Marazion and St Michael's Mount Tides

A brief explanation of tides and how they affect the people of Marazion and St Michael's Mount - Page 2

Continued from page 1

Neap tides occur shortly after the Moon is in its first and third Quarter, when these gravitational forces act at right angles to each other resulting in a lower than normal tidal range. The name Neap is believed to come from an old English word for "low" or "to nip".

There are many rocks in Mount's Bay that are only just covered at low spring tides; some are even uncovered at low tide around the Equinoctial Spring Tides. Neap tides can also cause problems. A couple of times a year, neap tides have such a small tidal range that the causeway from Marazion to St Michael's Mount does not open at all. Knowledge of the local Marazion and St Michael's Mount tides is, therefore, useful for sailing or boating or other activities in and around Mount's Bay.

The tide predictions on this web site may differ from those that you hear on the Radio or see on Television. This is because those predictions are based on information provided under license from the United Kingdom Hydrographic Office (UKHO), which uses proprietary tidal information to derive their predictions. If you need accurate predictions for navigation purposes, you should contact the UKHO.

Whichever tide predictions you use, you will need to take account of the weather, which affects tides more than you might expect.

Tide tables assume a standard pressure of 1013 millibars. A change in pressure of one millibar results in a change in the sea level by one centimetre. The effect is not, however, immediate and tends to average out over a wide area.

This means that in high summer in Marazion when the pressure may exceed 1040 millibars (quite high but not abnormal) the sea level could be nearly 30 centimetres lower than tide predictions. If you are boating, this could make the difference between crossing the Causeway and hitting it!

On the other hand, in a major storm the pressure may be only 960 millibars (the lowest pressure ever recorded in the British Isles was about 925 millibars) which would give sea levels more than half a metre above tide predictions.

The wind also affects sea levels, particularly in Mount's Bay. Strong onshore winds cause the sea level to be higher than predicted, while offshore winds have to a lesser extent the reverse effect. Under certain conditions, a 30mph onshore wind can increase tide height by up to 25 centimetres. This effect is known as "wind set-up".

The combination of wind setup and the pressure effect associated with storms can create a pronounced increase in sea level. A long surface wave travelling with the storm depression can further exaggerate this sea level increase. This is often called a storm surge. There have been such storm surges around Marazion and St Michael's Mount in recent years, usually in the autumn, and some have caused flooding on St Michael's Mount.