A spreading fire can be examined at many scales. For example, an incident commander working to contain a large fire observes the behavior of the fire at a level of spatial resolution that is no more detailed than a single watershed. A firefighter working on the same fire has less of an understanding of how the fire is moving through the various watersheds in the area and instead focuses only on the behavior of that portion of the fire front that is most nearby and (potentially) threatening. It is impractical to discuss the physics of fire spread from either of these scales. Instead, an idealized description of the fire and fuel bed is helpful. Three assumptions, listed below, facilitate the physical description. A good, but somewhat dated source, of information on the physics of fire spread is available in the book written by Drysdale (1985).
1. Assume that the material the fire is spreading through, termed the fuel bed, is completely homogeneous. The fuel bed can then be described as a collection of completely homogeneous fuel particles. Each atomic fuel particle has three measurable properties: mass of liquid water present, oven-dried mass, and temperature-dependent specific heat.
2. Assume that the fire is neither accelerating nor decelerating, and has thus reached an equilibrium rate of spread, termed steady-state. Fons (1946) was the first to observe that if four factors are held constant, the change in the rate of fire spread will be zero. Therefore, four factors must be held constant during steady-state conditions: meteorology, fuel bed moisture content, fuel bed composition, quantity, and arrangement, and topography.
3. Assume the existence of a fictitous unique temperature for every fuel particle in the fuel bed, termed the ignition temperature. The ignition temperature marks the exact onset of flaming combustion for the fuel particle.
1. Energy from combusting fuel particles at the fire front is transferred to an unignited fuel volume ahead of the fire front via three heat transfer mechanisms: radiation, convection, and conduction.
2. The energy transferred to the unignited fuel volume raises it's temperature from ambient to ignition and in the process volatilizes all the water present in the fuel.
3. Between the time of the onset of ignition and the reduction of the fuel particles in the fuel volume to char or tar, radiant energy is being emitted from two sources: the surface oxidation of the glowing fuel particles within the fuel bed, and a flame attached to the upper surface of the fuel volume.
4. If wind is present, then contemporaneous to the conditions described in iii, there will be heated gas and soot particulates from the oxidizing fuel particles and flame which displace the air around the unignited fuel bed creating strong temperature gradients between the fuel particles and the surrounding air.
5. The energy released while the flame is present, the fuel particles are combusting, and heated gases are emanating from the fuel is transferred to adjacent unignited fuel to repeat the process begining at i.