Description

An empirical relationship was derived between the ratio of total length to maximum width or breadth (L/B) of wind-driven forest fires on level terrain originating from a point source ignition and the international standard 10-m open wind (W). The relation is based on the wind component of the relative spread index in the Canadian fire-danger rating system. The resulting equation, a nonlinear regression exhibiting a rising curve of increasing slope (L/B=1.0+0.00120 W^2.154), yields L/B equal to 1.0 at zero wind and approx. 6.5 at 50 km/h, the upper limit of application. Wind direction is assumed to remain fairly constant. Comparisons of predicted L/B values with actual observations extracted from experimental fires and well-documented wildfires in a variety of coniferous forests (n=18) by and large show good agreement (r=0.865). The L/B versus surface wind speed function is suitable for use in conjunction with a simple fire growth model for calculating the approximate size (area and perimeter length) of free-burning elliptical-shaped surface and crown fires spreading through fuel types with an overstory tree canopy, given the time elapsed and forward spread rate.

[This publication is referenced in the "Synthesis of knowledge of extreme fire behavior: volume I for fire managers" (Werth et al 2011).]