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Ocean Love: the rhythm of tides

Tides are fascinating. They can drastically change the coastal landscape in a matter of hours⁠, they give us rock pools home to rich biodiversity, the ability to generate renewable energy, to visit tidal islands by land and many species of marine life depend on them for survival.

I'm no astronomer, but I love that tides are affected by forces outside of our planet, a reminder of how our amazing ocean ecosystems thrive on a balance of elements. 

Whether you enjoy coastal walks, swims, beach days or watersports, tides should not be underestimated and are important to know for your safety.

The Tide Cycle

Most of us know that the sea rises and falls in cycles, and it has something to do with the gravitational pull from the Moon, Sun and rotation of Earth.

 

The lunar day. Earth spinning on it's axis and the Moon

 

A lunar day is 24 hours and 50 minutes, made up of the 24 hours it takes for Earth to rotate, plus the small amount the Moon has orbited.  

During a lunar day there are typically two high and two low tides at intervals of just over six hours. This pattern is called a semidiurnal tide cycle. As the Earth spins on its axis, one side is closer to the Moon and its gravitational pull causes the first high tide. The centrifugal force of the earth spinning causes the second high tide on the farthest side from the Moon.

Tide cycles vary to location. The Mediterranean sees less tidal amplitude due to the narrow channel connecting the sea to the Atlantic Ocean. In the Gulf of Mexico there are diurnal tides with only one high and one low tide in a lunar day. Tidal patterns vary around the world, even in the UK, where coastlines and sea depth varies.

 

High and lows tides on Earth. The gravitational pull of the Moon.

 

The movement of the tide from low to high is the flood, and the high tide draining away is called the ebb. The middle two hours of flooding and ebbing tides can see the most movement of water, and stronger currents. It can be particularily difficult to swim back to shore during an ebbing tide, and there is an increased risk of rip currents as the build-up of water from the shoreline heads back out to sea.

An hour either side high and low tide is reached there is usually a period of slack water where there is less movement, and the water is 'unstressed'. ⁠

 Slack water can be enjoyed by swimmers as there is little tidal current. Snorkelers and scuba divers may find improved visibility due to less silt being churned up. The timing of slack water depends on location and local conditions so it's really important to do your research beforehand.

Whilst tidal cycles describe patterns, it is important to remember that open water can be unpredictable and factors including waves, currents, obstructions, and entry/exit into the water must be considered. 

Neap & Spring Tides

During the new and full Moon the Sun adds to the gravitational pull of the Moon which makes low tide even lower, and high tide greater. This is called a spring tide. Seven days later follows a neap tide when the Sun and Moon are at right angles, partially cancelling the gravitational pull resulting in a smaller tidal range. 

 

Spring and neap tides. The Moon, Earth and Sun.
Spring tides can see stronger tidal currents with the larger difference in water heights. A beach or cove that you found was accessible during neap tide may not be during spring tide, so it's worth checking when high tide occurs ahead of your adventures to ensure you don't get cut off. 

Extreme Tidal Ranges

During equinoxes in March and September, spring tides are even larger due to the Moon being closest to Earth in its elliptical orbit, called the perigean spring tide, or king tide. At low tide coast goers can view exciting rockpools and their inhabitants which would ordinarily be hidden. 

 

However you adventure by the waterside, check local information on tides and conditions at Met Office and the RNLI safety advice

 

 Low tide rock pools