How weather affects fires

Weather elements that affect bushfire behaviour

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

Wind, temperature, humidity and rainfall are key 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 affects how readily fires may ignite, and how fast they may spread.

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 quickly and become more flammable.

Wind

Strong gusty winds fan the flames and can cause a fire to spread faster. Strong winds can also 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 with a trough or cold front – also known as a cool change. This wind change can affect the behaviour and structure of existing fires.

In southern Australia, the worst fire days on record have happened when a cold front has moved over active bushfires. Hot, gusty north-east to north-west winds for much of the day turn to cooler south-westerly winds in the afternoon or evening. This causes long fire-fronts to expand rapidly.

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.

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.

Thunderstorms and lightning

Fires started by lightning account for a large amount of the total area burnt by bushfires.

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 also common.

Erratic 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 very dangerous bushfire behaviour. To learn more about fire-generated thunderstorms, view our How fires make thunderstorms page.

Temperature inversions

A temperature inversion is when a layer of warm air sits above a layer of cooler air near the ground.

This often occurs at night or early in the morning, when the ground cools. Because cold air is heavier, it stays near the ground. The warm air is lighter, so sits in a layer on top. As the layers are separate, this is a stable arrangement.

There are often strong winds in the warm air above inversions. These winds usually stay above but can reach the ground in some circumstances. For example, in mountainous areas.

As the sun heats the ground during the day, the cool layer warms and rises. The inversion weakens. Strong winds may begin to blow near the ground. On very hot days, this can happen very quickly. It can cause fires to suddenly become more intense during the morning.

But it usually takes most of the day for all the winds to reach the ground. This is one reason why many bushfires burn more fiercely in the afternoon.

Smoke

Temperature inversions also influence smoke and air quality.

When an inversion is strong, with light and variable winds at ground level, smoke is trapped close to the ground. The inversion acts as a lid. It lets the smoke build up, causing problems with air quality and visibility.

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

Climate change and bushfire risk in Australia

Climate change is influencing the frequency and intensity 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 (lightning without significant rainfall). This is an area of active research.

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 phenomenon in south-east Australia.

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