Fire Growth

The growth of fire follows a similar cycle regardless of fire type, size or intensity. Under perfectly uniform conditions wildland fire will move from ignition through an initiation stage, during which rate-of-spread accelerates to a steady state, explained below. Under real-life conditions the fire’s shape and rate of spread will continually change. Fuel, weather, and topography combine to influence a fires rate of spread and overall behavior. In the case of prescribed fires, ignition patterns can affect fire growth and intensity. Rarely, a fire can continue to increase its rate-of-spread and intensity, resulting in extreme fire behavior.

Initiation

The initiation stage of a fire is the period of time after ignition until a fire reaches a steady rate-of-spread. Wildland fire generally starts from a single ignition point (lightning strike, match, spark). During the initiation phase there is a buildup of heat and the fuel present at the point of origin is consumed. As radiant heat dries the adjacent fuels the fire will begin to expand its perimeter outward. The rate of spread accelerates initially, with the rate dependent on fuel type, load, compactness, continuity, and depth. Weather factors also affect initial rates of spread and the rate at which fire perimeter spread accelerates. In general, the characteristics that lead to the highest rates of spread include continuous, fine fuels, loosely packed, with low fuel moisture contents on windy days in late spring with high temperatures and low relative humidity.

Steady state

If all fuel conditions remain constant, the fire perimeter will expand at an accelerating rate for 10 to 30 minutes (depending on fuel type), at which point the rate of spread reaches a steady state. In a steady state, under uniform conditions, the flaming zone moves at a nearly constant rate with the same intensity, simply increasing the area that has been burned. The time required to reach steady state is independent of weather, while the actual rate of spread is a function of weather and fuel types. The steady state rate of spread is maintained as long as fuel, weather, and topographical conditions remain stable. Stable conditions are unlikely to occur in wildland fires.