- Published: Sunday, 28 December 2014 11:23
- Written by Lance Hartley
(Courtesy of CSIRO Bushfire Behaviour and Management Section)
Fire weather data is important because it describes the prevailing weather conditions affecting fire behaviour. The most important weather variables are wind, atmospheric humidity and temperature.
Wind is a major controlling factor that determines rate and direction of spread, and shape of fire. The following diagram illustrates the fire that can result from a change in wind direction. Notice that a change in wind direction from the NW to SW has caused the flank fire to become the new fire front — much larger and potentially more difficult to control than the original narrow front.
Fire whirlwinds can form during or after fires, and are created by surface heating and atmospheric instability. They can be problematic as they can carry burning debris outside the fire perimeter, sparking new fires. Wind can also influence the likelihood of spotting.
Spotting occurs when a fire produces sparks, embers or flaming material (such as loose bark) which are carried by the wind and start a fire ahead of the flaming front (outside the perimeter of the fire).
Relative humidity (RH) is a measure of how much moisture is in the air. Expressed as a percentage, it describes the amount of water vapour in the air compared to the amount needed for the air to be saturated (i.e. 100% RH). Humidity varies with temperature – as temperature increases humidity decreases (and vice versa). Humidity is important because it affects fuel moisture content and therefore its flammability. For example, in dry conditions, moisture from fuels is transferred to the atmosphere and the fuels become increasingly flammable. In the Tathra area, this means that surface fuels become more flammable as summer progresses.
Temperature affects fire behaviour indirectly through influence on fuel moisture content and local wind formation (e.g. sea-breezes). So, as air temperature increases, fuel moisture content tends to decrease, and vice versa.
Dew can form which can cause fire to slow or even be extinguished. Also, the higher the temperature of the fuel, the more easily it will reach ignition temperature. Fire weather information can be used in conjunction with fuel information to provide indices of fire hazard or danger rating.