How Wildfires Work

By: Kevin Bonsor
Fires like this one are more often than not the result of a careless human action. Watch these wildfire videos.
Photo courtesy Bureau of Land Management

­In just seconds, a spark or even the sun's heat alone sets off an inferno. The wildfire quickly spreads, consuming the thick, dried-out vegetation and almost everything else in its path. What was once a forest be­comes a virtual powder keg of untapped fuel. In a seemingly instantaneous burst, the wildfire overtakes t­housa­nds of acres of surrounding land, threatening the homes and lives of many in the vicinity.

An average of 5 million acres burns every year in the United States, causing millions of dollars in damage­. Once a fire begins, it can spread at a rate of up to 14.29 miles per hour (23 kph), consuming everything in its path. As a fire spreads over brush and trees, it may take on a life of its own -- finding ways to keep itself alive, even spawning smaller fires by throwing embers miles away. In this article, we will look at wildfires, exploring how they are born, live and die.


In 2000, this wildfire burned just north of Sula, Montana.
Photo courtesy Bureau of Land Management/John McColgan

On a hot summer day, when drought conditions peak, something as small as a spark from a train car's wheel striking the track can ignite a raging wildfire. Sometimes, fires occur naturally, ignited by heat from the sun or a lightning strike. However, the majority of wildfires are the result of human carelessness.

Common causes for wildfires include:

  • Arson
  • Campfires
  • Discarding lit cigarettes
  • Improperly burning debris
  • Playing with matches or fireworks
  • Prescribed fires

­Everything has a temperature at which it will burst into flames. This temperature is called a material's flash point. Wood's flash point is 572 degrees Fahre­nheit (300 C). When wood is heated to this temperature, it releases hydrocarbon gases that mix with oxygen in the air, combust and create fire.

There are three components needed for ignition and combustion to occur. A fire requires fuel ­to burn, air to supply oxygen, and a heat source to bring the fuel up to ignition temperature. Heat, oxygen and fuel form the fire triangle. Fire­fighters often talk about the fire triangle when they are trying to put out a blaze. The idea is that if they can take away any one of the pillars of the triangle, they can control and ultimately extinguish the fire.

­After combustion occurs and a fire begins to burn, there are several factors that determine how the fire spreads. These three factors include fuel, weather and topography. Depending on these factors, a fire can quickly fizzle or turn into a raging blaze that scorches thousands of acres.


Fuel Loads

Fuel is a major factor in determining a fire's intensity.
Photo courtesy Bureau of Land Management

­Wi­ldfires spread based on the type and quantity of fuel that surrounds it. Fuel can include everything from trees, underbrush and dry grassy fields to homes. The amount of flammable material that surrounds a fire is referred to as the fuel load. Fuel load is measured by the amount of available fuel per unit area, usually tons per acre.

A small fuel load will cause a fire to burn and spread slowly, with a low intensity. If there is a lot of fuel, the fire will burn more intensely, causing it to spread faster. The faster it heats up the material around it, the faster those materials can ignite. The dryness of the fuel can also affect the behavior of the fire. When the fuel is very dry, it is consumed much faster and creates a fire that is much more difficult to contain.


Here are the basic fuel characteristics that decide how it affects a fire:

  • Size and shape
  • Arrangement
  • Moisture content

Small fuel materials, also called flashy fuels, such as dry grass, pine needles, dry leaves, twigs and other dead brush, burn faster than large logs or stumps (this is why you start a fire with kindling rather than logs). On a chemical level, different fuel materials take longer to ignite than others. But in a wildfire, where most of the fuel is made of the same sort of material, the main variable in ignition time is the ratio of the fuel's total surface area to its volume. Since a twig's surface area is not much larger than its volume, it ignites quickly. By comparison, a tree's surface area is much smaller than its volume, so it needs more time to heat up before it ignites.

