Fuel Type and Wind Effects on Rate of Spread

Wind speed has one of the greatest effects on fire intensity and rates of spread. As wind blows across a fire, it pushes the flame forward and closer to the unburned fuel in front of the fire (see Flame Tilt). This increases convection and radiation, which dry the fuel and increase its combustibility. In general, the higher the wind speed, the further the flame leans and the faster it dries the fuels, increasing both fire intensity and rate of spread. Wind also adds oxygen to the existing fire, further increasing combustion rates in the flaming zone (Whelan 1995).

The following equation illustrates the direct relationship between fireline intensity (I, kw/m) and wind speed (U, m/s): I = 17.5U³ (Roberts 1976). This relationship has also been quantified in fire behavior prediction models such as BEHAVE, and therefore can also be seen by running fire behavior simulations under different wind conditions (see: Heat release rates under different fuel and weather conditions). The graph below similarly illustrates the effect of windspeed on rates of spread.

Wind also influences the direction of spread and can carry sparks and firebrands downwind of fires, greatly increasing spread rates (see Spotting). A shift in wind direction could rapidly turn a slower moving flanking fire to a head fire, increasing its rate of spread.

Weather conditions that control fuel moisture contents, such as temperature, relative humidity, and precipitation, are also important since fuel moisture determines fuel availability. Weather variations over a given time period generally have a greater effect on fire intensity than fuel variations in the stand. Weather changes can occur in a matter of minutes, whereas fuel variations take place over longer periods of time (Bessie and Johnson 1995).

The following table depicts how different fuel and weather conditions influence heat release and fireline intensity. Table: Fuel type, wind speed, and fuel moisture effects on fireline intensity compares total heat release and fireline intensity for three fuel classes under various wind speed and fuel moisture conditions. The results were generated by the Behave Plus 2.0.2 fire behavior simulator (Andrews and Bevins 2003). The fuel models for tall grass (FM 3), southern rough (FM 7), and light logging slash (FM 11) were used to represent typical fine, medium, and large fuels found in the southeast, respectively (Anderson 1982). These fuel models are summarized in (Table: Fuel Model Parameters).

For all fuels, the highest intensity fire (highest rate of heat release) occurs under conditions of high wind speed and low moisture content. Tall grass burns at a much higher intensity than the other fuels under the same weather conditions, due to the high surface area-to-volume ratio of grass, which allows it to lose moisture more rapidly than larger fuels, the higher loading of 1-hour (<.25 in. diameter) fuels, and the faster rate of spread in the grass fuels. On the other extreme, large fuels will not burn at 18% moisture content because they are unable to efficiently lose water. Total heat release per unit area of fuel does not change with wind speed but decreases with increased moisture content, because there is less available fuel per unit area as fuel moisture content increases. Another important consideration in interpreting the table is that fine fuel consumption is by flaming combustion whereas large fuels are consumed by glowing and smoldering combustion.

For more information on the effects of fuel and weather on heat release and fire intensity, detailed physical and thermochemical process equations can be found in Johnson and Miyanishi (2001) and Drysdale (1999).