Tide and Current Glossary

Silver Spring, MD

January 2000

U.S. DEPARTMENT OF COMMERCE

National Oceanic and Atmospheric Administration

National Ocean Service

Center for Operational Oceanographic Products and Services
Tide and Current Glossary

U.S. DEPARTMENT OF COMMERCE

Norman Mineta, Secretary

National Oceanic and Atmospheric Administration

D. James Baker, Administrator

National Ocean Service

Margaret Davidson, Assistant Administrator

Center for Operational Oceanographic Products and Services

David M. Kennedy, Acting Director
ii

Preface to 1999 Edition

The publication is a revision of the 1989 edition. This edition has been a group effort by

Steacy D. Hicks, Richard L. Sillcox, C. Reid Nichols, Brenda Via, and Evette C. McCray. It was

subsequently revised by Chris Zervas. Final formatting, layout, and printing has been under the

authority of Brenda Via.

 The author wishes to thank the following for their contributions: C. Reid Nichols (numerous

entries), Richard L. Sillcox, Jack E. Fancher, Dr. Robert G. Williams, Thomas J. Kendrick, Douglas

M. Martin, David B. Zilkoski, Richard Edwing, Michael Szabados, Steven Lyles, James Hubbard,

Michael Gibson, Steven Gill, William Stoney, Dr. Ledolph Baer, and Dr. Bruce B. Parker. Special

thanks is given to Dr. Kurt Hess for his numerous technical corrections and suggestions.
iii

Printing History of Tide and Current Glossary

Special Publication No. 228, Coast and Geodetic Survey, by Paul Schureman, 1941.

Special Publication No. 228, Coast and Geodetic Survey, by Paul Schureman revised by

E. C. McKay and F. J. Haight, 1949.

Special Publication No. 228, Coast and Geodetic Survey, by Paul Schureman, reprinted with

corrections, 1963.

National Ocean Survey, by Paul Schureman, revised by Steacy D. Hicks, 1975.

National Ocean Service, by Steacy D. Hicks, 1984.

National Ocean Service, by Steacy D. Hicks, 1989.

National Ocean Service, by Steacy D. Hicks, 1999.
iv

For further information on tides, sea level, tidal datums, record certifications,

and related publications, contact:

NOAA, National Ocean Service

CO-OPS, Products and Services N/OPS3

Attn: User Services

1305 East-West Highway 

Silver Spring, MD 20190-3281

Phone: (301)713-2877 Ext. 176

Fax: (301)713-4437

For further information on Great Lakes water levels, datums, and

 related publications, contact:

NOAA, National Ocean Service

CO-OPS, Requirements and Development N/OPS1

Attn: Great Lakes

1305 East-West Highway

Silver Spring, MD 20910-3281

Phone: (301)713-2902 Ext. 184

Fax: (301)713-4435

For further information on currents, tide and tidal current predictions,

 and related publications, contact:

NOAA, National Ocean Service

CO-OPS, Products and Services N/OPS3

Attn: Tidal Predictions

1305 East-West Highway

Silver Spring, MD 20910-3281

Phone: (301)713-2815 Ext. 119

Fax: (301)713-4500
1

Tide and Current Glossary

A

absolute mean sea level change—An eustatic change

in mean sea level relative to a conventional terrestrial

coordinate system with the origin at the center of mass of

the Earth. 

accepted values—Tidal datums and Greenwich high

and low water intervals obtained through primary determination or comparison of simultaneous observations

with a control tide station in order to derive the equivalent

value that would be obtained with a 19-year series.

acoustic Doppler current profiler (ADCP)—A current measuring instrument employing the transmission of

high frequency acoustic signals in the water. The current is

determined by a Doppler shift in the backscatter echo from

plankton, suspended sediment, and bubbles, all assumed to

be moving with the mean speed of the water. Time gating

circuitry is employed which uses differences in acoustic

travel time to divide the water column into range intervals,

called bins. The bin determinations allow development of a

profile of current speed and direction over most of the water

column. The ADCP can be deployed from a moving vessel,

tow, buoy, or bottom platform. In the latter configuration,

it is nonobtrusive in the water column and thus can be

deployed in shipping channels. 

ADR gauge—Analog to Digital Recording water level

gauge. A float or pressure-actuated water level gauge that

records heights at regular time intervals in digital format.

age of diurnal inequality—The time interval between

the maximum semimonthly north or south declination of the

Moon and the maximum effect of declination upon range of

tide or speed of the tidal current. The age may be computed

from the harmonic constants by the formula:

1 1 age of diurnal inequality = 0.911(5 ° – O °) hours.

age of Moon—The time elapsed since the preceding

new Moon

age of parallax inequality—The time interval between

perigee of the Moon and the maximum effect of parallax

upon range of tide or speed of the tidal current. This age

may be computed from the harmonic constants by the

formula: 

2 2 age of parallax inequality = 1.837(M ° – N °) hours. 

age of phase inequality—The time interval between

new or full Moon and the maximum effect of these phases

upon range of tide or speed of the tidal current. This age

may be computed from the harmonic constants by the

formula: 

2 2 age of phase inequality = 0.984(S ° – M °) hours.

age of tide—Same as age of phase inequality.

agger—Same as double 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 fundamental measurement is the time it

takes for the acoustic signal to travel from a transmitter to

the water surface and then back to the receiver. The distance

from a reference point to the water surface is derived from

the travel time. A calibration point is set at a fixed distance

from the acoustic transducer and is used to correct the measured distance using the calibrated sound velocity in the

tube. Air temperature sensors are located in the protective

well for the purpose of verifying uniformity of temperature

for measurements taken by the air acoustic ranging sensor.

Alaska Current—A North Pacific Ocean current

setting counterclockwise along the coasts of Canada and

Alaska in the Gulf of Alaska.

Alaskan Stream—A North Pacific Ocean current

setting westward along the south side of the Aleutian

Islands. It is an extension of the Alaska Current.

amphidromic point—A point of zero amplitude of the

observed or a constituent tide.

amphidromic region—An area surrounding an amphidromic point from which the radiating cotidal lines

progress through all hours of the tidal cycle.

amplitude (H)—One-half the range of a constituent

tide. By analogy, it may be applied also to the maximum

speed of a constituent current.

analog—A continuous measurement or a continuous

graphic display of data. See ADR gauge and marigram.

analysis, harmonic—See harmonic analysis.

analyzer, harmonic—See harmonic analyzer.

angular velocity of the Earth's rotation (S)—Time

rate of change of angular displacement relative to the fixed

stars. It is equal to 0.729,211 x 10 radian/second. -4

annual inequality—Seasonal variation in water level

or current, 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. The

anomalistic month is the average period of the revolution of

the Moon around the Earth with respect to lunar perigee,

and is approximately 27.554,550 days in length. The

anomalistic year is the average period of the revolution of

the Earth around the Sun with respect to perihelion, and is

approximately 365.259,6 days in length.

anomaly—As applied to astronomy, the anomaly is the

angle made at any time by the radius vector of a planet or

moon with its line of apsides, the angle being reckoned from

perihelion or perigee in the direction of the body's motion.

It is called the true anomaly when referred to the actual

position of the body, and mean anomaly when referred to a

fictitious body moving with a uniform angular velocity
2

equal to the average velocity of the real body and passing

perihelion or perigee at the same time.

Antarctic Circumpolar Current—The largest

permanent current in the world, setting eastward around the

Antarctic Continent south of Cape Horn, Cape of Good

Hope, Tasmania, and New Zealand. Through Drake

Passage, it transports approximately 200 x 10 m /s. Same 6 3

as West Wind Drift.

anticyclonic ring—A meander breaking off from the

main oceanic current and spinning in a clockwise direction

in the northern hemisphere (counter-clockwise in southern).

Antilles Current—A North Atlantic Ocean current

setting northwestward along the northeast coasts of the

Bahama Islands.

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 being in apogee. The apogean range

(An) of the tide is the average range occurring at the time of

apogean tides and is most conveniently computed from the

harmonic constants. It is smaller than the mean range, where

the type of tide is either semidiurnal or mixed, and is of no

practical significance where the type of tide is

predominantly diurnal.

apogee—The point in the orbit of the Moon or a

man-made satellite farthest from the Earth. The point in the

orbit of a satellite farthest from its companion body.

apparent secular trend—The nonperiodic tendency

of sea level to rise, fall, or remain stationary with time.

Technically, it is frequently defined as the slope of a leastsquares line of regression through a relatively long series of

yearly mean sea-level values. The word "apparent" is used

since it is often not possible to know whether a trend is truly

nonperiodic or merely a segment of a very long oscillation

(relative to the length of the series).

apparent time—Time based upon the true position of

the Sun as distinguished from mean time, which is measured

by a fictitious Sun moving at a uniform rate. Apparent time

is that shown by the sundial, and its noon is the time when

the Sun crosses the meridian. The difference between

apparent time and mean time is known as the equation of

time. Although quite common many years ago, apparent

time is seldom used now.

apsides—The points in the orbit of a planet or moon

which are the nearest and farthest from the center of

attraction. In the Earth's orbit these are called perihelion and

aphelion, and in the Moon's orbit, perigee and apogee. The

line passing through the apsides of an orbit is called the line

of apsides.

argument—See equilibrium argument.

astres fictifs—Fictitious celestial bodies which are

assumed to move in the celestial equator at uniform rates

corresponding to the speeds of the several harmonic

constituents of the tide producing force. Each astre fictif

crosses the meridian at a time corresponding to the

maximum of the constituent that it represents.

astronomical day—See astronomical time.

astronomical tide—Same as tide.

astronomical time—Time formerly used in

astronomical calculations in which the day began at noon

rather than midnight. The astronomical day commenced at

noon of the civil day of the same date. The hours of the day

were numbered consecutively from zero (noon) to 23 (11

a.m. of the following morning). Up to the close of the year

1924, astronomical time was in general use in nautical

almanacs. Beginning with the year 1925, the American

Ephemeris and Nautical Almanac and similar publications

of other countries abandoned the old astronomical time and

adopted Greenwich civil (mean) time for the data given in

their tables.

augmenting factor—A factor, used in connection with

the harmonic analysis of tides or tidal currents by stencils,

to allow for the fact that the tabulated hourly heights or

speeds used in the summation for any constituent, other than

S, do not in general occur on the exact constituent hours to

which they are assigned, but may differ from the same by as

much as a half hour.

automatic tide (water level) gauge—An instrument

that automatically registers the rise and fall of the water

level. In some instruments, the registration is accomplished

by recording the heights at regular time intervals in digital

format; in others, by a continuous graph of height against

time.

azimuth—Azimuth of a body is the arc of the horizon

intercepted between the north or south point and the foot of

the vertical circle passing through the body. It is reckoned

in degrees from either the north or south point clockwise

entirely around the horizon. Azimuth of a current is the

direction toward which it is flowing, and is usually reckoned

from the north point.

B

baroclinic—The condition and type of motion when

isobaric surfaces of a fluid are not parallel with isopycnal

surfaces.

barotropic—The condition and type of motion when

isobaric surfaces of a fluid are parallel with isopycnal

surfaces.

barycenter—The common center of mass of the SunEarth System or the Moon-Earth System. The distance from

the center of the Sun to the Sun-Earth barycenter is about

280 miles. The distance from the center of the Earth to the

Moon-Earth barycenter is about 2,895 miles. 

bench mark (BM)—A fixed physical object or mark

used as reference for a horizontal or vertical datum. A tidal

bench mark is one near a tide station to which the tide staff

and tidal datums are referred. A primary bench mark is the

principal mark of a group of tidal bench marks to which the

tide staff and tidal datums are referred. The standard tidal
3

bench mark of the National Ocean Service is a brass,

bronze, or aluminum alloy disk 3-1⁄2 inches in diameter

containing the inscription NATIONAL OCEAN SERVICE

together with other individual identifying information. A

geodetic bench mark identifies a surveyed point in the

National Spatial Reference System. Bench mark disks of

either type may, on occasion, serve simultaneously to

reference both tidal and geodetic datums. Numerous bench

marks of predecessor organizations to NOS, or parts of

other organizations absorbed into NOS, still bear the

inscriptions: U.S. COAST & GEODETIC SURVEY,

NATIONAL GEODETIC SURVEY, NATIONAL OCEAN

SURVEY, U.S. LAKE SURVEY, CORPS OF

ENGINEERS, and U.S. ENGINEER OFFICE.

Benguela Current—A South Atlantic Ocean current

setting northward along the southwest coast of Africa.

bore—Same as tidal bore.

Brazil Current—A South Atlantic Ocean current

setting southwestward along the central coast of South

America.

bubbler tide (water level) gauge—Same as gas

purged pressure gauge.

C

California Current—A North Pacific Ocean current

setting southeastward along the west coast of the United

States and Baja California.

Callippic cycle—A period of four Metonic cycles

equal to 76 Julian years, or 27,759 days. Devised by

Callippus, a Greek astronomer, about 350 B.C., as a

suggested improvement on the Metonic cycle for a period

in which new and full Moon would recur on the same day of

the year. Taking the length of the synodical month as

29.530,588 days, there are 940 lunations in the Callippic

cycle, with about 0.25 day remaining.

Canary Current—A North Atlantic Ocean current

setting southward off the west coast of Portugal and along

the northwest coast of Africa.

celestial sphere—An imaginary sphere of infinite

radius concentric with the Earth on which all celestial

bodies except the Earth are imagined to be projected.

centibar—The unit of pressure equal to 1 metric ton

(1000 kilograms) per meter per second per second. See

decibar.

chart datum—The datum to which soundings on a

chart are referred. It is usually taken to correspond to a

low-water elevation, and its depression below mean sea

level is represented by the symbol Z;. Since 1980, chart

datum has been implemented to mean lower low water for

all marine waters of the United States, its territories,

Commonwealth of Puerto Rico, and Trust Territory of the

Pacific Islands. See datum and National Tidal Datum

Convention of 1980.

Charybdis—Same as Galofaro.

chlorinity (Cl)—The total amount in grams of

chlorine, bromine, and iodine contained in one kilogram of

seawater, assuming the bromine and iodine to be replaced

by chlorine. The number giving the chlorinity in grams per

kilogram of a seawater sample is identical with the number

giving the mass in grams of atomic weight silver just

necessary to precipitate the halogens in 0.328,523,3

kilogram of the seawater sample. 

S(‰) = 1.806,55 x Cl(‰) 

where S(‰) is the salinity in parts per thousand. See

salinity.

civil day—A mean solar day commencing at midnight.

civil time—Time in which the day begins at midnight

as distinguished from the former astronomical time in which

the day began at noon.

classification—See type of tide.