As the fire progresses, it dries out the material just beyond it -- heat and smoke approaching potential fuel causes the fuel's moisture to evaporate. This makes the fuel easier to ignite when the fire finally reaches it. Fuels that are somewhat spaced out will also dry out faster than fuels that are packed tightly together, because more oxygen is available to the thinned-out fuel. More tightly-packed fuels also retain more moisture, which absorbs the fire's heat.


Weather's Role in Wildfires

Wildfires can produce winds that are 10 times stronger than the winds surrounding them.
Photo courtesy Bureau of Land Management

­Weather plays a major role in the birth, growth and death of a wildfire. Drought leads to extremely favorable conditions for wildfires, and winds aid a wildfire's progress -- weather can spur the fire to move faster and engulf more land. It can also make the job of fighting the fire even more difficult. There are three weather ingredients that can affect wildfires:

  • Temperature
  • Wind
  • Moisture

As mentioned before, temperature affects the sparking of wildfires, because heat is one of the three pillars of the fire triangle. The sticks, trees and underbrush on the ground receive radiant heat from the sun, which heats and dries potential fuels. Warmer temperatures allow for fuels to ignite and burn faster, adding to the rate at which a wildfire spreads. For this reason, wildfires tend to rage in the afternoon, when temperatures are at their hottest.


Wind probably has the biggest impact on a wildfire's behavior. It also the most unpredictable factor. Winds supply the fire with additional oxygen, further dry potential fuel and push the fire across the land at a faster rate.

Dr. Terry Clark, senior scientist at the National Center for Atmospheric Research, has developed a computer model that shows how winds move on a small scale. Since 1991, he's been converting that model to include wildfire characteristics, such as fuel and heat exchange between fires and the atmosphere.

"We look at what's called coupled fire atmosphere dynamics, where the fire and the atmosphere interact with each other," Clark said. "We've been looking at how fires interact with the environment and getting some of the characteristics of fire spread and fire behavior, through the modeling that we've been doing."

Clark's research has found that not only does wind affect how the fire develops, but that fires themselves can develop wind patterns. When the fire creates its own weather patterns, they can feed back into how the fire spreads. Large, violent wildfires can generate winds, called fire whirls. Fire whirls, which are like tornadoes, result from the vortices created by the fire's heat. When these vortices are tilted from horizontal to vertical, you get fire whirls. Fire whirls have been known to hurl flaming logs and burning debris over considerable distances.

"There's another way that you can tilt the vorticity. That is it can be titled without breaking into fire whirls, and basically be burst forward into what's called hairpin vortices or forward bursts," Clark said. "These are quite common in crown fires [fires at the top of trees], and so you see fires licking up hill sides." Forward bursts can be 20 meters (66 feet) wide and shoot out 100 meters (328 feet) at a speed of 100 mph (161 kph). These bursts leave a scorched region and lead to fire spread.

The stronger the wind blows, the faster the fire spreads. The fire generates winds of its own that are as many as 10 times faster than the ambient wind. It can even throw embers into the air and create additional fires, an occurrence called spotting. Wind can also change the direction of the fire, and gusts can raise the fire into the trees, creating a crown fire.

While wind can help the fire to spread, moisture works against the fire. Moisture, in the form of humidity and precipitation, can slow the fire down and reduce its intensity. Potential fuels can be hard to ignite if they have high levels of moisture, because the moisture absorbs the fire's heat. When the humidity is low, meaning that there is a low amount of water vapor in the air, wildfires are more likely to start. The higher the humidity, the less likely the fuel is to dry and ignite.

Since moisture can lower the chances of a wildfire igniting, precipitation has a direct impact on fire prevention. When the air becomes saturated with moisture, it releases the moisture in the form of rain. Rain and other precipitation raise the amount of moisture in fuels, which suppresses any potential wildfires from breaking out.


Fire on the Mountain

More often than not, fires travel faster up slopes. Once at the top of a hill, fires tend to burn out.
Photo courtesy Bureau of Land Management

­The third big influence on wildfire behavior is the lay of the land, or topography. Although it remains virtually unchanged, unlike fuel and weather, topography can either aid or hinder wildfire progression. The most important factor in topography as it relates to wildfire is slope.

