Tidal Glossary

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The following terms are often used in Tidal Science:


absolute mean sea level change: A eustatic change in mean sea level relative to the centre of the Earth.

ADR gauge: Analogue to digital recording tide gauge. The data acquired by, for example, a float or pressure tide gauge and recorded in digital format.

age of diurnal inequality: An old term for the time interval between the semi-monthly maximum declination of the Moon (north or south) and the maximum effect of that declination on tidal range or stream rate. This effect being manifested chiefly by an increase in the height or speed difference between the two high (or low) waters or flood (or ebb) streams during the day. The tides occurring at this time are called tropic tides. Can be calculated from the formula:

age of diurnal inequality (in hours) = 0.911(g of K1 - g of O1).

age of parallax inequality: An old term for the time interval between perigee of the Moon and the maximum effect of parallax on tidal range or stream rate. Can be calculated from the formula:

age of parallax inequality (in hours) = 1.837(g of M2 - g of N2)

age of the moon: The time elapsed since the last new moon.

age of the tide: Also known as age of phase inequality. An old term for the time interval between new or full Moon and the maximum spring tidal range or stream rate. Can be calculated from the formula:

age of the tide (in hours) = 0.984(g of S2 - g of M2)

agger: Same as double tide.

aliasing: A spurious tidal frequency appearing in an analysis when there is an occurrence of a sea level variation with a periodicity smaller than the sampling period. Usually presenting itself only when the observations are taken at intervals greater than one hour or when there is a seiche at the observation site.

amphidrome: A point in the sea with zero tidal amplitude of the observed or constituent tide. Cotidal lines radiate from the amphidromic point, and co-range lines encircle it. Also known as amphidromic point.

amplification: An increase in the amplitude of tide, particularly when approaching the coast. This is partly due to reflection and resonance (see also seiche), but more generally, caused by shoaling and funnelling.

amplitude (H): One half of the range of a constituent, tide, or of any wave in general. Occasionally applied to the maximum speed of a tidal stream.

analogue: The way of representing data by continuous graphic display.

analysis, harmonic: see harmonic analysis.

angles and factors method (of tidal prediction): A simplified harmonic prediction method utilising only four main tidal constituents (M2, S2, K1, O1) with factors modifying amplitude and angles modifying speed of these constituents to imply existence of 21 more constituents and thus improving accuracy of the predictions.

annual inequality: Seasonal variation in water level or tidal stream rate, more or less periodic, due chiefly to meteorological causes.

anomalistic: Pertaining to the periodic return of the Moon to its perigee, or the Earth to its perihelion. Hence anomalistic month and year.

aphelion: The point in the orbit of the Earth or other planet farthest from the Sun.

apogean tides or tidal streams: Tides of decreased range or tidal streams of decreased speed occurring monthly when the Moon is near apogee.

apogee: The point in the orbit of the Moon or other satellite farthest from the Earth.

apparent secular trend: The non-periodic tendency of sea level to rise, fall or remain stationary with time, as determined by the regression through a long series of yearly mean sea level values. The word "apparent" is used since it is frequently impossible to determine whether the trend is truly non-periodic or just a segment of a very long oscillation.

apparent time: Also known as true solar time. Time based on true position of the Sun as distinguished from mean time which is measured by a fictitious Sun moving at a uniform rate (see also equation of time).

apsides: The points in the orbit of a planet or the Moon which are nearest and farthest from the centre of attraction (see also aphelion, apogee, perigee and perihelion).

Aries, first point of: see equinoxes.

astres fictifs: Fictitious celestial bodies which are assumed to move in the celestial equator at uniform rates corresponding to the speeds of the relevant harmonic constituents of the tide-producing force.

astronomical argument: An angle, which together with equilibrium amplitude describes tide-producing potential of a constituent. It is a function of the mean longitudes of the Moon and Sun, and of the longitudes of: lunar perigee, lunar node and perihelion.

astronomical tide: Same as tide.

astronomical time: An old term for the solar time in a day that begins at noon (known as astronomical day). Now replaced by universal time.

atmospheric tides: Variations in the sea level caused by the regular variations of the atmospheric pressure, especially in the tropics.

Australian National Tide Tables (ANTT): The official annual publication of the Australian Hydrographic Office containing daily tidal predictions for standard ports in Australia and Papua New Guinea together with predictions of tidal streams in Torres Strait, constituent listings, time differences and tidal levels for standard and secondary ports, tables of angles and factors and of astronomical arguments, and general tidal information.

automatic tide gauge: An instrument that records the rise and fall of the tide automatically, either at regular time intervals in digital format or as a continuous graph of water height against time (analogue format).

azimuth: A horizontal angle reckoned 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 such a current or stream is flowing, and usually reckoned from the north point.


baroclinic: Referring to a condition and type of motion in which pressure is not constant on surfaces of constant density, eg. internal tides and other internal waves.

barometric pressure, effect of: see inverted barometer effect.

barotropic: Referring to a condition and type of motion in which pressure is constant on surfaces of constant density, eg. surface tides.

basin - tidal: A body of water located in a tidal area in which the water level can be kept at a desired height by means of a gate.

benchmark (BM): A fixed physical object or mark used as a reference for a vertical datum. A tidal benchmark is one near a tidal station to which a tide gauge/pole and tidal datums are referred. Sometimes spelled bench-mark.

bore (tidal): A tidal type wave which propagates up certain rivers, especially in wedge-shaped shoaling estuaries, at times of spring tides as a solitary wave with a steep leading edge. Common in the estuaries discharging into the Gulf of Papua. Also known elsewhere as eagre (England), pororóca (Brazil) or mascaret (France).

boundary layer: The layer of fluid in the immediate vicinity of a bounding surface, eg. sea bottom. At the boundary layer the frictional forces are significant and can lead to wave distortion.

bubbler tide gauge: Also known as gas purge pressure gauge. An analogue tide gauge based on the emission of gas (usually nitrogen) from a submerged tube which fluctuates due to changes in the water level.


calendar: A scheme devised in order to find one's position in time and to calculate the distance in time when the time scale exceeds one solar day. See Gregorian and Julian calendar.

capillary wave: A wave whose propagation is controlled by the surface tension of the water, usually of length less than 2cm.

celestial sphere: A notional sphere of infinite radius with the Earth at its centre.

change of tide (also known as turn of the tide): A reversal of the direction of motion (rising or falling) of a tide. This term is also sometimes applied to a reversal in the set of a tidal stream. See also stand of tide.

chart datum (or chart sounding datum): The datum or the plane of reference to which all charted depths and drying heights are related [but not other heights and elevations]. It is a level so low that the tide will not frequently fall below it. Usually defined in terms of low-water level such as LAT or ISLW. Chart datum is not a horizontal surface but may be considered as such over a limited local area. In areas where the tidal range is not appreciable, less than 0.3 metres as in some parts of PNG, a chart datum of Mean Sea Level is sometimes adopted. Chart Datum should preferably be the same as the datum of tidal predictions for an area. Some ports, particularly on the SW coast of Australia, employ a different chart datum for port charts than the chart datum used for the coastal series of charts because the water level in such ports is strongly affected by meteorological conditions and water level may fall significantly below that of the predicted tide.

civil year: A year of the Gregorian calendar of 365 days in common years, or 366 days in leap (or bissextile) years.

clock: An instrument for measuring and recording time or time intervals when time scale is below one solar day.

co-amplitude lines: Same as co-range lines.

common establishment: see establishment.

component: 1. Same as harmonic constituent. 2. Part of the tidal stream velocity which, after resolution into orthogonal vectors, acts in a specified direction (eg. north or east)

compound tide: A harmonic constituent of a tide with a speed equal to the sum or difference of the speeds of two or more elementary constituents. Usually associated with shallow water effects.

constants: see harmonic constants.

constituent: see harmonic constituent.

constituent day: The duration of one rotation of the Earth on its axis with respect to a fictitious star representing one of the periodic elements in tidal forces(see astres fictifs). It approximates the length of a lunar or solar day. The term is not applicable to the long-period constituents.

contact mark: A permanent reference mark of known altitude at a tide gauge, from which the water level can be directly measured to provide a check on the accuracy of the gauge. Commonly used with float well gauges.

control station: A tidal station with a reasonably long data set which serves to provide data for datum transfers, determination of long-period constituents, tidal predictions, etc. A primary control tidal station must have at least 19 years of continuous observations, a secondary one - at least one year.

co-phase lines: Same as cotidal lines.

co-range lines: Lines on a cotidal chart joining places which have the same tidal range or amplitude. Usually drawn for a particular constituent or tidal condition (eg. spring tides).

