Tidal phenomenon: high and low tides | Ponant Magazine

Calm, regular, powerful, complex: tides are a natural phenomenon that is both fascinating and mysterious. Before sailing, you have to take into account the tidal currents. Here are four vital things to know about the ebb and flow of the tide.

1. Definition

The tide is the rise and fall of the oceans and seas. It is the result of forces exerted by the Moon and the Sun on the sea combined with the rotation of the Earth which generates a centrifugal force. This so-called gravitational force varies depending on the position of the Moon and the Sun in relation to the Earth: if they are on the same side or directly opposite each other, there will be a high tide. If they are separated by 90°, there will be a low tide.

2. Tidal coefficient

The tidal coefficient is the size of the tide in relation to its mean. It usually varies between 20 and 120. The higher the tidal coefficient, the larger the tidal range – i.e. the difference in water height between high and low tide. This means that the sea level rises and falls back a long way. The mean value is 70. We talk of strong tides – called spring tides – from coefficient 95.  Conversely, weak tides are called neap tides.

3. Marine currents

A marine current is a regular, continuous, cyclic movement of sea water. It is created by the combined action of constant winds pushing the surface water, as well as by the tide and the Coriolis force which changes the direction of currents according to the hemisphere. As a reminder, the Coriolis principle is an effect whereby the trajectory of a moving object on the surface of the Earth is deflected. The current is also created by the numerous differences in temperature

4. Exceptional tides

The highest and most spectacular tides can be observed in France. For example, in Saint-Malo, the tidal range reaches more than 12 metres on average due to its geographic location. On 21 March 2015, the “tide of the century” occurred with a coefficient of 119. This phenomenon takes place every 18 years due to the alignment of the Sun, Earth and Moon and the spring equinox in the northern hemisphere.

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