Spotting

Spotting occurs when firebrands (sparks, embers or larger burning pieces of vegetation) are transported naturally by wind, convection, or gravity beyond the main perimeter of the fire. Showers of burning embers that fall within ¼ to ½ mile from the main fire can ignite large areas almost simultaneously creating a pseudo front (Brown and Davis 1973). This mode of spread is most often associated with large wildland fires (Pastor et al. 2003), but can occur anytime there are dry, windy conditions:

• Short-range spotting from firebrands can occur in any fuel type and are more likely to occur when the relative humidity is below 40% and are almost a certainty once the relative humidity reaches 25% or less (Weir 2003).

• Air temperatures above 60ºF (Bunting and Wright 1974) and wind speeds over 20 mph significantly increase the chance of firebrand ignitions (Wright and Bailey 1982).

Fuel types also influence the development of firebrands. Light fuels such as palmetto fronds and Spanish moss, which are easily picked up by convection may be carried long distances before they drop and ignite fresh fuels.

Long-range spotting indicators are large glowing firebrands, moderate winds and well developed convection columns. Convection columns form above a fire, with the height of the thermally-produced ascending column of gases, smoke, and debris dependent on air stability and heat released by the fire. As the column is pushed beyond the fire front, it may significantly contribute to fire spread by greatly increasing the likelihood that firebrands will drop onto fresh fuels and new fires will ignite. When air movement in the convection column interacts with the fire, fire whirls may form.