The well-know phenomenon of high water regularly occurs in Venice above all in autumn and winter. It is caused by higher-than-average tides which, by getting into the lagoon through its three inlets, flood some parts of the town. The combination of three different natural phenomena results in such event: tides, low pressure, strong winds and heavy rainfalls. Such extraordinary high tides are due to the very characteristics of the Adriatic Sea ploughed by winds such as scirocco wind coming from Syria and bora from Hungary. These two winds respectively press the sea towards the lagoon and, at the same time, avoid the tide itself from flowing back into the sea.
The limit of high water has been calculated on the level sea.
If there is  a one meter high water St. Mark’s Square and some parts of the town are covered by a level ranging from 5 to 40 centimetres of water. For these reasons the town and the most populated city points have been equipped with a number of footbridges which are arranged by AMAV operators (Environment Multi-services Company of Venice) early in the morning in order to help the use of routes.
Venetian people are warned of high tide by means of some sirens when the tide is rising (very early in the morning at about 5 o’ clock).