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2N2: Lunar elliptic semi diurnal second-order constituent. Speed = 2T– 4s + 2h + 2p = 27.895,354,8° pe
2Q1: Lunar elliptic diurnal, second order, constituent. Speed = T – 4s + h + 2p = 12.854,286,2° per so
Absolute Mean Sea Level Change: An eustatic change in mean sea level relative to a conventional terrestrial coordinate system with t
Absolute Mean Sea Level Change: An eustatic change in mean sea level relative to a conventional terrestrial coordinate system with t
Accepted Values: Tidal datums and Greenwich high and low water intervals obtained through primary de-termination or c
Acoustic Doppler Current Profiler (ADCP): Current measuring instrument employing the transmission of high frequency acoustic signals in the wa
ADR Gauge: Analog to Digital Recording water level gauge. A float or pressure-actuated water level gauge that r
Age Of Diurnal Inequality: The time interval between the maximum semimonthly north or south declination of the Moon and the max
Age Of Moon: The time elapsed since the preceding new Moon
Age Of Parallax Inequality: The time interval be tween perigee of the Moon and the maximum effect of parallax upon range of tide
Age Of Phase Inequality: The time interval between new or full Moon and the maximum effect of these phases upon range of tide
Agulhas Current: An Indian Ocean current setting southwestward along the southeast coast of Africa.
Air Acoustic Ranging Sensor: A pulsed, acoustic ranging device using the air column in a tube as the acoustic sound path. The fun
Alaska Current: A North Pacific Ocean current setting counterclockwise along the coasts of Canada and Alaska in the
Alaskan Stream: A North Pacific Ocean current setting westward along the south side of the Aleutian Islands. It is a
Amphidromic Point: A point of zero amplitude of the observed or a constituent tide.
Amphidromic Region: An area surrounding an am phidromic point from which the radiating cotidal lines progress through al
Amplitude (H): One-half the range of a constituent tide. By analogy, it may be applied also to the maximum speed of
Analog: A continuous measurement or a continuous graphic display of data. See ADR gauge and marigram.
Angular Velocity Of The Earths Rotation: Time rate of change of angular displacement relative to the fixed stars. It is equal to 0.729,211 x
Annual Inequality: Seasonal variation in water level or current, more or less periodic, due chiefly to me teorological
Anomalistic: Pertaining to the periodic return of the Moon to its perigee or the Earth to its perihelion. The ano
Anomaly: As applied to astronomy, the anomaly is the angle made at any time by the radius vector of a planet
Antarctic Circumpolar Current: The largest permanent current in the world, setting eastward around the Antarctic Continent south of
Anticyclonic Ring: A meander breaking off from the main oceanic current and spinning in a clockwise direction in the no
Antilles Current: A North Atlantic Ocean current setting northwestward along the northeast coasts of the Bahama Island
Aphelion: The point in the orbit of the Earth (or other planet, etc.) Farthest from the Sun.
Apogean Tides Or Tidal Currents: Tides of decreased range or currents of decreased speed occurring monthly as the result of the Moon
Apogee: The point in the orbit of the Moon or a man-made satellite farthest from the Earth. The point in the
Apparent Secular Trend: The non-periodic tendency of sea level to rise, fall, or remain stationary with time. Technically, i
Apparent Time: Time based upon the true position of the Sun as distinguished from mean time, which is measured by a
Apsides: The points in the orbit of a planet or moon which are the nearest and farthest from the center of at
Astres Fitifs: Fictitious celestial bodies which are assumed to move in the celestial equator at uniform rates corr
Astronomical Time: Time formerly used in astronomical calculations in which the day began at noon rather than midnight.
Augmenting Factor: A factor, used in connection with the harmonic analysis of tides or tidal currents by stencils, to a
Automatic Tide Gauge: An instrument that automatically registers the rise and fall of the water level. In some instruments
Azimuth: Azimuth of a body is the arc of the horizon intercepted between the north or south point and the foo
Barycenter: The common center of mass of the Sun-Earth System or the Moon-Earth System. The distance from the ce
Bench Mark (BM): A fixed physical object or mark used as reference for a horizontal or vertical datum. A tidal bench
Benguela Current: A South Atlantic Ocean current setting northward along the southwest coast of Africa.
Brazil Current: A South Atlantic Ocean current setting southwestward along the central coast of South America.
Bubbler Tide Gauge: Same as gas purged pressure gauge.
California Current: A North Pacific Ocean current setting southeastward along the west coast of the United States and Ba
Callippic Cycle: A period of four Metonic cycles equal to 76 Julian years, or 27,759 days. Devised by Callippus, a Gr
Canary Current: A North Atlantic Ocean current setting southward off the west coast of Portugal and along the northw
Celestial Sphere: An imaginary sphere of infinite radius concentric with the Earth on which all celestial bodies excep
Centibar: The unit of pressure equal to 1 metric ton (1000 kilograms) per meter per second per second. See dec
Chart Datum: The datum to which soundings on a chart are referred. It is usually taken to correspond to a low-wat
Chlorinity (CL): The total amount in grams of chlorine, bromine, and iodine contained none kilogram of seawater, assu
Civil Time: Time in which the day begins at midnight as distinguished from the former astronomical time in which
Coast And Geodetic Survey: A former name of the National Ocean Service. The organization was known as: Survey of the Coast from
Coast Line: The low water datum line for purposes of the Submerged Lands Act (Public Law 31). See shoreline.
Coastal: The term coastal zone means the coastal waters (including the lands therein and there under) and the
Coastal Boundary: The mean high water line (MHWL) or mean higher high water line (MHHWL) when tidal lines are used as
Cocurrent Line: A line on a map or chart passing through places having the same current hour.
