Warm moist air condenses at an altitude of 4000-5000 meters. Small ice pellets form and can cause great damage to land, buildings and cars.
Hail is among the most costly natural hazards in Switzerland. A list from the Association of Canton Fire Insurers (VKF) shows that between 1933 and 2012, hail damage to buildings – not including vehicles – amounted to around 1.6 billion Swiss francs.
In 2004 for example, a single hailstorm resulted in claims paid out to the amount of 100 million francs. There are no comprehensive statistics on hail damage to cars however. Conversely, the Swiss Society of Hail Insurance (Schweizer Hagel) has been keeping track of the damages to agricultural crops. In total, between 1972 and 2011 more than 1.5 billion francs were paid out in Switzerland for insured hail damage in the agricultural sector. Since the 1990s, the scale of the damage has significantly increased, according to Schweizer Hagel.
Some of the areas in Europe most under threat from hail are located in Switzerland. The danger is greatest in the foothills of the Alps from the canton of Bern into eastern Switzerland. On average, inhabitants here have to contend with damage caused by hailstones 1 cm in diameter once a year. Further danger zones are the western Bernese Oberland, southern Ticino, the Lake Geneva region and the central and eastern Jura Mountains.
The occurrence of hail first requires warm, humid air. On warm, sunny days, hot airrises, since it is lighter. As it rises, it cools. Above a certain height – the dew point – it condenses into tiny droplets, which are visible as clouds. Hereby the warmth is released. This forms an updraft, inside of whichmore water condenses and more heat is released. The updraft intensifies and can reach speeds of up to 100 kilometers an hour. Above a height of 4,000 – 5000 meters the droplets freeze and become hailstones. When falling into the clouds, they can be recaptured and sucked into the updraft as many as five times – all the while gradually increasing in size by gaining layers until they become too heavy and fall to the ground. The biggest hailstones found in Switzerland had a diameter of 10 cm.