How fires make thunderstorms

Learn how bushfires create thunderstorms, why this is dangerous and what we're doing to predict it.

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How bushfires make thunderstorms

Dangers of fire-generated thunderstorms

Predicting fire-generated thunderstorms

Climate change impact on fire-generated thunderstorms

 

How bushfires make thunderstorms

Weather conditions can increase the risk of bushfires and make them harder to fight. But fires can also create conditions that lead to thunderstorms 'pyrocumulonimbus clouds'.

In Australia, these clouds most commonly form in large and intense bushfire smoke plumes. The official name for them is 'flammagenitus', but they're also known as fire-generated thunderstorms.

Video: Pyrocumulonimbus at the Sedgerly fire, Queensland

How pyrocumulonimbus clouds form

Intense heat from the fire makes air rise rapidly in the smoke plume. The rising hot air is turbulent and draws in cooler air from outside the plume, which helps cool the plume as it rises.

Cooling creates cloud

As the plume rises higher, the atmospheric pressure reduces. This makes the plume air expand and cool even more. If it cools enough, moisture in the plume air turns from a gas to liquid (condenses) and forms cumulus cloud. Because it comes from the fire (pyro) plume, we call it 'pyrocumulus'.

Cloud rises

The condensation process releases heat. This makes the cloud warmer and more buoyant, driving air upwards. Further expansion and cooling means more condensation and the cloud air accelerates upwards even more. In the right conditions, the cloud reaches the upper troposphere before losing buoyancy. The cloud is now a ‘pyrocumulonimbus’.

Lightning sparks

Different types of ice particles collide in the very cold upper parts of these clouds. This may build electrical charge, which is released by giant sparks – lightning. Sometimes lightning does not develop in pyrocumulonimbus clouds. This is due to smoke in the clouds changing how the ice forms.

How pyrocumulonimbus clouds develop:

1. A plume of hot, turbulent air and smoke rises above a large area of intensely burning fire. This triggers the processes to create a thunderstorm.
2. Turbulence mixes cooler air into the plume, causing it to expand and cool as it rises.
3. When the plume rises high enough, low atmospheric pressure cools the air and cloud forms.
4. In an unstable atmosphere a thunderstorm can develop – a pyrocumulonimbus cloud.
5. Rain in the cloud evaporates and cools when it comes into contact with dry air, producing a downburst.
6. Lightning from the cloud can ignite new fires.

Dangers of fire-generated thunderstorms

These thunderstorms can be linked to dangerous and erratic fire behaviour that can make the fire more difficult and hazardous to fight.

Change in fire direction and intensity

When fire-generated thunderstorms form, there can often be intense updrafts – a rapidly moving wind that flows vertically upwards. Scientists are still investigating whether the thunderstorm causes these updrafts to intensify, or whether the intense updrafts make a fire-generated thunderstorm more likely to happen.

These powerful updrafts can draw in so much air that strong winds develop, drawn in from many directions towards the plume. This can cause the fire to burn hotter and sometimes spread faster. The inflowing winds can cause nearby fires – possibly caused by spot fires or lightning strikes – to change direction unexpectedly, as they are drawn into the parent fire.

Spot fires

The updrafts become very tall and because they are so strong, they can carry large burning embers and lift them to great heights. The embers are carried far downwind, where they can ignite new fires (spot fires). During Victoria's Black Saturday bushfires in 2009, one spot fire was recorded more than 30 km from its 'parent' fire.

Lightning

Lightning can form in these storms, which can cause new fires. In Victoria's Black Saturday fires, lightning from fire-generated thunderstorms started a new fire 100 km from the fire front.

You can read a research paper on this topic: Pyrocumulonimbus lightning and fire ignition on Black Saturday in southeast Australia – American Geophysical Union website.

Downbursts

Thunderstorms can produce intense downbursts that hit the ground and 'burst' outwards, producing very strong and gusty winds that can last 20 minutes or more. These winds can be strong enough to spread the fire in any direction.

Downbursts from thunderstorms near fires have been responsible for the deaths of firefighters, by blowing fire back toward them. Fire-generated thunderstorms could pose a similar threat.

Video: Weird Weather – Pyrocumulonimbus

Predicting fire-generated thunderstorms

Studying how fire-generated thunderstorms develop helps us predict their formation, intensity and danger.

Using very high resolution computer models to make virtual smoke plumes, we study:

• how the plumes behave in different wind environments
• how far they can transport embers.

For example, plumes tend to become taller in lighter background winds than in stronger winds. In stronger winds, they tend to bend over and have little turrets of smoke off the top.

Calculating firepower

We use thermodynamic (heat and energy) equations. These help us understand how air coming into the plume affects the height at which cloud can form.

Using atmospheric temperature and moisture information from a radiosonde (weather balloon) or computer weather model, we determine how:

• high a smoke plume must rise before cloud will form
• buoyant the plume needs to be for deep cloud, which may become pyrocumulonimbus, to develop
• much heat (firepower) a fire must produce to generate a thunderstorm. Forecasting firepower – when and where it will be greatest – helps fire agencies respond to the threat.

Understanding how embers travel

We're also using these equations to develop a simple model to predict ember transport. This will become part of fire spread models. 

Fire agencies use these models to help determine how fast a fire will move through the landscape.

Forecasting fire spread

With information from the fire plume computer models and thermodynamic equations, we're developing prediction tools. We're also working on a fire–atmosphere computer model.

These models can help us learn more about bushfires but need further development and testing. More powerful computers will also help deliver forecasts faster.

Climate change impact on fire-generated thunderstorms

Research continues into how climate change influences conditions that cause these thunderstorms.

With the National Environmental Science Programme, we've examined more than 30 years of data.

Black Summer bushfires 2019–20

The Black Summer fire season in south-eastern Australia contributed to an intense – super outbreak – of pyrocumulonimbus.

Our research shows increasing risk factors for outbreaks such as this. For details, see 'Future changes in extreme weather and pyroconvection risk factors for Australian wildfires', published in Nature scientific journal.

Canberra bushfires 2003 and Victorian Black Saturday bushfires 2009

We looked at conditions associated with pyrocumulonimbus events. This included data from the Canberra bushfires in 2003 and Victoria's Black Saturday bushfires in 2009.

The researchers found increased risk of fire-generated thunderstorms over recent decades in some areas. For example, for south-east Australia in spring and summer. This was based on when conditions were favourable for both very dangerous fires and thunderstorms. Their findings are available in the Pyroconvection Risk in Australia paper – American Geophysical Union website.

Likely higher risks for some Australian regions

Climate models suggest conditions associated with fire-generated thunderstorms may become more dangerous in some regions, such as south-east Australia.

For more information about climate change and bushfire risk, view our How weather affects fires page.

Fire weather warnings

View the National warnings summary.