow from high to low pressure centers. Wind maps have lines called isobars. “Iso” means “equal,” and a “bar” is a unit of pressure, so an isobar means "equal pressure.” The closer the isobars are drawn together, the more quickly the air pressure changes. The map on the right shows how close the lines were, prompting a red flag alert from the National Weather Service.

GET PREPARED

During the Palisades fire, roughly 100,000 people were given about an hour to evacuate their homes.

Get prepared now.

Preparation guide: To better understand the needs of first responders and other emergency workers, the 2017 American Housing Survey asked U.S. residents how prepared they are for disasters.

Percentages represent those who answered yes.

Emergency water supply?

At least 3 gallons or 24 bottles of water for each person in the household (58.6%)

Nonperishable emergency food?

Enough nonperishable food to sustain everyone in the household for three days (81.3%)

Prepared emergency evacuation kit? (52.9%)

Emergency meeting location? (36.5%)

Communication plan?

This must include a contingency for the disruption of cellphone service (26.4%).

Evacuation vehicle(s)?

Must be reliable and able to carry all household members, pets and supplies up to 50 miles away (91%).

Evacuation funds?

If you had to leave for a safe place at least 50 miles away, do you have financial resources to meet expenses of up to $2,000? (76.5%)

Generator present?

Asked of residents of single-unit buildings and those with two to four units (18.4%)

Access to financial information? (80.8%)

In total throughout Los Angeles County:

• 420,000 people were without power as of Thursday.

• About 180,000 had been evacuated.

• More than 9,000 structures had been destroyed.

• Estimates of damage were between $52 and $57 billion, making it one of the costliest natural disasters in U.S. history.

COURTESY OF CAL FIRE This 3D map from Cal Fire provides an idea of the steepness of the terrain for the largest of the L.A.-area blazes, the Palisades fire. As of Friday it had burned more than 20,000 acres. This is not the first Palisades fire –? a 1,202-acre blaze took place in the area in May 2021, believed to be arson.