Alexandre Flouttard is a forecasting engineer at Météo France Toulouse. “Storm hunter” by passion, he is also a specialist in tornadoes, the most intense of meteorological phenomena.
The Bihucourt tornado in Pas-de-Calais made the front page of all the newspapers Monday, October 24 morning. On the evening of Sunday October 23, several municipalities in the North of France suffered massive damage. The images circulating in the media or on social networks are impressive. Torn off roofs, collapsed walls, electricity pylons strewn on the ground, broken windows… The municipality of Bihucourt is one of the most affected since it was affected at 80%.
ud83cudf2a Pas-de-Calais: images – especially aerial ones – of #Bihucourta village of 350 inhabitants most affected by one of the #tornadoes of Sunday evening, are impressive. 3/4 of the town are devastated, houses have literally been gutted… #thunderstorm @LCI pic.twitter.com/NwnaWP0PPF
— Guillaume Woznica (@GWoznica) October 24, 2022
These climatic phenomena, qualified as exceptional such as floods or mega fires, are unfortunately becoming less and less so.
A “very short and intense, very strong” episode, explained Benoît-Vincent Caille, the mayor of Bihucourt. The first magistrate did not hesitate to speak of “razed houses”. The Pas-de-Calais prefecture itself described the phenomenon as “strong gusts of tornado-type winds.”
Very quickly, a controversy arose on social networks around this event against the term used mini-tornado. Center Presse Aveyron went to ask the question to Alexandre Flouttard, forecasting engineer at Météo France Toulouse.
The mini-tornado is a term that does not exist, it does not correspond to a meteorological phenomenon
A mini-tornado, does it exist?
“We are trying to fight against this term mini-tornado which is used wrongly and through. It is a term that does not exist, it does not correspond to a meteorological phenomenon. Either, it is a tornado, or it’s not.
ud83dude21 This Tweet is addressed to French media and journalists: STOP! It is unbearable to read, each time a tornadic phenomenon affects the country, articles using the term “mini-#tornado “. This term does not correspond to any pheno. weather report. Absolutely ban! pic.twitter.com/bHH8MmOmtu
— Nahel Belgherze (@WxNB_) October 24, 2022
A tornado is a very small, very localized phenomenon that is difficult to predict. We can simply tell upstream whether or not the conditions are conducive to the formation of a tornado. I’m going to caricature but it’s really like a cooking recipe. All the ingredients must be dosed to the nearest millimetre. It is a complex phenomenon to predict because of its small size, its short lifespan.
A tornado is a whirlwind of very strong winds which occurs under a particularly violent thunderstorm and which extends to the ground
Where does the tornado originate?
A tornado is a whirlwind of very strong winds which occurs under a particularly violent thunderstorm and which extends to the ground.
The Bihucourt tornado has been confirmed by several videos. There are two scenarios. Either, there is a video attesting to the phenomenon, as is the case here, thus showing the funnel, the cloud touching the ground with the damage swirling around. Or there are no witnesses.
In this case, we do a field survey to observe the alignment of the damage. For example, we will have trees that are lying down after having fallen in one direction, and 50 m further, we will have trees or other damage that have flown away lying in the other direction. This signals an aspect of variation in the wind over a very short distance, this is the whirlwind aspect of the tornado.
It takes a storm
Conversely, if all the damage is lying in the same direction, we will rather speak of downward gusts of wind. That is to say a more classic burst but without a whirlwind aspect. It is all these analyzed elements that will allow us to say whether or not there is a tornado.
What can cause a tornado?
You need a storm, what we call in our jargon, instability, that is to say rather warm and humid air present near the ground, and cool air at altitude. It is the contrast between the two that will give rise to instability. Concretely, the hot air, which is lighter, will tend to rise on contact with the cold air at altitude and create a storm.
To have a storm that will give rise to a tornado, it will be the variation of the wind with the altitude as much in direction as in force
To have a storm that will give rise to a tornado, it will be the variation of the wind with the altitude as much in direction as in force. We will therefore have a wind that varies, quickly increases in strength and can have a completely different direction. This is called vertical wind shear. It is the coupling of the difference in temperature, the surface of the altitude and the variation of the wind with the altitude which will give rise to the most powerful storms called super-cells.
That of Bihucourt was classified EF3 corresponding to gusts of wind between 220 and 270 km/h
This will cause the storm to rotate across its full width, what is called a mesocyclone, an area that is about ten kilometers in diameter. Then, with the particular conditions on the surface which will depend on the humidity, the variation of the wind, this vortex will be able to extend to the ground, shrinking and increasing in intensity until reaching a tornado stage. .
But, not all thunderstorms become a tornado.
How are tornadoes classified?
Tornadoes are rated on the Fujita scale, from EF0 for the weakest to EF5 for the most violent. You cannot measure the wind directly in a tornado. It very rarely passes over a weather station. This is why we are obliged to estimate the wind from the damage.
That of Bihucourt was classified EF3 corresponding to gusts of wind between 220 and 270 km/h.
In France, it is between 40 and 50 tornadoes per year
The particularity of the Bihucourt tornado is that it was on a very long trajectory…
Do not believe that France is a territory that is not affected by tornadoes. It’s not just about the United States. They have a little over 1,000 a year, we have between 40 and 50 a year. But the vast majority are of category EF0 or EF1, therefore of low intensity.
Reaching the EF3 stage as in Pas-de-Calais remains quite rare in France. Another peculiarity and rare fact, the fact that this happens at the end of October. These are phenomena that occur more in the summer season. There are of course cold season tornadoes occurring in winter but here we found ourselves with summer temperatures. It also reached a very long distance and must have lasted a few minutes. It is therefore a high intensity tornado compared to what we are used to seeing in France.
Are there favorable areas in France?
There is, for example, the North and the North-West of the country. It is a slightly more exposed area between the Charentes, the Cotentin, up to Belgium too. And then, the coastline around the Mediterranean.
Tornadoes don’t really like high ground
How do you explain it?
On average, the wind is a little stronger at altitude in the north of the country. So, as soon as there are thunderstorms, it can be a little more favorable. When on the coast, it is rather, in general, autumn or spring storms. We are therefore talking more about waterspouts, tornadoes that occur at sea. They can overflow on the coasts with a coupling of heat and humidity.
On the other hand, the tornadoes do not like the relief zones too much.