Almost every autumn, fierce desert winds called the Santa Anas sweep across California's mountains and into a landscape primed to burn by months, and sometimes years, of dry weather.
Southern California's wild lands have gone up in smoke again and again over the centuries. But now people have built their homes on those forested mountain slopes, amid flammable brush and in narrow canyons that act as wildfire funnels.
This confluence of forces - the weather, the wild landscape and the changes that humans have made to it - give Southern California's wildfires a destructive power that most Easterners can barely imagine.
Ten major fires were burning along a 40-mile front in four Southern California counties yesterday, and fire officials said they were far from gaining the upper hand in spite of a temporary slackening in the winds.
With 13 people dead, 900 homes destroyed and an additional 3,000 houses and apartments still in danger, the fires were the deadliest and most destructive since 1991, when a wildfire that swept through the scenic Oakland Hills above San Francisco Bay killed 25 and destroyed more than 3,000 homes.
Firefighters often can do little more than wait for the weather to change. Ten years ago this week, 20 wildfires were burning at once in Southern California. They burned for two weeks, killing four people and burning 1,000 homes.
Although they can be ignited by anything from lightning to stray sparks from a barbecue, the wildfires' most powerful ally is the hot, dry Santa Ana winds.
Meteorologists say winds like these can form only over deserts, and California lies alongside North America's biggest desert, the Great Basin of Nevada, Utah and southern Idaho.
The Santa Ana phenomenon begins when a strong high pressure system forms over the desert, said senior meteorologist Phillip Bothwell of the federal Storm Prediction Center. This can happen any time of year, but it's most common in autumn.
Normally, California's winds move from west to east, bringing Pacific Ocean moisture inland. But the high pressure system reverses that flow, Bothwell said, and squeezes the air, which gets hotter as it compresses. The pressure pushes the hot, dry air up and over California's mountain ranges from east to west.
"As this air descends toward the ocean, it replaces the moist air with hot, gusty, very dry air. It's coming from a high elevation into a low elevation, which compresses it even more as it's channeled into the valleys," Bothwell said.
Gauges in Southern California's mountain passes recorded gusts as high as 70 miles per hour over the weekend - almost hurricane force - with temperatures near 80 degrees, he said.
"It's just continually hot and dry and windy, hot and dry and windy, and it's allowed these fires to burn very actively," Bothwell said.
Normally, fire follows wind uphill, according Jack Cohen, a fire behavior expert at the U.S. Forest Service's fire sciences laboratory in Missoula, Mont. But during a Santa Ana, the wind carries the fire from the uninhabited highlands down into the passes, canyons and valleys where so many people live.
"These places have always been prone to burning," Cohen said, "but there is an urgency and a sense of anticipation [because] we live there and care about it now."
On the highest slopes, the winds encounter a prime source of wildfire fuel - dry pine forests where millions of trees have been killed over the past few years by a pine bark beetle epidemic.
Last summer, California Gov. Gray Davis declared a state disaster in three counties from the beetles' ravages. In parts of the San Bernardino National Forest, where the fires are now burning, the Forest Service reports that beetles have killed as much as 70 percent of the trees.
Although the beetles are always present in pine forests, healthy trees are able to fight off their infestations, Cohen said. Unfortunately, the Sierra Nevada's forests are under stress these days, though scientists disagree about the causes.
In July, the Los Angeles County agricultural commissioner cited a half-dozen problems. Topping the list were drought conditions in four of the past five years, which robbed the pines of their main defense, the ability to drown the beetles' larvae in a flood of sap.
Near-normal rainfall last winter eased drought conditions slightly - but it might have actually worsened the fire danger, according to forecaster Bothwell. "The first of the year they had some very nice rains," he said, "and things grew very nicely, and that creates more fuel for the fires later on."
The agriculture commissioner also blamed air pollution from the Los Angeles Basin, along with a century-long policy of putting out forest fires - which has left the pine stands unnaturally crowded and weakened the trees. He warned that the abundant deadwood in the region's forests would increase the likelihood of devastating fires this year.
About 40,000 people live in the San Bernardino National Forest, where the tree mortality is greatest, Cohen said. But at lower elevations, urban populations have expanded right to the edge of highly flammable shrub land known as chaparral.
Chaparral plants have evolved to survive California's climate, where rainy winters are followed by as many as nine rainless months. The native plants grow fibrous leaves that hold their shape even when they're dry, Cohen said. Mostly evergreen, they grow year-round and develop into dense stands.
Many chaparral natives have waxy leaves to help retain water and produce resins that retain nutrients. Most scientists believe those traits contributes to their flammability.
The result, Cohen said, is yet another source of ready fuel for a fire.
Some researchers think the region's fire patterns have changed for the worse since European settlement. In particular, they blame Spanish cattle, which imported fire-loving European grasses on their hoofs.
Chaparral plants can re-sprout after fire, but the grasses regenerate even faster and burn more often, speeding up the natural fire cycle, according to Forest Service and U.S. Geological Survey researchers.
Other scientists note that as more people inhabit the wild lands, they increase the odds that a car tailpipe, a flicked cigarette or an arsonist will set the woods ablaze.
Debates like these can quickly stray from science into politics and moralizing, said Cohen: "There are a huge number of factors, and to try to pin it on something is largely folly."
Although some argue that humans have altered the natural fire cycle, "The question is not, 'Is it unnatural?' It's basically, 'What can we do about it?'" Cohen said. "And the answer is, not much - we're just going to have to learn to live with it."
Some Southern California communities recommend or require nonflammable materials in new constriction. They're mandating brush-clearing, ordering sprinklers in larger buildings and retrofitting some older ones.
Attempts at fireproofing communities are "the only thing we can really do under extreme conditions," Cohen said. "Basically, we end up reducing the amount of fire occurring at any one time. It gives firefighters a better chance."