The science behind fire behaviour

Fire behaviour refers to the way a fire burns, such as how quickly it spreads, how much heat it gives off and how much vegetation it consumes. Three factors typically influence fire behaviour: weather, fuels and topography.

Weather

Hot, dry, windy days are more likely to experience increased bushfire activity.

• The hotter a day becomes the more moisture the air can hold, and so the relative humidity drops (how much moisture is in the air is relative to how much total moisture the air can hold). As night air usually holds more moisture than daytime, fuels will absorb moisture from the damp night which means a decrease in fire activity.
• The stronger the wind, the faster the spread of the fire. It does this by providing extra oxygen to feed the fire and it also lays over the flames, which results in pre-heating and drying the fuel ahead of the fire front. Wind also carries sparks and embers ahead of the main fire, causing spot fires.
• Atmospheric stability refers to the vertical movement of air in the atmosphere. A neutral or stable atmosphere will see no air movement or air sinking to the ground resulting in calmer fire behaviour. The more worrying condition is when the atmosphere is unstable. This causes warmer air at the ground to rise rapidly, sometimes for many kilometres into the sky. Increased winds occur as air is drawn in from surrounding areas resulting in more dangerous fire behaviour. An unstable atmosphere can also result in thunderstorm and lightning activity and influence the amount of rain that has fallen in recent days, which impacts on the likelihood of a fire starting.

Fuels

The amount of fuel influences fire behaviour, along with how it is arranged vertically and horizontally, at a location. The more fuel available to burn the more energy is released as heat.

• Most of the flames, intensity and rate of spread of a bushfire is driven by fine dead fuels less than 6mm in diameter (twigs, leaves, grass etc.) and living fuels to 2mm diameter.
• Larger branches and logs will burn more slowly and can throw off a lot of heat, but they do not contribute to the rate of spread of the fire.
• How easily fuels burn depends on the amount of moisture they contain and the amount of moisture in the soil below the fuels on the ground.
• Bark fuels can transport flames to the canopy and wind can carry burning bark fragments to spot ahead of the main fire. These fragments provide much of the material that contributes to spotting ahead of the main fire.
• Near-surface and elevated (higher) fine fuel layers can increase the rate of spread and intensity of a fire, and result in the fire moving to the tree canopy if flame heights are sufficient.
• When fuels are patchy, scattered or separated by natural barriers such as rocks, outcropping, streams, or areas of bare ground, the fire will spread more slowly unless wind speeds increase significantly to help the flames bridge these gaps.

Topography

• Fire moves faster uphill. In fact, for every 10 degree increase in slope a fire will double in speed. This is because the slope provides a similar effect to the wind, effectively laying the flames down into the slope and pre-heating the vegetation, allowing it to ignite more rapidly.
• Downhill fires slow down up to about 10 degrees on a slope. Beyond that they travel at about 0.6 times the speed of a fire on flat land, regardless of any further increase in slope.
• The aspect of a slope (direction that a sloping piece of land is facing) influences a fire's behaviour in a couple of ways; northern and western aspects receive more direct heat from the sun, drying the soil and vegetation more than on a southern or eastern slope. Therefore the fuels are usually drier and less dense on northern and western slopes than fuels on slopes with a different aspect.
• Terrain effects more hilly areas like the Mount Lofty or Flinders ranges in South Australia. Localised erratic winds caused by changes in the heating and cooling of the land during the day and at night can also have unpredictable effects on fire behaviour.