Glossary

Height of sea level relative to the centre of the Earth. See also relative sea level.

The delay in time between the transit of the moon and the highest spring tide. Normally one or two days, but it varies widely. In many places the maximum tidal range is one or two days after the new or full moon, and the minimum range is a day or two after first and third quarter.

A point in the sea which is a node with little or no tidal amplitude around which the tide rotates, for a particular tidal constituent.

The point in the elliptical orbit of Earth or other planet when it is furthest from the Sun.

The point in the Moon’s elliptical orbit when it is furthest from Earth. The tidal range tends to be reduced at this time – this is referred to as an apogean tide.

An angle, which is a function of the longitudes of the Moon, Sun, lunar perigee, lunar node and solar perihelion, that is used in the determination of the tide-generating potential of a particular constituent at a particular time.

Periodic variations of the atmosphere caused by gravitational and radiational effects, analogous to ocean tides. See also radiational tides.

A horizontal angle calculated clockwise from the meridian, especially the horizontal direction of a celestial point from a terrestrial point, expressed as the angular distance from a reference direction (hence often designated as true, magnetic, compass or relative, depending on the reference).

When applied to current or stream, it is a direction toward which the current or stream is flowing, and usually calculated from the north point. See also celestial sphere.

The mathematical representation of the spring–neap cycle is in terms of the two signals (in this example the semi-diurnal tides M₂ and S₂) going in and out of phase over the course of a fortnight.

This phenomenon is known as beats or beating and in the context of tides is not confined to M₂ and S₂. When any pair of slightly different frequencies (of similar amplitude) add together, their sum undergoes a regular cycle between near-zero magnitude (neaps), and a magnitude equal to the sum of the pair (springs).

The period between successive neaps is equal to the inverse of the absolute value of the difference between the two frequencies. The frequencies of M₂ and S₂ are 1.9322 cpd and 2.0 cpd respectively. Their difference is the beat frequency 0.0678 cpd, the inverse of which, 14.75 days, is the beat period – also known as the fortnightly or spring–neap cycle.

A fixed physical object or mark used as a reference for a vertical point. A tidal benchmark is a point near a tidal station used as a reference for a tide gauge and tide measurements.

A solitary tidal wave with a steep leading edge that travels up a river, estuary or bay against the direction of the river current. Also known as Tidal bore.

The point or plane of reference to which all charted depths and drying heights are related (but not other heights and elevations). It is usually defined in terms of a low-water level, such as lowest astronomical tide.

In some ports a local port datum might be adopted as chart datum, for instance where sea levels are strongly affected by meteorological conditions and may fall significantly below the predicted astronomical tide.

See co-tidal line.

Lines on a co-tidal chart joining places which have the same tidal range or amplitude. Usually drawn for a particular constituent or tidal condition, such as spring tides.

Lines on a co-tidal chart joining places where tide has the same phase, for example where high water occurs at the same time. Usually drawn for a particular constituent or tidal condition.

Part of the tidal stream velocity which, after resolution into orthogonal vectors, acts in a specified direction. For example North or East. Also known as harmonic constituent. See constituent.

A harmonic constituent of a tide with a speed equal to the sum or difference of the speeds of two or more elementary constituents. Compound tides usually occur in shallow water and result from the interaction of its two parent waves.

Elements that do not vary with time.

Harmonic tidal analysis represents the sea level record as the sum of cosine waves. Each wave (constituent) has a unique frequency (speed). For a given location, each frequency has an amplitude and phase which do not vary with time – these are called constants.

Tidal currents may be harmonically represented by tidal constants by converting them into north/south (along-stream) and east/west (across-stream) components.

One of the elements in a mathematical expression of the forces that produce a tide or tidal current. See constants.

A moving body on the surface of the Earth experiences a tendency to turn to the left in the southern hemisphere (right in the northern hemisphere) due to Earth's rotation. This tendency is a result of the rotating reference frame rather than an actual force.

Known as the Coriolis force or acceleration, it's only noticeable with larger scale motions such as ocean currents and winds.

The Coriolis force affects the direction with which the tide propagates around an amphidrome.

It can also affect the propagation of the tide as it moves up a broad channel. For example, by tilting the water surface to the left or right of the direction of propagation.