Coast and Geodetic Survey—A former name of the

National Ocean Service. The organization was known as:

Survey of the Coast from its founding in 1807 to 1836,

Coast Survey from 1836 to 1878, Coast and Geodetic

Survey from 1878 to 1970, and National Ocean Survey

from 1970 to 1982. In 1982 it was named National Ocean

Service. From 1965 to 1970, the Coast and Geodetic Survey

was a component of the Environmental Science Services

Administration (ESSA). The National Ocean Survey was a

component of the National Oceanic and Atmospheric

Administration (NOAA). NOAA became the successor to

ESSA in 1970. The National Ocean Service is a component

of NOAA, U.S. Department of Commerce.

coast line—The low water datum line for purposes of

the Submerged Lands Act (Public Law 31). See shoreline.

coastal boundary—The mean high water line

(MHWL) or mean higher high water line (MHHWL) when

tidal lines are used as the coastal boundary. Also, lines used

as boundaries inland of and measured from (or points

thereon) the MHWL or MHHWL. See marine boundary.

coastal zone (legal definition for coastal zone management)—The term coastal zone means the coastal waters

(including the lands therein and thereunder) and the

adjacent shorelands (including the waters therein and

thereunder), strongly influenced by each and in proximity to

the shorelines of the several coastal states, and includes

islands, transitional and intertidal areas, salt marshes,

wetlands, and beaches. The zone extends, in Great Lakes

waters, to the international boundary between the United

States and Canada and in other areas seaward to the outer

limit of the United States territorial sea. The zone extends

inland from the shorelines only to the extent necessary to

control shorelands, the uses of which have a direct and

significant impact on the coastal waters. Excluded from the

coastal zone are lands the use of which is by law subject

solely to the discretion of or which is held in trust by the

Federal Government, its officers, or agents.

coastline—Same as shoreline. See coast line.

cocurrent line—A line on a map or chart passing

through places having the same current hour. 
4

comparison of simultaneous observations—A reduction process in which a short series of tide or tidal current

observations at any place is compared with simultaneous

observations at a control station where tidal or tidal current

constants have previously been determined from a long

series of observations. The observations are typically high

and low tides and monthly means. For tides, it is usually

used to adjust constants from a subordinate station to the

equivalent value that would be obtained from a l9-year

series. See first reduction, standard method, modified-range

ratio method, and direct method.

compass direction—Direction as indicated by compass

without any correction for compass error. The direction

indicated by a compass may differ by a considerable amount

from true or magnetic direction.

compass error—The angular difference between a

compass direction and the corresponding true direction. The

compass error combines the effects of deviation and

variation. 

component—(1) Same as constituent. (2) That part of

a tidal current velocity which, by resolution into orthogonal

vectors, is found to flow in a specified direction.

compound tide—A harmonic tidal (or tidal current)

constituent with a speed equal to the sum or difference of

the speeds of two or more elementary constituents. The

presence of compound tides is usually attributed to shallow

water conditions.

constants, current—See current constants.

constants, harmonic—See harmonic constants.

constants, tidal—See tidal constants.

constituent—One of the harmonic elements in a

mathematical expression for the tide-producing force and in

corresponding formulas for the tide or tidal current. Each

constituent represents a periodic change or variation in the

relative positions of the Earth, Moon, and Sun. A single

constituent is usually written in the form y = A cos (at + "),

in which y is a function of time as expressed by the symbol

t and is reckoned from a specific origin. The coefficient A

is called the amplitude of the constituent and is a measure

of its relative importance. The angle (at + ") changes

uniformly and its value at any time is called the phase of the

constituent. The speed of the constituent is the rate of

change in its phase and is represented by the symbol a in the

formula. The quantity " is the phase of the constituent at the

initial instant from which the time is reckoned. The period

of the constituent is the time required for the phase to

change through 360° and is the cycle of the astronomical

condition represented by the constituent.

constituent day—The time of the rotation of the Earth

with respect to a fictitious celestial body representing one of

the periodic elements in the tidal forces. It approximates in

length the lunar or solar day and corresponds to the period

of a diurnal constituent or twice the period of a semidiurnal

constituent. The term is not applicable to the long-period

constituents.

constituent hour—One twenty-fourth part of a constituent day.

control current station—A current station at which

continuous velocity observations have been made over a

minimum period of 29 days. Its purpose is to provide data

for computing accepted values of the harmonic and

nonharmonic constants essential to tidal current predictions

and circulatory studies. The data series from this station

serves as the control for the reduction of relatively short

series from subordinate current stations through the method

of comparison of simultaneous observations. See current

station and subordinate current station (1).

control station—See primary control tide station,

secondary control tide station, and control current station.

corange line—A line passing through places of equal

tidal range.

Coriolis force—A fictional force in the hydrodynamic

equations of motion that takes into account the effect of the

Earth's rotation on moving objects (including air and water)

when viewed with reference to a coordinate system attached

to the rotating Earth. The horizontal component is directed

90° to the right (when looking in the direction of motion) in

the Northern Hemisphere and 90° to the left in the Southern.

The horizontal component is zero at the Equator; also, when

the object is at rest relative to the Earth. The Coriolis

acceleration = 2vS sin ø: where v is the speed of the object,

S is the angular velocity of the Earth, and ø is the latitude.

Named for Gaspard Gustave de Coriolis who published his

formulation in 1835.

corrected current—A relatively short series of current

observations from a subordinate station to which a factor is

applied to adjust the current to a more representative value

based on a relatively long series from a nearby control

station. See current and total current. 

cotidal hour—The average interval between the

Moon's transit over the meridian of Greenwich and the time

of the following high water at any place. This interval may

be expressed either in solar or lunar time. When expressed

in solar time, it is the same as the Greenwich high water

interval. When expressed in lunar time, it is equal to the

Greenwich high water interval multiplied by the factor

0.966. 

cotidal line—A line on a chart or map passing through

places having the same tidal hour.

countercurrent—A current usually setting in a direction opposite to that of a main current. See Equatorial

Countercurrent.

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 nontidal. Tidal

currents are caused by gravitational interactions between the

Sun, Moon, and Earth and are part of the same general

movement of the sea that is manifested in the vertical rise

and fall, called tide. Tidal currents are periodic with a net
5

velocity of zero over the particular tidal cycle. See tidal

wave. Nontidal currents include the permanent currents in

the general circulatory systems of the sea as well as

temporary currents arising from more pronounced

meteorological variability. Current, however, is also the

British equivalent of our nontidal current. See total current.

current constants—Tidal current relations that remain

practically constant for any particular locality. Current

constants are classified as harmonic and nonharmonic. The

harmonic constants consist of the amplitudes and epochs of

the harmonic constituents, and the nonharmonic constants

include the velocities and intervals derived directly from the

current observations. 

current curve—A graphic representation of the flow

of the current. In the reversing type of tidal current, the

curve is referred to rectangular coordinates with time

represented by the abscissa and the speed of the current by

the ordinate, the flood speeds being considered as positive

and the ebb speeds as negative. In general, the current curve

for a reversing tidal current approximates a cosine curve.

current diagram—A graphic table published in the

Tidal Current Tables showing the speeds of the flood and

ebb currents and the times of slacks and strengths over a

considerable stretch of the channel of a tidal waterway, the

times being referred to tide or tidal current phases at some

reference station. 

current difference—Difference between the time of

slack water (or minimum current) or strength of current in

any locality and the time of the corresponding phase of the

tidal current at a reference station for which predictions are

given in the Tidal Current Tables.

current direction—Same as set.

current ellipse—A graphic representation of a rotary

current in which the velocity of the current at different hours

of the tidal cycle is represented by radius vectors and

vectoral angles. A line joining the extremities of the radius

vectors will form a curve roughly approximating an ellipse.

The cycle is completed in one-half tidal day or in a whole

tidal day, according to whether the tidal current is of the

semidiurnal or the diurnal type. A current of the mixed type

will give a curve of two unequal loops each tidal day.

current hour—The mean interval between the transit

of the Moon over the meridian of Greenwich and the time

of strength of flood, modified by the times of slack water

(or minimum current) and strength of ebb. In computing the

mean current hour, an average is obtained of the intervals

for the following phases: flood strength, slack (or minimum)

before flood increased by 3.10 hours (one-fourth of tidal

cycle), slack (or minimum) after flood decreased by 3.10

hours, and ebb strength increased or decreased by 6.21

hours (one-half of tidal cycle). Before taking the average,

the four phases are made comparable by the addition or

rejection of such multiples of 12.42 hours as may be

necessary. The current hour is usually expressed in solar

time, but if lunar time is desired, the solar hour should be

multiplied by the factor 0.966.

current line—A graduated line attached to a current

pole formerly used in measuring the velocity of the current.

The line was marked in such a manner that the speed of the

current, expressed in knots and tenths, was indicated

directly by the length of line carried out by the current pole

in a specified interval of time. When marked for a

60-second run, the principal divisions for whole knots were

spaced at 101.33 feet and the subdivisions for tenths of

knots were spaced at 10.13 feet. The current line was also

known as a log line.

current meter—An instrument for measuring the

speed and direction or just the speed of a current. The

measurements are Eulerian when the meter is fixed or

moored at a specific location. Current meters can be

mechanical, electric, electromagnetic, acoustic, or any

combination thereof. 

current pole—A pole used in observing the velocity of

the current. The pole formerly used by the Coast and

Geodetic Survey was about 3 inches in diameter and 15 feet

long, and was weighted at one end to float upright with the

top about 1 foot out of water. Shorter poles were used when

necessary for shallow water. In use, the pole was attached

to the current line but separated from the graduated portion

by an ungraded section of approximately 100 feet, known as

the stray line. As the pole was carried out from an observing

vessel by the current, the amount of line passing from the

vessel during a specific time interval indicated the speed of

the current. The set was obtained from a relative bearing

from the vessel to the pole. The bearing was then related to

the ship's compass and converted to true. See pelorus.

current station—The geographic location at which

current observations are conducted. Also, the facilities used

to make current observations. These may include a buoy,

ground tackle, current meters, recording mechanism, and

radio transmitter. See control current station and

subordinate current station (1).

cyclonic ring—A meander breaking off from the main

oceanic current and spinning in a counter-clockwise

direction in the northern hemisphere (clockwise in

southern).

D

data collection platform (DCP)—A microprocessorbased system that collects data from sensors, processes the

data, stores the data in random access memory (RAM), and

provides communication links for the retrieval or

transmission of the data.

datum (vertical)—For marine applications, a base

elevation used as a reference from which to reckon heights

or depths. It is called a tidal datum when defined in terms of

a certain phase of the tide. Tidal datums are local datums

and should not be extended into areas which have differing

hydrographic characteristics without substantiating

measurements. In order that they may be recovered when

needed, such datums are referenced to fixed points known

as bench marks. See chart datum.
6

datum of tabulation—A permanent base elevation at

a tide station to which all water level measurements are

referred. The datum is unique to each station and is

established at a lower elevation than the water is ever

expected to reach. It is referenced to the primary bench

mark at the station and is held constant regardless of

changes to the water level gauge or tide staff. The datum of

tabulation is most often at the zero of the first tide staff

installed.

Davidson Current—A North Pacific Ocean countercurrent setting northward between the California Current

and the coasts of California, Oregon, and Washington

during the winter months.

day—The period of rotation of the Earth. There are

several kinds of days depending on whether the Sun, Moon,

or other object or location is used as the reference for the

rotation. See constituent day, lunar day, sidereal day, and

solar day.

daylight saving time—A time used during the summer

months, in some localities, in which clocks are advanced 1

hour from the usual standard time.

decibar—The practical unit for pressure in the ocean,

equal to 10 centibars, and is the approximate pressure

produced by each meter of overlying water

declination—Angular distance north or south of the

celestial equator, taken as positive when north of the

equator and negative when south. The Sun passes through

its declinational cycle once a year, reaching its maximum

north declination of approximately 23-1⁄2° about June 21 and

its maximum south declination of approximately 23-1⁄2°

about December 21. The Moon has an average declinational

cycle of 27-1⁄2 days which is called a tropical month. Tides

or tidal currents occurring near the times of maximum north

or south declination of the Moon are called tropic tides or

tropic currents, and those occurring when the Moon is over

the Equator are called equatorial tides or equatorial

currents. The maximum declination reached by the Moon in

successive months depends upon the longitude of the

Moon's node, and varies from 28-1⁄2° when the longitude of

the ascending node is 0°, to 18-1⁄2° when the longitude of the

node is 180° . The node cycle, or time required for the node

to complete a circuit of 360° of longitude, is approximately

18.6 years. See epoch (2).

declinational inequality—Same as diurnal inequality.

declinational reduction—A processing of observed

high and low waters or flood and ebb tidal currents to obtain

quantities depending upon changes in the declination of the

Moon; such as tropic ranges or speeds, height or speed

inequalities, and tropic intervals.

dens s,t,p ity, in situ (D )—Mass per unit volume. The

reciprocal of specific volume. In oceanography, the density

of sea water, when expressed in gm/cm , is numerically 3

equivalent to specific gravity and is a function of salinity,

temperature, and pressure. See specific volume anomaly,

thermosteric anomaly, sigma-t, and sigma-zero.

deviation (of compass)—The deflection of the needle

of a magnetic compass due to masses of magnetic metal

within a ship on which the compass is located. This

deflection varies with different headings of the ship. The

deviation is called easterly and marked plus if the deflection

is to the right of magnetic north, and is called westerly and

marked minus if it is to the left of magnetic north. A

deviation table is a tabular arrangementshowing the amount

of deviation for different headings of the ship. Each

compass requires a separate deviation table.

digital tide (water level) gauge—See automatic tide

(water level) gauge.

direct method—A tidal datum computation method.

Datums are determined directly by comparison with an

appropriate control, for the available part of the tidal cycle.

It is usually used only when a full range of tidal values are

not available. For example: Direct Mean High Water, when

low waters are not recorded.

direction of current—Same as set.

direction of wind—Direction from which the wind is

blowing.

diurnal—Having a period or cycle of approximately

one tidal day. Thus, the tide is said to be diurnal when only

one high water and one low water occur during a tidal day,

and the tidal current is said to be diurnal when there is a

single flood and a single ebb period of a reversing current

in the tidal day. A rotary current is diurnal if it changes its

direction through all points of the compass once each tidal

day. A diurnal constituent is one which has a single period

in the constituent day. The symbol for such a constituent is

the subscript 1. See stationary wave theory and type of tide.

diurnal inequality—The difference in height of the

two high waters or of the two low waters of each tidal day;

also, the difference in speed between the two flood tidal

currents or the two ebb currents of each tidal day. The

difference changes with the declination of the Moon and, to

a lesser extent, with the declination of the Sun. In general,

the inequality tends to increase with increasing declination,

either north or south, and to diminish as the Moon

approaches the Equator. Mean diurnal high water inequality

(DHQ) is one-half the average difference between the two

high waters of each tidal day observed over the National

Tidal Datum Epoch. It is obtained by subtracting the mean

of all the high waters from the mean of the higher high

waters. Mean diurnal low water inequality (DLQ) is

one-half the average difference between the two low waters

of each tidal day observed over the National Tidal Datum

Epoch. It is obtained by subtracting the mean of the lower

low waters from the mean of all the low waters. Tropic high

water inequality (HWQ) is the average difference between

the two high waters of each tidal day at the times of tropic

tides. Tropic low water inequality (LWQ) is the average

difference between the two low waters of each tidal day at

the times of tropic tides. Mean and tropic inequalities, as

defined above, are applicable only when the type of tide is
7

either semidiurnal or mixed. Diurnal inequality is

sometimes called declinational inequality.

diurnal range—Same as great diurnal range. 

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

separated by a relatively small depression, or a low water

consisting of two minima separated by a relatively small

elevation. Sometimes called an agger. See gulder.

drift (of current)—The speed of the current. 

drift current—Same as wind drift.

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 is the interval in which it is

ebbing, these intervals being reckoned from the middle of

the intervening slack waters or minimum currents. Together

they cover, on an average, a period of 12.42 hours for a

semidiurnal tidal current or a period of 24.84 hours for a

diurnal current. In a normal semidiurnal tidal current, the

duration of flood and duration of ebb each will be

approximately equal to 6.21 hours, but the times may be

modified greatly by the presence of nontidal flow. In a river

the duration of ebb is usually longer than the duration of

flood because of fresh water discharge, especially during

spring months when snow and ice melt are predominant

influences.

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 from high water to low water.

