Head, Flanks and Heel Spread at Different Rates Around a Fire Perimeter
A free-burning landscape fire is not a circle expanding evenly. Wind and slope push it into an elongated shape with distinct parts: the head, the fastest-moving forward edge where the flame leans over unburned fuel; the flanks along the sides, spreading sideways at a small fraction of the head's speed; and the heel or rear, backing slowly into the wind with short flames and often burning at low intensity. Fingers and pockets form where fuel or terrain varies, and the whole boundary is the perimeter. The rule to carry is that a quoted rate of spread almost always refers to the head, so applying it to the whole perimeter overstates the fire's growth badly, while ignoring the flanks understates the area burned. The confusion it resolves is treating a fire as a single number moving in a single direction. This anatomy is also what the elliptical growth models taught later in this path are describing mathematically. After this Concept you can identify the parts of a fire perimeter and say which one a spread figure refers to.
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A free-burning landscape fire is not a circle expanding evenly. Wind and slope push it into an elongated shape with distinct parts: the head, the fastest-moving forward edge where the flame leans over unburned fuel; the flanks along the sides, spreading sideways at a small fraction of the head's speed; and the heel or rear, backing slowly into the wind with short flames and often burning at low intensity. Fingers and pockets form where fuel or terrain varies, and the whole boundary is the perimeter. The rule to carry is that a quoted rate of spread almost always refers to the head, so applying it to the whole perimeter overstates the fire's growth badly, while ignoring the flanks understates the area burned. The confusion it resolves is treating a fire as a single number moving in a single direction. This anatomy is also what the elliptical growth models taught later in this path are describing mathematically. After this Concept you can identify the parts of a fire perimeter and say which one a spread figure refers to.
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