The Fire Triangle

Dwelling on heat, fuel, and oxygen

Heat, from one source or another, is what starts a wildfire. No matter how big or intense the fire becomes, it begins at a single point of ignition, where fuel, in the presence of oxygen, is somehow brought to its kindling temperature.

The data shows that it’s now mostly people who do this, whether intentionally or unintentionally. (In the past two decades, roughly 85 percent of all wildfires were started by people.) But there has also always been lightning. “On a statistical curve,” Norman Maclean observed in Young Men and Fire, “lightning seems to be a fairly fixed feature of the universe, as does the number of people who are careless with campfires.” 

I don’t think it’s the number of people who are careless with campfires that’s fixed, so much as the prevalence of carelessness itself. The more people, the more campfires. Campfires interpreted broadly, of course, to include dragging chains and downed power lines and botched gender reveal parties and flicked cigarettes—the sum total of our ignition-happy escapades across a combustible land. 

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On a brilliant July morning, smoke rises from the base of a large Ponderosa pine on the Payette National Forest, near McCall, Idaho. The tree is at least 100 feet tall and its DBH (Diameter at Breast Height) is about three feet. There are similarly sized trees farther up the hill, but this one stands alone. Within a radius of two or three chains (one chain equaling 66 feet), there are only patches of shrubs and young pines, and rocky clearings covered in dry needles, twigs, and leaf litter.

The tree’s canopy is smoking too. Whitish-grey wisps drift out of a dense ball of branches and needle clusters, beneath which a jagged, serpentine scar, black around the edges, slashes its way down a limbless trunk.

None of us know when the lightning struck. It could have been hours ago. Or it could have been days. In that time, the canopy would have smoldered slowly, occasionally dropping the live embers and firebrands that ultimately ignited the brush and duff below. If there was rain, it did little to cool the treetop down. More likely that the lightning was dry. Dry and hot. Now, a ground fire creeps through the understory, puffing whitish-grey smoke and attracting the attention of lookouts and firefighters.

Blood relays a command from Lalo: as soon as Gabe drops the tree, get up there and cut away the canopy before it lights anything else on fire.

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Between 400° and 500° Fahrenheit, wood starts to undergo pyrolysis, or heat-induced decomposition. (Pyro—fire. Lysis—loosening.) Free moisture evaporates. Constituent, complex hydrocarbons like cellulose and lignin break apart into smaller organic compounds. Some of these compounds are left behind as solid char or oily liquids. Others vaporize into flammable gases.

It’s the combustion of these gases—the exothermic reaction of volatile fuel molecules with molecular oxygen (O2)—that produces the visible, sensible heat and light we call a flame. And it’s this released heat that, pyrolyzing even more fuel and providing the activation energy for even more combustion, keeps the flame going—births and feeds itself. 

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Long after a flaming front has swept through an area, the fuel inside of it continues to burn—not as a gas, but as a solid. Though lower in temperature and intensity than flaming combustion, solid combustion, which is also known as smoldering, still emits heat and light. Think of glowing hot coals.

Smoldering combustion is also longer-lived than flaming combustion. “[O]ne November,” wrote Maclean, “I had gone back with my father to hunt deer in country close to where I had been on a big fire that summer, and to my surprise I had seen stumps and fallen trees still burning, with smoke coming out of blackened holes in the snow.”

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To help cool down smoldering fuels, wildland firefighters “mop up” after a fire. If this is done with handtools only, it is called dry-mopping. If there is water available, along with the apparatus necessary to get the water to the heat (a fire engine and a hoselay, usually), then it is called wet-mopping.

Mopping up is also one of the first tests of a rookie firefighter’s competence, and, on any given two or three-week run, can easily account for half or more of the shifts. These are long days spent trudging across charred moonscapes, stooping over every few steps to pat the ground with the back of an ungloved hand, crawling around in the ash and dirt, feeling, feeling, feeling for heat. It is tedious, mind-numbing work that nevertheless, requires attention to detail.

Of course, not every single pile of hot embers or smoldering root can be dug out and mixed with cool dirt and water. That would be a Sisyphean, not to mention pointless task. On a big fire, countless individual fuel particles deep in the interior of the “black,” or burned area, can be left to harmlessly burn themselves out. It’s only the outermost edge—a strip of ground usually between 50 and 200 feet wide, depending on time, terrain, directive, and morale—that needs to be mopped up thoroughly. The idea is that if the conditions are right for a smoldering piece of fuel to revert to flaming combustion and trigger what’s known as a “reburn,” we would rather that happen farther away from the perimeter of the black than closer, where the fire will have a better chance of jumping our line. 

