In a large part of central France, the weather in recent days has been characterized by stubborn banks of fog in the morning and bursts of sunshine in the afternoon. Explanations.
For around ten days, residents of central France have been waking up under a veil of persistent, humid mist, plunging towns and countryside into a fog that makes travel dangerous. This particularly stubborn grayness covers a large part of the territory – the west of France is not spared – and lasts until the end of the morning.
A radiation fog, typical of autumn
If this weather is common in autumn, it finds particularly favorable conditions here. “This type of fog, called radiation fog, appears especially in autumn and winter, explains David Bouche, forecaster at Météo France, in Aulnat (Puy-de-Dôme). It is mainly due to humidity, at low layer, and at high pressures, too. This phenomenon is seasonal.”
A widespread and persistent phenomenon
If this mist was able to settle in, it is primarily because of the damp soils. Frequent rains in September and October left the soils waterlogged. Another factor: the stable and widespread anticyclonic situation over a large portion of France. It should be understood that fog is a cloud in contact with the ground which forms particularly during anticyclonic periods, when the humidity level is high. Cold air, pressed to the ground by the high pressures above it, can cause condensation. Hence this feeling of humidity in the morning.
But in Clermont-Ferrand, what we see during the day, when it's 16°C, is also pollution, it's not just fog. Pollution is trapped by the fact that there is no air circulation. There is no wind and it is stagnant.
David Bouche (Forecaster at Meteo France)
A gradual return to more usual conditions
According to Météo-France, this grayness should disappear in the coming days with the arrival of disturbances coming from the south. A change in air mass and a resumption of winds should help dissipate the mist and return to clearer conditions. The night from Friday to Saturday could mark a turning point and end the low levels with precipitation forecast, sweeping away the anticyclonic conditions which block the fog.
While waiting for the sky to clear, fog forms more easily in the countryside thanks to the vegetation which allows better cooling of the soil and a higher humidity level, the first advice is to avoid country roads…
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