Learn how bushfires create thunderstorms, why this is dangerous and what we're doing to predict it.
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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.
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.
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'.
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. Ice particles collide in the very cold upper parts of these clouds. This builds electrical charge, which is released by giant sparks – lightning. Having produced a thunderstorm, the cloud is now a 'pyrocumulonimbus'.
How pyrocumulonimbus clouds develop:
These thunderstorms can be linked to dangerous and erratic fire behaviour that can make the fire more difficult and hazardous to fight.
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.
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 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.
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.
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:
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.
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:
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.
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.
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.
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.
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.
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.
View the National warnings summary.