E. Linacre and B. Geerts
Bushfires are driven forward by the wind, with an advancing line of burning (the fire front), at the edge of the tongue-shaped burnt area. Flames are usually about as thick as they are high. Forest fires normally travel at 1-3 km/h in Australia, and have flames 10-20 metres high and thick. This implies that the fire front normally passes a spot in less than a minute! However this is enough time to ignite a house, for instance. Severe forest fires travel at up to 12 km/h, with flames 100-l50 metres high and thick. Grass fires generally travel about 3-10 km/h, but speeds of around 25 km/h have been recorded. Stronger winds imply not only a faster movement, but also a more severe burning, because of the enhanced oxygen supply (1).
Experiments with grass fires and forest fires showed that the speed of the tongue’s lengthening depends partly on its breadth (2). A fire starting from a long line of flame advances more quickly than a narrow fire, which in turn extends faster than a fire from a point source. So the progress of the fire depends on the wind history; there is a sudden acceleration when a wind changes direction by 90 degrees, after creating a long thin fire from a point source, so that the fire is now wide across the wind. The speed of advance may increase threefold.
The Australian Bureau of Meteorology uses an index to assess the likelihood of bushfires, the Forest Fire Danger Index.