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 tide. In a normal semidiurnal tide, duration of rise

and duration of fall each will be approximately equal to

6.21 hours, but in shallow waters and in rivers there is a

tendency for a decrease in duration of rise and a

corresponding increase in duration of fall.

dynamic decimeter—See geopotential as preferred

term.

dynamic depth (height)—See geopotential difference

as preferred term. 

dynamic depth (height) anomaly—See geopotential

anomaly as preferred term.

dynamic meter (D)—The former practical unit for

geopotential difference (dynamic depth), equal to 10

geopotentials (dynamic decimeters). See geopotential

(dynamic depth) anomaly.

dynamic topography—See geopotential topography

as preferred term.

E

eagre (eager)—Same as tidal bore.

earth tide—Periodic movement of the Earth's crust

caused by gravitational interactions between the Sun, Moon,

and Earth.

East Africa Coast Current—Same as Somali Current.

East Australian Current—A South Pacific Ocean

current setting southward along the east coast of Australia.

East Greenland Current—A North Atlantic Ocean

current setting southward and then southwestward along the

east coast of Greenland.

ebb axis—Average set of the current at ebb strength.

ebb current (ebb)—The movement of a tidal current

away from shore or down a tidal river or estuary. In the

mixed type of reversing tidal current, the terms greater ebb

and lesser ebb are applied respectively to ebb tidal currents

of greater and lesser speed each day. The terms maximum

ebb and minimum ebb are applied to the maximum and

minimum speeds of a current running continuously ebb, the

speed alternately increasing and decreasing without coming

to a slack or reversing. The expression maximum ebb is also

applicable to any ebb current at the time of greatest speed.

See ebb strength.

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 strength (strength of ebb)—Phase of the ebb tidal

current at the time of maximum speed. Also, the speed at

this time. See strength of current.

eccentricity of orbit—Ratio of the distance from the

center to the focus of an elliptical orbit to the length of the

semimajor axis. The eccentricity of orbit = %1 - (B / A) :

2

where A and B are respectively the semimajor and

semiminor axes of the orbit.

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 which

it is associated.

edge waves—Waves moving between zones of high

and low breakers along the shoreline. Edge waves

contribute to changes in water level along the shoreface

which helps to control the spacing of rip currents. See

longshore current and rip current.

Ekman spiral—A logarithmic spiral (when projected

on a horizontal plane) formed by the heads of current

velocity vectors at increasing depths. The current vectors

become progressively smaller with depth. They spiral to the

right (looking in the direction of flow) in the Northern

Hemisphere and to the left in the Southern with increasing

depth. Theoretically, in deep water, the surface current

vector sets 45° and the total mass transport sets 90° from

the direction toward which the wind is blowing. Flow

opposite to the surface current occurs at the so-called "depth

of frictional resistance". The phenomenon occurs in wind

drift currents in which only the Coriolis and frictional forces

are significant. Named for Vagn Walfrid Ekman who,
8

assuming a constant eddy viscosity, steady wind stress, and

unlimited water depth and extent, derived the effect in 1905.

electric tape gauge—A gauge consisting of a

graduated Monel metal tape on a metal reel (with

supporting frame), voltmeter, and battery. Heights can be

measured directly by unreeling the tape into its stilling well.

When contact is made with the water's surface, the circuit is

completed and the voltmeter needle moves. At that moment

the length of tape is read against an index mark, the mark

having a known elevation relative to the bench marks.

elimination—One of the final processes in the

harmonic analysis of tides in which preliminary values for

the harmonic constants of a number of constituents are

cleared of the residual effects of each other.

epoch—(1) Also known as phase lag. Angular

retardation of the maximum of a constituent of the observed

tide (or tidal current) behind the corresponding maximum

of the same constituent of the theoretical equilibrium tide.

It may also be defined as the phase difference between a

tidal constituent and its equilibrium argument. As referred

to the local equilibrium argument, its symbol is 6. When

referred to the corresponding Greenwich equilibrium

argument, it is called the Greenwich epoch and is

represented by G. A Greenwich epoch that has been

modified to adjust to a particular time meridian for

convenience in the prediction of tides is represented by g or

by 6N. The relations between these epochs may be expressed

by the following formula: 

G = 6 + pL 

g = 6N = G – aS / 15

in which L is the longitude of the place and S is the

longitude of the time meridian, these being taken as positive

for west longitude and negative for east longitude; p is the

number of constituent periods in the constituent day and is

equal to 0 for all long-period constituents, 1 for diurnal

constituents, 2 for semidiurnal constituents, and so forth;

and a is the hourly speed of the constituent, all angular

measurements being expressed in degrees. (2) As used in

tidal datum determination, it is a 19-year cycle over which

tidal height observations are meaned in order to establish

the various datums. As there are periodic and apparent

secular trends in sea level, a specific 19-year cycle (the

National Tidal Datum Epoch) is selected so that all tidal

datum determinations throughout the United States, its

territories, Commonwealth of Puerto Rico, and Trust

Territory of the Pacific Islands, will have a common

reference. See National Tidal Datum Epoch.

equation of time— Difference between mean and

apparent time. From the beginning of the year until near the

middle of April, mean time is ahead of apparent time, the

difference reaching a maximum of about 15 minutes near

the middle of February. From the middle of April to the

middle of June, mean time is behind apparent time but the

difference is less than 5 minutes. From the middle of June

to the first part of September, mean time is again ahead of

apparent time with maximum difference less than 7 minutes.

From the first of September until the later part of December,

mean time is again behind apparent time, the difference

reaching a maximum of nearly 17 minutes in the early part

of November. The equation of time for each day in the year

is given in the American Ephemeris and Nautical Almanac.

Equatorial Countercurrent—A current setting

eastward between the North and South Equatorial Currents

of the Atlantic, Pacific, and Indian (in northern winter)

Oceans. In the Atlantic and Pacific, its axis lies about

latitude 7° north and in the Indian, about 7° south.

equatorial tidal currents—Tidal currents occurring

semimonthly as a result of the Moon being over the

Equator. At these times the tendency of the Moon to

produce a diurnal inequality in the tidal current is at a

minimum.

equatorial tides—Tides occurring semimonthly as a

result of the Moon being over the Equator. At these times

the tendency of the Moon to produce a diurnal inequality in

the tide is at a minimum.

Equatorial Undercurrent—A subsurface current setting eastward along the Equator in the Pacific, Atlantic, and

Indian Oceans. In the Pacific, its core of maximum velocity

lies at a depth of about 100 meters within the South

Equatorial Current.

equilibrium argument—The theoretical phase of a

constituent of the equilibrium tide. It is usually represented

by the expression (V + u), in which V is a uniformly

changing angular quantity involving multiples of the hour

angle of the mean Sun, the mean longitudes of the Moon

and Sun, and the mean longitude of lunar or solar perigee;

and u is a slowly changing angle depending upon the

longitude of the Moon's node. When pertaining to an initial

instant of time, such as the beginning of a series of

ob oservations, it is expressed by (V + u).

equilibrium theory—A model under which it is assumed that the waters covering the face of the Earth

instantly respond to the tide-producing forces of the Moon

and Sun to form a surface of equilibrium under the action of

these forces. The model disregards friction, inertia, and the

irregular distribution of the land masses of the Earth. The

theoretical tide formed under these conditions is known as

the equilibrium tide.

equilibrium tide—Hypothetical tide due to the tide

producing forces under the equilibrium theory. Also known

as gravitational tide.

equinoctial—The celestial equator.

equinoctial tides—Tides occurring near the times of

the equinoxes.

equinoxes—The two points in the celestial sphere

where the celestial equator intersects the ecliptic; also, the

times when the Sun crosses the equator at these points. The

vernal equinox is the point where the Sun crosses the

Equator from south to north and it occurs about March 21.

Celestial longitude is reckoned eastward from the vernal
9

equinox. The autumnal equinox is the point where the Sun

crosses the Equator from north to south and it occurs about

September 23.

equipotential surface—Same as geopotential surface.

establishment of the port—Also known as high water,

full and change (HWF&C). Average high water interval on

days of the new and full Moon. This interval is also

sometimes called the common or vulgar establishment to

distinguish it from the corrected establishment, the latter

being the mean of all the high water intervals. The latter is

usually 10 to 15 minutes less than the common

establishment.

estuary—An embayment of the coast in which fresh

river water entering at its head mixes with the relatively

saline ocean water. When tidal action is the dominant

mixing agent it is usually termed a tidal estuary. Also, the

lower reaches and mouth of a river emptying directly into

the sea where tidal mixing takes place. The latter is

sometimes called a river estuary.

Eulerian measurement—Observation of a current

with a device fixed relative to the flow.

eustatic sea level rate—The worldwide change of sea

level elevation with time. The changes are due to such

causes as glacial melting or formation, thermal expansion or

contraction of sea water, etc.

evection—A perturbation of the Moon depending upon

the alternate increase and decrease of the eccentricity of its

orbit, which is always a maximum when the Sun is passing

the Moon's line of apsides and a minimum when the Sun is

at right angles to it. The principal constituents in the tide

2 2 1

resulting from the evectional inequality are < , 8 , and D .

extreme high water—The highest elevation reached

by the sea as recorded by a water level gauge during a given

period. The National Ocean Service routinely documents

monthly and yearly extreme high waters for its control

stations.

extreme low water—The lowest elevation reached by

the sea as recorded by a water level gauge during a given

period. The National Ocean Service routinely documents

monthly and yearly extreme low water for its control

stations.

F

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 mean without adjustment to a long-term series

through comparison of simultaneous observations. 

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 (flood)—The movement of a tidal current toward the shore or up a tidal river or estuary. In the

mixed type of reversing current, the terms greater flood and

lesser flood are applied respectively to the two flood

currents of greater and lesser speed of each day. The

expression maximum flood is applicable to any flood

current at the time of greatest speed. See flood strength. 

flood interval—The interval between the transit of the

Moon over the meridian of a place and the time of the

following flood strength.

flood strength (strength of flood)—Phase of the flood

tidal current at the time of maximum speed. Also, the speed

at this time. See strength of current.

Florida Current—A North Atlantic Ocean current

setting northward along the south-east coast of the United

States. A segment of the Gulf Stream System, the Florida

Current extends from the Straits of Florida to the region off

Cape Hatteras.

flow—The British equivalent of the United States total

current. Flow is the combination of tidal stream and current.

flushing time—The time required to remove or reduce

(to a permissible concentration) any dissolved or suspended

contaminant in an estuary or harbor.

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

synodical month.

Fourier series—A series proposed by the French

mathematician Fourier about the year 1807. The series

involves the sines and cosines of whole multiples of a

varying angle and is usually written in the following form:

y B 12 3 1 2 =A + A sin x + A sin 2x + A sin 3x + ... B cos x + B

3

cos 2x + B cos 3x + ...

By taking a sufficient number of terms the series may be

assumed to represent any periodic function of x.

free wave—A wave that continues to exist after the

generating force has ceased to act. See gravity wave.

G

gage—See tide (water level) gauge.

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 constant rate. Fluctuations in

hydrostatic pressure due to changes in water level modify

the recorded emission rate. Same as bubbler tide (water

level) gauge.

gauge—See tide (water level) gauge.

geodetic datum—See National Geodetic Vertical

Datum of 1929 (NGVD 1929) and North American Vertical

Datum of 1988 (NAVD 1988).

geopotential—The unit of geopotential difference,

equal to the gravity potential of 1 meter squared per second

squared, m / s , or 1 joule per kilogram, J / kg. 2 2
10

geopotential anomaly ()D)—The excess in geopotential difference over the standard geopotential difference [at

a standard specific volume at 35 parts per thousand (‰) and

0 degrees C] between isobaric surfaces. See geopotential

and geopotential topography.

2

 P

)D = I *dp

1

 P

where p is the pressure and *, the specific volume anomaly.

P1 2 and P are the pressures at the two surfaces. 

geopotential difference—The work per unit mass

gained or required in moving a unit mass vertically from

one geopotential surface to another. See geopotential,

geopotential anomaly, and geopotential topography. 

geopotential (equipotential) surface—A surface that

is everywhere normal to the acceleration of gravity.

geopotential topography—The topography of an

equiscalar (usually isobaric) surface in terms of geopotential

difference. As depicted on maps, isopleths are formed by

the intersection of the isobaric surface with a series of

geopotential surfaces. Thus, the field of isopleths represents

variations in the geopotential anomaly of the isobaric

surface above a chosen reference isobaric surface (such as

a level of no motion).

geostrophic flow—A solution of the relative hydrodynamic equations of motion in which it is assumed that the

horizontal component of the Coriolis force is balanced by

the horizontal component of the pressure gradient force.

gradient flow—A solution of the relative hydrodynamic equations of motion in which only the horizontal

Coriolis, pressure gradient, and centrifugal forces are

considered.

gravitational tide—Same as equilibrium tide.

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 also be used in its contracted form,

diurnal range.

great tropic range (Gc)—The difference in height

between tropic higher high water and tropic lower low

water. The expression may also be used in its contracted

form, tropic range.

Greenwich argument—Equilibrium argument computed for the meridian of Greenwich.

Greenwich epoch—See epoch (1).

Greenwich interval—An interval referred to the

transit of the Moon over the meridian of Greenwich, as

distinguished from the local interval which is referred to the

Moon's transit over the local meridian. The relation in hours

between Greenwich and local intervals may be expressed by

the formula:

Greenwich interval = local interval + 0.069L

where L is the west longitude of the local meridian in

degrees. For east longitude, L is to be considered negative.

Gregorian calendar—The modern calendar in which

every year divisible by 4 (excepting century years) and

every century year divisible by 400 are bissextile (or leap)

years with 366 days. All other years are common years with

365 days. The average length of this year is, therefore,

365.242,5 days which agrees very closely with the length of

the tropical year (the period of changes in seasons). The

Gregorian calendar was introduced by Pope Gregory in

1582, and immediately adopted by the Catholic countries in

place of the Julian calendar previously in use. In making the

change it was ordered that the day following October 4,

1582, of the Julian calendar be designated October 15,

1582, of the Gregorian calendar; the 10 days being dropped

in order that the vernal equinox would fall on March 21.

The Gregorian calendar was not adopted by England until

1752, but is now in general use throughout the world.

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 the Equatorial Counter Current of the

Atlantic Ocean.

gulder—Local name given to the double low water

occurring on the south coast of England. See double tide.

Gulf Coast Low Water Datum (GCLWD)—A tidal

datum. Used as chart datum from November 14, 1977, to

November 27, 1980, for the coastal waters of the Gulf coast

of the United States. GCLWD is defined as mean lower low

water when the type of tide is mixed and mean low water

(now mean lower low water) when the type of tide is

diurnal. See National Tidal Datum Convention of 1980.

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 the elevation of Gulf Coast Low

Water Datum.

Gulf Stream—A North Atlantic Ocean current setting

northeastward off the east coast of the United States. A

segment of the Gulf Stream System, the Gulf Stream

extends from the region off Cape Hatteras to an area

southeast of the Grand Banks at about latitude 40° north,

longitude 50° west. It continues the flow of the Florida

Current to the North Atlantic Current.

Gulf Stream System—The continuous current system

composed of the Florida Current, Gulf Stream, and North

Atlantic Current.

H

h—Rate of change (as of January 1, 1900) in mean

longitude of the Sun. 

h = 0.041,068,64° per solar hour.

half-tide level—Same as mean tide level.

halocline—A layer in which the salinity changes

significantly (relative to the layers above and below) with

depth.
11

harmonic analysis—The mathematical process by

which the observed tide or tidal current at any place is

separated into basic harmonic constituents.

harmonic analyzer—A machine designed for the resolution of a periodic curve into its harmonic constituents.

