Fire Intensity

Fire intensity is a measure of the rate of heat released by a fire. It includes both radiant and convectional heat. There are several definitions and ways to measure fire intensity. The most common of these is fireline intensity. Other measures of fire intensity include reaction intensity, radiant intensity, convection intensity, total fire intensity (defined below).

Fire intensity is important for planning prescribed burns because it directly influences the smoke column height and flame heights during a burn, as well as the responses of an ecosystem to a particular burn (e.g., fire effects). Fire intensity is also useful in assessing the difficulty of wildfire containment (see Fireline Intensity).

Fire intensity is directly proportional to a fuels heat of combustion, the amount of fuel consumed, and a fires rate of spread. As such, fuels, weather, and topography are very important in determining the rate of heat released by a fire. Fuel properties that directly or indirectly affect fire intensity include fuel loading, moisture content, arrangement, chemical composition, and size. Wind speed and other weather conditions that influence fuel moisture also influence fire intensity. Exactly how these fuel, weather, and topographical conditions affect fire intensity and heat release is explained in the following pages:

• Effects of weather on fire intensity and rate of spread
• Effects of fuel on fire intensity and rate of spread
• Effects of topography on fire intensity and rate of spread

Measures of fire intensity

• Fireline intensity (also known as Byrams fireline intensity or frontal fire intensity) is the rate of heat energy released per unit time per unit length of fire front, regardless of the depth of the flame zone (Byram 1959).
• Reaction intensity (also called combustion rate or area-fire intensity) is the rate of heat release per unit area per unit time in the flame zone. Reaction intensity can be derived by dividing fireline intensity (kW/m) by flame depth (m). Under laboratory conditions, reaction intensity can be measured by recording weight loss and determining heat release through calorimetry. In the field, reaction intensity is calculated by estimating the amount of of fuel burned per second and assuming heat yields for the fuel (Chandler 1983, DeBano et al. 1998). It is usually expressed kW/m². Reaction intensity is calculated in Rothermels (1972) fire spread model which, in turn, is used for fire behavior calculations in the National Fire Danger Rating system (NFDRS) and BEHAVE (Wade 1986).
• Radiant intensity is the rate of thermal radiation emission either across the entire radiomagnetic spectrum or within specified wavelengths (spectral intensity). Radiant intensity is intercepted at or near the ground surface, or at some specified distance ahead of the flame front (Chandler et al. 1983, DeBano et al. 1998). The units are usually expressed in cal/cm²sec.
• Convection intensity is the heat from a fire that lifts gases and entrains air above the flame zone. Convective intensity is not determined directly, but is the difference between the total energy release rate and the sum of conductive and radiant heat losses (Chandler et al. 1983, DeBano et al. 1998). Convective intensity calculations are used to predict convection column phenomena. The units are usually expressed as kcal/m²min or as kW/m².
• Total fire intensity, the rate of heat output of the fire as a whole, is a function of the rate of area burned, fuel loading, and estimated heat yield. Its not useful in describing site-specific effects (Chandler et al. 1983, DeBano et al. 1998).