The angular distance of an astronomical body north or south of the celestial equator, taken as positive when north of the equator and negative when south.

The condition whereby daily high waters or low waters are of significantly different level. See also species.

A tide with only one high water and one low water occurring during a tidal day. Also known as single day tide.

Local South Australian term for a neap tide with minimal rise and fall over the course of a day or so.

A deformation of the solid earth in response to the gravitational tidal forces of the Sun and Moon. The largest effect is the semi-diurnal deformation nearly in phase with the transit of the Moon – its amplitude is less than 20 cm.

The tidal phase during which the water level is falling, or the tidal current is flowing seaward (ebb current). Opposite to flood.

Used in at least two different ways in tidal work – as a synonym for phase lag, and for a period of time (usually a nodal cycle) over which a mean is calculated as the basis for a tidal datum.

Theoretical model under which waters, uniformly covering the face of the Earth, respond instantly to the tide-producing forces of the Moon and Sun and form an equilibrium under action of these forces.

Friction, inertia, irregular distribution of the land masses are all disregarded under this theory, but the model provides an important theoretical basis for tidal science.

The hypothetical tide which would be produced by the lunar and solar gravitational forces under the equilibrium theory in the absence of any other ocean dynamics or terrestrial constraints.

Global changes of sea level taking place over many years. Sometimes associated with ocean volume changes or with globally synchronous changes.

However, such ocean surface displacements are now understood to be spatially irregular.

One of many deviations in the Moon's orbit caused by changes in the solar gravitational potential during the course of orbit, which produces evectional constituents. See also variation.

The tidal phase during which the water level is rising, or the tidal current is flowing inland (flood current). Opposite to ebb. See also streams.

A factor used to characterise the tides in an area as being mainly diurnal, semi-diurnal, or mixed.

A two-week tide cycle. In most parts of the world, the tides go through a fortnightly spring–neap cycle. These are beat phenomena rather than actual harmonics.

There are also some harmonics arising directly from the tide-generating potential which have a period of a fortnight.

The level surface that the ocean would take in the absence of tides, currents, water density variations and atmospheric effects. It is the surface of reference for astronomical observations and geodetic levelling. See also geopotential.

The earth’s gravitational potential is called the geopotential. A geopotential surface is one whose gravitational forces are equal everywhere. See also geoid.

See lunitidal interval.

The highest level of water which can be predicted to occur under average meteorological conditions and any combination of astronomical conditions.

An elevation depressed below mean sea level by the amount equal to the sum of amplitudes of the four main harmonic constituents: M₂, S₂, K₁ and O₁. Also known as ISLW.

A method used to infer the amplitude and phase of constituents where a tidal data set has insufficient duration.

To separate a pair of constituents of similar frequency, you can infer the amplitude and phase of one member of the pair (generally the weaker) on the basis of an analysis of a longer data set from a nearby location or the equilibrium relationships.

The inference relationships between the two constituents must be accounted for in the analysis.

Also called internal wave.

The ocean usually has a less dense upper layer overlying the much deeper, denser waters. Waves known as internal waves often occur on the interface between these two layers.

If the interface is gradual, the direction of wave propagation may have a vertical component, trapped by refraction within upper and lower limits.

Internal waves are usually caused by flow in the lower layer moving over an obstacle such as an undersea ridge. For example, semi-diurnal tidal flows into and out of a fjord. When this happens, a semi-diurnal internal wave, or internal tide, is produced.

Although these waves do not significantly affect the sea surface, they may be detected by satellite as bands of surface slicks due to the convergence of surface currents that are produced.

The part on a beach or coastal zone that lies between high and low tidal levels. It is exposed or inundated depending on the tide.

A symbol used in older texts on ocean tides, and modern texts on Earth tides, that refer to phase. This practice places the tidal phase lag and the reference signal in the local time zone.

The delay in the time of occurrence of high and low water due to changes in relative position of the Moon and the Sun. See also lunitidal interval.

The lowest of the low waters of any specified tidal day due to the declinational effects of the Moon and Sun.

The lowest tide level which can be predicted to occur under average meteorological conditions and under any combination of astronomical conditions. Increasingly used as chart datum, for example, for all new Australian charts. Also referred to as LAT.

A tidal constituent whose origins are a combination of lunar and solar – that is, by coincidence, there are identical force frequencies stemming from both sources.

