The African subtropical anticyclone is a dynamic anticyclonic area of a continental subtropical nature, which almost permanently affects the entire area of northern Africa occupied by the Sahara desert, where it guarantees continuous and persistent atmospheric stability.
Its characteristics are mainly identifiable in the high geopotential values, especially during the summer months, while the pressure on the ground does not have particularly high values, given that the area in which it stands has usually high temperatures.
In the summer months, relative lows of ground pressure are recurrent at medium-low altitudes due to the stationarity of very warm air masses in the lower layers. The northward upwelling of the African subtropical anticyclone up to the Mediterranean determines the formation of a high pressure promontory. In Europe, this scenario determines atmospheric stability with very pleasant temperature values in the winter season, and days that are always stable, but with a little heat in the central hours of the day in the intermediate seasons.
The consequences are much more annoying when this configuration occurs in the summer season: the warm currents coming from the Sahara, as well as bringing about a significant increase in temperatures above the average, as they cross the Mediterranean gradually become loaded with humidity, giving rise to conditions of sultry heat along the coastal areas, with increased humidity rates in the evening and at night even in the more inland areas, with consequent great discomfort for all subjects, a heat wave, and a very high level of risk especially for the elderly.
During the summer season, the stabilizing presence of the African subtropical anticyclone, which is always accompanied by very high geopotential height values, is responsible for the periods of widespread atmospheric stability with above-average temperatures and rather low daytime relative humidity values if there is no the simultaneous expansion to the ground of the Azores anticyclone, with subtropical oceanic characteristics.
The presence of the African subtropical anticyclone can be recognized by the almost total absence of cumulus activity in the inland areas, provided that there are no infiltrations of cooler air from the north, which can cause storm-type instability, due to the high atmospheric stabilization that occurs checks at all altitudes, effectively preventing infiltrations of cold air which could contrast with the rising of warm air following the intense heating of the soil; in winter, on the other hand, its expansion towards the north is rarer and can determine the formation of low clouds along the coastal areas and in the flat areas.