Why does Malibu seem to erupt in flames every fall, while most of Los Angeles, which has its share of houses clinging to brushy hillsides, does not?
The reason, according to a new study, is blowing in the wind.
Researchers have developed the first high-resolution map of Santa Ana wind events, showing that the hot, dry blasts don't sweep uniformly across the Southland and that the danger of large, wind-whipped wildfires is therefore greater in some parts of the region than others.
Guided by local topography, the seasonal Santa Anas follow certain corridors to the sea, consistently skirting other areas.
"Most people, think, 'Ah, it's a Santa Ana day, Southern California is in trouble,' and that is true," said Max Moritz, the study's lead author and co-director of the UC Berkeley Center for Fire Research and Outreach. "But there is much more spatial difference in that story, much more diversity."
The paper, published February in the online version of Geophysical Research Letters, includes a map marked with distinct bands outlining the favored Santa Ana routes.
"The Santa Monica Mountains and the Malibu area are just hammered," Moritz said. "Then the whole L.A. Basin to the south of there is actually in a sheltered window. You go farther south and you get another big band of high fire danger" in the Laguna Hills area of Orange County and then another in eastern San Diego County.
Pinpointing which parts of Southern California are the hardest hit carries implications for development, building standards, public lands management and property insurance.
"We have some of the most amazingly dangerous fire weather on the planet," Moritz said. "You could use a model like this to help make decisions about where and when you might want to restrict access so you don't get ignitions in the worst possible places at the worst possible times."
The maps also could aid in planning fuel-thinning projects: Locating them in areas protected from the winds would probably be more effective than placing them in a Santa Ana corridor, where ember-raining winds would sprint right over them.
Moritz and his co-authors used climate data and computer modeling to reconstruct weather conditions on a hyper-local scale for 18 Santa Ana days spanning nearly a decade.
They focused on dates in October, a month that has seen many of Southern California's most devastating wildfires.
The researchers then compared the results with Southland fire histories. They found that the areas pounded by extreme Santa Anas also experienced many of the region's biggest conflagrations -- among them, the 2003 Simi fire (107,000 acres), the 2003 Cedar (270,000 acres) and the 2007 Witch (162,000 acres).
In some ways, the study's results were not surprising. Anyone who has lived through a few fire seasons knows the desiccating winds spread flames like a fire-breathing dragon.
Certain mountain passes, especially Soledad, Cajon and San Gorgonio, act as funnels for the gusts, which heat up and gain speed as they are drawn from the interior West to the Pacific Coast by differences in atmospheric pressure.
But mapping the Santa Anas on a detailed enough level to show local variation in their pathways is new.
"Some people think you can just carve out the entire quadrant of Southern California and say Santa Anas occur. That's not really true," said Dave Sapsis, a wildland fire scientist with the California Department of Forestry and Fire Protection. "It isn't sufficient to say this entire region of the state is subject to the equivalent fire weather environment. But we wouldn't have known that without this kind of analysis."
He called the study "a good piece of work" but added it needed more validation by applying the modeling across a broader spectrum of time and landscape.
Alex Hall, a co-author of the paper and a UCLA associate professor of atmospheric and oceanic sciences, said the findings underscore the importance of weather, particularly wind speed, temperature and humidity, in the growth of monster blazes. "Wildfires are started all over the landscape, mostly by people," he said.
"The ones that survive and develop into large fires [do so] because of the meteorology. The fact that the study really clinches that is significant."
Some relatively protected parts of the Southland, such as the Santa Barbara area, have been hit repeatedly by big wildfires. There, Moritz said, a local version of the Santa Anas called sundowner winds are at play.