Anatomy of a wildfire: How fuel sources, weather and topography influence wildfire behavior
On the surface, wildfires seem simple.
There’s a spark, a few small twigs flare up, and it spreads throughout a forest landscape until it runs its course or is doused by firefighters. In the United States alone, we see it tens of thousands of times a year, from relatively innocuous burns deep in the wilderness — that most people won’t even hear about — to violent blazes along the wildland-urban interface that can consume everything in their paths, leaving only scorched earth and melted metal behind.
But how exactly do wildfires happen, and what factors determine whether a fire will stay calm or burst into an unpredictable and uncontrollable force of nature?
In reality, the causes and effects of wildland fires are complex, rippling through a wide-ranging network of trees, brush and wildlife inside a forest and broadly impacting its ecology and biology. But to start, we’ll look at the chemistry involved.
There’s an old saying that fire follows people, and the statistics back it up.
While fires caused by lightning and spontaneous combustion do occur naturally, almost all wildfires are caused by humans in one way or another.
“People don’t like to hear this, but eight out of 10 wildfires are human caused,” said Jeff Berino, former chief at Summit Fire & EMS and fire science instructor at Colorado Mountain College. “That includes everything from downed power lines, particles from a diesel engine landing on grass, out-of-control campfires or kids with matches. People often assume arson, but people do dumb things all the time and accidents happen.”
In order for an accident to turn into a full-blown wildfire, the environmental conditions have to be ripe, but to better understand how ambient conditions affect fire behavior, it helps to break down the chemistry of combustion.
At a base level, fire needs three components to sustain itself: fuel, oxygen and heat. But instead of a single reaction taking place, there are thousands, even tens of thousands, of reactions to produce self-sustaining flames.
On a molecular level, fuel sources like wood are complicated, made up in part by lignin, cellulose and hemicellulose molecules — essentially huge sugar molecules that have been interlinked.
“When you look at a piece of wood, what you’re really looking at is essentially sugar that has bonded to itself,” said Torben Grumstrup, Ph.D., a research mechanical engineer at the U.S. Forest Service’s Fire Sciences Laboratory in Missoula, Montana. “And just like glucose and fructose are good fuel for us, those serve as fuel for a fire. And taking apart these extremely complex molecules atom by atom requires a huge number of different reactions.”
As a heat source moves toward a piece of wood, the fuel eventually will hit a temperature — about 480-575 degrees Fahrenheit — when the molecules begin to break off the main body, a thermal decomposition process called pyrolysis. As a result, a stew of gas made up of broken-off sub-molecules reacts with the oxygen in the air, producing carbon dioxide, water, heat and light.
“When you see flames, it’s not actually the solid wood burning,” Grumstrup said. “It’s actually this gas that’s produced by the process of pyrolysis that’s burning.”
Once a flame is born, there are three primary variables that decide how it will act: fuel, weather and topography.
Fuel for fires
How quickly a fire starts and spreads is determined by myriad environmental factors, perhaps most notably the type of fuel and the moisture level inside.
“When you have a fuel — grass, pine needles, logs, any woody biomass — that’s full of water, when the fire burns that fuel, it not only has to heat up the fuel to get to the point where the chemistry of combustion is sustainable, it also has to heat up the water contained in the fuel and turn it from liquid into steam,” Grumstrup said. “And that takes a surprising amount of energy because water is a really phenomenal absorber of heat.”
In other words, the less moisture content you have in a fuel source, the easier it is for a fire to become self-sustaining and to spread.
Obviously, precipitation and the relative humidity in the air are the driving factors behind fuel moisture, which is measured at fire science labs by taking samples of different sizes and types of fuels, drying them out in an oven and determining the amount of water that was present.
Different size fuel sources also vary considerably in how they react to precipitation and humidity, with larger fuels requiring longer time periods to adapt to changes in the atmosphere. Fire officials take measurements on one-hour fuel sources like grass (less than 0.25 inches in diameter), 10-hour sources (0.25-1 inches), 100-hour sources (1-3 inches) and 1,000-hour sources like trees (3-plus inches).
Officials also measure dead and living fuel sources, which can return moisture levels of between zero to 30% and 30% to 300%, respectively, meaning living fuels can hold up to three times their weight in water. As a result, the critical values — when a fuel source is readily available for combustion — differ widely in various vegetations. For example, a dead fine fuel source like pine needles might hit the critical mark at around 6% fuel moisture content while living sagebrush can hit critical moistures at 100%.
But the moisture and size of fuel is only part of the equation in a wildfire. Fuel loading — the volume and density of fuel sources — also plays a major role.
“It impacts fire behavior fundamentally,” said Ross Wilmore, a former zone fire management officer with the White River National Forest and wildland fire specialist with the Greater Eagle Fire Protection District. “The heavier the fuel loading, the more fuel is available to increase the fire intensity.”
As a broad generalization, lighter fuels (in the one- and 10-hour classes) burn and spread faster, while heavier fuels burn slower and with more intensity. The composition of fuel types, along with density, helps to predict the type of fire.
A grass fire will spread quickly, but there will be very little left burning in its wake. Meanwhile, heavier fuel loads might leave massive amounts of burning material well behind the front line of the fire and burn hot enough to produce embers that can float up to a couple of miles away under the right conditions and start spot fires around the primary blaze.
The weather factor
Other weather factors aside from precipitation and humidity also are key in determining how a fire will behave. The biggest is wind.
