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Bush Fire Behaviour

Fire Behaviour is the reaction of fire to the environment. A good understanding of bush fire behaviour will facilitate better procedures and preparation for a bush fire event. The components necessary for a fire to burn and continue to burn can be illustrated by the 'fire triangle'.

The Fire Triangle

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

Fire Triangle 

Fuel

Fuel is anything that will burn under suitable fire conditions. It rates as one of the most important factors influencing the way a fire behaves and travels. Fuel is the common environmental factor which is manipulated in order to modify fire behaviour. There are 4 variables of fuel that will influence its contribution to fire behaviour. They are:

  • Type
  • Size and quantity
  • Arrangement
  • Moisture

Weather

Weather is another major factor that will influence the spread of a fire. The four key elements of weather that will influence fire behaviour are:

  • Air Temperature
  • Relative humidity
  • Wind
  • Atmospheric stability

Effect of Wind on Fire Behaviour

Wind speed is the most important factor in determining fire behaviour in dry fuels. Wind acts on a fire in the following ways:

  1. Tilts the flames forward and provides more effective radiation and pre-heating of the unburnt fuels.
  2. Increases the chances of direct flame contact with fuels ahead of the fire
  3. Maintains the oxygen supply to the combustion zone
  4. Shifts the convection column ahead of the fire so that the convective energy of the fire reinforces and increases the wind speed in the flame zone, providing additional momentum to fire spread
  5. Blows burning embers ahead of the fire to create spot fires.

Topography

The topography of the landscape also influence the speed at which a bush fire will spread (referred to as the rate of spread). Fire will travel faster up-slope than down-slope and with greater intensity because vegetation in front of the fire is pre-heated and will therefore more readily ignite.