How Tides Work
How tides work is a very large topic and this article is not replacing complex textbooks but tries at putting tides into the context of what we [the non-Scholar] need to consider when visiting the coast. So let us get started…
What Controls the Tides?
Most places in Britain experience two tides per day and the tide is fundamentally caused by gravitational interactions between the Sun, Moon, and Earth.
The Moon’s gravity pull is a little stronger on the surface of the Earth-facing the Moon and weaker on the hidden far side. This means that on the Moon side, the Moon tends to pull, whilst on the opposite side, there isn’t quite enough gravitational pull, so things move away. This means that on the Moon side water is pulled into a bulge towards the Moon, and on the far side, the water piles up into a bulge away from the Moon.
As the Earth rotates, different locations move under the two bulges of high water and experience high tides. This produces two tides a day around Britain, although in some parts of the world, local factors can mean only one tide a day, or even more than two.
Between the two bulges are two troughs of low water, producing two low tides a day, but the difference in height between consecutive low and high tides varies over time and from place to place. These tide movements are also known as the lunar tide as they are caused by the Moon.
The Moon does not orbit at a constant distance from the Earth and this impacts the tides. When the Moon’s orbit takes it furthest from the Earth smaller tidal ranges result, the opposite being true when the Moon passes closest to the Earth. The difference between low tide at one point of the cycle and high tide at another may be small.
The Sun also creates a very similar but smaller effect (solar tide) and it is how the lunar tide and solar tide interact that causes two special types of tides, Spring and Neap:
Spring tides have nothing to do with springtime, they are tides with ranges greater than the average monthly range. Spring tides happen just after every full and new Moon when the Sun, Moon and Earth are in line. You are actually getting the lunar tide added to the solar tide making a bigger total tide.
Neap tides are tides with ranges less than the average monthly range. They occur twice each month when the Moon is in the first or third quarter, so when the Sun, Earth and Moon form a right angle. In this case, you get the weaker solar tide to some degree cancelling out part of the lunar tide giving a smaller total tide.
Tides are affected by things such as weather and the shape of the coastline making it very important to get predicted times from as close to your location as possible and this is why on all our beach pages the high and low tide times always tell you where the value was taken and the distance away.
Geography also affects the tidal range, for all of Britain it’s clear that the height of the tide varies around the country. For example, the spring tidal range in the west at Avonmouth on the Severn Estuary is 12.2 m while at Lowestoft on the Suffolk east coast it’s only 1.9 m.
The weather can have a profound effect on tides with strong winds and abnormal atmospheric pressure being two of the main causes of altered tides. For example, a strong wind blowing onto land has the effect of pushing up the water, giving a higher than predicted tide.
Why do high and low tides change each day?
It takes 24 hours and 50 minutes (a lunar day) for the same location on Earth to re-align with the Moon and you would then expect in British waters because we have two high tides a day for these to occur approximately every 12 hours 25 minutes. Thus, the high or low tide point to move out by 50 minutes every day.
Although this is roughly true with the lunar day being a dominant factor actually many other factors and local variations have an impact, this is why if you have purchased a tide clock over the weeks it gets out of sync as it just works on the 12 hours 25 minutes assumption.
Thus, you can use the 50 minutes as a rough idea from one day to another but not when looking 7 or more days out, in this case, you should always consult a tide times book.
So why are we explaining about tides?
Tides are one of the most important aspects to consider when visiting a beach as they can impact your trip. The tide really impacts two items, the amount of beach that is available to enjoy and the condition of the sea.
The tide comes in and out twice a day, this means the beach that you arrived at in the morning can be a very different place only a few hours later.
When you head to some of the remote beaches that offer no facilities it is often great to explore the rock pools, caves and hopping around headlands to neighbouring coves. It is this situation when knowledge of that day’s tide and height is very important as the environment can change very quickly and you could have your return route blocked.
This is a great part of the British coastline and does not have to be risky. We would always recommend walking around headlands when the tide is going out rather than in and then the first thing to check in the new cove is alternative routes out.
The tide conditions will impact swimming conditions. At a simplistic level, the tide is at its slackest, lowest or no movement, around high and low tides and the tide is moving at its quickest at mid-tide. The speed that the water is travelling is then related to whether it is a Spring tide, a large difference between low and high or Neap tide with low variance.
A Springtide will have a greater range from higher to lower, so more water will move, so the speed of the running tide will be greater.
As well as the tide all beaches slope at different angles and an incoming tide (i.e. low to high) when linked to the weather and the gradient of the beach could create some big waves.
Do tides impact the waves on beaches?
Not really, waves are most commonly caused by the wind, created by the friction between wind and surface water. As the wind blows across the surface of the sea the continual disturbance creates a wave crest. This is very similar to the effect when you blow over a bowl of water.
As these deep-water waves travel toward the shallower shore water, the bottom part of the wave drags along the seafloor and so the upper visible part of the wave has more momentum than the rest of the wave and will begin to lift and tilt forward. At a point in time, the wave is travelling fast enough to curl over, creating what is called a breaker.
There are 4 basic types of breakers, depending on the type of shoreline. Spilling breakers occur on gently sloping beaches where the waves break slowly and over a long distance, with the crest spilling gently down the front of the wave. On steeper beaches, the waves slow down much more quickly and the crest curls way over the front of the wave and plunge down towards the base. This is a plunging breaker and is liked by experienced surfers. In some cases, where the beach is very steep, the wave builds up very suddenly and breaks or dumps right onto the beach. These are surging breakers. We also have a hybrid, the Collapsing breaker is a cross between plunging and surging, in which the wave gets steeper and then just collapses, resulting in foam.
The wave’s size depends on the wind speed, direction, and the area over which the wind is blowing. This variability leads to waves of all shapes and sizes. In the UK some of our best surfing beaches are those sloping bays on the west coast where the waves have been blown undisturbed across the Atlantic and these are often called Atlantic Rollers.
Rip currents are not really related to tides but should be mentioned. Beaches are not a uniform slope, as the waves come up onto the shore they lose momentum and energy and then have to go back toward the sea. Have you ever stood on the sand as a wave comes in and the power of the water rolling back to the sea tries to drag you along?
A rip current is this same effect but out in the sea where certain things on the seafloor funnel that water into a narrow stream. This stream is normally narrow and maybe travelling considerably faster out to sea than surrounding areas.
If you get caught in a rip current you should not try to swim straight back to shore, because you will be fighting the current. Instead, you need to be aware that it will be narrow and so should aim to get free by swimming parallel or diagonally to shore.
The relationship between tides and weather and a knowledge of how they work is a critical factor in a successful beach visit.
Tides are in the main predictable and formed because of the interaction of the gravitational forces between the Sun, Moon and Earth.
Waves are unpredictable and formed because of the force exerted by the wind on the water surface meeting shallower water.
Although tides can be closely predicted way ahead of any visit the complex relationship with coastal landscape and weather makes for a changing environment. Before heading out into the sea and waves, consult a lifeguard, local surfer/swimmer or other information sources at the beach to get details about that day’s tide times, weather conditions and if rip currents are expected.
Treat the sea with respect and you will have a great time!