Why will it be colder in the plain than in the mountains in the next few days? read also Weather News Weather videos
An anti-cyclonic blockade has occurred this week in the country. A powerful anticyclone at 1043 hPa centred on the north of France is thus an obstacle to any disturbance. However, the consequences for temperatures are very different depending on the plains, valleys and mountains.
An anticyclone loaded with subtropical air at altitude
Like what happened two weeks ago, a new anticyclone is coming up across our country and carrying air of subtropical origin up high. This mild air, on the other hand, will not reach the temperature levels at the end of December or the beginning of January. Two weeks ago, temperatures had reached 15 to 18 °C at 1500 m in the south of the country. This time, we will be around 5 to 8 °C, but this remains high for the season. That is why, as early as Wednesday, temperatures had sometimes turned positive in some mountain resorts. So, if you plan to ski in the next few days, the conditions will be rather pleasant with few frost in the morning, except in some cold holes, and the values will be clearly positive in the afternoon in the 1500-2000 m range. The 0 °C isotherm should even rise above 3000 meters on Friday in the Alpes-du-Nord. Don't worry, the snow will not melt too much, due to the dry weather and low frost observed on the ground during the night.
Crédit : La Chaîne MétéoAn anticyclone producing cold air in the lower layer
If this anticyclone is accompanied by mild air at altitude, it is the opposite in the lower layers since it produces cold air. This is a classic case in winter, where areas of high pressure place cold air in the first layers of the atmosphere. The clear sky under an anticyclone promotes radiative cooling. When night falls, the little heat (energy) received by the ground during the day is returned to the atmosphere, since there is no cloud cover to hold it back. As a result, the plains and valley bottoms cool very rapidly. In the last few days, you may have felt a clear cooling of the bottom of the air with almost generalized frosts from northeast to south-west and sometimes even as far as Provence. On average, the lowest values fell between -3 and -7 °C in these lowland areas. It should be noted that with lower temperatures the humidity in the lower layer tends to condense and form fog sheets. In this case, it is at the top of the fog that you can observe the lowest temperatures. The sun, which overlooks the fog, causes it to evaporate. This physical phenomenon consumes energy and therefore produces cold. Thus, in this case with fog, the strongest frost is observed not in the plain, but on the first slopes of the mountains.
Crédit : La Chaîne MétéoThermal deficit in the plain over the next few days and surplus in the mountains
This weekend, you will keep your warm clothes in the plains and valleys. Minimum temperatures will continue to fall with sometimes heavy frosts (
A winter anticyclone can thus produce its own cold air in the lower layer, while at higher altitudes it carries much softer air. These situations give rise to thermal inversions between cold and sometimes grey weather in the plain and sunny and mild in the mountains. That is the agenda for the next few days, and this could happen again next week.