Now performed by electronic digital computer.

harmonic constants—The amplitudes and epochs of

the harmonic constituents of the tide or tidal current at any

place.

harmonic constituent—See constituent.

harmonic function—In its simplest form, a quantity

that varies as the cosine of an angle that increases

uniformly with time. It may be expressed by the formula:

y = A cos at

in which y is a function of time (t), A is a constant

coefficient, and a is the rate of change in the angle at.

harmonic prediction—Method of predicting tides and

tidal currents by combining the harmonic constituents into

a single tide curve. The work is usually performed by

electronic digital computer.

harmonic reduction—Same as harmonic analysis. 

harmonic tide plane—Same as Indian spring low

water. 

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

tabulation for computation of tidal datums, head of tide is

the inland or upstream point where the mean range becomes

less than 0.2 foot. Tidal datums (except for mean water

level) are not computed beyond head of tide. 

high tide—Same as high water. 

high water (HW)—The maximum height reached by

a rising tide. The high water is due to the periodic tidal

forces and the effects of meteorological, hydrologic, and/or

oceanographic conditions. For tidal datum computational

purposes, the maximum height is not considered a high

water unless it contains a tidal high water. 

high water, full and change (HWF&C)—Same as

establishment of the port. 

high water inequality—See diurnal inequality. 

high water interval (HWI)—See lunitidal interval. 

high water line—The intersection of the land with the

water surface at an elevation of high water. 

high water mark—A line or mark left upon tide flats,

beach, or along shore objects indicating the elevation of the

intrusion of high water. The mark may be a line of oil or

scum on along shore objects, or a more or less continuous

deposit of fine shell or debris on the foreshore or berm.

This mark is physical evidence of the general height

reached by wave run up at recent high waters. It should not

be confused with the mean high water line or mean higher

high water line. 

higher high water (HHW)—The highest of the high

waters (or single high water) 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. 

Humboldt Current—Same as Peru Current. 

hydraulic current—A current in a channel caused by

a difference in the surface elevation at the two ends. Such a

current may be expected in a strait connecting two bodies of

water in which the tides differ in time or range. The current

in the East River, New York, connecting Long Island Sound

and New York Harbor, is an example. 

hydrographic datum—A datum used for referencing

depths of water and the heights of predicted tides or water

level observations. Same as chart datum. See datum.

I

incremental shaft encoder—A component of a water

level gauge for converting length to a shaft angle on a

rotating disk. The position of the rotating disk is determined

by single or dual optical or magnetic sensors to provide an

electrical output. No electro-mechanical components or

gears are used, so extremely low torque is required to move

the float wheel, wire, and float mechanism.

Indian spring low water—A datum originated by

Professor G. H. Darwin when investigating the tides of

India. It is an elevation depressed below mean sea level by

an amount equal to the sum of the amplitudes of he

h 2 2 1 1armonic constituents M , S , K , and O .

Indian tide plane—Same as Indian spring low water.

inequality—A systematic departure from the mean

value of a tidal quantity. See diurnal inequality, parallax

inequality, and phase inequality.

inertial flow—A solution of the relative hydrodynamic

equations of motion in which only the horizontal component

of the Coriolis and centrifugal forces are balanced. This

anticyclonic flow results from a sudden application and

release of a driving force which then allows the system to

continue on under its own momentum without further

interference. The period of rotation is 2B / 2S sin ø, where

S = 0.729,211 x 10 radians s and ø = latitude. -4 -1

internal tide—A tidal wave propagating along a sharp

density discontinuity, such as a thermocline, or in an area of

gradually changing (vertically) density.

International Great Lakes Datum (1985) [IGLD

1985]—Mean water level at Rimouski/Pointe-au-Pere,

Quebec, on the Gulf of St. Lawrence over the period 1970

through 1988, from which geopotential elevations

(geopotential differences) throughout the Great Lakes

region are measured. The term is often used to mean the

entire system of geopotential elevations rather than just the

referenced water level. See low water datum (1).

International Hydrographic Organization (formerly

Bureau)—An institution consisting of representatives of a

number of nations organized for the purpose of coordinating
12

the hydrographic work of the participating governments. It

had its origin in the International Hydrographic Conference

in London in 1919. It has permanent headquarters in the

Principality of Monaco and is supported by funds provided

by the member nations. Its principal publications include

the Hydrographic Review and special publications on

technical subjects.

intertidal zone—(technical definition) The zone between the mean higher high water and mean lower low

water lines.

interval—See lunitidal interval and lunicurrent interval.

inverse barometer effect—The inverse response of

sea level to changes in atmospheric pressure. A static

reduction of 1.005 mb in atmospheric pressure will cause

a stationary rise of 1 cm in sea level.

Irminger Current—A North Atlantic Ocean current

setting westward off the south-west coast of Iceland.

isanostere—An isopleth of either specific volume

anomaly or thermosteric anomaly.

isobar—An isopleth of pressure.

isobaric surface—A surface of constant or uniform

pressure.

isohaline—An isopleth of salinity. Constant or uniform in salinity.

isopleth—A line of constant or uniform value of a

given quantity. See isanostere, isobar, isohaline, isopycnic,

and isotherm.

isopycnic—An isopleth of density. Constant or uniform in density.

isotherm—An isopleth of temperature.

J

J1—Smaller lunar elliptic diurnal constituent. This

1

constituent, with M , modulates the amplitudes of the

d 1eclinational K , for the effect of the Moon's elliptical orbit.

Speed = T + s + h - p = 15.585,443,3° per solar hour.

Japan Current—Same as Kuroshio.

Julian calendar—A calendar introduced by Julius

Caesar in the year 45 B.C., and slightly modified by

Augustus a few years later. This calendar provided that the

common year should consist of 365 days and that every

fourth year, now known as a bissextile or leap year, should

contain 366 days, making the average length of the year

365.25 days. It differs from the modern or Gregorian

calendar in having every fourth year a leap year, while in

the modern calendar century years not divisible by 400 are

common years. See Gregorian calendar.

Julian date—Technique for the identification of successive days of the year when monthly notation is not

desired. This is especially applicable in computer data

processing and acquisition where indexing is necessary.

K

K1—Lunisolar diurnal constituent. This constituent,

wi 1th O , expresses the effect of the Moon's declination.

They account for diurnal inequality and, at extremes,

d 1iurnal tides. With P , it expresses the effect of the Sun's

declination.

Speed = T + h = 15.041,068,6° per solar hour.

K2—Lunisolar semi diurnal constituent. This con2

stituent modulates the amplitude and frequency of M and

S2 for the declinational effect of the Moon and Sun,

respectively.

Speed = 2T + 2h = 30.082,137,3° per solar hour.

kappa (6)—Name of Greek letter used as the symbol

for a constituent phase lag or epoch when referred to the

local equilibrium argument and frequently taken to mean the

same as local epoch. See epoch (1).

kappa prime (6t)— Name of Greek letter (with prime

mark) used as the symbol for a constituent phase lag or

epoch when the Greenwich equilibrium argument (G) has

been modified to a particular time meridian. Same as g. See

kappa (6) and epoch (1).

knot—A speed unit of 1 international nautical mile

(1,852.0 meters or 6,076.115,49 international feet) per hour.

Kuroshio—"Black Stream" in Japanese. A North

Pacific Ocean current setting northeastward off the east

coast of Taiwan and Japan from Taiwan to about latitude

35° north.

Kuroshio Extension—A North Pacific Ocean current

setting eastward from about longitude 145° east to about

160° east. It continues the flow of the Kuroshio to the North

Pacific Current.

Kuroshio System—The current system composed of

the Kuroshio, Tsushima Current, Kuroshio Extension, and

North Pacific Current.

L

L2—Smaller lunar elliptic semi diurnal constituent.

Thi 2s constituent, with N , modulates the amplitude and

2

frequency of M for the effect of variation in the Moon's

orbital speed due to its elliptical orbit.

Speed = 2T – s + 2h – p = 29.528,478,9° per solar hour.

Labrador Current—A North Atlantic Ocean current

setting southeastward along the east coasts of Baffin Island,

Labrador, and Newfoundland.

lagging of tide—The periodic retardation in the time

of occurrence of high and low water due to changes in the

relative positions of the Moon and Sun.

Lagrangian measurement—Observation of a current

with a device moving with the current.

2

lambda (8 )—Smaller lunar evectional constituent.

Thi 2 2 2s constituent, with < , : , and (S ), modulates the

2

amplitude and frequency of M for the effects of variation

in solar attraction of the Moon. This attraction results in a

slight pear-shaped lunar ellipse and a difference in lunar

orbital speed between motion toward and away from the

Sun 2 2 . Although (S ) has the same speed as S , its amplitude

is extremely small.

Speed = 2T – s + p = 29.455,625,3° per solar hour.
13

latitude—The angular distance between a terrestrial

position and the equator measured northward or southward

from the equator along a meridian of longitude.

leap year—A calendar year containing 366 days.

According to the present Gregorian calendar, all years with

the date-number divisible by 4 are leap years, except

century years. The latter are leap years when the

date-number is divisible by 400.

level of no motion—A level (or layer) at which it is

assumed that an isobaric surface coincides with a

geopotential surface. A level (or layer) at which there is no

horizontal pressure gradient force.

level surface—See geopotential surface as preferred

term.

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

oceanography, it is that part of the benthic division

extending from the high water line out to a depth of about

200 meters. The littoral system is divided into a eulittoral

and sublittoral zone, separated at a depth of about 50

meters. Also, frequently used interchangeably with

intertidal zone.

local epoch—See kappa (6) and epoch (1).

local time—Time in which noon is defined by the

transit of the Sun over the local meridian as distinguished

from standard time which is based upon the transit of the

Sun over a standard meridian. Local time may be either

mean or apparent, according to whether reference is to the

mean or actual Sun. Local time was in general use in the

United States until 1883, when standard time was adopted.

The use of local time in other parts of the world has also

been practically abandoned in favor of the more convenient

standard time.

log line—A graduated line used to measure the speed

of a vessel through the water or to measure the velocity of

the current from a vessel at anchor. See current line.

long period constituent—A tidal or tidal current

constituent with a period that is independent of the rotation

of the Earth but which depends upon the orbital movement

of the Moon or the Earth. The principal lunar long period

constituents have periods approximating a month and half

a month, and the principal solar long period constituents

have periods approximating a year and half a year.

long period waves (long waves)—Forced or free

waves whose lengths are much longer than the water depth.

See tidal wave and tsunami.

longitude—Angular distance along a great circle of

reference reckoned from an accepted origin to the

projection of any point on that circle. Longitude on the

Earth's surface is measured on the Equator east and west of

the meridian of Greenwich and may be expressed either in

degrees or in hours, the hour being taken as the equivalent

of 15° of longitude. Celestial longitude is measured in the

ecliptic eastward from the vernal equinox. The mean

longitude of a celestial body moving in an orbit is the

longitude that would be attained by a point moving

uniformly in the circle of reference at the same average

angular velocity as that of the body, with the initial position

of the point so taken that its longitude would be the same as

that of the body at a certain specified position in its orbit.

With a common initial point, the mean longitude of a body

will be the same in whatever circle it may be reckoned.

longshore current—A current paralleling the shore

largely within the surf zone. It is caused by the excess water

brought to the zone by the small net mass transport of wind

waves. Long shore currents feed into rip currents. See

progressive wave.

loop of stationary wave—That portion of the oscillating area where the vertical movement is greatest.

Loop Current—A current setting clockwise in the

Gulf of Mexico. It enters through the Yucatan Channel from

the Caribbean Sea and leaves through the Straits of Florida.

low tide—Same as low water.

low water (LW)—The minimum height reached by a

falling tide. The low water is due to the periodic tidal forces

and the effects of meteorological, hydrologic, and/or

oceanographic conditions. For tidal datum computational

purposes, the minimum height is not considered a low water

unless it contains a tidal low water.

low water datum (LWD)—(1) The geopotential

elevation (geopotential difference) for each of the Great

Lakes and Lake St. Clair and the corresponding sloping

surfaces of the St. Marys, St. Clair, Detroit, Niagara, and St.

Lawrence Rivers to which are referred the depths shown on

the navigational charts and the authorized depths for

navigation improvement projects. Elevations of these planes

are referred to IGLD 1985 and are Lake Superior—183.2

meters, Lakes Michigan and Huron—176.0 meters, Lake St.

Clair—174.4 meters, Lake Erie—173.5 meters, and Lake

Ontario—74.2 meters. (2) An approximation of mean low

water that has been adopted as a standard reference for a

limited area and is retained for an indefinite period

regardless of the fact that it may differ slightly from a better

determination of mean low water from a subsequent series

of observations. Used primarily for river and harbor

engineering purposes. Boston low water datum is an

example.

low water equinoctial springs—Low water springs

near the times of the equinoxes. Expressed in terms of the

harmonic constants, it is an elevation depressed below mean

sea level by an amount equal to the sum of the amplitudes

of 2 2 2 the constituents M , S , and K .

low water inequality—See diurnal inequality.

low water interval (LWI)—See lunitidal interval.

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 Moon and Sun.
14

lower low water (LLW)—The lowest of the low

waters (or single low water) of any specified tidal day due

to the declinational effects of the Moon and Sun.

lower low water datum (LLWD)—An approximation

of mean lower low water that has been adopted as a

standard reference for a limited area and is retained for an

indefinite period regardless of the fact that it may differ

slightly from a better determination of mean lower low

water from a subsequent series of observations. Used

primarily for river and harbor engineering purposes.

Columbia River lower low water datum is an example.

lowest astronomical tide—As defined by the

International Hydrographic Organization, the lowest tide

level that can be predicted to occur under average

meteorological conditions and under any combination of

astronomical conditions.

lunar cycle—An ambiguous expression which has

been applied to various cycles associated with the Moon's

motion. See Callippic cycle, Metonic cycle, node cycle, and

synodical month.

lunar day—The time of the rotation of the Earth with

respect to the Moon, or the interval between two successive

upper transits of the Moon over the meridian of a place.

The mean lunar day is approximately 24.84 solar hours in

length, or 1.035 times as great as the mean solar day.

lunar interval—The difference in time between the

transit of the Moon over the meridian of Greenwich and a

local meridian. The average value of this interval,

expressed in hours, is 0.069 L, where L is the local

longitude in degrees, positive for west longitude and

negative for east. The lunar interval equals the difference

between the local and Greenwich interval of a tide or

current phase.

lunar month—Same as synodical month.

lunar nodes—The points where the plane of the

Moon's orbit intersects the ecliptic. The point where the

Moon crosses in going from south to north is called the

ascending node and the point where the crossing is from

north to south is called the descending node. References are

usually made to the ascending node which, for brevity, may

be called the node.

lunar tide—That part of the tide on the Earth due

solely to the Moon as distinguished from that part due to

the Sun.

lunar time—Time based upon the rotation of the Earth

relative to the Moon. See lunar day.

lunation—Same as synodical month.

lunicurrent interval—The interval between the

Moon's transit (upper or lower) over the local or Greenwich

meridian and a specified phase of the tidal current

following the transit. Examples are strength of flood

interval and strength of ebb interval, which may be

abbreviated to flood interval and ebb interval, respectively.

The interval is described as local or Greenwich according

to whether the reference is to the Moon's transit over the

local or Greenwich meridian. When not otherwise specified,

the reference is assumed to be local. For a and b markings,

see lunitidal interval.

