Swiss storms produce hailstones the size of tennis balls

Storms that swept across Switzerland produced hailstones up to 6 centimetres in diameter, injured dozens of people and caused extensive damage, SRF reported. Meteorologist Simon Eschle of SRF Meteo said the weather situation was highly unusual.

A storm cell carrying hail travelled from Lake Biel to Lake Constance across Switzerland. Hailstones found after the storm were as large as tennis balls. At any specific location, an event of this kind occurs statistically only once every 30 to 50 years, Eschle said.

The storm cell was also unusual because it remained active for a long time. Large hailstones form when strong updrafts keep them inside a storm cloud, allowing them to grow.

The storms developed ahead of a cold front in conditions that combined sufficient moisture, an unstable atmosphere and a trigger that allowed air to rise. The storms also brought widespread damaging gusts. A wind gust of 142 kilometres per hour was measured in Schaffhausen, while high-voltage pylons were blown over in Aargau.

Cold air sinking inside thunderstorms creates strong winds. When rain evaporates in that cold air, it cools further, making the air heavier and causing it to sink faster. This can produce powerful, localised gusts known as downbursts.

Extreme weather events are generally becoming more frequent as a result of climate change, Eschle said. Heavy rainfall is already occurring more often and with greater intensity, and is expected to increase further. Climate change could also lead to larger hailstones.