Franklin Fire fanned by infamous Santa Ana winds. Here's how they work.

Santa Ana winds, one of the nation's most notorious wind events and an annual weather hazard in Southern California, have helped fuel the destructive Franklin wildfire in Malibu that's charred nearly 4,000 acres this week.

The winds, which occur most often in the fall and winter, push dry air from over the inland deserts of California and the Southwest toward the coast, the National Weather Service said.

They blow when high pressure builds over the Great Basin (the high plateau east of the Sierra Nevada), according to meteorologist Jan Null of Golden Gate Weather Services. Santa Anas then blow over the mountains between the deserts and coastal California. As the wind comes down the mountains, it's compressed and warms up.

Dryness provides better fuel for fires

As the air warms, its relative humidity also drops, sometimes to less than 20% or even less than 10%. The extremely low humidity helps dry out vegetation, making it a better fuel for fires.

"Nowhere else do such winds impact so many people with so much force and possess such extensive opportunity for damage and destruction," the National Weather Service has said.

Many of the winds that blow down mountain slopes in California are also named for their locale, according to Null. In addition to Santa Anas in southern California, northern California has the Diablo winds. These northeast winds tend to blow through the Diablo Range, which runs north-south on the east side of the San Francisco Bay.

Red flag warning issued

This current outbreak of gusty Santa Ana winds and dry conditions prompted the National Weather Service to issue a red flag warning for portions of southern California this week. Red flags signal dangerous weather conditions where wildfires can spark and spread unpredictably. Fire departments bring on additional staff while the warnings are in play.

Long fire season in California

Although Santa-Ana driven wildfires are a natural part of California’s landscape, the fire season in the state and across the West is starting earlier and ending later, according to CalFire.

Climate change is a key driver of the trend, CalFire said. Warmer spring and summer temperatures, reduced snowpack and earlier spring snowmelt create longer and more intense dry seasons that increase moisture stress on vegetation and make forests more susceptible to severe wildfires.

This article originally appeared on USA TODAY: What are Santa Ana winds? Franklin Fire fanned by infamous winds.