Tides - What Causes Them and How to Predict Them
Tides vary from day to day both in their timing and in height.
The tides at a particular location and time are principally the result of the gravitational influences between the Earth, the Moon and the Sun coupled with the effects of Earth rotation and the depths of local oceans, seas and estuaries.
The orbital details of the Earth, Moon and Sun and the relationships between them are known with a high degree of accuracy.
In addition to orbital information, tide prediction requires information (known as tidal constants) derived from an analysis of tides over a long period of time.
A computer programme using orbital information and tidal constants information can be used to calculate the predictions of future tides with a good degree of accuracy.
As well as times, tidal heights vary from day to day as the relative positions of the Earth, Moon and Sun change.
Spring tides occur shortly after New Moon and Full Moon, when the gravitational forces of the Sun and Moon reinforce each other, resulting in a higher than normal tidal range.
Spring tides are nothing to do with the season; the name may have been derived from a medieval word meaning to "leap up".
The largest spring tides occur shortly after the New Moon and Full Moon closest to the equinoxes; these tides are sometimes referred to as "Equinoctial Spring Tides".
The spring (or vernal) equinox occurs around 20/21 March, and the autumn equinox occurs around 22/23 September.
Neap tides occur shortly after the Moon is in its first and third Quarter, when these gravitational forces act at right angles to each other resulting in a lower than normal tidal range.
The name Neap is believed to come from an old English word for "low" or "to nip".
Whichever tide predictions you use, you will need to take account of the weather, which affects tides more than you might expect.
Tide tables usually assume a standard pressure of 1013 millibars.
A change in pressure of one millibar results in a change in the sea level of approximately one centimetre.
The effect is not, however, immediate and tends to average out over a wide area.
This means that in high summer in the United Kingdom when the pressure may exceed 1040 millibars (quite high but not abnormal) the sea level could be nearly 30 centimetres lower than tide predictions.
On the other hand, in a major storm the pressure may be only 960 millibars (the lowest pressure ever recorded in the British Isles was about 925 millibars) which would give sea levels more than half a metre above tide predictions.
The wind also affects sea levels.
Strong onshore winds cause the sea level to be higher than predicted, while offshore winds have to a lesser extent the reverse effect.
Under certain conditions, a 30mph onshore wind can increase tide height by up to 25 centimetres.
This effect is known as "wind set-up".
The combination of wind setup and the pressure effect associated with storms can create a pronounced increase in sea level.
A long surface wave travelling with the storm depression can further exaggerate this sea level increase.
This is often called a storm surge.
Although, computer programs can be used to predict tides for any date as far ahead into the future as required, such predictions take no account of the effects of future global sea level rises and changes in seabed topography, which for predictions many decades ahead may be significant.
Copyright 2010 Clive Goodhead
The tides at a particular location and time are principally the result of the gravitational influences between the Earth, the Moon and the Sun coupled with the effects of Earth rotation and the depths of local oceans, seas and estuaries.
The orbital details of the Earth, Moon and Sun and the relationships between them are known with a high degree of accuracy.
In addition to orbital information, tide prediction requires information (known as tidal constants) derived from an analysis of tides over a long period of time.
A computer programme using orbital information and tidal constants information can be used to calculate the predictions of future tides with a good degree of accuracy.
As well as times, tidal heights vary from day to day as the relative positions of the Earth, Moon and Sun change.
Spring tides occur shortly after New Moon and Full Moon, when the gravitational forces of the Sun and Moon reinforce each other, resulting in a higher than normal tidal range.
Spring tides are nothing to do with the season; the name may have been derived from a medieval word meaning to "leap up".
The largest spring tides occur shortly after the New Moon and Full Moon closest to the equinoxes; these tides are sometimes referred to as "Equinoctial Spring Tides".
The spring (or vernal) equinox occurs around 20/21 March, and the autumn equinox occurs around 22/23 September.
Neap tides occur shortly after the Moon is in its first and third Quarter, when these gravitational forces act at right angles to each other resulting in a lower than normal tidal range.
The name Neap is believed to come from an old English word for "low" or "to nip".
Whichever tide predictions you use, you will need to take account of the weather, which affects tides more than you might expect.
Tide tables usually assume a standard pressure of 1013 millibars.
A change in pressure of one millibar results in a change in the sea level of approximately one centimetre.
The effect is not, however, immediate and tends to average out over a wide area.
This means that in high summer in the United Kingdom when the pressure may exceed 1040 millibars (quite high but not abnormal) the sea level could be nearly 30 centimetres lower than tide predictions.
On the other hand, in a major storm the pressure may be only 960 millibars (the lowest pressure ever recorded in the British Isles was about 925 millibars) which would give sea levels more than half a metre above tide predictions.
The wind also affects sea levels.
Strong onshore winds cause the sea level to be higher than predicted, while offshore winds have to a lesser extent the reverse effect.
Under certain conditions, a 30mph onshore wind can increase tide height by up to 25 centimetres.
This effect is known as "wind set-up".
The combination of wind setup and the pressure effect associated with storms can create a pronounced increase in sea level.
A long surface wave travelling with the storm depression can further exaggerate this sea level increase.
This is often called a storm surge.
Although, computer programs can be used to predict tides for any date as far ahead into the future as required, such predictions take no account of the effects of future global sea level rises and changes in seabed topography, which for predictions many decades ahead may be significant.
Copyright 2010 Clive Goodhead