By Steff Gaulter
A tornado has torn through the Australian city of Sydney, causing millions of dollars worth of damage. The locals likened the sound to that of a freight train as the twister roared through the Kurnell neighbourhood on Wednesday, December 16. Roofs were ripped from buildings, trees were snapped like matches and windows were shattered. One resident described how the storm had deposited someone’s patio on his driveway, but he’d lost his shed. “It’s somewhere in Kurnell,” he reasoned.
In an unusual stroke of luck, the tornado passed over an automatic weather station. Tornadoes are fairly small weather features, usually not more than a kilometre across, so the chances of them passing over a weather station are very slight. This means that their wind speeds are very rarely measured directly. Instead the winds are normally estimated, based on the damage that they leave behind. The twister that hit Kurnell, however, was obliging enough to give us a direct, and therefore accurate, measurement of its wind speed.
The weather station at Kurnell has a rotating device, called an anemometer, which measured the wind speed as 213 kilometres per hour (132 miles per hour). This is the strongest gust ever recorded in New South Wales, and one of the strongest winds reported in the whole of Australia. Only cyclones have brought stronger winds to the country: an eye-watering 408 kph (253 mph) was reported in 1996 on Barrow Island during Cyclone Olivia. This is the highest wind speed ever recorded on earth, and with winds of that speed, I’m amazed that the anemometer didn’t break!
If the tornado had struck an isolated home in the countryside, the noise would have been very different. In the open air, a tornado simply sounds like loud swirling noise. In a built up area, however, people often describe a tornado as sounding like a ‘freight train’ due to all the debris which has been sucked up by the storm. The twister’s winds twist sheets of metal and rip branches from trees, meaning that what the residents actually hear is the tornado loudly destroying their neighbourhood.
The gust of wind that was reported at Kurnell suggests that the tornado was a F2 tornado on the Fujita F-Scale. The Fujita scale is used in Australia to rate the strength of tornados, running from F0, the weakest, to F5, the strongest. Even the weakest twister, those rated as F0, still have winds of between 62 and 117kph, so can cause significant damage. An F2 is extremely powerful, and it is rare for a tornado in Australia to be any stronger.
The tornado that hit Sydney in December was made even more dangerous by the type of storm from which it developed. Powerful tornadoes develop from thunderstorms, and Sydney’s storm also produced torrential rain and even some giant hailstones. The rain and the hail masked the tornado, meaning that the locals weren’t able to see it coming. The majority of people would never have known that a tornado was about to hit. They would have seen the rain and the hail, then suddenly the windows would have started to smash and the tiles would have started to fly from the roofs.
The reason that the storm that hit Sydney was so powerful was thanks to the setup of the atmosphere. Most importantly, there was a very cold patch of air in the upper atmosphere. This meant the air was very unstable. Most of us remember from Physics at school that hot air rises, so with cold air above, the hot air near the ground can rise a long way very quickly. This is the perfect condition for a thunderstorm to develop. At the same time, there were also large amounts of water in the air, which ensures the storm can form near the ground, and the winds at different heights in the atmosphere were blowing at different speeds and directions. This is important, because this encourages turning and rotation in the atmosphere, which is essential for the formation of a tornado.
The thunderstorm that developed had an element of rotation to it, and was therefore classed as a supercell thunderstorm. These are the most dangerous type of thunderstorms, and can be expected to bring hazardous weather. This could be huge hail, flash flooding, strong winds or sometimes a tornado. The surprising thing about this storm, however, was that it struck before lunchtime. This is rare, because usually the heat of the sun will supply the thunderstorm with the extra energy that it needs to turn severe. Strangely, in this case, the day hadn’t even reached its highest temperature when the storm struck.
The tornado lasted for just a few seconds, but the damage to some people’s homes was immense. It will take a long time for the building repairs to be completed, but will take even longer for the locals to recover from the shock.
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