The Anatomy of Wildfire

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Nov 2019

Fire: it dances romantically on the tips of candles, casts a flickering warmth from the fireplace. But for an entire week last year, fire also upended the daily lives of millions of people living in California. It spread across Spain, and France and Greece. And now it’s loose and reducing vast stretches of the Australian landscape to ash.

Everyone is familiar with fire. But what is it, exactly? What sends it rampaging wild? When it’s set loose, what makes it run, and drives its roaring heart? What determines where it will go, and what makes it stop?

Such questions keep fire experts busy, plotting, planning, moving resources. And as the scale of these wildfires grows in intensity, finding the answers is increasingly critical. Because what the fire watchers learn, might just help keep us safe.

Fire, down in its very microscopic core,

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Photo by Lucas Sandor on Unsplash

is actually a fairly simple thing. It’s a reaction between oxygen in the air, and other molecules that like to react with oxygen. We call these reactions oxidation, and they’re what turn a cut apple brown, or cause iron to rust or silverware to tarnish.

The same process that produces rust 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, 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. And if conditions are right, the reaction turns runaway and self-sustaining.

Then, all that radiant heat ignites fuel nearby, causing the fire to spread, molecule by molecule, in a rapid, expanding chain reaction. And should it escape, the 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: swirling flame towering over groves of trees and homes, leaping radiant orange into the night sky. The crackle and roar, even at a safe distance, is alarmingly loud.

Very few people react to the skin blistering 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 says, on the phone from Sacramento. McLean started fighting fires with CALFIRE in the early 90’s. He has three sons out on the fire line today, and he’s now CALFIRE’s Deputy Chief of Communications.

They train, he says, 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 stay vigilant,and constantly track the mix of vegetation, the lay of the land, weather, winds, humidity, air temperature.

In short, McLean says, they must stay aware of all the factors that shape a wildfire’s ever changing behavior.


The basics of controlling fire — how to start, contain and harness it — is something humans worked out at least a million years ago, archeological evidence suggests. It was a truly world-changing discovery. Cooking food using fire made it vastly more digestible, and made more of the nutritional value of what we ate available. That enhanced the size of our brains and bodies. In many ways, fire set the foundation for civilization itself.

But fire had been shaping life on earth long before we came along.

That’s particularly obvious in California. Fire has been a fixture in the Golden State 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.

That means the challenge now is to understand how wildfires develop and expand among dwellings, and find ways to harden our homes and businesses against them.


In a forest near Albany, Oregon, armed with a recent $2.1 million grant from the US Department of Defense, David Blunck has been running experiments to learn how wildfires spread. Blunck heads the aptly named Combustion, Ignition, Radiation, and Energy Laboratory at Oregon State University. But his interest is more than simply academic. In 2017, 43 miles west of downtown Portland, he watched the giant Eagle Creek Fire leap four miles across the waters of the Columbia River, and start new blazes on the distant shore.

The culprits were embers, smoldering bits of branch, shrub, bark and debris. Embers are generated, Blunck says, as fire breaks down larger pieces of wood or other material, separating them into smaller sections. Like barbecue charcoals, embers can stay burning hot for a long time. When they get pushed aloft by hot smoke 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.

That’s a critical topic, he says, “Because there’s no way to stop them”.

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Photo by Krystle Mikaere on Unsplash

The danger from embers is especially high considering wildfires generate their own powerful updrafts and hurricane force winds, which can send firebrands thousands of feet high and far down range.

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

From his studies of embers and how they work, Blunck says, he’s learned it’s not a good idea to think that creating a green space around your home means you’re safe. While such defensible spaces are vitally important, fire studies in East Coast labs have found, most homes aren’t set ablaze by flames, but by embers. We still need research, Blunck says, to tell us what types of features make homes the most vulnerable, so they can be hardened.

“Embers can be transported for miles,” he explains, “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 note, another potent source of embers in wildfires today are burning houses and garages themselves. They’re filled with highly flammable substances.


There’s no question, experts say, that residents in California, Australia and around the world will be visited by fire again.

“We don’t get to choose between fire and no fire.”

That’s how Monica Delmartini, a fire Specialist at the Sonoma County Agricultural Preservation and Open Space District puts it. Wildfires are not going away.

“The recent fires are really just restoring a process that was unnaturally suppressed for decades,” she says.

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, there’s a chance that our communities may become better prepared for living with it.