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Tidal range
Tidal range
from Wikipedia

Tidal range is the difference in height 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, by Earth's rotation and by centrifugal force caused by Earth's progression around the Earth-Moon barycenter. Tidal range depends on time and location.

Larger tidal range occur during spring tides (spring range), when the gravitational forces of both the Moon and Sun are aligned (at syzygy), reinforcing each other in the same direction (new moon) or in opposite directions (full moon). The largest annual tidal range can be expected around the time of the equinox if it coincides with a spring tide. Spring tides occur at the second and fourth (last) quarters of the lunar phases.

By contrast, 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 (neap range) is smallest. Neap tides occur at the first and third quarters of the lunar phases.

Tidal data for coastal areas is published by national hydrographic offices.[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 can cause ranges in excess of predicted values and can cause localized flooding. These weather-related effects 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]

Geography

[edit]
The M2 tidal constituent, peak amplitude indicated by color. White lines are cotidal lines spaced at phase intervals of 30° (a bit over 1 hr).[3] Amphidromic points are the dark blue areas where the lines come together. These are amphidromic points, NOT tidal ranges. Tidal ranges can vary from 4 to 10 times what's marked on this map.

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 a funnel amplifying or dispersing the tide.[5]

The world's largest mean tidal range of 11.7 metres (38.4 feet) occurs in the Bay of Fundy, Canada (more specifically, at Burntcoat Head, Nova Scotia).[4][6] The next highest, of 9.75 metres (32.0 feet), is at Ungava Bay, also in Canada,[4][7] and the next, of 9.60 metres (31.5 feet), in the Bristol 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 the National Oceanic and Atmospheric Administration of the United States.[4]

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.

Classification

[edit]

The tidal range has been classified[9] as:

  • Micro-tidal – when the tidal range is lower than 2 metres (6'6¾").
  • Meso-tidal – when the tidal range is between 2 metres and 4 metres (6'6¾" and 13'1½").
  • Macro-tidal – when the tidal range is higher than 4 metres (13'1½").

See also

[edit]
  • King tide, an informal term for an especially high spring tide

References

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia
from Grokipedia
The tidal range is the vertical difference in height between the high tide and the at a specific coastal , typically measured over a tidal cycle. This phenomenon results from the gravitational interactions between the , , and Sun, which cause periodic rises and falls in , with the Moon's influence being dominant due to its proximity. Tidal ranges can vary significantly, from less than 1 meter in some open ocean areas to over 16 meters in extreme cases, and are classified based on tidal patterns such as semidiurnal (two high and two low daily with roughly equal ranges), diurnal (one high and one low daily), or mixed. Tidal ranges are influenced by both astronomical and geographical factors. Astronomically, ranges increase during spring tides—when the Sun, , and align (at new or full moon)—producing greater gravitational pull, and decrease during neap tides when the Sun and are at right angles, resulting in smaller ranges. Additionally, the Moon's elliptical orbit causes higher ranges at perigee (closest approach) and lower at apogee, while Earth's orbital position relative to the Sun amplifies this during perihelion. Geographically, coastal plays a crucial role: funnel-shaped bays amplify ranges through , as seen in the between ’s and , where the world's highest recorded tidal range reaches up to 16 meters (53 feet) due to the basin's shape, depth, and a 12-hour oscillation period matching the tidal cycle. Other factors include shoreline configuration, shapes, river outflows, wind patterns, and , which can modify local ranges by several meters during storms. In contrast, the Pacific Ocean's vast size leads to generally smaller ranges compared to the Atlantic. The tidal range holds significant implications for coastal environments, human activities, and renewable energy. It shapes intertidal ecosystems by determining the extent of submersion and exposure, influencing biodiversity in zones like salt marshes and mudflats where species adapt to varying inundation periods. For navigation, precise knowledge of tidal ranges is essential to avoid grounding in shallow waters or to time port operations, as variations affect water depths and currents. In engineering, large ranges inform the design of harbors, bridges, and flood defenses, while in energy production, they enable tidal range technologies like barrages to harness the potential energy of water flow, with sites like the Bay of Fundy offering substantial hydroelectric potential equivalent to billions of cubic meters of water movement daily. Overall, understanding tidal ranges is vital for mitigating coastal hazards, sustainable development, and climate adaptation amid rising sea levels.

