Bushfire Weather
Fire Weather Warnings
The Bureau of Meteorology issues Fire Weather Warnings when the Fire Danger Index (FDI) is expected to reach or exceed a value of 50 (38 in Tasmania) either today or the next day. Warnings are broadcast on radio and television. In most States and Territories, fire authorities declare a Total Fire Ban based on a range of criteria including the Forecast FDI from the Bureau.
Further Information
Click on your State for more information about what to do in a bushfire.
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The Bushfire Threat
Large areas of Australia suffer from the threat of bushfires. The Australian climate is generally hot, dry and prone to drought. In the southeast, occasional strong winds often associated with summertime cold fronts can lead to extreme fire danger. The amount of rain in preceding months affects the amount of dry grasses. If good spring rains have resulted in abundant plant growth, late summer grass fires can be intense.
Many of Australia's native plants burn easily. The eucalypts' high oil content makes them particularly fire prone. The vast areas of dry grass common in mid-to-late summer also burn readily.
Most loss of life and property damage occurs around the fringes of the cities where homes are sometimes surrounded by flammable vegetation.
Varied fire seasons reflect different weather patterns. For most of southern Australia, the danger period is summer and autumn. For New South Wales and southern Queensland, the peak risk usually occurs in spring and early summer. Northern Australia experiences most of its fires in winter and spring.
Figure 1
Weather and fire
Low relative humidity, high winds and lack of rain all contribute to increased fire danger. Sunshine and high temperatures rapidly dry timber and grass (fuel) which burn very quickly. Hot air can lower the moisture content of forests and grasslands to around 5 per cent and in extreme cases to 2-3 per cent, greatly increasing the speed of the fire.
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.
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. However above about 50 km/h this relationship begins to break down, and above 80 km/h the rate of spread in grasslands reduces. This occurs because the head fire breaks up into narrow tongues, many of which become self extinguishing. Winds also carry burning embers downwind, which can start new fires. This is known as spotting.
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.
Anatomy of a Fire Danger Day
By world standards, Australia has a 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 2), while cooler southwesterlies follow the front (Figure 3), with significant effects on the behaviour and structure of existing fires.
Figure 2 (left), Figure 3 (right)
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.
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
High Risk Weather Patterns - Western Australia & the Northern Territory
The northern Australian fire season occurs during the warm, dry and sunny winter and spring, when the grasses are dead and the fuels have dried. In summer, a strong high pressure system over South Australia can bring strong southeast to northeast winds that increase the fire danger in the southern parts of western Australia (Figure 6).
Figure 6: Historic Chart Analysis at 8am WST on 4 April 1978 showing areas with extreme fire weather associated with the gales in the aftermath of Tropical Cyclone Alby.
The Third Dimension - the vertical profile of a severe fire day
A layer of warm air sitting over a layer of cold air is called a temperature inversion. Inversions are common during the night and early morning when cool night air collects close to the ground (Figure 7).This arrangement of air is stable because cold air near the ground is heavy and tends to stay near the ground. The warm air above the inversion is lighter and tends to stay above the inversion.
There are often strong winds in the warm air above inversions, but while the inversion lasts these winds cannot come down to ground level (Figure 8).
As the sun heats the ground during the day, the inversion weakens and strong winds may begin to blow near the ground. This is one reason why many bushfires burn more fiercely in the afternoon, as happened with the fires in Hobart in 1967.
Figure 7 (left) Figure 8 (right)
Last updated 01 October 2009 - Weather Services Branch
Further Information
- Changes to fire warnings
from October 2010
