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Property of CSC Library A ATMOS U.S. DEPARTMENT OF COMMERCE NOAh %C:,.-~.CO COASTAL SERVICES CENTER 2234 SOUTH HOBSON AVENUE CHARLESTON, SC 29405-~' X5 Tide and Current Glossar By Steacy D. Hicks Physical Oceanographer U.S. Department of Commerce Robert A. Mosbacher, Secretary National Oceanic and Atmospheric Administration Xz ~John A. Knauss, Administrator National Ocean Service Virginia K. Tippie, Assistant Administrator ; Dedication The Tide and Current Glossary is dedicated to Professor Raymond B. Montgomery; friend and advisor to the U.S. Coast and Geodetic Survey (now National Ocean Service), creator of oceanographic terms, units, and their rigorous definitions, and contributor to this glossary. Although falling short of his precision, this glossary attempts to approach Ray's standard of excellence. Hii Preface to 1989 Edition This publication is a revision of the 1984 edition. It contains 24 new entries, I deletion, major revisions to 21 entries, and numerous small changes and corrections. In addition, Appendix A has been updated with 10 new entries and Appendix B, with 5. The revision of terms in this edition reflects those changes brought about by the adoption and implementation of the National Tidal Datum Convention of 1980. The addition of terms is largely associated with the recent introduction of several new automated systems for the measurement, transmission, analysis, and presentation of tides, long-period sea-level variations, and currents. The National Water Level Observation Network is composed of tide and water level stations along the marine and Great Lakes coasts and islands of the United States. Since "water level" can be used as a generic term as well as a Great Lakes term, "ifwater level" can be substituted for "tide" and "tidal" in many of the marine entries. However, because the Tide and Current Glossary contains the official definitions of the National Ocean Service as "tide authority" of the United States, the terms 'tide" and "tidal" in the marine terms were retained for legal and international purposes. Preface to 1984 Edition This publication is an extensively revised version of the 1975 edition. The revision contains 84 new entries, 24 entry deletions, major revisions to 47 entries, and numerous small changes and corrections. This amounts to a total of 154 new entries, 73 entry deletions, and major revisions to 115 entries; or a 77 percent change in the entries by the present author since the last edition of Paul Schureman in 1949. The terms thermosteric anomaly and isanostere, introduced by Professors Ray- mond B. Montgomery and Warren S. Wooster ("Thermosteric anomaly and the analysis of serial oceanographic data," Deep-Sea Research, vol. 2, 1954, pp. 68-70), and the term naviface, introduced by Montgomery ("Comments on oceanic leveling," Sympo- sium on Mean Sea Level, Washington, DC, IAPSO, April 13, 1967, and Journal of Marine Research, vol. 27, no. 1, 1969, pp. 161-162), have been included. However, Montgomery's oxygen terms were not used due to the limited scope of the glossary. Professor Joseph L. Reid kindly helped the author by providing advice on the preferred names of the northcentral North Pacific currents. Geopotential, geopotential anomaly, geopotential difference, geopotential surface, and geopotential topography are listed as preferred terms over dynamic decimeter (and dynamic meter), dynamic depth anomaly, dynamic depth, level surface, and dynamic topography, respectively (Montgomery, R. B., "SI units in oceanography: some of the problems," Marine Geodesy, vol. 5, no. 4, 1982, pp. 325-334). In addition, heads of the variously named organizational elements responsible for the tide and current program of the National Ocean Service (Survey of the Coast from 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) since 1851, have been listed in Appendix B. This edition was prepared under the authorization of Captain (now Rear Admiral) Wesley V. Hull, NOAA, Acting Director, Office of Oceanography and Marine Serv- ices, National Ocean Service (now Director, Atlantic Marine Center). The Office of Oceanography and Marine Services, as Tide Authority (in the international and legal sense) of the United States, considers the tide, tidal datum, and tidal current terms in this glossary as official definitions of the National Oceanic and Atmospheric Admin- istration, U.S. Department of Commerce. With deep appreciation the author thanks Mrs. Betty V. Arozian for complete technical editing, Mrs. Elsa A. Bennett for typing the entire manuscript with its numerous revisions, and Mr. Thomas L. Allen for all production and printing arrange- ments. The Tide and Current Glossary by Paul Schureman was first published in 1941. Schureman (1876-1959) retired from the U.S. Coast and Geodetic Survey in 1945. In 1949, the publication was revised by E. C. McKay and F. J. Haight, and then reprinted (with corrections) in 1963. The first major revision was made by Steacy D. Hicks for the 1975 edition. Preface to 1975 Edition This publication is an extensively revised version of the 1949 Tide and Current Glossary (U.S. Coast and Geodetic Survey' Special Publication No. 228). The revision contains 61 new entries (of which only 18 are names of currents), 49 entry deletions, and major modifications to 63 definitions. Numerous small changes and corrections have also been made. In addition to general terms, the Tide and Current Glossary includes those accepted definitions intrinsic to certain standard procedures of the Oceanographic Division of the National Ocean Survey. These standard procedures are in the areas of observations, reductions, and predictions of tides and tidal currents, and in the deter- mination of tidal datums. Many of the definitions (such as "epoch" and "tropic intervals"), although written for tides, are equally applicable to tidal currents; to have included the words "tidal currents" in all instances would have rendered these definitions cumbersome. Therefore, these definitions should be thought of as representing the general tidal phenomenon. In composing the definitions of the major tidal constituents, the lucid explanations contained in Tides and Tidal Streams by Commander H. R. Hatfield, R.N. (Admiralty Manual of Hydrographic Surveying, Volume 2, Chapter 2, the Hydrographer of the Navy, Taunton, Somerset, England, N.P. 134b(2), 1969) were freely used and are acknowledged. The entries for radiational tides and response analysis, from Tidal Spectroscopy and Prediction by Walter H. Munk and David E. Cartwright, Philosophi- cal Transactions of the Royal Society of London, Series A, no. 1105, vol 259, pp. 533-581, May 19, 1966, are also acknowledged with appreciation. This edition was prepared under the authority of CAPT Robert C. Munson, NOAA, Associate Director, Office of Marine Surveys and Maps, National Ocean Survey, at the request of LCDR Carl W. Fisher, NOAA, Chief, Oceanographic Divison, Office of Marine Surveys and Maps, and Carroll I. Thurlow, Chief, Tides Branch, Oceanographic Division. 1The National Ocean Survey, a component of the National Oceanic and Atmospheric Administration, was known as the Coast and Geodetic Survey prior to 1970. Preface to 1949 Edition The principal changes involve those definitions pertaining to tidal datum planes and related terms for the purpose of bringing them into conformity with the latest accepted operational procedures of the Coast and Geodetic Survey. Some terms have been expanded to make them applicaable to tidal currents as well as tides. Minor changes have been made in some of the original descriptions and a few additional terms have been included. Changes and additions occurring in the revised (1949) edition were prepared by E. C. McKay, Chief, Section of Tides, and F. J. Haight, Chief, Section of Currents. Vii Preface to 1941 Edition This publication is designed primarily for use in connection with the tide and current work of the Coast and Geodetic Survey, and definitions and descriptions are given in reference to their application or relation to this work. In addition to words and phrases used to describe the tide itself, it contains other terms of technical significance which may have some bearing upon the subject. Also included are the names of a number of scientists of former years who have made important contributions to the development of tidal knowledge. In recording these names, the author is not unmindful of the valuable work of recent years which is still being carried on by men prominent in the scientific world'of today. Attention is called to the fact that a number of tidal terms refer to quantities that fluctuate from day to day but which may be reduced to mean values. The name of such quantity may apply to a single individual value or to its mean value. In usage, unless the context indicates otherwise, the name may be assumed to apply to the best determined mean value. Val 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. ix Tide and Current Glossary A air acoustic ranging sensor-A pulsed, acoustic- absolute mean sea level change-An eustatic change ranging device using the air column in a tube as the in mean sea level relative to the geographic center of acoustic sound path. The fundamental measurement is the Earth. the time it takes for the acoustic signal to travel from accepted values-Tidal datums and Greenwich high a transmitter to the water surface and then back to and low water intervals obtained through primary de- the receiver. The distance from a reference point to termination or simultaneous observational comparisons the water surface is derived from the travel time. A made with a primary control tide station in order to calibration point is set at a fixed distance from the derive the equivalent of a 19-year value. acoustic transducer and is used to correct the mea- acoustic Doppler current profiler (ADCP)-A cur- sured distance using the calibrated sound velocity in rent measuring instrument employing the transmission the tube. of high frequency acoustic signals in the water. The air temperature sensors-Sensors located in the pro- current is determined by a Doppler shift in the back- tective well for the purpose of verifying uniformity of scatter echo from plankton, suspended sediment, and temperature for measurements taken by the air acous- bubbles, all assumed to be moving with the mean tic ranging sensor. speed of the water. Time gating circuitry is employed Alaska Current-A North Pacific Ocean current which uses differences in acoustic travel time to divide setting counterclockwise along the coasts of Canada the water column into range intervals, called bins. The and Alaska in the Gulf of Alaska. bin determinations allow development of a profile of Alaskan Stream-A North Pacific Ocean current current speed and direction over the entire water col- setting westward along the south side of the Aleutian umn. The ADCP can be deployed from a moving Islands. It is an extension of the Alaska Current. vessel, tow, buoy, or bottom platform. In the latter amphidromic point-A point of no amplitude of the configuration, it is nonobtrusive in the water column observed or a constituent tide. and thus can be deployed in shipping channels. See amphidromic region-An area surrounding an am- remote acoustic Doppler sensing (RADS) current me- phidromic point from which the radiating cotidal lines ter system. progress through all hours of the tidal cycle. ADR gauge-Analog to Digital Recording tide amplitude (H)-One-half the range of a constituent gauge. A float or pressure actuated tide gauge that tide. By analogy, it may be applied also to the maxi- records the heights at regular time intervals in digital mum speed of a constituent current. format. analog-As used in the National Ocean Service, a age of diurnal inequality-The time interval between continuous measurement or a continuous graphic dis- the maximum semimonthly north or south declination play of data. See ADR gauge and marigram. of the Moon and the maximum effect of the declina- analysis, harmonic-See harmonic analysis. tion upon range of tide or speed of the current. The analyzer, harmonic-See harmonic analyzer. age may be computed from the harmonic constants by angular velocity of the Earth's rotation (Q2)-Time the formula: rate of change of angular displacement relative to the age of diurnal inequality=0.911(KI �-01') hours. fixed stars. It is equal to 0.729,211X 104 radian/sec- age of Moon-The time elapsed since the preceding ond. new Moon. annual inequality-Seasonal variation in water level age of parallax inequality-The time interval be- or current, more or less periodic, due chiefly to me- tween perigee of the Moon and the maximum effect of teorological causes. parallax upon range of tide or speed of the tidal anomalistic-Pertaining to the periodic return of the current. This age may be computed from the harmonic Moon to its perigee or the Earth to its perihelion. The constants by the formula: anomalistic month is the average period of the revolu- age of parallax inequality= 1.837(M2�--N2�) hours. tion of the Moon around the Earth with respect to age of phase inequality-The time interval between lunar perigee, and is approximately 27.554,550 days in new or full Moon and the maximum effect of these length. The anomalistic year is the average period of phases upon range of tide or speed of the tidal current. the revolution of the Earth around the Sun with re- This age may be computed from the harmonic con- spect to perihelion, and is approximately 365.259,6 stants by the formula: days in length. age of phase inequality=0.984(S2� -M2�) hours. anomaly-As applied to astronomy, the anomaly is age of tide-Same as age of phase inequality. the angle made at any time by the radius vector of a agger-Same as double tide. planet or moon with its line of apsides, the angle being Agulhas Current-An Indian Ocean current setting reckoned from perihelion or perigee in the direction of southwestward along the southeast coast of Africa. the body's motion. It is called the true anomaly when I referred to the actual position of the body, and mean astronomical time-Time formerly used in anomaly when referred to a fictitious body moving astronomical calculations in which the day began at with a uniform angular velocity equal to the average noon rather than midnight. The astronomical day com- velocity of the real body and passing perihelion or menced at noon of the civil day of the same date. The perigee at the same time. hours of the day were numbered consecutively from Antarctic Circumpolar Current-Same as West zero (noon) to 23 (11 a.m. of the following morning). Wind Drift. Up to the close of the year 1924, astronomical time Antilles Current-A North Atlantic Ocean current was in general use in nautical almanacs. Beginning setting northwestward along the northeast coasts of the with the year 1925, the American Ephemeris and Bahama Islands. Nautical Almanac and similar publications of other countries abandoned the old astronomical time and a onther planet, e thes frb o the E adopted Greenwich civil (mean) time for the data other planet, etc.) farthest from the Sun. given in their tables. apogean tides or tidal currents-Tides of decreased given in their tables. augmenting factor--A factor used in connection range or currents of decreased speed occurring month-aumnigfcoAfctrseinoncin range or currents of decreased speed occurring month- with the harmonic analysis of tides or tidal currents to ly as the result of the Moon being in apogee. The allow for the fact that the tabulated hourly heights or apogean range (An) of the tide is the average range speeds used in the summation for any constituent, occurring at the time of apogean tides and is most other than 5, do not in general occur on the exact conveniently computed from the harmonic constants. It constituent hours to which they are assigned, but may is smaller than the mean range, where the type of tide differ from the same by as much as a half hour. is either semidiurnal or mixed, and is of no practical automatic tide gauge-An instrument that automati- significance where the type of tide is predominantly cally registers the rise and fall of the tide. In some diurnal. instruments, the registration is accomplished by record- apogee-The point in the orbit of the Moon or ing the heights at regular time intervals in digital man-made satellite farthest from the Earth. The point format; in others, by a continuous graph of height in the orbit of a satellite farthest from its companion against time. The automatic gauges used by the Na- body. tional Ocean Service are of both types. apparent secular trend-The nonperiodic tendency azimuth-Azimuth of a body is the arc of the of sea level to rise, fall, or remain stationary with time. horizon intercepted between the north or south point Technically, it is frequently defined as the slope of a and the foot of the vertical circle passing through the least-squares line of regression through a relatively body. It is reckoned in degrees from either the north long series of yearly mean sea-level values. The word or south point clockwise entirely around the horizon. "apparent" is used since it is often not possible to Azimuth of a current is the direction toward which it know whether a trend is truly nonperiodic or merely a is flowing, and is usually reckoned from the north segment of a very long (relative to the length of the point. series) oscillation. apparent time-Time based upon the true position B of the Sun as distinguished from mean time, which is baroclinic--When isobaric surfaces of a fluid are measured by a fictitious Sub moving at a uniform rate. not parallel with density surfaces. Apparent time is that shown by the sundial, and its barotropic-When isobaric surfaces of a fluid are noon is the time when the Sun crosses the meridian. parallel with density surfaces. The difference between apparent time and mean time bench mark (BM)-A fixed physical object or mark is known as the equation of time. Although quite used as reference for