Tides - Definitions

About Tides

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.

Spring Tides

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.

Neap Tides

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.

Tidal Levels

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.

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).

Greenwich Mean Time

Greenwich Mean Time or Meridian Time has been a time standard since 1884. It was chosen because the Prime Meridian (the line of 0° Longitude) runs through Greenwich.

The Greenwich Meridian (Prime Meridian or Longitude Zero degrees) marks the starting point of every time zone in the World. GMT is Greenwich Mean (or Meridian) Time is the mean (average) time that the earth takes to rotate from noon-to-noon.

GMT is World Time and the basis of every world Time Zones of the time zone map. GMT sets current time or official time around the globe. Most time changes are measured by GMT. Although GMT has been replaced by atomic time (UTC) it is still widely regarded as the correct time for every international time zone.

British Summer Time

British Summer Time (BST) is the UK name for Daylight Saving Time

This means that in the spring we turn our clock forward one hour to GMT+1. In the autumn we turn the clock back one hour to GMT. This gives an extra hour of light at the start of the day in winter and at the end of the day in summer.

When do the clocks change to Summer Time?

From 2002 the rule has been:

the summer-time period shall begin, at 1.00 a.m., Greenwich Mean Time, on the last Sunday in March.

the summer-time period shall end, at 1.00 a.m., Greenwich Mean Time, on the last Sunday in October.

Tide Gauges

Bridge Heights

Published bridge heights can be found here

File Formats

CSV File

CSV is a simple file format used to store tabular data. Files in the CSV format can be exported from website loaded into programs such as Microsoft Excel or OpenOffice Calc. CSV stands for "comma-separated values".

Date,UT(GMT),Height(m)
2016-11-21,00:00,1.4
2016-11-21,00:10,1.5
2016-11-21,00:20,1.7
2016-11-21,00:30,1.8

XLSX File

XLSX is a file extension for an open XML spreadsheet file format used by Microsoft Excel. Microsoft introduced the open XML standard to Microsoft Office in 2007

XML - File

XML is a file extension for an Extensible Markup Language (XML) file format used to create common information formats and share both the format and the data on the World Wide Web, intranets, and elsewhere using standard ASCII text.

Date Format

The files use the International Standard ISO 8601 for numeric representation of date

The international standard date notation is

YYYY-MM-DD

where YYYY is the year in the usual Gregorian calendar, MM is the month of the year between 01 (January) and 12 (December), and DD is the day of the month between 01 and 31.

For example, the fourth day of February in the year 2017 is written in the standard notation as

2017-02-04