Fuels, Weather, and Fire Behavior-Overview

It is important to understand what controls fire behavior and how to predict it. This knowledge will help predict fire effects, conduct prescribed burns, predict wildfire risk, and control wildfires. Fire behavior is controlled by three interacting components: fuels, weather, and topography. Fuels provide the energy source for fire. Fuel availability which depends on both fuel arrangement and fuel moisture determines if fires will burn as ground, surface, or crown fires. Weather elements such as temperature, relative humidity, wind, precipitation, and atmospheric stability combine to influence fire behavior by regulating fuel moisture and rate of spread. Topography can influence fire indirectly, by mediating wind patterns, or directly- fires burning upslope spread faster than fire burning on flat land.

It is important to understand the physical-chemical process of fire to understand how heat is generated by fire. Fire releases heat through combustion. Oxygen, heat, and fuel- often called the fire triangle- must be present in the proper ratio for a fire to ignite and sustain combustion. Once a fire has ignited, the heat must be transferred to surrounding fuel in order for the fire to grow and spread. This occurs through one of several heat transfer processes, usually convection, radiation, and/or conduction, although vaporization and mass transport may also play roles.

Once a fire has ignited, its shape and rate of spread will continually change. Rarely, a fire can continue to increase its rate-of-spread and intensity, resulting in extreme fire behavior- a level of fire behavior that goes beyond human methods of fire control and prediction.

Characterizing flame attributes such as flame height, length, depth, angle, and char height can help predict fire effects and make comparisons among different fires possible. Fire intensity, which describes the rate of heat release, and rate of spread play significant roles in characterizing fire behavior.

Computer models can be used to predict fire behavior based on differences in fuels, weather, and topography. These fire behavior prediction systems are used to support fire management decisions, as a training tool to improve fire management skills, and can help display and explain fire behavior and fire management strategies to the general public. Fire danger rating systems produce qualitative and/or numeric indices of fire potential based on fuels, topography and weather. These rating systems allow fire managers to estimate present and future fire danger for a given area. Both fire behavior prediction systems and fire danger rating systems require mathematical descriptions of fuel models and their respective fuel properties as input.