A summer storm is a fascinating weather phenomenon. It purifies the air and brings welcome freshness. But often it also brings an unwanted hailstorm with it. So what is hail actually? And why does it come in summer when it's otherwise warm?

Clumps of ice with a diameter of 0.5 cm are considered hailstones, whereas anything smaller than that is sleet. They develop in summer storm clouds when supercooled water freezes into ice and is then enlarged layer by layer by upwinds within the cloud. Hailstones with a diameter of 2 cm or more can cause damage to cars, windowpanes or tents. The biggest hailstone ever recorded fell to Earth in 2010 in South Dakota, USA. It weighed 875 g, and with a diameter of 20 cm it was almost as big as a football.

People have been trying to prevent hail ever since ancient times. While ancient populations used sacrificial rituals or hail processions, more recently technological methods have been applied: The "injection" of silver iodide into clouds is supposed to reduce the size of the individual hailstones through the creation of additional crystallization cores and thus to make them more harmless - but its effectiveness is disputed. In agriculture, nets are sometimes strung up to protect plants from hail. As a private individual, you can protect yourself by keeping delicate objects in the house and your car under a roof. But that doesn't always work - because hailstorms generally occur unexpectedly.

What makes hail so hazardous? The bigger it is, the faster it falls: Small balls of ice travel at only around 35 km/h, whereas bigger balls of 2 cm in diameter or more can beat down on plants, roofs or cars at a speed of 70 kilometers per hour. Thus a brief hailstorm can quickly transform a car's bodywork into corrugated iron, mangle the new rattan lounger or reduce the coming autumn's apple harvest to pulp.

With cars, the cost of the repair of hailstones is covered by the single vehicle accidental damage insurance. Thanks to modern beating/pulling techniques, bodywork dents caused by hailstones can now generally be "evened out" within one or two days without the paint being damaged. The outdoor furniture on the terrace or the terracotta sculpture will be covered by the contents insurance, and a special additional module can be added for elaborately cultivated gardens or expensive ornamental plants. If hailstones are as big as hazelnuts they can even shatter single-glazed windows, for example on older buildings. Such claims are covered by the buildings insurance.