Comparison Of Simultaneous Observations: A reduction process in which a short series of tide or tidal current observations at any place is co
Compass Direction: Direction as indicated by com pass without any correction for compass error. The direction indicated
Compass Error: The angular difference between a compass direction and the corresponding true direction. The compass
Component: ( 1) Same as constituent.(2) That part of a tidal current velocity which, by resolution into orthogo
Compound Tide: A harmonic tidal (or tidal current) constituent with a speed equal to the sum or difference of the s
Constituent: One of the harmonic elements in a mathematical expression for the tide-producing force and in corres
Constituent Day: The time of the rotation of the Earth with respect to a fictitious celestial body repre senting one
Control Current Station: A current station at which continuous velocity observations have been made over a minimum period of
Coriolis Force: A fictional force in the hydrodynamic equations of motion that takes into account the effect of the
Corrected Current: A relatively short series of cur-rent observations from a subordinate station to which a factor is a
Cotidal Hour: The average interval between the Moon's transit over the meridian of Greenwich and the time of the f
Cotidal Line: A line on a chart or map passing through places having the same tidal hour.
Countercurrent: A current usually setting in a direc tion opposite to that of a main current. See Equatorial Counter
Crest: The highest point in a propagating wave. See high water and tidal wave.
Current: Generally, a horizontal movement of water. Currents may be classified as tidal and non-tidal. Tidal
Current Constants: Tidal current relations that re-main practically constant for any particular locality. Current const
Current Curve: A graphic representation of the flow of the current. In the reversing type of tidal current, the cur
Current Diagram: A graphic table published in the Tidal Current Tables showing the speeds of the flood and ebb curren
Current Difference: Difference between the time of slack water (or minimum current) or strength of current in any locali
Current Ellipse: A graphic representation of a rotary current in which the velocity of the current at different hours
Current Hour: The mean interval between the transit of the Moon over the meridian of Greenwich and the time of str
Current Line: A graduated line attached to a current pole formerly used in measuring the velocity of the current.
Current Meter: An instrument for measuring the speed and direction or just the speed of a current. The measurements
Current Pole: A pole used in observing the velocity of the current. The pole formerly used by the Coast and Geodet
Current Station: The geographic location at which current observations are conducted. Also, the facilities used to ma
Cyclonic Ring: A meander breaking off from the main oceanic current and spinning in a counter-clockwise direction i
Data Collection Platform (DCP): A microprocessor based system that collects data from sensors, processes the data, stores the data i
Datum (Vertical): For marine applications, a base elevation used as a reference from which to reckon heights or depths
Datum Of Tabulation: A permanent base elevation at a tide station to which all water level measurements are referred. The
Davidson Current: A North Pacific Ocean counter-current setting northward between the California Current and the coast
Day: The period of rotation of the Earth. There are several kinds of days depending on whether the Sun, M
Daylight Saving Time: A time used during the sum mer months, in some localities, in which clocks are advanced 1 hour from
Decibar: The practical unit for pressure in the ocean, equal to 10 centibars, and is the approximate pressure
Declination: Angular distance north or south of the celestial equator, taken as positive when north of the equato
Declinational Reduction: A processing of observed high and low waters or flood and ebb tidal currents to obtain quantities de
Deviation (Of Compass): The deflection of the needle of a magnetic compass due to masses of magnetic metal within a ship on
Direct Method: A tidal datum computation method. Datums are determined directly by comparison with an appropriate c
Diurnal: Having a period or cycle of approximately one tidal day. Thus, the tide is said to be diurnal when o
Diurnal Inequality: The difference in height of the two high waters or of the two low waters of each tidal day; also, th
Diurnal Tide Level: A tidal datum midway between mean higher high water and mean lower low water.
Double Ebb: An ebb tidal current having two maxima of speed separated by a smaller ebb speed.
Double Flood: A flood tidal current having two maxima of speed separated by a smaller flood speed.
Double Tide: A double-headed tide, that is, a high water consisting of two maxima of nearly the same height separ
Duration Of Flood And Duration Of Ebb: Duration of flood is the interval of time in which a tidal current is flooding, and duration of ebb
Duration Of Rise And Duration Of Fall: Duration of rise is the interval from low water to high water, and duration of fall is the interval
Dynamic Meter (D): The former practical unit for geopotential difference (dynamic depth), equal to 10 geopotentials (dy
Earth Tide: Periodic movement of the Earth's crust caused by gravitational interactions between the Sun, Moon, a
Ebb Current (Ebb): The movement of a tidal current away from shore or down a tidal river or estuary. In the mixed type
Ebb Interval: The interval between the transit of the Moon over the meridian of a place and the time of the follow
Ebb Strength (Strength Of Ebb): Phase of the ebb tidal current at the time of maximum speed. Also, the speed at this time. See stren
Eccentricity Of Orbit: Ratio of the distance from the center to the focus of an elliptical orbit to the length of the semi
Ecliptic: The intersection of the plane of the Earth's orbit with the celestial sphere.
Eddy: A quasi-circular movement of water whose area is relatively small in comparison to the current with
Edge Waves: Waves moving between zones of high and low breakers along the shoreline. Edge waves contribute to ch
Ekmanspiral : A logarithmic spiral (when projected on a horizontal plane) formed by the heads of current velocity
Electric Tape Gauge: A gauge consisting of a graduated Monel metal tape on a metal reel (with supporting frame), voltmete
Elimination: One of the final processes in the harmonic analysis of tides in which preliminary values for the har
Epoch: (1) Also known as phase lag. Angular retardation of the maximum of a constituent of the observed tid
Equation Of Time: Difference between mean and apparent time. From the beginning of the year until near the middle of
Equatorial Countercurrent: A current setting eastward between the North and South Equatorial Currents of the Atlantic, Pacific,
Equatorial Tidal Currents: Tidal currents occurring semimonthly as a result of the Moon being over the Equator. At these times
Equatorial Tides: Tides occurring semimonthly as a result of the Moon being over the Equator. At these times the tende
Equatorial Undercurrent: A subsurface current setting eastward along the Equator in the Pacific, Atlantic, and Indian Oceans.
