n. The distance between the leading edge of the fire and the trailing edge.
The challenge when observing flame depth is defining the trailing edge of the fire. One proposed definition of the trailing edge is the point at which 90 percent of the available fine fuel has been consumed. Flame depth is also defined as the product of rate of spread and flame residence time. In the case of laboratory and field fires, flame depth can be determined either from rate of spread and residence time measurements or from direct measurement. See Anderson and others (2006) for further discussion.