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