Equilibrium Argument: The theoretical phase of a constituent of the equilibrium tide. It is usually represented by the exp
Equilibrium Theory: A model under which it is assumed that the waters covering the face of the Earth instantly respond t
Equilibrium Tide: Hypothetical tide due to the tide producing forces under the equilibrium theory. Also known as gravi
Equinoxes: The two points in the celestial sphere where the celestial equator intersects the ecliptic; also, th
Establishment Of The Port: Also known as high water, full and change (HWF&C).Average high water interval on days of the new and
Estuary: An embayment of the coast in which fresh river water entering at its head mixes with the relatively
Eulerian Measurement: Observation of a current with a device fixed relative to the flow.
Eustaticsea Level Rate: The worldwide change of sea level elevation with time. The changes are due to such causes as glacial
Evection: A perturbation of the Moon depending upon the alternate increase and decrease of the eccentricity of
Extreme High Water: The highest elevation reached by the sea as recorded by a water level gauge during a given period. T
Extreme Low Water: The lowest elevation reached by the sea as recorded by a water level gauge during a given period. Th
Falkland Current: A South Atlantic Ocean current setting northeastward along the east coast of Argentina.
First Reduction: A method of determining high and low water heights, time intervals, and ranges from an arithmetic me
Float Well: A stilling well in which the float of a float-actuated water level gauge operates. See stilling well
Flood Axis: The average set of the tidal current at strength of flood.
Flood Current: The movement of a tidal cur-rent toward the shore or up a tidal river or estuary. In the mixed type
Flood Interval: The interval between the transit of the Moon over the meridian of a place and the time of the follow
Flood Strength: Phase of the flood tidal current at the time of maximum speed. Also, the speed at this time. See str
Florida Current: A North Atlantic Ocean current setting northward along the south-east coast of the United States. A
Flow: The British equivalent of the United States total current. Flow is the combination of tidal stream a
Flushing Time: The time required to remove or re-duce (to a permissible concentration) any dissolved or suspended c
Forced Wave: A wave generated and maintained by a continuous force.
Fortnight: The time elasped between the new and full moons. Half a synodical month or 14.765,294 days. See syno
Fourier Series: A series proposed by the French mathematician Fourier about the year 1807. The series involves the s
Free Wave: A wave that continues to exist after the generating force has ceased to act. See gravity wave.
Galofaro: A whirlpool in the Strait of Messina; at one time called Charybdis.
Gas Purged Pressure Gauge: A type of water level gauge in which gas, usually nitrogen, is emitted from a submerged orifice at a
Geodetic Datum: See National Geodetic Vertical Datum of 1929 (NGVD 1929) and North American Vertical Datum of 1988 (
Geopotential: The unit of geopotential difference, equal to the gravity potential of 1 meter squared per second sq
Geopotential (Equipotential) Surface: A surface that is everywhere normal to the acceleration of gravity.
Geopotential Anomaly: The excess in geopotential difference over the standard geopotential difference [at a standard speci
Geopotential Difference: The work per unit mass gained or required in moving a unit mass vertically from one geopotential sur
Geopotential Topography: The topography of an equiscalar (usually isobaric) surface in terms of geopotential difference. As d
Geostrophic Flow: A solution of the relative hydro-dynamic equations of motion in which it is assumed that the horizon
Gradient Flow: A solution of the relative hydrodynamic equations of motion in which only the horizontal Coriolis, p
Gravity Wave: A wave for which the restoring force is gravity.
Great Diurnal Range (GT): The difference in height between mean higher high-water and mean lower low water. The expression may
Great Tropic Range (GC): The difference in height between tropic higher high water and tropic lower low water. The expression
Greenwich Argument: Equilibrium argument computed for the meridian of Greenwich.
Greenwich Interval: An interval referred to the transit of the Moon over the meridian of Greenwich, as distinguished fro
Gregorian Calendar: The modern calendar in which every year divisible by 4 (excepting century years) and every century y
Guiana Current: An Atlantic Ocean current setting northwestward along the north-east coast of South America.
Guinea Current: An Atlantic Ocean current setting eastward along the west central coast of Africa. A continuation of
Gulder: Local name given to the double low water occurring on the south coast of England. See double tide.
Gulf Coast Low Water Datum (GCL WD): A tidal datum. Used as chart datum from November 14, 1977, to November 27, 1980, for the coastal wat
Gulf Coast Low Water Datum Line: The line on a chart or map which represents the intersection of the land with the water surface at t
Gulf Stream: A North Atlantic Ocean current setting northeastward off the east coast of the United States. A segm
Gulf Stream System: The continuous current system composed of the Florida Current, Gulf Stream, and North Atlantic Curre
Halocline: A layer in which the salinity changes significantly (relative to the layers above and below) with de
Harmonic Analysis: The mathematical process by which the observed tide or tidal current at any place is separated into
Harmonic Analyzer: A machine designed for the resolution of a periodic curve into its harmonic constituents. Now perfor
Harmonic Constants: The amplitudes and epochs of the harmonic constituents of the tide or tidal current at any place.
Harmonic Function: In its simplest form, a quantity that varies as the cosine of an angle that increases uniformly with
Harmonic Prediction: Method of predicting tides and tidal currents by combining the harmonic constituents into a single t
Head: The difference in water level at either end of a strait, channel, inlet, etc.
