Science probes how fires start, spread, move
Fire: it dances romantically on the tips of candles, and casts a flickering warmth from the fireplace. But for an entire week, fire also upended the daily lives of most people living in Sonoma County, and impacted hundreds of thousands more beyond.
Everyone is familiar with fire, but what is it, exactly? What sends it rampaging wildly? When it’s set loose in the landscape, what makes it move, and drives its roaring heart? What determines where it will go, and what makes it stop?
Those are questions fire experts are busy trying to answer, even as flames continue to burn in thousands of acres nearby. As the scale of California’s wildfires seem to be growing in severity and intensity, such efforts are increasingly important. Because what the fire watchers learn can help keep us safe.
Down in its very microscopic core, fire is actually a fairly simple thing. It’s a basic chemical reaction between oxygen in the air and molecules that readily react with oxygen. The reactions are called oxidation, and it’s what turns a cut apple brown, or causes pieces of iron or silverware to rust or tarnish.
Oxidation is also happening in fire, but with an important difference. On apples, iron and silver, oxidation is slow. But on materials that contain a lot of stored energy, like wood, grass or propane, the chemistry runs at extreme speed. If conditions are right, the reaction turns runaway and self-sustaining, releasing tremendous amounts of radiant heat, light and superheated gases.
Those glowing gases are what we see as flame, with soot and ash mixed in.
The heat can ignite nearby fuel, causing it to spread, molecule by molecule, in a wildly expanding chain reaction. And should it escape, fire will race and leap across entire landscapes, and burn for weeks or months, until it runs out of fuel, or is put out.
The images are shocking, and hard to forget: towering swirling flame among groves of trees and homes, leaping bright orange into the night sky. The crackle and roar, even at a safe distance, is alarmingly loud.
Very few people react to the white heat and choking smoke of wildfire by driving toward it. Those that do, do so with respect, and hard-earned knowledge about what wildfire is, and how it moves.
“We’re trained to look for indications that tell us what it might do,” Scott McLean said on the phone from Sacramento. McLean started fighting fires with Cal Fire in the early ’90’s. He has three sons out on the fire line, and he’s now Cal Fire’s deputy chief of communications.
They train, he said, to look: Look at the types of vegetation, to spot the plants that might burn faster or hotter. Grass growing on a hillside above a fire will preheat, and can flash-burst into flame all at once. Shrubs and trees tend to slow the flames’ speed of advance. But wind can turn a fire, send it racing down or up a funneling draw or drainage. Wind can make it burn hotter, and move faster, even start new fires out ahead, half a mile or more.
On the ground, for their own safety and to fight the flames, the fire fighters must look at vegetation, topography, weather, winds, humidity, air temperature.
In short, McLean said, the factors that shape a wildfire’s behavior are complex, and ever changing.