 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. |
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.
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.
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.
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).
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