a vertical datum. A tidal bench common many years ago, apparent time is seldom used mark is one near a tide station to which the tide staff now. and tidal datums are referred. A primary bench mark apsides-The points in the orbit of a planet or moon is the principal (or only) mark of a group of tidal which are the nearest and farthest from the center of bench marks to which the tide staff and tidal datums are referred. The standard tidal bench mark of the attraction. In the Earth's orbit these are called pe- ar e referred. The standard tidal bench mark of the National Ocean Service is a brass, bronze, or alu- helion and aphelion, and in the Moon's orbit, perigee National Ocean Service is a brass, bronze, or alu- minu'm alloy disk 3-�2 inches in diameter containing and apogee. The line passing through the apsides of an minum alloy disk 3- inches in diameter containing the inscription NATIONAL OCEAN SERVICE to- orbit is called the line of apsides. orbiticledthe equlibrm ardent. gether with other individual identifying information. A argument-See equilibrium argument. geodetic bench mark identifies a surveyed point in the astres fictifs-Fictitious celestial bodies which are National Geodetic Vertical Network. Most geodetic assumed to move in the celestial equator at uniform bench mark disks contain the inscription VERTICAL rates corresponding to the speeds of the several har- CONTROL MARK NATIONAL GEODETIC SUR- monic constituents of the tide producing force. Each VEY with other individual identifying information. astre fictif crosses the meridian at a time correspond- Bench mark disks of either type may, on occasion, ing to the maximum of the constituent that it repre- serve simultaneously to reference both tidal and geo- sents. detic datums. Numerous bench marks of predecessor astronomical day-See astronomical time. organizations to NOS, or parts of other organizations astronomical tide-Same as tide. absorbed into NOS, still bear the inscriptions: U.S. 2 COAST & GEODETIC SURVEY, NATIONAL named National Ocean Service. From 1965 to 1970, OCEAN SURVEY, U.S. LAKE SURVEY, CORPS the Coast and Geodetic Survey was a component of OF ENGINEERS, and U.S. ENGINEER OFFICE. the Environmental Science Services Administration Benguela Current-A South Atlantic Ocean current (ESSA). The National Ocean Survey was a component setting northward along the southwest coast of Africa. of the National Oceanic and Atmospheric Administra- bore-Same as tidal bore. tion (NOAA). NOAA became the successor to ESSA Brazil Current-A South Atlantic Ocean current in 1970. The National Ocean Service is a component setting southwestward along the central coast of South of NOAA, U.S. Department of Commerce. America. coast line-The low water datum line for purposes bubbler tide gauge-Same as gas purged pressure of the Submerged Lands Act (Public Law 31). See gauge. shoreline. coastal boundary-The mean high water line C (MHWL) or mean higher high water line (MHHWL) California Current-A North Pacific Ocean current when tidal lines are used as the coastal boundary. setting southeastward along the west coast of the Unit- Also, lines used as boundaries inland of and measured ed States and Baja Callifornia. from (or points thereon) the MHWL or MHHWL. See Callippic cycle-A period of four Metonic cycles marine boundary. equal to 76 Julian years, or 27,759 days. Devised by coastal zone (legal definition for coastal zone man- Callippus, a Greek astronomer, about 350 B.C., as a agement)-The term coastal zone means the coastal suggested improvement on the Metonic cycle for a waters (including the lands therein and thereunder) period in which new and full Moon would recur on the and the adjacent shorelands (including the waters same day of the year. Taking the length of the synodi- therein and thereunder), strongly influenced by each cal month as 29.530,588 days, there are 940 lunations and in proximity to the shorelines of the several coast- in the Callippic cycle, with about 0.25 day remaining. al states, and includes islands, transitional and inter- Canary Current--A North Atlantic Ocean current tidal areas, salt marshes, wetlands, and beaches. The setting southward off the west coast of Portugal and td rea t a es we s, t he te along the northwest coast of Africa. along thenorthwes coast o Africa.zone extends, in Great Lakes waters, to the interna- ialog shenortwe-t i ar s of ifrin tional boundary between the Unites States and Canada celestial sohere--An imaginary sphere of infinite and in other areas seaward to the outer limit of the radius concentric with the Earth, on which all celestial nd erra sea he ote la bodis ecepttheEart ar imaine to e pojeced. United States territorial sea. The zone extends inland bodies except the Earth are imagined to be projected. centibar-The unit of pressure equal to 1 ton per from the shorelines only to the extent necessary to meter per second per second. See decibar. control shorelands, the uses of which have a direct and chart datum-The datum to which soundings on a significant impact on the coastal waters. Excluded chart are referred. It is usually taken to correspond to from the coastal zone are lands the use of which is by a low-water elevation, and its depression below mean law subject solely to the discretion of or which is held sea level is represented by the symbol Z . Since 1989, in trust by the Federal Government, its officers, or chart datum has been implemented to mean lower low agents. water for all marine waters of the United States, its coastline-Same as shoreline. See coast line. territories, Commonwealth of Puerto Rico, and Trust cocurrent line-A line on a map or chart passing Territory of the Pacific Islands. See datum and Na- through places having the same current hour. tional Tidal Datum Convention of 1980. comparison of simultaneous observations-A reduc- Charybdis-Same as Galofaro. tion process in which a short series of tide or tidal chlorinity (Cl)--The number giving the chlorinity in current observations at any place is compared with grams per kilogram of a seawater sample is identical simultaneous observations at a control station where with the number giving the mass in grams of atomic tidal or tidal current constants have previously been weight silver just necessary to precipitate the halogens determined from a long series of observations. For in 0.328,523,3 kilogram of the seawater sample. tides, it is usually used to adjust constants from a S(�/..)= 1.806,55XC1(�/..) subordinate station to the equivalent of that which where S('/. .) is the salinity in parts per thousand. would be obtained from a 19-year series. See salinity. civiSda-A ean saolardninity. compass direction-Direction as indicated by com- civil day--A mean solar day commencing at mid- civilmensolardayconinghat pass without any allowances for compass error. The night. civil time-Time in which the day begins at mid- direction indicated by a compass may differ by a civil time- -Time in which the day begins at mid- cnieal mutfo reo antcdrcin night as distinguished from the former astronomical considerable amount from true or magnetic direction. time in which the day began at noon. compass error-The angular difference between a classification-See type of tide. compass direction and the corresponding true direc- Coast and Geodetic Survey-A former name of the tion. The compass error combines the effects of de- National Ocean Service. The organization was known viation and variation. as: The Survey of the Coast from its founding in 1807 component--() Same as constituent. (2) That part to 1836, Coast Survey from 1836 to 1878, Coast and of a tidal current velocity which, by resolution into Geodetic Survey from 1878 to 1970, and National orthogonal vectors, is found to act in a specified direc- Ocean Survey from 1970 to 1982. In 1982 it was tion. 3 compound tide-A harmonic tidal (or tidal current) the object is at rest relative to the Earth. The Coriolis constituent with a speed equal to the sum or difference acceleration=2v2 sin 9: where v is the speed of the of the speeds of two or more elementary constituents. object, 0 is the angular velocity of the Earth, and 4 is The presence of compound tides is usually attributed the latitude. Named for Gaspard Gustave de Coriolis to shallow water conditions. who published his formulation in 1835. constants, current-See current constants. corrected current-A relatively short series of cur- constants, harmonic-See harmonic constants. rent observations from a subordinate station to which a constants, tidal-See tidal constants. factor is applied to adjust the current to a more constituent-One of the harmonic elements in a representative value based on a relatively long series mathematical expression for the tide-producing force from a nearby control station. See current and total and in corresponding formulas for the tide or tidal current. current. Each constituent represents a periodic change cotidal hour-The average interval between the or variation in the relative positions of the Earth, Moon's transit over the meridian of Greenwich and the Moon, and Sun. A single constituent is usually written time of the following high water at any place. This in the form y=A cos (at+a), in which y is a function interval may be expressed either in solar or lunar time. of time as expressed by the symbol t and is reckoned When expressed in solar time, it is the same as the from a specific origin. The coefficient A is called the Greenwich high water interval. When expressed in amplitude of the constituent and is a measure of its lunar time, it is equal to the Greenwich high water relative importance. The angle (at+a) changes uni- interval multiplied by the factor 0.966. formly and its value at any time is called the phase of cotidal line-A line on a chart or map passing the constituent. The speed of the constituent is the through places having the same cotidal hour. rate of change in its phase and is represented by the countercurrent-A current usually setting in a direc- symbol a in the formula. The quantity a is the phase tion opposite to that of a main current. See Equatorial of the constituent at the initial instant from which the Countercurrent. time is reckoned. The period of the constituent is the current-Generally, a horizontal movement of wa- time required for the phase to change through 3600 ter. Currents may be classified as tidal and nontidal. and is the cycle of the astronomical condition repre- Tidal currents are caused by gravitational interactions sented by the constituent. between the Sun, Moon, and Earth and are part of the constituent day-The time of the rotation of the same general movement of the sea that is manifested Earth with respect to a fictitious celestial body repre- in the vertical rise and fall, called tide. Tidal currents senting one of the periodic elements in the tidal forces. are periodic with a net velocity of zero over the par- It approximates in length the lunar or solar day and ticular tidal cycle. See tidal wave. Nontidal currents corresponds to the period of a diurnal constituent or include the permanent currents in the general cir- twice the period of a semidiurnal constituent. The culatory systems of the sea as well as temporary cur- term is not applicable to the long-period constituents. rents arising from more pronounced meteorological constituent hour-One twenty-fourth part of a con- variability. Current, however, is also the British equiv- stituent day. alent of our nontidal current. See total current. control current station-A current station at which current constants-Tidal current relations that re- continuous velocity observations have been made over main practically constant for any particular locality. a minimum period of 29 days. Its purpose is to provide Current constants are classified as harmonic and non- data for computing accepted values of the harmonic harmonic. The harmonic constants consist of the am- and nonharmonic constants essential to tidal current plitudes and epochs of the harmonic constituents, and predictions and circulatory studies. The data series the nonharmonic constants include the velocities and from this station serves as the control for the reduction intervals derived directly from the current observa- of relatively short series from subordinate current sta- tions. tions through the method of comparison of simulta- current curve-A graphic representation of the flow neous observations. See current station and subordinate of the current. In the reversing type of tidal current, current station (1). the curve is referred to rectangular coordinates with control station-See primary control tide station, time represented by the abscissa and the speed of the secondary control tide station, and control current sta- current by the ordinate, the flood speeds being consid- tion. ered as positive and the ebb speeds as negative. In corange line-A line passing through places of general, the current curve for a reversing tidal current equal tidal range. approximates a cosine curve. Coriolis force-A term in the relative hydrodynamic current diagram-A graphic table showing the equations of motion that takes into account the effect speeds of the flood and ebb currents and the times of of the Earth's rotation on moving objects (including air slacks and strengths over a considerable stretch of the and water) when viewed with reference to a coordinate channel of a tidal waterway, the times being referred system attached to the rotating Earth. The horizontal to tide or tidal current phases at some reference sta- component is directed 90� to the right (when looking tion. in the direction of motion) in the Northern Hemi- current difference-Difference between the time of sphere and 90� to the left in the Southern. The hori- slack water (or minimum current) or strength of cur- zontal component is zero along the Equator; also, when rent in any locality and the time of the corresponding 4 phase of the tidal current at a reference station for relative bearing from the vessel to the pole. The bear- which predictions are given in the Tidal Current Ta- ing is then related to the ship's compass and converted bles. to true. See pelorus. current direction-Same as set. current station-The geographic location at which current ellipse-A graphic representation of a rotary current observations are conducted. Also, the facilities current in which the velocity of the current at dif- used to make current observations. These may include ferent hours of the tidal cycle is represented by radius a buoy, ground tackle, current meters, recording vectors and vectoral angles. A line joining the extrem- mechanism, and radio transmitter. See control current ities of the radius vectors will form a curve roughly station and subordinate current station (1). approximating an ellipse. The cycle is completed in Currents ABC-A subscription software program one-half tidal day or in a whole tidal day, according to providing users (with an IBM PC, AT, or compatible whether the tidal current is of the semidiurnal or the computer), on demand, access to observed currents in diurnal type. A current of the mixed type will give a graphic or tabular form. The service acquires real-time curve of two unequal loops each tidal day. current data at 10-minute intervals at several depths current hour-The mean interval between the tran- from selected stations using telephone telemetry, and sit of the Moon over the meridian of Greenwich and presents the information in a video or printed display. the time of strength of flood, modified by the times of slack water (or minimum current) and strength of ebb. D In computing the mean current hour, an average is data collection platform (DCP)-A microprocessor- obtained of the intervals for the following phases: flood based system that collects data from sensors, processes strength, slack (or minimum) before flood increased by the data, stores the data in random access memory 3.10 hours (one-fourth of tidal cycle), slack (or mini- (RAM), and provides communication links for the re- mum) after flood decreased by 3.10 hours, and ebb trieval or transmission of the data. strength increased or decreased by 6.21 hours (one-half datum (vertical)-For marine applications, a base of tidal cycle). Before taking the average, the four elevation used as a reference from which to reckon phases are made comparable by the addition or rejec- heights or depths. It is called a tidal datum when tion of such multiples of 12.42 hours as may be defined in terms of a certain phase of the tide. Tidal necessary. The current hour is usually expressed in datums are local datums and should not be extended solar time, but if lunar time is desired, the solar hour into areas which have differing hydrographic char- should be multiplied by the factor 0.966. acteristics without substantiating measurements. In or- current line-A graduated line attached to a current der that they may be recovered when needed, such pole used in measuring the velocity of the current. The datums are referenced to fixed points known as bench line is marked in such a manner that the speed of the marks. See chart datum. current, expressed in knots and tenths, is indicated Davidson Current-A North Pacific Ocean counter- directly by the length of line carried out by the cur- current setting northward between the California Cur- rent pole in a specified interval of time. When marked rent and the coasts of California, Oregon, and for a 60-second run, the principal divisions for the Washington during the winter months. whole knots are spaced at 101.33 feet and the subdivi- day-The period of rotation of the Earth. There are sions for tenths of knots are spaced at 10.13 feet. The several kinds of days depending on whether the Sun, current line is also known as a log line. Moon, or other object or location is used as the refer- current meter-An instrument for measuring the ence for the rotation. See constituent day, lunar day, speed and direction or just the speed of a current. The sidereal day, and solar day. daylight saving time--A time used during the