Hail stones the size of eggs have brought death and destruction to China's central Henan province.

Isobel Lang, Broadcast Meteorologist, looks at how hail forms and the world of giant hailstones.

Spring often sees changes in the weather. The atmosphere is still fairly cool but the sun is getting noticeably stronger and higher in the sky. With these marked contrasts the weather can often become rather lively with more widespread and heavy showers - often with hail.

Large hailstones fall from deep cumulonimbus or storm clouds. These dense, towering clouds go right up into the atmosphere - up to about 30,000 feet - where it turns icy. This icy section at the top can flatten out to form an anvil shape.

The strong updraughts and downdraughts within the cloud cause countless collisions between water droplets and ice crystals. Hailstones form when water freezes, time and time again, onto ice crystals and they build up several layers of clear and opaque ice. This process continues until the stone is so big that it falls to earth.

The largest hailstone recorded in the British Isles weighed 142 grams and fell at Horsham, West Sussex on the 5th September 1958. Hail can cause serious damage as you can imagine - denting cars and shattering greenhouses.

Even heavier and larger hailstones have been reported in other parts of the world. Such giant hail falls from supercell thunderstorms. The stronger updraughts help to hold larger ice particles aloft for longer and hence the hailstones grow bigger.

The most damaging hailstorm to date in Australian history occurred in Sydney, New South Wales on the evening of 14 April 1999. This violent storm produced hailstones 9cm across! The insurance payout was approximately $1.5 Billion.

One of the most notorious hail areas is that area of America from Texas to Montana, and from the foothills of the Rockies to the Mississippi River, known as "Hail Alley". Total hail damage each year in the US costs over $500 million and some incredibly large hail stones have been reported.

The largest hailstone ever recorded in the US measured about 44cm in circumference and weighed more than 750 grams (26 ounces)! It fell in Kansas on September 3rd, 1970.

Related links:
Weatherwise Factfiles: Hail
BBC News - China Hailstorm kills 22

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