October thunderstorms: Large hail in France

It may be October, but it's certainly feeling more like summer in the midst of all these thunderstorms.

Following damaging flash floods across parts of England on Thursday, France was also in the firing line with an outbreak of severe thunderstorms developing, particularly across central areas.

Large hail stones, like the ones pictured above from Mur-de-Sologne, southwest of Orléans, was particularly hard hit. Hailstones the size of golf balls rained down on people in the intense, thundery bursts, causing damage to buildings, cars, trees and homes.

On the WeatherRadar video above you can see the evolution of these thunderstorms from around 4pm on October 20th, as they joined to form one large cluster. Over 50,000 lightning strikes were recorded by the time the storms reached the border of Germany.

Normally, such strong thunderstorms tend to occur in summer, as warm air can absorb more water vapour. Water vapour is the energy source for showers and thunderstorms.

The more energy there is, the higher the risk of storms. Since it is usually cooler in October in France, thunderstorms are less likely. However, the cause was a low over the Atlantic that transported mild air from Spain to Central Europe.

The dew point was above 18C at the time across parts of France, providing an unusually large amount of energy available for the time of year.

In addition, the wind shear also plays a crucial role. Wind shear is the change in wind speed and direction with altitude. If this is large between the ground and about 6km above the ground, the risk of thunderstorms also increases.