sum- measurements are usually Eulerian, since the meter is daylight saving time-A time used during the sum- mer months, in some localities, in which clocks are most often fixed or moored at a specific location. The advanced 1 hour from the usual standard time. rotor or impeller is correlated with the speed of the decibar-The practical unit for pressure in the impinging current and the direction of flow is usually ocean, equal to 10 centibars. determined by an internal compass coupled with a declination-Angular distance north or south of the meter orienttion monitoing device.declination--Angular distance north or south of the meter orientation monitoring device celestial equator, taken as positive when north of the current pole-A pole used in observing the velocity equator and negative when south. The Sun passes of the current. The pole formerly used by the Coast through its declinational cycle once a year, reaching its and Geodetic Survey was about 3 inches in diameter maximum north declination of approximately 23-1/2 and 15 feet long, and was weighted at one end to float about June 21 and its maximum south declination of upright with the top about 1 foot out of water. Shorter approximately 23-�2� about December 21. The Moon poles were used when necessary for shallow water. In has an average declinational cycle of 27-'/3 days which use, the pole is attached to the current line but sepa- is called a tropical month. Tides or tidal currents rated from the graduated portion by an ungraded sec- occurring near the times of maximum north or south tion of approximately 100 feet, known as the stray declination of the Moon are called tropic tides or line. As the pole is carried out from an observing tropic currents, and those occurring when the Moon is vessel by the current, the amount of line passing from over the Equator are called equatorial tides or equato- the vessel during a specific time interval indicates the rial currents. The maximum declination reached by the speed of the current. The set is obtained from a Moon in successive months depends upon the longitude 5 of the Moon's node, and varies from 28-�/2 when the tracting the mean of the lower low waters from the longitude of the ascending node is 00, to 18-]/2 when mean of all the low waters. Tropic high water inequal- the longitude of the node is 1800. The node cycle, or ity (HWQ) is the average difference between the two time required for the node to complete a circuit of high waters of each tidal day at the times of tropic 3600 of longitude, is approximately 18.6 years. See tides. Tropic low water inequality (LWQ) is the aver- epoch (2). age difference between the two low waters of each declinational inequality-Same as diurnal inequality, tidal day at the times of tropic tides. Mean and tropic declinational reduction-A processing of observed inequalities, as defined above, are applicable only high and low waters or flood and ebb tidal currents to when the type of tide is either semidiurnal or mixed. obtain quantities depending upon changes in the dec- Diurnal inequality is sometimes called declinational lination of the Moon; such as tropic ranges or speeds, inequality. height or speed inequalities, and tropic intervals. ineual. density, in situ (p,,p)-Mass per unit volume. The diurnal range-Same as great diurnal range. reciprocal of specific volume. In oceanography, the diurnal tide level-A tidal datum midway between density of sea water is numerically equivalent to spe- mean higher high water and mean lower low water. cific gravity and is a function of salinity, temperature, double ebb-An ebb tidal current having two maxi- and pressure. See specific volume anomaly, ther- ma of speed separated by a smaller ebb speed. mosteric anomaly, sigma-t, and sigma-zero. double flood-A flood tidal current having two deviation (of compass)-The deflection of the needle maxima of speed separated by a smaller flood speed. of a magnetic compass due to masses of magnetic double tide-A double-headed tide, that is, a high metal within a ship on which the compass is located, water consisting of two maxima of nearly the same This deflection varies with different headings of the height separated by a relatively small depression, or a ship. The deviation is called easterly and marked plus low water consisting of two minima separated by a if the deflection is to the right of magnetic north, and relatively small elevation. Sometimes called an agger. is called westerly and marked minus if it is to the left See guider. of magnetic north. A deviation table is a tabular ar- drift (of current)-The speed of the current. rangement showing the amount of deviation for dif- drift current-Same as wind drift. ferent headings of the ship. Each compass requires a duration of flood and duration of ebb-Duration of separate deviation table. flood is the interval of time in which a tidal current is digital tide gauge-See automatic tide gauge. flooding, and duration of ebb is the interval in which it direction of current--Same as set. direction of current-Same as set. is ebbing, these intervals being reckoned from the direction of wind-Direction from which the wind is middle of the intervening slack waters or minimum middle of the intervening slack waters or minimum blowing. diurnal--Having a period or cycle of approximately currents. Together they cover, on an average, a period d~iurnal-Having-a period or cyle of approximately of 12.42 hours for a semidiurnal tidal current or a 1 tidal day. Thus, the tide is said to be diurnal when of 24 hours for a diurnal current a only one high water and one low water occur during a period of 24.84 hours for a diurnal current. In a tidal day, and the tidal current is said to be diurnal normal semidiurnal tidal current, the duration of flood when there is a single flood and a single ebb period of and duration of ebb each will be approximately equal a reversing current in the tidal day. A rotary current is to 6.21 hours, but the times may be modified greatly diurnal if it changes its direction through all points of by the presence of nontidal flow. In a river the dura- the compass once each tidal day. A diurnal constituent tion of ebb is usually longer than the duration of flood is one which has a single period in the constituent day. because of fresh water discharge, especially during The symbol for such a constituent is the subscript 1. spring months when snow and ice melt are predomi- See stationary wave theory and type of tide. nant influences. diurnal inequality-The difference in height of the duration of rise and duration of fall-Duration of two high waters or of the two low waters of each tidal rise is the interval from low water to high water, and day; also, the difference in speed between the two duration of fall is the interval from high water to low flood tidal currents or the two ebb currents of each water. Together they cover, on an average, a period of tidal day.-The difference changes with the declination 12.42 hours for a semidiurnal tide or a period of 24.84 of the Moon and, to a lesser extent, with the declina- hours for a diurnal tide. In a normal semidiurnal tide, tion of the Sun. In general, the inequality tends to duration of rise and duration of fall each will be increase with increasing declination, either north or approximately equal to 6.21 hours, but in shallow south, and to diminish as the Moon approaches the sout, an to iminsh a theMoonapprache thewaters and in rivers there is a tendency for a decrease Equator. Mean diurnal high water inequality (DHQ) is waters and in rivers there is a tendency for a decrease one-half the average difference between the two high in duration of rise and a corresponding increase in waters of each tidal day observed over the National duation of fall. Tidal Datum Epoch. It is obtained by subtracting the dynamic decimeter-See geopotential as preferred mean of all the high waters from the mean of the term. higher high waters. Mean diurnal low water inequality dynamic depth (height)-See geopotential difference (DLQ) is one-half the average difference between the as preferred term. two low waters of each tidal day observed over the dynamic depth (height) anomaly-See geopotential National Tidal Datum Epoch. It is obtained by sub- anomaly as preferred term. 6 dynamic meter (D)-The former practical unit for are significant. Named for Vagn Walfrid Ekman who, geopotential difference (dynamic depth), equal to 10 assuming a constant eddy viscosity, steady wind stress, geopotentials (dynamic decimeters). See geopotential and unlimited depth and extent, published on the ef- (dynamic depth) anomaly. fect in 1905. dynamic topography-See geopotential topography electric tape gauge-A gauge consisting of a gradu- as preferred term. ated Monel metal tape on a metal reel (with supporting frame), voltmeter, and battery. Heights can E be measured directly by unreeling the tape into its (ae)aeabore. stilling well. When contact is made with the water's eagre (eager)--Same as tidal bore. earth tide-aer iodic movement of the Earth's crust surface, the circuit is completed and the voltmeter earth tide--Periodic movement of the Earth's crust nel oe.A htmmn h egho aei needle moves. At that moment the length of tape is caused by gravitational interactions between the Sun, ed mst a n the lng a i Moon, and Earth. read against an index mark, the mark having a known Moon, and Earth. elevation relative to the bench marks. East Africa Coast Current-Same as Somali Cur- elevation relative to the bench marks. ~~~~~~~~~~~rent. ~elimination-One of the final processes in the har- East Australian Current-A South Pacific Ocean monic analysis of tides in which preliminary values for the harmonic constants of a number of constituents are current setting southward along the east coast of Aus- cleared of the residual effects of each other. tralia. East Greenland Current-A North Atlantic Ocean epoch--(1) Also known as phase lag. Angular re- tardation of the maximum of a constituent of the currenat setting southwA an t O e a tardation of the maximum of a constituent of thee ad cretalosng sthe east coast of Greenland. observed tide (or tidal current) behind the correspond- en axis-tAverage set of the current at ebb ing maximum of the same constituent of the theoreti- ebai-vrgstofterengtha ebcal equilibrium tide. It may also be defined as the ebb current (ebb)-The movement of a tidal current phase difference between a tidal constituent and its ebb current (ebb)--The movement of a tidal current away from shore or down a tidal river or estuary. In equilibrium argument. As referred to the local equilib- the mixed type of reversing tidal current, the terms rium argument, its symbol is K. When referred to the greater ebb and lesser ebb are applied respectively to corresponding Greenwich equilibrium argument, it is ebb tidal currents of greater and lesser speed each called the Greenwich epoch and is represented by G. day. The terms maximum ebb and minimum ebb are A Greenwich epoch that has been modified to adjust applied to the maximum and minimum speeds of a to a particular time meridian for convenience in the current running continuously ebb, the speed alternately prediction of tides is represented by g or by K'. The increasing and decreasing without coming to a slack or relations between these epochs may be expressed by reversing. The expression maximum ebb is also ap- the following formula: plicable to any ebb current at the time of greatest G=K+pL speed. See ebb strength. g=K'=G-aS/15 ebb interval-The interval between the transit of the in which L is the longitude of the place and S is the Moon over the meridian of a place and the time of the longitude of the time meridian, these being taken as following ebb strength. positive for west longitude and negative for east Ion- ebb strength (strength of ebb)-Phase of the ebb gitude; p is the number of constituent periods in the tidal current at the time of maximum speed. Also, the constituent day and is equal to 0 for all long-period speed at this time. See strength of current. constituents, 1 for diurnal constituents, 2 for semidiur- eccentricity of orbit-Ratio of the distance from nal constituents, and so forth; and a is the hourly center to focus of orbit to the semimajor axis. The speed of the constituent, all angular measurements eccentricity of orbit= VI/T-(7K): where A and B are being expressed in degrees. (2) As used in tidal datum respectively the semimajor and semiminor axes of the determinations, it is a 19-year cycle over which tidal orbit. height observations are meaned in order to establish ecliptic-The intersection of the plane of the the various datums. As there are periodic and apparent Earth's orbit with the celestial sphere. secular trends in sea level, a specific 19-year cycle (the eddy-A quasi-circular movement of water whose National Tidal Datum Epoch) is selected so that all area is relatively small in comparison to the current tidal datum determinations throughout the United with which it is associated. States, its territories, Commonwealth of Puerto Rico, Ekman spiral--A logarithmic spiral (when projected and Trust Territory of the Pacific Islands, will have a on a horizontal plane) formed by current velocity vec- common reference. See National Tidal Datum Epoch. tors at increasing depth intervals. The current vectors equation of time-Difference between mean and ap- become progressively smaller with depth. They spiral parent time. From the beginning of the year until near to the right (looking in the direction of flow) in the the middle of April, mean time is ahead of apparent Northern Hemisphere and to the left in the Southern time, the difference reaching a maximum of about 15 with increasing depth. Theoretically, the surface cur- minutes near the middle of February. From the middle rent vector sets 45� from the direction toward which of April to the middle of June, mean time is behind the wind is blowing. Flow opposite to the surface apparent time but the difference is less than 5 min- current occurs at the so-called "depth of frictional utes. From the middle of June to the first part of resistance." The phenomenon occurs in wind drift cur- September, mean time is again ahead of apparent time rents in which only the Coriolis and frictional forces with a maximum difference less than 7 minutes. From 7 the first of September until the later part of Decem- establishment of the port-Also known as high wa- ber, mean time is again behind apparent time, the ter, full and change (HWF&C). Average high water difference reaching a maximum of nearly 17 minutes interval on days of the new and full Moon. This in the early part of November. The equation of time interval is also sometimes called the common or vulgar for each day in the year is given in the American establishment to distinguish it from the corrected es- Ephemeris and Nautical Almanac. tablishment, the latter being the mean of all the high Equatorial Countercurrent-A current setting east- water intervals. The latter is usually 10 to 15 minutes ward between the North and South Equatorial Cur- less than the common establishment. rents of the Atlantic, Pacific, and Indian (in northern estuary-An embayment of the coast in which fresh winter) Oceans. In the Atlantic and Pacific, its axis river water entering at its head mixes with the rela- lies about latitude 70 north and in the Indian, about tively saline ocean water. When tidal action is the 70 south. dominant mixing agent it is usually termed a tidal equatorial tidal currents-Tidal currents occurring estuary. Also, the lower reaches and mouth of a river semimonthly as a result of the Moon being over the emptying directly into the sea where tidal mixing takes Equator. At these times the tendency of the Moon to place. The latter is sometimes called a river estuary. Eularian measurement-Observation of a current produce a diurnal inequality in the tidal current is at a wi a m evicenfixervation o u rrent with a device fixed relative to the flow. minimum. eustatic sea level rate--The worldwide change of sea equatorial tides-Tides occurring semimonthly as a eustatic sea level rate-The worldwide change of sea resultotheMonbengoequatoril level elevation with time. The changes are due to such result of the Moon being over the Equator. At these causes as glacial melting or formation, thermal expan- times the tendency of the Moon to produce a diurnal sion or contraction of sea water, etc. inequality in the tide is at a minimum. evection-A erturbation of the Moon deendin Equatorial Undercurrent-A subsurface current set- on the o dp ndig upon the alternate increase and decrease of the ec- ting eastward along the Equator in the Pacific, Atlan- centricity of its orbit, which is always a maximum tie, and Indian Oceans. In the Pacific, its core of when the Sun is passing the Moon's line of apsides and maximum velocity lies at a depth of about 100 meters a minimum when the Sun is at right angles to it. The within th South EuatorialCurrent.a minimum when the Sun is at right angles to it. The within the South Equatorial Current. principal constituents in the tide resulting from the equilibrium argument-The theoretical phase of a evectional inequality are v2, X2, and Pi* constituent of the equilibrium tide. It is usually repre- extreme high water-The highest elevation reached sented by the expression (V+u), in which V is a by the sea as recorded by a tide gauge during a given uniformly changing angular quantity involving mul- period. The National Ocean Service routinely docu- tiples of the hour angle of the mean Sun, the mean ments monthly and yearly extreme high waters for its longitudes of the Moon and Sun, and the mean lon- control stations. gitude of lunar or solar perigee; and u is a slowly extreme low water-The lowest elevation reached by changing angle depending upon the longitude of the the sea as recorded by a tide gauge during a given Moon's node. When pertaining to an initial instant of period. The National Ocean Service routinely docu- time, such as the beginning of a series of observations, ments monthly and yearly extreme low water for its it is expressed by (Vo+u). control stations. equilibrium theory-A model under which it is as- sumed that the waters covering the face of the Earth F instantly respond to the tide-producing forces of the Falkland Cnrrent--A South Atlantic Ocean current Moon and Sun to form a surface of equilibrium under t iad an th at ca ren setting northeastward along the east coast of Argen- the action of these forces. The model disregards fric- tina. tion, inertia, and the irregular distribution of the land nia first reduction--A method of determining high and masses of the Earth. The theoretical tide formed under low water heights, time intervals, and ranges from an low water heights, time intervals, and ranges from an these conditions is known as the equilibrium tide. arithmetic mean without adjustment to a long-term equilibrium tide-Hypothetical tide due to the tide- series through simultaneous observational comparisons. producing forces under the equilibrium theory. Also float well-A stilling well in which the float of a known as gravitational tide. float-actuated gauge operates. See stilling well. equinoctial-The celestial equator. flood axis-The average set of the tidal current at equinoctial tides-Tides occurring near the times of strength of flood. the equinoxes. flood current (flood)-The movement of a tidal cur- equinoxes-The two points in the celestial sphere rent toward the shore or up a tidal river or estuary. In where the celestial equator intersects the ecliptic; also, the mixed type of reversing current, the terms greater the times when the Sun crosses the equator at these flood and lesser flood are applied respectively to the points. The vernal equinox is the point where the Sun two flood currents of greater and lesser speed of each crosses the Equator from south to north and it occurs day. The expression maximum flood is applicable to about March 21. Celestial longitude is reckoned east- any flood current at the time of greatest speed. See ward from the vernal equinox. The autumnal equinox flood strength. is the point where the Sun crosses the Equator from flood interval-The interval between the transit of north to south and it occurs about September 23. the Moon over the meridian of a place and the time of equipotential surface-Same as geopotential surface. the following flood strength. 