How Wildfires Work

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.
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.