To alter the regularity of the astronomical tide by a way sometimes substantial, meteorological factors intervening and among them especially the wind and pressure.
In the case of the Adriatic Sea, long and narrow basin, closed in the upper part and open in the lower one, a strong wind blowing from the southeast (sirocco) along the longitudinal axis, produces an accumulation of water to the closed end. The phenomenon is favored by the long zone of action available to the wind ("fetch") and is further amplified because of the shallow waters of the northern Adriatic.
The contribution due to the wind can overcome even the meter and causes by itself flooding phenomena.
First figure shows an ideal scheme of cyclonic circulation that can induce the acqua alta phenomenon, whereas in the second is highlighted the particular shape of the Adriatic Sea.
Even the bora, which blows from the northeast with an intensity that often exceeds the sirocco, it tends to accumulate water to the lagoon, although to a lesser extent due to reduced fetch. When in the northern Adriatic bora is associated with the sirocco on low-and middle Adriatic, there is the so-called "scontraura", is the convergence of ocean currents caused by winds from the northeast and southeast.
The phenomenon, known since ancient times, has seen many events of high tides.
The atmospheric pressure alters the level of the sea with "inverse barometer effect": a decrease of pressure corresponds to an increase in the level and vice versa. Consequently, the passage of a disturbance characterized by a low pressure area over northern Adriatic and a high in the South results in a shift of the mass of water to the area of lower pressure, with a variation which can lead to 20 cm.