Below are frequently asked questions that we hope provide response and support to any questions you may have.
Where you cannot find an answer to your question, please contact us using email@example.com.
Q: How are tides predicted?
A: First the behaviour of the tide has to be recorded by a tide gauge at each particular location and the results subsequently analysed. The results of the analysis, known as tidal constants, are then used in a computer program to predict the behaviour of the tide for any period required.
Q: Can I add your tide times to my website?
A: We welcome links to or from other sites, providing they meet the criteria outlined in our Links Policy. Further details can be found here.
Q: I am interested in obtaining tidal predictions for the purposes of publishing my own tide table / placing predictions on my own website. Can EasyTide be used for this?
A: No. You can place a link to EasyTide on your own website provided that it meets the criteria outlined in our Inward Links Policy outlined here.
For details of bespoke supply of tidal predictions for commercial purposes, please visit our Tidal Prediction Service (TPS) website at https://tps.admiralty.co.uk/ for more information.
Q: What is Chart Datum?
A: Chart Datum is the plane below which all depths are published on a navigational chart. It is also the plane to which all tidal heights are referred, so by adding the tidal height to the charted depth, the true depth of water is determined. By international agreement Chart Datum is defined as a level so low that the tide will not frequently fall below it. In the United Kingdom, this level is normally approximately the level of Lowest Astronomical Tide.
The highest and lowest predicted tides that can occur are deemed Highest Astronomical Tide (HAT) and Lowest Astronomical Tide (LAT) respectively. These levels are the highest and lowest levels which can be predicted to occur under average meteorological conditions, and under any combination of astronomical conditions; these levels will not be reached every year. HAT and LAT are not the extreme levels which can be reached, as Storm Surges (wind-induced long period waves causing higher and lower-than-predicted levels to occur) and pressure effects can significantly alter the times and / or heights of the observed tide.
In order to determine the manner in which tidal levels vary along any given stretch of coastline, it is necessary to refer all levels to a common horizontal plane. Chart Datum, being dependent on the range of the tide, which varies from place to place, is not a suitable reference plane.
Ordnance Datum (Newlyn) can be regarded as a horizontal plane and, where comparisons of absolute heights are required on the mainland of England, Scotland and Wales, this should be used.
Ordnance Datum (Newlyn) is the datum of the land levelling system on the mainland of England, Scotland and Wales, and on some of the closer islands offshore; this datum was established at the level of the average value of Mean Sea Level at Newlyn for the six-year period 1915-21. Table III in Admiralty Tide Table Volumes 1A (NP201A) and 1B (NP201B) gives the connection between Chart Datum and Ordnance Datum (Newlyn or Local) for all Standard Ports and many Secondary Ports in the UK, while Table IV gives the equivalent figures for other countries.
Q: What are Spring Tides and Neap Tides?
A: The range between consecutive High and Low waters at a port changes over a full lunar month. When this range is at its greatest the tides are referred to as 'Spring tides' and when the range is at its smallest the tides are 'Neap tides'.
Q: When do Spring Tides occur?
A: In the main a spring tide occurs 2 days after the date of New and Full Moon. However, a Spring Tide can occur anywhere from 0 to 4 days after New and Full Moon - it just depends upon the geographical location of the tidal station concerned and how the tide-generating forces are affected by friction on the surface of the earth.
Furthermore, there are certain places, mainly in the Pacific, where Springs appear to occur before the date of New and Full Moon, but what is actually happening is that the Spring Tide occurs between 11 and 14 days after the previous New or Full Moon.
Fundamentally, it is the characteristics of the local tidal regime which result in the Spring Tide occurring after the instant at which the tide raising forces are at a maximum (i.e. at New and Full Moon).
From the harmonic analysis of tidal observations we can determine how the individual harmonic constituents (which combine to create the tidal regime) behave relative to their theoretical astronomical values. It then becomes a matter of cause and effect, where the resultant effect always occurs after the force has been imparted.
The constituent phase will always lag behind the theoretical (astronomical) value, and it is the combination of inertia and friction associated with the water mass which gives rise to the phase lag.
The two main constituents which influence our tides around the UK are the Moon’s semi-diurnal and the Sun’s semi-diurnal constituents, M2 and S2 respectively. The harmonic analysis provides us with the phase lag of these two constituents behind their astronomical values from which we can determine the occurrence of the maximum height of the Spring Tide after the time of New and Full Moon. This is also known as the Age of the Tide.
This occurs on average 2 days after New and Full Moon but, for example, it can be as much as 3 days after at London Bridge and only 1 day after at Dublin depending upon the local tidal regime.