The average duration of time between the high point of the Moon to the next high tide at any given location. Also called high water interval.

A tidal level. The average of all high waters observed over a sufficiently long period.

A tidal datum. The arithmetic mean of hourly heights of the sea at the tidal station observed over a period of time (preferably 19 years). Also referred to as MSL.

Periodical or quasi-periodical changes in water level caused by the daily or seasonal variations in local meteorological conditions. See also radiational tides.

The type of tide characterised by large diurnal inequalities in heights and/or times of successive high and/or low waters. In general, a type of tide intermediate between predominantly semi-diurnal and predominantly diurnal. See also form factor.

The tides of decreased range, or tidal streams of decreased speed, occurring semi-monthly near the times of the first and third quarters of the moon when the moon is in quadrature. See also fortnightly tides.

Small adjustments to the amplitudes and phases of harmonic constituents to allow for modulations over the 18.61-year nodal period. See regression of lunar nodes.

Tides produced in shallow water or by frictional effects in general, which have periods equivalent to 4, 6, 8 or more cycles per day.

A shallow water constituent with a speed that is a multiple of the speed of one of the basic constituents of the tide-producing force.

See perigee.

Tides of increased range as a result of the Moon being in perigee, particularly noticeable when perigee coincides with spring tides (perigean spring tides). The opposite situation is known as apogean.

The Moon's point of closest approach in its elliptical orbit around the Earth.

Over time, the orientation of the orbit within the orbital plane gradually rotates. As a consequence, the perigee circles the earth every 8.85 years. This period is known as the perigeal cycle (not to be confused with perigean tide) and designated 'p' in tidal literature.

This is distinct from the nodal cycle, in which the orbital plane itself rotates. The Moon is at perigee every 27.5546 days.

The point of closest approach in the Earth’s orbit around the Sun.

The Sun is at perihelion every 365.2596 days – currently this nearly coincides with the start of the year (coincidentally midsummer in the southern hemisphere).

The perihelion itself circles the Sun every 20,942 years in a rotation simlar to the perigeal cycle. The period is often designated 'p' or 'p₁' in tidal literature.

The lag of the phase of the maximum of a tidal constituent behind the phase of a reference wave (usually the phase of the corresponding equilibrium constituent at Greenwich). Also called tidal epoch.

Small tide of varying period (approximately 433 days, but varying) associated with changes in the Earth's axis of rotation known as the Chandler Wobble.

Considered a class of radiational tides, as the precession has been shown to be caused by oceanographic and meteorological variations, which redistribute water masses. This precession is independent of, and much smaller than, the precession of the equinoxes, which has a period of 26,000 years. See year.

See standard port.

The meridian of 0° longitude, also known as the Greenwich Meridian.

The earlier arrival of the time of occurrence of high and low water due to changes in relative position of the Moon and the Sun. See also lunitidal interval.

The condition whereby the angle formed by the Sun, Earth, and Moon is 90°. See also syzygy.

A quasi-periodic rise and fall of sea level caused by meteorological variability. Also known as meteorological tides.

The difference between the maximum and minimum water levels during a typical tidal cycle.

A criterion used in tidal analysis, which requires that only constituents which are separated by at least one complete period from their neighbouring constituents over the length of data available should be included in the harmonic analysis of a given time series. See also synodic period.

A tidal stream which flows alternately in approximately opposite directions with slack water at each reversal of direction. Encountered mainly in straits and channels. Also known as a reversing stream. See also streams.

The clockwise or western rotational direction of the lunar nodes around the ecliptic. This is opposite to that of most other rotations and orbits of the solar system.

Height of sea level above land at a particular location or point of reference. For example, a coastal benchmark.

The difference between the observed sea level and the tidal prediction for a given location.

Residuals are most commonly due to weather related effects, limitations of the harmonic model, harbour seiches and errors in measurement or data processing. All of which leave recognisable imprints in the data.

A result of the large amplitudes. Occurs when the frequency of a force is equal to the natural oscillation frequency of a system.

Tidal resonance occurs when the natural period of an ocean or sea is close to the period of a tide-producing force.

The representation of observed tidal variations as frequency-dependent amplitude and phase responses to the forcing functions, usually the gravitational and radiational tide-producing forces.

