Tides - Frequently Asked Questions

Q. How are tides predicted?
First the behaviour of the tide has to be recorded by a tidegauge at each particular location and the observations subsequently analysed. The results of these analyses, known as tidal constants, are then used in a computer program to predict the behaviour of the tide for any period required.

Q. How far ahead can the tide be predicted?
The tides are generated by gravitational influences of the Earth-Moon-Sun system, whose astronomical relationship and orbital details are known extremely accurately. Using this information, together with the tidal constants derived from the tidal analyses, the tides can be predicted for any date as far ahead into the future as required.

However, for predictions well into the future (many decades) the effects of global sea level rises and changes in seabed topography have to be carefully considered. These may have a significant effect upon local tidal behaviour, and the predictions can only be computed using the tidal data currently available on the UKHO database. So the further ahead we go from the date the tidal data is held on the database, the less reliable the predictions will become.

Q. What are "Spring" and "Neap" tides?
Spring and Neap tides are created by the relationship between the Earth and the Moon during each lunar month. Spring tides occur shortly after New and Full Moon, and Neap tides occur shortly after the Moon is in its First and Third Quarter.

Spring tides occur when the gravitational forces of the Sun and Moon reinforce each other resulting in a higher than normal tidal range. Neap tides occur when these gravitational forces act at right angles to each other resulting in a lower than normal tidal range. Spring tides are nothing to do with the season, but it is believed the name is derived from a medieval word to "leap up". Neap comes from an old English word for "low" or "to nip".

Q. When can we expect the largest spring tides each year?
The largest spring tides occur in this country shortly after the New and Full Moon closest to the equinoxes, and are sometimes referred to as "Equinoctial Spring Tides". The spring (or vernal) equinox occurs around 21st March, and the autumnal equinox occurs around 23rd September.

Q. Where are the highest tides around the world?
The largest tides occur in the Bay of Fundy in Canada. At Burncoat Head the range reaches nearly 16 metres. However, the Bristol Channel has the second highest tidal range in the world with ranges in excess of 15 metres, just above Avonmouth.

Q. Why is my local harbour not available?
Tidal predictions can only be computed for locations for which there is tidal data, derived from analysis of the tidal observations. If no tidal observations have been taken at your particular location then, unfortunately, no tidal predictions can be computed.

Q. Why is there a difference between the predictions on TV/Radio and those we get by using Admiralty Tide Tables?
It depends whether it is a Standard or Secondary Port as defined in Admiralty Tide Tables (ATT).

For Standard Ports it could simply be that the UKHO database had been updated since the ATT predictions were computed and it is the latest data that is used for the TV/Radio predictions.

For Secondary Ports it could be that ATT uses non-harmonic data (time and height differences) whereas TV/Radio predictions come off the UKHO's commercial prediction software using harmonic constants. Different methodology produces slightly different results.

Furthermore, for both Standard and Secondary Ports the degree of complexity of the local tidal regime can have a marked influence on the differences experienced between the predictions derived from the two methodologies. However, such differences should not be significant for normal navigational purposes.

Q. Where can I find tidal predictions for periods longer than 7 days?
UKHO's authoritative publication Admiralty Tide Tables contains a year's predictions and is re-published annually. However, most ports, harbours and havens around the UK publish their tide times for the year in booklet form and these are usually available from local shops etc. Sailing or nautical almanacs, on sale in bookshops and chandlers, reproduce a year's predictions (under licence from UKHO) and the best known of these are usually held by local public libraries.