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Two or three of us stand around a stump hole. Something—maybe a chunk of the root—is still skunking away deep inside of it. We know it’s there, even though we can’t see it, because of how hot all the dirt and rocks and ash on top of it are. Whoever discovered the hole would have scooped up a little bit of the whitish-beige ash with the blade of his gravy hoe, patted it lightly, jerked his hand back, and muttered, Oh shit, that’s hot, or something like that. He would have called the rest of us over immediately, and each one of us would have poked knowingly around in the hole, and confirmed, individually, that shit, it really is pretty hot.

We take turns jumping in and digging, each turn lasting a minute, if that. When my turn is over, I jump back out and do a little jig. I shake one foot, then the other, and jog in place. The soles of my boots are hot griddles. The toebox is an oven. My feet feel like they are cooking.

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Fuel is what determines a wildfire’s energy output and where it can go—is what carries fire across a landscape. The carrying can be slow, as when a ground fire creeps across the surface duff on the forest floor. Or it can be fast, as when a crown fire spreads through the canopy layer, leaping from one treetop to another without ever touching the ground. It can be even faster, when a large, intense fire creates spot fires, or, as Maclean put it, “[plays] leapfrog by throwing small fires ahead of the fire’s main front.” In this case, the burning fuels may not be in direct contact or even adjacent. Still, a fire can only leapfrog to where there is fuel.

When fuels carry a fire quickly in any particular direction, we say that the fire is “running.”

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The first time I saw a crown run was from a great distance, which is not the most dramatic, but certainly the preferred distance from which to see one. Maclean tells of an apocryphal, young ranger in the early days of the US Forest Service, who “made himself famous by answering the big question on an exam, ‘What would you do to control a crown fire?’ with the one-liner, ‘Get out of the way and pray like hell for rain.’” Nowadays, we can make the rain with helicopters, which drop several-hundred gallon buckets of water, and Air Tankers, which are ordinary airplanes outfitted to carry many thousands of gallons of water or fire retardant instead of passengers. And nowadays, instead of just praying for it, we often get to call the rain in and watch it come down—again, from a distance. It’s a heavy rain, and not one you want to get caught in.

But when I saw my first crown run, it was dusk, and we were done making rain for the day, and there didn’t seem to be much else to pray for either, so we just watched it instead. There were five or six of us leaning against a fence—some brushing teeth, others talking in low voices, and the rest of us just staring silently. The fire was running up towards a distant ridge-top, far enough away for individual flames to be undiscernible and for it to seem like a continuous, undifferentiated, red-orange lesion on the horizon. From that distance, and if I stared long and hard enough and ignored all sense of scale, I thought the fire looked and behaved like the tide lapping against the gradual slope of a beach. Every few seconds, unburned trees would catch and torch, and these would register as momentary flares in brightness—little tendrils of light licking at the blue-gray mountainside, before the main front of the lesion caught up to and engulfed them all again. And this was happening all along the fire’s uphill-spreading front—this flaring and licking and engulfing, so that the whole perimeter looked like it was twinkling and pulsating and creeping, like foaming rollers breaking on the sand. Only no, they were flames rippling through the treetops, and I watched them climb and climb until they had reached up to touch and join themselves with what was left of the fire in the sky.

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Rain, whether man- or nature-made, suppresses fire by cooling it down. Oxygen is too invisible and omnipresent to do much about. The only remaining leg of the fire triangle that wildland firefighters can manipulate to any immediate effect is fuel.

Actually, there isn’t just one fire triangle, comprising heat, fuel, and oxygen. This is the essential and original one because it lays out the three things that all fires need, in the correct proportions, in order to sustain themselves (remove any one of them, and combustion immediately ceases). But as a tool for predicting and managing fire behavior, this original triangle is best suited to describing flame burning in a human- or room-sized space over the course of seconds or minutes. If we are interested in what a wildfire might do across days or weeks and entire landscapes, we might think instead about topography, fuel, and weather. And yet another step above these are ignitions, vegetation, and climate, which together define a fire regime, or, the overall pattern of wildfire occurrence and intensity that prevails throughout an entire region for years or even decades.

So heat and its sources are aggregated into ignitions, but not before taking a detour through topography (which, to be sure, affects the distribution of both heat and oxygen). And oxygen ends up being just a small part of the big picture of weather and climate, which affect a fire in many other ways besides supplying it with air. But fuel stays fuel, scaling up to vegetation in the widest frame of reference so as to account for the way different floras burn differently, but more or less retaining its basic character: that which burns.

