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| Why does it hail at the peak of a heatwave? | Why does it hail at the peak of a heatwave? |
| (2 months later) | |
| So this is what it has come to. Three weeks of blazing heat – the kind where you go to bed knowing that it will be hot all night and hot still in the morning, the kind where you can actually plan to go to the beach in a few days and not worry about checking the forecast in between – and it all ends in the middle of the night with hailstones in your bed. At least, in the beds of those who had left their windows open and thrown off their duvets in parts of south London on Monday night. The Met Office is expecting more on Tuesday. Thunder and lightning are familiar punctuation marks to heatwaves – isn't this always how they end, not with a whimper but a bang – but how come there is hail when it's so hot? | So this is what it has come to. Three weeks of blazing heat – the kind where you go to bed knowing that it will be hot all night and hot still in the morning, the kind where you can actually plan to go to the beach in a few days and not worry about checking the forecast in between – and it all ends in the middle of the night with hailstones in your bed. At least, in the beds of those who had left their windows open and thrown off their duvets in parts of south London on Monday night. The Met Office is expecting more on Tuesday. Thunder and lightning are familiar punctuation marks to heatwaves – isn't this always how they end, not with a whimper but a bang – but how come there is hail when it's so hot? |
| Predictably, you need to look to the skies to find the answer. "Even on a sunny day, the temperature falls rapidly as you go up," explains Dr Pete Inness, teaching fellow in meteorology at the University of Reading. "It can be 30C on the ground but the temperature falls by about 10C for every kilometre you go up – so the freezing level is around 3,000m above the ground. Hail forms in a cloud that is somewhere between 3km and 10km above ground. At the top of the cloud, 10km above ground, the temperature could be -59C or -60C." | Predictably, you need to look to the skies to find the answer. "Even on a sunny day, the temperature falls rapidly as you go up," explains Dr Pete Inness, teaching fellow in meteorology at the University of Reading. "It can be 30C on the ground but the temperature falls by about 10C for every kilometre you go up – so the freezing level is around 3,000m above the ground. Hail forms in a cloud that is somewhere between 3km and 10km above ground. At the top of the cloud, 10km above ground, the temperature could be -59C or -60C." |
| The hail sits in the cloud, biding its time, getting bigger and bigger. "Eventually, it gets big enough to fall out," says Inness. "It is melting on the way down, but if it formed in a cloud that is cold enough, it will be frozen still when it hits the ground." It takes a hailstone around a minute and a half to reach land. | The hail sits in the cloud, biding its time, getting bigger and bigger. "Eventually, it gets big enough to fall out," says Inness. "It is melting on the way down, but if it formed in a cloud that is cold enough, it will be frozen still when it hits the ground." It takes a hailstone around a minute and a half to reach land. |
| Apparently, not only is a hailstorm not a freak occurrence in summer, but, to those in the know, it is the meteorologically unexciting endnote to a meteorologically unexciting summer. "I wouldn't say anything out of the ordinary has happened this summer," says Inness. "The temperatures haven't been excessive. We haven't broken any records. We are not jumping up and down saying: 'Yeah, 2013, isn't it amazing?'" It's good to get these things in perspective. | Apparently, not only is a hailstorm not a freak occurrence in summer, but, to those in the know, it is the meteorologically unexciting endnote to a meteorologically unexciting summer. "I wouldn't say anything out of the ordinary has happened this summer," says Inness. "The temperatures haven't been excessive. We haven't broken any records. We are not jumping up and down saying: 'Yeah, 2013, isn't it amazing?'" It's good to get these things in perspective. |
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