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Fallstreak holes decorate sizzling central Queensland skies with hearts and angel wings

Jodie van de Wetering, Monday October 9, 2017 - 08:14 EDT
ABC licensed image
A fallstreak hole captured above the Keppel islands. - ABC licensed

Unusual cloud formations have been decorating central Queensland's skies with hearts and angel wings, framed by holes in the surrounding cloud cover.

They're fallstreak holes, or hole-punch clouds, triggered by freezing water droplets high in the atmosphere.

ABC Open contributor and writer Helene Young spotted one while sailing around the Keppel islands, east of Rockhampton off the Queensland coast.

"It was probably the result of an aircraft taking off from Rockhampton, heading north, and passing through the layer of cloud," she said.

"I prefer to think we had a visit from an angel."

Harry Clark from the Bureau of Meteorology says fallstreak holes form when ice crystals start forming in clouds made of supercooled water droplets — water that remains liquid despite being below freezing point.

"They're quite a fascinating phenomenon, not overly uncommon but still really exciting when you get to see one," Mr Clark said.

"Once those water crystals start freezing, they start falling out, and that's that little wispy bit that comes out.

"As they freeze they eat away at the parent cloud, leaving the hole.

"So really what you're seeing is a cloud almost cannibalising itself as it turns to ice crystals."

Mr Clark agrees with Ms Young that aircraft are a common trigger for fallstreak holes.

"Sometimes just a plane flying through the cloud is enough to stir it up and start the process," Mr Clark said.

"Sometimes they can start spontaneously if there's a little bit of wind or some sort of perturbation.

"Once it gets going it just gets bigger and bigger, and that's why those holes look quite circular and form in an outwards direction."

Too high for hot weather

Even though Queensland has sweltered through some record high temperatures in recent weeks, Mr Clark said the high altitudes where fallstreak holes formed were still icy cold.

"These clouds form quite high up in the atmosphere, usually around 10,000 feet [3,000 metres] or higher, because that's where the freezing level is.

"Even though it can be quite warm at the surface, these clouds still exist quite high up in the atmosphere at below freezing levels."

While central Queensland's recent fallstreak holes have spread heart and wing shapes across the sky, Mr Clark said the shape of the wispy centre was entirely random.

"It's really up to the chaotic nature of how it formed," he said.

"The prevailing wind at that height can have an impact on which way the crystals form, but it's all a bit of a mystery.

"It's one of those beautiful things that you can explain with theory, but getting into the intricacies can be quite difficult.

"They are pretty stunning, especially on sunset; if you get the bottom part illuminated it can be a really spectacular sight."


© ABC 2017

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