From what i am seeing on the fires in LA, trees are also a factor there, as they can catch a wind driven ember, catch fire, and then that fire can spread to adjacent houses.
There are many trees left standing while everything around has burned to the ground, particularly taller trees. It's the wind-carried embers that are the big issue. With a tall tree, the ember drops to the ground. But when the ember lands on something combustible on the ground, then the fire spreads. And, of course, the brush/chapparal grows close to the ground.
The embers are not small, either. A couple of years ago I was watching a night-time live video stream from a fire in southern California. They zoomed in on a house that was burning near the top of a nearby hill. The material that was being lofted away and carried by the wind appeared to be large pieces of wood, such as 2x4 studs up to two feet long, fully engulfed in flame. That's not really an ember; it's a fire bomb.
One result is that there really isn't a continuous advancing flame front. It's more like a moving ignition zone that is a couple of miles wide, within which spot fires break out, grow, and merge. That spot outbreak pattern is what leads to random structures that are seemingly untouched while everything around them is completely burned. It also makes fighting the fire much harder. When there is an advancing flame front, you can attack the front. You can stop it, or hinder the advance in certain directions. But when disconnected spot fires are breaking out all over the place, you don't enough people and resources to fight everyone of those areas. So there is little ability to direct or control the fire's advance.
Another important factor is that, while the fire may have started in open country, once it enters residential areas it becomes fueled by structures. That really makes it more like an urban fire, similar to the fires that periodically broke out in cities in the 19th century, or like the war-time fires in cities set off by firebombs. That was something that I noted with the Oakland Hills fire in 1991, when I was living in the East Bay. The fire started in grassland at the top of the Oakland Hills, but as it went downhill, it entered residential areas, and just skipped along - it didn't burn house to house. Again, there were the flying embers that made up the fire front.
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This was an interesting article. It's from the LA Times, which is a paywall site. The link below is from the Seattle Times, which usually allows access to a limited number of free articles per month. So it should be more accessible.
While Stephen Pyne focuses on our cultural relationship with fire, Jack Cohen looks at fire from a scientific perspective. Both suggest that we have more control over fire disasters than we think.
www.seattletimes.com