Unlike humans, fires usually travel uphill much faster than downhill. The steeper the slope, the faster the fire travels. Fires travel in the direction of the ambient wind, which usually flows uphill. Additionally, the fire is able to preheat the fuel further up the hill because the smoke and heat are rising in that direction. Conversely, once the fire has reached the top of a hill, it must struggle to come back down because it is not able to preheat the downhill fuel as well as the uphill.


Dr. Clark says that fires travelling slower uphill are an exception to the rule, but it does happen. Winds can work against a fire that is trying to move up a slope.

"It depends on which way the wind's blowing," he said. "For example, I have a case study in Australia where the wind was blowing down the mountain side, blowing the fire away from the hill until a front came through. Then it went uphill."

In addition to the damage that fires cause as they burn, they can also leave behind disastrous problems, the effects of which might not be felt for months after the fire burns out. When fires destroy all the vegetation on a hill or mountain, it can also weaken the organic material in the soil and prevent water from penetrating the soil. One problem that results from this is extremely dangerous erosion that can lead to debris flows.

An example of this occurred following a July 1994 wildfire that burned about 2,000 acres of forest and underbrush on the steep slopes of Storm King Mountain, near Glenwood Springs, Colorado. Two months after the fire, heavy rains caused debris flows that poured tons of mud, rock and other debris onto a 3-mile stretch of Interstate 70, according to United States Geological Survey. These debris flows engulfed 30 cars and swept two into the Colorado River.

While we often look at wildfires as being destructive, many wildfires are actually beneficial. Some wildfires burn the underbrush of a forest, which can prevent a larger fire that might result if the brush were allowed to accumulate for a long time. Wildfires can also benefit plant growth by reducing disease spread, releasing nutrients from burned plants into the ground and encouraging new growth.


Battling the Blaze

Firefighters build firebreaks like this one to remove potential fuel from a wildfire's path.
Photo courtesy Bureau of Land Management

­I­magine being inside an oven, wearing heavy clothing with smoke filling your lungs, and you can only begin to understand what it's like to fight a raging wildfire. Every year, thousands of firefighters put their lives at risk to battle merciless blazes. The elite, ground-based firefighters fit into two categories:

  • Hotshots - Working in 20-person teams, the main job of these highly trained firefighters is to build a firebreak around the fire to keep it from spreading. A firebreak is a tract of land that has been stripped of any possible fuel for the fire. Hotshots are employed by the U.S. Forest Service.
  • Smokejumpers - These firefighters are the paratroopers who jump out of planes to get to small blazes located in remote areas. Their job is to suppress small fires before they are able to spread into larger ones. Smokejumpers use the same firefighting techniques as the Hotshots once they have landed on the ground. There are only a few hundred smokejumpers in the entire United States, all employed by either the Bureau of Land Management (BLM) or the U.S. Forest Service.

In addition to building firebreaks and dousing the fire with water and fire retardant, ground crews may also use backfires. Backfires are fires started by the ground crew that advance toward the burning wildfire. The goal of setting a backfire is to burn up any potential fuel in the path of the progressing wildfire.


An air tanker drops water and fire retardant onto a wildfire.
Photo courtesy Bureau of Land Management

While the Hotshots, Smokejumpers and other support crews fight the battle on the ground, they are given a lot of support from the air. Air tankers are often used to drop thousands of gallons of water and retardant onto fires. The red stuff that you often see being dropped from planes and helicopters is a chemical retardant that contains phosphate fertilizer, which helps to slow and cool down the fire.

Helicopters are also used as a method of attacking the fire from above. Carrying buckets that can hold hundreds of gallons of water, these aircraft fly over the fire and drop water bombs. Helicopters are also valuable for transporting firefighters to and from the fire.

Wildfires are powerful forces of nature that can burn for as long as they have fuel, oxygen and heat. The job of the firefighters is to eliminate one, if not all three, sides of the fire triangle to prevent further damage.

For additional information on wildfires and related topics, check out the links on the next page.