Fire danger ratings
Fire Danger Ratings are determined daily for NSW Fire Areas by the Bureau of Meteorology (BOM). Fire Danger Ratings are measured as low, moderate, high, very high or extreme and are based on weather conditions such as low relative humidity, high winds and lack of rain which all contribute to increase fire danger.
Humidity
Relative humidity is the most commonly used measure of atmospheric moisture and is defined as the ratio of the amount of water vapour actually measured to that which air could hold at saturation. Very low relative humidity of, say, less than 20 per cent, causes fuels to dry out and become more flammable (www.bom.gov.au/index.shtml, 2007).
Wind
Air movement provides the oxygen the fire needs to keep burning. Higher winds mean more oxygen and more intense flames. Doubling the wind speed will quadruple the rate of spread of the fire. Winds also carry burning embers downwind, which can start new fires. This is known as spotting (www.bom.gov.au/index.shtml, 2007).
Rainfall
Dry grass, parched native shrubs and dead leaves and twigs are fire's basic fuel. During droughts and in very hot, windy weather, even heavy fuels like large logs and the green leaves and smaller branches of large trees can become dry and flammable (www.bom.gov.au/index.shtml, 2007).
The BOM also maintain a site on their web page called 'Australian Climate Extremes'. This site described how climatic evens such as El Nino affect the number and severity of bushfires in Australia.
When fire danger conditions become extreme, the Bureau of Meteorology issues Fire Weather Warnings. Warnings are broadcast on radio and television. In most States and Territories, fire authorities will declare a Total Fire Ban when they receive a Fire Weather Warning from the Bureau.
NSW Fire Areas are determined by the Rural Fire Service and have different boundaries to the Bureau of Meteorology's Weather Forecast Districts. Further information on NSW Fire Areas can be found at www.rfs.nsw.gov.au or by calling the Rural Fire Service on 1800 679 737.