While a brief heat peak affected the south at the end of last week, quickly closed by violent thunderstorms, very cool air is now invading our country. This cold air, coming directly from the Arctic, will even plunge as far as Morocco via the Iberian Peninsula. If this situation is common in winter and spring, it is atypical in June, but can occur occasionally, even in the heart of summer.
Cold air from the arctic
The principle of polar stall © The Weather Channel
The term “polar shift” refers to the detachment of an Arctic air mass that plunges southward. This situation can happen anywhere, but currently western Europe is in the path of these cold air descents. Note that when cold air rushes somewhere, hot air rises elsewhere due to the effect of the communicating vessels. The driving force behind this mechanism is the behavior of jet streamof which undulations cause the movement of air masses. When cold air reaches far south, it causes a sharp drop in temperatures. Likewise, the contrast in temperatures causes instability due to the cold air present at altitude, which is called a “cold drop“.
Consequences: drastic drop in temperatures and showers
This type of weather is more common in winter and spring, when cold air masses cover the entire polar zone. Stalls occur more easily as long as the flow turns to the north or northwest. The air, of maritime origin, does not bring snowfall to the plains, except in the form of sleet. In the mountains, this type of weather is conducive to late snow showers at low altitude.
But, in June, the air is no longer cold enough to bring showers of snow to the plains. The showers fall in stormy form, and bring snow to our mountains, above 2000 m altitude. Even in the middle of summer, this is possible. The feeling is therefore cool and gloomy, with a noticeable north wind. The air mass conflict between cold and warm air can cause violent thunderstorms, like this weekend in the south, with hail. A polar shift is therefore synonymous with dangerous weather (storms, hail, cold and snow in the mountains).
An atypical phenomenon in summer
Recent years have been marked by very hot Junes: we had lost the habit of these late bursts of coolness. The last similar Junes were in 2020, 2016 and 2013. During the 1980s, several “rotten” summers had occurred, with polar shifts in the heart of summer, during which snow had fallen, sometimes, up to on the mountains of Auvergne, while the coasts of the Channel suffered from the cold northwest wind. Now, although these situations are rarer, natural climate variability still produces this type of weather episodically. According to certain studies, global warming could affect the behavior of the jet stream, which, by undulating, causes cold spells as well as heat strokes, which is currently the case over Europe.
The polar shift is therefore a rather usual weather phenomenon in spring. In summer, it becomes much rarer (once every 5 to 8 years in France) and causes very cool and gloomy weather. Such recurring types of weather result in “rotten summers”. But, according to our long-term forecasts, this recurrence should stop during July with the prospect of a summer which should settle in for the long term in our country. For those who are waiting for the heat, we will have to be a little more patient, and that these forecasts for such a long time frame are confirmed.