A more comprehensive explanation can be found by consulting any authoritative textbook on tidal theory. Our own Admiralty Manual of Tides (NP120) may possibly be found in a local reference library, otherwise it can be obtained via your local Admiralty chart agent or chandler.
Q: Why do Spring High Tides always seem to occur at the same time of day?
A: The reason is that when the New and Full Moon occur, the two major semi-diurnal constituents (M2 and S2) of the tide raising force are precisely in phase. Consequently, High Water Springs will always occur at about the same time of day.
Q: The time of the Spring High Water at my local port always appears to occur at 0000 / 1200 hrs, and I have been told that this is always the case. Why is this?
A: To say that the Spring High Water (after New and Full Moon) should occur at local midday/midnight is purely theoretical, even though, coincidentally, this may be the case at your local port. For example, the Spring High Water (HW) occurs at the following rudimentary times in each of these major UK ports: Plymouth around 0700; London around 1600; Greenock around 1330. It should be stressed these are very coarse times but it is clear that there is a wide variation, and even a cursory inspection of the predictions displayed by EasyTide, or Admiralty Tide Tables Volumes 1A (NP201A) and 1B (NP201B), will provide further ample proof.
The tidal predictions themselves are generated using the harmonic constants derived from the analysis of actual tidal observations taken over any given period of time (30 days minimum to 19 years maximum). These tidal constants are unique to each location, which is why predictions quoted for individual locations may appear somewhat illogical on occasions at first glance.
In a purely theoretical progression the time of HW should advance by 50 minutes each day, but in practice you can expect the times to differ markedly from this concept simply as a direct result of the tidal cycle on the given day being at either Springs or Neaps (or in between).
We publish an excellent graphical representation of the tidal variations around the British Isles and adjacent waters, namely Co-Tidal Chart 5058, showing the Mean High Water Interval (MHWI) (combining Springs and Neaps) and Mean Spring Range. MHWI is defined as the mean time interval between the passage of the Moon over the Meridian of Greenwich and the time of the next High Water at the place concerned.
Q: What does 'ML inferred' mean?
A: Where no substantial data is available to compute Mean Sea Level, the value has to be inferred from data held for adjacent tidal stations in order to enable a tidal prediction to be computed using the Simplified Harmonic Method.
Q: On using EasyTide Predictions, I notice a difference between the Times / Heights shown on the website compared with the Times / Heights on the day. Why is this?
A: It is important to note that the data displayed on EasyTide are predicted values only. They do not take into account any meteorological effects which may be prevalent on the day(s) in question. Such effects (such as wind and pressure effects) can significantly alter the observed tide, causing it to deviate considerably from the predicted values.
Q: Your predictions don’t agree with my observations of the tide. What can be done about this?
A: With our obligations for safety and liability for all the data we publish we must remain guarded whenever considering changes to long-established tidal data, which we have published in good faith from the outset.
Nevertheless, we are only too prepared to re-evaluate our tidal data at any time in the light of reported discrepancies. But experience has shown that visual observations alone can be misleading because the tidal characteristics can vary significantly depending upon whether it is a Spring or Neap tide, and whether or not the meteorological conditions are excessive. Consequently, the only reliable way in which we could consider amending our existing tidal database would be through a detailed analysis of continuous structured tidal observations over a complete lunar month (actually 30 days), which can subsequently be held up to rigorous scrutiny.
Regrettably new tidal observations at existing locations are not always easy to come by, especially if the original data came from a hydrographic survey of the area, which means that a tide gauge would only have been established for the duration of the survey, plus the necessary number of extra days in order to resolve a lunar month. Unfortunately the UKHO is not resourced to commission new tidal observations where we are not sponsoring a hydrographic or tidal survey, and so often new analyses are simply not possible.
Q: I have a locally-produced tide table - it shows different predictions compared to EasyTide. Which is correct?
A: We do not have access to the underlying database which has been used by other independent tide table publishers. Therefore, we cannot comment directly on the sources of the data they use in order to compute their tidal predictions.
Suffice to say that any difference between two sets of tidal predictions will be accounted for by the following:-
So it is inevitable that there will be differences between any two sets of predictions based on the above facts. Thus the two sets of predictions are simply different, and neither is necessarily wrong. The best way to quantify the accuracy of any predictions would be to carry out a rigorous statistical analysis of the observed tide against the predicted tide.
Q: Where can I obtain information about Tidal Streams?
A: All tidal stream data, where available, is contained on the largest scale Admiralty Charts of the particular area concerned. In the main it takes the form of the position marked by a Tidal Stream Diamond, which then refers to a table of Mean Spring and Mean Neap Rates and Directions for the whole tidal cycle, that is, from 6 hours before to 6 hours after the time of High Water at a suitable reference port (Standard Port). The predictions for the port can be obtained from EasyTide or the relevant volume of Admiralty Tide Tables.
