Some of Australia's most devastating bushfires have fuelled their own fury by creating thunderstorm clouds capable of producing dry lightning, violent winds, black hail and even a fire tornado.

The unpredictable changes brought on by the storms can be extremely dangerous, and make fighting fires even harder.

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

How do fires create their own weather?

Fire-fuelled thunderstorm clouds are called flammagenitus, but they are commonly known by the name pyrocumulonimbus.

Pyrocumulonimbus clouds form when the intense heat from a fire causes air to rapidly rise in the smoke plume.

Fires can create their own thunderstorms in the form of a pyrocumulonimbus cloud. ( Supplied: BOM )

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

When the plume rises high enough, low atmospheric pressure will cause its air to cool even further and the moisture in the plume air will condense and form into cloud.

In an unstable atmosphere, pyrocumulonimbus clouds will form.

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How dangerous can these firestorms be?

Pyrocumulonimbus clouds present a huge challenge for firefighters and forecasters by triggering dangerous and unpredictable changes in fire behaviour.

"The prediction of fire weather in terms of wind is critical and when you've got a highly variable wind environment as you do with a thunderstorm, if you have that in the fire environment, those winds become very, very difficult to predict," Bureau of Meteorology spokesman Neil Bennett said.

"The other issue that you have is if it does get to the state where a thunderstorm can form, you are then going to have lightning, [which] can then set off fires in areas where there were none before — it can be an ignition point.

"And then the other problem that we have is that the embers can be swept up and deposited a long way away, so spotting can occur a long way away from the thunderstorm and from the original fire front.

"So you've got a number of real problems if one of these systems develops. It can create major headaches for the authorities if they're trying to fight the fire."

A pyrocumulonimbus cloud hovers over bushfires in the Grampians in Victoria in 2014. ( Twitter: @MountZeroOlives )

Dry lightning — sparks, but no rain relief

Melbourne University Associate Professor Kevin Tolhurst, who specialises in fire ecology and management, said lightning had been known to spark new fires several kilometres away.

"We've certainly seen a number of instances in the past where new fires have started 20 or 30 kilometres downwind of the main fire from lightning that's been created by the pyrocumulonimbus cloud that was created by the fire itself.

"So the fire is creating this electrical storm that's igniting new fires," he said.

Lightning can spark fires far from the main blaze. ( Audience submitted: Jordan Cantelo )

Mr Bennett said this phenomenon was referred to as "dry lightning" because it was rarely accompanied by rainfall.

"Typically in these sorts of environments, you have very dry air in the lower part of the atmosphere because for a fire to really take hold, you need low humidity," he said.

"So if you do get rain that forms out of these [weather systems], and it's not unheard of, it's possible that it would just evaporate before it hits the ground."

Intense, unpredictable winds as damaging as the fire itself

Pyrocumulonimbus clouds can generate intense updrafts that draw in so much air that strong winds develop, coming from all directions towards the plume.

"When you have a large bushfire burning a lot of fuel at once, the rapid rising heat draws in air around it, so when you have a very large fire with a big convection column or smoke plume, the air gets drawn in," Associate Professor Tolhurst said.

"What we saw for example on Black Saturday in Victoria in 2009, was there were over 200 areas where trees had been snapped off or pulled out of the ground because the winds were so strong.

"And for those sorts of damages to occur would require winds of around 120 or 140 kilometres per hour.

Lightning strikes were blamed for many of the fires which tore across the Tasmanian landscape in January. ( Supplied: Nelle Degrassi )

"If the winds are getting up to 120–140 kilometres per hour, you can see actual physical damage to buildings whether or not the fire even gets to them … so roofs lifted off and windows broken."

Fire-induced winds could cause a blaze to spread faster or cause nearby fires, which may have been sparked by spotting or lightning strikes, to change direction unexpectedly as they are drawn into the main fire.

It gets worse — firenadoes and black hail

In 2003 the world's first-known fire tornado or "firenado" was observed in Australia, when it carved a path of destruction during the Canberra bushfires in which four lives were lost and almost 500 homes destroyed.

"A fire tornado is where the air is not only rising but it also starts to spin," Associate Professor Tolhurst said.

"In even relatively small fires you'll see what we call fire whirls, which are like mini tornadoes, and they will move across the ground and they may throw embers and spread the fire, and that can be quite scary if you see them but they don't last very long and they're only a few metres across.

"A fire tornado is usually hundreds of metres across and will persist for much longer.

"The Canberra fire tornado, for example, basically went up to the top of the troposphere, so it was more than 10,000 metres high and it was 200–300 metres wide … and it travelled for more than 20 kilometres across the ground damaging trees and houses as it went."

This fire whirl was snapped at the Queensland town of Thargomindah in 2012. ( Supplied: Sherilee Honnery )

Firestorms also have the potential to produce what is known as "black hail".

"In the convection column you've got about 40 per cent moisture, but there's also a lot of ash and soot in the smoke column as well, it's part of the reason the smoke column is so dense you can't see through it," Associate Professor Tolhurst said.

"So in the process of the water vapour condensing, as it rises further it goes from a liquid into a solid — an ice particle.

"And those ice particles can then aggregate into a hail stone and the hail stones will incorporate the soot and the ash that's in the column as well, so you can get dirty hail stones."