Coriolis force (acceleration): A composite centrifugal force, due to rotation of the Earth, acting on objects (including air and water) moving in relation to the Earth. It causes deflection of such moving objects to the left in the southern hemisphere, and to the right in the northern hemisphere.

corrected establishment: see lunitidal interval.

cotidal chart: A chart of a major harmonic constituent (or of a semi-diurnal tide) constructed to illustrate the propagation patterns and to assist in the interpolation of tidal conditions and datum assessment at locations where no observations exist (especially offshore). Now frequently replaced by numerical modelling.

cotidal lines: Lines on a cotidal 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.

counter current: A secondary current setting in a direction opposite to that of a main current.

critical tidal level: A level on the shore where the emersion/submersion tidal characteristics change drastically, and which probably controls zonation of plants and animals in the intertidal zone.

critical length: A length of a gulf or an enclosed sea at which tidal resonance takes place.

crosslines: Sounding lines that cross the main system of lines at either right angles or at oblique angle serving to check the accuracy of the work, including tidal reduction of soundings.

current: A non-periodical, generally horizontal movement of water (or other fluid) due to many causes such as prevalent winds, temperature differential etc., but not to gravitational forces of the Moon and Sun (see tidal stream)

current: diurnal, ebb, flood, mixed, reversing, rotary, semidiurnal: see under the relevant stream entry.

current, geostrophic: A current resulting from the balance of horizontal pressure gradient and the Coriolis force.

current, nontidal: Any current caused by other than tide-producing forces.

current, residual: The mean current with periodic (mostly tidal) components removed.

current meter: An instrument to measure direction and/or speed of a current or tidal stream.

current profile: A detailed presentation of the variation of current speed and direction between the sea-bed and sea surface.


daily mean: A traditional but incorrect representation of the mean of 24 consecutive hourly observations of the water level as the mean sea level for that day, unperturbed by tide. To obtain a correct mean sea level value for the day, 25 consecutive hourly readings are required (see daily retardation), otherwise severe aliasing will occur due to truncation of lunar constituents.

daily retardation: The amount of time by which corresponding tidal phases are later from day to day (approximately 50 minutes).

Darwin, Sir George (1845-1912): An early developer of harmonic methods of tidal analysis, father of modern tidal theory and practice. Established the Indian spring low water datum.

data reduction: A process of checking, calibration and preparation necessary to convert raw tidal measurements into a form suitable for analysis and application.

datum: 1. Any numerical or geometrical quantity which may serve as a reference or base for other quantities. In marine applications, a base elevation or plane used as a reference from which to reckon heights or depths, plural: datums. 2. A single piece of information, plural: data.

datum, chart: see chart datum.

datum, sounding: see sounding datum.

datum, tidal: see tidal datum.

datum, vertical control: see vertical control datum.

datum of tide predictions: The level from which the heights of tide are referenced in the tide tables. See also chart datum.

day: The duration of one rotation of the Earth on its axis. There are several kinds of day, depending on the object or location of the reference point. See constituent day, lunar day, solar day.

daylight saving time: A variation of zone time, usually one hour ahead of that time, used during summer months in some localities. Also known as summer time.

declination: 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 Sun moves through its declination cycle, between 23.5°N and 23.5°S, once a year, and the Moon moves through its cycle in 27.21 mean solar days. The lunar declination varies from 28.5° to 18.5° over an 18.61 year period or a regression of lunar nodes cycle.

declinational inequality: Same as diurnal inequality.

degenerate amphidrome: A terrestrial point on a co-tidal chart from which co-tidal lines seem to radiate.

degree (o): 1. A unit of angular or circular arc measurement, equal to 1/360 of a circle. 2. A unit of measurement of temperature in any scale.

digital: Method of representing information by discrete, discontinuous, mostly numerical data.

digital tide gauge: An automatic tide gauge recording water level at regular intervals in digital form.

digitisation: The process of converting analogue data into digital format.

direction of current: The direction toward which a current is flowing, usually known as the set of the current. Also called current direction.

direction of wind: The direction from which the wind is blowing. Also called wind direction.

dispersion: The separation of a complex surface gravity wave into its component parts.

distortion: An undesired change in waveform. In gravity waves usually caused by friction in the boundary layer. See also shallow water effects.

diurnal: Having a period or cycle of approximately one tidal day.

diurnal inequality: Also known as declinational inequality. The difference between the heights of the two high waters (high water inequality) or the two low waters (low water inequality) during a tidal day, or a difference in speed (2) between the two flood or the two ebb flows during a tidal day. This difference varies with the declination of the Moon (increases with increasing declination), and to a lesser degree, with the declination of the Sun. Mean diurnal high water inequality and mean diurnal low water inequality are half the average differences between the two high waters and the two low waters respectively, of each tidal day over a 19 year period (see epoch).

diurnal tidal stream: A tidal stream with only one ebb and one flood cycle of a reversing stream during a tidal day.

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

divergence: A horizontal flow of water, in different directions, from a common centre, line or area; often associated with upwelling.

Doodson's number: A six digit number, with each digit describing a different characteristic of tide according to a system developed by Doodson in 1921.

double tide: Also known as agger or gulder. A double-headed tide with a high water consisting of two maxima of similar height separated by a small depression (double high water), or a low water consisting of two minima separated by a small elevation (double lowwater).

drift: The speed of water due to ocean currents or tidal streams.

drifting logship: A logship (weighted wooden pole attached to a bridle to make it float in vertical position) used for measuring tidal streams and currents. It is allowed to drift freely while its position is fixed at regular intervals from an accompanying boat or from shore stations.

drogue: A device for measuring currents and tidal streams, consisting of a weighted parachute and attached surface buoy.

duration of flood and ebb: The time interval during which a tidal stream is flooding or ebbing, respectively, these intervals being reckoned from the middle of the intervening slack waters or minimum streams. Together they cover, on average, a period of 12.42 hours for a semidiurnal tidal stream or a period of 24.84 hours for a diurnal one. The duration of flood and ebb are generally equal, but can be modified considerably by non-tidal flows, eg. by a river discharge which usually produces an extended ebb and correspondingly shorter flood stream.

duration of rise and fall: The time interval from low water to high water, and high water to low water, respectively. Together they cover, on an average, a period of 12.42 hours for a semidiurnal tide or a period of 24.84 hours for a diurnal one. In general, the duration of rise and duration of fall are about equal, but in shallow water and in rivers there is a tendency for a decrease in duration of rise and a corresponding increase in duration of fall.

duration of tide: The time interval between low water and the following high water or between high water and the following low water.


earth tides: Periodic vertical movements of the Earth's crust caused by gravitational forces of the Moon and Sun.

Eastern Australia Current (EAC): A section of the south Pacific ocean current setting southward along the east coast of Australia.

ebb: Usually the same as ebb stream, but sometimes used with reference to the vertical tidal movement. Opposite to flood.

ebb interval: The interval between the transit of the Moon over the meridian of a place and the time of the following ebb strength.

ebb stream: The horizontal movement of water associated with falling tide, usually setting seaward or in the direction opposite to tide progression. Also known as ebb, ebb current or outgoing stream.

ebb strength: Phase of the ebb stream at the time of maximum velocity, or such a velocity.

ebb tide: Also known as falling tide, the portion of tidal cycle between high water and the following low water.

ecliptic: The intersection of the plane of the Earth's orbit around the Sun (or apparent orbit of the Sun around the Earth) and the celestial sphere.

eddy: A circular movement of water, usually formed where currents or tidal streams pass obstructions, along the edge of a current or stream, or between two adjacent flows.

Ekman spiral: A horizontal projection of the extremities of the current velocity vector at different (increasing) depths. At depth of frictional resistance the flow is in a direction opposite to that on the surface. Applicable only to flows where only Coriolis and frictional forces are significant.

