Rate of Spread Measures How Fast a Fire Front Advances
Rate of spread, usually written \( R \), is the speed at which a fire front moves through fuel, quoted in metres per minute or kilometres per hour, and conventionally measured at the head. It is obtained by observation rather than derived: repeated mapping of the perimeter from aircraft or satellite, timed marks between known points on the ground, or time-stamped imagery, with the spread taken along the direction of travel over a stated interval. Reported values span an enormous range, from a few metres per minute in a backing fire in litter to several kilometres per hour in wind-driven grass. Two cautions make the number usable: it is an average over the interval measured, so a fire that ran hard for five minutes and stalled for twenty reports a modest figure, and it is direction-specific, so a head rate cannot be applied to the flanks. The confusion it resolves is treating a single spread figure as a property of the fire rather than of a place, direction and time window. After this Concept you can read a rate of spread figure and state what it does and does not cover.
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Rate of spread, usually written \( R \), is the speed at which a fire front moves through fuel, quoted in metres per minute or kilometres per hour, and conventionally measured at the head. It is obtained by observation rather than derived: repeated mapping of the perimeter from aircraft or satellite, timed marks between known points on the ground, or time-stamped imagery, with the spread taken along the direction of travel over a stated interval. Reported values span an enormous range, from a few metres per minute in a backing fire in litter to several kilometres per hour in wind-driven grass. Two cautions make the number usable: it is an average over the interval measured, so a fire that ran hard for five minutes and stalled for twenty reports a modest figure, and it is direction-specific, so a head rate cannot be applied to the flanks. The confusion it resolves is treating a single spread figure as a property of the fire rather than of a place, direction and time window. After this Concept you can read a rate of spread figure and state what it does and does not cover.
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