The threat of giant hailstones

Many homes were damaged by a hailstone storm in Sydney in 1999

Giant hailstones, which badly damaged a jet flying over Germany on Monday, are very rare but often wreak havoc when they appear.

The stones created a hole the size of a football in the nose of the BMI plane and also cracked its windscreen as it was flying from Cyprus to Manchester.

But giant hailstones have not just caused damage to unfortunate aircraft in the past.

Livestock have been killed, crops damaged and roofs and power lines destroyed by the freak storms.

In 1999, hailstones the size of cricket balls fell on Sydney, Australia, causing damage costing an estimated 150m Australian dollars and ruining 100,000 cars.

There have been records of hail damaging crops and smashing through conservatory roofs in this country

Paul Mott, meteorologist

House fires broke out across the city as the power lines were brought down while 15,000 homes were left without electricity.

The Belgian town of Ghent was also hit by hailstones in the same year with 19,000 brand new cars being damaged, seriously disrupting car supplies for Honda.

And in eastern China earlier this year, 105 people were injured and damage estimated at £4m was caused to homes in the Zhejiang province.

Meteorologists say the stones are formed when temperatures drop below freezing and small water droplets in clouds collide and join together.

'Very rare'

They normally fall to earth when they become too heavy but giant hailstones appear when big updrafts stop the effects of gravity.

Paul Mott, a meteorologist at the Press Association's weather centre, said: "I would think that it would be very rare indeed to get hail of a size that would actually damage an aircraft.

"For hail to do such damage is rare because clouds usually cannot hold hail of that size."

But Mr Mott warned that it was more common in the UK than some people would realise.

He said: "There have been records of hail damaging crops and smashing through conservatory roofs in this country."