This chapter explores fire as a physical process, including combustion, fuel characteristics, fuel models, fire weather, ignition sources, mechanisms for fire spread, and fire effects. In wildland fuels, combustion occurs in three phases: preheating, gaseous, and smoldering. Fuel is characterized by physical and chemical properties that affect combustion and fire behavior. Its characteristic classes are defined for a vegetation type and contain data for fuels in up to six strata representing potentially independent combustion environments. Fire weather includes air temperature, atmospheric moisture, atmospheric stability, and clouds and precipitation. Sufficient fuel, conducive weather, and an ignition are necessary ingredients for a fire. In line with this, this chapter investigates how these factors, combined with topography, cause a fire to spread. The chapter also introduces the physical parameters of fire behavior that affect fire severity, spotting, tree scorch height, plant mortality, biomass consumption, and microclimate.
