how could a tornado form in Mayenne?

Impressive images. A tornado appeared late in the day on Sunday September 18 in the Mayenne department between Ernée and Saint-Pierre-des-Landes, municipalities located northwest of Laval. There were no injuries or homes affected, only the roofs of several agricultural buildings were blown away. A relief for residents shocked to see such a phenomenon occurring near their homes.

>> “At first, I didn’t believe it”: this resident testifies to the passage of a tornado in Mayenne

How are tornadoes formed?

A tornado appears during a storm, Météo-France points out, unlike land or sea waterspouts which generally form without. The tornado is a whirlwind of violent winds which will develop under a cumulonimbus – the cloud characteristic of storm phenomena – and extend to the earth.

The shape of a tornado is reminiscent of a funnel. It is generally visible by the droplets of condensation present within it. The dust or debris forms what is called a “bush,” at the base of the tornado. But be careful, as the French observatory Keraunos reminds us, the visible appearance of a tornado is very variable and should not be the only criterion to define it. In summary, even if the snorkel is not visible, we can still speak of a tornado.

It is customary to summarize the formation of a tornado by the sudden encounter between hot air and cold air, but it is “an abusive simplification”, underlines Keraunos. A tornado will form with the storm cloud which will suck up the hot and humid air near the ground, summarizes the president of the French observatory to TF1 Info. It will send it to altitude where it is cooler and drier. This suction will start to rotate when the winds change direction and intensity at altitude.

This so-called wind shear phenomenon and atmospheric instability are the elements necessary for the formation of tornadoes. The most devastating tornadoes appear within superstorm cells, which are more violent, larger and more durable than so-called “classic” storm cells. These storms are particularly unstable where with altitude, the temperature drops quickly and the wind varies greatly.

Six intensity categories

When the phenomenon occurs, it is quite brief and very localized. The intensity of a tornado is measured using the Enhanced Fujita Scale. This international rating system has been used since 2007 and incorporates 28 criteria to determine the strength of a tornado.

It was developed from the scale created by the Japanese Theodore Fujita in 1971, then was improved in the 2000s by the American National Weather Service. This classification abbreviated as EF includes six categories which range from EF0, that is to say a very weak tornado with winds of 105 to 13 km/h up to EF5, a very violent tornado with winds greater than 322 km/h /h.

Around forty tornadoes in France each year

If the frequency does not reach that observed in the United States in the famous “Tornado Alley”, the phenomenon is not so rare in France. Other tornadoes than the one observed on Sunday in Mayenne occurred in France. The Keraunos observatory estimates that there are between 40 and 50 tornadoes each year in France, most of them with low intensities EF0 or EF1. Some, however, have exceeded the EF1 level. At the beginning of March, a tornado, classified EF2 by Keraunos, devastated the village of Pontarion in the Creuse, causing significant material damage. Meteo-France also cites the example of Hautmont in the North in 2008 with an EF4 level tornado, and recalls that only two tornadoes of EF5 intensity were recorded in France at Palluel (Pas-de-Calais) in 1967 and in Montville (Seine-Maritime) in 1845.

But will tornadoes be more numerous or more intense with global warming? It is impossible to say. As recalled Science and Futurethe conditions for the formation of tornadoes may be fueled by global warming, but scientists prefer to remain cautious on a direct link.

Recently, more and more images show tornadoes on French soil, however improved technologies and social networks encourage greater and rapid sharing of these phenomena.