What happens in a bushfire

Bushfires can be killers, and climate scientists predict we will have more and more of them as global warming proceeds. These fires are fundamentally different to ones that might burn in your hearth or in your barbecue. JEFF GLORFELD explains why.

MOST OF US HAVE sat gazing at an open fire, into the crackling flames, the deep crimson embers at its heart. We heat our homes with fire, cook with it, admire its endlessly shifting shapes and hues. But a bushfire is one of the most destructive forces on Earth.

HEAT

The hottest temperatures in most fireplace wood fires are in those red embers. These range from 650ºc to 815ºc. Inside the unbridled flames of a bushfire, the temperature in the reaction zone – where volatile gases released from combusting vegetation mix with oxygen in the air – can reach 1,600ºc.

In experimental bushfires set to measure flame temperature in a range of dry eucalypt forests, flames were found to be hotter in tall shrubs than in low ones. The maximum temperature observed was about 1,100ºc near the flame base, decreasing at the visible flame tip to about 300ºc. Temperature was affected by flame height, how fast the fires were spreading and the amount of surface fuel available.

The type of forest in a bushfire is also a key factor. Pine wood gives off 21.28 megajoules per kilogram (Mj/kg) of energy in the form of heat. Eucalypts put out 19.98 Mj/kg. But the eucalypt’s oil yields 37.20 Mj/kg compared to 35.13 Mj/kg from the pine’s sticky sap. Wayne Padgett, a former ecologist with the US Forest Service and Bureau of Land Management in Washington DC, says pine forests don’t have the volatile oils that occur in eucalyptus forests. “That, combined with the shredding bark and the leaf litter that builds up in these ecosystems, make eucalyptus forest fires extreme,” he explains.

INTENSITY

Fire intensity is generally expressed in kilowatts per metre (kw/m), which is the amount of energy released from each metre of a wind-driven fire’s leading edge. Rating a fire’s intensity takes into account the fuel consumed and the fire’s rate of spread.

A mild fire produces up to 350 kw/m. An intense fire produces 2,000 kw/m or more.

A kilogram of dry vegetation contains enough energy to power a 100-watt light globe for 50 hours. In a bushfire that energy is released in only a few seconds.

The average radiant heat from the Sun at midday in summer is about 1 kw/m2. In the midst of a highintensity fire, radiant heat can be as high as 150 kw/m2.