Conduction

Conduction is the transfer of heat energy by the motion of adjacent molecules, from a region of high temperature to a region of lower temperature. It occurs in all solid materials, with greatest efficiency in dense solids such as metal. Dense materials conduct heat better because the molecules are much closer together. Wood, soil, and water are considered poor conductors, while material such as steel or bronze readily conduct heat. A dense material conducts heat better into the interior of the substance so the surface heats up more slowly. This explains why solid, dense wood requires a lot of heat to ignite, but rotten wood can be ignited with a spark. Since the wildland fire environment is generally devoid of dense materials (except logs and tree stems) conduction is usually not a primary method of heat transmission except during the combustion of logs, duff, soil organic matter, and roots (Countryman 1976).

Conduction is expressed by the equation:

q”_x = -k (DT/Dx)

where q”_x = heat transferred per unit time, k is a constant that represents thermal conductivity of a particular material, DT is temperature change over time, and Dx is change in distance over time.