8 flood strength (strength of flood)-Phase of the geopotential topography-The topography of an flood tidal current at the time of maximum speed. equiscalar (usually isobaric) surface in terms of Also, the speed at this time. See strength of current. geopotential difference. As depicted on maps, isopleths Florida Current-A North Atlantic Ocean current are formed by the intersection of the isobaric surface setting northward along the south-east coast of the with a series of geopotential surfaces. Thus, the field United States. A segment of the Gulf Stream System, of isopleths represents variations in the geopotential the Florida Current extends from the Straits of Florida anomaly of the isobaric surface above a chosen refer- to the region off Cape Hatteras. ence isobaric surface (such as a level of no motion). flow-The British equivalent of the United States geostrophic flow-A solution of the relative hydro- total current. Flow is the combination of their tidal dynamic equations of motion in which it is assumed dynamic equations of motion in which it is assumed stream and current. stream and current. that the horizontal component of the Coriolis force is flushing time--The time required to remove or re- flushing me-The time required to remove or re- balanced by the horizontal component of the pressure duce (to a permissible concentration) any dissolved or suspended contaminant in an estuary or harbor. gradient force. forced wave-A wave generated and maintained by gradient flow-A solution of the relative hydrody- a continuous force. namic equations of motion in which only the horizontal Fourier series-A series proposed by the French Coriolis, pressure gradient, and centrifugal forces are mathematician Fourier about the year 1807. The series considered. involves the sines and cosines of whole multiples of a gravitational tide-Same as equilibrium tide. varying angle and is usually written in the following great diurnal range (Gt)-The difference in height form: between mean higher high water and mean lower low y=Ho+A, sin x+A2 sin 2x+A3 sin 3x water. The expression may also be used in its con- +... B. cos x+B2 cos 2x+B3 cos 3x+... tracted form, diurnal range. By taking a sufficient number of terms the series may great tropic range (Gc)-The difference in height be made to represent any periodic function of x. between tropic higher high water and tropic lower low free wave-A wave that continues to exist after the water. The expression may also be used in its con- generating force has ceased to act. tracted form, tropic range. Greenwich argument-Equilibrium argument com- G puted for the meridian of Greenwich. gage-See tide gauge. Greenwich epoch-See epoch (1). Galofaro-A whirlpool in the Strait of Messina; at Greenwich interval-An interval referred to the one time called Charybdis. transit of the Moon over the meridian of Greenwich as gas purged pressure gauge-A type of analog tide distinguished from the local interval which is referred gauge in which gas, usually nitrogen, is emitted from a to the Moon's transit over the local meridian. The submerged tube at a constant rate. Fluctuations in relation in hours between Greenwich and local inter- hydrostatic pressure due to changes in tidal height vals may be expressed by the formula: modify the emission rate for recording. Same as bub- Greenwich interval=local interval+0.069 L bier tide gauge. where L is the west longitude of the local meridian in gauge-See tide gauge. degrees. For east longitude, L is to be considered geodetic datum-See National Geodetic Vertical negative. Datum of 1929. Gregorian calendar-The modern calendar in which geopotential-The unit of geopotential difference, every year divisible by 4 (excepting century years) and equal to the gravity potential of 1 meter squared per every century year divisible by 400 are bissextile (or second squared, m2/s2, or 1 joule per kilogram, J/kg. leap) years with 366 days. All other years are common geopotential anomaly (AD)-The excess in geopoten- years with 365 days. The average length of this year tial difference over the standard geopotential differ- is, therefore, 365.242,5 days which aggrees very close- ence [at a standard specific volume at 35 parts per ly with the length of the tropical year (the period of thousand (=/ .) and 0 degrees C] between isobaric changes in the seasons). The Gregorian calendar was surfaces. See geopotential and geoopotential topogra- introduced by Pope Gregory in 1582, and immediately phy. introduced by Pope Gregory in 1582, and immediately phy. adopted by the Catholic countries in place of the Julian calendar previously in use. In making the p change it was ordered that the day following October where p is the pressure and 6, the specific volume 4, 1582, of the Julian calendar be designated October anomaly. 15, 1582, of the Gregorian calendar; the 10 days being geopotential difference-The work per unit mass dropped in order that the vernal equinox would fall on gained or required in moving a unit mass vertically March 21. The Gregorian calendar was not adopted by from one geopotential surface to another. See England until 1752, but is now in general use through- geopotential, geopotential anomaly, and geopotential out the world. topography. Guiana Current-An Atlantic Ocean current setting geopotential (equipotential) surface-A surface that northwestward along the north-east coast of South is everywhere normal to the acceleration of gravity. America. 9 Guinea Current-An Atlantic Ocean current setting harmonic reduction-Same as harmonic analysis. eastward along the west central coast of Africa. A harmonic tide plane-Same as Indian spring low continuation of the Equatorial Counter Current of the water. Atlantic Ocean. head of tide-The inland or upstream limit of water guider-Local name given to the double low water affected by the tide. For practical application in the occurring on the south coast of England. See double tabulation for computation of tidal datums, head of tide. tide is the inland or upstream point where the mean Gulf Coast Low Water Datum (GCLWD)-A tidal range becomes less than 0.2 foot. Tidal datums (except datum. Used as chart datum from November 14, 1977, for mean water level) are not computed beyond head to November 27, 1980, for the coastal waters of the of tide. Gulf coast of the United States. GCLWD is defined high tide-Same as high water. as mean lower low water when the type of tide is high water (HW)-The maximum height reached by mixed and mean low water (now mean lower low a rising tide. The high water is due to the periodic water) when the type of tide is diurnal. See National tidal forces and the effects of meteorological, hydro- Tidal Datum Convention of 1980. logic, and/or oceanographic conditions. For tidal da- Gulf Coast Low Water Datum line-The line on a tum computational purposes, the maximum height is chart or map which represents the intersection of the not considered a high water unless it contains a tidal land with the water surface at the elevation of Gulf high water. Coast Low Water Datum. high water, full and change (HWF&C)-Same as Gulf Stream-A North Atlantic Ocean current set- establishment of the port. ting northeastward off the east coast of the United high water inequality-See diurnal inequality. States. A segment of the Gulf Stream System, the high water interval (HWI)-See lunitidal interval. Gulf Stream extends from the region off Cape Hat- high water line-The intersection of the land with teras to an area southeast of the Grand Banks at about the water surface at an elevation of high water. latitude 400 north, longitude 50� west. It continues the h water rae a leton i e high water mark--A line or mark left upon tide flow of the Florida Current to the North Atlantic flow of the Florida Current to the North Atlantic flats, beach, or alongshore objects indicating the eleva- Current. ~~~~~~~~~Current. ~tion of the intrusion of high water. The mark may be a Gulf Stream System--The continuous current sys- Gulf Stream System-The continuous current sys- line of oil or scum on alongshore objects, or a more or tern composed of the Florida Current, Gulf Stream, t em composed of the Florida Current, Gulf Stream, less continuous deposit of fine shell or debris on the and North Atlantic Current. foreshore or berm. This mark is physical evidence of the general height reached by wave runup at recent _H high waters. It should not be confused with the mean h-Rate of change (as of January 1, 1900) in mean high water line or mean higher high water line. longitude of the Sun. higher high water (HHW)-The highest of the high h=0.041,068,64� per solar hour. waters (or single high water) of any specified tidal day half-tide level-A tidal datum. The arithmetic mean due to the declinational effects of the Moon and Sun. of mean high water and mean low water. Same as higher low water (HLW)-The highest of the low mean tide level. waters of any specified tidal day due to the declina- halocline-A layer in which the salinity changes tional effects of the Moon and Sun. significantly (relative to the layers above and below) Humboldt Current-Same as Peru Current. with depth. hydraulic current-A current in a channel caused harmonic analysis-The mathematical process by by a difference in the surface elevation at the two which the observed tide or tidal current at any place is ends. Such a current may be expected in a strait separated into basic harmonic constituents. connecting two bodies of water in which the tides harmonic analyzer-A machine designed for the res- differ in time or range. The current in the East River, olution of a periodic curve into its harmonic constitu- New York, connecting Long Island Sound and New ents. Now performed by electronic digital computer. York Harbor, is an example. harmonic constants-The amplitudes and epochs of hydrographic datum-A datum used for referencing the harmonic constituents of the tide or tidal current depths of water and the heights of predicted tides or at any place. water level observations. Same as chart datum. See harmonic constituent-See constituent. datum. 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 for- I mula: incremental shaft encoder-A component of a tide y =A cos at gauge for converting length to a shaft angle on a in which y is a function of time (t), A is a constant rotating disk. The position of the rotating disk is deter- coefficient, and a is the rate of change in the angle at. mined by single or dual optical or magnetic sensors to harmonic prediction-Method of predicting tides provide an electrical output. No electro-mechanical and tidal currents by combining the harmonic constitu- components or gears are used, so extremely low torque ents into a single tide curve. The work is usually is required to move the float wheel, wire, and float performed by electronic digital computer. mechanism. 10 Indian spring low water-A datum originated by isohaline-An isopleth of salinity. Constant or uni- Professor G. H. Darwin when investigating the tides of form in salinity. India. It is an elevation depressed below mean sea isopleth-A line of constant or uniform value of a level by an amount equal to the sum of the amplitudes given quantity. See isanostere, isobar, isohaline, isopyc- of the harmonic constituents M2, S2, K1, and 01. nic, and isotherm. Indian tide plane-Same as Indian spring low water. isopycnic-An isopleth of density. Constant or uni- inequality-A systematic departure from the mean form in density. value of a tidal quantity. See diurnal inequality, par- isotherm-An isopleth of temperature. allax inequality, and phase inequality. inertial flow-A solution of the relative hydrody- namic equations of motion in which only the horizontal J component of the Coriolis and centrifugal forces are J--Smaller lunar elliptic diurnal constituent. This balanced. This anticyclonic flow results from a sudden constituent, with Ml, modulates the amplitudes of the application and release of a driving force which then declinational K, for the effect of the Moon's elliptical allows the system to continue on under its own momen- orbit. turn without further interference. Speed=T+s+h-p = 15.585,443,3� per solar hour. internal tide-A tidal wave propagating along a Japan Current-Same as Kuroshio. sharp density discontinuity, such as a thermocline, or Julian calendar-A calendar introduced by Julius in an area of gradually changing (vertically) density. Caesar in the year 45 B.C., and slightly modified by International Great Lakes Datum (1955) [IGLD Augustus a few years later. This calendar provided (1955)]-Mean water level at Pointe-au-Pere, Quebec, that the common year should consist of 365 days and on the Gulf of St. Lawrence over the period 1941 that every fourth year, now known as a bissextile or through 1956, from which geopotential elevations leap year, should contain 366 days, making the aver- (geopotential differences) throughout the Great Lakes age length of the year 365.25 days. It differs from the region are measured. The term is often used to mean modern or Gregorian calendar in having every fourth the entire system of geopotential elevations rather than year a leap year, while in the modern calendar century just the referenced water level. See low water datum years not divisible by 400 are common years. See (1). Gregorian calendar. International Hydrographic Organization (formerly Julian date-Technique for the identification of suc- Bureau)-An institution consisting of representatives of cessive days of the year when monthly notation is not a number of nations organized for the purpose of a number of nations organized for the purpose of desired. This is especially applicable in computer data coordinating the hydrographic work of the participat- ing governments. It had its origin in the International Hydrographic Conference in London in 1919. It has necessary. permanent headquarters in the Principality of Monaco and is supported by funds provided by the member K nations. Its principal publications include the Hydro- K1-Lunisolar diurnal constituent. This constituent, graphic Review and special publications on technical with O0, expresses the effect of the Moon's declination. subjects. They account for diurnal inequality and, at extremes, international low water-A hydrographic datum diurnal tides. With PI it expresses the effect of the originally suggested for international use at the Inter- Sun's declination. national Hydrographic Conference in London in 1919, Speed=T+h= 15.041,068,6� per solar hour. and later discussed at the Monaco Conference in 1926. K2-Lunisolar semidiurnal constituent. This con- The proposed datum, which has not yet been generally stituent modulates the amplitude and frequency of M2 adopted, was to be "a plane so low that the tide will and S2 for the declinational effect of the Moon and but seldom fall below it." This datum was the subject of the International Hydrographic Bureau's Special Publication No. 5 (March 1925) and No. 10 (Januaryer 1926), reproduced in the Hydrographic Review for kappa ()-Name of Greek letter used as the sym- May 1925 and July 1926. bol for a constituent phase lag or epoch when referred intertidal zone-(technical definition) The zone be- to the local equilibrium argument and frequently taken tween the mean higher high water and mean lower low to mean the same as local epoch. See epoch (1). water lines. knot-A speed unit of 1 international nautical mile interval-See lunitidal interval and lunicurrent inter- (1,852.0 meters or 6,076.115,49 international feet) per val. hour. Irminger Current-A North Atlantic Ocean current Kuroshio-A North Pacific Ocean current setting setting westward off the south-west coast of Iceland. northeastward off the east coast of Taiwan and Japan isanostere-An isopleth of either specific volume from Taiwan to about latitude 35� north. anomaly or thermosteric anomaly. Kuroshio Extension-A North Pacific Ocean cur- isobar-An isopleth of pressure. rent setting eastward from about longitude 145� east isobaric surface-A surface of constant or uniform to about 160� east. It continues the flow of the pressure. Kuroshio to the North Pacific Current. 