Fire Shape

Wildfires that originate from a single ignition point (lightning, arson, or accidental) are often referred to as point source fires. Fire will spread out from this point in a circular pattern in an area with uniform fuel distribution, on level ground, with no wind present. In the presence of wind, the fire will begin to spread fastest in the direction of the prevailing winds or in cases of steep topography, upslope. This area will become the front or head of the fire. The lateral and upwind (or downslope) portions of the fire will spread at a slower rate and are referred to as the flanks and rear of the fire, respectively. The resulting fire perimeter is an elongated or elliptical shape. The more uniform the fuel, terrain, and weather conditions, the more the fire will resemble an ellipse. However, conditions are usually variable and as the fire perimeter spreads its shape will continually change.

Fuel types, topography, weather conditions, barriers, and spotting can influence and alter the shape of the fire. As spot fires ignite outside the perimeter of the main fire, they are incorporated into the expanding main fire line resulting in ‘fingers’ around the edge of the fire. The effect of spotting on fire growth will depend on the fire intensity. Under extreme fire conditions, firebrands can ignite new fires one or more miles downwind from the original fire perimeter. The distance of the main fire from the spot fire will affect fire shape as the spot fire will likely be drawn into the main fire perimeter by the convective column.

The fire perimeter in fine fuels, such as grasses and leaf litter, will change shape more rapidly and with more variance than in heavier fuels. As wind and slopes increase the length-to-width ratio of the elliptical perimeter will also increase. The uniform perimeter disappears as wind direction and fuel type or load change. Changes in these conditions can cause the fire perimeter to develop U-shaped indentions known as pockets or protrusions of fire known as fingers. Former flanks can become the head or rear of a fire for some period of time. Gradually, the fire perimeter will become very irregular.

With prescribed fires, the pattern of fire spread may be very different because of ignition patterns that use lines of fire or multiple spots. A line ignition is created by continuously dripping fuel from a hand held, ATV-mounted, or helicopter drip torch. The resulting fire includes a line of fire moving with the wind and another line backing into the wind (if not ignited along a fire break). Multiple spot or grid ignitions are a method of igniting fires in which ignition points are set individually at predetermined spacing with predetermined timing throughout the area to be burned. These spot fires will expand elliptically until they burn into each other, with a brief increase in fire intensity as the individual fire perimeters merge. See also: Firing Techniques.