Part 2 - Fire Initiation and Spread

Fire models used in this analysis predict expected fire behavior at the interface of the advancing fire and unburned fuel. Therefore, it is essential to understand the dynamics and processes affecting a fire's propagation. Wildland fire behavior is determined by three interacting variables: fuels, topography, and weather (Countryman 1972). Fuels include all of the live and dead vegetation through which a fire can propagate. This study, focused on fuels occurring on the surface of the forest floor including leaf and needle litter, downed woody debris, bark , tree cones and smaller living plants. Aerial fuels, occurring in the strata just above the surface fuels, include all parts of tree and very tall shrub crowns. Topographical considerations such as slope and aspect also directly influence fire behavior. Slope is an extremely important factor in fire behavior as the flames of a fire burning upslope are positioned closer to the fuels ahead of the fire. Since most of the heat energy released during combustion rises vertically, this horizontal component dries and preheats the fuels at a greater rate than if they were on flat terrain. Fuels on South and west facing aspects receive more solar radiation and will likely have drier fuels than north or east aspects (NFES 2394, Fire Effects Guide 2001). The final component of the triad of variables is weather. Weather is the most transient of the three interacting variables, and in many circumstances will have the greatest affect on fire behavior at that moment . Two components of weather that have the greatest affect on fire behavior are temperature and windspeed. Ambient atmospheric temperature affects fuel temperature. Also, the probability and ease of ignition, the amount of heating required to raise fuel to ignition temperature (320° C.; 608° F.) (Burgan and Rothermel 1984), depends on initial fuel temperature. The ambient temperature has the greatest affect on relative humidity, a controlling factor in dead fuel moisture content prevalent throughout all fuel models. Wind is a large component in determining fire spread and intensity. Wind provides oxygen to the fuel and, like slope, determines which way the fire moves. Wind bends the flame forward and causes direct flame contact with fuel ahead of the fire (Burgan and Rothermel 1984). These fuels are preheated and dried by this increased transfer of radiant and convective heat.