1

lunisolar tides—Harmonic tidal constituents K , and

K2, which are derived partly from the development of the

lunar tide and partly from the solar tide, the constituent

speeds being the same in both cases. Also, the lunisolar

synodic fortnightly constituent MSf.

lunitidal interval—The interval between the Moon's

transit (upper or lower) over the local or Greenwich

meridian and the following high or low water. The average

of all high water intervals for all phases of the Moon is

known as mean high water lunitidal interval and is

abbreviated to high water interval (HWI). Similarly, mean

low water lunitidal interval is abbreviated to low water

interval (LWI). The interval is described as local or

Greenwich according to whether the reference is to the

transit over the local or Greenwich meridian. When not

otherwise specified, the reference is assumed to be local.

When there is considerable diurnal inequality in the tide,

separate intervals may be obtained for the higher high

waters, lower high waters, higher low waters, and lower low

waters. These are designated respectively as higher high

water interval (HHWI), lower high water interval (LHWI),

higher low water interval (HLWI), and lower low water

interval (LLWI). In such cases, and also when the tide is

diurnal, it is necessary to distinguish between the upper and

lower transit of the Moon with reference to its declination.

Intervals referred to the Moon's upper transit at the time of

its north declination or the lower transit at the time of south

declination are marked a. Intervals referred to the Moon's

lower transit at the time of its north declination or to the

upper transit at the time of south declination are marked b.

M

M1—Smaller lunar elliptic diurnal constituent. This

1

constituent, with J , modulates the amplitude of the

d 1eclinational K , for the effect of the Moon's elliptical orbit.

A 1 1 slightly slower constituent, designated (M ), with Q ,

modulates the amplitude and frequency of the declinational

O1, for the same effect.

Speed = T – s + h + p = 14.496,693,9° per solar hour.

M2—Principal lunar semidiurnal constituent. This

constituent represents the rotation of the Earth with respect

to the Moon.

Speed = 2T – 2s + 2h = 28.984,104,2° per solar hour.

M3—Lunar terdiurnal constituent. A shallow water

compound constituent. See shallow water constituent. 

Speed = 3T – 3s + 3h = 43.476,156,3° per solar hour.

M , 4 M , 6 8 M —Shallow water overtides of the principal

lunar constituent. See shallow water constituent.

Speed o 4 2 f M = 2M = 4T – 4s + 4h = 57.968,208,4° per

solar hour.
15

Speed o 6 2 f M = 3M = 6T – 6s + 6h = 86.952,312,7° per

solar hour. 

Speed o 8 2 f M = 4M = 8T – 8s + 8h = 115.936,416,9° per

solar hour. 

Maelstrom—Famous whirlpool off the coast of Norway in the Lofoten Islands between Moskenesoy and

Mosken.

magnetic azimuth—Azimuth reckoned from the

magnetic north or magnetic south. See magnetic direction.

magnetic declination—Same as variation. 

magnetic direction—Direction as indicated by a

magnetic compass after correction for deviation but without

correction for variation. 

marigram—A graphic record of the rise and fall of

water level. The record is in the form of a curve in which

time is generally represented on the abscissa and the height

of the water level on the ordinate. See tide curve. 

marine boundary—The mean lower low water line

(MLLWL) when used as a boundary. Also, lines used as

boundaries seaward of and measured from (or points

thereon) the MLLWL. See coastal boundary.

mascaret—French for tidal bore. 

mean current hour—Same as current hour. 

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 average of all the high water heights observed over the National

Tidal Datum Epoch. For stations with shorter series,

comparison of simultaneous observations with a control

tide station is made in order to derive the equivalent datum

of the National Tidal Datum Epoch. 

mean high water line (MHWL)—The line on a chart

or map which represents the intersection of the land with

the water surface at the elevation of mean high water. See

shoreline. 

mean higher high water (MHHW)—A tidal datum.

The average of the higher high water height of each tidal

day observed over the National Tidal Datum Epoch. For

stations with shorter series, comparison of simultaneous

observations with a control tide station is made in order to

derive the equivalent datum of the National Tidal Datum

Epoch. 

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 the elevation of mean higher

high water. 

mean low water (MLW)—A tidal datum. The average of all the low water heights observed over the National

Tidal Datum Epoch. For stations with shorter series,

comparison of simultaneous observations with a control

tide station is made in order to derive the equivalent datum

of 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 the elevation of mean low water. 

mean low water springs (MLWS)—A tidal datum.

Frequently abbreviated spring low water. The arithmetic

mean of the low water heights occurring at the time of

spring tides observed over the National Tidal Datum Epoch.

It is usually derived by taking an elevation depressed below

the half-tide level by an amount equal to one-half the spring

range of tide, necessary corrections being applied to reduce

the result to a mean value. This datum is used, to a

considerable extent, for hydrographic work outside of the

United States and is the level of reference for the Pacific

approaches to the Panama Canal. 

mean lower low water (MLLW)—A tidal datum. The

average of the lower low water height of each tidal day

observed over the National Tidal Datum Epoch. For stations

with shorter series, comparison of simultaneous

observations with a control tide station is made in order to

derive the equivalent datum of the National Tidal Datum

Epoch. 

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 the elevation of mean lower low

water. 

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 tide. It may be computed by adding half

the duration of rise to the mean low water interval,

subtracting the semidiurnal tidal period of 12.42 hours when

greater than this amount. The mean rise interval may be

either local or Greenwich according to whether it is referred

to the local or Greenwich transit. 

mean river level—A tidal datum. The average height

of the surface of a tidal river at any point for all stages of

the tide observed over the National Tidal Datum Epoch. It

is usually determined from hourly height readings. In rivers

subject to occasional freshets, the river level may undergo

wide variations and, for practical purposes, certain months

of the year may be excluded in the determination of the tidal

datum. For charting purposes, tidal datums for rivers are

usually based on observations during selected periods when

the river is at or near a low water stage. 

mean sea level (MSL)—A tidal datum. The arithmetic

mean of hourly heights observed over the National Tidal

Datum Epoch. Shorter series are specified in the name; e.g.,

monthly mean sea level and yearly mean sea level. 

mean sun—A fictitious sun which is assumed to move

in the celestial equator at a uniform speed corresponding to

the average angular speed of the real Sun in the ecliptic, the

mean sun being alternately in advance and behind the real

Sun. It is used as a reference for reckoning mean time, noon

of mean local time corresponding to the time of the transit

of the mean sun over the local meridian. See equation of

time and mean time. 

mean tide level (MTL)—A tidal datum. The

arithmetic mean of mean high water and mean low water.

Same as half-tide level.
16

mean time—Time based upon the hour angle of the

mean sun as distinguished from apparent time which is

based upon the position of the real Sun. The difference

between apparent and mean time is known as the equation

of time.

mean water level (MWL)—A datum. The mean

surface elevation as determined by averaging the heights of

the water at equal intervals of time, usually hourly. Mean

water level is used in areas of little or no range in tide.

mean water level line (MWLL)—The line on a chart

or map which represents the intersection of the land with

the water surface at the elevation of mean water level.

meteorological tides—Tidal constituents having their

origin in the daily or seasonal variations in weather

conditions which may occur with some degree of periodicity. The principal meteorological constituents recogni l

zed in the tides are Sa, Ssa, and S . See storm surge.

Metonic cycle—A period of almost 19 years or 235

lunations. Devised by Meton, an Athenian astronomer who

lived in the fifth century B.C., for the purpose of obtaining

a period in which new and full Moon would recur on the

same day of the year. Taking the Julian year of 365.25 days

and the synodical month as 29.530,588 days, we have the

l9-year period of 6,939.75 days as compared with the 235

lunations of 6,939.69 days, a difference of only 0.06 day.

M f—Lunar fortnightly constituent. This constituent

expresses the effect of departure from a sinusoidal

declinational motion.

Speed = 2s = 1.098,033,1° per solar hour.

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

flood strengths and/or greater and lesser ebb strengths. See

flood current and ebb current.

mixed (tide)—Type of tide characterized by a conspicuous diurnal inequality in the higher high and lower

high waters and/or higher low and lower low waters. See

type of tide.

M m—Lunar monthly constituent. This constituent

expresses the effect of irregularities in the Moon's rate of

change of distance and speed in orbit.

Speed = s – p = 0.544,374,7° per solar hour.

modified epoch—See kappa prime (6t) and epoch (1).

modified-range ratio method—A tidal datum

computation method. Generally used for the East Coast,

Gulf Coast, and Caribbean Island stations. Values needed

are mean tide level (MTL), mean diurnal tide level (DTL),

mean range of tide (MN), and great diurnal range (GT) as

determined by comparison with an appropriate control.

From those, the following are computed:

MLW = MTL - (0.5*MN)

MHW = MLW + MN

MLLW = DTL - (0.5*GT)

MHHW = MLLW + GT

Monsoon Current (Southwest Monsoon Current)—An Indian Ocean current setting in a generally

eastward to southeastward direction off India and Ceylon.

It replaces the North Equatorial Current, reversed by wind

stress of the south-west monsoons, in August and

September.

month—The period of the revolution of the Moon

around the Earth. The month is designated as siderial,

tropical, anomalistic, nodical, or synodical according to

whether the revolution is relative to a fixed star, vernal

equinox, perigee, ascending node, or Sun. The calendar

month is a rough approximation to the synodical month.

MSf—Lunisolar synodic fortnightly constituent.

Speed = 2s – 2h = 1.015,895,8° per solar hour.

mu (:2)—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

several staffs will form a continuous scale referred to the

same datum.

N

N—Rate of change (as of January 1, 1900) in mean

longitude of the Moon's node.

N = – 0.002,206,41° per solar hour.

N2—Larger lunar elliptic semi diurnal constituent. See

L2

Speed = 2T – 3s + 2h + p = 28.439,729,5° per solar hour.

2N2—Lunar elliptic semi diurnal second-order constituent.

Speed = 2T – 4s + 2h + 2p = 27.895,354,8° per solar hour.

National Geodetic Vertical Datum of 1929 [NGVD

1929]—A fixed reference adopted as a standard geodetic

datum for elevations determined by leveling. The datum

was derived for surveys from a general adjustment of the

first-order leveling nets of both the United States and

Canada. In the adjustment, mean sea level was held fixed as

observed at 21 tide stations in the United States and 5 in

Canada. The year indicates the time of the general

adjustment. A synonym for Sea-level Datum of 1929. The

geodetic datum is fixed and does not take into account the

changing stands of sea level. Because there are many

variables affecting sea level, and because the geodetic

datum represents a best fit over a broad area, the

relationship between the geodetic datum and local mean sea

level is not consistent from one location to another in either

time or space. For this reason, the National Geodetic

Vertical Datum should not be confused with mean sea level.

See North American Vertical Datum of 1988 (NAVD

1988).

National Spatial Reference System (NSRS)—A

consistent national coordinate system that defines latitude,

longitude, height, scale, gravity, and orientation throughout

the nation, and how these values change with time. The

NSRS is developed and maintained by the National
17

G eod eti c S urv ey using a d vanced geodetic ,

photogrammetric, and remote sensing techniques.

National Tidal Datum Convention of 1980—Effective November 28, 1980, the Convention: (1) establishes

one uniform, continuous tidal datum system for all marine

waters of the United States, its territories, Commonwealth

of Puerto Rico, and Trust Territory of the Pacific Islands,

for the first time in history; (2) provides a tidal datum

system independent of computations based on type of tide;

(3) lowers chart datum from mean low water to mean lower

low water along the Atlantic coast of the United States; (4)

updates the National Tidal Datum Epoch from 1941

through 1959, to 1960 through 1978; (5) changes the name

Gulf Coast Low Water Datum to mean lower low water; (6)

introduces the tidal datum of mean higher high water in

areas of predominantly diurnal tides; and (7) lowers mean

high water in areas of predominantly diurnal tides. See

chart datum.

National Tidal Datum Epoch—The specific l9-year

period adopted by the National Ocean Service as the

official time segment over which tide observations are

taken and reduced to obtain mean values (e.g., mean lower

low water, etc.) for tidal datums. It is necessary for

standardization because of periodic and apparent secular

trends in sea level. The present National Tidal Datum

Epoch is 1960 through 1978. It is reviewed annually for

possible revision and must be actively considered for

revision every 25 years.

National Water Level Observation Network

(NWLON)—The network of tide and water level stations

operated by the National Ocean Service along the marine

and Great Lakes coasts and islands of the United States.

The NWLON is composed of the primary and secondary control tide stations of the National Ocean Service.

This Network provides the basic tidal datums for coastal

and marine boundaries and for chart datum of the United

States. Tide observations at a secondary control tide station

or tertiary tide station are reduced to equivalent l9-year

tidal datums through comparison of simultaneous

observations with a primary control tide station. In addition

to hydrography, nautical charting, and delineation of

coastal and marine boundaries, the Network is used for

coastal processes and tectonic studies, tsunami and storm

surge warnings, and climate monitoring.

The National Water Level Observation Network also

includes stations operated throughout the Great Lakes

Basin. The network supports regulation, navigation and

charting, river and harbor improvement, power generation,

various scientific activities, and the adjustment for vertical

movement of the Earth's crust in the Great Lakes Basin.

neap range—See neap tides.

neap tides or tidal currents—Tides of decreased

range or tidal currents of decreased speed occurring

semimonthly as the result of the Moon being in quadrature.

The neap range (Np) of the tide is the average range

occurring at the time of neap tides and is most conveniently

computed from the harmonic constants. It is smaller than the

mean range where the type of tide is either semidiurnal or

mixed and is of no practical significance where the type of

tide is predominantly diurnal. The average height of the

high waters of the neap tide is called neap high water or

high water neaps (MHWN) and the average height of the

corresponding low waters is called neap low water or low

water neaps (MLWN).

Next Generation Water Level Measurement System

(NGWLMS)—A fully integrated system encompassing new

technology sensors and recording equipment, multiple data

transmission options, and an integrated data processing,

analysis, and dissemination subsystem.

nodal line—A line in an oscillating body of water

along which there is a minimum or zero rise and fall of the

tide.

nodal point—The zero tide point in an amphidromic

region.

node—See lunar nodes.

node cycle—Period of approximately 18.61 Julian

years required for the regression of the Moon's nodes to

complete a circuit of 360° of longitude. It is accompanied

by a corresponding cycle of changing inclination of the

Moon's orbit relative to the plane of the Earth's Equator,

with resulting inequalities in the rise and fall of the tide and

speed of the tidal current.

node factor (f)—A factor depending upon the longitude of the Moon's node which, when applied to the mean

coefficient of a tidal constituent, will adapt the same to a

particular year for which predictions are to be made.

nodical month—Average period of the revolution of

the Moon around the Earth with respect to the Moon's

ascending node. It is approximately 27.212,220 days in

length.

nonharmonic constants—Tidal constants such as

lunitidal intervals, ranges, and inequalities which may be

derived directly from high and low water observations

without regard to the harmonic constituents of the tide. Also

applicable to tidal currents.

nontidal current—See current.

normal tide—A nontechnical term synonymous with

tide; i.e., the rise and fall of the ocean due to the

gravitational interactions of the Sun, Moon, and Earth

alone. Use of this term is discouraged.

North American Vertical Datum of 1988 [NAVD

1988]—A fixed reference for elevations determined by

geodetic leveling. The datum was derived from a general

adjustment of the first-order terrestrial leveling nets of the

United States, Canada, and Mexico. In the adjustment, only

the height of the primary tidal bench mark, referenced to the

International Great Lakes Datum of 1985 (IGLD 1985)

local mean sea level height value, at Father Point,

Rimouski, Quebec, Canada was held fixed, thus providing

minimum constraint. NAVD 1988 and IGLD 1985 are not
18

identical. However, NAVD 1988 bench mark values are

given in Helmert orthometric height units while IGLD 1985

values are in dynamic heights. See International Great

Lakes Datum of 1985, National Geodetic Vertical Datum

of 1929, and geopotential difference.

