After months of rain, low clouds and fog. If a large southern part of France is experiencing sunshine and temperatures well above normal, the north remains in gray. A situation which should continue for several more days.
For many days, a northern half of France has been waking up in fog, plunging the inhabitants into a veil of humid mist which persists even during the day. The city of Nantes has not experienced a single minute of sunshine since October 28 and is approaching the 1956 record which recorded 12 consecutive days without sunshine.
If this meteorological situation is rather classic in autumn, several conditions must be met to have such persistent fog and low clouds.
Anticyclonic conditions
First of all, the good news with this current fog is that it's almost not raining. Indeed, the long period of rain experienced by France ended at the end of October thanks to anticyclonic conditions which blocked the arrival of new precipitation.
Anticyclone, unlike depression, is an area where atmospheric pressure is higher than the average atmospheric pressure at sea level. If such conditions are associated with calm, dry and sunny weather in summer, in winter, the situation can be very different, as we are currently observing.
“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. .
A lid that blocks low clouds
With the longer nights in fall, the air near the ground cools quickly. Fog forms when water vapor condenses in the air layer near the ground. The colder the air, the less water vapor it can hold. Thus, when this air cools, it becomes saturated with water vapor and the latter condenses to form droplets and therefore fog.
“Physically, fogs are indistinguishable from stratus clouds or clouds. They are made up of tiny water droplets that float in the air and reduce visibility. The higher the number of droplets, the more reduced the visibility,” explains the Swiss meteorological service.
Fog is therefore a cloud stuck on the ground. With the anticyclone above us, there is a lot of mild air at altitude which pushes the cold air to the ground, the warm air being less heavy and therefore rising, unlike the cold masses. This acts as a lid that traps ambient moisture close to the ground and prevents gray matter from dissipating.
We speak of fog when it is present at ground level and visibility is less than 1 kilometer. According to Météo-France, we speak of haze when visibility is between 1 and 5 kilometers.
Not enough sun or wind
Generally, the fog can dissipate in the morning under the action of solar radiation. As the air temperature increases, that air can suddenly contain more water vapor and the droplets begin to evaporate.
However, in autumn, the sun is low in the sky and shines for less time. This prevents it from warming the air enough to remove moisture. The sun does not, in fact, have enough energy to dry the air and the sky remains overcast.
The wind can mix the air mass and dissipate the fog but it is currently not strong enough to clear away the gray.
To find the sun, you have to go to higher altitudes
In the city, the fog can dissipate more quickly in the morning because it is generally warmer there: the stored heat is greater than in the countryside due to buildings and human activities. Thus, the air warms and dries more quickly and low clouds do not persist as elsewhere.
Another exception in this northern half of France plunged into grayness: the reliefs. Indeed, the lucky ones who can escape to altitude find sunny weather and milder mercury as they pass above the clouds, which remain low due to the strong pressures that force the cold air into the plains.
This Thursday, part of Brittany benefited from a break in the stratus, these low clouds. This was very visible from space and was captured by NASA or Eumetsat satellites.
Such local exceptions can be explained by a stronger wind than elsewhere but also the presence of relief, as explained above. In Finistère, the proximity to the Monts d'Arrée allows the sun to shine through.
While the summits are sunny, this warms the surrounding air and allows the clouds “to gradually dissipate towards the south under the action of a slight drying northeast flow”, explains the Télégramme in particular. Thus, we measured just 10°C in Rennes when it was almost 20°C in Brest.
Deterioration of air quality
If this meteorological 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, and lead to a deterioration in air quality. Currently, we are observing a significantly degraded level of air quality, particularly in Hauts-de-France.
In addition, if fog can sometimes weigh on morale, it is also dangerous because the poor visibility it brings constitutes a risk for traffic on the roads but also for trains or even planes. It is therefore important to reduce your speed and check the condition of your headlights to see and be seen.
Until when?
According to weather forecasts, these low clouds will persist for a few more days in the north of France. Over the next week, France will shift into a north-easterly flow and a new cold drop will position itself over Germany.
We should thus get rid of the fog but gain instability with possible showers, more wind and above all a notable drop in mercury, “because of a disturbed descent from the north”, explains Météo-France.
However, according to the meteorological institute, “the outlook for the first part of November does not seem conducive to a rapid return of western disturbances.” “Across the country, we could therefore have a dry period of several weeks in most regions, which has not happened since the beginning of autumn 2023,” he predicts.
