Heuristic approaches to problem solving, commonly called rules of thumb, employ practical, quick, in the moment, methods that are not intended to be strictly accurate or reliable in every situation but sufficient for most decision making situations, especially when there is insufficient time for a detailed assessment. Forecasts of wildfire spread into wildland-urban interface areas are of critical importance in alerting members of the public of the potential threat. We have devised a rule of thumb for obtaining a first approximation of a fire’s spread rate that wildland fire operations personnel may find valuable in such situations. It is based on the premise that under certain conditions wind speed is the dominant factor in determining a wildfire’s forward rate of advance.

The published spread rates associated with 118 high-intensity wildfire runs that have occurred in temperate shrublands, Australian dry eucalypt forests, and North American conifer forests were compared to the average wind speeds observed at a 10-m height in the open. The rates of fire spread and corresponding wind speeds ranged from 0.29-10.5 km/h and 5-95 km/h, respectively. This comparison led us to the realization that a simple linear relationship exists between the two that can serve as a suitable model or guide for very quickly estimating a wildfire’s forward rate of spread, independent of the fuel type(s) considered. The resulting rule of thumb can be given as a simple expression that lends itself to an easy mental calculation.  A wildfire’s forward rate of spread (R) can be estimated as follows: R = 10% of the average 10-m open wind speed (e.g., for an open wind speed of 30 km/h, R = 3 km/h).