But I was speaking of what we can manipulate.

Again, it depends on scale and context. We are capable of moving mountains and warming the climate, but only as whole societies, acting together over long periods of time. A firefighter, on the other hand, is stuck with the weather overhead and the terrain underfoot. We mind both because our lives depend on it. But all we can actually control in the moment is fuel—how much, what kind, and where. With few exceptions, the sole purpose of our various fireline duties, including line-digging and cutting, brushing and prepping, and backfiring and burning-out, is to deprive a fire of its fuel.

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On the Caldwell Fire, near the Lava Beds National Monument in northern California, we brushed road for days on end without a fire in sight. The fire was somewhere that way, we were told, and this road needed to be prepped as a containment line. Which meant: within 30 feet of the edge of the road on its “black” side (the side where the fire would come from), any dead fuel thicker than a stick, along with any live fuel skinnier than a flagpole, needed to be cut or hauled out, dragged across the road, and dumped at least 20 feet into the “green.” Logs, limbs, saplings, young pines, shrubs—it all had to come out. No surface or ladder fuels could remain, otherwise the fire might climb up into the crowns of the mature trees and leap the road. And once in the green, the fuel couldn’t all be jackpiled either. What if a stray ember drifted across, landed in said jackpile, and blew everything up at once? It had to be spread out, which meant that as the shifts wore on, the job only got tougher, because we had to carry the cut-out fuel deeper and deeper into the green and climb over everything that had been dropped along the way. 

There’s a term for this hauling and dumping of fuel. It’s called “swamping,” and any saw team has at least one swamper swamping for the sawyer. As in, Riley get in there and swamp for Baby Bird! Nobody likes swamping because it’s heavy, tedious work, but it’s also absolutely necessary and more or less the only way to earn a chance at becoming a sawyer yourself. It’s also another effective way, in addition to mopping up, to gauge a rookie’s ego and patience. Just have them drop their tool and pick up sticks for 12 hours a day, day in, day out. (Even better: make them try to keep up with a sawyer who really knows what they’re doing. Somebody like Bird or Gabe, who will cut down two armfuls of fuel for every one that you haul out.) Do they start dragging their feet and picking up less than they can carry? Or do they work like they mean it? Do they grumble and complain as they get bored and fatigued? Or do they keep a good attitude? Do they start strong and fade quick? Or do they pace themselves throughout the shift? 

Bird was my crew boss on the Caldwell, which turned out to be the first and last time I ever got to work alongside him. The following spring, after over a decade on the fireline, he quit firefighting and took a job as a stewardship forester for the Oregon Department of Forestry. He was beloved. But now he gets to go home to his family most nights.

One day in March, right about when he must have been tying up loose ends and getting through the last of his crew boss paper shuffle, I got a call from an unknown number. It was Bird. He’d called to ask: Do you want to be a sawyer?

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Maclean was surprised when he saw smoke coming out of blackened holes in the snow, as was I the first time I heard from a squad boss that residual heat from a wildfire can linger for weeks or even months. How?

It has to do with a combination of fuel diameter, moisture content, and exposure. The wetter the fuel, the more energy you have to spend evaporating excess moisture out of it, before it can finally undergo pyrolysis and eventually, combustion. If you’ve ever struggled to light a campfire with damp firewood, you already know this from experience. You can’t just hold a match to a moist log and expect it to catch. You have to start with tinder: needles, moss, grass, strips of bark—wispy, hair-like stuff. Because of the high surface area-to-volume ratio of these fine, flashy fuels, it’s easy to drive excess moisture out of them, even with short-lived, pocketable ignition sources. They’re consumed just as quickly and easily though. So you layer kindling on top of them: twigs and small sticks, mostly. These take a little longer to catch, but also burn a little longer. Long enough, perhaps, to dry out and ignite even thicker fuels, and so on and so forth, until you can finally put on your split logs.

In the forest fires of the American West, the thickest, most massive fuels are typically large-diameter trunks, limbs, roots, and stumps. Because their surface-area-to-volume ratio is low, and because wood is actually a poor conductor of heat, only the outer, surface layers of these fuels smolder and skunk, while the inner layers stay insulated. This can continue well into the wintertime, so long as the fuel stays buried or otherwise sheltered from precipitation. If it’s beneath ground, a layer of snow may even provide additional insulation and prevent the heat from dissipating.