Head Of Tide: The inland or upstream limit of water affected by the tide. For practical application in the tabulat
High Water (Hw): The maximum height reached by a rising tide. The high water is due to the periodic tidal forces and
High Water Mark: A line or mark left upon tide flats, beach, or along shore objects indicating the elevation of the i
Higher High Water (HHW): The highest of the high waters (or single high water) of any specified tidal day due to the declinat
Higher Low Water (HLW): The highest of the low waters of any specified tidal day due to the declinational effects of the Moo
Horizontal Pressure Gradient Force: The horizontal component of the product of the specific volume and the rate of decrease in pressure
Hydraulic Current: A current in a channel caused by a difference in the surface elevation at the two ends. Such a curre
Hydrographic Datum: A datum used for referencing depths of water and the heights of predicted tides or water level obser
Incremental Shaft Encoder: A component of a water level gauge for converting length to a shaft angle on a rotating disk. The po
Indian Spring Low Water: A datum originated by Professor G. H. Darwin when investigating the tides of India. It is an elevati
Inequality: A systematic departure from the mean value of a tidal quantity. See diurnal inequality, parallax ine
Inertial Flow: A solution of the relative hydrodynamic equations of motion in which only the horizontal component o
Internal Tide: A tidal wave propagating along a sharp density discontinuity, such as a thermocline, or in an area o
International Great Lakes Datum: Mean water level at Rimouski/Pointe-au-Pere, Quebec, on the Gulf of St. Lawrence over the period 197
International Hydrographic Organization: An institution consisting of representatives of a number of nations organized for the purpose of coo
Intertidal Zone: (technical definition) The zone between the mean higher high water and mean lower low water lines.
Inverse Barometer Effect: The inverse response of sea level to changes in atmospheric pressure. A static reduction of 1.005 mb
Irminger Current: A North Atlantic Ocean current setting westward off the south-west coast of Iceland.
Julian Calendar: A calendar introduced by Julius Caesar in the year 45 B.C., and slightly modified by Augustus a few
Julian Date: Technique for the identification of successive days of the year when monthly notation is not desired
K1: Lunisolar diurnal constituent. This constituent, with O1, expresses the effect of the Moon's declina
K2: Lunisolar semi diurnal constituent. This constituent modulates the amplitude and frequency of M2 and
Kappa: Name of Greek letter used as the symbol for a constituent phase lag or epoch when referred to the lo
Kappa Prime: Name of Greek letter (with prime mark) used as the symbol for a constituent phase lag or epoch when
Knot: A speed unit of 1 international nautical mile (1,852.0 meters or 6,076.115,49 international feet) pe
Kuroshio: Black Stream in Japanese. A North Pacific Ocean current setting northeastward off the east coast of
Kuroshio Extension: A North Pacific Ocean current setting eastward from about longitude 145° east to about 160° east.
Kuroshio System: The current system composed of the Kuroshio, Tsushima Current, Kuroshio Extension, and North Pacific
L2: Smaller lunar elliptic semi diurnal constituent. This constituent, with N2, modulates the amplitude
Labrador Current: A North Atlantic Ocean current setting southeastward along the east coasts of Baffin Island, Labrado
Lagging Of Tide: The periodic retardation in the time of occurrence of high and low water due to changes in the relat
Lagrangian Measurement: Observation of a current with a device moving with the current.
Lambda (82): Smaller lunar evectional constituent. This constituent, with <2, :2, and (S2), modulates the amplitu
Latitude: The angular distance between a terrestrial position and the equator measured northward or southward
Leap Year: A calendar year containing 366 days. According to the present Gregorian calendar, all years with the
Level Of No Motion: A level (or layer) at which it is assumed that an isobaric surface coincides with a geopotential sur
Littoral Current: A current in the littoral zone such as a long shore or rip current.
Littoral Zone: In coastal engineering, the area from the shoreline to just beyond the breaker zone. In biological o
Local Time: Time in which noon is defined by the transit of the Sun over the local meridian as distinguished fro
Log Line: A graduated line used to measure the speed of a vessel through the water or to measure the velocity
Long Period Constituent: A tidal or tidal current constituent with a period that is independent of the rotation of the Earth
Long Period Waves: Forced or free waves whose lengths are much longer than the water depth. See tidal wave and tsunami.
Long Shore Current: A current paralleling the shore largely within the surf zone. It is caused by the excess water broug
Longitude: Angular distance along a great circle of reference reckoned from an accepted origin to the projectio
Loop Current: A current setting clockwise in the Gulf of Mexico. It enters through the Yucatan Channel from the Ca
Loop Of Stationary Wave: That portion of the oscillating area where the vertical movement is greatest.
Low Water (LW): The minimum height reached by a falling tide. The low water is due to the periodic tidal forces and
Low Water Datum (LWD): (1) The geopotential elevation (geopotential difference) for each of the Great Lakes and Lake St. Cl
Low Water Equinoctial Springs: Low water springs near the times of the equinoxes. Expressed in terms of the harmonic constants, it
Low Water Line: The intersection of the land with the water surface at an elevation of low water.
Lower High Water (LHW): The lowest of the high waters of any specified tidal day due to the declinational effects of the Moo
Lower Low Water (LLW): The lowest of the low waters (or single low water) of any specified tidal day due to the declination
Lower Low Water Datum (LLWD): An approximation of mean lower low water that has been adopted as a standard reference for a limited
Lowest Astronomical Tide: As defined by the International Hydrographic Organization, the lowest tide level that can be predict
Lunar Cycle: An ambiguous expression which has been applied to various cycles associated with the Moon's motion.
Lunar Day: The time of the rotation of the Earth with respect to the Moon, or the interval between two successi
Lunar Interval: The difference in time between the transit of the Moon over the meridian of Greenwich and a local me
Lunar Nodes: The points where the plane of the Moon's orbit intersects the ecliptic. The point where the Moon cro
Lunar Tide: That part of the tide on the Earth due solely to the Moon as distinguished from that part due to the
Lunar Time: Time based upon the rotation of the Earth relative to the Moon. See lunar day.
Lunicurrent Interval: The interval between the Moon's transit (upper or lower) over the local or Greenwich meridian and a
Lunisolar Tides: Harmonic tidal constituents K1, and K2, which are derived partly from the development of the lunar t
Lunitidal Interval: The interval between the Moon's transit (upper or lower) over the local or Greenwich meridian and th
M1: Smaller lunar elliptic diurnal constituent. This constituent, with J1, modulates the amplitude of th
M2: Principal lunar semidiurnal constituent. This constituent represents the rotation of the Earth with
M3: Lunar terdiurnal constituent. A shallow water compound constituent. See shallow water constituent. S
M4, M6, M8: Shallow water over tides of the principal lunar constituent. See shallow water constituent. Speed of
Maelstrom: Famous whirlpool off the coast of Nor-way in the Lofoten Islands between Moskenesoy and Mosken.
