In France, weather is associated with an “atmosphere river”.

The warm and humid air blowing over France this weekend is coming from a “long cloud railway” from Bermuda and passing through the Azores.

21°C in Biarritz, 18°C ​​in Clermont-Ferrand, 17°C in Rennes, 14.3°C at dawn on Saturday morning in Paris… Like most of the 365 days of the year, according to this latest Météo France, 2022 ” with exceptional gentleness”.

Monthly records were broken in Lille, Rennes and Strasbourg, while Entzheim airport recorded 18.6°C. François Jobard Institute meteorologists on Twitter and Etienne Kapikian.

“The 3rd hottest day of December”

As for the day, “this New Year’s Eve is one of the three warmest Decembers nationwide,” trailing December 16, 1989, and December 4, “more than 8 degrees above normal,” compared to seasonal norms. , 1953, adds Météo France.

Across the country, the night between December 30 and 31 was “the mildest night on record in December”, particularly for Paris and the large north-west quarter of the country, Météo France said. Le Mans (13.9°C vs 14.2°C on 18 December 1987), Isle of Noirmoutier (13.9°C vs 6°C on 12 December 1961), Tours (14.2° C, previous record 13.3°C on 11 December 1961) or Orleans (13.9°C vs. 12.9°C on 11 December 1961).

These record temperatures are accompanied by heavy rain in several departments in northern France, such as Finistère, which has an orange warning for the risk of rain, wind and flooding for more than 24 hours by Météo France.

Since the rainy episode started 48 hours ago, 86mm of rain had fallen in Coray or 78.8mm in Brennilis by noon this Saturday. At the tip of Brittany, the wind speed exceeded 100 km/h.

This bad weather is caused by a famous meteorological phenomenon: the “atmospheric river”. These bands of warm, moist air carry large amounts of water vapor and can cause very heavy precipitation.

A rain corridor trapped between two air masses

These “rivers” have a rather limited width, only a few kilometers, which sometimes allows them to stretch for thousands of kilometers on the other side and transport very high water flows.

For this end of 2022, this corridor is squeezed between unstable fresh air from the North American continent and warm subtropical air from the African continent.

CNRS research director Christophe Cassou explained in messages posted on Twitter that the rains affecting northern France came directly from the Caribbean Sea or the western tropical Atlantic, “warm and very humid” starting from Bermuda before joining the European continent and reaching the Azores. .

“It is this continuity of the junction of fronts and troughs that allows the atmospheric river to form,” he adds. “A strong and persistent filament and rail of westerly/southwesterly wind clouds carrying the constantly replenishing moisture very efficiently.”

“The penetration effect of mild and moist ocean air is reinforced by the lack of ground snow across Eastern Europe,” he adds, with the lack of snow being particularly pronounced in the Pyrenees later in the year.

The researcher notes that the so-called “human influence” is “amplifying not only for temperatures but also for precipitation. A 1°C warmer atmosphere can have 7% more water vapor.”

Atmospheric rivers, also widespread in the Pacific Ocean, play a major role in the water cycle and ecosystems around the world: they account for about 90% of the global water vapor transport from north to south.

But they are also the cause of extreme climate events such as severe floods. For example, mid-latitude regions such as the west coast of North America are regularly affected.

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