Understanding fire weather

The Bureau of Meteorology issues fire weather warnings when forecast weather conditions are likely to be dangerous. The Bureau’s Fire Weather Knowledge Centre provides information about bushfires and fire weather as well as information about the Bureau’s fire weather warnings.

The Bureau’s ‘Understanding Fire Weather’ video explains how the weather influences fire behaviour. Weather conditions influence the size, intensity, speed and predictability of bushfires – and how dangerous they can be to the community.

How does weather influence fire behaviour?

Weather conditions influence the size, intensity, speed, and predictability of bushfires and how dangerous they can be to the community.

Vegetation growth can be encouraged by periods of wet weather, increasing the amount of fuel available (grass, leaf litter, twigs, bark). When the weather is hot, the humidity is low and there's been little recent rain, this vegetation dries out and becomes more flammable. A fire is more likely to start, and continue to burn, in hot, dry and windy weather.

Humidity

Relative humidity is commonly used to measure atmospheric moisture. It is defined as the ratio of the amount of water vapour measured to what the air could hold at saturation point. Very low relative humidity, say less than 20%, causes fuels to dry out and become more flammable.

Wind

Strong gusty winds help fan the flames and cause a fire to spread faster across the landscape. Strong winds can carry hot embers long distances - these can start spot fires many kilometres ahead of the main fire front.

A change in wind direction can bring a period of dangerous bushfire activity, this is often seen as a trough or cold front - also known as a cool change.

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, while cooler southwesterlies follow the front, with significant effects on the behaviour and structure of existing fires.

Rainfall

Dry grass, parched native shrubs and dead leaves and twigs are a 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.

Heavy rainfall might quench a fire, but it can make it difficult for firefighters to access burnt areas. Where fires have been intense, or occur over steep terrain, heavy rain might cause severe erosion and affect water quality when rivers carry ash and sediment along.

Lightning

In some parts of Australia lightning is the main source of bushfire ignition. The winds during thunderstorms can make it hard to predict the behaviour and movement of a bushfire.

Large fires can also create their own thunderstorms known as pyrocumulonimbus. They can cause erratic more intense and dangerous bushfire behaviour. Find out more about pyrocumulonimbus at the Bureau Blog.

Temperature inversions

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. 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.

As the sun heats the ground during the day, the inversion weakens and strong winds may begin to blow near the ground.

Smoke

Inversions also have an influence on the smoke produced by a fire. In stable conditions, when an inversion is strong, smoke is trapped close to the ground causing issues with air quality and visibility. Once the inversion breaks, typically in the late morning, the smoke becomes more mixed with clear air and disperses through the atmosphere. New smoke produced by the fire forms a neat plume that moves downstream away from the fire.

The mixing height, which is available to view on MetEye under the Wind Forecasts section, is an indicator of whether smoke will be trapped on a particular day. When there is low mixing height (less than 1000 metres), smoke will stay near the ground. When the mixing height is higher (greater than 2000 metres), smoke will disperse higher into the atmosphere and air quality will improve at the ground.

More information

Learn more about fire weather at the Bureau’s Fire Weather Knowledge Centre at:
http://www.bom.gov.au/weather-services/fire-weather-centre

(Information source: Bureau of Meteorology)