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Fire requires fuel, heat and oxygen in order to burn. Consideration of the fuels,
terrain and weather of a location can give an indication of areas that may be more at risk during a major wildland fire. Fires
can burn very intensely at their flaming fronts. However, they do not burn everything in their paths and can have pockets
of lesser intensities as well as various rates of spread. Fire needs fuel---something to burn. In a wildland fire, fuel is
generally vegetation but can also include homes and other structures.
The arrangement of fuels
known to wildland fire professionals as "ladder fuels" is one of the more significant ways wildfires gain strength
and power. In this scenario, a wildfire starts in light fuel---often grass. It burns rapidly through the grass to nearby trees.
Once at the base of a tree, it moves into low branches and climbs to the top, or "crowns". The slope of a terrain
plays an important role in the rate of a wildfire's spread. Generally, a fire moving up a slope moves faster and has longer
flames than one on level ground. This is because hot gases rise in front of it, pre-heating its path.
Wind is
also a major factor in the spread of a wildfire. The wind provides oxygen. Fires need air to continue burning, and large
fires need a lot of air. Wind can cause wildfires to grow quickly, to die down, to change direction, or even move downhill
as fast as they do uphill.
Many different kinds of wildland fires can develop during a fire event. Crown fires were so named because they burn in the
tops of trees. Once started, they are very difficult to control because of their intensity.
Surface fires burn in grasses and shrubs up to 4 feet tall,
or in the lower branches of trees. A surface fire often moves rapidly. Ease of control depends on the fuel and the wind involved.
Ground
fires burn in natural litter, duff, roots or sometimes highly organic soils. Once started, they are very difficult to detect
and control. One of the difficulties with a ground fire is that it can, and often does, rekindle.
Crown fires, wind and the local topography can produce spotting.
When this phenomenon occurs, large burning embers called firebrands are blown ahead of the main fire. Once spotting begins,
a fire is very difficult to control.
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