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High tides

The Saint Malo tides are amongst the biggest in Europe…

Effectively, when the enormous body of water that is the Atlantic rushes into the bottleneck of the English Channel, the flow of water is extremely rapid and extremely strong. The tidal range (i.e. the difference between high tide and low tide) can reach an average of over 13 metres on the shore surrounding the town walls. Predicting the magnitude of the tides is achieved by using tide coefficients. This ‘range’ goes from 20, the weaker end of the scale (‘neap tide’), to 120, the highest or equinoctial tide (‘spring tide’). Anything from 90 is classed as a spring tide.. 

Difference maree

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And here’s why…

Tidal range is the variation in the height of sea level caused by the gravitational pull of the moon and the sun combined with the force of inertia due to the Earth rotating around the centre of gravity in the Earth-Moon system. During a full moon and a New Moon (i.e. when the Earth, Moon and Sun re roughly on the same axis), the effect/presence of celestial bodies builds resulting in tides of greater amplitude (a spring tide). At the other end of the scale, during the first and last phases when the Earth, Moon and Sun are in quadrature (when the moon is 90° from the sun as viewed from the earth), the amplitude is at its weakest (a neap tide). The current of the rising tide is called the flux or the swell whereas the current of a receding tide is called the reflux or ebb tide. Depending on the position of the Earth, the flux/reflux cycle can take place once per day (diurnal tide), twice per day (semidiurnal) or it can be a mixed cycle. When the sea reaches its highest level in a tide cycle, it is known as ‘high sea’ or high tide. At the other extreme, the lowest level is called ‘low sea’ or low tide. When the sea reaches its highest or lowest level and can go no further, we refer to the sea as being ‘slack’ (neither rising nor receding). This is what we usually call ‘high tide’ or ‘low tide’ although the word tide usually indicates movement. The weakest tides of the year usually occur in the summer solstice or the winter solstice whereas the strongest occur in the spring or autumn equinoxes.  The movement of the tides is not limited to the water but affects all the Earth’s crust - what we refer to as ‘crustal tide’ - albeit to a lesser degree. As a result, what we see on the coast is in fact the difference between the crustal tide and the ocean tide. Generally speaking, celestial bodies that are close to other bodies are affected by tidal forces (for example, Io, one of Jupiter’s moons is affected by colossal tidal forces).




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