Knowable Magazine has an interesting article by Alexandra Witze on a variety of physics principles that affect wildland fires. She covers the research by Michael Gollner of the University of Maryland on how embers start spot fires, how Janice Coen, an atmospheric scientist who studies wildland fires at the National Center for Atmospheric Research in Boulder, Colorado monitored the start of the Camp Fire as she sat in the back of a room at a conference, and the real time radar signature of the firenado (fire tornado) at the Carr Fire.
Below is an excerpt from a section about embers propagating spot fires.
It turns out that a single ember, or a handful of embers, can’t build up that much heat if it lands on a material such as a deck or a roof. But put one or two dozen embers into Gollner’s device and the heat flux goes up dramatically, he and his colleagues report in the March Fire Safety Journal. “You start to have re-radiation between them,” he says. “It glows, under the wind — it’s just beautiful.”
Just a small pile of embers can generate about 40 times the heat you’d feel from the sun on a hot day. That’s as much heating, and sometimes more, as comes from the fire itself. It’s also enough to ignite most materials, such as the wood of a deck.
So if there are a lot of embers flying ahead of a fire, but those embers land relatively far from one another, they may not build up the radiative heat needed to generate a spot fire. But if the embers pile up, perhaps blown by the wind into a crevice of a deck, they can smolder together and then trigger an ignition, Gollner says. Most homes that burn in the wildland-urban interface ignite from these embers, often hours after the fire front itself has passed.
Understanding the heat flux at these small scales can illuminate why some houses burn while others don’t. During the Tubbs fire, homes on one side of some streets were destroyed while those on the other side had hardly any damage. That may be because the first house that ignited radiated energy to its neighbor, which then burned neighboring homes like dominoes because of the radiative heat. When houses are closely packed together, there’s only so much homeowners can do to mitigate the danger by clearing brush and flammable material around the house.