Sharing knowledge – Tides: questions and answers

This is not as obvious as it sounds – the rise and fall of the water in our seas and oceans are not just because of tides (although tides are one of the contributing factors in this rise and fall). A tide is the regular and predictable movement of water caused by astronomical phenomena – the way the earth, moon and sun move in relation to each other and the attractive force of gravity – these are the values published in tide tables.

Movement of water caused by meteorological effects (for example, winds and atmospheric pressure changes) are called surges. They are not easily predictable and require powerful computers and sophisticated software to predict just 36 hours in advance. These are the reasons why tide table predictions do not always agree with observations. The National Oceanography Centre in Liverpool developed the storm surge models for flood forecasting that have been run at the Met Office since 1978. (A large positive storm surge can add a few metres to the predicted water level.)

There are also wave movements which are purely wind-generated and impossible to predict accurately, therefore statistical values are used, such as significant wave height, the average of the highest one-third of waves.

Since the tide is caused by the astronomy of the earth-moon-sun system, which is known very accurately and can be predicted well into the future, the tides can also be predicted well into the future. So if you want to plan your sailing club events for the next year, get in touch and (for a small fee) the Applications Team will provide you with the relevant tide table.

When trying to predict tides well into the future, we have to take into account the rise in global sea level. The further into the future we try to predict, the more significant this effect can become.

Tides are caused by the effects of gravity in the earth-moon-sun system, and the movement of the three bodies within the system. If you imagine that the earth is completely covered in water, there are two bulges of water – one towards the moon and another on the opposite side (see question 4). The rise and fall in sea-level is caused by the earth rotating on its axis underneath these bulges of water. There are two tides a day because it passes under two bulges for each rotation in 24 hours (see question 7). This is called the lunar tide.

Two bulges of water are also caused by the sun, called the solar tide - and these can either reinforce or partially cancel out the lunar tide to give spring and neap tides (see question 8).

Many people think the moon rotates round the earth. In reality, the earth and the moon rotate about a common centre just inside the earth's surface (indicated by the light blue dot on the diagram). At the centre of the earth the two forces acting are: gravity towards the moon, and a rotational force away from the moon. These are perfectly in balance, otherwise the earth and moon would not stay in this orbit.

The 'tide-generating' force is the difference between these two forces. On the surface of the earth nearest the moon, gravity is greater than the rotational force, and so there is a net force towards the moon causing a bulge towards the moon. On the opposite side of the earth, gravity is less as it is further from the moon, so the rotational force is dominant. Hence there is a net force away from the moon that creates this second bulge. On the surface of the earth, the horizontal tide generating forces are more important than the vertical forces in generating the tidal bulges.

You might expect that as Britain passes under the bulge of water, the time of high water would be roughly the same for all points on the coast – but it isn't, because the land 'gets in the way' of the moving water. As the earth rotates, the water has to move to generate the high tides but because of the shape of coastlines and the variation in the depth of the sea bed (bathymetry), there is a lag. Every location has a unique coastline and bathymetry, which gives each location its unique tidal pattern.

In UK waters, high tides occur approximately every 12 hours 25 minutes. You may wonder why it is not exactly 12 hours, but you need to consider that the moon is also orbiting around the earth. By the time a point on the earth's surface has rotated from point x to point y (12 hours) the moon has also moved a small amount, so the earth has to rotate for an extra 25 minutes from point y to point z to be under the high water bulge.

No. Although most coastal locations in Britain experience approximately two tides a day (semi-diurnal) there are some places which experience what is known as a double-high water (for example, Southampton) or double-low water (for example, Weymouth). This is caused by the shape of the coastline and the shape of the bathymetry (water depth).

The diagrams below are tidal curves for Liverpool on the west coast, Lowestoft on the east coast and Weymouth on the south coast.

Liverpool

Lowestoft

Weymouth

In some parts of the world there is only one high and one low water each day (diurnal) – for example in Karumba, Australia. In other places, it varies between semi-diurnal and diurnal as in Musay-id in the Arabian Gulf.

When the earth, moon and sun are in line (during new and full moon), the bulges of water caused by the moon and sun occur in the same place on the earth's surface. The lunar tide and the solar tide are reinforcing each other – which leads to higher than average high tides, and lower than average low tides. These are called spring tides.

