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.
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.
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 high-intensity fire, radiant heat can be as high as 150 kW/m2.
HOW A FIRE BEHAVES
The behaviour of a fire is influenced by three main factors: fuel, weather and topography.
Fuel is anything that will burn under suitable conditions. It is also the only element in a bushfire we can control. The primary fuel for bushfires is the fine dead vegetation on the forest floor – leaves, bark and small branches, along with dried grasses.
Moisture in the fuel is a key factor. If it contains more than 30% moisture by weight, it won’t ignite; at less than 20% it can be readily ignited; when it is less than 10%, combustion is rapid and fires can spread easily; when it is less than 5%, fire behaviour is highly erratic and fire spread is rapid.
‘Volatile oils, combined with the shredding bark and leaf litter that builds up in these ecosystems, make eucalyptus forest fires extreme.’
Bushfires typically move in a front, a line of burning grass or forest that advances as new material catches alight. Radiant heat from the fire front warms the air ahead, drying out fuel and causing volatile gases inside wood to expand and escape, thus priming new food for the approaching fire.
Grass fires are generally wind-driven and spread quickly, moving at more than 20 km/h. Fires in forests will burn hotter but they advance more slowly because there is more fuel. Bushfires burning through dense, uninterrupted fuels move at about 3-4 km/h.
Fires move more quickly uphill than down, because when going uphill the flames are much closer to new fuel and spread easily. For every 10 degrees of uphill slope, the fire will double its speed. For example, if a fire is travelling at 20 km/h along flat ground and hits a 10-degree slope, it will accelerate to 40 km/h. As it increases in speed, the fire also increases in intensity, becoming even hotter.
The opposite effect applies to a fire travelling downhill. The flames reach less fuel, and less radiant heat pre-warms the fuel in front of the fire. Every 10 degrees of downhill slope will halve the fire’s speed.
Wind-blown fires can throw embers ahead of the front into new dry fuels. This phenomenon, called “spotting”, can overcome the potential handicap of a downward slope or a break in the fuel.
Weather is the third key factor that affects the spread of a fire. There are four key elements of weather influencing fire behaviour: air temperature, relative humidity, wind and atmospheric stability.
Wind speed is the most important of these. It tilts the flames forward and provides more effective radiation and pre-heating of unburnt fuels. It also increases flame contact with fuels ahead of the fire and maintains the oxygen supply necessary for burning.
Wind also blows burning embers ahead of the fire to create spot fires. It is a complex, self-sustaining process. As the wind pushes the fire, its own energy reinforces and increases the wind speed in the flame zone, providing additional momentum to fire spread.
This last point is also worth noting. The heat and smoke given off from an intense bushfire can create pyrocumulonimbus clouds that form over the burning area. Sometimes these clouds create thunderstorms, fuelling lightning strikes that can start new fires.