THE TIDES - 1st rhythm
How many tides per day?
This is the first rhythm that is easiest to observe.
- The water level rises or falls 4 times a day:
- 2 high tides or high tides
- 2 low tides or low tides
Is the rate of rise and fall of the water regular?
No, the tide rises and falls in stages; its movement is irregular. It's important to know this for safety reasons, especially on coastlines where the tidal range is significant.
To simplify, in a 24-hour day, the water level rises in approximately 6 hours and falls in approximately 6 hours. Note that this is only a guideline, as in reality this time period is never exactly 6 hours. It can be more or less.
THE TIDES - 2nd rhythm
How many tide periods are there per month?
This is the second tidal pattern observed along our coastline. Periods of neap tides (when the water level rises and falls only slightly) alternate with periods of spring tides (when the water level rises and falls more significantly). Indeed, along our coastline, water levels are not always the same. The water level rises and falls to varying degrees depending on these periods.
On a monthly scale there are:
- 2 periods of spring water
- 2 periods of neap tides
This second rhythm is linked to the phases of the moon and the position of the stars in relation to each other.
The sun is shining:
Its influence is added to that of the moon at the time of the full moon and the new moon = period of spring water.
Its influence counteracts that of the moon at the time of the first and last quarters = period of neap tides
THE TIDES - 3rd rhythm
The third rhythm occurs at the equinox every six months. The spring equinox generally occurs in March, the autumn equinox in September. The sun amplifies its effect, and the coefficients are generally higher.
The coefficients
This is a French tool used only in metropolitan France. Historically, we use coefficients calculated for each high tide. These coefficients are established for the port of Brest and are used all along the Atlantic, English Channel, and North Sea coastlines.
They tell us about the relative importance of the tidal range: the difference in water level between two successive tides, high and low. The coefficient makes it easier to read and indicates the magnitude of the water movement. The greater the tidal range, the higher the coefficient.
The higher the coefficient, the more the water rises and falls. During periods of neap tides with a coefficient below 45, the water rises and falls very little, for example.
This second rhythm is linked to the phases of the moon and the position of the stars in relation to each other.
The sun is shining:
Its influence is added to that of the moon at the time of the full moon and the new moon = period of spring water.
Its influence counteracts that of the moon at the time of the first and last quarters = period of neap tides
