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  • German study: hail damage could increase by as much as 50% by 2070
  • Science: link between increase of hailstorms and global warming
  • ‘Hail Alley’: the area in the US receives the highest frequency of large hail in North America
  • Preventative measures for farmers, manufacturers and home owners

The Himalayan Mountains have long kept a dark secret. In 1942, hundreds of human skeletons dating back to the 9th century were discovered around an upland lake in northern India. They had all died at the same time. But not until 2007 did scientists offer an explanation for their mysterious demise.

All the bodies showed similar wounds: deep cracks in the skull. Scientists came up with a stunning explanation. They were killed by cricket-ball-sized hailstones.

Incredible as it may sound, the story of ‘Skeleton Lake’ is a warning of what we might expect in the future.

As climate change warms the atmosphere, climatologists expect to see increasingly violent weather increasingly often. That could mean bigger hailstones falling more of the time.

Hailstones measure at least 5mm in diameter – smaller stones are sometimes called sleet or ice pellets. Hailstones with a diameter of 2.5cm (1 inch) or more can leave small dents in the bodywork of cars, or damage in roof tiles or house facades. In extreme cases, hailstones can be the size of tennis balls, baseballs or grapefruits.

This is one reason why hail damage to people’s homes could increase by as much as 50% by 2070, according to a study conducted by the Association of German Insurers (GDV). In the US, the insurance claims resulting from hailstorm damage increased 84% in 2012 from their 2010 level, according to the National Insurance Crime Bureau.

Hail can occur anywhere in the world. But there are certain regions that experience more severe or more frequent storms, especially between May and September: namely the US, central Europe, western China and northern India.

The five costliest and largest hailstorms worldwide

The climatic conditions that produce thunderstorms and hail are very similar in these regions: air rises up a mountain range, intensifying the updrafts required for hail to form. If very warm, damp masses of air form at the same time, this produces extreme weather events like tornados, thunderstorms or hail. In Germany, southern parts of the states of Bavaria and Baden-Württemberg are hit particularly often.

The most recent major hailstorm in Germany happened in Baden-Wuerttemberg at the end of July 2013. The tennis ball-sized hailstones de-leaved trees, smashed windows and even roofs. The storm left thousands of damaged cars in its wake. People had to be hospitalized because of head injuries. The reinsurer Munich RE put the damage at about €600 million ($790 million)

There are various other reasons for the increase in the number of claims caused by extreme weather. First, studies like that published by the US National Oceanic and Atmospheric Administration in the journal ‘Geophysical Research Letters’ indicate a link between global warming and more extreme precipitation.

Second, the volume of insured assets is on the rise -- especially in Asia, which is experiencing strong population growth and prosperity levels.

Finally, technical developments such as solar panels mounted on roofs or housing insulation can make residential buildings more susceptible to hail damage than they were in the past, meaning that smaller hailstones can cause more damage

Whether a refreshing summer rain, a menacing thunderstorm or a full-blown hailstorm -- even the weather experts can only make an educated guess at what a particular cloud in the sky really has in store.

Radar forecasts can provide hour-by-hour images of thunderstorm cells forming and can generate storm warnings. The strength of the radar signal depends on the volume and form of precipitation – strong signals suggest that hail is on its way.

"Hail usually comes hand-in-hand with a violent thunderstorm," says Markus Stowasser, meteorologist and climate expert at Allianz SE Reinsurance. "But even when using modern forecasting methods, hail forecasting still bears considerable uncertainties."

In the US, Colorado, Nebraska and Wyoming are known as ‘Hail Alley.’ The area receives the highest frequency of large hail in North America. Residents in Colorado experience three or four catastrophic hailstorms with insured damages of more than $25 million every year, according to the Rocky Mountain Insurance Information Association. In Germany, Allianz pays out an average of around €140 million ($185 million) a year to its customers to compensate them for hail damage

People who take prevention measures to protect their property can limit the potential damage. Individuals can park their car in the garage when they hear a storm warning. Farmers can cover particularly delicate plants or move equipment into a shed. Farmers or manufacturers in susceptible regions can also take preventative action by not growing certain types of plants, or planning enough roofed or indoor areas. Weather warnings via mobile phone can also contribute to mitigation of damage as well.

Meanwhile, insurance companies together with engineers in the US are exploring ways to protect houses from hail. To test protective materials for roofs and walls they are building model houses and then bombarding them with artificial hail stones.

The largest ever recorded hailstone in US history fell on July 23, 2010, in Vivian, South Dakota, measuring about 20cm (8 inches) in diameter and weighing 900 grams (1.94 pounds).

It is very likely that the people from Skeleton Lake were bombarded by similar sized hailstones. Without any protection beyond the tree line and no warning they had no chance

Michael Grimm

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