When the earth, moon and sun form a right angle (at 90°) the high water caused by the lunar tide coincides with the low water of the solar tide. This produces lower than average high waters and higher than average low waters which are called neap tides. They occur approximately seven days after spring tides.

Neap means 'low' – so that's an easy one. Spring tides can be confusing because they have nothing to do with the season. It is not exactly known where the word 'spring' comes from in this context, but there are two possible origins – one is a Scandinavian word meaning to 'leap up'; another is related to the natural feature of a spring, which is a place where water wells up from the earth.

In Canada there has been some rivalry between Arctic Quebec and the Canadian Maritimes over which one has the world's highest ocean tides. The Canadian Hydrographic Service has declared a tie between the famous tides of the Bay of Fundy and those of Ungava Bay on the northern coast of Quebec.

It has long been recognised that the tides at Burntcoat Head on the shore of Minas Basin, Bay of Fundy can in extreme reach a range of 17 metres. After 200 days of measurements at Leaf Basin in the southwest corner of Ungava Bay it is estimated that in the extreme the tides there could have a range of 16.8 metres. It is of course possible that points near Burntcoat Head or Leaf Basin, as yet unmeasured by tide gauges, could have slightly higher tides so lacking further data a dead heat has been declared.

The next highest tides are in the Bristol Channel where the extreme range at Avonmouth is just over 15 metres.

No. There are similarities – for example every 18.6 years, we experience larger than average tides – but they never actually repeat.

A day or two after the full or new moon nearest to the equinoxes. The spring equinox is usually the 21st March, and the autumn equinox, the 23rd September.

Some years have tides that are notably higher than other years. 1997 was a significant year, as will be the year 2015. For really favourable conditions – you will have to wait around until the year 3182. Even then, the tides may only be 1 or 2cm higher than in 1997.

The tidal force generated by a planet is based on two things – the mass of the planet and its distance from the earth – and it is the latter of these that is far more important. The nearest approach of Venus to earth is still more than a hundred times further away than the moon. Hence the tidal force is approximately 0.000054 times that of the moon. The next most significant effect is from Jupiter, with a tidal force of 0.000005 times that of the moon. That's why the effects of the planets are negligible.

Even if all the planets line up such that their effects are combined, the additional force would be minuscule. On 3rd May 2000, Mercury, Venus, Mars, Jupiter and Saturn lined up with the sun and moon. At the time a rumour circulated that the collective gravitational pull would initiate earthquakes, tidal waves and volcanic eruptions, which of course never happened.

There are many different steps involved in obtaining the final numbers that go into a tide table. Before a tidal prediction can be made for a port, a long sequence (or time series) of tidal observations for that port are needed. This time series will include all the astronomical effects and local coastline/depth effects which make up the tide, as well as the weather-induced effects called the surge (see question 1).

As highlighted in questions 6 and 8, there are certain frequencies that are known to occur in the tide. Some tidal periods (period = 1/frequency) are listed below:

12 hour (12:00:00) repeated pattern (cycle) due to the gravity of the sun,
12:25:14.164 cycle due to the gravity of the moon,
24:00:00 and 24:50:28.328 cycles caused by the differences in the two tidal bulges,
27.2122 day cycle caused by change in lunar declination (moon's angle to the earth),
27.5546 day cycle caused by a regular change in the earth-moon distance,
29.5306 day cycle caused by the phases of the moon (see question 8).

Each of these cycles is called a tidal harmonic constituent and the frequencies of these are known very accurately. Therefore it is easy to find them in a sequence of observations using a method called tidal analysis. Once each constituent is identified, its size (amplitude) and time of 'arrival' (phase) are stored. These pairs of values (known as a harmonic constants) are unique for every location.

The amplitude and phase for each constituent combined with the fixed speed of that constituent allows us to predict its contribution to the overall tide forward or backward in time almost indefinitely. Adding up the effects of all the constituents at a given location lets us predict the overall tide at any time in the future or past. See the Applications Group's Tidal Prediction Service.

Most tide tables just list the time and height when the water is at a maximum and minimum level in each tidal cycle. This leads to approximately two high waters and two low waters every 24 hours and 50 minutes (or four tide table entries a day on most days with three entries about every 7 or 8 days).