The predictable component of water level, produced by the tide-generating forces and shaped by local basin response, as represented by harmonic constants. It is what tide tables show. The real water level adds meteorological and other non-tidal effects on top, so flood studies separate the astronomical tide from surge to understand how often given levels will be reached.
Astronomical tide alone peaks at 2.1 metres, it's the surge on top that turns that into a flood.
Also heard as
- predicted tide
- gravitational tide
Related terms
Tidal prediction
A forecast of tide heights or currents computed from harmonic constants and astronomy, with no weather included.
Residual
The observed water level minus the predicted tide, showing the effect of weather, rivers and other non-tidal causes.
Storm tide
The full water level reached in a storm: astronomical tide and storm surge added together.
LAT
Lowest Astronomical Tide: the lowest level that tide predictions reach when the weather is held at average.