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Pyrocumulus cloud: How fires can create their own weather systems

Mark Rigby, Monday July 10, 2017 - 15:58 EST
ABC licensed image
A pyrocumulus cloud forms over a fire near Batchelor in the Northern Territory. - ABC licensed

As the old saying goes, where there's smoke, there's fire. But when conditions are just right, where there is fire there will also be cloud.

Clouds forming above fires are a phenomena commonly called pyrocumulus but officially they are known as flammagenitus.

Bureau of Meteorology (BOM) senior forecaster Chris Kent said pyrocumulus clouds were formed by a rising column of hot air coming off fires.

"It's rising rather rapidly at times, and as it rises it's also cooling," he said.

"If there's a lot of moisture in the air then you get water vapour starting to condense into clouds above the fires."

Like other clouds, pyrocumulus clouds that get large and heavy enough will eventually drop their moisture in the form of rain.

"It doesn't generally happen up here in the [Northern] Territory because there's usually only a shallow layer of moisture and the clouds don't really grow too big," Mr Kent said.

"But there are cases in the southern states where you actually get storms developing off the top of fires, especially in the bigger bushfires."

Unusual but not unique

Mr Kent said there was no real formula that stated a fire had to be a certain size to create pyrocumulus.

"If the column of air is large enough and there's enough moisture in the air then it will condense," he said.

"And if it does reach a reasonable height in which it can condense, then a cloud will form on top of the smoke plume."

He said in that sense pyrocumulus were not entirely different from other forms of cloud.

"They're still formed by rising air, it's just that with pyrocumulus you can see the smoke underneath it.

"If you took away that smoke you'd still have a rising air column and it would just look like a normal cloud."

Popular little puffs

Mr Kent said the rarity of pyrocumulus, particularly in the Northern Territory, made them popular with weather watchers.

"They're definitely not something you see every day, so when it does occur it does strike a chord with people," he said.

"A lot of the time during the dry season we don't have that moisture in the air so the fire's warm air doesn't get the chance to condense.

"And we don't see fires burning too much in the wet season when there's a lot of that moisture around."


© ABC 2017

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