Weather report. How do hailstones as big as a tennis ball form?

Broken windshields, broken glass, damaged ceilings … Violent hailstorms the size of a tennis ball fell in various departments of France. The bad weather linked to the violent storms that followed the heat wave that hit France in recent days. But how do such large hailstones form? Let’s take stock.

Non-standard sizes

Hailstones more than 8 cm in diameter fell in places in France, especially in the west and center, on the night of Sunday 22 May to Monday 23 May 2022.

In Niort, in Deux-Sèvres, hailstones the size of a petanque ball fell, a journalist reported on Twitter.

In Châteauroux, the Météo Center association is also transmitting the damage to cars whose windshields and windows were shattered by hail.

Hail formation

Hail is formed within a thunderstorm, called a cumulonimbus cloud. This large cloud develops vertically above a base, located at an altitude of about 1 to 2 km, and rises to an altitude of 12 to 13 km in France. Within this column of water or ice, which extends about 10 km thick, there are upward and downward movements between the base (10 to 15 ° C) and the top (approximately – 50 ° C).

“Inside, ice cores can circulate, small imperfections conducive to the formation of ice crystals”, explains Jean-Yves Choplin, forecaster of Météo France. Caught in upward movements, these “dust” rise in altitude when you find water “overcooled”, that is, liquid water but at negative temperatures. “There will be frost and we will form a hailstone that gets bigger as it moves through the cloud and is found with supercooled water droplets. Our hailstone continues to rise in an upward motion and there comes a time when it will weigh enough to go down and rush to the ground. ”

A phenomenon linked to the heat episode

In general, as they fall to the bottom of the cloud, hailstones find positive temperatures that melt it, so hail phenomena are still rare.

“If the hail doesn’t have time to melt, it’s because it’s too big or trapped in a downward current, which accelerates its fall and doesn’t have time to melt.”says Jean-Yves Choplin.

The phenomena observed in France during the night from Sunday to Monday testify to the intensity of the storm episodes, linked to the heat wave observed in recent days with temperature records that exceeded 30 degrees.

Conditions that provided enough energy to cause a super cumulonimbus. “Sunday was the climax of this stormy situation with an unstable mass of air, due to the presence of very hot air on the ground, especially at the end of the day, throughout central France, and much of the east and south. in the east of France, and this very fresh air that came in from the Atlantic through Brittany at high altitude ”.Météo France forecaster continues.

“The longer the hail remains inside the cumulonimbus, the more it will grow. Yesterday, in the Châteauroux region, we had a super cumulonimbus, which allowed these hailstones to last long enough to grow a lot, about 8 cm, and they could weigh about 250 g ”. add.

Rare phenomena, difficult to predict accurately

In all the storms we observe in France, there are only between 5 and 10% where we observe in addition to hail. “It’s pretty weird,” informs Jean-Yves Choplin. “We can predict situations of hail in this or that area, but we can not predict the exact places where there will be hail, or its intensity. You should know the route of the hail, it is very complex “, says the forecaster.

According to Météo France, the last significant episode of the hail phenomenon in France dates back to June 19, 2021, with a hail of 10 cm and a weight of 400 g, in the Haute Jura, recalls Jean-Yves Choplin. “It’s pretty exceptional, once a year, on average, almost more”.

Weather report. How do hailstones as big as a tennis ball form?

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