Weather: what is the "barometric swamp", the phenomenon that divides the France in two?

Alexandre Dalifard / Photo credit: SEBASTIEN LAPEYRERE / HANS LUCAS / HANS LUCAS VIA AFP 18:00 pm, June 01, 2023

But what happens in France at the level of the sky? Wednesday, four departments of the South-West (Gers, Landes, Hautes-Pyrénées and Pyrénées-Atlantiques) were placed in "orange" vigilance by Météo France because of the risk of violent thunderstorms. If the south of the France will be affected by stormy weather for the beginning of the meteorological summer that starts this Thursday and ends on August 31, the north will be hit by a summer sun, and this, for the next 15 days.

For two weeks, the Hexagon will be cut in two with a sunny north and a south threatened by many storms, despite high temperatures. This meteorological phenomenon is known and has a name, the "barometric swamp".

Deadlock situation + barometric marsh = situation conducive to stormy developments. These sometimes strong #orages are currently developing in the south and over Spain and can locally give hail, strong gusts and strong accumulations. Caution in these areas. pic.twitter.com/E0oh99Va50

— The Weather Channel (@lachainemeteo) May 31, 2023

Areas between an anticyclone and a depression

How can these degradations be explained? "A barometric marsh refers to a meteorological situation found over regions generally located between two centers of action, an anticyclone and a depression, and an atmospheric pressure that varies little and is slightly low," gives as definition the site The Weather Channel. Concretely, this phenomenon accentuates the formation of thunderstorms and stormy showers when the atmosphere is humid and the sunshine is important. In this situation, we then speak of showers of diurnal evolution.

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Unlike nocturnal, the word "diurnal" refers to what happens during the day. To define, the "diurnal evolution" corresponds to the gradual increase of the mercury during the day. This causes the development of sometimes stormy clouds that occur most often on the relief. These thunderstorms occur when cold air at altitude is mixed with warm air near the ground. This coupling favors the design of a temperature differential and the development of these stormy clouds.

Difficulties in anticipating climate

In definition, the France is located between a powerful anticyclone, centered in Ireland and which benefits the north of the Hexagon by a great sun, and depressions descended towards the Mediterranean basin, which favor thunderstorms after sunshine in the morning. "Within this mixed zone, the atmospheric pressure varies little and is slightly low-pressure," says The Weather Channel. Normally, thunderstorms are already a difficult phenomenon to anticipate. In a barometric swamp situation, they are even more so.