Magnetic Azimuth: Azimuth reckoned from the magnetic north or magnetic south. See magnetic direction.
Magnetic Direction: Direction as indicated by a magnetic compass after correction for deviation but without correction f
Marigram: A graphic record of the rise and fall of water level. The record is in the form of a curve in which
Marine Boundary: The mean lower low water line (MLLWL) when used as a boundary. Also, lines used as boundaries seawar
Mean Diurnal Tide Level (MDTL): A tidal datum. The arithmetic mean of mean higher high water and mean lower low water.
Mean High Water (MHW): A tidal datum. The aver-age of all the high water heights observed over the National Tidal Datum Epo
Mean High-Water Line (MHWL): The line on a chart or map which represents the intersection of the land with the water surface at t
Mean Higher High Water (MHHW): A tidal datum. The average of the higher high water height of each tidal day observed over the Natio
Mean Higher High Water Line (MHHWL): The line on a chart or map which represents the intersection of the land with the water surface at t
Mean Low Water (MLW): A tidal datum. The average of all the low water heights observed over the National Tidal Datum Epoch
Mean Low Water Line (MLWL): The line on a chart or map which represents the intersection of the land with the water surface at t
Mean Low Water Springs (MLWS): A tidal datum. Frequently abbreviated spring low water. The arithmetic mean of the low water heights
Mean Lower Low Water (MLLW): A tidal datum. The average of the lower low water height of each tidal day observed over the Nationa
Mean Lower Low Water Line (MLLWL): The line on a chart or map which represents the intersection of the land with the water surface at t
Mean Range Of Tide (Mn): The difference in height between mean high water and mean low water.
Mean Rise: The height of mean high water above the elevation of chart datum.
Mean Rise Interval (MRI): The average interval between the transit of the Moon and the middle of the period of the rise of the
Mean River Level: A tidal datum. The average height of the surface of a tidal river at any point for all stages of the
Mean Sea Level (MSL): A tidal datum. The arithmetic mean of hourly heights observed over the National Tidal Datum Epoch. S
Mean Sun: A fictitious sun which is assumed to move in the celestial equator at a uniform speed corresponding
Mean Tide Level (MTL): A tidal datum. The arithmetic mean of mean high water and mean low water. Same as half-tide level.
Mean Time: Time based upon the hour angle of the mean sun as distinguished from apparent time which is based up
Mean Water Level (MWL): A datum. The mean surface elevation as determined by averaging the heights of the water at equal int
Mean Water Level Line (MWLL): The line on a chart or map which represents the intersection of the land with the water surface at t
Meteorological Tides: Tidal constituents having their origin in the daily or seasonal variations in weather conditions whi
Metonic Cycle: A period of almost 19 years or 235 lunations. Devised by Meton, an Athenian astronomer who lived in
MF: Lunar fortnightly constituent. This constituent expresses the effect of departure from a sinusoidal
Midextreme Tide: An elevation midway between extreme high water and extreme low water occurring in any locality.
Mixed (Current): Type of tidal current characterized by a conspicuous diurnal inequality in the greater and lesser fl
Mixed (Tide): Type of tide characterized by a conspicuous diurnal inequality in the higher high and lower high wat
MM: Lunar monthly constituent. This constituent expresses the effect of irregularities in the Moon's rat
Modified-Range Ratio Method: A tidal datum computation method. Generally used for the East Coast, Gulf Coast, and Caribbean Islan
Monsoon Current (Southwest Monsoon Current): An Indian Ocean current setting in a generally eastward to southeastward direction off India and Cey
Month: The period of the revolution of the Moon around the Earth. The month is designated as siderial, trop
MSF: Lunisolar synodic fortnightly constituent. Speed = 2s – 2h = 1.0 15,895,8° per solar hour.
MU: Variational constituent. See lambda. Speed = 2T – 4s + 4h = 27.968,208,4° per solar hour.
Multiple Tide Staff: A succession of tide staffs on a sloping shore so placed that the vertical graduations on the severa
N: Rate of change (as of January 1, 1900) in mean longitude of the Moon's node. N = – 0.002,206,41°
N2: Larger lunar elliptic semi diurnal constituent. Speed = 2T– 3s + 2h + p = 28.439,729,5° per solar
National Geodeticvertical Datum: A fixed reference adopted as a standard geodetic datum for elevations determined by leveling. The da
National Spatial Reference System (NSRS): A consistent national coordinate system that defines latitude, longitude, height, scale, gravity, an
National Tidal Datum Convention: Effective November 28, 1980, the Convention: (1) establishes one uniform, continuous tidal datum sys
National Tidal Datum Epoch: The specific l9-year period adopted by the National Ocean Service as the official time segment over
National Water Level Observation Network (NWLON): The network of tide and water level stations operated by the National Ocean Service along the marine
Neap Tides Or Tidal Currents: Tides of decreased range or tidal currents of decreased speed occurring semimonthly as the result of
Next Generation Water Level Measurement System (NGWLMS): A fully integrated system encompassing new technology sensors and recording equipment, multiple data
Nodal Line: A line in an oscillating body of water along which there is a minimum or zero rise and fall of the t
Nodal Point: The zero tide point in an amphidromic region.
Node Cycle: Period of approximately 18.61 Julian years required for the regression of the Moon's nodes to comple
Node Factor (F): A factor depending upon the longitude of the Moon's node which, when applied to the mean coefficient
Nodical Month: Average period of the revolution of the Moon around the Earth with respect to the Moon's ascending n
Non-Harmonic Constants: Tidal constants such as lunitidal intervals, ranges, and inequalities which may be derived directly
Normal Tide: A nontechnical term synonymous with tide; i.e., the rise and fall of the ocean due to the gravitatio
North American Vertical Datum: A fixed reference for elevations determined by geodetic leveling. The datum was derived from a gener
North Atlantic Current: A North Atlantic Ocean current setting northeastward from southeast of the Grand Banks at about lati
North Cape Current: An Arctic Ocean current setting eastward off the north coast of Scandinavia in the Barents Sea.
