Tides - Definitions and Notes
The gravitational forces of the moon and sun create areas of high and low water on the earth's surface. As the earth rotates the location of high and low tide changes. The moon has the greatest effect on the water compared with the sun due to its proximity to the earth and the configuration of the sun and moon, whether aligned or offset, has an effect on the tidal range.
The tides of increased range occurring near the times of full moon and new moon. The gravitational forces of the moon and the sun act to reinforce each other. Since the combined tidal force is increased the high tides are higher and the low tides are lower than average. Spring tides is a term which implies a welling up of the water and bears no relationship to the season of the year.
The tides of decreased range occurring near the times of first and third quarter phases of the moon. The gravitational forces of the moon and the sun counteract each other. Since the combined tidal force is decreased the high tides are lower and the low tides are higher than average. Neap comes from a Greek word meaning scanty.
Interpreting Tide Tables
Tide tables give the high and low water times and heights for particular locations as shown by the example table below. All times are given in Greenwich Mean Time so please remember to add an hour during British Summer Time. All heights are in metres above local chart datum.
Height of Tide (m)
The tidal levels are defined below. For the table of chart datums and standard levels in the Port of London, please click here. For an introduction to chart datums and charting on the Thames please click here.
HAT (Highest Astronomical Tide) & LAT (Lowest Astronomical Tide).
|The highest and lowest levels respectively 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 may cause considerably higher and lower levels to occur.
MHWS (Mean High Water Springs) & MLWS (Mean Low Water Springs)
The height of mean high water springs is the average of the heights of two successive high waters during those periods of 24 hrs (approximately once a fortnight) when the range of the tide is greatest. The height of mean low water springs is the average height obtained by the two successive low waters during the same period, i.e. -
MHWS The average HEIGHT of the HIGH WATERS of SPRING TIDES above Chart Datum.
MLWS The average HEIGHT of all LOW WATERS of SPRING TIDES above Chart Datum.
MHWN (Mean High Water Neaps) & MLWN (Mean Low Water Neaps)
The height of mean high water neaps is the average, throughout a year as defined above, of the heights of two successive high waters during those periods (approximately once a fortnight) when the range of the tide is least. The height of mean low water neaps is the average height obtained from the two successive low waters during the same periods, i.e.
MHWN The average HEIGHT of the HIGH WATERS of NEAP TIDES above Chart Datum.
MLWN The average HEIGHT of the LOW WATERS of NEAP TIDES above Chart Datum.
The values of MHWS, MHWN, MLWN and MLWS vary from year to year in a cycle of approximately 18.6 years. In general the levels are computed from at least a year’s predictions and are adjusted for the long period variations to give values which are the average over the whole cycle. The values of Lowest Astronomical Tide (LAT) and Highest Astronomical Tide (HAT) are determined by inspection over a span of years.
MHW (Mean High Water) & MLW (Mean Low Water)
The height of mean high water is the average hight between mean high water springs and mean high water neaps as defined above. The height of mean low water is the average height between mean low water springs and mean low water neaps. i.e.
MHW The average HEIGHT of the HIGH WATERS of SPRING and NEAP TIDES above Chart Datum.
MLW The average HEIGHT of the LOW WATERS of SPRING and NEAP TIDES above Chart Datum.
MSL (Mean Sea Level)
Mean Sea Level is the average level of the sea surface over a long period, normally 19 years, or the average level which would exist in the absence of tides, i.e.
The average HEIGHT of the surface of the SEA at a TIDE STATION for all stages of the TIDE over a 19 year period, usually determined from hourly height readings measured from a fixed predetermined reference level (Chart Datum).
Frequenty Asked Questions - Thames Tides
Q) The predicted tide values in the PLA tidebook are sometimes slightly different to those published on the PLA website. Is there a reason for this?
A) Yes, there can be slight differences in the order of a few minutes for predicted times or up to 0.1m for predicted heights between the values printed in the "Tide Tables and Port Information" booklet and those published on the PLA website. These differences are due to the prediction software that produces them. The printed tide tables, hourly heights and predictions used in the Port Control Centres are generated by the Admiralty using their prediction software where as the web based predictions are generated in house by the PLA using Geotide to give predictions for up to 5 years in advance and for all tide stations. If these slight differences are critical to your passage plan, live tidal observations are broadcast on the port VHF channels and are available on our website.
Q) Occasionally there are multiple consecutive low waters predicted at Richmond for a given day. Is this an error or can it be explained?
A) This phenomenon is not an error but due to the prolonged low water period that exisits at Richmond. The prediction programme looks for the turning points in the tidal cycle (i.e. where the tide changes from rising to falling). At Richmond this means that slight variations in the prediction can result in consecutive low waters. The adjacent graph helps to explain this. The multiple low waters have not been edited out of the tide tables so as to illustrate the low water stand that occurs.
Further Information on Tides