A long gray column connecting the sky to the earth swirled Saturday in the village of Saint-Nicolas-de-Bourgueil (Indre-et-Loire).
In its passage, the stone bell tower of the church collapsed and part of the roof of the religious building flew away.
“You would have thought there had been a bomb,” said the parish priest, Benoît Laurens.
"Very localized", the tornado did not cause any injuries.
But the images posted on social networks are impressive.
Where does this intense weather phenomenon come from?
Is it more and more frequent in France?
We take stock.
What is a tornado?
A tornado is a whirlwind of strong winds that develops under the base of a storm cloud - a cumulonimbus cloud - and extends toward the earth's surface.
It takes the form of an outgrowth of this cloud, looking like a funnel, and sucks up what it finds in its path.
The phenomenon is brief and localized, which makes it almost impossible to predict.
"In France, their diameter varies from a few dozen to several hundred meters, for a course of a few kilometers and a lifespan rarely exceeding fifteen minutes," explains Météo France.
The tornado is "considered to be the most intense of meteorological phenomena", specifies the organization.
With winds that can exceed 400 to 500 km / h, the destructive power of these vortices is “greater than that of a tropical cyclone”.
Its effects, on the other hand, are much more limited in time and space.
Why is it happening?
For a tornado to form, the cumulonimbus must be spinning.
And for it to run, “two elements must be brought together at the same time, in the same place,” explains Parisian Emmanuel Wesolek, president of Keraunos, the French observatory for tornadoes and violent thunderstorms.
“Warm and humid air must be present near the ground.
It creates instability, which allows it to rise.
"Second element:" Winds must have different directions and speeds between the ground and the altitude ".
“This is what happened on Saturday, notes the specialist, with a northeast wind near the ground relatively weak while at altitude south, southwest winds were blowing at more than 200 km / h.
"
Once these conditions are met, the cloud starts to rotate.
If the thunderstorm is strong enough, it can start to create a tornado.
Is the phenomenon frequent in France?
The tornado is a rare phenomenon, but not exceptional.
Each year, "there are about thirty in the country," says Emmanuel Wesolek.
Probably "about fifty", counting "those that escape us, because they occur in an uninhabited place or are of too low an intensity", adds the specialist.
Compared to the number of thunderstorms, "that represents a marginal portion".
Their frequency is highest in summer, due to thunderstorm activity, with more than a third observed between June and August.
Fall is the second most suitable season for tornado development - nearly one in three cases.
Then spring comes, then winter.
VIDEO.
Saint-Nicolas-de-Bourgueil: the church tower torn off by a tornado
Certain regions, from the Center-West to the North-West, are more affected.
“For a tornado to form, there shouldn't be too many obstacles.
This is why they are very rare in the relief areas ”, explains the president of Keraunos.
“They appear especially in lowland areas.
"
The Mediterranean rim, in particular the Var region, is affected by waterspouts - maritime tornadoes - which run aground on the coast, sometimes causing great damage.
They form especially in the fall, "because the sea is warm compared to the temperatures at altitude".
How are they classified?
The intensity of a tornado is established using the improved Fujita scale, from EF0 to EF5. To classify them, meteorologists do not base themselves directly on the strength of the winds but on the damage caused. Investigators are sent to the area after the tornado has passed to identify the types of damage (vegetation, buildings, cars, etc.). These data then make it possible to establish an estimate of wind speeds and to classify the phenomenon.
Very low (EF0) to low (EF1) tornadoes are the most common in mainland France.
Those of significant intensity (EF2) occur on average "two or three times a year", indicates Emmanuel Wesolek.
For higher intensities, the phenomenon is even rarer and more distant.
The last tornado classified as EF3 took place on June 19, 2013 in Étrochey (Côte-d'Or).
“They occur approximately every ten years, against thirty years for the EF4 intensity and approximately once a century for an EF5 tornado,” explains the specialist.
Is the phenomenon increasing?
The number of tornadoes has not increased over the past fifty years. "The trend is even rather downward", indicates Emmanuel Wesolek. Admittedly, more and more tornadoes are recorded. But it is “the social media effect”, he explains. “If someone loses three tiles, we know it within a minute. Whereas before, half a village had to be demolished for it to appear in the press and be seen. "
Does global warming have consequences for the appearance of tornadoes? "There is no correlation between the warmer years which would be more tornadic," notes the expert. Tornadoes tend to occur in dynamic situations with, for the example of France, cold air descending over the Atlantic. However, in long-lasting periods of hot air, generally anticyclonic, the polar air no longer descends. As soon as we no longer have this contrast, the situations are less dynamic and therefore less conducive to tornadoes. "