Australia Weather News

The fire burning in north-east Victoria near Walwa was so fierce itgenerated its own weather on Thursday afternoon, including a substantial pyrocumulonimbus cloud with lightning and thunder.

Pyrocumulonimbus clouds (pyroCb), also known as are fire generated thunderstorms, are a dangerous fire behaviour that occurs during extreme bushfires.

Radar images showed the fire-generated storm building through the early hours of the afternoon at Mt Lawson, in the north-east corner of Victoria, with lightning strikes. 

It weakened a few hours later, but radar images showed another may have formed just after 7pm.

But what is a firestorm, and why does it present such a challenge for firefighters and forecasters?

How do pyroCBs form?

Fire-fuelled thunderstorm clouds form when the intense heat from a fire causes air to rapidly rise in the smoke plume.

The rising, turbulent air draws in cooler air, which helps to cool the plume.

Just like the formation of a normal thunderstorm, when the plume rises high enough, low atmospheric pressure will cause its air to cool, and the moisture in the plume will condense and form into a cloud.

In an unstable atmosphere, pyrocumulonimbus clouds will form and begin to discharge lightning strikes.

What conditions do pyroCBs form in?

University of New South Wales bushfire scientist Rick McRaehasdone extensive research on pyroCB clouds.  

He said to see a pyroCB cloud meant the landscape was very dry and ready to burn with a lot of energy. 

"I saw yesterday that the river flow in the [Albury] area is drying out catastrophically, " he said. 

"That says there's no water in the landscape. And that means that all the burnable biomass is fully flammable. So a large log lying on the ground will turn to white ash in two hours," he said.

"So that [would be] adding a lot of extra heat to the fire that you don't normally get."

How dangerous are pyroCBs?

Pyrocumulonimbus clouds can make what is already a dangerous fire situation less predictable and harder to manage.

Professor McRae said this was for a few reasons.

The first was theerratic winds.

The clouds can generate intense updrafts, which then cause other air to be drawn in from all directions to replace it.

Professor McRae said this changed the winds that were pushing the flames around on the fireground, making it very hard to know where it would move.The powerful winds can also cause the fire to burn hotter and sometimes spread faster.

"When you've got winds that are really, really complicated, you get a lot of spot fires, and these spot fires tend to try and merge with each other," he said.

"You don't know where the wind is pushing the fire."

They also cause lightning,which can start new spot fires well away from the main fireground.

"By the time the plume has risen and cooled (enough to cause lightning) it could be 40, 50, 100 kilometres downwind," Professor McRae said.

Lightning but no rain

Unfortunately, Professor McRae said they don't help produce rain.

This is because there are so many dust particles in the plume that tiny water droplets stay aloft, instead of growing heavy enough to fall as rain.

"With a cloud, you've got dust particles and the water vapour in the air condenses on the dust particles if it cools, and it forms a water droplet," he said.

"Normally, these dust particles are in short supply, so the water droplets can get big enough for gravity to grab them and pull them down, and we call that rain.

"However, in a smoke plume, there's a billion times more condensation nuclei, so the water droplets are all tiny, and gravity can't grab them, so they keep rising."

Fire tornadoes and black hail

In very rare circumstances, the thunderstorm has also been known to produce a tornado and something known as black hail.

This happened during the 2003 Canberra bushfires, in which four lives were lost and almost 500 homes destroyed.

It was the world's first fire-generated tornado to be documented, with the tornado attached to the cloud base, meaning it was able to lift off the ground multiple times.

"[Fire-generated thunderstorms are] a very complicated situation," Professor McRae said. 

How long do pyroCBs last?

Professor McRae said they tended to have a life span of about three hours before the feedback loop of the storm breaks down.

But McRae said you could often see more than one popping up.

"You can have a fire where another one will start elsewhere on the fire. You might have, say, three fires on the landscape and another fire will kick in. Or they can just literally pulse if the bad conditions continue," he said.

How common are they?

Increasingly so, according to Professor McRae.

While they're not a regular occurrence, Australia has seen this kind of fire behaviour develop on a number of occasions, the first of which was during the Ash Wednesday fire in 1983. 

Since then, they've been witnessed in a number of different states, including the ACT, Victoria, NSW and south-west WA.

Professor McRae runs a website called the pyroCB register, which keeps track of instances of fire-generated thunderstorms.

It shows that before the year 2000, Australia had seen fire thunderstorms but they were rare, with seven in the space of 35 years.

But since the year 2000, there have been over 100 fire thunderstorms, according to the register.

"During Black Summer, the count of fire thunderstorms doubled. That's just in one season," he said.

He said this was making the need for rapid adaptation more urgent.

"We're evolving, but it's rapid adaptation because climate change keeps accelerating the evolution of the problem," he said.

What should people on the ground do?

Fire-generated thunderstorms can sound very scary.

But Professor McRae said what was most important for people on the ground was to listen to and follow the advice of fire authorities.

"The most important message is do what the fire service says you should do," he said.

"If the fire service says go, you pay attention to that message."