All about tides

Tides are the alternating rise and fall of the sea surface. They are due mainly to the gravitational attraction (pull) of the moon and sun on the rotating earth. Two high and two low tides occur daily around Britain and, with average weather conditions, their movements can be predicted with considerable accuracy.

Tides – the earth, the moon and the sun

Both the moon and sun affect the tides, but since the moon is much closer to the earth (384,400 km instead of 149,600,000 km), it has more than twice the effect of the sun, even though it is much smaller. So, to understand tides it's best to start with the moon and the lunar tide.

The earth isn't fixed rigidly in space, and as the moon orbits, it attracts the earth round in a monthly orbit of its own. So the earth has a small orbit caused by the moon in addition to its annual one round the sun. It is the gravitational pull of the moon on the earth which keeps the earth in its monthly orbit.

This is similar to when you whirl a weight round on a piece of string. It is only by constantly pulling on the string that you prevent the object from flying off. If you stop pulling completely, by letting go on the string, the object does fly off. Gravity acts in the same way as you pulling on the string, and prevents the earth from flying off. However, unlike the string, the attractive pull of gravity gets weaker as the distance between the objects gets larger (and it becomes stronger, of course, the closer they get). The earth is large, so the pull of gravity on the side of the earth nearer the moon is stronger than the pull on the side of the earth farther from the moon.

The pull of the moon's gravity is just enough to keep the earth in its monthly orbit, but it is a bit stronger on the surface of the earth facing the moon (near side) and weaker on the far side. This means that on the near side, the moon tends to pull anything that's free to move towards it. In the same way, on the far side, there isn't quite enough gravitational pull, so that anything that's free to move tends to fly off, away from the moon. So on the near side they are pulled into a bulge towards the moon, and on the far side, they pile up into a bulge away from the moon.

As the earth spins, different parts of the world move under the two bulges of high water and experience high tides, giving the familiar two tides a day around Britain. In some parts of the world, local effects can mean only one tide a day, or even none.

Between the two bulges are two troughs of low water, producing two low tides a day. The sun also creates a very similar though smaller effect (the solar tide) and it is the interaction of the lunar and solar tides that causes spring and neap tides.

Springs and neaps

Tides change in height – low water level and high water level vary throughout the month. The tides build up to a maximum and fall to a minimum twice a month. The tides with the biggest difference between high and low water are called springs and those with the smallest are called neaps.

Spring tides happen just after every full and new moon, when the sun, moon and earth are in line. That's when lunar and solar tides line up and reinforce each other, making a bigger total tide. Neap tides occur when the moon is in the first or third quarter - when the sun, earth and moon form a right angle. The lunar high tide coincides with the solar low tide and they partly cancel out, giving a small total tide.

The regular motion of the sun, moon and earth cause spring tides to occur roughly 36 to 48 hours after the full or new moon, and for any given location, always at roughly the same time of day. For example at Liverpool, the spring tides are generally around midnight and midday.

Not all spring tides are the same size. Springs nearest the equinoxes (21 March and 21 September – when day and night are of equal length all over the world) are slightly bigger. The explanation for this is very complex and won't be given here.

Worldly effects

Back on earth, tides are affected by more mundane things, such as weather and the shape of the coastline. In the upper reaches of estuaries the river bed and shape of the estuary can distort the tidal pattern, producing long flood times and short ebb times. Though the time from high tide to high tide is the same as usual – roughly 12 hours 25 minutes – low tide is not half way between them. At some places, the flood tide rises quickly after a period of low water lasting maybe four or five hours. This kind of geographical effect is very important, and because of it you should use local tide tables (available from local newsagents and tackle shops) when possible.

Cromer, Lowestoft and Aldeburgh are fairly close to each other on the East Anglian coast. However, high tide is hours apart at these places. This is precisely the opposite to Southend and Herne Bay, which face each other across the mouth of the Thames. Their tides differ by only a few minutes. Without detailed geographical information, it is impossible to predict these differences, so keep to local tide tables.

Geography also affects the tidal range. Looking at tide tables for all of Britain, it's clear that the height of the tide varies around the country. For example the spring tidal range at Avonmouth is 12.2 m while at Lowestoft it's only 1.9 m.

Weather which can have a profound effect on the tide, is impossible to predict when calculating tide tables. Strong winds and abnormal atmospheric pressure are two of the main causes of altered tides. For example, a strong wind blowing on to land has the effect of piling up the water, giving a higher than predicted tide.

Original article by Valerie Doodson