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How hailstones are formed

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Derek Brockway Derek Brockway | 13:04 UK time, Wednesday, 14 December 2011

The weather is certainly a big talking point at the moment with more rain, hail, thunder and snow in places today.

The air over us has come from Greenland so it's cold but as it crosses the relatively warm sea around the British Isles it becomes very unstable.

The air rises sharply forming large, towering cumulonimbus clouds. These clouds produce hail and thunder. If you love clouds, check out the cloud appreciation society.

Hail showers can occur at any time of the year but are quite common during the spring in Britain and vary greatly in size.

If a hailstone is cut open, a layered structure like an onion is sometimes apparent. A large hailstone may consist of several layers of clear and opaque ice.

How hail is formed

Large hailstones fall from deep cumulonimbus clouds. The cloud base may be 3,000 feet (900m) above the ground with tops as high as 60,000 feet (18,000m).

Much of the cloud will be composed of supercooled water droplets. As the hailstone falls it will collect water droplets which instantly freeze and form a layer of ice.

It may then be caught in a vigorous updraught and, as it is carried back higher into the cloud, it collects more water droplets or ice particles to form another layer of ice.

So layers build up on the hailstone (made of alternate layers of clear and opaque ice) and the cycle may be repeated until the stone is so big that it falls to earth.

Hail history

Really large hailstones originate in thunderstorms during the summer. In July 1968 a hailstone the size of a tennis ball fell at Cardiff Airport.

An enormous hailstone from 1968. Copyright R K Pilsbury.

A 3.5" hailstone which fell on Cardiff, South Wales, during a thunderstorm on July 1, 1968. Copyright R K Pilsbury.

The largest hailstone recorded in the British Isles weighed 141 grams (5 oz) and occurred at Horsham, West Sussex on September 5, 1958.

Certainly anything approaching golf-ball size is remarkable, but hailstones can grow large enough to dent cars, shatter greenhouses, injure, and even kill people. In late October 2008 a massive hail and thunderstorm hit Ottery St. Mary in Devon.

The USA, Canada, central Europe, India and China all experience large hail. So too do land areas in the southern hemisphere.

In July 1984, a shower of giant hailstones caused about £750,000,000 worth of damage in Munich, Germany.

The heaviest hailstone (in the Guinness Book of Records) occurred in a hailstorm in the Gopalanj district of Bangladesh on April 14, 1986. The hailstones weighed up to 1kg (2lb 3oz) and were reported to have killed 92 people.

While in Nebraska, a hailstone almost the size of a bowling ball fell on June 22 2003. Measuring 17.8cm in diameter - the largest hailstone ever recorded!


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