Fire behaviour

• Crown fires


  Fires that spread through the crowns of trees
• Three progressive stages of crown fires

  1. Passive

  2. Active

  3. Independent
• Passive crown fire occurs when individual trees within the crown branches are ignited, creating a series of torching trees
• Active crown fire spreads to the top of trees where it moves through the canopy, ignited by heat and embers from the surface fire
• Independent crown fire spreads through the crowns of trees, creating a continuous sheet of flames that can be seen from a distance and can cause damage to forests and the trees
• Factors affecting crown fire development

  • Steepness of slope
  • Crown closure
  • Intensity of surface fire
  • Wind at upper crown level
  • Dead fuel moisture content
• Fire behaviour


  The manner in which fire moves, spreads and changes in intensity, influenced by factors such as fuel, weather and topography
• Factors that affect combustion

  • Compactness and arrangement of fuel
  • Fuel moisture content
  • Fuel temperature
  • Size and shape of fuel
• Factors influencing fire behaviour

  • Topography
  • Weather
  • Heat transfer
• Heat transfer

  1. Log preheated by Conduction and Convection

  2. Tree trunk preheated by Radiation

  3. Branches heated by Convection and Radiation

  4. Duff heated by Conduction and Radiation

  5. Bush heated by Radiation
• Short term effect of wind

  Decreases fire temperature
• If temperature increases


  Relative humidity decreases
• When fire behaviour increases

  • Cumulus cloud development increases
  • Local wind increases
  • Fire temperature increases
  • Atmospheric instability increases
• Forces causing spotting


  • Gravity
  • Wind
  • Convection
• Factors influencing spot fire conditions

  • Fire intensity
  • Fire temperature
  • Fuel moisture
  • Surface fire characteristics
  • Wind speed
  • Distance of spotting
• Factors influencing fire spread

  • Size and shape of fire
  • Fuel moisture content
  • Arrangement and compactness of fuel