North Atlantic Current—A North Atlantic Ocean

current setting northeastward from southeast of the Grand

Banks at about latitude 40° north, longitude 50° west, to the

British Isles. A segment of the Gulf Stream System, the

North Atlantic Current continues the flow of the Gulf

Stream to the Norwegian and Canary Currents.

North Cape Current—An Arctic Ocean current setting eastward off the north coast of Scandinavia in the

Barrents Sea.

North Equatorial Current—A current setting westward in the North Atlantic and North Pacific Oceans and in

the Indian Ocean from about October to July. It occurs

immediately north of the Equatorial Counter Current.

North Pacific Current—A North Pacific Ocean current setting eastward from about 160° east to somewhat

beyond about 150° west. It continues the flow of the

Kuroshio Extension, sending branches to the south.

Norwegian Current—A North Atlantic Ocean current

setting northeastward off the coast of Norway.

nu 2(< )—Larger lunar evectional constituent. See

lambda.

Speed = 2T – 3s + 4h – p = 28.512,583,1° per solar hour.

O

O1 1 —Lunar diurnal constituent. See K . 

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

inclination of the Moon's orbit to the plane of the Earth's

Equator.

obliquity of the ecliptic—The angle which the ecliptic

makes with the plane of the Earth's Equator. Its value is

approximately 23.45°.

obliquity of the Moon's orbit—The angle which the

Moon's orbit makes with the plane of the Earth's Equator.

Its value varies from 18.3° to 28.6°, depending upon the

longitude of the Moon's ascending node; the smaller value

corresponding to a longitude of 180° and the larger one, to

a longitude of 0°.

oceanography—Oceanography is the science of all

aspects of the oceans, in spite of its etymology. The term,

oceanography, implies the interrelationships of the various

marine sciences of which it is composed. This connotation

has arisen through the historical development of marine

research in which it has been found that a true

understanding of the oceans is best achieved through

investigations based on the realization that water, its

organic and inorganic contents, motions, and boundaries

are mutually related and interdependent.

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 be synonymous with mean. Thus, ordinary high (low)

water is the equivalent of mean high (low) water. The use of

ordinary in tidal terms is discouraged.

orifice—See stilling well and protective well.

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 of

one of the fundamental constituents derived from the

development of the tide-producing force. The presence of

overtides is usually attributed to shallow water conditions.

The overtides usually considered in tidal work are the

harmonics of the principal lunar and solar semidiurnal

2 2 constituents M and S , and are designated by the symbols

M468 4 6, M , M , S , S , etc. The magnitudes of these harmonics

relative to those of the fundamental constituents are usually

greater in the tidal current than in the tide.

Oyashio—A current setting southwestward along the

Siberian, Kamchatka, and Kuril Islands coasts in the Bering

Sea and North Pacific Ocean.

P

p—Rate of change (as of January 1, 1900) in mean

longitude of lunar perigee. 

p = 0.004,641,83° per solar hour.

p1—Rate of change (as of January 1, 1900) in mean

longitude of solar perigee. 

p1 = 0.000,001,96° per solar hour.

P1 1 —Solar diurnal constituent. See K .

Speed = T – h = 14.958,931,4° per solar hour. 

parallax—In tidal work, the term refers to horizontal

parallax, which is the angle formed at the center of a

celestial body between a line to the center of the Earth and

a line tangent to the Earth's surface. Since the sine of a small

angle is approximately equal to the angle itself in radians,

it is usually taken in tidal work simply as the ratio of the

mean radius of the Earth to the distance of the tide-producing body. Since the parallax is a function of the distance of

a celestial body, the term is applied to tidal inequalities

arising from the changing distance of the tide-producing

body. 

parallax inequality—The variation in the range of tide

or in the speed of a tidal current due to changes in the

distance of the Moon from the Earth. The range of tide and

speed of the current tend alternately to increase and

decrease as the Moon approaches its perigee and apogee,

respectively, the complete cycle being the anomalistic

month. There is a similar but relatively unimportant

inequality due to the Sun, the cycle being the anomalistic

year. The parallax has little direct effect upon the lunitidal

intervals but tends to modify the phase effect. When the

Moon is in perigee, the priming and lagging of the tide due

to the phase is diminished and when in apogee the priming

and lagging is increased. 
19

parallax reduction—A processing of observed high

and low waters to obtain quantities depending upon

changes in the distance of the Moon, such as perigean and

apogean ranges. 

parallel plate intake—Intake of a stilling or protective

well with two parallel plates attached below. The plates are

typically three times the diameter of the well and are spaced

three inches apart. The plates are used to minimize

current-induced draw-down (Bernoulli effect) error in water

level measurements. 

pelorus—An instrument formerly used on a vessel in

connection with a current line and current pole to obtain the

set of the current. In its simplest form, it was a disk about

8 inches in diameter and graduated clockwise for every 5°

or 10° . It was mounted rigidly on the vessel, usually with

the 0° mark forward and the diameter through this mark

parallel with the keel. Bearings were then related to the

vessel's compass and converted to true. 

perigean tides or tidal currents—Tides of increased

range or tidal currents of increased speed occurring

monthly as the result of the Moon being in perigee. The

perigean range (Pn) of tide is the average range occurring

at the time of perigean tides and is most conveniently

computed from the harmonic constants. It is larger than the

mean range where the type of tide is either semidiurnal or

mixed, and is of no practical significance where the type of

tide is predominantly diurnal. 

perigee—The point in the orbit of the Moon or a

man-made satellite nearest to the Earth. The point in the

orbit of a satellite nearest to its companion body. 

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 body or

the time between two consecutive like phases of the tide or

tidal current. A period may be expressed in angular

measure as 360°. The word also is used to express any

specified duration of time. 

permanent current—A current that runs continuously

and is independent of tides and other temporary causes.

Permanent currents include the general surface circulation

of the oceans. 

Peru Current—A South Pacific Ocean current setting

northward along the west coast of South America. It has

sometimes been called the Humboldt Current because an

early record of its temperature was taken by the German

scientist Alexander von Humboldt in 1802. It has also been

called the Peruvian or Chilean Current. The name Corriente

de Peru was adopted by a resolution of the Ibero-American

Oceanographic Conference at its Madrid-Malaga meeting

in April 1935. 

phase—(1) Any recurring aspect of a periodic phenomenon, such as new Moon, high water, flood strength,

etc. (2) A particular instant of a periodic function expressed

in angular measure and reckoned from the time of its

maximum value, the entire period of the function being

360°. The maximum and minimum of a harmonic

constituent have phase values of 0° and 180°, respectively.

phase inequality—Variations in the tides or tidal

currents due to changes in the phase of the Moon. At the

times of new and full Moon the tide-producing forces of the

Moon and Sun act in conjunction, causing the range of tide

and speed of the tidal current to be greater than the average,

the tides at these times being known as spring tides. At the

times of the quadratures of the Moon these forces are

opposed to each other, causing neap tides with diminished

range and current speed.

phase lag—Same as epoch (1). 

phase reduction—A processing of observed high and

low waters to obtain quantities depending upon the phase of

the Moon, such as the spring and neap ranges of tide. At a

former time this process was known as second reduction.

Also applicable to tidal currents. 

pororoca—Brazilian for tidal bore. 

PORTS—Physical Oceanographic Real Time System.

A national system of current, water level, and other

oceanographical and meteorological sensors telemetering

data in real-time to central locations for storage, processing,

and dissemination. Available to pilots, mariners, the U.S.

Coast Guard, and other marine interests in voice or digital

form. First introduced in Tampa Bay. 

potential, tide-producing—Tendency for particles on

the Earth to change their positions as a result of the

gravitational interactions between the Sun, Moon, and

Earth. Although gravitational attraction varies inversely as

the square of the distance of the tide producing body, the

resulting potential varies inversely as the cube of the

distance. 

predicting machine—See tide predicting machine.

pressure gauge—A water level gauge that is operated

by the change in pressure at the bottom of a body of water

due to the rise and fall of the water level. See gas purged

pressure gauge. 

pressure gradient force, horizontal—The horizontal

component of the product of the specific volume and the

rate of decrease in pressure with distance. 

pressure sensor—A pressure transducer sensing device for water level measurement. A relative transducer is

vented to the atmosphere and pressure readings are made

relative to atmospheric pressure. An absolute transducer

measures the pressure at its location. The readings are then

corrected for barometric pressure taken at the surface.

primary control tide station—A tide station at which

continuous observations have been made over a minimum

of 19 years. Its purpose is to provide data for computing

accepted values of the harmonic and nonharmonic constants

essential to tide predictions and to the determination of tidal

datums for charting and for coastal and marine boundaries.

The data series from this station serves as a primary control

for the reduction of relatively short series from subordinate
20

tide stations through the method of comparison of

simultaneous observations and for monitoring long-period

sea level trends and variations. See tide station, secondary

control tide station, tertiary tide station, and subordinate

tide station (1).

primary tidal bench mark—See bench mark.

prime meridian—The meridian of longitude which

passes through the original site of the Royal Observatory in

Greenwich, England and used as the origin of longitude.

Also known as the Greenwich Meridian.

priming of tide—The periodic acceleration in the time

of occurrence of high and low waters due to changes in the

relative positions of the Sun and Moon.

progressive wave—A wave that advances in distance

along the sea surface or at some intermediate depth.

Although the wave form itself travels significant distances,

the water particles that make up the wave merely describe

circular (in relatively deep water) or elliptical (in relatively

shallow water) orbits. With high, steep, wind waves, a

small overlap in the orbital motion becomes significant.

This overlapping gives rise to a small net mass transport.

See long shore current and rip current. Progressive waves

can be internal, traveling along a sharp density

discontinuity, such as the thermocline, or in a layer of

gradually changing density (vertically).

protective well—A vertical pipe with a relatively large

opening (intake) in the bottom. It is used with the air

acoustic ranging sensor and electronic processing (filtering)

technique to minimize the nonlinear characteristics of the

stilling well. Its purpose is also to shield the sensing

element from physical damage and harsh environment.

Unlike a stilling well, damping of high frequency waves is

not a critical requirement. See stilling well.

pycnocline—A layer in which the density increases

significantly (relative to the layers above and below) with

depth.

Q

Ql—Larger 1 lunar elliptic diurnal constituent. See M .

Speed = T – 3s + h + p = 13.398,660,9° per solar hour.

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

Speed = T – 4s + h + 2p = 12.854,286,2° per solar hour. 

quadrature of Moon—Position of the Moon when its

longitude differs by 90° from the longitude of the Sun. The

corresponding phases are known as first quarter and last

quarter.

R

R2—Smaller solar elliptic constituent. This constituent,

wi 2 2 th T , modulates the amplitude and frequency of S for

the effect of variation in the Earth's orbital speed due to its

elliptical orbit.

Sp 1eed = 2T + h – p = 30.041,066,7° per solar hour.

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

semidaily (solar) cycle in barometric pressure, daily (solar)

land and sea breezes, and seasonal (annual) changes in

temperature. Other changes in sea level due to

meteorological changes that are random in phase are not

considered radiational tides. 

range of tide—The difference in height between

consecutive high and low waters. The mean range is the

difference in height between mean high water and mean low

water. The great diurnal range or diurnal range is the

difference in height between mean higher high water and

mean lower low water. For other ranges see spring, neap,

perigean, apogean, and tropic tides; and tropic ranges. 

real-time—Pertains to a data collecting system that

monitors an on-going process and disseminates measured

values before they are expected to have changed

significantly.

rectilinear current—Same as reversing current. 

red tide (water)—The term applied to toxic algal

blooms caused by several genera of dinoflagellates

(Gymnodinium and Gonyaulax) which turn the sea red and

are frequently associated with a deterioration in water

quality. The color occurs as a result of the reaction of a red

pigment, peridinin, to light during photosynthesis. These

toxic algal blooms pose a serious threat to marine life and

are potentially harmful to humans. The term has no

connection with astronomic tides. However, its association

with the word "tide" is from popular observations of its

movements with tidal currents in estuarine waters.

reduction factor (F)—Reciprocal of node factor (f). 

reduction of tides or tidal currents—A processing of

observed tide or tidal current data to obtain mean values for

tidal or tidal current constants. 

reference station—A tide or current station for which

independent daily predictions are given in the "Tide Tables"

and "Tidal Current Tables," and from which corresponding

predictions are obtained for subordinate stations by means

of differences and ratios. See subordinate tide station (2)

and subordinate current station (2). 

relative mean sea level change—A local change in

mean sea level relative to a network of bench marks

established in the most stable and permanent material

available (bedrock, if possible) on the land adjacent to the

tide station location. A change in relative mean sea level

may be composed of both an absolute mean sea level

change component and a vertical land movement change

component. 

residual current—The observed current minus the

astronomical tidal current.

response analysis—For any linear system, an input

i 0 function X (t) and an output function X (t) can be related

according to the formula:

X0 o i(t) = I X (t – J)W(J)dJ + noise(t) 4

where W(J) is the impulse response of the system and its

Fourier transform:

Z(f o ) = I W (J)e = R(f)e 4 -2BifJ iN(f)

is the system's admittance (coherent output/input) at
21

frequency f. In practice, the integrals are replaced by

i

summations; X , W, and Z are generally complex. The

d 0iscrete set of W values are termed response weights; X (t)

i

is ordinarily an observed tidal time series and X (t) the tide

potential or the tide at some nearby place. A future

prediction can be prepared by applying the weights to an

i

appropriate X (t) series. In general:

* Z * = R(f) and Tan(Z) = N(f)

measure the relative magnification and phase lead of the

station at frequency f.

reversing current—A tidal current which flows alternately in approximately opposite directions with a slack

water at each reversal of direction. Currents of this type

usually occur in rivers and straits where the direction of

flow is more or less restricted to certain channels. When the

movement is towards the shore or up a stream, the current

is said to be flooding, and when in the opposite direction,

it is said to be ebbing. The combined flood and ebb

movement (including the slack water) covers, on an

average, 12.42 hours for a semidiurnal current. If

unaffected by a nontidal flow, the flood and ebb

movements will each last about 6 hours, but when

combined with such a flow, the durations of flood and ebb

may be quite different. During the flow in each direction

the speed of the current will vary from zero at the time of

slack water to a maximum about midway between the

slacks. 

reversing falls—A name applied to falls which flow

alternately in opposite directions in a narrow channel in the

St. John River above the city of St. John, New Brunswick,

Canada, the phenomenon being due to the large range of

tide and a constriction in the river. The direction of flow is

upstream or downstream according to whether it is high or

low water on the outside, the falls disappearing at the

half-tide level. 

1

rho (D )—Larger lunar evectional diurnal constituent.

Speed = T – 3s + 3h – p = 13.471,514,5° per solar hour. 

rip—Agitation of water caused by the meeting of

currents or by a rapid current setting over an irregular

bottom. Termed tide rip when a tidal current is involved.

See overfalls. 

rip current—A narrow intense current setting seaward

through the surf zone. It removes the excess water brought

to the zone by the small net mass transport of waves. It is

fed by longshore currents. Rip currents usually occur at

points, groins, jetties, etc., of irregular beaches, and at

regular intervals along straight, uninterrupted beaches. 

river current—The gravity-induced seaward flow of

fresh water originating from the drainage basin of a river.