Trunks, limbs, roots, and stumps are nothing, though, compared to the longest-burning, naturally-occurring fuels in the world: organic soils and sedimentary deposits. In the boreal forests of Siberia, Alaska, and Northern Canada, subterranean fires winter, like hibernating bears, in the carbon-rich peat before bursting back into open flame come spring. And in New South Wales, Australia, beneath a hill commonly known as Burning Mountain, an underground coal seam has been smoldering for ages, as have been too many others to count all over the world. The mine beneath Centralia, Pennsylvania started burning in 1962. The Smoking Hills in Canada have been smoking for centuries. But Burning Mountain’s is the oldest fire of all: 6,000 years.

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On the Juniper Fire, in the heart of the Modoc Plateau, the fuel was grass, sagebrush, and gnarled old Junipers, and nothing but. 

The specific part of the plateau where the fire was burning is called The Devil’s Garden, and it’s easy to see why: flat, rocky lava-steppe as far as the eye can see, and exceedingly dry and hot during the summertime. The horizon wavered in the heat and on the day we arrived, we had to pull the rigs over to the side of the narrow two-track that led to our drop point in order to let a Forest Service engine by. It was transporting a code yellow—a firefighter who had gone down with heat stroke—and as it passed, we could see the others working on him in the back of the cab.

Juniper trees, with their flaky, papery bark, lacy foliage, and resinous sap, are notoriously receptive to fire. One day towards the end of the run, while gridding through the burn’s interior, a few of my squad mates and I came across an unscathed specimen at the base of which a few tiny flames were dancing in the duff. We probably would have snuffed them out and mopped up, but the shift was about to be over and we were at least several hundred feet deep into the black anyways, so we let it be.

The next morning there was a pile of ash where the tree had stood. It was light and fine and rose and blew away as we passed.

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Oxygen is what makes a fire hot. Sure, you need an initial ignition source to raise the fuel temperature to the point where combustion can begin. But once it has, oxygen is the reason the flames and firebrands themselves stay hot.

It isn’t elemental oxygen, but rather molecular oxygen, O2, that enables combustion. The two oxygen atoms are double-bonded to each other, and are commonly represented like this: 

O=O 

That double bond is key. During the combustion reaction of, say, methane (CH4), which is one of the primary gaseous products of the pyrolysis of wood, the double bonds oxygen forms with itself in O2 get converted into double bonds with carbon in the carbon dioxide molecule (CO2), and single bonds with hydrogen in the water molecule (H2O). All together, the balanced equation for the combustion of methane looks like this: 

CH4 + 2O2 —> CO2 + 2H2O + energy 

These new carbon-oxygen and hydrogen-oxygen bonds are both much, much stronger—which is to say, more stable and lower in energy—than the old oxygen-oxygen double bond. And that energy differential gets released in the form of heat and light. 

As for the bond energies in methane—they hardly contribute to this energy differential at all. The combined energy of the carbon-hydrogen bonds in CH4 are almost the same as that of the carbon-oxygen bonds in CO2. And this is true regardless of the fuel’s composition—regardless of whether we start with methane, or any of the other products of pyrolysis. Combusting them is always exothermic, thanks to the relative instability of molecular oxygen.

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On the Buffalo Pasture Fire, near the head of the Little Bull Elk Canyon in southeastern Montana, we spiked out at elevations upwards of 8,000 feet. The wide open plains of the Crow Reservation sprawled out before us, creased and wrinkled like the hands of the elderly, and at nighttime we could see the cell towers and city lights of St. Xavier flickering on the horizon, maybe 25 or 30 miles to the north. It got cold and windy, as it always does at altitude, even in the dead of summer. But until the arrival of the rainstorm that soaked the fire’s smoldering remnants and turned the last few days of that run into an over-equipped backpacking trip, there was always a handful of us who didn’t bother to pitch tents. One thing fewer to break down and pack up in the morning, one layer fewer between our faces and the stars and the crisp, mountain air.

The different gases that make up the Earth’s atmosphere exist in more or less constant proportions until about 60 miles up, or nine times the cruising altitude of a commercial airliner. So mountain or not, any air you could possibly breathe outside is always going to be roughly 21% O2. But what does change with elevation is barometric pressure. It declines exponentially as you ascend, as does the pressure inside of your respiratory tract, which is always continuous with the outside air.

What this means is that the concentration of oxygen (and all other atmospheric gases) decreases the higher up you go. The air thins out, and you have to breathe more of it at altitude than at sea level in order to take in the same amount of oxygen. Your body has a number of homeostatic mechanisms it can deploy to make this happen. But there’s usually a lag of at least a few days between the body reaching altitude and these physiological compensations kicking in. In the meantime—and even after you’ve supposedly acclimatized—it can be tough to get enough oxygen, or at least the amount that your body is accustomed to, and the result can be altitude sickness. Headaches, fatigue, and general malaise set in. Maybe nausea too. What would otherwise be a minor exertion turns into an exhausting struggle.

