How Wildfires Work

­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:

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 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:

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.

­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.

­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:

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.

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.