Announced by ayellow alert Arpa Piemonte for thunderstorms, this afternoon a new one downpour hit Turin and its surroundings. The phenomenon developed in the context of a vast storm system technically called “multicellular”, that is, a group of several closely spaced storm cells in continuous regeneration, which shortly before had produced violent downpours of rain and problems for road traffic also between Val Chisone and Pinerolo.
Heavy rain
The ARPA meteorological stations have in fact detected remarkable rainfall of 105 mm in Pra Catinat (Fenestrelle), 118 a Taluccobehind Pinerolo, 56 mm at Vallere Park on the border between Turin and Moncalieri, along with strong gusts of wind. It was the fourth intense episode in just two weeks in the city, after the destructive hailstorm of August 2nd and other strong storms of the August 7th and this Tuesday evening.
Why hail hits Turin
In summer the Turin plain is often affected by cloudburstsmostly in the afternoon and evening, for a number of reasons:
- It’s posted at the foot of the Western Alpsfrom which storms migrate, driven by the prevailing westerly or south-westerly winds;
- The position at the mouth of the Val di Susa favors convergences between the diurnal breezes rising from the plain (East) and descending currents from the valley (West), the latter sometimes generated right at the exit of storms formed in the mountains
- As for the city, the urban heat island and the presence of the hill sometimes represent a further invitation to climb of warm, humid air, thus triggering intense vertical movements which, in certain conditions of atmospheric instability, lead to the development of imposing cumulonimbus clouds within a few dozen minutes.
Habitual phenomena, therefore, but the novelty is in an increase in frequency and the intensity of the cloudbursts that is beginning to be seen in the observed data, which is also in line with what is expected based on atmospheric physics and climate models in response to the increase in temperatures (the Clausius-Clapeyron law of physics states that for every degree Celsius of thermal increase, the air can contain 7% more water vapor, which can translate into more extreme precipitation). Over the last twenty-five years in central Turin the frequency of short and intense showers (at least 35 mm of rain in an hour, already capable of causing flooding) has increased by 2.5 timesThe series of hourly resolution data is still short by the standards of climatology, which requires at least thirty years for significant evaluations, but it is a first signal to take into account in the management of the territory and cities.
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