Tides

A calculated forecast of tidal height or stream for a specified place and time. Predictions use harmonic or numerical methods based largely on astronomical forcing and historical observations. Actual conditions can differ because of wind, pressure, river flow and local changes, so real-time information may be needed.

A local rotating or reversing current formed where tidal flow separates around a headland, bank, structure or channel edge. Its direction and timing may differ from the main stream and change through the tidal cycle. Eddies can assist or hinder pilotage but should be used only with current local information and observation.

The roughly fortnightly variation between larger spring tides and smaller neap tides. Spring tides occur near new and full moon when lunar and solar tidal effects reinforce one another; neaps occur near the quarter moons when they partly oppose. Local timing and range are modified by coastal geography and weather.

The direction toward which a current flows. Set is expressed as a true direction and is paired with drift, the current’s rate. This differs from wind direction, which is conventionally named for the direction from which the wind comes.

A tidal current whose direction changes progressively through the tidal cycle rather than simply reversing along one axis. Successive hourly vectors form a pattern often represented by a tidal ellipse. Both direction and rate must therefore be taken for the required time rather than inferred only from flood and ebb labels.

The average height of low waters observed over a defined period at a particular place. Its formal definition and epoch depend on the responsible hydrographic or surveying authority. Individual low waters may be higher or lower because of the spring–neap cycle and meteorological effects.

The average height of high waters observed over a defined period at a particular place. The exact tidal-datum definition may specify which high waters and observation epoch are used. It supports coastal boundaries and height references but is not the height of every individual high tide.