Glossary

Every struggle with all the odd sounding language used to discuss tides?  Well fear not, here we shed light on what is actually quite straightforward.

Tide
The cyclical changes in local sea levels caused by astronomical events.

Surge
Changes in sea level locally, not cyclical but caused by weather conditions such as wind and changes in atmospheric pressure.

Tidal Range
The difference between consecutive high and low water heights. Click HERE to see some ranges for ports around the British Isles.

Spring Tides
Tides with a greater tidal range which are produced when the gravitational effects of the moon and sun are in line with each other. The maximum effect is seen a day or two after a full or new moon.

Neap Tides
Tides with a lesser tidal range which are produced when the gravitational effects of the moon and sun are at right angles to each other. They occur a day or two after the quarter moons.

Spring-Neap Cycle
A single cycle of spring and neap tides lasts approximately 14.8 days.

Equinoctial Spring Tides
Above average tidal ranges caused when the moon and sun are lower in the sky (around the equinoxes: 21 March and 21 September). The highest and lowest tides of the year usually occur in either of these two months.

HAT and LAT
The Highest or Lowest Astronomical Tide is the highest or lowest tide that can be predicted using average meteorological conditions. This figure can be exceeded in times of storm.

Datum
Tide heights are always given for a particular reference point known as the datum. Published predictions are invariably related to Chart Datum which is the depth shown on the largest scale Admiralty chart. In the British Isles, Chart Datum is approximately the level of the Lowest Astronomical Tide (LAT).

Mean Sea Level
The level of the sea above or below chart datum determined as an average of measurements taken over a long period. Known technically as Zo.

Tidal Analysis
Tide gauges record the movement of the tide and from this raw data scientists can extract the details of the harmonising waveform patterns. These are an important part of the calculation when making predictions. The longer the period recorded the more harmonics can be extracted.

Tidal Regime
The typical pattern of tides at a particular location. The British Isles has high and low water occuring approximately twice a day (semidiurnal). In some parts of the world the regime can be daily (diurnal) or a mixture of both.