LOS ANGELES  — Santa Ana winds are common in Southern California during cooler months. Here’s a look at the often-fierce winds that topple power lines and trees and can turn a spark into a raging wildfire:

What are Santa Ana winds?

Santa Anas are dry, warm and gusty northeast winds that blow from the interior of Southern California toward the coast and offshore, moving in the opposite direction of the normal onshore flow that carries moist air from the Pacific into the region.

RELATED: Cal Fire moves resources to Southern California as agencies brace for extreme winds

How do they form?

Santa Anas are created by high pressure over the Great Basin — the vast desert interior of the West overlapping several states. The sinking air loses its moisture and flows in a clockwise direction toward Southern California, where it must get past towering mountain ranges that separate the desert from the metropolitan region lining the coast. Like a slow-moving river that suddenly narrows and turns into rapids, the air speeds up as it squeezes through mountain passes and canyons, becoming drier and warmer as it descends.

Why do they create wildfire danger?

Related Articles

Harsh winds, big surf to give way to calm and sunny Bay Area weather conditions

Santa Cruz surfer Nat Young shreds during chilly afternoon session – wearing a bikini

Cal Fire moves resources to Southern California as agencies brace for 80-100 mile per hour winds

Weather service issues tornado warning in Northern California

Santa Cruz Wharf to reopen to the public after partial collapse

Humidity levels often plunge to single-digit percentages during a Santa Ana wind. The extreme lack of humidity in the air causes vegetation — living and dead — to significantly dry out and become susceptible to fire. The tremendous wind speeds can stoke any spark — from a fallen power line, for example — into a rapidly spreading conflagration. Santa Anas are linked to some of the worst wildfires Southern California has experienced.

How did they get their name?

A commonly accepted explanation is that the name is linked to Santa Ana Canyon in Orange County. Other theories persist, along with other nicknames such as “devil winds.”

Do the winds affect people?

Santa Ana winds can sweep urban pollution away, creating sparkling vistas. At the same time, the extreme lack of moisture dries out lips, noses, throats and skin. In the short story “Red Wind,” Raymond Chandler captured the emotional effect: “There was a desert wind blowing that night. It was one of those hot dry Santa Anas that come down through the mountain passes and curl your hair and make your nerves jump and your skin itch.”