After a month of severe weather across swathes of eastern Australia, a leading expert on hail formation has explained how tiny stones can grow to the record-breaking size of a mango.
Tornadoes spawned by supercell thunderstorms over the past fortnight have ripped off roof panels and sent cargo pods tumbling at Brisbane’s airport and flipped cars and felled power lines at Armidale in northern NSW.
Wild winds and heavy rain have also torn through Noosa, parts of the Gold Coast, Sunshine Coast and Moreton Bay in Queensland and areas of regional NSW.
Victoria has most recently copped it, after destructive winds moved across the state on Thursday night, tearing down trees, damaging homes and causing widespread blackouts.
On October 19, however, it was hail which inflicted the most damage as freak storms lashed Queensland’s Mackay region.
A stone measuring 16 centimetres in diameter, which fell near Yalboroo, eclipsed the Australian record of 14 centimetres set in 1991 in northern NSW at Kempsey and again in 2020 in southeast Queensland.
The day after the Mackay episode, such an accumulation of smaller pellets blanketed the ground at Coffs Harbour on the NSW midnorth coast, and looked like snowfall.
Stones sometimes reach the size of a golf ball or even a cricket ball, says the Bureau of Meteorology’s Joshua Soderholm.
But for truly large hail to form, conditions must be just so.
“Hail starts as water vapour near the ground,” he said.
“This condenses into tiny water droplets, cooling as it rises inside a thunderstorm updraft, collecting really cold water that’s below freezing.
“This freezing water is called supercooled water and the longer the droplets – and eventually ice – fly through the supercooled water within the hailstorm updraft, the bigger it grows.
“So, whenever you see giant or large amounts of hail on the ground, a lot has had to happen for it to get there. It is also why there is not hail in every storm.”
Dr Soderholm says hail can occur at any time of the year but major hailstorms are most common in spring and early summer.
“This is a time of the year when surface temperatures are warm enough to promote the development of intense thunderstorms,” Dr Soderholm said.
“But you also need the accompanying ingredients of strong winds and cool air in the upper atmosphere to support hail growth.”