Convection

Convection is the transfer of heat from one point to another by movement of the surrounding fluid or gas. In the case of wildland fire, convection is exclusively the movement of heated air. “Free convection” occurs when the fluid motion of the liquids or gases creates convection currents because of differences in densities caused by temperature differences of two similar substances.

Wind acts as a force that drives convection, and the stronger the winds are the further away fuels are pre-heated by convection. While convection currents always rise (as does all heated air), they may be directed laterally by wind, and may be trapped (slowed in their dissipation) by thermoclines (air stratification along thermal levels). See also: Convective Winds.

Convection is the primary mode of heat transfer to shrubs and other tall vegetation in wildland fires. Rising convective currents influence the spread of fires from the understory and midstory to tree canopies. In high-wind situations and on slopes, convection may carry heat into adjacent fuels, greatly increasing the rate of fire spread. Convection is also critical in its ability to carry firebrands (sparks or embers) up and out from a fire front and depositing them outside the fire’s perimeter, causing spot fires. In general, convection currents can be tracked by the flow of particulates from a flame front (smoke).

Convection is described by the equation:

q”_x = h DT

where, h is a non-constant coefficient that varies under different conditions and DT is temperature change over time.