Tidal range is the vertical difference in height between consecutive high and low waters over a tidal cycle (Figure 1). The range of the tide varies between locations and also varies over a range of time scales. Differences in tidal range are important, as they are often related to variations in coastal processes and morphology. Around the UK the tidal range varies between a minimum of 0.5 m and a maximum of 15 m in the Bristol Channel.
Figure 1: Tidal range – vertical difference in height between consecutive high and low waters.
Spatially, tidal range varies according to the hydrodynamic response of a particular ocean basin, shelf sea, bay or estuary to astronomical tidal forcing. Tidal ranges are usually largest in semi-enclosed seas and funnel-shaped entrances of bays and estuaries, such as the Bristol Channel in the UK or the Bay of Fundy in Canada, or regions where a continental shelf has the right combination of depth and width for tidal resonance to occur, such as on the northwest Australian shelf. Conversely, tidal ranges are typically smallest in the open ocean, along open ocean coastlines and in almost fully enclosed seas, such as the Mediterranean.
Temporally, tidal range varies due to changes in the position and alignment of the Moon and Sun relative to Earth. Every fortnight, the largest tidal ranges in semi-diurnal regions occur during spring tides, when the Moon and Sun are in phase around times of new or full Moon; while the smallest tidal ranges occur during neap tides, when the Moon and Sun are out of phase. In diurnal regions, the largest tidal ranges occur every fortnight during equatorial tides, when the Moon is over the tropics; while the smallest tidal ranges occur during tropic tides, when the Moon is over the equator. Over a month, tidal range changes as the Moon moves from its closest (perigee) approach to Earth, to its furthest approach (apogee) and back. Over annual time scales, changes in tidal range occur as the Sun’s position varies north or south of the equator, and as it moves from its closest (perihelion) to furthest approach (aphelion) to Earth and back. The largest semi-diurnal tidal range occurs in March and September during the equinoxes, while the largest diurnal tidal range occurs in June and December during the solstices. Over longer time scales, variations in tidal range arise as a result of the 8.85-year cycle of lunar perigee, which influence tides as a quasi-4.4 year cycle, and the 18.61-year lunar nodal cycle.
Tides are often crudely classified by their mean range: macrotidal (> 4 m); mesotidal (2 to 4 m); and microtidal (< 2 m).