A wildland fire is a complex phenomenon driven by the interactions of a number of fundamental chemical and physical processes involved in the combustion and the release of heat from burning vegetative fuels. In this second article of a two-part series summarising our state of knowledge of these processes, the focus is on the physical processes involved in the transfer of the liberated heat (the generation of which is discussed in the first article) to adjacent fuel and its subsequent ignition which provides the key mechanism for sustained spread of the fire.

The competitive thermal degradation reactions (volatilisation versus charring) occur simultaneously at all points around a wildland fire perimeter but the extent to which one pathway dominates the other is determined by the transfer of the heat released from combusting fuel to adjacent fuel. This transfer occurs by the combination of advection, radiation and transport of burning material and may be moderated or compounded by interactions with the surrounding atmosphere or topography and by vegetation condition.

The primary heat transfer processes in a wildland fire are advection (incorporating buoyancy and convection), radiation, direct flame contact and transport of burning solid material such as embers and firebrands (i.e. spotting). Other key processes involved in the transfer of heat to adjacent fuel necessary for sustained fire spread include fuel moisture, atmospheric and topographic effects, and the interactions of these with the fire. Gaps in our knowledge that limit our ability to predict the seeming capricious behaviour of wildland fires are also highlighted. The series is concluded with a discussion of how these physical processes interact with the combustion chemistry processes discussed in the first article.