Heat is transferred to unburned fuels by four methods: convection, radiation, conduction and mass transport.
Convection is the upward movement of heated smoke, gases and air. It causes fuels to become preheated up-slope or downwind from a fire. Thus, preheated fuels will ignite and burn faster than normal. Most of the heat in a fire is transferred by convection.
Radiation is the movement of heat through space from a warmer object to a cooler object. The sun heating the surface of the earth and the warmth of a campfire on your face are examples. Radiation also preheats and dries fuels ahead of the fire. It emits energy equally in all directions. The enormous amount of energy released by prescribed fires can make it unbearably hot and drive crews back from the fireline. It can become a safety risk leading to exhaustion, heat stroke and physical injury. It can also damage equipment left too close to the fire.
Conduction is the transfer of heat through or within an object, like the metal spoon in the cooking pot that’s too hot to pick up even though the handle is sticking out of the pot. Since wood and grass are poor conductors of heat, conduction is not an important factor in the spread of fire.
Mass transport is the movement of burning material from one place to another. Leaves blowing across firebreaks, or sparks and embers in a fire whirl, are examples of mass transport. Burning logs rolling downhill and burning trees that fall or drop burning materials or brands are also examples.
