Heat

A significant amount of heat is needed to break the chemical bonds in the fuel and begin the reaction. Fuels like grass and leaves require an initial heat source of at least 400° F before ignition occurs. This heat source may be natural as in the case of lightning, or it may be human caused as with matches or a drip torch. The actual temperature and amount of heat needed varies depending on the condition of the fuel. Wet fuels require more heat than dry fuels; large fuels require more heat than fine fuels. The amount of heat needed to ignite the fuel and sustain combustion is called the heat of ignition. Once ignition is achieved, the combustion process releases an enormous amount of heat which can easily sustain the reaction as long as the other elements of the fire triangle are available. Transfer of this heat to unburned fuels affects the spread of fire and occurs by conduction, radiation, convection and mass transport.