“Acqua alta” (high water) is a phenomenon which generally takes place in Venice in winter time, when a combination of astronomical tide, strong south wind (scirocco) and seiche (the periodic movement of sea waters, a sort of long wave which washes all Adriatic coasts) can cause a larger inflow of water into the Venetian Lagoon.
Acqua alta occurs when certain events coincide, such as:
- A very high tide (usually during a full or new moon).
- Low atmospheric pressure.
- A scirocco wind blowing up the narrow, shallow Adriatic Sea, which forces water into the Venetian Lagoon.
By official definition, acqua alta occurs when the tide is 90 cm (35.4 inches) above normal high tide.
The phenomenon is most likely to take place between late September and April, and especially in the months of November, December, and October, in that order. The Comune di Venezia’s Acqua Alta publication explains: “In the remaining months, frequency is very low, while there has never been an event from June to August.”
Not all parts of the city are equally susceptible to flooding, as the following chart from the Comune di Venezia indicates. Also, the actual depth of water in the streets is far less than the “level of tide” might suggest. (See the “extreme case” below, where 135 cm of flooding translated into 40 cm of water in the Piazza San Marco.)