Weather Effects on Fire Intensity and Rates of Spread

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).