Fire Behavior Triangle Explained

Unlike the Fire Triangle (heat, oxygen, fuel), The Fire Behavior Triangle explains how a fire acts after it is already ignited. The Fire Behavior Triangle is similar in that it is comprised of 3 parts - Weather, Topography, and Fuels – but the complexity of how these components interact on a large scale makes this mnemonic device much more important in the education of wildland firefighters and homeowners alike.

Weather.  The aspects of weather that have the greatest influence on fire behavior are wind, humidity, temperature, and precipitation. Wind has played a vital role in every catastrophic fire in history. It empowers fire by injecting oxygen for combustion and pushing the flames onto unburned fuel. Humidity and precipitation both influence the degree to which fuels are saturated with water, which is important because dry fuels burn easily, while wet fuels are difficult, often impossible, to ignite. Temperature plays a lesser role, but along with low humidity, temperature dries out fuel for a more intense burn.

Topography. Simply put, topography can be described as “the lay of the land”. It may seem odd, but fires don’t burn the same on all landscapes - fires burn much faster uphill because the rising heat from the fire dries out the fuels above it, priming them for easy and rapid combustion. This effect of fire behavior, along with the inaccessibility of mountainous areas to firefighters, makes it significantly harder to fight flames in steep terrains. Fire behavior is also greatly affected by the aspect, or direction the slope faces. Here on the Northern Hemisphere it is the southern-facing slopes that get the most direct sunlight, and therefore are the driest side of any mountain or foothill. Rivers, deserts, and lakes that impede the path of a fire are also included in this category, and are used by firefighters to help contain wildfires.

 Fuels. Anything that can burn in a fire is considered a fuel – grass, trees, piles of leaves, even your home! Areas with a high volume of fuel per acre have the greatest potential to burn intensely in the case of a fire. Dry fuels will burn easier. Fuels that are close together allow fire to spread quickly. In Fire Ecology there are 3 main types of fuels: Ground fuels, surface fuels, and canopy fuels. Ground fuels are combustibles that lie just under the surface, like buried logs or roots, and burn slowly because of the higher moisture and lower oxygen levels. Surface fuels lie on top of or just right above the surface and can include anything from pine needles, leaves, grass, shrubs, or even your back porch. Crown fuels, which typically refer to the crowns, or tops, of trees, are far and away the most dangerous fuel type. If a fire can make its way up to the crown it is significantly harder to suppress, and crown fires can actually spread separately from the ground fire it originated from.

      The Fire Behavior Triangle is a convenient mnemonic device used by fire professionals to quickly and easily teach the fundamentals of fire behavior to just about anyone. Although it’s great for the layman, the Fire Behavior Triangle isn’t exclusively for beginners - it is still taught to wildland firefighters today who need to be constantly aware of the risky, unpredictable nature of fire behavior. Fire behavior is complicated and hard to predict even for experts, but when broken down into its 3 most basic components it is possible to understand, on a very basic level, how fires act in different ecosystems, seasons, weather, or terrains.