Blame the wind, if you want. In Southern California they call it the Santa Ana; in the north, the Diablos. Every autumn, from 4,000 feet up in the Great Basin deserts of Nevada and Utah, air drops down over the mountains and through the canyons. By the time it gets near the coast it’s hot, dry, and can gust as fast as a hurricane.

Or blame lightning, or carelessness, or downed power lines. No one yet knows the cause of the more than a dozen fires ablaze around California, but fires start where humans meet the wild forests, where people build for solitude or space or beauty. Things go wrong in those liminal spaces, at the interface between the wilds and the built.

So blame sprawl, or civilization’s cycling of wilderness into rural into exurban into suburban—urban agglomerations with an ever-expanding wavefront.

Blame all of it. There’s a reason the great Californian writer Raymond Chandler called it the Red Wind—winds “that come down through the mountain passes and curl your hair and make your nerves jump and your skin itch.” Those winds blast down from the mountains and fan small fires into infernos, and sometimes those infernos maim or kill a city. In 1991 it was in the hills of Oakland. And this past weekend it was Napa and Sonoma, and the town of Santa Rosa. At least 15 people are dead. More than 1,500 houses are gone. The skies of the West are full of dust and ash.

Smoke and water stream through the roof of a home destroyed by fire in Anaheim Hills. Stuart Palley/Los Angeles Times/Getty Images

Pushed by the wind, fires can throw burning embers a mile and a half ahead. The fire front starts moving faster than anyone can respond, jumping from ridgeline to ridgeline.

A fire’s progress through the forests and wildlands of North America isn’t exactly formulaic, but scientists understand it reasonably well. In the city, though? “Most wildland firefighters are not trained in structural protection, but the urban fire departments are not trained to deal with dozens or hundreds of houses burning at the same time,” says Volker Radeloff, a forestry researcher at the University of Wisconsin. “When these areas with lots of houses burn, the fires become very unpredictable.”

Buildings, the material bits of cities, don’t burn like woodlands. “A wildfire typically doesn’t last in one spot more than a minute or two. In grass it can be like 10 seconds,” says Mark Finney, a US Forest Service researcher at the Missoula Fire Sciences Laboratory. “But structures can burn for a long time. That means they have a long time to be able to spread the fire, to be able to ignite adjacent structures.” They throw off embers as they decompose, and those wide walls emit and transfer heat.