The 10% wind speed rule of thumb for estimating a wildfire's forward rate of spread in forests and shrublands

Key messageThe collective analysis of a relatively large number of wildfire observations documented in conifer forests, dry eucalypt forests and temperate shrublands revealed that the forward rate of fire spread is roughly 10% of the average 10-m open wind speed, provided both are expressed in the same units (e.g. km h(-1)).ContextKnowledge of a wildfire's forward spread rate is a prerequisite for defining adequate fire suppression strategies and to ensure timely public warnings.AimsWe wanted to investigate the possibility that a simple relationship exists that could be used as a first approximation for quickly estimating a wildfire's spread rate simply from the open wind speed alone.MethodsWe analysed data from a number of high-intensity wildfire observations (n = 118) documented in temperate shrublands, Australian dry eucalypt forests and North American conifer forests to examine the suitability and soundness of a relationship between wind speed and rate of fire spread. We also contrasted the performance of the best function against established fire spread rate models for the three fuel types.ResultsThe resulting rule of thumb is that the forward rate of spread of wildfires burning in forests and shrublands in relatively dry conditions is approximately equal to 10% of the average 10-m open wind speed, where both values are expressed in the same units.ConclusionThe rule of thumb gives the most accurate results for dry fuel and high wind speed conditions with reduced bias and mean relative errors lower than 50%. Under these conditions, the error statistics are comparable to those obtained by the establishedfire spread rate models. The rule is not applicable to grasslands.