Par
Alexandra Second
Published on
Jan 12, 2025 at 10:21 a.m.
Between the arrival of an anticyclone and cold continental air, coming from Germany and Scandinavia, this weekend of January 11 and 12, 2025 promises to be cold and dry. Even very cold, due to local winds.
Indeed, in the same way that it is hotter in the sun, the presence (or not) of wind can affect the way we perceive temperatures: the more it blows, the more the cold seems biting to us. In this case, we use a precise index, le « Windchill ».
Eight departments on yellow “very cold” vigilance on Sunday
For the first time in winter 2024-2025, Météo France has chosen to trigger “very cold” vigilance, for the day of Sunday January 12, 2025. Eight departments are placed in yellow: Aveyron, Cantal, Corrèze , Creuse, Doubs, Haute-Loire, Loire and Lozère.
Distinguish between real temperatures and felt temperatures
When we consult the weather forecast, we can base ourselves on two elements, namely actual temperatures and felt temperatures.
As we explained in this article, the first correspond to the scrupulously measured temperatures via a thermometer and a probe 1.5 m above the ground, in a shelter. It is therefore an accurate and rational amount.
Conversely, the temperatures felt relate more to a perception specific to each person and weather conditions that can affect this sensation: the sun which warms the atmosphere, humid rain which causes the cold to rise from the ground… or even the wind which makes the cold more bitter.
L’indice Windchill
To quantify this cold felt, meteorologists use an index: the Windchill or “wind chill index”, which depends on “wind and air temperature”. In other words, the windier it is, the colder it feels, the lower the Windchill will be.
Let’s take the example of a actual temperature recorded at 5°C. Here, according to the Windchill index, the potential temperatures felt depending on the wind speed:
- 0 km/h wind: 5°C felt
- 10 km/h wind: 3°C felt
- 20 km/h wind: 1°C felt
- 30 km/h wind: 0°C felt
- 40 km/h wind: 0°C felt
- 50 km/h wind: -1°C felt
- 60 km/h wind: -2°C felt.
Another example, this time with a real temperature of -5°C:
- 0 km/h wind: -5°C felt
- 10 km/h wind: -9°C felt
- 20 km/h wind: -12°C felt
- 30 km/h wind: -13°C felt
- 40 km/h wind: -14°C felt
- 50 km/h wind: -15°C felt
- 60 km/h wind: -16°C felt.
For the record: on the same model, there is an index to quantify the heat felt. It is about the Humidexwhich defines discomfort “due to heat and humidity”, agrees Météo France.
Between -4 and 0°C in reality, but a feeling of up to -10°C
Also, when Météo France forecasts for the day of Sunday January 12, 2025 report a minimum temperature between -4 and 0°C in the majority of the country in the morning (actual temperatures), it is possible that some French people find the cold more intense.
And it is entirely justified: “between the north-east wind, or the wind in the center of France, the mistral or the tramontane, we estimate the Windchill index between -5 and -10°C on two thirds of France”, confirms to actu.fr meteorologist Guillaume Séchet.
Météo France mentions “gusts [de mistral] reaching 100/120 km/h south of the Rhône valley and 90 km/h elsewhere”, as well as “a sensitive tramontane all day long with gusts between 70/90 km/h”. It will also blow over Corsica, with “between 70/90 km/h towards Porto-Vecchio and on the terrain”.
Due to this drop in temperatures and a low Windchill index, Météo France has also triggered “very cold” vigilance for all day Sunday. Eight departments are concerned: Aveyron, Cantal, Corrèze, Creuse, Doubs, Haute-Loire, Loire and Lozère.
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