Ekman transport: The total movement of water in wind-driven currents. On the surface, the drift in the southern hemisphere is at 45° to the left of the direction toward which the wind is blowing, and over the whole water column there is a net transport of water 90o to the left of it (to the right in northern hemisphere). Important when considering the sea level variations due to winds parallel to the shore and coastal upwellings.

endogenous mechanism: An internal biochemical or physiological mechanism for adjusting biological activity to natural rhythms such as tidal cycle.

epoch: In tidal datum determination, a specific 19 year cycle (approximation of the 18.61 year cycle of the regression of lunar nodes) which covers all tidal variations associated with lunar declination, and over which tidal height observations are meaned to establish the various datums.

equation of time: The difference between mean and apparent time.

equatorial tides: Tides occurring approximately every fortnight, when the Moon is over the equator. At these times, the Moon produces minimum diurnal inequality in the tide.

equilibrium theory: A theoretical model under which the waters covering uniformly 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 here, but the model provides an important theoretical basis for tidal science.

equilibrium tide: 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.

equinoctial tides: Tides occurring near the times of equinoxes. If the full or new moon and the Sun have little or low declination at the same time, the resultant spring tides have a greater than average range, especially if the Moon is also at or nearly in perigee, and are known as equinoctial spring tides.

equinoxes: The two points in the celestial sphere where the celestial equator intersects the ecliptic; also, the times at which the Sun crosses the equator at these points (vernal equinox or first point of Aries about March 21 and autumnal equinox or first point of Libra about September 23).

establishment: Also known as high water lunitidal interval, an old term for the interval of time between the transit of the Moon and the next high water at a place. The average establishment at the time of full or new moon is called vulgar or common establishment, or high water full and change.

estuary: An embayment of the coast in which fresh river water entering at its head mixes with saline sea water. When tidal action is the main mixing agent it is known as a tidal estuary.

Eulerian current observations: A method to monitor and measure the flow at a fixed point with, eg., a moored current meter.

eustatic sea level change: The worldwide change of sea level elevation with time due to such causes as glacial melting or formation, thermal expansion or contraction of sea water, post-glacial rebound, etc.

evection: Also known as lunar inequality. A perturbation of the Moon in its orbit due to attraction of the Sun.

exclusive economic zone: An area, usually not exceeding 200 nautical miles from the baselines from which the breadth of the territorial sea is measured, subject to specific legal regime under which the coastal state has certain rights and jurisdiction. The baselines are usually low-water tidal datums.

extra tides: In the Australian National Tide Tables, those tides which exceed 6 per day, and are shown separately due to restricted amount of space for each day's predictions.

extreme high or low water: The highest or lowest elevation reached by the sea during a given period.


falling tide: Same as ebb tide.

first point of Aries (g): see equinoxes.

first quarter: see phases of the moon.

float well: A stilling well in which a float of a float-type tide gauge operates.

flood: Usually the same as flood stream, but sometimes used with reference to the vertical tidal movement. Opposite to ebb.

flood interval: The interval between the transit of the Moon over the meridian of the place and the time of the following flood strength.

flood stream: The horizontal movement of water associated with rising tide, usually setting toward the shore or in the direction of the tide progression. Also known as flood, flood current or incoming stream.

flood strength: Phase of the flood stream at the time of maximum velocity, or such a velocity.

flood tide: Also known as rising tide, the portion of tidal cycle between low water and the following high water.

flow: 1. The motion characteristic of a fluid. 2. Combination of tidal stream and current, also known in US as total current.

flushing time: The time required to remove any dissolved or suspended matter (eg. a contaminant) from an estuary or a harbour, a function of tidal and estuarine processes.

Foucault's pendulum: A laboratory experiment demonstrating the effect of Earth rotation on a body in motion (Coriolis force) where the vertical plane of the oscillations of a free swinging pendulum is observed to rotate through 360o in a period related to the latitude of the location (pendulum day).

Fourier series: The mathematical basis of harmonic analysis of tides and of tidal predictions, a series of sinusoids of different frequencies representing different tidal harmonic constituents. First presented in 1807 by Fourier as a tool for representing any periodic function.

free wave: A wave that continues to exist after the generating force has ceased to act, in contrast with a forced wave. Most ocean surface waves except tidal wave are free waves.

frequency (f ): The number of cycles or vibrations in unit time, a characteristic parameter of a wave, including tide wave and its constituents.

full moon: see phases of the moon.

funnelling: The effect of increasing tide height when a tide propagates into an inlet whose width diminishes toward the head. One of the shallow water effects.


g : see phase lag

gage: US spelling of gauge.

gauge: see tide gauge.

geoid: an equipotential surface (ie. having the same potential gravity at each point) that would be assumed by the sea level in the absence of tides, currents, water density variations and atmospheric effects. It is the surface of reference for astronomical observations and geodetic levelling.

geostrophic current: A steady flow where the Coriolis force balances exactly the horizontal pressure gradient.

GLOSS: Global sea level observing system, a worldwide network of tide gauges developed under the auspices of the International Oceanographic Commission. One purpose is to monitor the long-term variations in mean sea level (see greenhouse effect).

GPS: Satellite based Global Positioning System, capable of accurately locating points in three dimensions using pseudo ranging techniques from a constellation of orbiting satellites. Can be utilised in tidal measurements of the open oceans.

gravitation, Newton's law of: The physical basis of tides, it states that all particles in the universe are attracted to other particles with a force which is proportional to the product of their masses and inversely proportional to the square of their distance apart.

gravitational tide: Same as equilibrium tide.

gravity: The force which attracts all particles to the Earth, thus giving them weight. It is a resultant of two opposing forces: gravitation and centrifugal force due to rotation of the Earth.

gravity wave: A wave whose propagation is controlled by gravity (as opposed to capillary wave). All waves longer than approximately 2cm such as surface chop, sea, swell, tsunamis, tides are gravity waves.

great diurnal range: The difference in height between mean higher high water and mean lower low water, often shortened to diurnal range.

great tropic range: The difference in height between tropic higher high water and tropic lower low water, often shortened to tropic range (see tropic tides)

greenhouse effect: The effect, similar to that in a greenhouse, whereby the Earth's surface is maintained at a temperature much higher than appropriate to the balance of solar irradiation alone. This is caused by carbon dioxide, ozone, water vapour, trace gases and aerosols in the atmosphere trapping some of the longer wave-length return radiation. The increasing concentration of atmospheric carbon dioxide, attributed to the burning of fossil fuels, could lead to a steady increase of global temperatures. The resultant thermal expansion of oceans and melting of polar icecaps could increase sea levels leading to a possibility of coastal flooding.

Greenwich mean time (GMT): Now called universal time, time expressed with respect to the Greenwich meridian (0°), often used as the standard for comparison of global phenomena. Also known as Zulu time (military term).

Greenwich phase lag: Phase lag referred to the corresponding Greenwich tidal equilibrium argument. Also known as Greenwich epoch.

Gregorian calendar: The modern calendar in which every year divisible by 4 (except century years) and every century year divisible by 400 are leap (or bissextile) years of 366 days while all other, common years have 365 days. The resultant average year of 365.2425 days agrees very closely with the length of the astronomical tropical year (the period of changes in the seasons). Introduced by Pope Gregory in 1592 to replace the Julian calendar, it differed by 10 days from the latter to make the vernal equinox fall on March 21, and thus causing a "loss" of the days from 5th to 14th (both inclusive) of October 1592 from the calendar. Accepted by catholic countries immediately on its introduction, by England in 1752, and by Russia and other orthodox Christian countries after World War I, it is now in general use throughout the world.

gulder: see double tide.


H: see amplitude.

half-tide level: Same as mean tide level.

harbour, tidal: See tidal harbour.

harbour datum: An arbitrary datum defined by a local harbour authority, from which levels and tidal heights are measured by that authority, frequently different from chart datum.

harmonic analysis: The mathematical process by which the observed tide or tidal stream at any place is separated into basic harmonic constituents. A long and repetitive process, nowadays usually performed by computer. See also Fourier series.

harmonic constants: The amplitudes and phase lags of the harmonic constituents of the tide or tidal stream at any place. Also known as tidal constants.

harmonic constituent: One of the harmonic elements in a mathematical expression of the tide-producing force, and in corresponding formulae for the tide or tidal stream. Each constituent represents a periodic change of relative position of the Earth, Sun and Moon. Also known as tidal constituent or component.

harmonic prediction: Method of predicting tides and tidal streams by combining the harmonic constituents into a single tidal curve. This inversion of harmonic analysis used to be performed by the mechanical tide predicting machines, but now usually by computers.

head of tide: The inland or upstream limit of water affected by the tide.

height of tide: The vertical distance from the chart datum to the level of the water at any time.

high tide: Same as high water.

high water (HW): The highest level reached by the water during one tidal cycle. Also called high tide.

