Thermal inversion is a technical expression identifying this spectacular phenomenon, typical in the valleys of the Langhe region. The warmth goes up and cold goes down, and the result is that going up, the fog gets less dense. Look what happens!
Thermal inversions are usually more frequent during the winter and during the high pressure periods and scant circulation of air: during the clear and windeless nights, because of the quick loss of warmth from the days irradiation of the layers closest to the soil, there forms a cushion of frozen air.
Besides, on the mountains, at sunset, the slopes tend to cool down more quickly than the warmer downvalley, but the colder, more dense and thicker air descends to the valley provoking the thermal inversion in the bottom of the valley, where the temperatures in the mornings are lower.