How weather affects fires

Find out how weather influences bushfires and the impact of climate change.

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Weather elements that affect bushfire behaviour

Climate change and bushfire risk in Australia

Weather elements that affect bushfire behaviour

Bushfire activity varies across Australia with the changes in the seasonal weather patterns. Weather conditions influence the size, intensity and speed of bushfires and how dangerous they can be to the community.

Wind, temperature, humidity and rainfall are weather elements that affect fire behaviour. For example:

  • Wet weather encourages vegetation growth, 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

The amount of moisture in the air can affect the fuel available for a bushfire.

The measure of this moisture is relative humidity. It's shown as a percentage of the maximum moisture the air could hold at that temperature.

Very low relative humidity, say less than 20%, causes fuels to dry out and become more flammable.

Wind

Strong gusty winds fan the flames and can cause a fire to spread faster. Strong winds can carry hot embers long distances. The embers 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, the worst fire days on record have happened when a cold front has moved through the area. Hot, gusty north-east to north-west winds for much of the day turn to cooler south-westerly wind in the afternoon or evening.

As a cold front passes, the wind direction can swing significantly. This can affect the behaviour and structure of existing fires.

A cold front is a boundary where warm and cold air masses meet. The strong temperature contrast provides an energy source that generates the frequently associated strong winds. Learn more about frontal systems.

Video: Fire weather

Rain

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 large logs, the green leaves and smaller branches of large trees can become dry and flammable.

Heavy rain might put out a fire. It can also make it difficult for firefighters to access burnt areas. Where fires have been intense, or happened over steep terrain, heavy rain might cause severe erosion and affect water quality when rivers carry ash and sediment.

Lightning

In some parts of Australia, lightning is the main way that bushfires start (ignition) – particularly in tropical and central areas with low population density. In southern and eastern Australia, lightning ignitions are common. Fires started by lightning account for a large amount of the total area burnt by bushfires. The winds during thunderstorms can make it difficult to predict the behaviour and movement of a bushfire.

Large fires can also create their own thunderstorms known as pyrocumulonimbus clouds. These can cause erratic more intense and dangerous bushfire behaviour. To learn more about fire-generated thunderstorms, view our How fires make thunderstorms.

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 denser 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. While the inversion lasts these winds generally stay above the inversion, though they can come down in some circumstances (for example, in mountain terrain).

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.

Smoke

Temperature inversions also influence smoke and air quality due to a fire.

In stable conditions, when an inversion is strong with light and variable winds, smoke is trapped close to the ground. The inversion acts as a lid, causing problems with air quality and visibility.

Once the inversion breaks and winds increase, typically in the late morning, the smoke mixes with clear air. It disperses through the atmosphere. New smoke produced by the fire moves downstream away from the fire.

Aerial photo
looking from the ocean over the coastline to forested hills in Victoria's Otway
region. Smoke rises from a prescribed burn at Big Hill Track.

Smoke from a prescribed burn at Big Hill Track in the Otways on 4 April 2017 after an inversion has broken. Credit: Nathan Christian, Forest Fire Management Victoria, Mildura.

Climate change and bushfire risk in Australia

Climate change is influencing the frequency and risk of dangerous bushfire conditions in Australia and other regions of the world. It affects temperature, environmental moisture, weather patterns and fuel conditions.

More dangerous bushfire weather conditions

There have been significant changes observed in recent decades towards more dangerous bushfire weather conditions for various regions of Australia. Observed changes in southern and eastern Australia include:

  • more extreme conditions during summer
  • an earlier start to the bushfire season, with dangerous weather conditions significantly earlier in spring than before.

The trends towards more dangerous bushfire conditions can be attributed to human-caused climate change. This includes through increased temperatures and more frequent dry conditions.

Northern Australia has significant fire activity during the dry season. It has experienced increases in monsoonal rainfall that have increased fuel growth in recent decades – a key factor influencing fire danger in that region.

Dry lightning and fire-generated thunderstorms

There is some indication that climate change could influence the risk of fires starting from dry lightning. This is lightning without significant rainfall. However, there are relatively large uncertainties in our current understanding of dry lightning risk.

There have also been several devastating fire events in Australia associated with thunderstorm development in a fire plume (pyrocumulonimbus clouds). Recent research indicates a long-term trend towards increased risk factors for this in south-east Australia.

Bushfire weather conditions in future years are projected to be more severe for many regions of Australasia. This includes more extreme heat events, with the rate and magnitude of change increasing with greenhouse gas concentrations (and emissions).

Fire weather warnings

View the National warnings summary