11 Kuroshio System-The current system composed of log line-A graduated line used to measure the the Kuroshio, Tsushima Current, Kuroshio Extension, speed of a vessel through the water or to measure the and North Pacific Current. velocity of the current from a vessel at anchor. See current line. L long period constituent-A tidal or tidal current constituent with a period that is independent of the L2-Smaller lunar elliptic semidiurnal constituent. rotation of the Earth but which depends upon the This constituent, with N2, modulates the amplitude orbital movement of the Moon or the Earth. The and frequency of M2 for the effect of variation in the principal lunar long period constituents have periods Moon's oribital speed due to its elliptical orbit. pr oximati ng the month and half month, and the approximating the month and half month, and the Speed=2T-s+2h-p =29.528,478,9N per solar hour. principal solar long period constituents have periods Labrador Current-A North Atlantic Ocean current approximating the year and half year. approximating the year and half year. setting southeastward along the east coasts of Baffin longitude-Angular distance in a great circle of Island, Labrador, and Newfoundland. Isl, Lr ad . reference reckoned from an accepted origin to the lagging of tide-The periodic retardation in the . . lagging of tide-The periodic retardlation in the projection of any point on that circle. Longitude on the time of occurrence of high and low water due to Earth's surface is measured on the Equator east and changes in the relative positions of the Moon and Sun. west of the meridian of Greenwich and may be ex- Lagrangian measurement-Observation of a current pressed either in degrees or in hours, the hour being with a device flowing with the current. taken as the equivalent of 15� of longitude. Celestial lambda (X2)-Smaller lunar evectional constituent. longitude is measured in the ecliptic eastward from the This constituent, with V2, uL2, and (S2), modulates the vernal equinox. The mean longitude of a celestial body amplitude and frequency of M2 for the effects of moving in an orbit is the longitude that would be variation in solar attraction of the Moon. This attrac- attained by a point moving uniformly in the circle of tion results in a slight pear-shaped lunar ellipse and a reference at the same average angular velocity as that difference in lunar orbital speed between motion to- of the body, with the inital position of the point so ward and away from the Sun. Although (S2) has the taken that its longitude would be the same as that of same speed as S2, its amplitude is extremely small. the body at a certain specified position in its orbit. Speed=2T-s+p=29.455,625,3' per solar hour. With a common initial point, the mean longitude of a leap year-A calendar year containing 366 days. body will be the same in whatever circle it may be According to the present Gregorian calendar, all years reckoned. with the date-number divisible by 4 are leap years, longshore current-A current paralleling the shore except century years. The latter are leap years when largely within the surf zone. It is caused by the excess the date-number is divisible by 400. water brought to the zone by the small net mass level of no motion-A level (or layer) at which it is transport of wind waves. Longshore currents feed into assumed that an isobaric surface coincides with a rip currents. See progressive wave. geopotential surface. A level (or layer) at which there loop of stationary wave-That portion of the os- is no horizontal pressure gradient force. cillating area where the vertical movement is greatest. level surface-See geopotential surface as preferred low tide-Same as low water. term. low water (LW)-The minimum height reached by a littoral current-A current in the littoral zone such falling tide. The low water is due to the periodic tidal as a longshore or rip current. forces and the effects of meteorological, hydrologic, littoral zone-In coastal engineering, the area from and/or oceanographic conditions. For tidal datum com- the shoreline to just beyond the breaker zone. In putational purposes, the minimum height is not consid- biological oceanography, it is that part of the benthic ered a low water unless it contains a tidal low water. division extending from the high water line out to a low water datum (LWD)-(1) The geopotential depth of about 200 meters. The littoral system is elevation (geopotential difference) for each of the divided into a eulittoral and sublittoral zone, separated Great Lakes and Lake St. Clair and the corresponding at a depth of about 50 meters. Also, frequently used sloping surfaces of the St. Marys, St. Clair, Detroit, interchangeably with intertidal zone. Niagara, and St. Lawrence Rivers to which are re- local time-Time in which noon is defined by the ferred the depths shown on the navigational charts and transit of the Sun over the local meridian as distin- the authorized depths for navigation improvement pro- guished from standard time which is based upon the jects. Elevations of these planes are referred to IGLD transit of the Sun over a standard meridian. Local (1955) and are Lake Superior-600.0 feet, Lakes time may be either mean or apparent, according to Michigan and Huron-576.8 feet, Lake St. whether reference is to the mean or actual Sun. Local Clair-571.7 feet, Lake Erie-568.6 feet, and Lake time was in general use in the United States until Ontario-242.8 feet. (2) An approximation of mean 1883, when standard time was adopted. The use of low water that has been adopted as a standard refer- local time in other parts of the world has also been ence for a limited area and is retained for an indefinite practically abandoned in favor of the more convenient period regardless of the fact that it may differ slightly standard time. from a better determination of mean low water from a 12 subsequent series of observations. Used primarily for current following the transit. Examples are strength of river and harbor engineering purposes. Boston low wa- flood interval and strength of ebb interval, which may ter datum is an example. be abbreviated to flood interval and ebb interval, re- low water equinoctial springs-Low water springs spectively. The interval is described as local or Green- near the times of the equinoxes. Expressed in terms of wich according to whether the reference is to the the harmonic constants, it is an elevation depressed Moon's transit over the local or Greenwich meridian. below mean sea level by an amount equal to the sum When not otherwise specified, the reference is assumed of the amplitudes of the constituents M2, S2, and K2. to be local. For a and b markings, see lunitidal inter- low water inequality-See diurnal inequality. val. low water interval (LWI)-See lunitidal interval. lunisolar tides-Harmonic tidal constituents K, and low water line-The intersection of the land with K2, which are derived partly from the development of the water surface at an elevation of low water. the lunar tide and partly from the solar tide, the lower high water (LHW)-The lowest of the high constituent speeds being the same in both cases. Also, waters of any specified tidal day due to the declina- the lunisolar synodic fortnightly constituent MSf. tional effects of the Moon and Sun. lunitidal interval-The interval between the Moon's lower low water (LLW)-The lowest of the low transit (upper or lower) over the local or Greenwich waters (or single low water) of any specified tidal day meridian and the following high or low water. The due to the declinational effects of the Moon and Sun. average of all high water intervals for all phases of the lower low water datum (LLWD)-An approximation Moon is known as mean high water lunitidal interval of mean lower low water that has been adopted as a and is abbreviated to high water interval (HWI). Simi- standard reference for a limited area and is retained larly, mean low water lunitidal interval is abbreviated for an indefinite period regardless of the fact that it to low water interval (LWI). The interval is described may differ slightly from a better determination of as local or Greenwich according to whether the refer- mean lower low water from a subsequent series of ence is to the transit over the local or Greenwich observations. Used primarily for river and harbor en- meridian. When not otherwise specified, the reference gineering purposes. Columbia River lower low water is assumed to be local. datum is an example. When there is considerable diurnal inequality in the lunar cycle-An ambiguous expression which has tide, separate intervals may be obtained for the higher been applied to various cycles associated with the high waters, lower high waters, higher low waters, and Moon's motion. See Callippic cycle, Metonic cycle, lower low waters. These are designated respectively as node cycle, and synodical month. higher high water interval (HHWI), lower high water lunar day-The time of the rotation of the Earth interval (LHWI), higher low water interval (HLWI), with respect to the Moon, or the interval between two and lower low water interval (LLWI). In such cases, successive upper transits of the Moon over the merid- and also when the tide is diurnal, it is necessary to ian of a place. The mean lunar day is approximately distinguish between the upper and lower transit of the 24.84 solar hours in length, or 1.035 times as great as Moon with reference to its declination. Intervals re- the mean solar day. ferred to the Moon's upper transit at the time of its lunar interval-The difference in time between the north declination or the lower transit at the time of transit of the Moon over the meridian of Greenwich south declination are marked a. Intervals referred to and a local meridian. The average value of this inter- the Moon's lower transit at the time of its north val, expressed in hours, is 0.069 L, where L is the declination or to the upper transit at the time of south local longitude in degrees, positive for west longitude declination are marked b. and negative for east. The lunar interval equals the difference between the local and Greenwich interval of M a tide or current phase. MI-Smaller lunar elliptic diurnal constituent. This lunar month-Same as synodical month. constituent, with J1, modulates the amplitude of the lunar nodes-The points where the plane of the declinational K1 for the effect of the Moon's elliptical Moon's orbit intersects the ecliptic. The point where orbit. A slightly slower constituent, designated (M1), the Moon crosses in going from south to north is called with Q., modulates the amplitude and frequency of the the ascending node and the point where the crossing is declinational 01 for the same effect. from north to south is called the descending node. Speed=T-s+h+p =14.496,693,9� per solar hour. References are usually made to the ascending node M2-Principal lunar semidiurnal constituent. This which, for brevity, may be called the node. constituent represents the rotation of the Earth with lunar tide-That part of the tide on the Earth due respect to the Moon. solely to the Moon as distinguished from that part due Speed =2T-2s+2h=28.984,104,2� per solar hour. to the Sun. M3-Lunar terdiurnal constituefit. A shallow water lunar time-Time based upon the rotation of the compound constituent. See shallow water constituent. Earth relative to the Moon. See lunar day. Speed=3T-3s+3h=43.476,156,3� per solar hour. lunation-Same as synodical month. M4, M6, M6-Shallow water overtides of principal lunicurrent interval-The interval between the lunar constituent. See shallow water constituent. Moon's transit (upper or lower) over the local or Speed of M4=2M2=4T-4s+4h Greenwich meridian and a specified phase of the tidal =57.968,208,4� per solar hour. 13 Speed of M6=3M2=6T-6s+6h time of spring tides observed over the National Tidal =86.952,312,7� per solar hour. Datum Epoch. It is usually derived by taking an eleva- Speed of M8=4M2 =8T-8s+8h tion depressed below the half-tide level by an amount = 115.936,416,9� per solar hour. equal to one-half the spring range of tide, necessary Maelstrom-Famous whirlpool off the coast of Nor- corrections being applied to reduce the result to a way in the Lofoten Islands between Moskenesoy and mean value. This datum is used, to a considerable Mosken. magnti aimthAzmuh eckoned frextent, for hydrographic work outside of the United magnetic azimuth-zimuth reckoned from the States and is the level of reference for the Pacific magnetic north or magnetic south. See magnetic direc- approaches to the Panama Canal. tion. ~~~~~~~~~~~approaches to the Panama Canal. tion. magnetic declination-Same as variation. mean lower low water (MLLW)-A tidal datum. magnetic declination--Same as variation. magnetic direction-Direction as indicated by a The average of the lower low water height of each magnetic compass after correction for deviation but tidal day observed over the National Tidal Datum without correction for variation. Epoch. For stations with shorter series, simultaneous marigram-A graphic record of the rise and fall of observational comparisons are made with a control tide the water. The record is in the form of a curve in station in order to derive the equivalent datum of the which time is generally represented on the abscissa National Tidal Datum Epoch. and the height of the tide on the ordinate. See tide mean lower low water line (MLLWL)-The line on curve. a chart or map which represents the intersection of the marine boundary-The mean lower low water line land with the water surface at the elevation of mean (MLLWL) when used as a boundary. Also, lines used lower low water. as boundaries seaward of and measured from (or as boundaries seaward of and measured from (or mean range of tide (Mn)-The difference in height points thereon) the MLLWL. See coastal boundary. between mean high water and mean low water. mascaret-French for tidal bore. m ean risbetween mean high water and mean low water. mascaret--French for tidal bore. mean current hour--Same as current hour. mean rise-rThe height of mean high water above mean current hour-Same as current hour.thelvioofcatdum the elevation of chart datum. mean diurnal tide level (MDTL)-A tidal datum. The arithmetic mean of mean higher high water and mean rise interval (MRI)-The average interval be- mean lower low water. tween the transit of the Moon and the middle of the mean high water (MHW)-A tidal datum. The aver- period of the rise of the tide. It may be computed by age of all the high water heights observed over the adding half the duration of rise to the mean low water National Tidal Datum Epoch. For stations with shorter interval, rejecting the semidiurnal tidal period of 12.42 series, simultaneous observational comparisons are hours when greater than this amount. The mean rise made with a control tide station in order to derive the interval may be either local or Greenwich according to equivalent datum of the National Tidal Datum Epoch. whether it is referred to the local or Greenwich transit. mean high water line (MHWL)-The line on a chart mean river level-A tidal datum. The average height or map which represents the intersection of the land of the surface of a tidal river at any point for all with the water surface at the elevation of mean high stages of the tide observed over the National Tidal water. See shoreline. meanhig hwater. (A stidl . Datum Epoch. It is usually determined from hourly mean higher high water (IMHHW)--A tidal datum. Tmeave higher high water htidt oeat. height readings. In rivers subject to occasional fresh- The average of the higher high water height of each tidal day observed over the National Tidal Datum ets, the river level may undergo wide variations and, tidal day observed over the National Tidal Datum frpatclproecranmnh fteya a Epoch. For stations with shorter series, simultaneous for practical purposes, certain months of the year may observational comparisons are made with a control tide be excluded in the determination of the tidal datum. station in order to derive the equivalent datum of the For charting purposes, tidal datums for rivers are usu- National Tidal Datum Epoch. ally based on observations during selected periods mean higher high water line (MHHWL)-The line when the river is at or near a low water stage. on a chart or map which represents the intersection of mean sea level (MSL)-A tidal datum. The the land with the water surface at the elevation of arithmetic mean of hourly heights observed over the mean higher high water. National Tidal Datum Epoch. Shorter series are speci- mean low water (MLW)-A tidal datum. The aver- fled in the name; e.g., monthly mean sea level and age of all the low water heights observed over the earl mean sea level. National Tidal Datum Epoch. For stations with shorter mean sun-A fictitious sun which is assumed to mean sun--A fictitious sun which is assumed to series, simultaneous observational comparisons are move in the celestial equator at a uniform speed cor- made with a control tide station in order to derive the o in the ae ra au r speed or responding to the average angular speed of the real equivalent datum of the National Tidal Datum Epoch. mean low wqater line (MWL)-The line ona a mchart Sun in the ecliptic, the mean sun being alternately in mean low water line ( M LWL)--The line on a chart avneadbhn h elSn ti sda or map which represents the intersection of the land advance and behind the real Sun. It is used as a with the water surface at the elevation of mean low reference for reckoning mean time, noon of mean local water. time corresponding to the time of the transit of the mean low water springs (MLWS)-A tidal datum. mean sun over the local meridian. See equation of Frequently abbreviated spring low water. The arith- time and mean time. metic mean of the low water heights occurring at the mean tide level (MTL)-Same as half-tide level. 