ANATOMY OF A FIRE DANGER DAY

By world standards, Australia has a fairly low and very unreliable rainfall, and droughts are a significant feature of the Australian climate. Dry spells create a high fire risk, particularly if the dry spell follows a period of good rain that has encouraged lush growth.

Temperature, relative humidity, wind speed, drought and the amount of fuel can be combined into a fire danger index.

Weather systems like the highs, lows and cold fronts that appear on weather maps control the temperature, humidity and wind. Because Australia spans a large range of latitudes, from tropical to temperate, these weather systems work differently in different regions of the country. 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.

As a cold front passes, the wind direction will often swing about 90 degrees. Usually, the winds ahead of a cold front are hot northwesterlies (Figure 3), while cooler southwesterlies follow the front (Figure 4), with significant effects on the behaviour and structure of existing fires.

Figure 4