Tides are quite a complex phenomenon that arouses many questions. In a previous article, we explained to you why there are such things as high tides and low tides. In some areas, water moves just a dozen of centimetres while in other areas it can be dozens of metres ! Tides can be rather impressive, especially at the equinoxes. Another phenomenon that needs an explanation !
As we approach equinox periods, coastline dwellers are overwhelmed with both excitement and apprehension. Large tides can be beautiful to watch but its violence and speed can turn out to be dangerous, so it is better to watch them from a long distance.
Before going to watch this phenomenon, check out the tide schedule first to avoid bad surprises. With our tide indicating device , you will know at a glance whether it is high or low tide thanks to its simplified dial.
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While watching the landscape change, have you already wondered why tides are so strong at that time of year ? Let us explain to you why it is so.
Before talking about strong tides, let us tell you what equinoxes are about. They appear at times of the year when daytime is as long as night-time.
This phenomenon appears depending on the position of the Earth relative to the Sun. Indeed, at times of equinoxes, the equator faces the sun.
Why Are Equinox Tides Stronger Than Normal Tides ?
These times happen twice a year : the Spring Equinox happens around the 20th of March whereas the Autumn Equinox takes place around the 22nd of September, depending on the year. But what does it have to do with tides ?
If you have already read our previous article on tides phenomenon, you know that the pull exerted on the Earth by the Moon and Sun triggers tides.
During equinoxes, the Sun exerts a stronger pull on the Earth than the rest of the year, because of the alignment between the sun and the equator. Consequently, the water surface is strongly attracted by the Sun, which accentuates tides, we call them “great tides”.
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If on top of that there is also an alignment of the Moon with the Earth and Sun, then tides will get even stronger as the water will be twice as pulled by these two stars.
On the contrary, around the summer solstice (the longest day of the year ) and the winter solstice (the shortest day), tides get weaker. Indeed, the Earth is tilted to the Sun, the exerted pull thus gets a lot weaker and tidal coefficients get lower.
Let us leave space and put our feet back on Earth ! Basically, what happens to our coasts during equinox tide ?
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At equinox times, our coastal landscape changes radically and in very little time due to a strong tidal range. This means that the difference of height between low tide and high tide is stronger than usual.
For instance, in the Mont-Saint-Michel bay, the average tidal range is between 8 and 9 metres but it can reach 13 metres at times of strong coefficient like during equinox tide.
During equinoctial tides, you can enjoy admiring the endless stretches of sand caused by very low tides. It is a time much appreciated by people who enjoy gathering seafood by hand and walking along the seaside.
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However, beware of high tides going up very fast as they can cause floods. If you wish to admire strong equinox tides, we suggest you go to the town of Saint-Malo and the bay of Mont-Saint-Michel which have the highest tides in Europe.Often between 6-8 times a year, the new or full moon coincides closely in time with the perigee of the moon — the point when the moon is closest to the Earth. These occurrences are often called 'perigean spring tides.' High tides during perigean spring tides can be significantly higher than during other times of the year.
In order to understand the phenomenon called a 'perigean spring tide, ' you first have to know that the gravitational pull of the moon and the sun cause tides. Tides are actually long-period waves that roll around the planet as the ocean is 'pulled' back and forth as the moon and the sun interact with the Earth in their monthly and yearly orbits.
The next thing you need to know is that the moon follows an elliptical path around the Earth in its monthly orbit, and the Earth follows an elliptical path in its yearly orbit around the sun. This means that, at times, the moon and the sun are closer to Earth. At other times, they are farther away. What happens when the moon and the sun are close to the Earth? You guessed it: the gravitational pull they exert is stronger, resulting in slightly higher tides.
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While both the moon and the sun influence tides, the moon plays a much larger role because it is so close to the Earth. Its gravitational pull is about twice as strong as that of the sun. Now consider these two cases:
During full or new moons — which occur when the Earth, sun, and moon are nearly in alignment — average tidal ranges are slightly larger. This occurs twice each lunar month (about 29.5 days on average). The moon appears new (dark) when it is between the Earth and the sun. The moon appears full when the Earth is between the moon and the sun. In both cases, the gravitational pull of the sun is ‘added’ to the gravitational pull of the moon on Earth, causing the oceans to bulge a bit more than usual. This means that high tides are higher and low tides are lower than average. These are called 'spring tides.'
There are many factors that cause the tides to be higher than what is normally seen from day to day. Our monthly bulletin tells you when you may experience higher than normal high tides where you live. We also publish annual high tide flooding reports that present a broad outlook of what to expect for a given year in terms of high tide flooding, as well as a summary of high tide flooding events for the previous calendar year.
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Once about every 28 days, the moon reaches a 'perigee, ' its closest point of approach to the Earth. This is the point at which the gravitational pull of the moon is strongest. During these periods there will be an increase in the average range of tides. Conversely, about 14 days following the perigee, the moon reaches an ‘apogee’, its furthest point of approach to the Earth. This is the point at which the gravitation pull of the moon is weakest. During these periods there will be a decrease in the average range of tides.
Typically between 6-8 times each year, the new or full moon coincides closely in time with the perigee of the moon — the point when the moon is closest to the Earth. These occurrences are often called 'perigean spring tides.' The difference between perigean spring tides and spring tides that occur closer to the moon’s apogee are location dependent and significantly influenced by tidal range, but can be quite large. It is not uncommon for high tides during a perigean spring tide to be more than a foot higher than high tides during ‘apogean spring tides’. In places like Anchorage, Alaska, which has a tidal range over 30 feet, the difference between spring tides can be 3 feet or more at high tide!
It's also important to note that other factors influence the height of the tide as well. Seasonal effects on mean water level and the tide, like higher water level due to the thermal expansion of warmer water, can sometimes mean that some of the highest tides of the year are not perigean spring tides.
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Coastal flooding doesn't always occur whenever there is a perigean spring tide. However, perigean spring tides combined with seasonal changes in the tide and mean sea level may cause minor coastal flooding in some low-lying areas, often referred to as “high tide flooding” or “nuisance flooding”. Major coastal flooding typically occurs in response to strong onshore winds and barometric pressure changes from a coastal storm. If a storm strikes during a perigean spring tide, flooding could be significantly worse than it otherwise would have been. In some instances, perigean spring tides have coincided with a shift in offshore ocean circulation patterns and large scale shifts in wind that have resulted in unexpected coastal flooding. It is expected that occurrences of minor high tide flooding at the times of perigean spring tides will increase even more as sea level rises relative to the land. NOAA’s tide and tidal current predictions take into account astronomical considerations due to the position of the moon and the sun.I have always been fascinated with the tides. For me, the place at the edge of the ocean, where the earth, air and water converge, is where the magic and bioluminescence are found. When I was in college, a few of my courses contained lessons on what drives the tides, and while most of the learning from the courses of that
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