Hailstones in summer? Actually, it’s not so weird…
Digital News Editor
Tuesday, July 29, 2014, 11:57 GMT -
Huge thunderstorms, which passed through areas such as Brighton and Hove, delivered torrential downpours and, to many people’s surprise, a thick, white scattering of hailstones.
At first, hailstones in summer seem very strange. How can ice, falling from the sky, occur in July? What is happening to our weather?
Actually, it is perfectly normal for hailstorms to occur during the summer. In fact, most hail observed in the UK happens during the warmest months of the year.
Why does hail happen in summer?
Hailstones form in the godfather of all clouds – the giant, towering cumulonimbus. These enormous storm clouds, which we often link to thunderstorms, rise in the sky to great heights.
These clouds grow especially large during the summer, as the strong sun heats the ground, causing warm air, known as ‘thermals’, to rise and form the cloud as the water vapour within the thermal condenses. Within the cloud, air circulates up and down and this movement of air is known as an updraught or a downdraught.
Layers of ice form
As cumulonimbus clouds extend so high into the atmosphere, the top parts of the cloud become extremely cold, allowing ice crystals to form. These ice crystals start to fall within the cloud, and, as they do so, they make contact with water droplets, which freeze on contact with the ice crystal.
However, the updraughts in a cumulonimbus can be so strong that the ice crystal does not fall out of the cloud altogether; it gets carried back upwards again, allowing yet more water droplets to freeze onto the now-forming hailstone.
A hailstone can go up and down within a cloud a number of times, steadily growing in size. Eventually, the mass of the hailstone will become so large that the cloud can no longer support it, and it falls down to the earth.
This process of layering is known as riming, and is clearly evident if a hailstone is cut in half and examined under a microscope: layers of clear and opaque ice appear, just like an onion.
But it’s really hot outside - why doesn’t it melt?
Hailstones generally fall too fast to melt before reaching the earth, even if the temperature of the air around them is much higher than 0C. Snow, on the other hand, falls a lot more slowly than hail, so during the summer months will melt well before it reaches the ground.
What the hail?
- Some hailstones can be as large as a grapefruit, weighing up to 1kg. Sometimes, when hailstones stick together, they can be as heavy as 4kg!
- A hailstone, which fell in Nebraska in 1970, weighed 758g and measured 17.5 inches in circumference.
- Largest hailstone in the UK fell in Horsham on 5th September 1958 and weighed 142 grams.
- In 1959, a 10-mile area of northwestern Kansas was covered, by an accumulation of hailstones, which were 18 inches deep!
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