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The first time I learned about cellular respiration, my eighth grade science teacher explained to the class that in essence, combustion was happening inside of our cells.

Combustion? I asked. As in, fire?

The release of energy inside our bodies was far more gradual and stepwise, she explained. And while a significant portion of it was also lost as heat, the whole point of cellular respiration was to capture and store energy for later use by the body.

But yes, apart from that, the way we metabolized our food was fundamentally identical to the way a fire burned: fuel and oxygen in, carbon dioxide, water, and energy out. Our cells were microscopic furnaces.

No matter how many hours I’ve since spent poring over equations and diagrams—no matter how solid a grasp I have of the biochemical reality—I’ve still never been able to erase the totally unscientific image that flashed across my eighth grade mind, of a single cell sparking and fizzling like a firecracker, of a trillion cells humming in incandescent unison, of my body as a pincushion of flame. 

🜂

It may be misleading to characterize oxygen as “invisible” and “omnipresent” because there are situations in which the absence or presence of O2 becomes very clear. 

For instance, describing what might happen when a fire gets so intense that it cooks all the oxygen out of the air in its immediate vicinity, Maclean writes:

The unburned smoke boils up until it reaches oxygen, then bursts into gigantic flames on top of its cloud of smoke in the sky. The new firefighter, seeing black smoke rise from the ground and then at the top of the sky turn into flames, thinks that natural law has been reversed. The flames should come first and the smoke from them. The new firefighter doesn’t know how his fire got way up there. He is frightened and should be.

Or in other words, track the other two legs of the triangle, and it should be obvious where they meet the third.

🜂

Most days on the Buffalo Pasture, we had to switchback and sidehill down the sides of steep slopes, draws, and cliffs in order to get to our assigned sections of the line. My guess is that we usually worked at around 7,400 feet, plus or minus a few hundred feet, which meant that on any given day, we lost and gained well over 1,000 feet in elevation—maybe even 2,000 or more, if you take into account the amount of hiking around and coming and going we did throughout a shift. 

For instance: one afternoon, Cody rounded up me and three other members of the crew whom he trusted could really book it if shit hit the fan, and led us along the creek bed at the bottom of the canyon in search of a flareup that had been reported near the edge of the green. It turned out to be nothing more than a few ground flames chewing through some dead-and-down, without any torching or spotting yet. But even so, this was certainly a “lookout situation,” if only because of its topographical dicey-ness. Wedged in a narrow creek bed at the bottom of a heat- and wind-channeling canyon is generally not where you want to be in the face of open flame. Moreover, to even find the creek bed in the first place, we had to hike out of the neighboring bowl where we’d been working all morning, cross an open meadow, and climb over an intervening ridge. This took about 20 to 30 minutes and probably brought us to our lowest elevation yet, though there were plenty of ups and downs along the way. 

Still, after days of gridding and mopping up, the prospect of going direct on an actual fire was a welcome thrill, and the circumstantial dicey-ness only provided an additional boost of adrenaline. With Cody calling in the helicopter and the rest of us cutting and digging in five-minute intervals in between bucket drops (yet another motivator), our squad was able to put a line around the fire in short order.

We still had to get out of there though. And as we sat in the ash, eating the last of our snacks and watching steam rise from our soaked, contained fire, we realized we had two choices: slog back out along the creek bed, the way we had come in, or bomb straight up the side of the canyon and make a bee-line for the ridge and the meadow and our rigs just beyond. The tiredness hit us then. Was it even a choice?

About halfway up the side of the canyon, with the ridge in sight, we stopped to take a break. Hunter, our sawyer, had been altitude-sick for days. He lowered the saw slowly to the ground and stood back up, looking ashen and pale.“Want me to carry that?” Cody asked. The saw changed hands wordlessly. 

At the ridge-top, another break. “Switch off?” I asked Cody. “Yeah, right” he said. We grinned at each other.

In the 8,000 foot-high meadow, we paused yet again—unexpectedly, this time. We took our packs and helmets off, felt the breeze chill our damp yellows. We drank water, breathed deep—deep as we could—and sat down amidst the tall grass and the wildflowers. One hundred yards in front of us: a grizzly bear. It was ambling along, aware of us, but apparently not minding us, and perfectly at ease. Below it: a burnt canyon, creased and wrinkled plains. Above it: crisp, mountain air and the wide, wide Montana sky.