We also have a series of Tidal Stream Atlases, covering the north west European Continental Shelf, displaying structured observed tidal stream rates and directions, embellished by schematic flow diagrams, intended for navigation. Please see Tidal Stream Atlases for details.
Q: I am interested in mean tidal levels as opposed to predictions - where can I obtain these?
A: All volumes of Admiralty Tide Tables contain mean tidal levels at locations for which we hold the necessary tidal data. They can be purchased from any Admiralty Distributor listed at How To Buy; or may be possibly be viewed free of charge in a good reference library.
Q: There seems to be a strange progression in the times of High / Low Water at my port on certain days - can this be correct?
A: Often the key to explaining the seemingly illogical tabulated predictions lies in an investigation of the predicted tidal curve on the day(s) in question. It is often useful to display the predicted curve for just one day at a time in order to appreciate the issues involved.
For example, certain ports exhibit a complex tidal regime, such as double High Waters (at Southampton) or double Low Waters (at Portland), or High / Low Water 'stands' where the predicted height of the water remains unchanged for a long period of time (at Invergordon).
Furthermore, at Neap tides, (where the amount of tidal energy is reduced), very shallow predicted curves can occur making the turning points less precisely defined.
Q: I have noticed that the predictions for my port show consistently different heights of successive High Waters in a 24-hour period. Why is this?
A: The cause of the difference in height between successive High (and indeed Low) Waters at any port is simply due to the diurnal inequality resulting from the declination of the Moon on any particular day. Were the Moon to remain on the celestial equator (which of course it does not) then the successive High Waters would be identical.
Q: Why is my local harbour not available?
A: Tidal predictions can only be computed for locations at which there are tidal data that have been derived from the analysis of the tidal observations. If no tidal observations have been taken at your particular location then, unfortunately, no tidal predictions can be computed.
Q: During what year can we expect to find the largest tides?
A: Extreme tidal forces are prevalent during March/April and September/October, during the Equinoxes (21st March being the Vernal Equinox; 23rd September being the Autumnal Equinox). At these times the Spring tides (generally those occurring shortly after the Full or New Moon) are usually significantly larger than usual due to the fact that the Solar Semi-Diurnal forces are maximised.
By simple arguments it is evident that extreme tidal forces will occur when the Moon and Sun are in line with the Earth and at their closest respective distances, amplified further when the Moon and Sun have zero declination.
There are several times when zero Solar declination (the Equinox) and Lunar Perigee occurs almost simultaneously with zero Lunar Declination. The significant date when this occurred recently was 8 April 2020, with the next situation not occurring again until 11 March 2028.
Of course, as illustrated above the level of Highest Astronomical Tide (HAT; the highest level which can be predicted to occur under average meteorological effects, and under any combination of astronomical conditions) will not be reached every year. It can also be further amplified by weather effects (such as pressure and wind effects) causing significantly higher tidal levels to occur. Storm Surges can greatly affect the levels, as was the case in 1953 along the east coast of the UK and the Netherlands coast, where predicted High Waters were exceeded by between 2.4 to 3.0m.
Q: How far in advance can you predict tides?
A: Theoretically, once the necessary tidal constants have been derived for a particular location from the analysis of raw tidal observations, predictions can be obtained indefinitely at that place. However, realistically, the natural environment in which tides operate is continually changing. Topographical changes to the seabed and man-made constrictions such as harbour developments can all have an effect on the tidal regime experienced at a location. It is for this reason that, wherever possible, we maintain a continuous program of tidal analysis which refines and updates the underlying data, over a continuous period of 19 years (the Metonic Cycle). Hence although predictions can be computed indefinitely, it is not practicable to say how much the tidal regime may change at any given location, thus rendering predictions computed for many years into the future less reliable than those computed nearer the time.
Q: How often do tides occur?
A: There are basically three types of tidal regime. Diurnal tides are those where one High and one Low Water occur during a period of 24 hours mainly experienced in the Pacific Ocean. Semi-Diurnal tides are characterised by having two High and Low Waters in a 24 hour period mainly experienced in the Atlantic Ocean. The third type is Mixed, where the tidal regime exhibits characteristics of both Diurnal and Semi Diurnal tides experienced in areas such as the Indian Ocean, Gulf of Mexico, and Australia.
Q: Why are there different tidal ranges in different parts of the world?
