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How did the massive hailstones that fell over southern Queensland form?

Sunday December 15, 2019 - 01:10 EDT
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Massive hailstones that fell on the Sunshine Coast on Friday. - ABC

Some of the largest hailstones ever seen in Australia are thought to have plummeted to Earth over southern Queensland on Friday afternoon.

It is .

Yet Bureau of Meteorology (BOM) senior forecaster Sam Campbell said hailstones this big were rare.

"Some really significant, in fact possibly historic, giant hailstones around parts of the Sunshine Coast and the Wide Bay, Burnett — some hail of 11-13 centimetres in diameter recorded there and that's some of the biggest hail that's ever been seen in Australia," he said.

It's hot — why does it hail?

Hailstones are clumps of layered ice which form in the air that is rising within thunderstorms, known as updraughts.

They start as water vapour condensing into tiny water droplets that exist inside the updraught.

Temperatures near the top of thunderstorm clouds are very cold, dropping as low as –60 degrees Celsius, even during summer months.

How big do hailstones get?

BOM senior research scientist Harald Richter said really large hail forms from a hail embryo — a tiny ball of ice.

"This ball is flying through a thunderstorm updraught that's collecting really cold water that's way below freezing, it's called super-cooled water," Dr Richter said.

"The longer it [the ice ball] flies through that super-cooled water, the bigger it grows.

"If it has a lot of time and strong updrafts, it can form a golf ball and in really strong updraughts it can even form a cricket ball."

Most hailstones are smaller than 25mm — about the size of a 10-cent piece.

But Dr Richter said in particularly intense storms, the upward air motion inside the updraught is so strong that even larger hailstones are suspended in the air, or fall very slowly.

In these storms, hail has more time to collect super-cooled water droplets to swell in size.

Will we see it again soon?

Mr Campbell said those conditions were unlikely to be repeated in southern Queensland on Saturday.

"I think we can safely say that won't happen today, we had an incredible environment in terms of the storms yesterday," he said.

"It's very rare that we see such instability and winds as well in the atmosphere, so nothing like that today, you don't have to worry about giant hail."



© ABC 2019

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