Extraordinary weather phenomenon: giant hailstones

The thunderstorms the most violent are capable of phenomena worthy of Hollywood disaster films: tornadosudden winds over 100 km/h, torrential rains that flood cities in the space of a few minutes, but also, giant hailstones. The phenomenon is not uncommon in regions of the world known to harbor the most violent storms on the planet: United States, South America, Australia and Africa. Between 2010 and 2020, about 10 hailstorms around the world produced hailstones larger than 15 centimeters, according to observations reported to theAmerican Meteorological Society.

The world record for the largest hailstone ever observed dates from February 2018: a 23.7 centimeter hailstone (the size of a volleyball) is said to have fallen in the city of Villa Carlos Paz, Argentina, during a violent thunderstorm. In 1959, a hailstone of 1.9 kg would have been noted in Kazakhstan. In the United States, the record is held by South Dakota, with an 20.3 cm hailstone recorded in 2010. More recently, the city of San Antonio in Texas was devastated by 16.2 cm hailstones on the 28 April 2021. The American record for the largest hailstone is, on the other hand, held by another state, South Dakota, with a diameter of 20.3 cm observed in 2010.

A contrast of temperatures at the origin of the giant hail

In the supercells, the most violent storm clouds, strong updrafts occur. The water vapor condenses until it forms water droplets which rise in altitude. This giant hail therefore occurs under thunderstorms characterized by strong ascending currents laden with humidity: these currents propel the hail towards the top of the cloud several times in a row and the ice clumps together with each rebound in the cloud, until the when it becomes so heavy that it ends up falling. Why do some storms produce giant hailstones and others more classic hailstorms? The greater the temperature difference between the ground (hot) and the tops of the clouds (icy), the higher the clouds rise: the extreme size of these hailstones testifies both to the immense height of the storm cloud, and violent currents that were present inside.

If it is not possible to accurately predict the size of the hailstones that will fall during a stormy degradation, the forecasting services weather report can still raise the possibility of a risk of giant hail. In France, this risk is very present when the ground is overheated at the end of spring or in summer, and a depression bringsair cooler from the Atlantic: this is the reason why the risk of destructive hail is especially present on the line which stretches from the south-west to the north-east of the country.

Giant hail is not a recent phenomenon in France

Between the space of a month, from the beginning of May to the beginning of June 2022, theorganization Keraunos recorded about 2,000 hailstorms with a diameter greater than 2 cm, and 150 others with a diameter greater than 5 cm. The link with the climate change is studied, but has not yet been proven with regard to hailstorms.

Although many impressive hailstorms have followed one another in recent weeks, the phenomenon is not new in France. The record for the largest hailstone observed in France dates from August 11, 1958, under a stormy degradation that some newspapers of the time had dubbed “the apocalypse of August 11”: a hailstone the size of a fist weighing 972 grams was recovered near Strasbourg that day. Going back even further in the past, the stormy episode of July 13, 1788 marked the history of the weather: according to the reports of the time, the hail accumulated on 60 to 80 cm thick in the department of Essonne and 10 cm hailstones, weighing 700 grams, were reportedly observed.