High Water Full and Change (HWF&C): see establishment.

high water line: Same as high water mark.

high water mark: 1. The mark left by the tide at high water. Also the line or level reached, usually the highest. Also known as high water line. 2. A permanent mark which indicates the maximum observed level of tide.

High Water (Lunitidal) Interval (HWI): see lunitidal interval.

High Water Quadrature (HWQ): The average high water interval when the Moon is at quadrature. See lunitidal interval.

high water stand: see stand of tide.

higher high water (HHW): The highest of the high waters of any specified tidal day due to the declinational effects of the Moon and Sun.

higher low water (HLW): The highest of the low waters of any specified tidal day due to the declinational effects of the Moon and Sun.

Highest Astronomical Tide (HAT): The highest level of water which can be predicted to occur under any combination of astronomical conditions.

hour: A 24th part of a day.

hydraulic current: A current in a channel caused by a difference in surface elevations at the two ends, as in a strait connecting two bodies of water in which tides differ in time or range, eg. Torres Strait.

hydrodynamic levelling: The transfer of survey datum levels by comparing mean sea level at two locations, and adjusting them for gradients on the sea surface due to currents, water density, winds and atmospheric pressure. See also datum transfer.

hydrographic datum: See chart datum.

Hydrographic Service: A national authority responsible for the publication of official charting information in support of safe navigation. In Australia, the Australian Hydrographic Office, which is a part of the Department of Defence, is the national hydrographic authority. The Tidal Section is a part of the Australian Hydrographic Office.

hydrographic survey: A survey of a body of water to determine several classes of data pertaining to it, such as depth, configuration and nature of the bottom, direction and force of the currents, heights and times of tides, location of fixed objects for navigation purposes, etc. These data form the basis for a navigational chart.


Indian Spring Low Water (ISLW): A tidal datum originated by Sir G. Darwin when investigating tides of India. 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.

Indian tide plane: Same as Indian Spring Low Water.

interface: A surface forming a boundary between adjacent solids, spaces or fluids. The waves such as tides propagate along interfaces; surface waves along sea/air interface, and internal waves along interface between two water masses of different densities.

Intergovernmental Oceanographic Commission (IOC): A UNESCO body in charge of international oceanographic cooperation, including tidal and sea level observations (see GLOSS).

internal tides: Tidal waves which propagate at the density interfaces within the ocean. They travel slowly compared with surface tides and have a much shorter wavelength (only a few tens of kilometres compared with several hundreds of kilometres for surface tidal waves), but their amplitude can be tens of metres.

International Hydrographic Bureau: Founded in 1921 to establish a close and permanent association between the hydrographic offices of its member states; located in Monaco; since 1970 the headquarters of the International Hydrographic Organisation.

International Hydrographic Organisation: Coordinates the activities of national hydrographic offices, promotes standards and provides advice in all fields of hydrographic surveying (including tides and sea level observations) and production of nautical charts and publications.

intertidal zone: The zone between mean high water and mean low water, subject to regular submersions and emersions, important for species zonation.

inverted barometer effect: An adjustment of the sea level to changes in barometric pressure; an increase of barometric pressure of 1mb corresponds with a fall in sea level of 1 cm.

isopleth: A line of constant or uniform value of a given quantity, eg. an isobar is an isopleth of pressure.


J1: Smaller lunar elliptic diurnal constituent, modulates the amplitude of K1 for the effects of the Moon's elliptical orbit.

Julian calendar: A calendar introduced by Julius Caesar in 45BC in which a common year had 365 days, and every fourth year 366 days, thus creating an average year of 365.25 year. It was in use until at least 1592. Differs from the modern or Gregorian calendar by 0.0075 days per year.

Julian date: Method of identifying successive days of the year without monthly notation, by numbering days sequentially from January 1, useful in some computer applications.


K1: Lunisolar diurnal constituent. Together with O1 it expresses the effect of the Moon's declination which accounts for diurnal inequality and, in extreme, diurnal tides. With P1 it expresses the effects of the Sun's declination.

K2: Lunisolar semidiurnal constituent. Modulates the amplitude and frequency of M2 and S2 for the declinational effects of the Moon and Sun respectively.

Kelvin, Lord (Sir William Thomson, 1824 - 1907): A tidal theoretician (between his many other achievements). Devised a harmonic method of tidal analysis and prediction (1867), and invented a mechanical tide-predicting machine (1872).

Kelvin wave: A long wave in the oceans whose characteristics have been changed by the rotation of the Earth (see also Coriolis force). In the southern hemisphere the amplitude of a Kelvin wave decreases from left to right along the crest, when viewed in the direction of wave travel, thus creating for example, a variation of tidal range across a channel.

king tide: In popular usage, any high water well above average, commonly applied to high water at springs.

knot: A nautical unit of speed of one nautical mile (1852 m) per hour. The name is derived from the knots in a log line.

kymatology: The science of waves and wave motion.


L2: Smaller lunar elliptic semidiurnal constituent; together with N2 it modifies the amplitude and frequency of M2 for the effects of variation in the Moon's orbital speed due to its elliptical orbit.

lagging of tide: The retardation in the time of occurrence of high and low water due to changes in relative position of the Moon and the Sun.

Lagrangian current: The movement of particles of water through space as measured by drogues or drifting logships or buoys.

lambda (l2): Smaller lunar evectional constituent. One of the constituents modifying the amplitude and frequency of M2 for the effects of variation in solar attraction of the Moon resulting in a pear shaped lunar orbit. See also NU2.

last quarter: see phases of the moon.

latitude: 1. An angular distance between the direction of gravity and the plane of the equator. 2. Celestial latitude: an angular distance between a point on the celestial sphere and the ecliptic.

leap year: A calendar year containing 366 days (see Gregorian calendar).

lee (or leeward) tide: A tidal stream setting in the same direction as that toward which the wind is blowing. Also known as leeward tidal current.

littoral current: A current in the littoral zone, eg. a longshore current.

littoral zone: 1. In coastal engineering , the area from the shoreline to just beyond the breaker zone. 2. In marine biology, the part of benthic (bottom) division extending from the high water mark to the depth of about 200 m. Often used interchangeably with intertidal zone.

local time: 1. Time based upon the transit of the Sun over the local meridian, as distinguished from that based upon a zone meridian (standard time), or on meridian of Greenwich (Universal time). 2. Any time kept locally.

log line: A graduated (usually with knots) line attached to a log, used to measure the speed of a vessel through the water, or to measure the speed of a current or tidal stream from a vessel at anchor.

long period constituent: A tidal constituent with a period independent of the rotation of the Earth but depending on orbital movement of the Moon or of the Earth. The principal lunar long period constituents have periods of one month and a fortnight, the solar ones - periods of one year and six month.

longitude: In general, an angular distance along a primary great circle from an adopted reference point. On the Earth's surface, the angle between the Greenwich meridian (0o) and the meridian of a point on the Earth, measured eastward or westward from the former through 180o and labelled E or W to indicate the direction of measurement.

longshore current: A current paralleling the shoreline, largely within the surf zone. Caused by the net mass transport of wind waves. Tides control the positioning of the surf zone and of the longshore current.

long wave: A wave whose wavelength from crest to crest is long compared with the water depth. All Tidal waves propagate as long waves.

low tide: Same as low water.

low-tide elevation: A naturally formed area of land submerged at high water but emerged and surrounded by water at low water (or low tide).

low water (LW): The lowest level reached by the water during one tidal cycle. Also called low tide.

low water datum: An approximation of mean low water adopted as the reference level for a limited area, irrespective of better determinations at a later date. Used mostly in harbour and river engineering.

low water equinoctial springs: Low water springs near the time of equinoxes, an elevation depressed below mean sea level by the amount equal to the sum of amplitudes of the constituents M2, S2 and K2.

Low Water Full and Change (LWF&C): An old term for the average interval of time between the transit of the full or new Moon and the next low water.

low water line: Same as low water mark.

Low Water (Lunitidal) Interval (LWI): see lunitidal interval.

low water mark: The line along the coast to which the sea recedes at low water. Also known as low water line.

Low Water Quadrature (LWQ): The average low water interval when the Moon is at quadrature. See lunitidal interval.

low water stand: see stand of tide.

lower high water (LHW): The lowest of the high waters of any specified tidal day due to the declinational effects of the Moon and Sun.

lower low water (LLW): The lowest of the low waters of any specified tidal day due to the declinational effects of the Moon and Sun.

