HOW A FIRE BEHAVES

The behaviour of a fire is influenced by three main factors: fuel, weather and topography.

1. FUEL

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.

2. TOPOGRAPHY

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 winddriven 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.

3. WEATHER

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, selfsustaining 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.

‘ Volatile oils, combined with the shredding bark and leaf litter that builds up in these ecosystems, make eucalyptus forest fires extreme.’