Storms and bad weather have been happening one after another in France in recent days and it’s not over yet. They have already caused damage, without causing any casualties, particularly between Marseille (Bouches-du-Rhône) and the Alpes-Maritimes, on the shores of the Mediterranean.
The current atmosphere is particularly conducive to this situation. It results from a cyclical element, namely the presence of a cold drop which is a source of instability, and a more structural element, namely the surface temperature of the Mediterranean which acts as a fuel.
Air instability
When thunderstorms form, they mean conflicts between different air masses. This is what is happening this week in France. A cold drop is rotating near the French coast along the Atlantic. This phenomenon corresponds to the isolated presence of a cool air mass in the middle of a larger warm air mass. Being away from the dominant air circulations, whether oceanic, polar or tropical air, it has erratic behavior, which increases the instability of the air around it.
France is the area most exposed to bad weather. As shown on this map of temperatures at 1500 m altitude produced by the European Centre for Medium-Range Weather Forecasts (ECMWF), it is under a cool influence in the middle of two warm masses, particularly towards the east of the country. Central and Eastern Europe are currently experiencing temperatures well above seasonal averages.
Beyond the principle of the cold drop, the beginning of autumn, which begins on September 1st for meteorologists, is in any case very favorable to the formation of violent storms. As Météo-villes reminds us, after two months of decreasing daylight hours, cold air accumulates in quantity in the northernmost part of the northern hemisphere. It can therefore more easily drop off towards the latitudes of France.
Sea temperature and humidity difference
Above all, the surface temperature of the Mediterranean Sea plays an accelerator role. It has had time to heat up significantly during the summer, especially when the air temperature is very high, which was the case in July and August. “When flows of cool air reach the Mediterranean basin, there is therefore a very large thermal contrast between the surface temperature and that at altitude (sometimes 20°C difference over 1500m). Added to the humidity of the sea, this thermal contrast then creates an environment very favorable to the development of storms, which can lead to major rain and storm episodes in the south-east of France”, explains Météo-villes.
After a lull on Friday, new storms are expected to arrive on Saturday from the south of France. The forecast remains uncertain for Sunday due to the unstable behavior of the cold drop, but the probability of experiencing a new significant rainy and stormy episode near the Mediterranean is increasing.
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