Tide

The periodic change in sea level produced principally by the gravitational interaction of the Moon, Sun and Earth and by the Earth’s rotation.

Astronomical tide is the predictable sum of many constituents whose periods and amplitudes vary by location because of basin shape, depth and coastline. It produces high and low waters and is related to, but distinct from, the horizontal movement called tidal stream.

Observed water level also includes meteorological and oceanographic effects such as atmospheric pressure, wind, waves, river flow and longer-period changes. Predictions therefore do not guarantee the actual level at a particular moment, especially during severe weather.