what is a tide?
On some parts of the coastline, it is possible to observe the sea going out and coming back in several hours later, in a ceaseless to-and-fro motion. The sea level can therefore be seen to vary along certain parts of the French coastline, and this cyclical movement of the sea level corresponds to what we call tides. These tides can be seen quite clearly on the Atlantic and Channel and North Sea coasts, but remain very subtle in the Mediterranean.
Parts played by the Moon and Sun
Tides are explained by the movement of the seas and oceans, which fall under the gravitational pull of the Moon and Sun (that is the force of attraction that the Sun and Moon exercise on the oceans’ water masses). The Sun and the Moon move round the Earth, and the intensity of the tides changes based on their relative position to the Earth:
When the Sun, Earth and Moon are aligned, the pull exercised by the Moon and the Sun is added together, and the sea, and ocean water masses are subject to these stars’ additional pull, the tides are strongest as a result. These are called spring tides.
When the Moon, Earth, and Sun form a right angle, the tides are weakest and are called neap tides.
Between high and low tides
The difference in height between high and low tide is called “the tidal range”. And this tidal range is not the same everywhere. It can be weaker in the Mediterranean, with a tidal range of around 40 centimeters, or stronger in the Channel, on the Baie du Mont Saint-Michel side, with a tidal range of around 14 meters. And, viewed from a given position on the coastline, the tidal range can also vary according to the time of year (and more precisely according to the tidal cycle). The tidal range is very helpful for giving an idea of the importance of the tide for a given place. However it is not enough when comparing the importance of two tides in two different places.
How to know if the tides are important or not?
In order to compare the force of the tides on any site, the Service Hydrographique et d’Océanographie de la Marine (SHOM) has defined a tidal coefficient for France. This consists of a number, between 20 and 120, which gives an idea of the size of a tide.
- 20 = exceptional neap tide
- 45 = a medium neap tide
- 70 = a medium tide
- 95 = a medium spring tide
- 120 = an exceptional spring tide
These tidal coefficients are available free on the Internet, or at the local tourism office.
Don’t get caught out by the rising tide
Going out along the coastline is not risk-free, particularly on the Atlantic and Channel and North Sea coastlines, where the tides are quite pronounced. Some lookout and safety rulesneed to be kept in mind when visiting an intertidal zone that becomes submerged due to tidal change. There is a danger, for example, if you are not careful, that you could become engulfed on a rocky islet by the rising tide. Also, despite your vigilance, you could be caught out by the speed of the rising tide in the particular site where you are.
To avoid these risky situations in general, pay attention to the following instructions:
- Have you checked the times of the tides for the site that you are going to visit (at what time does the tide go out? At what time is it at its lowest (what is called the “slack tide”?) And at what time does it start to come in again?),
- Take a watch with you and set an alarm if you have one.
- Start back at the latest when the sea is low. Do not take the risk of waiting for the rising tide before starting to return to land.
To learn more, consult:
The Service Hydrographique et d’Océanographie de la Marine (SHOM)