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Tides questions and answered
Questions answered

Below is a selection of questions that have been asked by members of the public. The normal NERC disclaimers apply to any advice given.



  1. What is a tide? (What is a storm surge?)
  2. How far ahead can the tide be predicted?
  3. What causes tides?
  4. If gravity is always pulling towards the moon, what causes the bulge on the opposite side of the earth?
  5. Why are the tides not the same all round the coast of Britain?
  6. How often do high tides occur?
  7. Are there always two high tides a day?
  8. What are spring tides and neap tides?
  9. Why are they called spring and neap tides?
  10. Where are the highest tides in the world?
  11. Do the tides follow a repeated pattern?
  12. When during a year can we expect to find the largest tides?
  13. Do the planets have any affect on the tides?
  14. How can tide tables be produced so accurately?

1. What is a tide?

This is not as obvious as it may sound - it is not just the rise and fall of the water in our seas and oceans (tides are only 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 force of gravity. These are the values that you can see in tide tables.

Movement of water caused by meteorological effects (for example winds and atmospheric pressure changes) are called surges.  These 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. POL develops storm surge models for flood forecasting that have been run at the Met Office since 1978. (A large positve storm surge can add a few metres to the predicted water level.)

There is also wave movement which is purely wind generated and impossible to predict accurately. Therefore statistical values are used such as significant wave height which is the average of the highest 1/3 of waves.


2. How far ahead can the tide be predicted?

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 year 2005, get in touch and (for a small fee) Applications Group will provide you with the tide table.

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


3. What causes tides?

Tides are caused by the effects of gravity in the earth-moon-sun system, and the movement of those 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 (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).


4. If gravity is always pulling towards the moon, what causes the bulge on the opposite side of the earth?

Most 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: gravity towards the moon and a rotational force away from the moon are perfectly in balance. They have to be otherwise the earth and moon would not stay in this orbit.

Earth-moon system

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. It is this that creates the second bulge away from the moon. On the surface of the earth, the horizontal tide generating forces are more important than the vertical forces in generating the tidal bulges.

forces at the surface

5. Why are the tides not the same all round the coast of Britain?

You might expect that as Britain passes under the bulge of water, time of high water would be roughly the same for all points on the coast, but it isn't. The problem is caused by the land that '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 sea depth (bathymetry), there is a lag. Every location has a unique coastline and bathymetry - which gives each location its unique tidal pattern.


6. How often do high tides occur?

In UK waters, approximately every 12 hours 25 minutes. You may wonder why it is not exactly 12 hours, but you must remember 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.

Earth-moon system

7. Are there always two high tides a day?

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

The diagrams below show a typical tidal curve for three places round the UK coast.

Liverpool Tides  Lowestoft Tides  Portland Tides

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.


8. What are spring tides and neap tides?

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 7 days after spring tides.


9. Why are they called spring and neap tides?

Neap means low - so that is 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 possible source is a Scandinavian word meaning to 'leap up'. Another possibility is that it is related to the natural feature of a spring - which is a place where water wells up from the earth.


10. Where are the highest tides in the world?

Burncoat Head in the Bay of Fundy, Canada has an average spring tidal range of 12.9 m. The second largest tides in the world occur at Avonmouth in the Bristol Channel where the average spring range is 12.3 m.
(The average spring tidal range is the average difference between high and low waters during spring tides).


11. Do the tides follow a repeated pattern?

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


12. When during a year can we expect to find the largest tides?

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 2 cm higher than in 1997.


13. Do the planets have any affect on the tides?

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 significant. The nearest approach of Venus to earth is still more than 100 times further away than the moon. Hence the tidal force is approximately 0.000054 times that of the moon. The next most significant planet is Jupiter, where the tidal force is 0.000005 times that of the moon. So as you can see, the effect of the planets is negligible.

Even if all the planets line up such that their effects are combined, the additional force would be minuscule. During 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, something which never happened.


14. How can tide tables be produced so accurately?

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 of tidal observations for that port are needed (called a time series). 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 shown in questions 6 and 8, there are certain frequencies that are known to occur in the tide. Some of these are listed below:

12 hour (12:00:00.0) repeated pattern (cycle) due to the gravity of the sun.
12:25:14.164 cycle due to the gravity of the moon.
24:00 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 (Moons 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 as you can see, the frequencies of these is 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) is stored. These two values (known as a harmonic constant) 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 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 2 high waters and 2 low waters every 24 hours and 50 minutes (or 4 tides a day on most days with 3 tides about every 7 or 8 days).


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