In the fresh water portion of the river below head of tide,

the river current is alternately increased and decreased by

the effect of the tidal current. After entering a tidal estuary,

river current is the depth-averaged mean flow through any

cross-section. See head of tide and estuary. 

river estuary—See estuary. 

rotary current—A tidal current that flows continually

with the direction of flow changing through all points of the

compass during the tidal period. Rotary currents are usually

found offshore where the direction of flow is not restricted

by any barriers. The tendency for the rotation in direction

has its origin in the Coriolis force and, unless modified by

local conditions, the change is clockwise in the Northern

Hemisphere and counterclockwise in the Southern. The

speed of the current usually varies throughout the tidal

cycle, passing through the two maxima in approximately

opposite directions and the two minima with the direction

of the current at approximately 90° from the directions of

the maxima.

S

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 respect

to the Sun.

Speed = 2T = 30.000,000,0° per solar hour.

S , 4 6 S —Shallow water overtides of the principal solar

constituent.

Sp 4 2 eed of S = 2S = 4T = 60.000,000,0° per solar hour. 

Sp 6 2 eed of S = 3S = 6T = 90.000,000,0° per solar hour.

Sa—Solar annual constituent. This constituent, with

Ssa, accounts for the nonuniform changes in the Sun's

declination and distance. In actuality, they mostly reflect

yearly meteorological variations influencing sea level.

Speed = h = 0.041,068,64° per solar hour.

Ssa—Solar semiannual constituent. See Sa.

Speed = 2h = 0.082,137,3° per solar hour.

salinity (S)—The total amount of solid material in

grams contained in 1 kilogram of sea water when all the

carbonate has been converted to oxide, the bromine and

iodine replaced by chlorine, and all organic matter

completely oxidized. The following is approximate.

S(‰) = 1.806,55 x Cl (‰)

Where Cl(‰) is chlorinity in parts per thousand. See

chlorinity.

Sargasso Sea—The west central region of the subtropical gyre of the North Atlantic Ocean. It is bounded by

the North Atlantic, Canary, North Equatorial, and Antilles

Currents, and the Gulf Stream. It is characterized by the

absence of any well-marked currents and by large quantities

of drifting Sargassum, or gulfweed.

Saros—A period of 223 synodic months corresponding

approximately to 19 eclipse years or 18.03 Julian years, and

is a cycle in which solar and lunar eclipses repeat

themselves under approximately the same conditions.

sea level datum (SLD)—An obsolete term. See National Geodetic Vertical Datum of 1929 and mean sea level.
22

second reduction—Same as phase reduction.

secondary control tide station—A tide station at

which continuous observations have been made over a

minimum period of 1 year but less than 19 years. The series

is reduced by comparison with simultaneous observations

from a primary control tide station. This station provides

for a 365-day harmonic analysis including the seasonal

fluctuation of sea level. See tide station, primary control

tide station, tertiary tide station, and subordinate tide

station (1).

secular trend—See apparent secular trend as preferred

term. 

seiche—A stationary wave usually caused by strong

winds and/or changes in barometric pressure. It is found in

lakes, semi-enclosed bodies of water, and in areas of the

open ocean. The period of a seiche in an enclosed

rectangular body of water is usually represented by the

formula:

Period (T) = 2L / %gd

in which L is the length, d the average depth of the body of

water, and g the acceleration of gravity. See standing wave.

seismic sea wave—Same as tsunami. 

semidiurnal—Having a period or cycle of approximately one-half of a tidal day. The predominant type of tide

throughout the world is semidiurnal, with two high waters

and two low waters each tidal day. The tidal current is said

to be semidiurnal when there are two flood and two ebb

periods each day. A semidiurnal constituent has two

maxima and two minima each constituent day, and its

symbol is the subscript 2. See type of tide. 

sequence of current—The order of occurrence of the

four tidal current strengths of a day, with special reference

as to whether the greater flood immediately precedes or

follows the greater ebb. 

sequence of tide—The order in which the four tides of

a day occur, with special reference as to whether the higher

high water immediately precedes or follows the lower low

water. 

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 account for the change in the form of a tide

wave resulting from shallow water conditions. Shallow

water constituents include the overtides and compound

tides. 

shallow water wave—A wave is classified as a shallow water wave whenever the ratio of the depth (the

vertical distance of the still water level from the bottom) to

the wave length (the horizontal distance between crests) is

less than 0.04. Such waves propagate according to the

formula:

C = %gd

where C is the wave speed, g the acceleration of gravity,

and d the depth. Tidal waves are shallow water waves. 

shear—A quasi-horizontal layer moving at a different

velocity relative to the layer directly below and/or above.

shoreline (coastline)—The intersection of the land

with the water surface. The shoreline shown on charts

represents the line of contact between the land and a

selected water elevation. In areas affected by tidal

fluctuations, this line of contact is the mean high water line.

In confined coastal waters of diminished tidal influence, the

mean water level line may be used. See coast line. 

sidereal day—The time of the rotation of the Earth

with respect to the vernal equinox. It equals approximately

0.997,27 of a mean solar day. Because of the precession of

the equinoxes, the sidereal day thus defined is slightly less

than the period of rotation with respect to the fixed stars,

but the difference is less than a hundredth part of a second.

sidereal month—Average period of the revolution of

the Moon around the Earth with respect to a fixed star,

equal to 27.321,661 mean solar days.

sidereal time—This is usually defined by astronomers

as the hour angle of the vernal equinox. The sidereal day is

the interval between two successive upper transits of the

vernal equinox. It is to be noted that when applied to the

month and year the word sidereal has reference to motion

with respect to the fixed stars, while the word tropical is

used for motion with respect to the vernal equinox. Because

of the precession of the equinox there is a slight difference.

sidereal year—Average period of the revolution of the

Earth around the Sun with respect to a fixed star. Its length

is approximately 365.256,4 mean solar days.

t

sigma-t (F )—An expression of density as a function of

temperature and salinity (at atmospheric pressure) in a

convenient numerical form. See density.

Ft s,t,p = (D – 1)1,000

o

sigma-zero (F )—An expression of density as a function of salinity (at atmospheric pressure and 0°C) in a

convenient numerical form. See density.

Fo s,t,o = (D – 1)1,000

slack; ebb begins (slack before ebb)—The slack water immediately preceding the ebb current. 

slack; flood begins (slack before flood)—The slack

water immediately preceding the flood current. 

slack water (slack)—The state of a tidal current when

its speed is near zero, especially the moment when a

reversing current changes direction and its speed is zero.

The term also is applied to the entire period of low speed

near the time of turning of the current when it is too weak to

be of any practical importance in navigation. The relation of

the time of slack water to the tidal phases varies in different

localities. For a perfect standing tidal wave, slack water

occurs at the time of high and of low water, while for a

perfect progressive tidal wave, slack water occurs midway

between high and low water. See slack; ebb begins and

slack; flood begins. 

small diurnal range (Sl)—Difference in height between mean lower high water and mean higher low water.
23

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

the rotation with respect to the mean Sun. The solar day

commencing at midnight is called a civil or calendar day,

but if the day is reckoned from noon it is known as an

astronomical day because of its former use in astronomical

calculation. 

solar tide—(1) The part of the tide that is due to the

tide-producing force of the Sun. (2) The observed tide in

areas where the solar tide is dominant. This condition

provides for phase repetition at about the same time each

solar day.

solar time—Time measured by the hour angle of the

Sun. It is called apparent time when referred to the actual

Sun and mean time when referred to the mean Sun. It is

also classified as local, standard, or Greenwich according

to whether it is reckoned from the local, standard, or

Greenwich meridian.

solitary wave—A wave of translation consisting of a

single crest rising above the undisturbed water level

without any accompanying trough. The rate of advance of

a solitary wave depends upon the depth of the water and is

usually expressed by the formula:

C = %g(d + h)

in which C = rate of advance, g = acceleration of gravity, d

= depth of water, and h = height of wave, the depth and

height being measured from the undisturbed water level.

solstices—The two points in the ecliptic where the Sun

reaches its maximum and minimum declinations; also the

times when the Sun reaches these points. The maximum

north declination occurs on or near June 21, marking the

beginning of summer in the Northern Hemisphere and the

beginning of winter in the Southern. The maximum south

declination occurs on or near December 22, marking the

beginning of winter in the Northern Hemisphere and the

beginning of summer in the Southern.

solstitial tides—Tides occurring near the times of the

solstices. The tropic range may be expected to be especially

large at these times.

Somali (East Africa Coast) Current—An Indian

Ocean current setting southwestward along the coast of

Somalia. The current reverses and sets to the northeast

during the Southwest Monsoon.

South Equatorial Current—A current setting westward along and south of the Equator in the Atlantic and

Pacific Oceans, and south of the Equator in the Indian

Ocean. It occurs immediately south of the Equatorial

Counter Current.

Southwest Monsoon Current—Same as Monsoon

Current.

species of constituent—A classification depending

upon the period of a constituent. The principal species are

semidiurnal, diurnal, and long-period.

specific volume anomaly, or steric anomaly

(*)—The excess in specific volume over the standard

specific volume at 35 ‰, 0°C, and the given pressure. See

thermosteric anomaly and specific volume.

*="s,t,p 35,o,p – "

s,t,p specific volume, in situ (" )— Volume per unit

mass. The reciprocal of density (specific gravity). The

specific volume of sea water as a function of salinity,

temperature, and pressure. See specific volume anomaly and

thermosteric anomaly.

speed (of constituent)—The rate of change in the

phase of a constituent, usually expressed in degrees per

hour. The speed is equal to 360° divided by the constituent

period expressed in hours.

speed (of current)—The magnitude of velocity. Rate

at which the current flows. Usually expressed in knots or

centimeters per second.

Spitsbergen Atlantic Current—A current setting

northwestward off the southwest coast of Spitsbergen in the

Greenland Sea.

spring high water—Same as mean high water springs

(MHWS). See spring tides.

spring low water—Same as mean low water springs

(MLWS). See spring tides and mean low water springs.

spring range (Sg)—See spring tides.

spring tides or tidal currents—Tides of increased

range or tidal currents of increased speed occurring

semimonthly as the result of the Moon being new or full.

The spring range (Sg) of tide is the average range occurring

at the time of spring tides and is most conveniently

computed from the harmonic constants. It is larger than the

mean range where the type of tide is either semi diurnal or

mixed, and is of no practical significance where the type of

tide is predominantly diurnal. The average height of the

high waters of the spring tides is called spring high water or

mean high water springs (MHWS) and the average height of

the corresponding low waters is called spring low water or

mean low water springs (MLWS).

stand of tide—Sometimes called a platform tide. An

interval at high or low water when there is no sensible

change in the height of the tide. The water level is stationary

at high and low water for only an instant, but the change in

level near these times is so slow that it is not usually

perceptible. In general, the duration of the apparent stand

will depend upon the range of tide, being longer for a small

range than for a large range, but where there is a tendency

for a double tide the stand may last for several hours even

with a large range of tide.

standard method—A tidal datum computation

method. Generally used for the West Coast and Pacific

Island stations. Values needed are mean tide level (MTL),

mean range of tide (MN), great diurnal range (GT), and

mean diurnal high and low water inequalities (DHQ and

DLQ) as determined by comparison with an appropriate

control. From those, the following are computed:
24

MLW = MTL - (0.5*MN)

MHW = MLW + MN

MLLW = MLW - DLQ

MHHW = MHW + DHQ

standard time—A kind of time based upon the transit

of the Sun over a certain specified meridian, called the time

meridian, and adopted for use over a considerable area.

With a few exceptions, standard time is based upon some

meridian which differs by a multiple of 15° from the

meridian of Greenwich. The United States first adopted

standard time in 1883 on the initiative of the American

Railway Association, and at noon on November 18 of that

year the telegraphic time signals from the Naval

Observatory at Washington were changed to this system.

standing (stationary) wave—A wave that oscillates

without progressing. One-half of such a wave may be

illustrated by the oscillation of the water in a pan that has

been tilted. Near the axis, which is called the node or nodal

line, there is no vertical rise and fall of the water. The ends

of the wave are called loops and at these places the vertical

rise and fall is at a maximum. The current is maximum near

the node and minimum at the loops. The period of a

stationary wave depends upon the length and depth of the

body of water and, for a simple rectangular basin, may be

expressed by the formula:

T = 2L / %gd

in which T is the period of wave, L the length of the basin,

d the depth of water, and g the acceleration of gravity. A

stationary wave may be resolved into two progressive

waves of equal amplitude and equal speeds moving in

opposite directions.

station datum—See datum of tabulation.

stationary wave theory—An assumption that the basic tidal movement in the open ocean consists of a system

of stationary wave oscillations, any progressive wave

movement being of secondary importance except as the tide

advances into tributary waters. The continental masses

divide the sea into irregular basins, which, although not

completely enclosed, are capable of sustaining oscillations

which are more or less independent. The tide-producing

force consists principally of two parts, a semidiurnal force

with a period of approximately half a day and a diurnal

force with a period of approximately a whole day. Insofar

as the free period of oscillation of any part of the ocean, as

determined by its dimensions and depth, is in accord with

the semidiurnal or diurnal tide-producing forces, there will

be built up corresponding oscillations of considerable

amplitude which will be manifested in the rise and fall of

the tide. The diurnal oscillations, superimposed upon the

semidiurnal oscillations, cause the inequalities in the

heights of the two high and the two low waters of each day.

Although the tidal movement as a whole is somewhat

complicated by the overlapping of oscillating areas, the

theory is consistent with observational data.

stencils—Perforated sheets formerly used with the

tabulated hourly heights of the tide or speeds of the tidal

current for the purpose of distributing and grouping them

into constituent hours preliminary to summing for harmonic

analysis. See Coast and Geodetic Survey Special

Publication No. 98, Manual of Harmonic Analysis and

Prediction of Tides. This analysis is now performed on

electronic digital computers.

steric anomaly—Same as specific volume anomaly.

stilling well—A vertical pipe with a relatively small

opening (intake) in the bottom. It is used in a gauge

installation to dampen short period surface waves while

freely admitting the tide, other long period waves, and sea

level variations; which can then be measured by a water

level gauge sensor inside. See float well and protective well.

storm surge—The local change in the elevation of the

ocean along a shore due to a storm. The storm surge is

measured by subtracting the astronomic tidal elevation from

the total elevation. It typically has a duration of a few hours.

Since wind generated waves ride on top of the storm surge

(and are not included in the definition), the total

instantaneous elevation may greatly exceed the predicted

storm surge plus astronomic tide. It is potentially

catastrophic, especially on low lying coasts with gently

sloping offshore topography. See storm tide.

storm tide—As used by the National Weather Service,

NOAA, the sum of the storm surge and astronomic tide. See

storm surge.

stray line—Ungraduated portion of line connected

with the current pole formerly used in taking current

observations. The stray line was usually about 100 feet long

and permitted the pole to acquire the velocity of the current

at some distance from the disturbed waters in the immediate

vicinity of the observing vessel, before the current velocity

was read from the graduated portion of the current line.

strength of current—Phase of tidal current in which

the speed is a maximum; also the speed at this time.

Beginning with slack before flood in the period of a

reversing tidal current (or minimum before flood in a rotary

current), the speed gradually increases to flood strength and

then diminishes to slack before ebb (or minimum before ebb

in a rotary current), after which the current turns in

direction, the speed increases to ebb strength and then

diminishes to slack before flood, completing the cycle. If it

is assumed that the speed throughout the cycle varies as the

ordinates of a cosine curve, it can be shown that the average

speed for an entire flood or ebb period is equal to 2/B or

0.636,6 of the speed of the corresponding strength of

current. 

strength of ebb—Same as ebb strength. 

strength of flood—Same as flood strength. 