North Equatorial Current: A current setting west-ward in the North Atlantic and North Pacific Oceans and in the Indian Ocean f
North Pacific Current: A North Pacific Ocean cur-rent setting eastward from about 160° east to somewhat beyond about 150°
Norwegian Current: A North Atlantic Ocean cur-rent setting northeastward off the coast of Norway.
Nu: Larger lunar evectional constituent. See lambda. Speed = 2T – 3s + 4h – p = 28.512,583,1° per s
O1: Lunar diurnal constituent. See K1. Speed = T – 2s + h = 13.943,035,6° per solar hour.
Obliquity Factor: A factor in an expression for a constituent tide (or tidal current) involving the angle of the incli
Obliquity Of The Ecliptic: The angle which the ecliptic makes with the plane of the Earth's Equator. Its value is approximately
Obliquity Of The Moons Orbit: The angle which the Moon's orbit makes with the plane of the Earth's Equator. Its value varies from
Oceanography: Oceanography is the science of all aspects of the oceans, in spite of its etymology. The term, ocean
OO1: Lunar diurnal, second-order, constituent. Speed = T + 2s + h = 16.139,101,7° per solar hour.
Ordinary: With respect to tides, the use of this nontechnical word has, for the most part, been determined to
Overfalls: Breaking waves caused by the meeting of currents or by waves moving against the current. See rip.
Overtide: A harmonic tidal (or tidal current) constituent with a speed that is an exact multiple of the speed
Oyashio: A current setting southwestward along the Siberian, Kamchatka, and Kuril Islands coasts in the Berin
P: Rate of change (as of January 1, 1900) in mean longitude of lunar perigee. P = 0.004,641,83° per so
P1: Rate of change (as of January 1, 1900) in mean longitude of solar perigee. P1 = 0.000,001,96° per s
Parallax: In tidal work, the term refers to horizontal parallax, which is the angle formed at the center of a
Parallax Inequality: The variation in the range of tide or in the speed of a tidal current due to changes in the distance
Parallax Reduction: A processing of observed high and low waters to obtain quantities depending upon changes in the dist
Parallel Plate Intake: Intake of a stilling or protective well with two parallel plates attached below. The plates are typi
Pelorus: An instrument formerly used on a vessel in connection with a current line and current pole to obtain
Perigean Tides Or Tidal Currents: Tides of increased range or tidal currents of increased speed occurring monthly as the result of the
Perigee: The point in the orbit of the Moon or a man-made satellite nearest to the Earth. The point in the or
Perihelion: The point in the orbit of the Earth (or other planet, etc.) Nearest to the Sun.
Period: Interval required for the completion of a recurring event, such as the revolution of a celestial bod
Permanent Current: A current that runs continuously and is independent of tides and other temporary causes. Permanent c
Peru Current: A South Pacific Ocean current setting northward along the west coast of South America. It has someti
Phase: (1) Any recurring aspect of a periodic phenomenon, such as new Moon, high water, flood strength, etc
Phase Inequality: Variations in the tides or tidal currents due to changes in the phase of the Moon. At the times of n
Phase Reduction: A processing of observed high and low waters to obtain quantities depending upon the phase of the Mo
Physical Oceanographic Real Time System (PORTS): A national system of current, water level, and other oceanographical and meteorological sensors tele
Pressure Gauge: A water level gauge that is operated by the change in pressure at the bottom of a body of water due
Pressure Sensor: A pressure transducer sensing de-vice for water level measurement. A relative transducer is vented t
Primary Control Tide Station: A tide station at which continuous observations have been made over a minimum of 19 years. Its purpo
Prime Meridian: The meridian of longitude which passes through the original site of the Royal Observatory in Greenwi
Priming Of Tide: The periodic acceleration in the time of occurrence of high and low waters due to changes in the rel
Progressive Wave: A wave that advances in distance along the sea surface or at some intermediate depth. Although the w
Protective Well: A vertical pipe with a relatively large opening (intake) in the bottom. It is used with the air acou
Pycnocline: A layer in which the density increases significantly (relative to the layers above and below) with d
QL: Larger lunar elliptic diurnal constituent. See M1. Speed = T – 3s + h + p = 13.398,660,9° per sol
Quadrature Of Moon: Position of the Moon when its longitude differs by 90° from the longitude of the Sun. The correspon
R2: Smaller solar elliptic constituent. This constituent, with T2, modulates the amplitude and frequency
Race: A very rapid current through a comparatively narrow channel.
Radiational Tide: Periodic variations in sea level primarily related to meteorological changes such as the semi daily
Range Of Tide: The difference in height between consecutive high and low waters. The mean range is the difference i
Real-Time: Pertains to a data collecting system that monitors an on-going process and disseminates measured val
Red Tide (Water): The term applied to toxic algal blooms caused by several genera of din flagellates (Gymnodinium and
Reduction Of Tides: A processing of observed tide or tidal current data to obtain mean values for tidal or tidal current
Reference Station: A tide or current station for which independent daily predictions are given in the 'Tide Tables' and
Relative Mean Sea Level Change: A local change in mean sea level relative to a network of bench marks established in the most stable
Residual Current: The observed current minus the astronomical tidal current.
Response Analysis: For any linear system, an input function Xi(t) and an output function X0(t) can be related according
Reversing Current: A tidal current which flows alternately in approximately opposite directions with a slack water at e
Reversing Falls: A name applied to falls which flow alternately in opposite directions in a narrow channel in the St.
RHO: Larger lunar evectional diurnal constituent. Speed = T – 3s + 3h – p = 13.471,514,5° per solar
Rip: Agitation of water caused by the meeting of currents or by a rapid current setting over an irregular
Rip Current: A narrow intense current setting sea-ward through the surf zone. It removes the excess water brought
River Current: The gravity-induced seaward flow of fresh water originating from the drainage basin of a river. In t
Rotary Current: A tidal current that flows continually with the direction of flow changing through all points of the
S: Rate of change (as of January 1, 1900) in mean longitude of Moon. S = 0.549,016,53° per solar hour.
