France cut in two since the beginning of November

Due to anticyclonic conditions, the north of France is plunged into grayness and fog, with the exception of the reliefs and to a lesser extent the Paris basin. Conversely, it is very hot in the south, with temperatures well above seasonal norms.

Although the month of November is not particularly known for being synonymous with sunshine, for residents of the northern half of France, the weather may seem long this Thursday, November 7.

For several days, gray weather has dominated, accompanied by persistent fog in the morning. Since November 1, the city of Strasbourg has experienced 9:08 hours of sunshine, or -83% compared to the 1991-2020 normal.

With its 8h27 of sunshine, it's -88% for Paris. For a week, the inhabitants of Cherbourg have seen the sun for… four minutes.

Grayness linked to anticyclonic conditions

The cause: an anticyclonic situation. If such conditions are associated with dry and sunny weather in summer, in winter they can be synonymous with grayness, low clouds and humidity on the ground. “The ground cools during anticyclonic nights in the absence of cloud cover, the cold air, pressed to the ground by the high pressures under the warmer air above it, can cause condensation,” explains Météo-France. . The sky therefore remains overcast.

“We have come out of the precipitation situation that we experienced for a year,” underlines Matthieu Sorel, climatologist at Météo-France.

This situation will continue for several more days, with ups and downs but no lasting improvement before next week.

Mild temperatures on the reliefs

As the sun is unable to break through and warm the air, we have been witnessing a cold anomaly in the northern half of France for several days. But, as Matthieu Sorel notes, this is not the case everywhere since the reliefs experience a very hot anomaly, from the very first heights.

For example, while the Lyon metropolitan area has only experienced 21 hours 41 minutes of sunshine since November 1, i.e. -73% compared to normal, the commune of Sauvages, located at an altitude of 833m just 40km from Lyon, totals 47h55 of sunshine.

This difference is due to what is called thermal inversion. First, cold air being denser than warm air, it will tend to concentrate in the valleys and plains, here blocked by low clouds. Then, the height allows you to pass above the mass of low clouds and therefore benefit from sunshine conducive to warming the air.

On the map below, we can easily identify small reliefs: the Monts d'Arrée, the Norman hills, the Morvan, the Côte-d'Or, the Langres plateau or even the Vosges and the Jura. All these reliefs experience much higher temperatures than in the plains.

This thermal inversion situation therefore traps cold air in the lower layers of the atmosphere. It can thus, at the same time, trap pollutants by acting as a cover, which can lead to a deterioration in air quality.

The particularity of the Paris basin and its urban heat island

Another exception in this situation of grayness and fresh anomaly: the Paris basin. In large cities, stored heat is greater than in the countryside.

Many factors prevent urban space from cooling, such as the urbanization model, floor coverings, lack of vegetation or water in public spaces. Tall buildings and the density of walls slow down the circulation of air, the building stores heat Building materials such as concrete, brick or stone easily capture heat during the day, through solar radiation, and gradually release it into the atmosphere at night, preventing air. to cool down…”, explains Météo-France.

In Paris, the urban heat island can reach +6.5°C. In the current meteorological configuration, this breaks the thermal inversion known elsewhere in the north of France with warming and drying of the air. As it is warmer, the low clouds do not persist like elsewhere, which allows the sun to break through and therefore cause the mercury to rise.

This phenomenon tends to spread “depending on the direction and strength of the winds towards regions around Paris”, indicates Matthieu Sorel.

An even more significant warm anomaly to the south

As the map above shows very well, if the northern half is in a cold anomaly under the gray, the southern half, sunnier, is in a very mild air mass.

Such a difference is explained by the position of the anticyclone, rather located over Northern Europe. “In the south, we are at +5°C compared to seasonal norms, sometimes more locally,” explains Matthieu Sorel.

Consequence: at the national level, the average temperature is well above seasonal norms. Indeed, if it is relatively cool in the north, this anomaly is much smaller compared to the averages than the warm anomaly experienced in the south. They therefore do not compensate each other.

In short, on the scale of France, the cold does not weigh heavily compared to the heat and we thus observe an anomaly of +2.1°C in the country between November 1 and 5. “We find ourselves in a classic meteorological situation but boosted by climate change,” concludes Matthieu Sorel.