Lowest Astronomical Tide (LAT): 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.

lunar day: The duration of one rotation of the Earth on its axis with respect to the Moon or the interval between two consecutive transits of the Moon over the local meridian. Its average length is 24h 50m of mean solar time. Also called tidal day.

lunar inequality: Variation in the orbit of the Moon caused by gravitational attraction by other bodies of solar system. See also evection, perturbation.

lunar interval: The difference in time between the transit of the Moon over the Greenwich meridian and a local meridian.

lunar nodes: The points where the Moon's orbit intersects the ecliptic. Where the Moon crosses from south to north it is called the ascending node (an astronomical reference point, often called the node), and where it crosses from north to south it is called the descending node.

lunar tide: That part of the tide which is due solely to the tide-producing forces of the Moon, as opposed to that part produced by the forces of the Sun (see solar tide).

lunar time: Time based upon the rotation of Earth in relation to the Moon (see lunar day).

lunatic: Anyone considering a career in tidal science.

lunation (or lunar month): Same as synodical month.

lunicurrent interval: The time interval between the Moon's transit over the local or Greenwich meridian and a specified phase of the tidal stream following the transit, eg. flood interval or ebb interval. It can be described as local or Greenwich, depending on the transit reference meridian; assumed to be local if not otherwise specified.

lunisolar tides: Harmonic constituents K1 and K2 which are derived partly from the development of the lunar and partly solar tide. Also included under this heading is the lunisolar synodic fortnightly constituent MSf.

lunitidal interval: The time interval between the transit of the Moon over the local or Greenwich meridian and the next local high water or low water. It can be described as local or Greenwich, depending on the transit reference meridian; assumed to be local unless otherwise specified. The average of all high water intervals is called mean high water lunitidal interval, high water interval, or corrected establishment. The average of all low water intervals is called mean low water lunitidal interval, or low water interval. The expressions higher high, lower high, higher low and lower low water intervals can be used when there is considerable diurnal inequality. See also establishment.


M1: Smaller lunar elliptic diurnal constituent. Together with J1, it modulates the amplitude of the declinational K1 for the effects of the Moon's elliptical orbit.

M2: Principal lunar semidiurnal constituent. It represents the rotation of the Earth with respect to the Moon.

M3: Lunar terdiurnal, shallow water constituent.

M4, M6, M8: Shallow water overtides of principal lunar constituent.

maelstrom: A tidal whirlpool between Moskenesy and Mosken in the Lofoten Islands of northern Norway. Term usually applied to other tidal whirlpools.

marigram: A graphic record of the rise and fall of the water level, same as tide curve.

mean high water (MHW): A tidal level. The average of all high waters observed over a sufficiently long period.

Mean High Water Neaps (MHWN): A tidal level. The average of all high water observations at the time of neap tide over a period time (preferably 19 years). Applicable in semi-diurnal waters only.

Mean High Water Springs (MHWS): A tidal level. The average of all high water observations at the time of spring tide over a period time (preferably 19 years). Applicable in semi-diurnal waters only.

Mean Higher High Water (MHHW): A tidal level. The mean of the higher of the two daily high waters over a period of time (preferably 19 years). Applicable in mixed and diurnal waters.

Mean Higher Low Water (MHLW): A tidal level. The mean of the higher of the two daily low waters over a period of time (preferably 19 years). Applicable in mixed and diurnal waters.

mean low water: A tidal level. The average of all low waters observed over a sufficiently long period.

Mean Low Water Neaps (MLWN): A tidal level. The average of all low water observations at the time of neap tide over a period of time (preferably 19 years). Applicable in semi-diurnal waters only.

Mean Low Water Springs (MLWS): A tidal level. The average of all low water observations at the time of spring tide over a period of time (preferably 19 years). Applicable in semi-diurnal waters only.

Mean Low Water Ordinary Springs (MLWOS): A chart datum commonly used before the Indian spring low water was developed, similar to Mean low water springs.

Mean Lower High Water (MLHW): A tidal level. The mean of the lower of the two daily high waters over a period of time (preferably 19 years). Applicable in mixed and diurnal waters.

Mean Lower Low Water (MLLW): A tidal level. The mean of the lower of the two daily low waters over a period of time (preferably 19 years). Applicable in mixed and diurnal waters.

mean neap rise: The height of mean high water neaps above the chart datum.

mean sea level (MSL): 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).

mean sea level trends: Changes of mean sea level at a location over long periods of time (decades or longer), also called secular sea level changes. See also eustatic sea level change.

mean spring rise: The height of mean high water springs above the chart datum.

mean tide level (MTL): The arithmetic mean of all heights of low and high waters over a period of time. Must not be confused with mean sea level.

mean time: Also known as mean solar time. Time based on the position of a fictitious Sun moving at a uniform rate as distinguished from apparent time which is based on the position of the real Sun. See also equation of time.

mean water level: A tidal datum. An average surface level of a body of water; used mainly in the areas of little or no tidal range (see non-tidal seas).

meteorological effects: Changes in water level due to the local meteorological conditions, such as barometric pressure, wind stress, etc (see also inverted barometer effect, storm surge and wind set-up).

meteorological tides: Periodical or quasi-periodical changes in water level caused by the daily or seasonal variations in local meteorological conditions. They are recognised principally by Sa, Ssa, and S1 constituents.

metonic cycle: A period of almost 19 years, after which the phases of the moon recur on approximately the same days of the year as in the previous cycle. Devised by Meton, an Athenian astronomer in the fifth century BC.

Mf: Lunar fortnightly constituent which expresses the effect of departure from a sinusoidal declinational motion of the moon.

mixed tide: 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 semidiurnal and a predominantly diurnal.

Mm: Lunar monthly constituent, expresses the effect of irregularities in the moon's rate of change of distance and speed in orbit.

modulation: Variation of a characteristic of a wave, such as tide, by the instantaneous values of another, modulating wave, eg. a by a satellite constituent. Amplitude and frequency modulation are the usual effects of such variations.

month: A measure of time based on the period of revolution of the Moon around the Earth. Can be designated as sidereal, tropical, anomalistic, nodical (also known as dracontic) or synodical (also known as lunation or lunar month) depending whether the revolution is relative to a fixed star, vernal equinox, Moon's perigee, ascending node or Sun. The calendar month is a rough approximation of the synodical month.

MSf: Lunisolar synodic fortnightly constituent.

MU2 (m2): A variational constituent. One of constituents modifying the amplitude and frequency of M2 for the effects of variation in solar attraction of the Moon resulting in a pear shaped lunar orbit.

multiple tide pole: A succession of tide poles on a sloping shore placed so that the vertical graduations on the several poles form a continuous scale referred to the same datum.


N2: Larger lunar elliptic semidiurnal constituent; together with L2 it modifies the amplitude and frequency of M2 for the effects of variation in the Moon's orbital speed due to its elliptical orbit.

2N2: Lunar elliptic semidiurnal second order constituent.

National Tidal Centre (N.T.C.): The Australian centre of excellence of tidal science and research. Located in Adelaide, S.A., and associated with the Bureau of Meteorology, it is responsible for most predictions and analyses for Australia's standard ports. It also operates the baseline network of high accuracy tide gauges dedicated to research into the greenhouse effect.

nautical mile: A unit of length used principally in navigation. Originally equal to the length of one minute of arc of a meridian but as this varies slightly with latitude, it is now internationally agreed as equal to 1852m.

neap range: The mean range of tide when neap tides are occurring; the mean difference in height between neap high water and neap low water.

neap tides or streams: The tides of decreased range or streams of decreased speed occurring semimonthly near the times of first and last quarter of the Moon.

negative surge: A pronounced decrease in sea level created by the combination of wind setdown and the inverted barometer effect associated with high air pressure systems. See also storm surge.

Neptune: Roman equivalent of Poseidon.

new moon: see phases of the moon.

Newton, Sir Isaac (1642-1727): The father of tidal theory. Having discovered the laws of gravitation, he carried on further research in the matter and developed the scientific basis of the equilibrium tide theory used to present days. He was also first to state that the inertia of the water must cause modifications of the equilibrium theory, and showed that variations of the tides are indicated, qualitatively at least, by the variations of the equilibrium tide.

nodal corrections: Same as nodal factors.

nodal factors: 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.

nodes: see lunar nodes.

non-harmonic constants: Tidal or tidal stream constants derived directly from high and low water observations without regard to the harmonic constituents, eg. lunitidal intervals, ranges, inequalities etc.

non-harmonic method: An approximate tide prediction method based on the principle that the "tide follows the Moon" and using the relationship between the time of tide and the Moon's meridian transit.

