After an episode of cold and snow, the thermometer has been showing very warm temperatures for the season since Sunday. Natural oscillations in weather and temperature are particularly being pushed upwards due to global warming.
From winter to spring. France experienced a taste of winter last week with even snow on Thursday in a northern part of the country. However, this Monday, November 25, the French woke up to exceptionally high temperatures.
At 7 a.m., it was 24.6°C in Pau, reports Gaétan Heymes, forecaster at Météo-France, on the Bluesky network. “Since temperatures were recorded (…) in 1945, this is the 5th highest temperature observed at 6 a.m. UTC, all months combined,” he wrote. We also recorded 16.6°C in Paris at 6 a.m., or 10 degrees higher than what we should have in the afternoon for the end of November.
Between Saturday morning and Sunday afternoon, in Mouthe, in Doubs, the thermometer went from -11°C to 14°C. A difference of more than 25°C in one day.
A mass of warm air coming from the south
How can we explain such a radical change in mercury? As detailed by La Chaîne Météo, “these sudden temperature variations are due to the ripples of the jet stream”, a very powerful upper-altitude wind axis circulating around the Earth from west to east. By its shape, sometimes undulating like the meanders of a river, it determines the major types of atmospheric circulation in mid-latitudes.
The winter weather was due to a northwest-oriented jet, bringing polar air down towards our latitudes. The flow shifted during the weekend: the jet is now oriented south/southwest. Consequently, it is a mass of hot air which rises from the south towards France.
Foehn effect near reliefs
Mercury is also boosted by a parallel phenomenon: the Foehn effect. The air and wind arriving from the Iberian Peninsula encounter the Pyrenean relief on their way. “Upwind of the relief, these air parcels rise. Subjected to relaxation (drop in pressure), they cool,” explains Météo-France.
Clouds will then form, sometimes until precipitation appears, evacuating part of the water carried by the air flow. Once the summits have been crossed, the air on the contrary undergoes compression (increase in pressure) which heats it and dries it out. It is thus warmer once the reliefs have passed.
The Foehn effect explains the remarkable temperatures in the South-West, such as Pau, Tarbes and Biarritz, where the 20°C mark was well exceeded this Monday morning.
As Ouest-France notes, the Foehn effect is also felt in Brittany with a southerly wind which meets the reliefs of the Armorican Massif.
A summer night in November
All these conditions combined, exceptionally high temperatures were recorded overnight from Sunday to Monday.
Up to 23°C was recorded near Saint-Jean-de-Luz and the Basque Country, but also 19°C in the La Rochelle region, 17°C in Bordeaux. On the Mediterranean rim, 19°C was reported in Perpignan, 15°C in Toulon, 17°C in Sète and even 16°C in Montpellier. These high nighttime temperatures also affected the northern half of the country: 15°C in Paris and Rouen, 16°C in Dunkirk and 17°C in Tours at 4 a.m.
Climate: what does a 1.5°C rise in global temperature mean?
According to Météo France forecasts, the mercury will drop slightly throughout the day. Afternoon temperatures range from 12 to 15°C over a small northwest quarter of the country. We expect 15 to 19°C over a large southeastern half and 20 to 23 at the foot of the Pyrenees and on the coast of Corsica.
Global warming
Everywhere in the metropolitan area, these values are well above seasonal norms. If the southerly flow and the Foehn effect boost mercury, these conditions are also exacerbated by climate change.
The latter is the consequence of the emission of greenhouse gases linked to human activities, in particular the exploitation of fossil fuels. The more these emissions continue and accumulate year after year, the more the warming power strengthens.
Thus, the natural oscillations of weather and temperature are pulled upwards due to this warming power. With an average temperature of almost +3 degrees compared to the beginning of the 20th century, 2022 was the hottest year ever recorded in France, followed by 2023.
According to the European Copernicus Service, the year 2024 will be the hottest ever recorded on Earth.
