September 11, 2001

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Fire...what is it? Facts

When you hear the word fire, what comes to mind? Were you ever afraid of it? Mesmerized by it? Comforted by it? No matter where or how you have experienced fire, it is essentially the same. Fire is a chemical reaction.

Fire TriangleIn order for fire to burn, three elements must be present. Oxygen, fuel and heat combine to make what is called the "Fire Triangle".

Oxygen, fuel of some sort, and heat are around us all the time. Why don't we see fire more often? The answer lies in the details.

Oxygen is pretty easy. Fire needs air that is about 16% oxygen. The earth's atmosphere is 21% oxygen. Fuel is anything that will burn. In the outdoors, that often includes wood, grass, shrubs, pine needles and the like. The presence of heat will vary. Wood needs about 617 degrees F to burn.

Forest FireIf any one of these elements is missing or not present in the right form or amount, one "leg" of the fire triangle collapses and no burning occurs. So, once a fire is ignited and there is enough fuel and oxygen for it to burn, the fire will create all the heat it needs to sustain itself. The more fuel, the higher the temperature. The higher the temperature, the faster the fire spreads. The more the fire spreads, the more it "preheats" or heats the fuels around it increasing its size and the temperature around it. And the race is on!

Fuel Fact
All of us have seen fire. Technically, the fuel you see burning isn't really on fire. Instead, the fuel is being converted into a gas. It's the gas produced by the fuel that's burning. Next time you are watching a log burning in your fireplace, see if you can see a space between the log and the flame. I'll bet you can!

A Fire is a Fire is a Fire......Not So….

If you listen to the news or if you talk to people who work with fire, you will hear it described in several ways. Here are some terms that will help you understand what is going on.

Wildland fire is one of nature's oldest phenomena. Evidence of free-burning fires has been found in petrified wood and coal deposits formed as early as the Paleozoic Era, about 350 million years ago. Wildland fire is any fire burning in wildlands, including wildfires and all prescribed fires.

Forest FireA wildfire is a fire is one that is out of control and generally viewed as undesirable by land managers. It needs to be put out or suppressed. An example of a wildfire might be one that is burning the habitat of an endangered animal like the sagegrouse as has been the case in Southern Idaho the past few years. Managers would call for fire fighters to suppress this fire.

Prescribed BurnA prescribed fire is one that is considered to desirable by managers because it meets some management objective. They can be naturally ignited, such as those that are started by lightning, or they can by lit by land managers to accomplish a specific task. Burning logging debris following a logging operation would be one example of a time that managers might ignite a fire. Allowing a lightning-caused fire to burn because it is clearing out dead branches and needles on the forest floor of a Ponderosa Pine Forest would be an example of a prescribed natural fire.

Fire in Ecosystems
It is important to remember that fire behaves differently in different ecosystems. The lodgepole pine forest depends on fire to survive because the lodgepole cones need fire to open them so seeds can be released.

SagebrushRepeated fire in sagebrush-steppe country can destroy the sagebrush, an important part of that system. Ponderosa Pine forests benefit from an occasional ground fire to help clear the forest floor of competing grasses and young trees. A healthy Ponderosa Pine forest has trees that are spaced far apart so that sun can reach the ground and grasses and shrubs can grow.

Fire Spread
Fires spread in three general patterns: ground fires, surface fires and crown fires.

Ground fires burn organic material in the soil beneath the litter on the surface. They burn by glowing combustion.

Surface Fires have a flaming front and burn leaf litter, fallen branches and other materials on the ground.

Crown fires are the hottest and most intense. They are often difficult to control, need strong winds, steep slopes and lots of fuel to keep burning. Crown fires burn the top layer of foliage on the tree.

Fire Behavior

WildfireOnce a wildfire is started, the way it behaves is determined by the current weather conditions, the amount of humidity in the air, the type and amount of fuel available to the fire and the topography of the land. Because live plants contain so much water, they are less likely to burn than dry logs and branches or stems.

High winds can create small fires out in front of a large fire by blowing embers into the unburned fuel. These fires are called spot fires and may burn some trees and shrubs and leave others untouched.

A large fire can even create its own wind. As the fire heats the air around a fire it quickly rises. Cool air rushes in to replace the hot air which creates a wind and increases the supply of oxygen to the fire.

Trees can even explode if water deep inside the tree turns quickly to steam.

After the Fire

After a fire, the hard work of rehabilitating the landscape begins. Most of it needs to be done quickly because there is often little to hold the soil in place and erosion can be a big problem. This is especially true if the burn is on a steep slope. This was the case Boise in 1959 and 1996 when the Boise Foothills burned.

Learn about some of the techniques land managers used to stabilize the soil and rehabilitate the land.

Saving the land

BLM LogoBLM LogoPermission granted to use this material for educational purposes only. The photographic images were provided by the Bureau of Land Management
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