The vertical thermal gradient

Air temperature decreases by about 0.6°C every 100 m you climb, a value that may be considered the normal thermal gradient in the lower strata of the atmosphere, but that can register local variations. In particular, when air masses move vertically, a new situation can arise in which there is imbalance with the surrounding air temperature, determining anomalous zones that are either colder or warmer.
At times situations of so-called thermal inversion can take place when the temperature, instead of decreasing, increases with altitude.
This situation can take place when, for example, many days of good stable weather tend to make the air stratify according to its density, with the cold heavier air touching the ground and the warm lighter air at a higher altitude: this phenomenon occurs frequently in winter on the plain of the Po river resulting in persistent and widespread areas of fog. Thermal inversion can also occur in valleys that are not very windy as, for example, the Valtellina valley that is set out perpendicularly respect to the prevailing winds and where the air stratifies, with colder air on the valley floor. Yet another example of thermal inversion takes place when a mass of cold air wedges itself under a mass of warmer air as in the case of the cold front of a perturbation. In the air that covers large cities a situation of thermal inversion can prevent the dispersion of pollutants, giving rise to smog: it is not surprising that the alarm signals warning that the pollution levels have been exceeded in our big cities are more frequent in winter. The word smog derives from the words smoke and fog: it is in fact a mixture of drops of water and solid particles (generally made up of dusts and residual combustion products).

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