The African anticyclone is about to bring a storm on Thursday 8th August new heat wave in Italy with peaks of up to 40 °C in the South and 38 °C in the Centre and North, while for today Wednesday 7 August thunderstorms are possible, especially the so-called “heat storms” typical between late spring and early autumn, when the air at ground level is particularly warm and humidity increases making the heat muggy.
Heat storms, or more properly thermoconvective stormsare the classic summer or late spring thunderstorms that break out in the late afternoon or evening after a hot and sunny day, discharge heavy rain and lightning in rather limited areas and end quickly, sometimes even just 30 minutes later. They are typical of the central areas of the plains or near the reliefs and in Italy they occur mainly from May to September. Like any thunderstorm, they are heralded by imposing towering clouds with a flat base and strong vertical development: the cumulonimbusTo form, thermoconvective thunderstorms need two fundamental ingredients: warm, moist air on the ground (a condition not difficult to find in the warmer months) and cold air at high altitude.
The phenomenon that triggers thermoconvective thunderstorms is heat on the ground caused by solar radiation. On hot, sunny days, the sun’s rays heat the ground from the early hours of the day. The air immediately above the ground becomes increasingly warm and, as the temperature increases, its density decreases. At this point, Archimedean thrust comes into play, and these “bubbles” of hot air begin to rise upwards like a balloon, creating a convective motion in the form of ascending currents which can also be quite intense. From this you understand why these storms are called “thermoconvective”: because they originate from convective motions due to the high temperature of the ground.
As the warm air rises it meets the cold air at altitude and its temperature drops. This causes the water vapor contained in the air itself to condense. When there is a lot of humidity on the ground, this humidity transforms into countless droplets of liquid water that condense to form large clouds called heaps. A pile, when it becomes congestioncan cause showers but not thunderstorms, because it does not produce lightning (in fact, let us remember that we speak of thunderstorms only in the presence of electrical activity).
To have electrical activity, and therefore thunderstorms, cumulus clouds must develop vertically – thanks to ascending currents – up to the edges of the troposphere (about 10 km above sea level), where water solidifies into tiny ice crystals. It is here that cumulus clouds evolve into cumulonimbus and the storms begin, which on average last 30-40 minutes along their route (but in a specific location they can last only a few minutes).
With precipitation we witness a real tug of war between the ascending currents in the cumulonimbus (updraft) that feed the storm cell and the downdrafts (downdraft) that discharge the rain rather violently to the ground. These descending currents of cold air are the reason why “storms cool down”, that is, the temperatures on the ground drop considerably (even by 5-6 °C) even if temporarily. The prevalence of descending currents in the area where the precipitation falls also prevents the entry of new warm and humid air into the cumulonimbus, thus “turning off the engine” of the thunderstorm phenomenon.
