Wildland Fire Triangle

The fire triangle taught in our Fire Behavior article applies to wildland fires as well. Fuel, Heat, and Oxygen are all required components for a wildland fire to sustain combustion (and there is plenty of all 3 once a fire is started in a grassland). Wildland fires are affected by a Wildland Fire Triangle that consists of weather, topography, and fuel.

WEATHER

Moisture's involvement in wildland fires will present itself in the form of precipitation or relative humidity. Low humidity implies dry vegetation, which of course burns more easily. The humidity in an area will usually be higher before the sun rises, and decrease as the temperature increases during the day. Because water has an incredible ability to absorb heat, changes in humidity can have a significant effect on a wildland fire's ability to spread.

Wind is fire's best friend. A fire can spread by itself, but adding wind to the equation will push a fire in any given direction at an alarming rate. It will help heat transfer from fuel to fuel, and increase the amount of oxygen available to the combustion process. The largest fires in American history have wind to thank as a contributor.

TOPOGRAPHY

Topography will be a large determining factor in a wildland fire's spread. Any change in elevation falls within the realm of topography, be it manmade or natural. Any fire will have most of the excess heat from combustion disperse into the atmosphere above, but a fire located near a hill will have that excess heat moving up the hill right into more vegetation, spreading the fire that much faster. Conversely, fire burns downhill more slowly than on flat ground or uphill for the same reasons. Any natural barrier such as a rock wall, a lake, or a river will deter fire spread, at least for a short amount of time.

FUEL

Wildland fires will obviously be using brush and trees as their primary sources of fuel. There are a few terms to describe wildland fuel's physical properties:

Fuel Density- The rate at which a fuel will burn. A property similarly described by the fuel's STMR

Fuel Continuity- The relative closeness of fuels. Objects that are closer together will have fire spread amongst them more quickly

Fuel Volume- The quantity of fuel available in an area

Fuel Moisture- The amount of moisture in a fuel. This characteristic will determine how much heat is absorbed, ignition speed, etcetera

Now, we split the actual fuel type into different categories to describe their fire behavior:

Fine Fuels- Fuels with a high STMR, and thus burn very rapidly. They are the primary source of fuel in ground cover fires and include twigs, shrubs, needles, grass, etcetera. Fine fuels also include ground duff, which is the material littered about on the forest floor, some of which is partly decomposed.

Heavy Fuels- Fuels with a low STMR, and will burn for much longer. Examples of a heavy fuel would be stumps, thick branches, or slash (whatever a logging operation has left behind- logs, bark, etc).

Subsurface Fuels- Any combustible fuel underground in a wildland scenario. Roots, for example. These can consist of both fine and heavy fuels.

Surface Fuels- Fuels directly on or low to the ground such as grass, saplings, leaves, or slash. These can also consist of both fine or heavy fuels. This can also include ladder fuels.

Aerial Fuels- Also called canopy fuels. Any fuel more than six feet high. This will usually consist of branches, leaves, and needles. We're talking about trees here, people!