How fuel sources, weather and topography influence nature
Summit Daily News
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, 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 behind only scorched earth and melted metal.
But how 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 network of trees, brush and wildlife inside a forest and broadly impacting its ecology and biology.
The flashpoint
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 humancaused,” 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.”
For an accident to turn into a 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. Instead of a single reaction taking place, there are thousands, even tens of thousands, of reactions to produce self-sustaining flames.
“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, a research mechanical engineer at the U.S. Forest Service’s Fire Sciences Laboratory in Missoula, Mont. “And just like glucose and fructose are good fuel for us, those serve as fuel for a fire.”
As a heat source moves toward a piece of wood, the fuel eventually will hit a temperature — about 480 to 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 selfsustaining and to spread.
Precipitation and the relative humidity in the air are the driving factors behind fuel moisture.
Different size fuel sources also vary considerably in how they react to precipitation and humidity, with larger fuels requiring longer times to adapt to changes in the atmosphere.
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 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 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.
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 has 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.
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.
The aspect of slopes also will impact how susceptible the area is to a wildfire, largely because of temperature and which areas are receiving direct sunlight.
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.