How Do Wildfires Behave?

Article Written by:

  • Glenn Nader, University of California Cooperative Extension, Yuba City, CA
  • Max Moritz, University of California Cooperative Extension, Berkeley, CA
  • Stephen L. Quarles, University of California Cooperative Extension, Richmond, CA
  • Michael Kuhns, Utah State University, Logan, UT

What You Need to Understand to Protect Your Property from Fire

A basic knowledge of fire will greatly help you understand what you need to do to protect your house and property from a wildfire.

Fire is driven by heat transfer. The fiber of wood or any solid combustible material does not ignite. All solid material requires heating to a point that thermal degradation of the material results in the production of combustible gases. It is these gases that ignite. This is called “preheating.” The size of the material and its moisture content determine how much heat is required before sufficient combustible gases are produced. The moisture content of the material also is important, as its temperature must get hot enough to first remove the moisture before getting the material to volatilize gas.

Branch Diameter 300w72dpi.jpg
Smaller diameter material (see top of picture with pencil) not only dries quicker when exposed to heat but also requires less heat when dry to volatilize gas. A grass fire generally moves quicker across the land than a tree fire for the following reasons:

  • grass has a small diameter;
  • grass is a less dense fuel,that is, it requires less heat to produce combustible gases;
  • there is sufficient air around each stem of grass to burn efficiently.

For fire to occur, it must have fuel, heat that preheats the material to the point where combustible gases are generated and oxygen for combustion of the gases. Remove either of these factors, and the fire will go out. In fighting a fire, water is applied to lower the temperature of the material so that it stops producing combustion gases.

The term “fire behavior” is used to describe the magnitude, direction and intensity of fire spread. It is affected by weather, topography and fuel. These three factors play an important role in the efficiency of heat transfer and resulting ignitions.


Dry, hot weather reduces fuel moisture. This in turn reduces the amount of heat necessary for material to reach temperatures needed to generate combustible gases. Windy weather conditions not only increase the drying but also increase the horizontal transfer of heat. With increasing wind speed, the wind bends the flame from vertical to horizontal. This means the heat changes from vertical — going up in the air — to horizontal transfer — going to fuel next to the combustion. Wind also causes more burning fine fuels, or embers, to be lofted and carried to fuel at other locations and igniting them. An example of this is when embers blow up against a woodpile and ignite it.


Topography changes the preheating of fuels. Most of the heat from a fire transfers vertically or rises up. The steeper the slope, the more efficient fuels in front of the fire are preheated as heat rises upslope. This is why fires occurring in the same kind of fuel move faster going up a steep slope than on flat ground.


The amount of fuel and how it is arranged vertically and horizontally affects fire behavior. Areas with no fuel produce no additional heat and provide air that decreases the transfer of heat required for ignition. Homeowners cannot change the weather or affect topography, but they can decrease the horizontal and vertical spacing of fuels to reduce the chance of the wildfire reaching their home and the vegetation growing nearby.

From a wildfire fuel standpoint, vegetation is often described in terms of its vertical and horizontal arrangement. Sometimes the arrangement is described in terms of vertical or horizontal fuel continuity. Closely spaced vertical fuels can also be referred to as “ladder” fuels (Figure 1). As seen in this figure, vertical continuity can depend on vegetation that is closely spaced horizontally.

Most wildfires will not reach the crown of a tree if the vertical fuel continuity has been eliminated by increasing spacing by thinning (affecting horizontal continuity) and pruning (affecting vertical continuity). Once the fire reaches the crown of a tree, the heat intensity increases. This increases the combustibility of the surrounding vegetation. Therefore, it is important to prune lower branches of taller trees to eliminate ladder fuels and create enough space so that the heat from a ground fire cannot preheat the branches to the point that they will ignite.

Horizontal fuel continuity allows a fire to spread across the landscape. Breaking up this continuity through wider spacing of the vegetation can greatly reduce fire intensity (Figure 2). The wider the spacing between plants, the greater the wind speed must be to spread the fire. The actual distance required between plants depends on the height of the plants and the slope of the property.

Embers are usually relatively small pieces of burning or glowing debris and are the primary cause of home ignition in a wildfire. Large numbers of embers can be generated during wildfires. When the number of embers is similar to snow or rain in a snowstorm or rainstorm, this can be described as an ember storm. Embers can easily ignite fine fuels such as pine needles or a fiber door mat located near the home. If ignited, these fuels will provide a radiant heat or flame contact exposure to your home, depending on proximity. Because of their relatively small size, embers have less energy than a flame front and must find smaller or drier fuels. The key is to reduce the presence of fine plant material around the home that is easy for incoming embers to ignite.

Wind, temperature and humidity are out of your control. The effect of topography can be controlled to some extent by careful placement of buildings and plants within the landscape. However, you can definitely reduce fuel availability and distribution by manipulating the amount and continuity or spacing of fuel and, by irrigation, the moisture content of the plants. Controlling these factors is the key to making your house safer during a wildfire.

Also see: How Wildfire Threatens a House