S1: Solar diurnal constituent. Speed = T = 15.000,000,0° per solar hour.
S2: Principal solar semidiurnal constituent. This constituent represents the rotation of the Earth with
S4, S6: Shallow water over tides of the principal solar constituent. Speed of S4 = 2S2 = 4T = 60.000,000,0°
Sa: Solar annual constituent. This constituent, with Ssa, accounts for the non-uniform changes in the Su
Salinity (S): The total amount of solid material in grams contained in 1 kilogram of sea water when all the carbon
Sargasso Sea: The west central region of the sub-tropical gyre of the North Atlantic Ocean. It is bounded by the N
Saros: A period of 223 synodic months corresponding approximately to 19 eclipse years or 18.03 Julian years
Sea Level Datum (SLD): An obsolete term. See National Geodetic Vertical Datum of 1929 and mean sea level.
Secondary Control Tide Station: A tide station at which continuous observations have been made over a minimum period of 1 year but l
Seiche: A stationary wave usually caused by strong winds and/or changes in barometric pressure. It is found
Semidiurnal: Having a period or cycle of approximately one-half of a tidal day. The predominant type of tide thro
Sequence Of Current: The order of occurrence of the four tidal current strengths of a day, with special reference as to w
Sequence Of Tide: The order in which the four tides of a day occur, with special reference as to whether the higher hi
Set (Of Current): The direction towards which the current flows.
Shallow Water Constituent: A short-period harmonic term introduced into the formula of tidal (or tidal current) constituents to
Shallow Water Wave: A wave is classified as a shallow water wave whenever the ratio of the depth (the vertical distance
Shear: A quasi-horizontal layer moving at a different velocity relative to the layer directly below and/or
Shoreline (Coastline): The intersection of the land with the water surface. The shoreline shown on charts represents the li
Sidereal Day: The time of the rotation of the Earth with respect to the vernal equinox. It equals approximately 0.
Sidereal Month: Average period of the revolution of the Moon around the Earth with respect to a fixed star, equal to
Sidereal Time: This is usually defined by astronomers as the hour angle of the vernal equinox. The sidereal day is
Sidereal Year: Average period of the revolution of the Earth around the Sun with respect to a fixed star. Its lengt
Sigma-T (Ft): An expression of density as a function of temperature and salinity (at atmospheric pressure) in a co
Sigma-Zero (Fo): An expression of density as a function of salinity (at atmospheric pressure and 0°C) in a convenien
Slack Water (Slack): The state of a tidal current when its speed is near zero, especially the moment when a reversing cur
Small Diurnal Range (SL): Difference in height between mean lower high water and mean higher low water.
Small Tropic Range (SC): Difference in height between tropic lower high water and tropic higher low water.
Solar Day: The period of the rotation of the Earth with respect to the Sun. The mean solar day is the time of t
Solar Tide: (1) The part of the tide that is due to the tide-producing force of the Sun. (2) The observed tide i
Solar Time: Time measured by the hour angle of the Sun. It is called apparent time when referred to the actual S
Solitary Wave: A wave of translation consisting of a single crest rising above the undisturbed water level without
Solstices: The two points in the ecliptic where the Sun reaches its maximum and minimum declinations; also the
Solstitial Tides: Tides occurring near the times of the solstices. The tropic range may be expected to be especially l
Somali (East Africa Coast) Current: An Indian Ocean current setting southwestward along the coast of Somalia. The current reverses and s
South Equatorial Current: A current setting west-ward along and south of the Equator in the Atlantic and Pacific Oceans, and s
Species Of Constituent: A classification depending upon the period of a constituent. The principal species are semidiurnal,
Specific Volume: Volume per unit mass. The reciprocal of density (specific gravity). The specific volume of sea water
Specific Volume Anomaly, Or Steric Anomaly: The excess in specific volume over the standard specific volume at 35 ‰, 0°C, and the given press
Speed (Of Constituent): The rate of change in the phase of a constituent, usually expressed in degrees per hour. The speed i
Speed (Of Current): The magnitude of velocity. Rate at which the current flows. Usually expressed in knots or centimeter
Spitsbergen Atlantic Current: A current setting northwestward off the southwest coast of Spitsbergen in the Greenland Sea.
Spring Tides Or Tidal Currents: Tides of increased range or tidal currents of increased speed occurring semimonthly as the result of
Ssa: Solar semiannual constituent. See Sa. Speed = 2h = 0.082,137,3° per solar hour.
Stand Of Tide: Sometimes called a platform tide. An interval at high or low water when there is no sensible change
Standard Method: A tidal datum computation method. Generally used for the West Coast and Pacific Island stations. Val
Standard Time: A kind of time based upon the transit of the Sun over a certain specified meridian, called the time
Standing Wave: A wave that oscillates without progressing. One-half of such a wave may be illustrated by the oscill
Stationary Wave Theory: An assumption that the basic tidal movement in the open ocean consists of a system of stationary wav
Stencils: Perforated sheets formerly used with the tabulated hourly heights of the tide or speeds of the tidal
Stilling Well: A vertical pipe with a relatively small opening (intake) in the bottom. It is used in a gauge instal
Storm Surge: The local change in the elevation of the ocean along a shore due to a storm. The storm surge is meas
Storm Tide: As used by the National Weather Service, NOAA, the sum of the storm surge and astronomic tide. See s
Stray Line: Ungraduated portion of line connected with the current pole formerly used in taking current observat
Strength Of Current: Phase of tidal current in which the speed is a maximum; also the speed at this time. Beginning with
Submerged Lands: Lands covered by water at any stage of the tide. See tidelands.
Subordinate Current Station: (1) A current station from which a relatively short series of observations is reduced by comparison
Subordinate Tide Station: (1) A tide station from which a relatively short series of observations is reduced by comparison wit
Summer Time: British name for daylight saving time.
