Radiation

Radiation is the transfer of heat by electromagnetic waves. These waves travel at the speed of light away from the heat source and transfer their energy to matter they impact (the denser and more opaque the matter, the greater the energy transfer rate). Unlike conduction, the heat of one body does not have to be in contact with another body to transfer heat by radiation.

Radiation plays a key role in the preheating of fuels in forest fires, especially in the absence of wind, where heat transfer by convection would be reduced. Radiation travels in straight lines so intensity depends on the angle of the objects absorbing the electromagnetic waves. A perpendicular angle will yield the most intense heat transfer. This is also important for fuel arrangements and the effect of slopes on wildland fire.

The intensity of radiant energy can be summed up in the equation:

q”_x = tsT⁴

where t is the emissivity, a measure of how well the surface performs as a radiator, s is the Stefan-Boltzmann constant (5.67 x 10^-8 W/m² K^-4), and T equals temperature in degrees Kelvin. For a point source of radiation, the radiation intensity decreases inversely with the square of the distance from a specific point. This means that the radiation intensity 10m from the source is only ¼ that at 5m. As the distance from the source increases, the same total amount of radiation is spread over a greater area and so the amount of heat received per unit area decreases.