Stop to bad weather and snow, weather changes from Monday 13 December

The weather in Italy changes from Monday 13 December. After several days with cold, bad weather and even snow up to the plains, over the next week the weather conditions will undergo a good shake and there will be consequences for the whole of Italy. The trend for the central part of December in fact offers days that are in some ways anomalous and also a particular phenomenon that will affect some of our regions. The new week will start as early as Monday in the sign of greater atmospheric stability thanks to the eastward expansion of the Azores anticyclone that will embrace a large part of the Mediterranean basin. An old acquaintance will therefore come back to visit us, after a fairly prolonged period of absence, during which bad weather and dynamism dominated. What will be the concrete consequences? There will certainly be room for more sunshine and also for generally rising thermal values, especially during the central hours of the day. These conditions will accompany us at least until Friday 17 December, without any particular hitches, but be careful, as often happens in winter, high pressure does not only mean good weather and clear skies. On the plains of the North and in the valleys of the Center, in fact, due to the stagnation in the lower layers of the atmosphere of more humid and colder air, together with the poor ventilation, the fogs could return, in some cases even rather dense and locally persistent even in the Central hours of the day: Piedmont, Lombardy, Emilia Romagna and Veneto will be particularly at high risk. In these sectors, temperatures will remain rather low and widely below zero at night and early in the morning. On the mountainous areas the opposite will happen with afternoon values ​​well above the reference climatic averages. In the mountains, therefore, it will be “hotter” than in the plains! But why does this happen? The technical term to define this particular atmospheric phenomenon is thermal inversion: as a rule, as the altitude rises, the air cools (about 6.5 ° every 1000 meters), because it is subjected to a lower pressure than that of the ground. where the air is warmer. In thermal inversion the opposite happens, that is, in simple words, it is colder on the ground and warmer at higher altitudes. In fact, during this season there are two factors that make the earth’s surface irradiation capacity lower: firstly, the duration of the day is considerably shorter than the duration of the night and secondly, the sun’s rays are more inclined than the ground. There are two conditions that are absolutely necessary for the formation of a thermal inversion: 1) absolute calm of the wind is required thanks to the high pressure field, 2) the formation of a temporary vertical homothermal layer (same temperatures). For this reason, on clear winter nights, when the ground has lost heat due to radiation (the ground always tends to release heat upwards, i.e. it radiates, loses heat by introducing electromagnetic radiation in the direction of the atmosphere), the layers of air close to the ground they cool down faster than in the upper layers. This cold air is very dense and heavy and settles in the valley floors or precisely in the lowland areas surrounded by mountains (Valpadana).