A narrow shearing current flowing offshore through the surf zone. Sometimes (misleadingly) called a rip tide. Rips are a part of a circulation cell forced by surface wave transport, and have little to do with tides.

A tidal stream that flows continually with the direction of flow changing through all points of the compass during a tidal cycle. Usually found offshore where there are no restricting barriers.

This natural tendency of tidal flows (they become rectilinear only when restricted) is due to the Coriolis force. See also streams.

Remote sensing of the ocean surface height by satellite-mounted microwave radar.

Techniques have been developed for extracting the tidal constants for the larger constituents from the satellite data, despite its sampling interval being about twenty times longer than the semi-diurnal period.

It provides an accurate global ocean database of tidal constants, which was previously restricted to areas close to coastal tide gauges and to numerical models constrained over wide areas of the ocean surface.

In tide tables, a port for which predictions are required, but for which predictions from the nearest standard port must be used (with suitable corrections). Used when insufficient data for a reliable harmonic analysis is available.

Also called a subordinate port.

A standing wave in an enclosed or semi-enclosed body of water set off by weather, seismicity, or incident ocean waves.

Seiches are primarily a resonance phenomenon, whose wavelength and period are determined by the geometry. A characteristic feature is the existence of nodes – points of minimum water level disturbance, but greatest horizontal currents – and antinodes, where the reverse occurs.

The description of the tide-generating potential was simplified in several ways, two of which were by ignoring the continuous re-distribution of mass that occurs as the water moves in response to the tidal forces, and by ignoring the warping of the elastic solid earth surface as the water level varies. These effects are commonly combined into the term 'self-attraction and loading' (SAL).

A tide, having a period or cycle of approximately one-half of a tidal day.

The direction towards which the current or tidal stream is flowing.

The state of a tidal stream when its speed is or nearly zero. Especially the moment when the reversing stream changes its direction. Also known as slack tide or slack. See also streams.

Tides occurring near the times of summer and winter solstice, when the sun is overhead at the Tropic of Cancer or Capricorn.

A classification of tidal constituents according to their period. The principle species are semidiurnal, diurnal, and long period.

The rate of change in the phase of a constituent expressed in degrees per hour. The speed is equal to 360° divided by the constituent period expressed in hours.

The rate at which a stream or current flows. Usually expressed in knots or metres per second.

See tidal plane.

The tides of increased range, or tidal streams of increased speed, occurring semi-monthly near the times of full moon and new moon. See also fortnightly tides.

The condition at high or low water when there is no perceptible change in the height of tide for a period of time. Sometimes called platform tide. See also streams.

In the context of tide tables, a port for which sufficient data is available in order for a set of official predictions to be produced. Also known as a primary port, especially in North America.

A well that is connected to the sea through a small opening which filters out any high frequency waves while admitting the long period tidal (and other) level variations. Used by some types of tide gauges to measure sea level, traditionally via a float or some other means.

An abnormal rise in sea level over and above the normal (astronomical) tide levels. It can be thought of as the change in the water level due to the presence of a storm.

These powerful ocean movements are caused by strong winds piling water up against the coast as a cyclone or other low pressure system approaches.

The water level that results from the combination of the storm surge and the normal (astronomical) tide.

The high water mark on a shoreline, typified by the presence of flotsam and jetsam (types of marine debris associated with vessels).

Also known as tidal currents. Sometimes used to refer exclusively to the tidal currents along the main directions of ebb and flood. 

The set of the current is the direction in which it flows. On the incoming tide, the streams are said to be in flood and the outgoing streams are in ebb. The stand of the tide occurs near high and low water when the water level is unchanging.

Tidal streams which flow back and forth along a line are rectilinear, whereas those that follow an elliptical circuit (due to the Coriolis force) are rotary. See also rotary flow.

The minimum length of data necessary to separate a pair of constituents according to the Rayleigh criterion.

The average period of the revolution of the Moon around the Earth with respect to the Sun or the average interval between corresponding phases of the Moon. The synodical month is 29.5305888 mean solar days long. Also known as lunation or lunar month.

The condition whereby the Sun, Earth and Moon are in alignment. See also quadrature.

A harmonic constituent with three periods in a constituent day. See also species.

A layer in which the temperature decreases significantly (relative to the layers above and below) with depth. The principal ones are designated diurnal, seasonal, and main thermocline. A common feature of thermoclines is the presence of internal tides.

