Fire speed – faster than a speeding bullet?
Reports of wildfires moving quickly enough to outrun a car are common – but can they really go that fast?
6 May 2008 | Updated 14 October 2011
Faster than a speeding bullet – well, at least a beat-up Kingswood on a country road.
After every wildfire there are reports of fires spreading at phenomenal rates.
People describe their experience of driving down the road at 80 or 90 km/h and not being able to keep pace with the fire.
On later analysis though, when information has been drawn from all observers, the pattern of fire spread often reveals a predictable pedestrian rate of 16–20 km/h for the maximum fire speed.
So what explains this convincing illusion?
Grassfires burn in a direct line across the countryside that is determined by the wind direction.
This is usually at an angle to the road network, which is often set out on a square grid.
So a person driving down the flank of the fire is likely to be driving at times at an angle towards, and at times away from, the fire front.
The driver has to negotiate right-angled bends, sometimes tortuous creek crossings and other obstacles, so the distance travelled is likely to be considerably longer than the path taken by the head of the fire.
The pattern of fire spread often reveals a predictable rate of 16–20 km/h for the maximum fire speed.
Hence considerable speed is required to catch up with the head.
Another feature of fire behaviour that can give the illusion of high rates of spread is waves of flames in the flank fires.
As the wind oscillates, its change in direction progressively picks up the flames on the flank, giving the appearance of a wave of flame actively travelling down the edge of the fire.
This wave will appear to travel at the speed of the gust, perhaps as fast as 80 km/h.
Also, any switch in wind direction which hits the back of the fire first and then travels down the flank will pick up the flames at the rate that the change in wind direction hits the flank.
This can also make it appear that the flames themselves are travelling very rapidly.
When first arriving at a fire, it is a good idea to take a few minutes to observe its behaviour and note the movement of the flames compared with the speed at which the flame front is actually travelling.
It will soon become clear that, while a rate of spread of 18–20 km/h is certainly devastatingly fast, no miraculous rates of spread vastly exceeding this figure are maintained for any length of time.
Fire speed – faster than a speeding bullet? - was taken from the book Grassfires: Fuel, Weather and Fire Behaviour.
Learn more about our bushfire research in Bushfires.
Cheney P, Sullivan, A. 1997. Grassfires: Fuel, Weather and Fire Behaviour. CSIRO Publishing, Melbourne.