Cold, snow, frost, black ice… What is the “meteorological winter” which begins on December 1st?

While the first snowfalls are expected in part of France this Thursday, this week marks the entry into the winter meteorological season. Explanations.

“We can expect the first winter sequence of the year this week, with a drop in temperatures expected this Thursday, as well as snowfall in the plains as far as the Central region, or even the south of the Île-de-France. -France”, predicts Météo France. This winter-like end of November is not surprising for meteorologists. She marks “entry into meteorological winter” which begins on December 1st.

This “meteorological winter corresponds to the coldest months observed in weather”, explains Élodie Callac, Météo France forecaster at Radio France. It runs from December 1 to the end of February. During these three months, the average temperatures in France (excluding overseas territories), which are also called normal, are 6.06°C in December, 5.46°C in January and 6 .1°C in February, explains Élodie Callac. These average temperatures are higher in November and March, around 9°C. December, January and February correspond “at the coldest time of the year in the northern hemisphere” For Meteo France.

The peak of winter is usually mid-January.

Thus, the meteorological seasons differ from the astronomical and calendar seasons, recalls Météo France. The calendar, or astronomical, seasons are based on the equinoxes (spring and autumn) and the solstices (summer and winter). Winter thus begins on the day of the winter solstice (December 21, 22 or even 23 depending on the year and astronomical calculations). When we think in terms of weather seasons, winter corresponds to the coldest time of the year. At this time, the sun’s rays are less effective. The minimum sunshine duration is around the winter solstice, which occurs on December 22 in 2023. But it’s not until about three weeks later that the average temperature is usually at its minimum, due to “the inertia of the atmosphere”. So “the peak of winter is usually mid-January”indicates Météo France.

This “inertia of the atmosphere corresponds to the reaction time of the atmosphere to a changeexplains Élodie Callac. It’s like when you turn off the heating in a room, it takes time to cool down.” The peak of winter is January 15, which is why meteorological winter begins in early December and ends at the end of February.

A snowy episode at the end of November is not uncommon

These first snows expected early at low altitude and up to the plain did not “nothing remarkable”, estimates Météo France. The organization recalls that historically, “snowy episodes are relatively common in November, especially towards the end of the month”. It was two years ago, between November 27 and 29, that the Auvergne-Rhône-Alpes region, the Pyrenees and Limousin experienced an orange snow alert. Even recently, on November 14 and 15, 2019, significant snowfall covered the Rhône-Alpes region, 23 cm of snow fell on Grenoble-Saint-Geoirs airport.

Even earlier snowy episodes occurred as early as October: October 29 and 30, 2018 during the passage of storm Adrian and we have to go back to 1975 for it to snow in the plains from mid-October.