A level of the sea defined by some phase of the tide, from which depths of water and heights of tide are calculated.

The various tidal water levels, such as low water or mean sea level for example, are known collectively as tidal planes. Also known as tidal datums, tidal levels, tidal elevations or datum planes.

Tidal planes are defined at the tide gauge – a specific location – but for practical purposes they are considered points on a continuous surface. The full list of tidal planes is extensive and different countries define the tidal planes differently. For legal definitions, the appropriate regulatory authority or document should be consulted.

Where the tide moves up and down the lower reaches of a river, a volume, known as the tidal prism, of fresh water is displaced each tidal cycle.

The tidal prism takes its name from the fact that the front between fresh and salt water is often inclined to the vertical, with the downstream edge of the fresh water riding over the salt.

This term is used in various contexts, including:

• Coastal aquifers – in this case, the rise and fall of the tide is often accompanied by a delayed and reduced oscillation of water level in nearby wells.
• The bringing of nutrient-rich offshore water into the shallower regions – this usually involves a relatively large flood tide bringing water up and into a bay or other semi-enclosed area, where it mixes with water from previous high tides before draining more slowly back to the open ocean. Submarine canyons across the continental shelf may also cause a rectified flow with a net increase of nutrients in the upper layer.

A generic term for a type of large-scale wave generated by the gravitational forces of the Sun and Moon on the ocean. Sometimes this term is used incorrectly as a synonym for tsunami.

A device for measuring sea level.

At latitudes near the maximum declination of the Moon (which varies between 18.3° and 28.6° latitude north and south over the course of the nodal cycle) the diurnal tides are greatest when the moon is near maximum declination.

These so-called tropic tides are the equivalent of the more common spring tides, with the beat frequencies being diurnal instead of semi-diurnal. The range (peak to peak distance between high and low tide) at the time when these diurnal tides are greatest is known as the tropic range.

Time as defined by the rotation of the Earth and determined from diurnal motions reflecting this rotation.

It is expressed in respect of 0° meridian, and used as a standard for comparison of global phenomena. Also known as Greenwich Mean Time (GMT) and Zulu time (military term). See Coordinated Universal Time (UTC).

In the context of tides, upwelling (upwards movement of water) can occur as a result of periodic flow over uneven topography, especially submarine canyons on the continental shelf.

Upwelling is more often associated with alongshore winds combined with the Coriolis effect, or spatially divergent wind fields over the open ocean. Tidal upwelling can also lead to significant flux of nutrients into the photic zone.

A test designed to detect flaws in the mechanical operation of tide gauge chart recorders.

A measurement is taken of the positive distance between a fixed point near the top of the gauge, down through the stilling well to water level.

The sum of this distance, which is a maximum at low tide, and the tide gauge reading should be constant through a full tidal cycle. The sum when plotted against the measured distance (with the latter plotted on the vertical axis) should therefore be a vertical line. Deviations from the straight line can be interpreted as faults such as backlash in the gauge mechanism or scaling error.

See dodge tide.

One of many deviations in the Moon's orbit caused by changes in the solar gravitational potential during the course of orbit, which produces variational constituents. See also evection.

See evection and variation.

Four different types of year are of significance to tides.

Sidereal year – the period taken by earth complete a single orbit of the sun, 365.2564 mean solar days (msd).

Tropical year – measured in relation to the beginnings of the various seasons (successive vernal equinoxes). It is slightly shorter than the sidereal year as a consequence of precession. The axis of the earth is tilted at about 23½° degrees to the perpendicular of the orbital plane. The axis slowly precesses about the perpendicular, in the manner of a 'sleeping top'. Each precession takes 26,000 years. If a single precession was completed in just one day, then we would experience four seasons in a single day. This means that the seasons advance 1/26,000^th part per sidereal year faster than they would without precession, and the tropical year is therefore only 365.2422 msd.

Anomalistic year – the period between successive perihelions. Just as the anomalistic month is slightly longer than a sidereal month, the anomalistic year, 365.2596 msd, is slightly longer than a sidereal year. Of the three types of year, the anomalistic is of greatest importance in tides. The longitude of the sun undergoes a complete cycle in one tropical year.

Julian year – defined as 365.25 days.