Thermal Inversion from the Biellese Prealps

January 31, 2022


Photographer: Paolo Bardelli

Summary Authors: Paolo Bardelli; Cadan Cummings

These two images feature a thermal inversion that occurred on October 26, 2021 in the autumn colored foothills of the Biellese Alps. The valley is located between the Monte Rosa massif to the north and the Po Valley to the south. Thermal inversions are typical in this region during the autumn and winter due to the intense and prolonged periods of high pressure created when colder air layers near the land surface are trapped under warmer air layers being heated by the Sun. This difference in pressure combined with colder temperatures near the surface causes the air to drop below dew point forming haze and permanent mist. Above 3,280 feet (1,000 meters) elevation, the sky becomes clearer, and the temperature is decidedly more pleasant, which is also due to the thermal inversion.

The terrain of the Po Valley further exacerbates the inversions because it is closed in on the north by the Alps and partially by the Apennines to the south. As a result, mist can stagnate for several days in the region and only fades once either an atmospheric perturbation occurs or a “barrage” situation is created north of the Alps with the entry of hot and dry wind (also known as a foehn). In the photos above, the Monviso mountain (12,600 ft / 3,841 m) stands out on the horizon. From the northern slopes of this mountain rises the Po River, the main river that crosses the Po Valley from west to east.

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