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

non-tidal current: Any current produced by other than tide producing forces.

non-tidal seas: The seas, such as Baltic, Black or Eastern Mediterranean Sea, which have natural periods of oscillation which make them unresponsive to either the diurnal or semi-diurnal tide-producing forces, and thus responding only faintly to some shorter period forces and having either no perceptible tides or tides of very small range.

Notice to Mariners: Periodical notices published by hydrographic offices, or other competent authorities, notifying changes in aids to navigation, dangers to navigation, important new soundings, and, in general, all information affecting nautical charts and nautical publications, including tide tables.

NU2 (n2): Larger lunar evectional constituent. One of the constituents modifying the amplitude and frequency of M2 for the effects of variation in solar attraction of the Moon resulting in a pear shaped lunar orbit. See also lambda.

numerical modelling: The calculations of the hydrodynamic responses of the real seas and oceans to physical forces, such as tide-producing forces, by representing them as a set of discrete connected elements operating within simplified boundaries, and then solving the hydrodynamic equations for each element in sequence. This methodology can represent tides in the open oceans with a relatively high accuracy but it is not as successful near the shoreline where the complexity of the interactions is still too high for current computers.


O1: Lunisolar diurnal constituent. See K1.

obliquity factor: A factor in an expression for a partial tide allowing for the angle of the inclination of the Moon's orbit to the plane of the Earth's equator which varies from 18.3°to 28.6°.

observed tide: The data from tide observing equipment such as tide poles, gauges, etc. Sometimes referred to as actual tides.

oceanic tides: The periodic rise and fall of the Earth's oceans caused by the gravitational interactions between the Sun, Moon and Earth. The tides in the open ocean are closest to the theoretical equilibrium tides.

oceanography: The study of the sea, embracing all the relevant knowledge of its physical, chemical, biological and geological aspects. Tidal science is a part of physical oceanography.

OO1: Lunar diurnal second-order constituent.

oscillation: The natural frequency at which almost every physical system moves when disturbed from rest; it can be free or forced. See also resonance and seiche.

overfalls: Short, breaking waves occurring when a strong current or stream passes over a shoal or other submarine obstruction or meets a contrary current or stream or wind. See also rip.

overtides: 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.


P1:Solar diurnal constituent. See K1.

parallax inequality: The variation in the range of tide or in the speed of a tidal stream due to changes in the distance of the Moon from the Earth.

parallax reduction: Processing of observed tides to obtain quantities depending on changes in the distance of the Moon, eg. perigean and apogean range.

partial tide: Also known as constituent tide, one of the harmonic components comprising the tide at any point and time. The periods of partial tides result from various combinations of the angular velocities of Earth, Sun and Moon relative to each other. See also harmonic constituent.

perigean tides or tidal streams: Tides of increased range, or tidal streams of increased speed occurring when the Moon is near perigee.

perigee: The point in the orbit of the Moon or other satellite nearest to the Earth.

perihelion: The point in the orbit of Earth or other planet nearest to the Sun.

period: 1. The interval needed to complete a cycle of a recurring event such as time between two consecutive like phases of the tide. 2. Any specific duration of time.

perturbation: Disturbance in the orbit of a celestial body due to attraction of another body. See also lunar inequality.

phase: The amount by which a particular cycle, such as a harmonic constituent, progresses from a specified origin, usually expressed in angular measure.

phase inequality: Variations in the tides or tidal streams due to changes in the phase of the moon.

phase lag (g): 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.

phases of the Moon: The various appearances of the Moon during different parts of the synodical month. The cycle begins with new moon and the visible part of the waxing Moon increases in size during the first half of the cycle until full Moon appears, after which the visible part of the waning Moon decreases for the remainder of the cycle. First quarter occurs when the waxing Moon is at east quadrature, last quarter when the waning Moon is at west quadrature.

Poincaré wave: A log period gravity wave resultant from superimposition of incident and reflected plane waves with a composite wave front perpendicular to the coast; used in tidal analysis to study ocean tides and tides in open seas and gulfs.

pole tide: Small variations in sea level due to Chandler Wobble of the axis of rotation of the Earth. Has a period of about 436 days and an amplitude of a few millimetres (maximum of 30mm found in Gulf of Bothnia, Baltic Sea).

Poseidon: Greek god of the sea, and thus probably of tides.

potential, tide producing: See tide-producing potential.

precession of the equinoxes: The conical movement of the Earth's axis about the vertical to the plane of the ecliptic caused by gravitational forces of the Sun, Moon and other planets, and resulting in the continual change of the position of vernal equinox along the celestial equator; it has a period of 26,000 years.

prediction of tides: The mathematical process in which the values of the basic harmonic constituents for a given location are projected into the future, and their amplitudes added for any given time to obtain the predicted tidal height; nowadays performed mostly by computers; mechanical tide predicting machines having been used frequently in the past. See also Fourier series.

pressure tide gauge: A tide gauge operated by the change in pressure at the bottom of a body of water due to the rise and fall of the tide.

priming of tide: The acceleration in the time of occurrence of high and low water due to changes in relative position of the Moon and the Sun.

progressive wave: A wave which is manifested by the progressive movement of its form while the water particles making up the wave merely describe circular orbits. Also known as a travelling wave.


Q1: Larger lunar elliptic diurnal constituent.

2Q1: Lunar elliptic diurnal, second order, constituent.

quadrature of moon: Position of the Moon when its longitude differs by 90° from the longitude of the Sun. The corresponding phases of the moon are known as first quarter and last quarter.

quarter-diurnal tides: The tides resulting from the distortion of the normal tide in shallow water with four high waters and four low waters during one day.


R2: Smaller solar elliptic semi-diurnal constituent; together with T2 it modulates the amplitude and frequency of S2 for the effects of the variation in the Earth's orbital speed due to its elliptical orbit.

race, tidal: A very rapid tidal stream through a relatively narrow channel.

radiational tides: Periodic variations in sea level generated by regular periodic meteorological forces, usually associated with solar radiation.

range of tide: The difference in height between consecutive high and low waters. The mean range is the difference in height between mean high water and mean low water.

Rayleigh criterion: 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.

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

recorder: A man on a survey party who records observational data, including tidal data. Also a recording instrument or part of an automatic tide gauge, which records the time and height of tide.

red tide: The reddish discolouration of surface waters, usually coastal, caused by high concentration of certain micro-organisms, some of which can be toxic and can result in mass killing of other marine animals. It is often transported by real tides.

reduction of soundings: The process of correcting an observed depth for the height of tide above or below the reference datum at the time of measurement. Usually, this term does not apply to corrections other than those due to tides.

reduction of tides: The processing of observed tidal data to obtain mean values of harmonic constants.

reference station: A place where tide or tidal stream constants have been determined from observations (usually long-term), and which is used as a standard for comparison of simultaneous observations at a secondary station.

reflection: The process by which the direction of a wave encountering the coastline is changed.

refraction: The process by which the direction of movement of a long wave is changed as the wave front travels over shallower water and slows down.

regression of lunar nodes: Rotational motion of the nodes of the Moon which make a complete westerly revolution in approximately 18.6 years.

relative mean sea level change: A local change in mean sea level relative to the network of benchmarks on the adjacent land. It may be composed of both, the absolute mean sea level change, and a vertical land movement component.

residual current: A portion of the tidal FLOW not accounted for by the tidal stream.

resonance: A phenomenon of the large amplitudes occurring 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. See also seiche.

response analysis: 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.

return period: The average time between events such as flooding of a particular level. Its inverse is the statistical probability of such an event occurring in any individual year.

reversing stream: Same as rectilinear stream.

reversing falls: The falls which flow in a narrow river channel alternately in opposite directions, depending on the height of tide due to a large tidal range and a constriction in a river. Best known example is in the St. John's River in New Brunswick, Canada.

RHO (r1): Larger lunar evectional diurnal constituent.

rip, tidal: Agitation of water caused by the meeting of tidal streams and wind, or by tidal stream setting over an irregular bottom. See also overfalls.

ripple: The ruffling of the surface of water, a small wave controlled to a significant degree by both surface tension and gravity.

rise of tide: The height of high water above datum. Also known as tidal rise.

rising tide: Same as flood tide.

rotary flow: 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) has its origins in the Coriolis force and thus it tends to rotate counterclockwise in the southern hemisphere.

runoff: The amount of precipitation not absorbed into the ground which flows into the sea. It can cause fluctuations in the sea level, for example in a harbour at the mouth of a river. Sea level along open coasts is not noticeably affected by it.


