Ignition normally takes place in the dead component of the fine fuels. Three distinct steps must be considered in this ignition process. Living material in the fine fuel complex reduces the efficiency of ignition. The closer the initial temperature of the fuel is to the ignition temperature, the more likely a fire will result when a firebrand in introduced into the fine fuel complex, since not a much energy is required to raise the fuel particle to its ignition temperature. The Spread Component is a numerical value derived from a mathematical model that integrates the effects of wind and slope with fuel bed and fuel particle properties to compute the forward rate of spread at the head of the fire.
Output is in units of feet per minute.
A Spread Component of 31 indicates a worst-case, forward rate of spread of approximately 31 feet per minute. Since the characteristics through which the fire is burning are so basic in determining the forward rate of spread of the fire front, a unique SC table is required for each fuel type. The Energy Release Component is defined as the potential available energy per square foot of flaming fire at the head of the fire and is expressed in units of BTUs per square foot. Like the Spread Component, the Energy Release Component is calculated using tables unique to each fuel model. The rate of combustion is almost totally dependent on the same fuel properties as are considered in the SC calculation.