Science probes how fires start, spread, move

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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.

Controlling fire — how to start, contain and harness it — is something humans worked out at least a million years ago, archaeological evidence suggests. It was a providential discovery. Using fire to cook food made it vastly more digestible, and made more of the nutritional value of what we ate available, thereby altering the size of our brains and bodies. In multiple ways, fire set the foundation for civilization itself.

But fire was shaping life on earth long before that, and particularly in the place we call California.

Fire has been a fixture here for so long, an amazing 54% of the state’s ecosystems now depend on fire to survive and regenerate, and most of the rest have adapted ways of surviving its regular passage, according to Stephen Pyne, a fire historian at Arizona State University.

The reason we consider it so destructive, in the view of those like McLean who fight to keep wildfire out of our homes and communities, is that we’ve decided to build flammable houses where fires have always roamed. The challenge now is to understand how they develop, expand, and find ways to harden our dwellings and businesses against them.

In a forest near Albany, Oregon, armed with a recent $2.1 million grant from the U.S. Department of Defense, David Blunck has been running experiments to learn how wildfires spread. Blunck heads the Combustion, Ignition,

Radiation and Energy Laboratory at Oregon State University, but his interest is more than academic. In 2017, he watched the giant Eagle Creek Fire, 43 miles west of downtown Portland, leap four miles across the flowing Columbia River, and start new blazes on the distant shore.

The culprits were embers, smoldering bits of branch, shrub, bark and debris. They’re generated, Bluncksaid, as fire breaks down larger pieces of wood or other material, separating them into smaller sections.

Like barbecue charcoal, embers can stay burning hot a long time. And when pushed aloft and spread away by wind, they will ignite anything flammable they happen to land on – dry grass, trees, decks, roofs.

Researchers like Blunck are still trying to understand what conditions generate the most embers. It’s a critical topic, he said, “Because there’s no way to stop them++.”

The danger from embers is especially high considering wildfires generate their own powerful updrafts and winds, sending the firebrands thousands of feet high and far down range.

Embers are also the main reason firefighters and emergency authorities dread high winds.

Having studied embers and how they work, Blunck said, it’s not a good idea to think that just creating a green space around your home means you’re safe. Fire studies in East Coast labs have found that most homes are set ablaze by embers. We need research, he said, to tell us, what types of features make homes the most vulnerable, so they can be hardened.

“Embers can be transported for miles,” he explained, “so if they land on a cedar roof, or among leaves and debris in roof gutters, or where needles gather around ridge corners, or cracks in decks, studies show, they’ll ignite.” Clearing such ignition sources away is essential.

Unfortunately, Blunck and McLean noted, another potent source of embers in wildfires today are burning houses and garages, filled with highly flammable substances.

There’s no question, experts said, California residents will be visited by fire again.

“We don’t get to choose between fire and no fire,” is how Monica Delmartini, a fire Specialist at the Sonoma County Agricultural Preservation and Open Space District put it.

“The recent fires are really just restoring a process that was unnaturally suppressed for decades,” she said. That’s the perspective of all who work with wildfires face to face.

With their help, and better understanding of this essential force of nature, California’s communities may become better prepared for living with it.

Stephen Nett is a Bodega Bay-based Certified California Naturalist, writer and speaker. Contact him at

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