Synodical Month: The average period of the revolution of the Moon around the Earth with respect to the Sun, or the av
Syzygy: With respect to tides, whenever the Moon is lined up with the Earth and Sun in a straight Sun-Moon-E
T: Rate of change of hour angle of mean Sun at place of observation. T = 15° per mean solar hour.
T2: Larger solar elliptic constituent. See R2. Speed = 2T – h + p1 = 29.958,933,3° per solar hour.
Telemetry: The capability of transmitting or retrieving data over long distance communication links, such as sa
Terdiurnal: Having three periods in a constituent day. The symbol of a terdiurnal constituent is the subscript 3
Tertiary Tide Station: A tide station at which continuous observations have been made over a minimum period of 30 days but
Thermocline: A layer in which the temperature de-creases significantly (relative to the layers above and below) w
Thermosteric Anomaly: The specific volume anomaly (steric anomaly) that would be attained if the water were changed isothe
Tidal Bench Mark State Index Map: A state map which indicates the locations for which tidal datums and tidal bench mark descriptions a
Tidal Benchmark Description: A published, concise description of the location, stamped number or designation, date established, a
Tidal Bore: A tidal wave that propagates up a relatively shallow and sloping estuary or river with a steep wave
Tidal Characteristics: Principally, those features relating to the time, range, and type of tide.
Tidal Constants: Tidal relations that remain practically constant for any particular locality. Tidal constants are cl
Tidal Current: A horizontal movement of the water caused by gravitational interactions between the Sun, Moon, and E
Tidal Current Chart Diagrams: A series of monthly diagrams to be used with the Tidal Current Charts. Each diagram contains lines t
Tidal Current Charts: Charts on which tidal current data are depicted. Tidal Current Charts for a number of important wate
Tidal Current Tables: Tables which give daily predictions of the times and velocities of the tidal currents. These predict
Tidal Difference: Difference in time or height between a high or low water at a subordinate station and a reference st
Tidal Wave: A shallow water wave caused by the gravitational interactions between the Sun, Moon, and Earth. Esse
Tidal Zoning: The practice of dividing hydrographic survey area into discrete zones or sections, each one possessi
Tide: The periodic rise and fall of a body of water resulting from gravitational interactions between Sun,
Tide Curve: A graphic representation of the rise and fall of the tide in which time is usually represented by th
Tide Gauge: An instrument for measuring the rise and fall of the tide (water level). See ADR gauge, automatic ti
Tide Predicting Machine: A mechanical analog machine especially designed to handle the great quantity of constituent summatio
Tide Reducers: Height corrections for reducing soundings to chart datum (MLLW). A tide reducer represents the heigh
Tide Staff: A water level gauge consisting of a vertical graduated staff from which the height of the water leve
Tide Station: The geographic location at which tidal observations are conducted. Also, the facilities used to make
Tide Tables: Tables which give daily predictions of the times and heights of high and low waters. These predictio
Tide-Producing Force: That part of the gravitational attraction of the Moon and Sun which is effective in producing the ti
Tide-Producing Potential: Tendency for particles on the Earth to change their positions as a result of the gravitational inter
Tidelands: The zone between the mean high-water and mean low water lines. It is identical with intertidal zone
Tidewater: Water activated by the tide generating forces and/or water affected by the resulting tide, especiall
Tideway: A channel through which a tidal current flows.
Time: Time is measured by the rotation of the Earth with respect to some point in the celestial sphere and
Time Meridian: A meridian used as a reference for time.
Total Current: The combination of the tidal and non-tidal current. The United States equivalent of the British flow
Tractive Force: The horizontal component of a tide-producing force vector (directed parallel with level surfaces at
Transit: The passage of a celestial body over a specified meridian. The passage is designated as upper transi
Tropic Currents: Tidal currents occurring semi-monthly when the effect of the Moon's maximum declination is greatest.
Tropic Inequalities: Tropic high water inequality (HWQ) is the average difference between the two high waters of the day
Tropic Intervals: Tropic higher high water interval (tchhwi) is the lunitidal interval pertaining to the higher high w
Tropic Ranges: The great tropic range (Gc),or tropic range, is the difference in height between tropic higher high
Tropic Speed: The greater flood or greater ebb speed at the time of tropic currents.
Tropic Tides: Tides occurring semi-monthly when the effect of the Moon's maximum declination is greatest. At these
Tropical Month: The average period of the revolution of the Moon around the Earth with respect to the vernal equinox
Tropical Year: The average period of the revolution of the Earth around the Sun with respect to the vernal equinox.
Trough: The lowest point in a propagating wave. See low water and tidal wave.
True Direction: Direction relative to true north (0°) which is the direction of the north geographic pole. See comp
Tsunami: A shallow water progressive wave, potentially catastrophic, caused by an underwater earthquake or vo
Tsushima Current: A North Pacific Ocean current setting northeastward in the East China Sea (in summer) and Sea of Jap
Type Of Tide: A classification based on characteristic forms of a tide curve. Qualitatively, when the two high wat
Universal Time (UT): Same as Greenwich meantime (GMT). See time, kinds.
Uplands: Land above the mean high water line (shoreline) and subject to private ownership, as distinguished f
Upwelling: An upward flow of subsurface water due to such causes as surface divergence, offshore wind, and wind
Vanishing Tide: In a predominantly mixed tide with a very large diurnal inequality, the lower high water and higher
Variation (Of Compass): Difference between true north as determined by the Earth's axis of rotation and magnetic north as de
Variational Inequality: An inequality in the Moon's motion due mainly to the tangential component of the Sun's attraction.
West Australian Current: An Indian Ocean current setting northward along the west coast of Australia.
West Greenland Current: A North Atlantic Ocean current setting northward along the west coast of Greenland.
Wind Drift: An ocean current in which only the Coriolis and frictional forces are significant. The wind drift em
ZO: Symbol recommended by the International Hydrographic Organization to represent the elevation of mean