14 mean time-Time based upon the hour angle of the Speed=2s-2h= 1.015,895,8� per solar hour. mean sun as distinguished from apparent time which is mu (#2)-Variational constituent. See lambda. based upon the position of the real Sun. The dif- Speed=2T-4s+4h =27.968,208,4� per solar hour. ference between apparent and mean time is known as multiple tide staff-A succession of tide staffs on a the equation of time. sloping shore so placed that the vertical graduations on mean water level (MWL)-A tidal datum. The mean the several staffs will form a continuous scale referred surface elevation as determined by averaging the to the same datum. heights of the water at equal intervals of time, usually hourly, over the National Tidal Datum Epoch. Mean N water level is used in areas of little or no range in tide. N-Rate of change (as of January 1, 1900) in mean mean water level line (MWLL)-The line on a chart longitude of the Moon's node. or map which represents the intersection of the land N=-0.002,206,41 per solar hour. with the water surface at the elevation of mean water N2-Larger lunar elliptic semidiurnal constituent. level. See L2. meteorological tides-Tidal constituents having their Speed=2T-3s+2h +p=28.439,729,5' per solar origin in the daily or seasonal variations in weather hour. conditions which may occur with some degree of pe- 2N2-Lunar elliptic semidiurnal second-order con- riodicity. The principal meteorological constituents rec- stituent. ognized in the tides are Sa, Ssa, and S,. See storm Speed=2T-4s+2h+2p =27.895,354,8� per solar surge. hour. Metonic cycle-A period of almost 19 years or 235 National Geodetic Vertical Datum of 1929 (NGVD lunations. Devised by Meton, an Athenian astronomer (1929)1-A fixed reference adopted as a standard geo- who lived in the fifth century B.C., for the purpose of detic datum for elevations determined by leveling. The obtaining a period in which new and full Moon would datum was derived for surveys from a general adjust- recur on the same day of the year. Taking the Julian ment of the first-order leveling nets of both the United year of 365.25 days and the synodic month as States and Canada. In the adjustment, mean sea level 29.530,588 days, we have the 19-year period of was held fixed as observed at 21 tide stations in the 6,939.75 days as compared with the 235 lunations of United States and 5 in Canada. The geodetic datum 6,939.69 days, a difference of only 0.06 day. now in use in the United States is the National Geo- Mf-Lunar fortnightly constituent. This constituent detic Vertical Datum. The year indicates the time of expresses the effect of departure from a sinusoidal the general adjustment. A synonym for Sea-level Da- declinational motion. tum of 1929. The geodetic datum is fixed and does not Speed=2s= 1.098,033,1� per solar hour. take into account the changing stands of sea level. midextreme tide-An elevation midway between ex- Because there are many variables affecting sea level, treme high water and extreme low water occurring in and because the geodetic datum represents a best fit any locality. over a broad area, the relationship between the geo- mixed (current)-Type of tidal current characterized detic datum and local mean sea level is not consistent by a conspicuous diunal inequality in the greater and from one location to another in either time or space. lesser flood strengths and/or greater and lesser ebb For this reason, the National Geodetic Vertical Datum strengths. See flood current and ebb current. should not be confused with mean sea level. mixed (tide)-Type of tide characterized by a con- National Tidal Datum Convention of 1980-Effec- spicuous diurnal inequality in the higher high and tive November 28, 1980, the Convention: (1) estab- lower high waters and/or higher low and lower low lishes one uniform, continuous tidal datum system for waters. See type of tide. all marine waters of the United States, its territories, Mm-Lunar monthly constituent. This constituent Commonwealth of Puerto Rico, and Trust Territory of expresses the effect of irregularities in the Moon's rate the Pacific Islands, for the first time in its history; (2) of change of distance and speed in orbit. provides a tidal datum system independent of com- Speed=s-p=0.544,374,7� per solar hour. putations based on type of tide; (3) lowers chart datum Monsoon Current (Southwest Monsoon Cur- from mean low water to mean lower low water along rent)-An Indian Ocean current setting in a generally the Atlantic coast of the United States; (4) updates eastward to southeastward direction off India and Cey- the National Tidal Datum Epoch from 1941 through lon. It replaces the North Equatorial Current, reversed 1959, to 1960 through 1978; (5) changes the name by wind stress of the south-west monsoons, in August Gulf Coast Low Water Datum to mean lower low and September. water; (6) introduces the tidal datum of mean higher month-The period of the revolution of the Moon high water in areas of predominantly diurnal tides; and around the Earth. The month is designated as siderial, (7) lowers mean high water in areas of predominantly tropical, anomalistic, nodical, or synodical according to diurnal tides. See chart datum. whether the revolution is relative to a fixed star, vernal National Tidal Datum Epoch-The specific 19-year equinox, perigee, ascending node, or Sun. The calendar period adopted by the National Ocean Service as the month is a rough approximation to the synodical official time segment over which tide observations are month. taken and reduced to obtain mean values (e.g., mean MSf-Lunisolar synodic fortnightly constituent. lower low water, etc.) for tidal datums. It is necessary 15 for standardization because of periodic and apparent node-See lunar nodes. secular trends in sea level. The present National Tidal node cycle-Period of approximately 18.61 Julian Datum Epoch is 1960 through 1978. It is reviewed years required for the regression of the Moon's nodes annually for possible revision and must be actively to complete a circuit of 360� of longitude. It is accom- considered for revision every 25 years. panied by a corresponding cycle of changing inclina- National Water Level Observation Network tion of the Moon's orbit relative to the plane of the (NWLON)-The network of tide and water level sta- Earth's Equator, with resulting inequalities in the rise tions operated by the National Ocean Service along and fall of the tide and speed of the tidal current. the marine and Great Lakes coasts and islands of the node factor (f)-A factor depending upon the Ion- United States. gitude of the Moon's node which, when applied to the The NWLON is composed of the primary and sec- mean coefficient of a tidal constituent, will adapt the ondary control tide stations of the National Ocean same to a particular year for which predictions are to Service. Distributed along the coasts of the United be made. States, this Network provides the basic tidal datums nodical month-Average period of the revolution of for coastal and marine boundaries and for chart datum the Moon around the Earth with respect to the Moon's the Moon around the Earth with respect to the Moon's of the United States. Tide observations at a secondary ascending node. It is approximately 27.212,220 days in control tide station or tertiary tide station are reduced length. to equivalent 19-year tidal datums through the com- nonharmonic constants-Tidal constants such as nonharmonic constants--Tidal constants such as parison of simultaneous observations with a primary lunitidal intervals, ranges, and inequalities which may lunitidal intervals, ranges, and inequalities which may control tide station. In addition to hydrography and be derived directly from high and low water observa- be derived directly from high and low water observa- nautical charting, and to coastal and marine bound- tions without regard to the harmonic constituents of tions without regard to the harmonic constituents of aries, the Network is used for coastal processes and the tide. Also applicable to tidal currents. tectonic studies, tsunami and storm surge warnings, nontid al current-See current . nontidal current--See current. and climate monitoring. The National Water Level Observation Network normal dde-A nontechnical term synonymous with also includes stations operated throughout the Great tide; i.e., the rise and fall of the ocean due to th Lakes Basin. The primary network is composed of 54 Lakes Basin. The primary network is composed of 54 gravitational interactions of the Sun, Moon, and Earth sites with 139 seasonal gauge sites selectively operated alone. Use of this term is discouraged. 4 months annually for the maintenance of IGLD. The North Atlantic Current-A North Atlantic Ocean network supports regulation, navigation and charting, current setting northeastward from southeast of the river and harbor improvement, power generation, var- Grand Banks at about latitude 40 north, longitude ious scientific activities, and the adjustment for verti- 50 west, to the British Isles. A segment of the Gulf cal movement of the Earth's crust in the Great Lakes Stream System, the North Atlantic Current continues Basin. the flow of the Gulf Stream to the Norwegian and naviface-The interface between atmosphere and Canary Currents. ocean; air-sea interface; sea surface. Adjective, navi- North Cape Current-An Arctic Ocean current set- facial. ting eastward off the north coast of Scandanavia in the neap range-See neap tides. Barrents Sea. neap tides or tidal currents-Tides of decreased North Equatorial Current-A current setting west- range or tidal currents of decreased speed occurring ward in the North Atlantic and North Pacific Oceans, semimonthly as the result of the Moon being in and in the Indian Ocean from about October to July. quadrature. The neap range (Np) of the tide is the It occurs immediately north of the Equatorial Counter average range occurring at the time of neap tides and Current. is most conveniently computed from the harmonic con- North Pacific Current-A North Pacific Ocean cur- stants. It is smaller than the mean range where the rent setting eastward from about 160� east to some- type of tide is either semidiurnal or mixed and is of no what beyond about 1500 west. It continues the flow of practical significance where the type of tide is pre- the Kuroshio Extension, sending branches to the south. dominantly diurnal. The average height of the high Norwegian Current-A North Atlantic Ocean cur- waters of the neap tide is called neap high water or rent setting northeastward off the coast of Norway. high water neaps (MHWN) and the average height of nU (v2)-Larger lunar evectional constituent. See the corresponding low waters is called neap low water lambda. or low water neaps (MLWN). Speed=2T-3s+4h-p =28.512,583,10 per solar Next Generation Water Level Measurement System hour. (NGWLMS)-A fully integrated system encompassing new technology sensors and recording equipment, mul- tiple data transmission options, and an integrated data O processing, analysis, and dissemination subsystem. 0--Lunar diurnal constituent. See K,. nodal line-A line in an oscillating body of water Speed=T-2s+h=13.943,035,6� per solar hour. along which there is a minimum or no rise and fall of obliquity factor-A factor in an expression for a the tide. constituent tide (or tidal current) involving the angle nodal point-The no-tide point in an amphidromic of the inclination of the Moon's orbit to the plane of region. the Earth's Equator. 16 obliquity of the ecliptic-The angle which the eclip- Earth's radius divided by the distance of the celestial tic makes with the plane of the Earth's Equator. Its body, or, since the sine of a small angle is approxi- value is approximately 23.45�. mately equal to the angle itself in radians, it is usually obliquity of the Moon's orbit-The angle which the taken in tidal work simply as the ratio of the mean Moon's orbit makes with the plane of the Earth's radius of the Earth to the distance of the tide-produc- Equator. Its value varies from 18.3� to 28.6�, depend- ing body. Since the parallax is a function of the ing upon the longitude of the Moon's ascending node; distance of a celestial body, the term is applied to the smaller value corresponding to a longitude of 180� tidal inequalities arising from the changing distance of and the larger one, to a longitude of 0�. the tide-producing body. oceanography-Oceanography is the science of all parallax inequality-The variation in the range of aspects of the oceans, in spite of its etymology. The tide or in the speed of a tidal current due to changes term, oceanography, however, implies the interrelation- in the distance of the Moon from the Earth. The range ships of the various marine sciences of which it is of tide and speed of the current tend alternately to composed. This connotation has arisen through the increase and decrease as the Moon approaches its historical development of marine research in which it perigee and apogee, respectively, the complete cycle has been found that a true understanding of the oceans being the anomalistic month. There is a similar but is best achieved through investigations based on the relatively unimportant inequality due to the Sun, the realization that water, its organic and inorganic con- cycle being the anomalistic year. The parallax has tents, motions, and boundaries are mutally related and little direct effect upon the lunitidal intervals but little direct effect upon the lunitidal intervals but interdependent. 001-Lunar diurnal, second-order, constituent. tends to modify the phase effect. When the Moon is in Speed=T+2s+h=16.139,101,7 s per solar hour. perigee, the priming and lagging of the tide due to the ordinary-With respect to tides, the use of this phase is diminished and when in apogee the priming nontechnical word has, for the most part, been deter- and lagging is increased. mined to be synonymous with mean. Thus, ordinary parallax reduction-A processing of observed high high (low) water is the equivalent of mean high (low) and low waters to obtain quantities depending upon water. The use of ordinary in tidal terms is discour- changes in the distance of the Moon, such as perigean aged. and apogean ranges. orifice-See stilling well and protective well. parallel plate intake-Intake of a stilling or protec- overfalls-Breaking waves caused by the meeting of tive well with two parallel plates attached below. The currents or by waves moving against the current. See plates are typically three times the diameter of the rip. well and are spaced three inches apart. The plates are overtide-A harmonic tidal (or tidal current) con- used to minimize current-induced draw-down (Ber- stituent with a speed that is an exact multiple of the noulli effect) error in water level measurements. speed of one of the fundamental constituents derived pelorus-An instrument used on a vessel in connec- from the development of the tide-producing force. The tion with a current line and current pole to obtain the presence of overtides is usually attributed to shallow set of the current. In its simplest form, it is a disk water conditions. The overtides usually considered in about 8 inches in diameter and graduated clockwise tidal work are the harmonics of the principal lunar and for every 5� or 10�. It is mounted rigidly on the solar semidiurnal constituents M2 and S2, and are des- vessel, usually with the 0� mark forward and the ignated by the symbols M4, M6, MS, S4, S6, etc. The diameter through this mark parallel with the keel. magnitudes of these harmonics relative to those of the Bearings are then related to the vessel's compass and fundamental constituents are usually greater in the converted to true. tidal current than in the tide. perigean tides or tidal currents-Tides of increased Oyashio-A current setting southwestward along range or tidal currents of increased speed occurring the Siberian, Kamchatka, and Kuril Islands coasts in monthly as the result of the Moon being in perigee. the Bering Sea and North Pacific Ocean. The perigean range (Pn) of tide is the average range occurring at the time of perigean tides and is most p conveniently computed from the harmonic constants. It p-Rate of change (as of January 1, 1900) in mean is larger than the mean range where the type of tide is longitude of lunar perigee. either semidiurnal or mixed, and is of no practical p=0.004,641,83� per solar hour. significance where the type of tide is predominantly pi-Rate of change (as of January 1, 1900) in mean diurnal. longitude of solar perigee. perigee-The point in the orbit of the Moon or p,=0.000,001,96� per solar hour. man-made satellite nearest to the Earth. The point in Pi-Solar diurnal constituent. See K,. the orbit of a satellite nearest to its companion body. Speed=T-h=14.958,931,4� per solar hour. perihelion-The point in the orbit of the Earth (or parallax-In tidal work, the term refers to horizon- other planet, etc.) nearest to the Sun. tal parallax, which is the angle formed at the center of period-Interval required for the completion of a a celestial body between a line to the center of the recurring event, such as the revolution of a celestial Earth and a line tangent to the Earth's surface. It may body or the time between two consecutive like phases also be expressed as an angle whose sine equals the of the tide or tidal current. A period may be expressed 17 in angular measure and is then taken as 3600. The absolute transducer measures the pressure at its loca- word also is used to express any specified duration of tion. The readings are then corrected for barometric time. pressure taken at the surface. permanent current-A current that runs fairly con- primary control tide station-A tide station at tinuously and is independent of tides and other