The r in Eq. 1 refers to the linear rate of advance rather than perimeter increase or area growth rates, by a surface head fire, a crown fire, or a backfire. The first two fire types are spreading with the wind while the latter is burning against the wind. It can be readily observed that identical intensities can be arrived at by fires of varying rates of spread and fuel consumption. Thus, a more complete description of fire behavior could be given by quoting the rate of spread as well as the fire intensity (Van Wagner 1962, 1965a). Although r in Eq. 1 in metres per second is required in the calculation of
intensity, metres per minute is recommended when quoting rate of spread for its own sake (Van Wagner 1978). Rate of spread provides a better mental image in metres per minute than in metres per second, especially for slow-moving fires and short lapse times.