S1: Solar diurnal constituent.

S2: Principal solar semi-diurnal constituent; represents the rotation of the Earth with respect to the Sun.

S4, S6: Shallow water overtides of the principal solar constituent.

Sa: Solar annual constituent; together with Ssa, it represents the non-uniform changes in Sun's distance and declination. In Australia however, they are more of a radiational character, and represent the yearly meteorological forces, usually associated with solar radiation, and influencing sea level.

saros: Period of 18.03 years, or 223 synodical months, at the end of which the Sun, Moon, and the line connecting the lunar nodes return to approximately the same relative position; the cycle in which solar and lunar eclipses repeat themselves under approximately the same conditions.

satellite: A relatively small celestial body, natural or man-made, revolving around a planet.

satellite constituent: A harmonic constituent generated by the movement of a fictitious satellite. See also astres fictifs.

sea level: The observed level of the sea surface relative to a predefined datum at any instant of time.

secondary port: One of the places listed in Tide Tables for which tide (or tidal stream) is determined by applying relevant corrections to the predictions of a standard port.

secondary (or subordinate) station: A tidal or tidal stream station at which only a short term series of observations was made which were reduced by comparison with simultaneous observations at a reference station.

secular sea level change: see apparent secular trend.

sediment: Any mineral or organic material accumulating in a loose unconsolidated form. May be precipitated chemically or produced biologically in situ, or transported by ice, wind or water and deposited on the bottom. Includes ballast and other materials dumped into the sea by man. When solidified, sediments form sedimentary rocks.

sediment transport: The total sediment transported by a flow of water, consisting of bedload, material supported by the bed as it bounces and rolls, and suspended load, carried in the water.

seiche: A resonant standing wave in an enclosed or semi-enclosed water body that continues to oscillate after the cessation of the originating force, which may have been seismic, atmospheric, tidal or wave induced.

seismic sea wave: Same as tsunami.

semidiurnal: Having a period or cycle of approximately one-half of a tidal day.

semidiurnal constituents: A harmonic tidal constituent having two maxima and two minima each constituent day; its symbol being the subscript 2. See also type of tide and species of constituent.

semidiurnal tide or stream: A tide or tidal stream having a period of approximately 12 hours, ie. having two high waters and two low waters (or two ebb and two flood cycles of a reversing stream) during a tidal day.

sequence of tide or stream: The order in which the tides or tidal stream strengths of the day occur, with special reference to whether the higher high water (or the greater flood) immediately precedes or follows the lower low water (or the greater ebb).

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

sexagesimal: An angular division system where a circle is divided in 360 degrees. A degree is divided in 60 minutes, and one minute is divided in 60 seconds.

shallow water: Water of such depth that surface waves are noticeably affected by bottom topography, customarily water of depth less than half the surface wave length.

shallow water constituents: A short period harmonic constituent introduced into the formula of tidal (or tidal stream) constituents to take account of the change in form of a tide wave resulting from shallow water conditions. Shallow water constituents include overtides and compound tides.

shallow water corrections: Quantities to be applied to a tidal prediction to correct for quarter and sixth-diurnal tides.

shallow water effects: Modifications of the form of tide wave resulting from shallow water conditions such as bottom friction and other physical processes depending on the square or higher powers of the tidal amplitude; in tidal analysis and predictions they are expressed by the shallow water constituents.

shallow water wave: A wave propagated in conditions where the ratio of the depth to the wave length is less than 0.04. Tides are shallow water waves as their length in the open ocean is of the order of 9000km for semidiurnal tides and 18000 km for diurnal ones.

shoaling: An alteration of a wave proceeding from deep water to shallow water.

sidereal: Of or pertaining to the stars; when applied to time or its units (eg. sidereal day, month, etc.), it refers the motion to vernal equinox.

simultaneous observations: Observing tides at a reference and secondary stations at the same time, usually for the purpose of transferring a datum from the former to the latter station.

single day tide: Same as diurnal tide.

sixth-diurnal constituents: Shallow water constituents representing the distortion of the normal tide with six high waters and six low waters during one tidal day.

slack water: The state of a tidal stream when its speed is zero or nearly so, especially the moment when the reversing stream changes its direction. Also known as slack tide or slack.

solar day: 1. The duration of one rotation of the Earth on its axis with respect to the Sun. 2. The duration of one apparent rotation of the Sun around Earth.

solar tide: 1. The part of the tide that is due to the tide-producing force of the Sun. 2. The observed tide in the areas where solar tide (1) is dominant.

solitary wave: A non-oscillatory wave (or wave of translation) consisting of a single crest rising above the undisturbed water level without any accompanying trough.

solstices: 1. The two points of the ecliptic farthest from the celestial equator where the Sun reaches its maximum north or south declination. 2. The times when the Sun reaches these points: about June 21 and December 22.

solsticial tides: Tides occurring near the times of solstices when the tropic range is especially large.

sounding: Measured or charted depth of water or the measurement of such depth.

sounding datum: The horizontal plane or tidal datum to which soundings on a hydrographic survey are reduced.

sounding reduction: see reduction of soundings.

species of constituent: A classification of tidal constituents depending upon their period. The principal species are semidiurnal, diurnal, and long period.

speed (of constituent): 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.

speed (of stream or current): The rate at which a stream or current flows; usually expressed in knots or metres per second.

spring tides or streams: The tides of increased range or tidal streams of increased speed occurring semimonthly near the times of full moon and new moon.

Ssa: Solar semiannual constituent. See Sa.

stand of tide: 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.

standard port: A place for which independent daily predictions are given in the tide or stream tables, from which corresponding predictions are obtained for other locations, known as secondary ports, by means of differences or factors.

standard time: A land based variation of zone time, with irregular but well defined zone limits (eg. state boundaries).

standing wave: Also called stationary wave, a type of resonant wave in which the surface of the water oscillates vertically between fixed points (nodes), without progression. A result of two equal progressive waves moving in opposite directions.

steric level differences: Sea level differences due to differences in water density.

still water level: The level that the sea surface would assume in the absence of wind waves (not to be confused with Mean Sea Level or Mean Tide Level).

stilling-well gauge: A tide gauge which measures the sea level via a float moving in a well connected to the sea through a small orifice which filters out any high-frequency waves whilst admitting the long period tidal (and other) level variations.

storm surge: A rise of the water level along the coast due to the action of wind stress alone or its combination with the reduction of atmospheric pressure accompanying it. The effects of a storm surge are more severe when it occurs in conjunction with high tide. Also known as storm tide, storm wave, tidal wave.

stream: A steady current in the sea (or a river), especially the middle or most rapid part of tide or current. See also tidal stream.

stream rate: Same as speed of tidal stream.

strength of stream: The phase of a tidal stream at which the speed is at maximum, also the speed at this time.

surge: 1. A large change in sea level generated by extreme meteorological events, for example, cyclones; may be positive or negative depending on whether the sea level is higher or lower than predicted. 2. Wave motion with a period intermediate between that of a wind wave and that of tide, that is, from about 12 to 60 minutes. 3. A short period horizontal oscillation of water accompanying a seiche.

surge level: A non-predictable, meteorologically induced component of tide, sometimes called non-tidal residual. May have some statistical regularity.

survey, hydrographic: See hydrographic survey.

survey datum: The datum to which levels on land surveys are related, often defined in terms of mean sea level.

synodic period: The minimum length of data necessary to separate a pair of constituents according to Rayleigh criterion.

synodical month: 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.

syzygy: Either of two opposing points in the orbit of a planet or satellite but particularly of the Moon when it is new or full, ie. in conjunction or in opposition to the Sun. Also an old term for the tide which takes place on the afternoon of the day when syzygy occurs.