Not only does wind help supply a fire with more oxygen enabling the faster combustion of fuels, it also drives flames toward one direction, pressing the fire closer to the ground and enhancing how quickly heat is being transmitted to different fuel sources on the forest floor.
“Wind is the biggest driver of wildfires,” Berino said. “Think of it like blowing on a campfire. It has the same effect of giving the fire more oxygen, but it’s also pushing the fire toward adjacent vegetation and starting to preheat it.”
By the time a flame reaches the new fuel source, it’s already absorbed a considerable amount of heat and is ready to combust.
Oxygen availability also affects how hot a flame burns and, in turn, what the fire looks like. With sufficient oxygen to burn up the entirety of a fuel source, wildfires can burn at about 1,800 degrees Fahrenheit and will show a yellow flame and white smoke. Without enough oxygen to thoroughly burn fuels, wildfires likely will burn at between 1,100 and 1,300 degrees, with red and orange flames and darker smoke.
And while weather certainly plays a big role in predicting a fire’s path, wildfires can create their own weather systems in extreme cases, including changing wind patterns.
If a wildfire breaks out in an area with a considerable fuel source — heavy, downed trees that will burn for an extended period — the fire is releasing a tremendous amount of energy. As the fire continues to heat up, the buoyant flow of hot gas creates a low-pressure area that draws in air from its surroundings.
“When fuels are burning, they are sucking in oxygen through the convection process,” Wilmore said. “You’ve got a low pressure area above the fire as hot air rises, and it creates a vacuum in the area of the fire that gets filled by surrounding air. … From both a micro and macro sense — within a couple miles of the fire — it can actually change the weather and alter wind directions.”
As the fire draws in ambient air from the sides, known as an indraft, a column of pyrocumulus clouds of particulate matter and water vaporizing from fuel sources rises into the atmosphere, creating a weather system capable of generating its own rain, hail and lightning, along with erratic winds.
“The bigger a fire gets and the faster it’s able to burn, the more it is going to have a strong influence on the surrounding weather,” Grumstrup said.
The slope effect
The final major contributing factor in determining a wildfire’s behavior is the topography in which it’s burning, and the most important variable in the landscape is slope.
Fire tends to burn more aggressively moving uphill. Similar to the effects of wind, steep slopes allow for the preheating of nearby vegetation. If a fire ignites on a mountainside, it initially will burn the same as though it were on flat ground, with smoke and flames heading straight into the air. But once the fire has established itself, the flames naturally will begin laying over toward the uphill side of the slope.
“The fire is trying to draw in air from all directions,” Grumstrup said. “But when there’s a hill, the fire isn’t able to draw in air from the uphill side, creating a gentle pressure from the wind coming in that pushes the flame to the uphill side. Just like with wind, when you have flames in close proximity to unburned fuel on the ground, it’s naturally enhancing the rate of transmission of heat to that fuel. It rises in temperature faster, and it thermally decomposes and pyrolyses faster.”
The phenomenon is even more strongly apparent in canyons, where there are slopes on both sides, along with landscaping that can funnel wind directly to the flames. Called the chimney effect, a wildfire can induce winds from the bottom of a canyon through a convection current, pushing flames deeper into the hillsides.
The aspect of slopes also will impact how susceptible the area is to a wildfire, largely due to temperature and which areas are receiving direct sunlight.
“The hotter the ambient temperature is, the easier it is for wildfires to grow and spread,” Berino said. “The fuel is already heated up. If you’re up here in Summit on a hot summer day, things get really hot in the direct sunlight. That’s why north-facing slopes don’t burn as aggressively as south-facing slopes. That aspect of the mountain is getting cooked all day and slowly heating that fuel up.”
This means that the time of day plays a role in when fires are most active, with fire danger typically peaking during the hottest and driest part of the day at around 2 p.m.
Altitude also affects fire danger. Just as humans have more difficulty breathing with less oxygen at higher elevations, wildfires are subject to the same effect.
A nonlinear problem
Fuels, weather and topography are the three main factors in determining how and why a wildfire behaves the way it does. But it’s only in combining all three variables that we’re able to truly assess the type of fire we’ll see, along with how quickly it spreads and with what intensity.
There are three types of wildfires: ground fires, surface fires and crown fires. Ground fires burn up to a few feet underground in soils rich in wood fiber and can smolder for weeks or months until conditions are favorable to emerge to the surface.
Surface fires are the main drivers behind wildfire behavior, burning up available fuel sources on the forest floor. And in more severe conditions, surface fires can create heat and flame lengths significant enough to climb the ladder of branches and create crown fires burning along the forest’s canopy.
“The rate of spread ties back to all these factors,” Berino said. “If there is no wind, fires are relatively easy to jump on. But once you add wind, it makes it more complicated. And once you add slopes and a dry fuel package, then you have a recipe for a fast-spreading wildfire.”
But even when considering all of the factors, wildfires can be extremely difficult to predict as even the smallest changes in conditions can have massive effects on behavior.
“At around 2 p.m., when the temperature is at its highest and humidity is lowest, it seems like the fire will suddenly start burning very aggressively,” said Grumstrup.” That’s because of the inherent nonlinearity of fire, where a small change in input creates a huge change in output. Even a very small change in humidity, temperature or wind speeds can create big changes in the fire behavior and the rate of spread.”
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