Fundamentals

Definition

The tidal range refers to the vertical difference in between consecutive high and low tides at a specific coastal . This metric captures the amplitude of the tidal oscillation over a single tidal cycle, typically spanning about 12 to 24 hours depending on the local tidal pattern. Tidal range is commonly measured in meters or feet using tide gauges, which record continuous water level variations relative to a fixed benchmark on land. In modern applications, satellite altimetry supplements these observations by providing global data on sea surface heights, from which tidal ranges can be derived through . The mean tidal range, a standardized value, is calculated as the difference between mean high water (the average of all high water heights over a 19-year ) and mean low water (the average of all low water heights over the same period). Within the broader tidal cycle, the range connects high and low water levels, forming the basis for reference datums like mean high water and mean low water, which serve as benchmarks for nautical charting and . These datums account for long-term averages to mitigate short-term variations, such as those during spring tides (larger ranges) or neap tides (smaller ranges). The concept of tidal range emerged in 19th-century through systematic observations, where researchers tabulated differences between high and low water heights to quantify tidal behavior. This approach, pioneered in coastal monitoring efforts across and , laid the foundation for modern tidal analysis.

Physical Causes

The tidal range, which measures the vertical difference between high and low tides, arises primarily from the gravitational attractions exerted by on 's oceans. The Moon's gravitational pull is the dominant force due to its proximity to , despite the Sun's much greater mass; the Moon is about 390 times closer to than the Sun is, resulting in a approximately twice as strong as the Sun's, as tidal effects scale inversely with the cube of the distance between bodies. These gravitational forces create a two-bulge in Earth's oceans: one bulge forms on the side facing the due to direct gravitational attraction pulling toward it, while the second bulge appears on the opposite side because the from the Earth-Moon orbital motion exceeds the Moon's gravitational pull there, causing to lag behind. Earth's rotation beneath these relatively stationary bulges (aligned with the ) produces the observed semidiurnal tidal cycle, with locations experiencing high twice daily as they pass through each bulge. In the equilibrium tide theory, the shape of these ocean bulges is determined by the tidal potential, which describes the gravitational perturbation from the (or Sun). The basic form of this potential at a point on or near Earth's surface is given by Φtide=GMd(rd)2P2(cosγ),\Phi_\text{tide} = -\frac{GM}{d} \left( \frac{r}{d} \right)^2 P_2(\cos \gamma), where GG is the , MM is the mass of the (or Sun), dd is the distance from Earth's center to the 's (or Sun's) center, rr is the distance from Earth's center to the point (approximately Earth's radius), γ\gamma is the angle between the position vectors to the point and the , and P2(cosγ)=12(3cos2γ1)P_2(\cos \gamma) = \frac{1}{2} (3 \cos^2 \gamma - 1) is the second-degree Legendre polynomial capturing the quadrupolar deformation. The equilibrium ocean surface aligns with equipotential surfaces of this potential plus Earth's own gravity, yielding theoretical tide heights of about 0.5 meters for the alone. The tidal range varies due to changes in the Moon's declination (its angular position relative to Earth's equator) and orbital alignments with the Sun. During new and full moons, when the Moon and Sun are aligned with Earth, their gravitational forces constructively interfere, producing spring tides with greater ranges; conversely, at quarter moons, the forces partially cancel, resulting in neap tides with smaller ranges. Declination effects further modulate this: maximum lunar declination (up to 28.5°) tilts the bulges away from the equator, reducing ranges at low latitudes and enhancing them at higher ones, while perigee (Moon's closest approach) amplifies forces by up to 20% compared to apogee. Beyond equilibrium theory, dynamic effects in the oceans modify the tidal range through wave propagation and interactions with the seafloor. In shallow waters, where depth is much less than the tidal wavelength, friction and reduced wave speed (c=ghc = \sqrt{gh}
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