Tidal range is the difference in height betweenhigh tide andlow tide.Tides are the rise and fall ofsea levels caused bygravitational forces exerted by theMoon andSun, byEarth's rotation and bycentrifugal force caused by Earth's progression around the Earth-Moonbarycenter. Tidal range depends on time and location.
Larger tidal range occur duringspring tides (spring range), when the gravitational forces of both the Moon and Sun are aligned (atsyzygy), reinforcing each other in thesame direction (new moon) or inopposite directions (full moon). The largest annual tidal range can be expected around the time of theequinox if it coincides with a spring tide. Spring tides occur at the second and fourth (last) quarters of thelunar phases.
By contrast, duringneap tides, when the Moon and Sun's gravitational force vectors act inquadrature (making aright angle to theEarth's orbit), the difference between high and low tides (neap range) is smallest. Neap tides occur at the first and third quarters of the lunar phases.
Tidal data for coastal areas is published bynational hydrographic offices.[1] The data is based on astronomical phenomena and is predictable. Sustained storm-force winds blowing from one direction combined with lowbarometric pressure can increase the tidal range, particularly in narrowbays. Such weather-related effects on the tide can cause ranges in excess of predicted values and can cause localizedflooding. These weather-related effects are not calculable in advance.
Mean tidal range is calculated as the difference betweenmean 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 1 metre (3 feet) – mapped in blue and green at right. Mean ranges near coasts vary from near zero to 11.7 metres (38.4 feet),[4] with the range depending 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 afunnel amplifying or dispersing the tide.[5]
The world's largest mean tidal range of 11.7 metres (38.4 feet) occurs in theBay of Fundy, Canada (more specifically, atBurntcoat Head, Nova Scotia).[4][6] The next highest, of 9.75 metres (32.0 feet), is atUngava Bay, also in Canada,[4][7] and the next, of 9.60 metres (31.5 feet), in theBristol Channel, between England and Wales.[4] The highest predicted extreme (not mean) range is 17.0 metres (55.8 feet), in the Bay of Fundy.[7] The maximum range in the Bristol Channel is 15 metres (49 feet).[8] The fifty coastal locations with the largest ranges worldwide are listed by theNational Oceanic and Atmospheric Administration of the United States.[4]
Some of the smallest tidal ranges occur in theMediterranean,Baltic, andCaribbean Seas. A point within a tidal system where the tidal range is almost zero is called anamphidromic point.
The tidal range has been classified[9] as:
