Bushfire behaviour is determined by three factors:
If all the relevant data in relation to these factors were known, then the passage and characteristics of a bushfire would be predictable making its control a relatively straightforward process. However, in most fires although the topography is obvious and the fuel load easily estimated, the weather is the element that creates the greatest difficulty in predicting bushfire behaviour.
The combination of these factors creates a dynamic entity capable of huge variations in behaviour in relatively short times frames with relatively small variations in the elements which control it.
Although fuel may be present in large quantities it is the condition of the fuel that, to a large extent, determines its flammability. The factors affecting the flammability of fuel are:
- quantity (tonnes per hectare)
- fuel moisture content (Percentage Oven-Dried Weight)
It is the difficulty in predicting fire behaviour that greatly increases the inherent dangers of bushfire fighting. The effects of weather can cause a bushfire to be unpredictable. It has the ability to cause a fire to increase in intensity and rate of spread, change in direction and fiercely erupt.
Fire behaviour can be altered by the effects of:
- relatively humidity
- atmospheric stability
- frontal movement
- effects of drought
More information on bushfire weather can be found on the Bureau of Meteorology website.
Topography can have a great impact on a bushfire's behaviour. During fires at Jindabyne and adjacent areas it was common knowledge to local residents that the surrounding mountain range had the potential to change a Westerly wind at the Southern tip of the ranges to a South/Westerly and further Northward along the Eastern side of the range to a South/Easterly to Easterly wind. Such is the affect of the mountain range. It requires a great deal of experience and local knowledge to be able to interpret the effects of such winds so as to accurately determine a fire's movements. A fire's progress will be affected to a large extent by the lay of the land. Slope will cause a fire to slow if it is burning downhill or accelerate if it is moving uphill.
The extent of the fire's progress, apart from the fuel load and weather conditions, will be determined by the angle of the slope. For each 10 degrees of uphill slope the fire will double in rate of spread. For each additional 10 degrees of slope, the rate of spread will double again. Similarly, for downhill slopes, for every 10 degrees of negative slope the rate of spread will halve, an additional 10 degrees again halving the fire's effects.