Tidal range

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Tidal range is the height difference between high tide and low tide. Tides are the rise and fall of sea levels caused by gravitational forces exerted by the Moon and Sun and the rotation of Earth. Tidal range is not constant but changes depending on the locations of the Moon and Sun.


The most extreme tidal range occurs during spring tides, when the gravitational forces of both the Moon and Sun are aligned (syzygy), reinforcing each other in the same direction (new moon) or in opposite directions (full moon). During neap tides, when the Moon and Sun's gravitational force vectors act in quadrature (making a right angle to the Earth's orbit), the difference between high and low tides is smaller. Neap tides occur during the first and last quarters of the Moon's phases. The largest annual tidal range can be expected around the time of the equinox if it coincides with a spring tide.

Tidal data for coastal areas is published by national hydrographic services. [1] The data is based on astronomical phenomena and is predictable. Sustained storm-force winds blowing from one direction combined with low barometric pressure can increase the tidal range, particularly in narrow bays. Such weather-related effects on the tide, which can cause ranges in excess of predicted values and can cause localized flooding, are not calculable in advance.

Mean tidal range is calculated as the difference between Mean High Water (i.e., the average high tide level) and Mean Low Water (the average low tide level). [2]


The typical tidal range in the open ocean is about 0.6 metres (2 feet)(blue and green on the map on the right). Closer to the coast, this range is much greater. Coastal tidal ranges vary globally and can differ anywhere from near zero to over 16 metres. [3] The exact range depends on the volume of water adjacent to the coast, and the geography of the basin the water sits in. Larger bodies of water have higher ranges, and the geography can act as a funnel amplifying or dispersing the tide. [4] The world's largest tidal range of 16.3 metres (53.5 feet) occurs in Bay of Fundy, Canada, [3] [5] and the United Kingdom regularly experiences tidal ranges up to 15 metres (49 feet) between England and Wales in the Severn Estuary. [6]

The fifty coastal locations with the largest tidal ranges worldwide are listed by the National Oceanic and Atmospheric Administration of the United States. [3]

Some of the smallest tidal ranges occur in the Mediterranean, Baltic, and Caribbean Seas. A point within a tidal system where the tidal range is almost zero is called an amphidromic point.

The M2 tidal constituent, amplitude indicated by color. White lines are cotidal lines spaced at phase intervals of 30deg (a bit over 1 hr). Amphidromic points are the dark blue areas where the lines come together. M2 tidal constituent.jpg
The M2 tidal constituent, amplitude indicated by color. White lines are cotidal lines spaced at phase intervals of 30° (a bit over 1 hr). Amphidromic points are the dark blue areas where the lines come together.


The tidal range has been classified [8] as:

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Amphidromic point point of zero amplitude of one harmonic constituent of the tide

An amphidromic point, also called a tidal node, is a geographical location which has zero tidal amplitude for one harmonic constituent of the tide. The tidal range for that harmonic constituent increases with distance from this point.

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The lunitidal interval measures the time lag from lunar culmination to the next high tide at a given location. It is also called the high water interval (HWI). Sometimes a term is not used for the time lag, but instead the terms age or establishment of the tide are used for the entry that is in tide tables.

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Chart datum The level of water from which depths displayed on a nautical chart are measured

A chart datum is the water level that depths displayed on a nautical chart are measured from. A chart datum is generally derived from some phase of the tide. Common chart datums are lowest astronomical tide and mean lower low water. In non-tidal areas, e.g. the Baltic Sea, mean sea level (MSL) is used.

Tide table Tabulated data used for tidal prediction which show the daily times and heights of high water and low water, usually for a particular location

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Tide clock Specially designed clock that keeps track of the Moons apparent motion around the Earth

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Theory of tides science of interpretation and prediction of deformations of astronomical bodies and their atmospheres and oceans under the gravitational loading of other astronomical bodies

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Burntcoat Head, Nova Scotia human settlement in Nova Scotia, Canada

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King tide colloquial term for an especially high spring tide, such as a perigean spring tide.

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Supermoon coincidence of a full moon with the closest approach the Moon makes to the Earth

A supermoon is a full moon or a new moon that nearly coincides with perigee—the closest that the Moon comes to the Earth in its elliptic orbit—resulting in a slightly larger-than-usual apparent size of the lunar disk as viewed from Earth. The technical name is a perigee syzygy or a full Moon around perigee. The term supermoon is astrological in origin and has no precise astronomical definition.

Syzygy (astronomy) Straight-line configuration of three celestial bodies in astronomy

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  1. Hydrographic and Oceanographic Agencies
  2. NOAA. "Tidal Datums" . Retrieved 26 Mar 2019.
  3. 1 2 3 NOAA. "FAQ2 Where are the highest tides?" . Retrieved 27 Jan 2011.
  4. NOAA. "It appears that the range of the tides gets larger the further the location from the equator. What causes this??" . Retrieved 27 Jan 2011.
  5. NOAA. "The highest tide in the world is in Canada" . Retrieved 23 January 2014.
  6. "Tidal range".
  8. Masselink, G.; Short, A. D. (1993). "The effect of tidal range on beach morphodynamics and morphology: a conceptual beach model". Journal of Coastal Research. 9 (3): 785–800. ISSN   0749-0208.