A: Every body of water of any size has a natural frequency of oscillation. The tide-raising forces which act upon the surface of the earth will produce an imposed-force on that body of water. If the frequency of the imposed force is the same as the natural frequency, resonance will occur and very large movements of water (or displacements) will occur from very small applied forces.
The natural period of oscillation is the decisive factor in determining whether the water mass responds by one of three regimes, namely diurnal (one High and One Low Water per lunar day), semi-diurnal (two High and Low Waters per lunar day) or mixed (features of both, where the diurnal inequality is relatively large).
Where the oceanic tide enters the shallow water on the continental margins, the tidal range is amplified accordingly. Depending on the shape of the bordering land masses, this can further compound the tidal wave, constricting and funnelling it in sympathy with its natural resonance, creating huge tidal ranges. For example, the Bay of Fundy in Nova Scotia, Canada, has a natural resonance period of around 12.5 hours, exactly coincident with the semi-diurnal tidal cycle, and thus experiences the worlds greatest tidal range. Similarly this effect is prevalent in the Bristol Channel (second-highest tidal range in the world). Conversely, the English Channel experiences much smaller ranges indeed Avonmouth and Portland have longitudes which differ by less than 1, yet their Mean Spring Ranges are 12.2m and 2.0m respectively. Furthermore, the Mediterranean and Baltic are of such a size and shape that they prevent any appreciable tide to be generated.
Q: Why are there limited tidal ranges in the Mediterranean & Baltic Seas?
A: All the tides we experience on Earth are generated in the Great Oceans as a result of their response to the tide raising forces generated by the gravitational attraction between the Earth, the Moon and the Sun.
If the natural resonance period of the body of water matches the frequency of the tide raising forces then it will respond accordingly and a tide is generated. For example, the Atlantic has a natural period of resonance in the order of 12.5 hours and so responds vigorously to the twice daily (semi-diurnal) tide raising forces. The Pacific on the other hand has a natural period of resonance closer to 25 hours and so responds vigorously to the daily (diurnal) tide raising forces. There are, of course, exceptions to both in unique locations in either ocean but to understand why you would need to consult any authoritative textbook on tidal theory.
Other large bodies of water such as the Baltic, Black Sea, Caspian Sea and indeed the Mediterranean do not have natural periods of resonance which align closely with either the diurnal or semi-diurnal tide raising forces. Hence they do not respond vigorously and, consequently, no significant tide is generated. They do respond marginally, however, but the tide is only what I would call centimetric and so we deem these areas to have no appreciable tide which for safe navigational purposes can be taken to be less than one foot or 0.3 metre.
Nevertheless, there is a one metre tide at Gibraltar due to the direct influence of the Atlantic tide spilling into the Mediterranean at that point. There is also a one metre tide at Venice as the top end of the Adriatic responds more vigorously with a harmonic of the semi-diurnal frequency. Finally, the largest tide in the Mediterranean is at Sfax in the Gulf of Gabes with a 1.5m tidal range for reasons similar to those at Venice.
Q: What causes the Double High Water at Southampton and the Double Low Water at Portland? I have been led to believe that the Double High Water at Southampton is caused by the influence of the Isle of Wight - is this correct?
A: The double High Water (HW) at Southampton has nothing whatsoever to do with the tide entering simultaneously via the East and West Solent - which is a common misconception. It is entirely due to the ultimate manifestation of the Shallow Water Tide (a distortional effect upon the astronomically generated tide caused by shallow waters), which begins probably somewhere off Start Point, and which first manifests itself as a double LW at Portland and then becomes a double HW at Southampton.
The technical reason for the migration from a double LW to a double HW is the rapid change in the phase of the principle lunar semi-diurnal harmonic constituent (M2) between Portland and Southampton, which then directly affects the higher harmonic constituents of the Shallow Water Tide. The presence of the degenerate amphidromic point* lying to the northwest of Bournemouth results in further complexities of the local tidal regime.
Any authoritative textbook on tidal theory should give a comprehensive explanation of this phenomenon. Our own Admiralty Manual of Tides (NP120) gives a thoroughly comprehensive explanation, and may possibly be found in a local reference library; otherwise it can be obtained via your local Admiralty chart agent or chandler.
*Amphidromic Point: a point where there is no tidal range, caused by the coriolis effect, from which co-tidal lines radiate. An amphidromic point is said to be degenerate when its centre appears to be located over land rather than water.
Q: Predictions are referred to being computed for average barometric pressure. What is average, and how do tides react to differences from this average?
A: Tidal predictions are computed for average barometric pressure at the particular place concerned. The average barometric pressure for certain places is given in Admiralty Sailing Directions and information is also given in some instances concerning the changes in level which can be expected under different conditions.
