Geography lessons

Bushfire Weather (extract from BOM)

Each part of Australia has its own special combination of weather systems that produces severe bushfire conditions, but in all cases these conditions result from hot, dry winds blowing from Australia’s central arid region. In southern Australia, cold fronts are probably the most powerful influence on our fire weather. Cold fronts occur at the junction of warm and cold air masses where the strong temperature contrast provides the energy source that generates the frequently associated strong winds.

High Risk Weather Patterns – Southeast Australia

Summer and autumn are the most dangerous times of year in southeast Australia. The highest temperatures occur during these seasons and in most years the grass and forests have dried out by mid-summer.

A typical dangerous fire situation occurs in southeastern Australia when a vigorous cold front approaches a slow-moving high in the Tasman Sea, causing very hot, dry, northwesterly winds. Figure 4 shows the situation associated with the Victorian Ash Wednesday fires of 16 February 1983. The passage of the cold front can cause the winds to suddenly change direction, shifting fire direction abruptly.

Fires driven by a strong, steady wind are usually long and narrow. When the wind changes with the passage of a cold front, the long side of the fire can suddenly become the fire front.

Figure 4: Historic Chart Analysis at 11am EDT on “Ash Wednesday”, Wed 16th February 1983 showing areas with extreme fire weather.

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High Risk Weather Patterns – New South Wales & southern Queensland

The fire season for most of Australia’s east coast extends from spring to mid-summer. The greatest danger occurs after the dry winter/spring period, before the onset of the rainy weather common in summer. The worst conditions occur when deep low-pressure systems near Tasmania bring strong, dry, westerly winds to the coast, as occurred in the major New South Wales fires in January 1994 (Figure 5).

Figure 5: Historic Chart Analysis at 11am EDT on 7 January 1994, showing areas with extreme fire weather that resulted in serious bushfires in NSW and southern Queensland

For further information and explanation see See www.bom.gov.au Bushfire Weather

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