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Fire behaviour

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

Read our fast facts to find out more.

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 consequently the relative humidity (how much moisture is in the air relative to how much total moisture the air can hold) drops. Air is usually drier during the day than at night, so fuels absorb moisture from the damp night with a resulting 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 of the fuel ahead of the fire front. Wind can also carry sparks and embers well 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 either no air movement or air sinking to the ground and results in calmer fire behaviour. The more worrying condition from a fire behaviour perspective is when the atmosphere is unstable. In this case, warmer air at the ground can rise rapidly, sometimes for many kilometres into the sky. Increased winds occur at the fire 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 can influence the amount of rain that has fallen in recent days, which both have an impact on the likelihood of a fire starting.

Fuels

  • The amount of fuel available also influences fire behaviour, and how it is arranged both vertically and horizontally at a location. The more fuel available to burn the more energy that will be released as heat.
  • Most of the flames, intensity and the rate of spread of a bushfire is driven by the fine dead fuels less than 6mm in diameter (twigs, leaves, grass etc.) and living fuels to 2mm diameter.
  • Larger branches and logs will burn much 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 will burn is dependent on the amount of moisture they contain and the amount of moisture in the soil below the fuels on the ground.
  • Bark fuels can also be important for fire behaviour as they can transport the flames to the canopy and wind-driven burning bark fragments provide much of the material that contributes to spotting ahead of the main fire. The near surface and elevated fine fuel layers can provide a significant increase in the rate of spread and intensity of a fire when present, and can result in the fire moving to the canopy if the flame heights are sufficient.
  • When fuels are patchy, scattered or separated by natural barriers such as rock, 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 degrees 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 more rapidly ignite.
  • Downhill fires will slow down on a slope up to about -10 degrees. 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 both the soil and the vegetation more than on a southern or eastern slopes. The fuels are therefore usually drier and less dense on northern and western slopes than fuels on slopes with a different aspect.
  • Terrain effects occur in more hilly areas like in 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 have unpredictable effects on fire behaviour.
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