Understanding the equinox tides


What is a tide?

The tide refers to the daily rising and falling movement of the oceans and seas. The phenomenon of ebb and flow of water takes place twice a day during a 12-hour cycle and is repeated ad infinitum.

The tide is the result of the attraction of the moon and the sun on the sea, but also of the rotation of the earth which generates a centrifugal force.

The Sun, although its mass is much greater and due to its greater distance, exerts a weaker attraction on the Earth than that of the Moon. Indeed, although smaller than the Sun, the Moon exerts a stronger attraction on the Earth because it is located closer to the Earth. It is this cumulative gravitational attraction which has the effect of deforming the surface of the oceans by inducing a lifting of the waters..

This so-called gravitational attraction varies depending on the position of the moon and the sun in relation to the earth: if these two stars are on the same side or diametrically opposed, there will be an ascending movement, we then speak of low tide. If they are located at 90° from each other the effect of their attractions is subtracted, it will be a downward movement, we then speak of low tide.

The amplitude of water movement varies according to the position of the sun and moon in relation to the Earth's axis. The more they are aligned with the Earth, the greater the amplitude of the tide will be.

What are tidal coefficients?

Tidal coefficients are used to measure the size of a tide. Coefficients range from 20 to 120 and are calculated by taking into account the height of high water and the average sea level.

Medium-low tides have a coefficient of 45, while medium-high tides have a coefficient of 75. High tides recorded on the coasts are those which exceed 90. Coefficient 119 has been reached twice, in 1918 and 1993.

What is tidal range?

The tidal range is the difference in water height between high and low tide. The stronger the tides, the greater the tidal range. The size of the tides also depends to a large extent on the tidal range.

On the French coast, the tidal range is most pronounced on the Atlantic Ocean, with high tides rising to over 10 meters at Mont Saint-Michel. During the equinoxes, the tidal range is of the order of 15 meters, and can reach 16 meters in exceptional spring tide conditions.

Why are equinox tides more important?

The high equinox tides occur every six months. During the equinoxes, the Sun exerts a stronger attraction on the Earth than the rest of the year: the Sun, the Moon and the equator (the axis of the Earth) being perfectly aligned.

This explains why the highest tides occur at the autumn (around 22 September) and spring (around 20 March) equinoxes, while the lowest tides occur at the winter (when the day is shortest) and summer (when the day is longest) solstices.

The Sun and Moon are perfectly aligned with the Equator during the high tides of the equinox, and the Equator is tilted in relation to the Moon and Sun during the low tides of the solstice.

As for the equinox tides known as "The Tides of the Century", they only occur every 18 years, with particularly high coefficients reaching at least 119. This is because the orbits of the Sun, Moon and Earth return to exactly the same configuration every 18 years. The last one occurred on April 21, 2015, the next ones are scheduled for March 3, 2033, and March 14, 2051...

Weather conditions that amplify high tides

There are three factors that can amplify the importance of a tide:

  • Atmospheric pressure, which varies the height of the water. An anticyclone, or high pressure, "presses on the sea" and lowers the water level. Conversely, a depression, or low pressure, raises the water level. Tide forecasts are always based on a pressure of 1013 hpa. Each hectopascal higher or lower corresponds to 1 cm higher or lower water level. Between a barometric pressure of 1013 and 1003 hectopascals, the sea will be 10 cm higher.
  • In the event of a storm, waves or swells can also cause water to rise higher than forecast.
  • A sea wind can push the sea up, particularly at the bottom of a bay where the sea bed rises gently, increasing the height of the water. Conversely, an onshore wind will lower the water level.

The combination of these three factors, associated with a high equinox tide, can result in flooding, submersive waves and a tidal bore.

A tidal bore is the meeting of the current of a river flowing into the ocean and the force of a large tide flowing in the opposite direction. The push of water from the tide moving inland creates waves that travel up the river.

High equinox tides: what photographic interest?

Whatever the event being photographed, it's recommended to be prepared: understand the phenomenon being photographed, scout the location, check the weather forecast, tide times, sunrise and sunset times and the orientation of the sun - all of which vary with the seasons - so that you can determine the best light conditions. This preparation ensures that you're in the right place at the right time on the big day...

When it comes to the equinox tides, the potential range of shots is endless: submerged quays and shores, crashing waves, tidal bores, and at low tide the sea uncovers seabeds that are usually submerged and inaccessible the rest of the year.

The three photos below illustrate this last example: the "Etel bar" on a normal tide.

The second made during the equinox tide of March 12, 2024, with a coefficient of 117, uncovering the dike and the numerous breakwaters usually submerged.

The third, taken on the normally inaccessible dike, with in the foreground the imposing concrete tripods, almost 2 meters high, which act as breakwaters. And who seemed very small in the first photo!