A difference from the average of 34 hectopascals can cause a difference in height of about 0.3m. A low barometer will tend to raise sea level and a high barometer will tend to depress it. The water level does not, however, adjust itself immediately to a change of pressure and it responds, moreover, to the average change in pressure over a considerable area. Changes in level due to barometric pressure seldom exceed 0.3m but, when mean sea level is raised or lowered by strong winds or by Storm Surges (wind-induced long period waves causing higher and lower-than-predicted levels to occur), this effect can be important.
Q: It states that 'Tidal Predictions are in the Standard Local Time Zone of the place concerned'. What does this mean, and how do these Time Zones work?
A: Universal Time (UT) is the mean solar time of the prime meridian obtained from direct astronomical observation and corrected for the effects of small movements of the Earth relative to the axis of rotation. Greenwich Mean Time (GMT) is based on the hour angle of the mean sun and for all tidal predictions may be taken as the same as UT. The term UT(GMT) is used throughout this volume.
There are twenty four Time Zones in the world each of which covers 15° of longitude. The zero time zone, in which the time kept corresponds to Greenwich Mean Time, is centred on the prime meridian and extends from 7.5°W. to 7.5°E. The other zones, in which the time kept differs from GMT by an integral number of hours, are sequentially numbered and have either a negative prefix if east of Greenwich or a positive prefix if west of Greenwich.
To convert Zone Time to GMT, the number of hours as given by the zone number is added to or subtracted from the Zone Time, e.g. in Zone - 0400 the time kept is 4 hours in advance of GMT and so at 2000 local time it is 1600 GMT, i.e. to obtain GMT apply zone number and its sign.
On land, a uniform time is adopted for convenience throughout a given country even though its boundaries may not wholly lie within a time zone. The Standard Time or Legal Time is in most cases that of the zone in which the country mainly lies. Countries having a longitudinal extent greater than a time zone may adopt more than one Standard Time, e.g. Eastern Standard Time, Pacific Standard Time in the United States.
Daylight Saving Time (Summer Time), introduced to prolong the hours of daylight in the evening, may in certain countries be the Legal Time for part of the year. The Standard Time of the zone to the eastward is normally adopted during such periods, e.g. BST (British Summer Time) is Zone - 0100. In certain countries this advanced time has been made Standard Time throughout the year. On EasyTide messaging on the Prediction Dashboard indicates when Daylight Saving has been applied.
The times of port predictions are given in the normal Standard Time kept by the port. When using EasyTide it should be verified that this is the same as the time which is actually being kept. Changes in Zone Times are not always reported in sufficient time for inclusion in EasyTide. For the latest information consult Admiralty List of Radio Signals Vol.2 (NP282) corrected by Section VI of the weekly edition of Admiralty Notices to Mariners.
Q: I have heard the term tidal bulge to account for the fact that there are two High Waters a day. What is this bulge?
A: The tidal "bulge" is a purely theoretical phenomenon relating to the theoretical Equilibrium Tide, which would exist only if the world were covered entirely by water.
The tides actually experienced around the world are all generated in the great oceans, which oscillate as a result of the tide raising forces acting in concert with the natural period of resonance of each ocean. The oceanic tides are then modified by the shallow water tides generated in the continental margins thereby resulting in the tides actually experienced at the coast.
Any authoritative textbook on tidal theory should give a comprehensive explanation of the tide raising forces and their impact on the waters around the world. Our own Admiralty Manual of Tides (NP120) may possibly be found in a local reference library; otherwise it can be obtained via your local Admiralty chart agent or chandler.
Q: I see that many Admiralty Charts refer vertical clearance heights above Highest Astronomical Tide (HAT). Where can I find values of HAT?
A: Values of HAT can only be accurately obtained at specific locations via the investigation of daily predictions at those places over a significantly long period of time (ideally the full Metonic cycle of 18.6 years). For all Standard Ports, values of HAT are published in Table V, Part 1 of the relevant volume of Admiralty Tide Tables (ATT) - for the UK this is Volumes 1A (NP201A) and 1B (NP201B). For Secondary Ports, you will find these values in Table V, Part 2
You can also utilise the information provided in the relevant volume of ATT to obtain inferred levels of HAT at Secondary Ports, using the linear relationships in High Water Height Differences between the Secondary Port, and its reference Standard Port. Information on how to do this is given in the preliminary pages of the relevant volume of ATT.
In addition, our commercial tidal predictions software TotalTide displays values of HAT at all port locations for which the necessary underlying data are available, thus negating the need for you to have to compute it yourself. For further details about TotalTide, please see www.ukho.gov.uk/ProductsandServices/DigitalPublications/Pages/ATT.aspx.