tem- which continuous observations have been made over a porary causes. Permanent currents include the general minimum of 19 years. Its purpose is to provide data surface circulation of the oceans. for computing accepted values of the harmonic and Peru Current-A South Pacific Ocean current set- nonharmonic constants essential to tide predictions and ting northward along the west coast of South America. to the determination of tidal datums for charting and It has sometimes been called the Humboldt Current for coastal and marine boundaries. The data series because an early record of its temperature was taken from this station serves as a primary control for the by the German scientist Alexander von Humboldt in reduction of relatively short series from subordinate 1802. It has also been called the Peruvian or Chilean tide stations through the method of comparison of tide stations through the method of comparison of Current. The name Corriente de Peru was adopted by Current. T he name Corriente de Peru was adopted by simultaneous observations and for monitoring long-pe- a resolution of the bero-American Oceanographic riod sea level trends and variations. See tide station, 1935. Conference at its Madrid-Malaga meeting in April secondary control tide station, tertiary tide station, and phase--() Any recurring aspect of a periodic phe- subordinate tide station (1). nomenon, such as new Moon, high water, flood primary tidal bench mark-See bench mark. strength, etc. (2) A particular instant of a periodic priming of tide-The periodic acceleration in the function expressed in angular measure and reckoned time of occurrence of high and low waters due to from the time of its maximum value, the entire period changes in the relative positions of the Sun and Moon. of the function being taken as 360'. The maximum progressive wave-A wave that advances in distance and minimum of a harmonic constituent have phase along the sea surface or at some intermediate depth. values of 00 and 180�, respectively. Although the wave form itself travels significant dis- phase inequality-Variations in the tides or tidal tances, the water particles that make up the wave currents due to changes in the phase of the Moon. At merely describe circular (in relatively deep water) or the times of new and full Moon the tide-producing elliptical (in relatively shallow water) orbits. With forces of the Moon and Sun act in conjunction, caus- high, steep, wind waves, a small overlap in the orbital ing the range of tide and speed of the tidal current to motion becomes significant. This overlapping gives rise be greater than the average, the tides at these times to a small net mass transport. See longshore current being known as spring tides. At the times of the and rip current. Progressive waves can be internal, quadratures of the Moon these forces are opposed to traveling along a sharp density discontinuity, such as each other, causing neap tides with diminished range the thermocline, or in an area of gradually changing and current speed. density (vertically). phase lag--Same as epoch (1). phase lag-Samen- epocesih ad protective well-A vertical pipe with a relatively phase reduction--A processing of observed high and phas redctin-A rocssin ofobsevedhighandlarge opening (intake) in the bottom. It is used with low waters to obtain quantities depending upon the large opening (intake) in the bottom. It is used with low wtersto otainquanitie depndin upo thethe air acoustic ranging sensor and electronic process- phase of the Moon, such as the spring and neap ranges the air acoustic ranging sensor and electronic process- of tide. At a former time this process was known as ig (filtering) technique to m ze the nonlinear second reduction. Also applicable to tidal currents. characteristics of the stilling well. Its purpose is also to poror6ca-Brazilian for tidal bore. shield the sensing element from physical damage and potential, tide-producing-Tendency for particles on harsh environment. Unlike a stilling well, damping of the Earth to change their positions as a result of the high frequency waves is not a critical requirement. See gravitational interactions between the Sun, Moon, and stilling well. Earth. Although the gravitational attraction varies in- pycnocline-A layer in which the density increases versely as the square of the distance of the tide- significantly (relative to the layers above and below) producing body, the resulting potential varies inversely with depth. as the cube of the distance. predicting machine-See tide predicting machine. pressure gauge-A tide gauge that is operated by the change in pressure at the bottom of a body of Q1-Larger lunar elliptic diurnal constituent. See water due to the rise and fall of the tide. See gas M1. purged pressure gauge. Speed=T-3s+h+p =13.398,660,9� per solar hour. pressure gradient force, horizontal-The horizontal 2Qi-Lunar elliptic diurnal, second order, constitu- component of the product of the specific volume and ent. the rate of decrease in pressure with distance. Speed=T-4s+h+2p =12.854,286,2' per solar hour. pressure sensor-A pressure transducer sensing de- quadrature of Moon-Position of the Moon when its vice for water level measurement. A relative trans- longitude differs by 900 from the longitude of the ducer is vented to the atmosphere and pressure Sun. The corresponding phases are known as first readings are made relative to atmospheric pressure. An quarter and last quarter. 18 R Z(f)= fOO W(7)e2-if"dr =R(f)ei'o) R2-Smaller solar elliptic constituent. This constitu- ent, with T2, modulates the amplitude and frequency is the system's admittance (coherent output/input) at of S2 for the effect of variation in the Earth's orbital frequency f. In practice, the integrals are replaced by speed due to its elliptical orbit. summations; Xi, W, and Z are generally complex. The Speed=2T+h-p,= 30.041,066,7� per solar hour. discrete set of W values are termed response weights; race-A very rapid current through a comparatively Xo(t) is ordinarily an observed tidal time series and narrow channel. Xi(t) the tide potential or the tide at some nearby radiational tides-Periodic variations in sea level place. A future prediction can be prepared by applying primarily related to meteorological changes such as the the weights to an appropriate Xi(t) series. In general: semidaily (solar) cycle in barometric pressure, daily I 7 I=R(f) and Avg(z) =(f) (solar) land and sea breezes, and seasonal (annual) measure the relative magnification and phase lead of changes in temperature. Other changes in sea level due the station at frequency f. to meteorological changes that are random in phase reversing current-A tidal current which flows al- are not considered radiational tides. ternately in approximately opposite directions with a range of tide-The difference in height between slack water at each reversal of direction. Currents of range of tide--The difference in height between this type usually occur in rivers and straits where the consecutiver heigh and low waters. The mean range is direction of flow is more or less restricted to certain the difference in height between mean high water and channels. When the movement is towards the shore or channels. When the movement is towards the shore or mean low water. The great diurnal range or diurnal up a stream, the current is said to be flooding, and range is the difference in height between mean higher when in the opposite dire nt is said to be ebbing. when in the opposite direction, it is said to be ebbing. high water and mean lower low water. For other The combined flood and ebb movement (including the ranges see spring, neap, perigean, apogean, and tropic slack water) covers, on an average, 12.42 hours for a tides; and tropic ranges. semidiurnal current. If unaffected by a nontidal flow, real-time-Pertains to a data collecting system that the flood and ebb movements will each last about 6 controls an on-going process and delivers its outputs hours, but when combined with such a flow, the dura- (or controls its inputs) not later than the time when tions of flood and ebb may be quite different. During recthese arcurre needed for effectiver cntrol. the flow in each direction the speed of the current will reductilinear cur(-Reciprocal of node factor rent. vary from zero at the time of slack water to a maxi- reduction factor (F)--Reciprocal of node factor (f). mum about midway between the slacks reduction of tides or tidal currents-A processing of reversing falls-A name applied to falls which flow observed tide or tidal current data to obtain mean alternately in opposite directions in a narrow channel values for tidal or tidal current constants. in the St. John River above the city of St. John, New reference station-A tide or current station for Brunswick, Canada, the phenomenon being due to the which independent daily predictions are given in the large range of tide and a constriction in the river. The "Tide Tables" and "Tidal Current Tables," and from direction of flow is upstream or downstream according which corresponding predictions are obtained for sub- to whether it is high or low water on the outside, the ordinate stations by means of differences and ratios. falls disappearing at the half-tide level. See subordinate tide station (2) and subordinate cur- rho (pl)-Larger lunar evectional diurnal constitu- rent station (2). ent. relative mean sea level change-A local change in Speed=T-3s+3h-p =13.471,514,5� per solar hour. mean sea level relative to a network of bench marks rip-Agitation of water caused by the meeting of established in the most stable and permanent material currents or by a rapid current setting over an irregular available (bedrock, if possible) on the land adjacent to bottom. Termed tide rip when a tidal current is in- the tide station location. A change in relative mean sea volved. See overfalls. level may be composed of both an absolute mean sea rip current-A narrow intense current setting sea- level change component and a vertical land movement ward through the surf zone. It removes the excess change component, together. water brought to the zone by the small net mass remote acoustic Doppler sensing (RADS) current me- transport of waves. It is fed by longshore currents. Rip ter system-The name used by the Estuarine and currents usually occur at points, groins, jetties, etc., of Ocean Physics Branch of the National Ocean Service, irregular beaches, and at regular intervals along NOAA for the acoustic Doppler current profiler and straight, uninterrupted beaches. its associated instrumentation and data transmission river estuary-See estuary. equipment. See acoustic Doppler current profiler rotary current-A tidal current that flows contin- (ADCP). ually with the direction of flow changing through all response analysis-For any linear system, an input points of the compass during the tidal period. Rotary function Xi(t) and an output function Xo(t) can be currents are usually found offshore where the direction related according to the formula: of flow is not restricted by any barriers. The tendency Xo(t)= fcoXi (t-r)W(r)dTr+noise(t) for the rotation in direction has its origin in the Cor- iolis force and, unless modified by local conditions, the where W(r) is the impulse response of the system and change is clockwise in the Northern Hemisphere and its Fourier transform: counterclockwise in the Southern. The speed of the 19 current usually varies throughout the tidal cycle, pass- secular trend-See apparent secular trend as pre- ing through the two maxima in approximately opposite ferred term. directions and the two minima with the direction of seiche-A stationary wave usually caused by strong the current at approximately 90� from the directions winds and/or changes in barometric pressure. It is of the maxima. found in lakes, semienclosed bodies of water, and in areas of the open ocean. The period of a seiche in an S enclosed rectangular body of water is usually repre- s-Rate of change (as of January 1, 1900) in mean sented by the formula: longitude of Moon. Period (T)=2L/ gvd s=0.549,016,53' per solar hour. in which L is the length, d the average depth of the SI-Solar diurnal constituent. body of water, and g the acceleration of gravity. See Speed=T= 15.000,000,0" per solar hour. standing wave. S2-Principal solar semidiurnal constituent. This seismic sea wave-Same as tsunami. constituent represents the rotation of the Earth with semidiurnal-Having a period or cycle of approxi- respect to the Sun. mately one-half of a tidal day. The predominant type Speed=2T=30.000,000,0� per solar hour. of tide throughout the world is semidiurnal, with two S4, S6-Shallow water overtides of the principal high waters and two low waters each tidal day. The solar constituent. tidal current is said to be semidiurnal when there are Speed of S4=2S2=4T =60.000,000,0� per solar hour. two flood and two ebb periods each day. A semidiur- Speed of S6=3S2=6T =90.000,000,0� per solar hour. nal constituent has two maxima and two minima each Sa-Solar annual constituent. This constituent, with constituent day, and its symbol is the subscript 2. See Ssa, accounts for the nonuniform changes in the Sun's type of tide. declination and distance. In actuality, they mostly re- sequence of current-The order of occurrence of the flect yearly meteorological variations influencing sea four tidal current strengths of a day, with special level. reference as to whether the greater flood immediately Speed=h=0.041,068,64' per solar hour. precedes or follows the greater ebb. Ssa-Solar semiannual constituent. See Sa. sequence of tide-The order in which the four tides Speed=2h=0.082,137,3� per solar hour. of a day occur, with special reference as to whether salinity (S)-The total amount of solid material in the higher high water immediately precedes or follows grams contained in 1 kilogram of sea water when all the lower low water. the carbonate has been converted to oxide, the bro- set (of current)-The direction towards which the mine and iodine replaced by chlorine, and all organic current flows. matter completely oxidized. shallow water constituent-A short-period harmonic S(7 /. )= 1.806,55XC1 (X / ) term introduced into the formula of tidal (or tidal Where Cl(�/..) is chlorinity in parts per thousand. current) constituents to take account of the change in See chlorinity. the form of a tide wave resulting from shallow water Sargasso Sea--The west central region of the sub- conditions. Shallow water constituents include the over- tropical gyre of the North Atlantic Ocean. It is boun- tides and compound tides. ded by the North Atlantic, Canary, North Equatorial, shallow water wave-A wave is classified as a shal- and Antilles Currents, and the Gulf Stream. It is low water wave whene ver the ratio of t he depth (the characterized by the absence of any well-marked cur- characterized by the absence of any well-marked cur- vertical distance of the still water level from the bot- rents and by large quantities of drifting Sargassum, or tom) to the wave length (the horizontal distance be- Saros-gulfweed period of 223 synodic months correspond- tween crests) is less than 0.04. Such waves propagate ing approximately to 19 eclipse years or 18.03 Julian according to the formula: years, and is a cycle in which solar and lunar eclipses C VW repeat themselves under approximately the same con- where C is the wave speed, g the acceleration of ditions. gravity, and d the depth. Tidal waves are shallow sea level datum (SLD)-An obsolete term. See Na- water waves. tional Geodetic Vertical Datum of 1929 and mean sea shoreline (coastline)-The intersection of the land level. with the water surface. The shoreline shown on charts second reduction-Same as phase reduction. represents the line of contact between the land and a secondary control tide station-A tide station at selected water elevation. In areas affected by tidal which continuous observations have been made over a fluctuations, this line of contact is the mean high minimum period of 1 year but less than 19 years. The water line. In confined coastal waters of diminished series is reduced by comparison with simultaneous ob- tidal influence, the mean water level line may be used. servations from a primary control tide station. This See coast line. station provides for a 365-day harmonic analysis in- sidereal day-The time of the rotation of the Earth cluding the seasonal fluctuation of sea level. See tide with respect to the vernal equinox. It equals approxi- station, primary control tide station, tertiary tide sta- mately 0.997,27 of a mean solar day. Because of the tion, and subordinate tide station (1). precession of the equinoxes, the sideral day thus de- 20 fined is slightly less than the period of rotation with solar time-Time measured by the hour angle of respect to the fixed stars, but the difference is less the Sun. It is called apparent time when referred to than the hundredth part of a second. the actual Sun and mean time when referred to the sidereal month-Average period of the revolution of mean Sun. It is also classified as local, standard, or the Moon around the Earth with respect to a fixed Greenwich according to whether it is reckoned from star, equal to 27.321,661 mean solar days. the local, standard, or Greenwich meridian. sidereal time-This is usually defined by astrono- solitary wave-A wave of translation consisting of a mers as the hour angle of the vernal equinox. The single crest rising above the undisturbed water level sidereal day is the interval between two successive without any accompanying trough. The rate of advance upper transits of the vernal equinox. It is to be noted of a solitary wave depends upon the depth of the water that when applied to the month and year the word and is usually expressed by the formula: sidereal has reference to motion with respect to the C = /g(d+h) fixed stars, while the word tropical is used for motion in which C=rate of advance, g=acceleration of grav- with respect to the vernal equinox. Because of the ity, d=depth of water, and h=height of wave, the precession of the equinox there is a slight difference. depth and height being measured from the undisturbed sidereal year-Average period of the revolution of water level. the Earth around the Sun with respect to a fixed star. solstices-The two points in the ecliptic where the Its length is approximately 365.256,4 mean solar days. Sun reaches its maximum and minimum declinations; sigma-t (at)-An expression of density as a function also the times when the Sun reaches these points. The of temperature and salinity (at atmospheric pressure) maximum north declination occurs on or near June 21, in a convenient numerical form. See density. marking the beginning of summer in the Northern at=(p',t- 1)1,000 Hemisphere and the beginning of winter in the South- sigma-zero (o.)