T2: Larger solar elliptic constituent. Together with R2 it modulates the amplitude and frequency of S2 for the effects of the variation in the Earth's orbital speed due to its elliptical orbit.

telemetry: The technique of measuring, transmitting, receiving and/or retrieving data over long distances, usually using long distance communication links such as satellite or telephone.

terdiurnal (or third-diurnal) constituents: A harmonic constituent with three periods in a constituent day, indicated by subscript 3.

thermocline: A layer in a body of water in which the temperature decreases with depth at an appreciably greater rate than above and below, also such a gradient of temperature. Also called discontinuity or thermal layer.

tidal characteristics: The features relating to the time, range and type of tide.

tidal constant: see harmonic constant.

tidal constituent: see harmonic constituent.

tidal current: US equivalent of tidal stream.

tidal cycle: A complete set of tidal conditions as those occurring during a tidal day, lunar month or metonic cycle.

tidal day: Same as lunar day.

tidal datum: A level of the sea defined by some phase of the tide, from which depths of water and heights of tide are reckoned.

tidal difference: Difference in time or height of a high or low water at a secondary port and standard port for which predictions are given in the tide tables. The difference, when applied to the predictions at a nominated standard port, gives the corresponding time or height at a secondary port.

tidal epoch: see epoch and phase lag.

tidal flat: A marsh or sandy or muddy coastal flatland which is covered and uncovered by the rise and fall of the tide.

tidal frequency: The number of cycles of tide during one day.

tidal harbour: A harbour affected by tides, as opposed to a harbour incorporating a BASIN in which the water level is maintained by caissons or gates.

tidal level: The part of the observed sea level which is coherent with tide-producing forces, and thus predictable from a set of harmonic constants.

tidal lights: Lights shown at the entrance of a harbour to indicate tide and tidal stream conditions within.

tidal movement: The vertical rise and fall of the sea level, and the horizontal flow of the tidal streams associated with astronomical tide-producing forces of the Moon and Sun.

tidal prism: A volume of water exchanged between an estuary or a lagoon and the open sea during one tidal period.

tidal stream: The horizontal movement of water associated with the rise and fall of the tide caused by tide-producing forces. Also called tidal current.

tidal wave: In popular usage, an unusually high, and hence destructive, water level along the shore; usually refers to a storm surge or, more commonly but erroneously, to a tsunami.

tide: The periodic rise and fall of the water of oceans, seas, bays, etc., caused mainly by the gravitational interactions between the Earth, Moon and Sun.

tide-bound: Referring to a vessel which is unable to proceed because of insufficient water depth due to tidal action.

tide crack: The fissure between an immovable ice wall and fast ice which is subjected to the rise and fall of the tide.

tide curve: A graphic representation of the tide. For most tides it approximates a sine curve. Also called a marigram.

tide gate: A narrow passage through which water runs with great speed due to tidal action.

tide gauge: Any instrument used for measuring the level of the tide, including a tide pole.

tide gauge bench mark (TGBM): A benchmark near a tide gauge used to check its levelling.

tide mark: Same as high water mark.

tide pole : A graduated pole positioned vertically in the water from which the height of tide can be read at any time, the simplest tide gauge. Also known as tide staff. See also multiple tide pole.

tide predicting machine: A mechanical analogue instrument used to compute the times and heights of a tide by mechanically summing its harmonic constituents. Now superseded by computers.

tide-producing force: Also called tide-generating force. That part of gravitational attraction of a celestial body (mainly the Sun and the Moon) which is effective in producing the tides on Earth. It varies proportionally to the mass of the attracting body, and inversely to the cube of its distance.

tide-producing potential: A tendency for particles on Earth to change their positions as a result of the gravitational interactions between the Sun, Moon and Earth; varies inversely to the cube of the distance from the tide-producing body.

tide station: A place where tidal observations are obtained. In general, a primary tide station is established for log-term continuous observations (preferably no less than 19 years) to determine the tidal characteristics of the locality, a secondary tide station is established over shorter periods for specific purposes.

tide tables: Tables listing, usually a year in advance, daily predictions of the times and heights of the tide for the standard ports. Frequently, these predictions are supplemented by tidal differences and constituent listings enabling predictions for numerous other locations called secondary ports to be obtained.

tide wave: A long-period wave associated with the tide-producing forces of the Moon and Sun; identified with the rising and falling of the tide. Also known as tide bulge. Not to be confused with tidal wave.

tidelands: Same as intertidal zone

tidewater: Water activated by tides, especially in coastal or estuarine areas.

tideway: A channel through which a tidal stream runs.

time: 1. A natural phenomenon of continuing progress through past, present and future, preventing everything from happening at once. 2. The measurable aspect of duration, an elapsed interval. 3. The hour of the day.

tractive force: The horizontal component of a tide-producing force vector, responsible for the horizontal displacement of water.

transfer of datum: A method of determining a chart datum by comparison with a place near the a survey area which already has a chart datum established.

tropic of Cancer: The northern parallel of declination, approximately 23° 27' from the celestial equator, reached by the Sun at its maximum northerly declination (about June 21), or the corresponding parallel on Earth.

tropic of Capricorn: The southern parallel of declination, approximately 23° 27' from the celestial equator, reached by the Sun at its maximum southerly declination (about December 22), or the corresponding parallel on Earth.

tropic tides and streams: Tides and tidal streams occurring semi-monthly when the effect of Moon's maximum declination north or south of the equator is greatest. Associated terms: tropic inequalities, tropic intervals, tropic ranges, tropic speed, etc.

tropical year: The period of one revolution of the Earth around the Sun with respect to the vernal equinox, about 20 minutes shorter than the sidereal year. The calendar year is adjusted to this unit through introduction of extra days on leap years.

tsunami: A wave generated by seismic activity. Also called seismic sea wave, or, inaccurately, a tidal wave. Barely discernible in the open ocean, their amplitude may increase to several metres in the shallow coastal regions. Most common in the Pacific.

type of tide: The characteristic feature of the tide (or tidal stream) determined from the combination of its diurnal and semidiurnal components. Tides are generally classified as semidiurnal, mixed and diurnal, but the limits between the groups are not clearly defined and may vary depending on application, the mixed classification being sometimes subdivided or dropped altogether.


underkeel clearance: The distance between the ship's lowest point, normally on the keel, and the sea bottom.

Universal Time (UT): 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).

upwelling: An upward flow of subsurface water, usually a result of divergence, offshore currents, winds and wind drift transport.


vanishing tide: The phenomenon occurring when a higher low and lower high water meld to produce a period of several hours with a nearly constant water level. This occurs occasionally in waters with very strong diurnal inequalities at times of extreme declinations (for example at Wallaroo, SA).

variation: The angle between the magnetic and geographical meridians at any place, expressed in degrees east or west to indicate the direction of magnetic north from true north.

vernal equinox: see equinoxes.

vertical clearance: The minimum vertical space available for passage.

vertical control datum: Any level surface taken as reference from which to reckon elevations. In Australia, this is generally the Australian Height Datum which approximates the mean sea level in most locations.

VLBI: Very Long Baseline Interferometry - a very accurate technique of three-dimensional fixing position of points on Earth by comparing the arrival times of radio signals from distant extra-terrestrial sources (eg. some stars). It is used in fixing the position of selected tide gauge bench marks to differentiate between the vertical land movements and variations of the sea level at the sites.

vulgar establishment: see establishment.


wave: 1. A periodic disturbance which moves through or over the surface of the medium (eg. water) with speed dependent upon the properties of the medium. It is characterised by frequency, length, and amplitude. 2. A ridge-like deformation, or undulation of a liquid or solid surface.

weather (or windward) tide: A tidal stream flowing to windward, that is, in the direction from which the wind blows.

Wentworth scale: International scale used to classify sediments in terms of their particle sizes.

wind drift: An ocean current in which only Coriolis and frictional forces are significant. See also Eckman spiral.

wind tide: Wind set-up on the ocean or other large body of water.

wind set-up: The vertical rise in the still water level on the leeward side of a body of water caused by the wind stress on the water surface, the resulting difference in still water levels between windward and leeward sides of such a body of water. This term is used mainly for smaller bodies of water. See also wind tide and meteorological effects and tides.

wind wave: A wave resulting from the action of the wind on the surface of water.


xerotide: Dry tide, an obsolete term for the earth tide.


year: A period of one revolution of the Earth around the Sun. May be anomalistic, sidereal or tropical depending whether the revolution is relative to the Earth's perihelion, Sun, or vernal equinox. See also civil year.


Zo: International symbol of the elevation of mean sea level above chart datum

zonation: The pattern of colonisation of the sea shore by plants and animals where the abundance of the individual species is associated with particular tidal levels.

zone time: The local mean time of a reference (or zone) meridian, whose time is kept throughout a designated zone. The zone meridian is usually the nearest meridian whose longitude is divisible by 15°.

Zulu time: Military term for Universal Time (Z for zero time zone adjustment).