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 with simultaneous observations
25

from a control current station. See current station, control

current station, and reference station. (2) A station listed in

the Tidal Current Tables for which predictions are to be

obtained by means of differences and ratios applied to the

full predictions at a reference station. See reference station.

subordinate tide station—(1) A tide station from

which a relatively short series of observations is reduced by

comparison with simultaneous observations from a tide

station with a relatively long series of observations. See tide

station, primary control tide station, secondary control tide

station, and tertiary tide station. (2) A station listed in the

Tide Tables from which predictions are to be obtained by

means of differences and ratios applied to the full

predictions at a reference station. See reference station. 

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 average interval between corresponding phases of the

Moon. The synodical month is approximately 29.530,588

days in length. 

syzygy—With respect to tides, whenever the Moon is

lined up with the Earth and Sun in a straight Sun-MoonEarth or Sun-Earth-Moon configuration. At these times the

range of tide is greater than average. See spring tides or

tidal currents.

T

T—Rate of change of hour angle of mean Sun at place

of observation.

T = 15° per mean solar hour. 

T2—L 2arger solar elliptic constituent. See R . 

Sp 1eed = 2T – h + p = 29.958,933,3° per solar hour. 

tape gauge—See electric tape gauge. 

telemetry—The capability of transmitting or retrieving

data over long distance communication links, such as

satellite, VHF radio, or telephone. 

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 less than 1 year. The series is reduced

by comparison with simultaneous observations from a

secondary control tide station. This station provides for a

29-day harmonic analysis. See tide station, primary control

tide station, secondary control tide station, and subordinate

tide station (1). 

thermocline—A layer in which the temperature decreases significantly (relative to the layers above and

below) with depth. The principal ones are designated

diurnal, seasonal, and main thermocline. 

T s,t thermosteric anomaly(* , )', or ) )—The specific

volume anomaly (steric anomaly) that would be attained if

the water were changed isothermally to a standard pressure

of one atmosphere. The specific volume anomaly with

pressure terms omitted. See isanostere. 

tidal bench mark—See bench mark. 

tidal bench mark description—A published, concise

description of the location, stamped number or designation,

date established, and elevation (referred to a tidal datum) of

a specific bench mark. 

tidal bench mark state index map—A state map

which indicates the locations for which tidal datums and

tidal bench mark descriptions are available. 

tidal bore—A tidal wave that propagates up a relatively shallow and sloping estuary or river with a steep wave

front. The leading edge presents an abrupt rise in level,

frequently with continuous breaking and often immediately

followed by several large undulations. An uncommon

phenomenon, the tidal bore is usually associated with very

large ranges in tide as well as wedge shaped and rapidly

shoaling entrances. Also called eagre, eager (for Tsientan,

China bore), mascaret (French), pororoca (Brazilian), and

bore. 

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

classified as harmonic and nonharmonic. The harmonic

constants consist of the amplitudes and epochs of the

harmonic constituents, and the nonharmonic constants

include the ranges and intervals derived directly from the

high and low water observations. 

tidal constituent—See constituent. 

tidal current—A horizontal movement of the water

caused by gravitational interactions between the Sun, Moon,

and Earth. The horizontal component of the particulate

motion of a tidal wave. Part of the same general movement

of the sea that is manifested in the vertical rise and fall

called tide. The United States equivalent of the British tidal

stream. See tidal wave, tide, and current. 

Tidal Current Chart Diagrams—A series of monthly

diagrams to be used with the Tidal Current Charts. Each

diagram contains lines that indicate the specific tidal current

chart to use for a given date and time, and the speed factor

to apply to that chart.

Tidal Current Charts—Charts on which tidal current

data are depicted. Tidal Current Charts for a number of

important waterways are published by the National Ocean

Service. Each consists of a set of charts giving the speed

and direction of the current for each hour or equal interval

of the tidal cycle, thus presenting a comprehensive view of

the tidal current movement.

tidal current constants—See current constants.

tidal current station—See current station.

Tidal Current Tables—Tables which give daily predictions of the times and velocities of the tidal currents.

These predictions are usually supplemented by current

differences and constants through which predictions can be

obtained for numerous other locations.

tidal datum—See datum.

tidal day—Same as lunar day.
26

tidal difference—Difference in time or height between

a high or low water at a subordinate station and a reference

station for which predictions are given in the Tide Tables.

The difference, when applied according to sign to the

prediction at the reference station, gives the corresponding

time or height for the subordinate station.

tidal epoch—See National Tidal Datum Epoch and

epoch.

tidal estuary—See estuary.

tidal stream—British equivalent of United States tidal

current.

tidal wave—A shallow water wave caused by the

gravitational interactions between the Sun, Moon, and

Earth. Essentially, high water is the crest of a tidal wave

and low water, the trough. Tidal current is the horizontal

component of the particulate motion, while tide is

manifested by the vertical component. The observed tide

and tidal current can be considered the result of the

combination of several tidal waves, each of which may vary

from nearly pure progressive to nearly pure standing and

with differing periods, heights, phase relationships, and

direction.

tidal zoning—The practice of dividing a hydrographic

survey area into discrete zones or sections, each one

possessing similar tidal characteristics. One set of tide

reducers is assigned to each zone. Tide reducers are used to

adjust the soundings in that zone to chart datum (MLLW).

Tidal zoning is necessary in order to correct for differing

water level heights occurring throughout the survey area at

any given time. Each zone of the survey area is

geographically delineated such that the differences in time

and range do not exceed certain limits, generally 0.2 hours

and 0.2 feet respectively; however, these limits are subject

to change depending upon type of survey, location, and

tidal characteristics. The tide reducers are derived from the

water levels recorded at an appropriate tide station, usually

nearby. Tide reducers are used to correct the soundings

throughout the hydrographic survey area to a common,

uniform, uninterrupted chart datum. See tide reducers.

tide—The periodic rise and fall of a body of water

resulting from gravitational interactions between Sun,

Moon, and Earth. The vertical component of the particulate

motion of a tidal wave. Although the accompanying

horizontal movement of the water is part of the same

phenomenon, it is preferable to designate this motion as

tidal current. See tidal wave.

tide curve—A graphic representation of the rise and

fall of the tide in which time is usually represented by the

abscissa and height by the ordinate. For a semidiurnal tide

with little diurnal inequality, the graphic representation

approximates a cosine curve. See marigram.

tide datum—See datum.

tide (water level) gauge—An instrument for

measuring the rise and fall of the tide (water level). See

ADR gauge, automatic tide gauge, Next Generation Water

Level Measurement System, gas purged pressure gauge,

electric tape gauge, pressure gauge, and tide staff.

tide predicting machine—A mechanical analog

machine especially designed to handle the great quantity of

constituent summations required in the harmonic method.

William Ferrel's Maxima and Minima Tide Predictor

(described in Manual of Tides, U.S. Coast and Geodetic

Survey, Appendix 10, Report for 1883) was the first such

machine used in the United States. Summing only 19

constituents, but giving direct readings of the predicted

times and heights of the high and low waters, the Ferrel

machine was used for the predictions of 1885 through 1914.

A second machine, developed by Rollin A. Harris and E. G.

Fischer and summing 37 constituents, was used for the

predictions of 1912 through 1965 (described in Manual of

Harmonic Analysis and Prediction of Tides by Paul

Schureman, U.S. Coast and Geodetic Survey Special

Publication No. 98, 1958). Predictions are now prepared

using an electronic digital computer.

tide-producing force—That part of the gravitational

attraction of the Moon and Sun which is effective in

producing the tides on the Earth. The force varies

approximately as the mass of the attracting body and

inversely as the cube of its distance. The tide-producing

force exerted by the Sun is a little less than one-half as great

as that of the Moon. A mathematical development of the

vertical and horizontal components of the tide-producing

forces of the Moon and Sun will be found in Coast and

Geodetic Survey Special Publication No. 98.

tide reducers—Height corrections for reducing

soundings to chart datum (MLLW). A tide reducer

represents the height of the water level at a given place and

time relative to chart datum. Tide reducers are obtained

from one or more tide stations within or nearby the survey

area. Often, due to differing tidal characteristics over the

survey area, the tide reducers obtained directly from a tide

station must be corrected to adjust for time and range of tide

differences in the various zones of the hydrographic survey

area. See tidal zoning.

tide rip—See rip.

tide staff—A water level gauge consisting of a vertical

graduated staff from which the height of the water level can

be read directly. It is called a fixed staff when secured in

place so that it cannot be easily removed. A portable staff is

one that is designed for removal from the water when not in

use. For such a staff a fixed support is provided. The

support has a metal stop secured to it so that the staff will

always have the same elevation when installed for use. See

electric tape gauge.

tide (water level) station—The geographic location at

which tidal observations are conducted. Also, the facilities

used to make tidal observations. These may include a tide

house, tide (water level) gauge, tide staff, and tidal bench

marks. See primary control tide station, secondary control

tide station, tertiary tide station, and subordinate tide station

(1).
27

Tide Tables—Tables which give daily predictions of

the times and heights of high and low waters. These

predictions are usually supplemented by tidal differences

and constants through which predictions can be obtained

for numerous other locations.

tide wave—See tidal wave.

tidelands—The zone between the mean high water and

mean low water lines. It is identical with intertidal zone

(technical definition) when the type of tide is semidiurnal

or diurnal.

tidewater—Water activated by the tide generating

forces and/or water affected by the resulting tide, especially

in coastal and estuarine areas. Also, a general term often

applied to the land and water of estuarine areas formed by

postglacial drowning of coastal plain rivers.

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 may

be designated as sidereal, solar, or lunar, according to

whether the measurement is taken in reference to the vernal

equinox, the Sun, or the Moon. Solar time may be apparent

or mean, according to whether the reference is to the actual

Sun or the mean Sun. Mean solar time may be local or

standard, according to whether it is based upon the transit

of the Sun over the local meridian or a selected meridian

adopted as a standard over a considerable area. Greenwich

time is standard time based upon the meridian of

Greenwich. In civil time the day commences at midnight,

while in astronomical time, as used prior to 1925, the

beginning of the day was reckoned from noon of the civil

day of the same date. The name universal time is now

applied to Greenwich mean civil time.

time meridian—A meridian used as a reference for

time.

total current—The combination of the tidal and nontidal current. The United States equivalent of the British

flow. See current.

tractive force—The horizontal component of a tideproducing force vector (directed parallel with level surfaces

at that geographic location). 

transit—The passage of a celestial body over a

specified meridian. The passage is designated as upper

transit or lower transit according to whether it is over that

part of the meridian lying above or below the polar axis. 

tropic currents—Tidal currents occurring semimonthly when the effect of the Moon's maximum

declination is greatest. At these times the tendency of the

Moon to produce a diurnal inequality in the current is at a

maximum. 

tropic inequalities—Tropic high water inequality

(HWQ) is the average difference between the two high

waters of the day at the times of tropic tides. Tropic low

water inequality (LWQ) is the average difference between

the two low waters of the day at the times of tropic tides.

These terms are applicable only when the type of tide is

semidiurnal or mixed. See tropic tides. 

tropic intervals—Tropic higher high water interval

(TcHHWI) is the lunitidal interval pertaining to the higher

high waters at the time of the tropic tides. Tropic lower low

water interval (TcLLWI) is the lunitidal interval pertaining

to the lower low waters at the time of the tropic tides.

Tropic intervals are marked a when reference is made to the

upper transit of the Moon at its north declination or to the

lower transit at the time of south declination, and are

marked b when the reference is to the lower transit at the

north declination or to the upper transit at the south

declination. See tropic tides. 

tropic ranges—The great tropic range (Gc), or tropic

range, is the difference in height between tropic higher high

water and tropic lower low water. The small tropic range

(Sc) is the difference in height between tropic lower high

water and tropic higher low water. The mean tropic range

(Mc) is the mean between the great tropic and the small

tropic range. Tropic ranges are most conveniently computed

from the harmonic constants. See tropic tides. 

tropic speed—The greater flood or greater ebb speed

at the time of tropic currents. 

tropic tides—Tides occurring semimonthly when the

effect of the Moon's maximum declination is greatest. At

these times there is a tendency for an increase in the diurnal

range. The tidal datums pertaining to the tropic tides are

designated as tropic higher high water (TcHHW), tropic

lower high water (TcLHW), tropic higher low water

(TcHLW), and tropic lower low water (TcLLW). 

tropical month—The average period of the revolution

of the Moon around the Earth with respect to the vernal

equinox. Its length is approximately 27.321,582 days. 

tropical year—The average period of the revolution of

the Earth around the Sun with respect to the vernal equinox.

Its length is approximately 365.242,2 days. The tropical

year determines the cycle of changes in the seasons, and is

the unit to which the calendar year is adjusted through the

occasional introduction of the extra day on leap years.

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

compass direction and magnetic direction.

tsunami—A shallow water progressive wave, potentially catastrophic, caused by an underwater earthquake or

volcano.

Tsushima Current—A North Pacific Ocean current

setting northeastward in the East China Sea (in summer) and

Sea of Japan. A segment of the Kuroshio System.

type of tide—A classification based on characteristic

forms of a tide curve. Qualitatively, when the two high

waters and two low waters of each tidal day are

approximately equal in height, the tide is said to be

semidiurnal; when there is a relatively large diurnal
28

inequality in the high or low waters or both, it is said to be

mixed; and when there is only one high water and one low

water in each tidal day, it is said to be diurnal.

Quantitatively (after Dietrich), where the amplitude ratio of

K1 1 2 2 + O to M + S is less than 0.25, the tide is classified as

semidiurnal; where the ratio is from 0.25 to 1.5, the tide is

mixed, mainly semidiurnal; where the ratio is from 1.5 to

3.0, the tide is mixed, mainly diurnal; and where greater

than 3.0, diurnal.

U

universal time (UT)—Same as Greenwich mean time

(GMT). See time, kinds.

uplands— Land above the mean high water line

(shoreline) and subject to private ownership, as distinguished from tidelands, the ownership of which is prima

facie in the state but also subject to divestment under state

statutes. See tidelands.

upwelling—An upward flow of subsurface water due

to such causes as surface divergence, offshore wind, and

wind drift transport away from shore.

V

V +0 u—See equilibrium argument.

vanishing tide—In a predominantly mixed tide with a

very large diurnal inequality, the lower high water and

higher low water become indistinct (or vanish) at times of

extreme declinations.

variation (of compass)—Difference between true

north as determined by the Earth's axis of rotation and

magnetic north as determined by the Earth's magnetism.

Variation is designated as east or positive when the

magnetic needle is deflected to the east of true north and as

west or negative when the deflection is to the west of true

north. The variation changes with time. Also called

magnetic declination.

variational inequality—An inequality in the Moon's

motion due mainly to the tangential component of the Sun's

attraction.

velocity (of current)—Speed and set of the current.

vernal equinox—See equinoxes.

vulgar establishment—Same as establishment of the

port.

W

water level gauge—See tide gauge.

water level station—See tide station.

wave height—The vertical distance between crest and

trough. See range of tide.

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.

West Wind Drift—Same as Antarctic Circumpolar

Current.

wind drift—An ocean current in which only the

Coriolis and frictional forces are significant. The wind drift

embodies an Ekman spiral.

Z

ZO—Symbol recommended by the International Hydrographic Organization to represent the elevation of mean

sea level above chart datum.