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Over the weekend, parts of south-east and southern Queensland were pummelled by giant hailstones, which measured between 8 and 9 centimetres in the hardest-hit areas. In parts of the Darling Downs, as well as the town of Esk to Brisbane's north-west, hail wrecked solar panels, cars, windows and garden furniture in its path. Windows of homes were smashed, sending hailstones and glass flying, and causing what residents anticipate will be thousands of dollars worth of damage. Many are still counting the costs. What caused the weekend's giant hail? Over both Saturday and Sunday, Bureau of Meteorology senior meteorologist Angus Hines said the atmospheric conditions in south-east Queensland were "exceptional" for thunderstorm development and "pretty ripe" for giant hail. Supercell storms formed over parts of New South Wales and Queensland. Mr Hines said these last longer than typical storms, letting hailstones grow for a longer period of time. Australian National University climatologist Janette Lindesay put it this way: "To get really large hailstones, what you need is an extremely large thunderstorm," she said."Hail forms from rain drops within a cloud, or ice crystals within the cloud conglomerating together and freezing ..."To get really large hailstones ... they have to stay in the cloud above the freezing level for a long period of time." Professor Lindesay said hailstones are usually layers of ice formed around an initial drop of frozen water. Odd-shaped hailstones were more likely to be lots of smaller hailstones which had got stuck together and frozen into one large ball of ice. What counts as giant hail? Unsurprisingly, perhaps, the BOM does have an official definition for hailstone sizes. Giant hail is anything larger than 5 centimetres (in diameter). Mr Hines said that is about the size of a golf ball if that's easier to imagine. Hail between 2 to 5 centimetres is classified as large hail (that is more like the size of a marble). Not this time. Mr Hines said hailstones can occasionally crack double digits. In 2020, 14-centimetre hailstones were recorded around the Brisbane area. Further north, in Mackay in 2016, there were reports of 14-centimetre hail. Professor Lindesay said some of the largest to ever be recorded around the world measured up to 20 centimetres, which she said was "almost unimaginable". Can we expect to see more this summer? While Mr Hines said giant hailstones of the sizes seen over the weekend were pretty "unusual", he said it was likely Queensland would see similar storms over the summer. And Professor Lindesay agreed, saying global heating meant more energy in the atmosphere, which was needed to fuel supercell thunderstorms. "We're looking at a lot of extra heat out there, a lot of evaporated moisture, with all of this energy in it," she said. "And that is what is driving these really intense storms." Why is hail so damaging? Giant hail can cause "significant" injuries and damage cars, windows, homes and even crops which end up in its path. Mr Hines explained that is mostly to do with the height from which they fall from the base of a thunderstorm, combined with their size and weight. Professor Lindesay said it's also due to the fact hailstones reach terminal velocity – or the fastest an object can fall – as they travel towards the ground. A 5-centimetre hailstone, for example, travels at a speed of 32 metres per second, or 115 kph, she said. Professor Lindesay said when you added wind to that equation, it meant hailstones made an even greater impact when they hit a windscreen, or a solar panel, or anything else. No surprise then, that garden furniture did not stand a chance on Saturday afternoon.
