After the sweltering heat come the thunderstorms: how the heat turns into hail and lightning
Thermoconvective thunderstorms do not originate from Atlantic weather systems but from the overheating of the ground: when the ground temperature exceeds 30 degrees, the air in contact with it rises by convection and, upon encountering colder air at higher altitudes, generates intense but short-lived cumulonimbus clouds
The subtropical high-pressure system that has been keeping Italia in temperatures above 35 degrees for days does not rule out — indeed, in some cases it sets the stage for — violent thunderstorms that strike, in particular, the Alpine, Pre-Alpine and Apennine regions in the afternoon. These are known as heat storms, or thermoconvective storms: phenomena that do not arise from Atlantic disturbances but are triggered by the overheating of the ground.
How it works
The mechanism is relatively simple. On days when solar radiation is intense and winds are light, the ground accumulates heat until it exceeds 30 degrees. The air in contact with the ground heats up, becomes lighter and begins to rise by convection — in effect, it behaves like a bubble that rises due to the difference in density compared with the surrounding air.
If there are layers of colder air at higher altitudes, the rising air mass cools rapidly: water vapour condenses, forming cumuliform clouds with great vertical extent – cumulonimbus clouds – with their characteristic ‘anvil-shaped’ tops that can extend to the edge of the tropopause, at an altitude of over 10–12 kilometres.
Rapid and intense thunderstorms
The result is a rapid and intense storm — usually lasting no more than 30–40 minutes — with localised showers, lightning and, at its peak, hail and gusts of wind that can be violent. Unlike frontal thunderstorms, which accompany the passage of weather systems and affect large areas, thermoconvective thunderstorms are spatially confined and do not bring about a lasting drop in temperature: once they have run their course, they leave the atmosphere still warm and more humid.
Mountainous terrain favours their formation because the upward movement of air finds an additional lift on the mountain slopes, which allows it to bypass the temperature inversion layers. This is why the frequency of these events has historically been higher in the Alps, the Pre-Alps and along the Apennine ridge. The inland plains, which are far from the sea, are, in contrast, more exposed than the coasts, where the water moderates daily temperature fluctuations.
