Geography lessons

Australian Bushfires

Bushfires are common in Australia and are associated with the explosive combustion of our eucalypt forests and woodlands in the southwest and southeast Australia.

What factors affect the occurrence and spread of bushfires?

  • Fuel load: high fuel loads with disastrous potential for bushfire if ignited.  Dry shrubs and grasses, the shed bark of eucalypt trees, branches and leaves are highly combustible.
  • High air temperatures: Australian summer temperatures in the high thirties and forties dry out fuel, make spontaneous combustion more likely.  Summer storms with lightning strikes can ignite fires.
  • Prolonged drought: dries out the fuel load, making it more easily combustible.
  • Low humidity: hot, dry air with humidity below 25% dries out the fuel and creates critical bushfire conditions.
  • Strong winds: air movement provides oxygen to keep the fire burning and accelerates the speed at which fires spread.  A doubling of wind speed results in a four-fold increase in the rate that the fire spreads.
  • Terrain: Fires spread more quickly up hillsides and slopes.  For every 10 degrees of slope, fires will double their forward speed.  (20 degrees, four times the speed, 30 degrees, eight times the speeds).

CSIRO Bushfire Environment

Bushfire Weather

The Fire danger Rating (FDR) is an assessment of the potential fire behaviour, the difficulty of suppressing a fire and the potential impact on the community, should a fire occur on a given day.

The FDR is determined by the Fire Danger Index (FDI).

The Fire Danger Index is  a function of wind speed and fuel moisture content.  Fuel moisture content is affected by high air temperatures, low humidity and drought.

Case studies:

Major Historic Bushfires

Black Saturday Victoria 2009 Black Saturday

Canberra Firestorm 2003


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