-An expression of density as a func- ern. The maximum south declination occurs on or near tion of salinity (at atmospheric pressure and 0�C) in a December 22, marking the beginning of winter in the convenient numerical form. See density. Northern Hemisphere and the beginning of summer in = (~P.". - 1) 1,000 the Southern. slack; ebb begins (slack before ebb)-The slack wa- solstitial tide s-Tides occurring near the times of solstitial tides--Tides occurring near the times of ter immediately preceding the ebb current. slack; flood begins (slack before flood)-The slack the solstices. The tropic range may be expected to be slack; flood begins (slack before flood)--The slack especially large at these times. water immediately preceding the flood current. y Somali (East Africa Coast) Current--An Indian slack water (slack)--Th e of a tidal current slac.wate r (s le stte of near zcially ent .Ocean current setting southwestward along the coast when its speed is near zero, especially the moment when a reversing current changes direction and its of Somalia. The current reverses and sets to the north- when a reversing current changes direction and its speed is zero. The term also is applied to the entire east during west period of low speed near the time of turning of the South Equatorial Current-A current setting west- current when it is too weak to be of any practical ward along and south of the Equator in the Atlantic importance in navigation. The relation of the time of and Pacific Oceans, and south of the Equator in the slack water to the tidal phases varies in different Indian Ocean. It occurs immediately south of the localities. For a perfect standing tidal wave, slack Equatorial Counter Current. water occurs at the time of high and of low water, Southwest Monsoon Current- while for a perfect progressive tidal wave, slack water Current. occurs midway between high and low water. See slack; species of constituent-A classification depending ebb begins and slack; flood begins, upon the period of a constituent. The principal species small diurnal range (S1)-Difference in height be- are semidiurnal, diurnal, and long-period. tween mean lower high water and mean higher low specific volume anomaly, or steric anomaly (a)-The water. excess in specific volume over the standard specific small tropic range (Sc)-Difference in height be- volume at 35 �/o , 0�C, and the given pressure. See tween tropic lower high water and tropic higher low thermosteric anomaly and specific volume. water. 6=astp-a 35,o,p solar day-The period of the rotation of the Earth specific volume, in situ (a,,,t)-Volume per unit with respect to the Sun. The mean solar day is the mass. The reciprocal of density (specific gravity). The time of the rotation with respect to the mean Sun. The specific volume of sea water as a function of salinity, solar day commencing at midnight is called a civil or temperature, and pressure. See specific volume anom- calendar day, but if the day is reckoned from noon it aly and thermosteric anomaly. is known as an astronomical day because of its former speed (of constituent)-The rate of change in the use in astronomical calculation. phase of a constituent, usually expressed in degrees per solar tide--(1) The part of the tide that is due to hour. The speed is equal to 360� divided by the the tide-producing force of the Sun. (2) The observed constituent period expressed in hours. tide in areas where the solar tide is dominant. This speed (of current)-The magnitude of velocity. Rate condition provides for phase repetition at about the at which the current flows. Usually expressed in knots same time each solar day. or centimeters per second. 21 Spitshergen Atlantic Current-A current setting in which T is the period of wave, L the length of the northwestward off the southwest coast of Spitsbergen basin, d the depth of water, and g the acceleration of in the Greenland Sea. gravity. A stationary wave may be resolved into two spring high water-Same as mean high water progressive waves of equal amplitude and equal speeds springs (MHWS). See spring tides, moving in opposite directions. spring low water-Same as mean low water springs stationary wave theory-An assumption that the ba- (MLWS). See spring tides and mean low water sic tidal movement in the open ocean consists of a springs, system of stationary wave oscillations, any progressive spring range (Sg)-See spring tides. wave movement being of secondary importance except spring tides or tidal currents-Tides of increased as the tide advances into tributary waters. The con- range or tidal currents of increased speed occurring tinental masses divide the sea into irregular basins, semimonthly as the result of the Moon being new or which, although not completely enclosed, are capable full. The spring range (Sg) of tide is the average range of sustaining oscillations which are more or less in- occurring at the time of spring tides and is most dependent. The tide-producing force consists princi- pally of two parts, a semidiurnal force with a period conveniently computed from the harmonic constants. Ito parts, a semidiurnal force with a period approximately the half-day and a diurnal force with a is larger than the mean range where the type of tide is prim he day n a t fre i period of a whole day. Insofar as the free period of either semidiurnal or mixed, and is of no practical io of ay ar the e erid oscillation of any part of the ocean, as determined by significance where the type of tide is predominantly its dimensions and depth, is in accord with the semi- diurnal. The average height of the high waters of the diurnal or diurnal tide-producing forces, there will be diurnal or diurnal tide-producing forces, there will be spring tides is called spring high water or mean high built up corresponding oscillations of considerable am- water springs (MHWS) and the average height of the plitude which will be manifested in the rise and fall of corresponding low waters is called spring low water or the tide. The diurnal oscillations, superimposed upon mean low water springs (MLWS). the semidiurnal oscillations, cause the inequalities in stand of tide-Sometimes called a platform tide. An the heights of the two high and the two low waters of interval at high or low water when there is no sensible each day. Although the tidal movement as a whole is change in the height of the tide. The water level is somewhat complicated by the overlapping of oscillating stationary at high and low water for only an instant, areas, the theory is consistent with observational data. but the change in level near these times is so slow that stencils-Perforated sheets used with the tabulated it is not usually perceptible. In general, the duration of hourly heights of the tide or speeds of the tidal current the apparent stand will depend upon the range of tide, for the purpose of distributing and grouping them into being longer for a small range than for a large range, consitituent hours preliminary to summing for har- but where there is a tendency for a double tide the monic analysis. See Coast and Geodetic Survey Spe- stand may last for several hours even with a large cial Publication No. 98, Manual of Harmonic Analysis range of tide. and Prediction of Tides. This analysis is now per- standard time-A kind of time based upon the tran- formed on electronic digital computers. sit of the Sun over a certain specified meridian, called steric anomaly-Same as specific volume anomaly. the time meridian, and adopted for use over a consid- stilling well-A vertical pipe with a relatively small erable area. With a few exceptions, standard time is opening (intake) in the bottom. It is used in a gauge based upon some meridian which differs by a multiple installation to dampen short period surface waves while of 15' from the meridian of Greenwich. The United freely admitting the tide, other long period waves, and States first adopted standard time in 1883 on the sea level variations; which can then be measured by a initiative of the American Railway Association, and at tide gauge senor inside. See float well and protective well. noon on November 18 of that year the telegraphic ell storm surge--A departure from a normal elevation time signals from the Naval Observatory at Washing- ti inas wfrom e chan O rtor atWhis of the sea due to the piling up of water against a coast ton were changed to this system. staiona) wae chand toshis tem by strong winds such as those accompanying a hur- standing (stationary) wave--A wave that oscillates ricane or other intense storm. Reduced atmospheric without progressing. One-half of such a wave may be pressure often contributes to the departure in height illustrated by the oscillation of the water in a pan that dpeurn rricanes. It is potentially catastrophic, espe- during hurricanes. It is potentially catastrophic, espe- has been tilted. Near the axis, which is called the node cially in deltaic regions with onshore winds at the time or nodal line, there is no vertical rise and fall of the of high tide and extreme wind wave heights. water. The ends of the wave are called loops and at stray line-Ungraduated portion of line connected these places the vertical rise and fall is at a maximum. with the current pole used in taking current observa- The current is maximum near the node and minimum tions. The stray line is usually about 100 feet long and at the loops. The period of a stationary wave depends permits the pole to acquire the velocity of the current upon the length and depth of the body of water and, at some distance from the disturbed waters in the for a simple rectangular basin, may be expressed by immediate vicinity of the observing vessel before the the formula: current velocity is read from the graduated portion of T=2L/vig the current line. 22 strength of current-Phase of tidal current in which tertiary tide station-A tide station at which con- the speed is a maximum; also the speed at this time. tinuous observations have been made over a minimum Beginning with slack before flood in the period of a period of 30 days but less than 1 year. The series is reversing tidal current (or minimum before flood in a reduced by comparison with simultaneous observations rotary current), the speed gradually increases to flood from a secondary control tide station. This station strength and then diminishes to slack before ebb (or provides for a 29-day harmonic analysis. See tide sta- minimum before ebb in a rotary current), after which tion, primary control tide station, secondary control the current turns in direction, the speed increases to tide station, and subordinate tide station (1). ebb strength and then diminishes to slack before flood thermistor-Temperature sensor. completing the cycle. If it is assumed that the speed thermocline-A layer in which the temperature de- throughout the cycle varies as the ordinates of a cosine creases significantly (relative to the layers above and curve, it can be shown that the average speed for an below) with depth. The principal ones are designated entire flood or ebb period is equal to 2/7r or 0.636,6 of diurnal, seasonal, and main thermocline. the speed of the corresponding strength of current. thermosteric anomaly (OT, a, or As,,)-The specific strength of ebb-Same as ebb strength. volume anomaly (steric anomaly) that would be at- strength of flood-Same as flood strength. tained if the water were changed isothermally to a submerged lands-Lands covered by water at any standard pressure of one atmosphere. The specific vol- stage of the tide. See tidelands. ume anomaly with pressure terms omitted. See subordinate current station--(l) A current station isanostere. from which a relatively short series of observations is tidal bench mark-See bench mark. reduced by comparison with simultaneous observations tidal bench mark description-A published, concise from a control current station. See current station, description of the location, stamped number or des- control current station, and reference station. (2) A ignation, date established, and elevation (referred to a station listed in the Tidal Current Tables for which tidal datum) of a specific bench mark. predictions are to be obtained by means of differences tidal bench mark state index map-A state map and ratios applied to the full predictions at a reference which indicates the locations for which tidal datums station. See reference station. and tidal bench mark descriptions are available. subordinate tide station--() A tide station from tidal bore-A tidal wave that propagates up a rela- which a relatively short series of observations is re- tively shallow and sloping estuary or river in a solitary duced by comparison with simultaneous observations wave form. The leading edge presents an abrupt rise in from a tide station with a relatively long series of level, frequently with continuous breaking and often observations. See tide station, primary control tide immediately followed by several large undulations. An station, secondary control tide station, and tertiary tide uncommon phenomenon, the tidal bore is usually asso- station. (2) A station listed in the Tide Tables from ciated with very large ranges in tide as well as wedge- which predictions are to be obtained by means of shaped and rapidly shoaling entrances. Also called differences and ratios applied to the full predictions at eagre, eager (for Tsientan, China bore), mascaret a reference station. See reference station. (French), poror6ca (Brazilian), and bore. summer time-British name for daylight saving tidal characteristics-Principally, those features re- time. lating to the time, range, and type of tide. synodical month-The average period of the revolu- tidal constants-Tidal relations that remain prac- tion of the Moon around the Earth with respect to the tically constant for any particular locality. Tidal Sun, or the average interval between corresponding constants are classified as harmonic and nonharmonic. phases of the Moon. The synodical month is approxi- The harmonic constants consist of the amplitudes and mately 29.530,588 days in length. epochs of the harmonic constituents, and the nonhar- syzygy-Position of the Moon when it is new or monic constants include the ranges and intervals de- full. rived directly from the high and low water observa- tions. tidal constituent-See constituent. T tidal current-A horizontal movement of the water T-Rate of change of hour angle of mean Sun at caused by gravitational interactions between the Sun, place of observation. Moon, and Earth. The horizontal component of the T= 15� per mean solar hour. particulate motion of a tidal wave. Part of the same T2-Larger solar elliptic constituent. See R2. general movement of the sea that is manifested in the Speed=2T-h+p,= 29.958,933,3� per solar hour. vertical rise and fall called tide. The United States tape gauge-See electric tape gauge. equivalent of the British tidal stream. See tidal wave, telemetry-The capability of transmitting or retriev- tide, and current. ing data over long distance communication links, such Tidal Current Chart Diagrams-A series of 12 as satellite or telephone. monthly diagrams to be used with the Tidal Current terdiurnal-Having three periods in a constituent Charts. Each diagram contains lines that indicate the day. The symbol of a terdiurnal constituent is the specific tidal current chart of each series to use, and subscript 3. the speed factor to apply to that chart. 23 Tidal Current Charts-Charts on which tidal cur- motion of a tidal wave. Although the accompanying rent data are depicted. Tidal Current Charts for a horizontal movement of the water is part of the same number of important waterways are published by the phenomenon, it is preferable to designate this motion National Ocean Service. Each consists of a set of as tidal current. See tidal wave. charts giving the speed and direction of the current for tide curve-A graphic representation of the rise and each hour or equal interval of the tidal cycle, thus fall of the tide in which time is usually represented by presenting a comprehensive view of the tidal current the abscissa and height by the ordinate. For a semi- movement. diurnal tide with little diurnal inequality, the graphic tidal current constants-See current constants. representation approximates a cosine curve. See marig- tidal current station-See current station. ram. Tidal Current Tables-Tables which give daily pre- tide datum-See datum. dictions of the times and velocities of the tidal cur- tide (water level) gauge-An instrument for measur- rents. These predictions are usually supplemented by ing the rise and fall of the tide (water level). See ADR current differences and constants through which pre- gauge, automatic tide gauge, Next Generation Water dictions can be obtained for numerous other locations. Level Measurement System, gas purged pressure tidal datum-See datum. gauge, electric tape gauge, pressure gauge, and tide tidal day-Same as lunar day. staff. tidal difference-Difference in time or height be- tide predicting machine-A mechanical analog ma- tween a high or low water at a subordinate station and chine especially designed to handle the great quantity a reference station for which predictions are given in of constituent summations required in the harmonic the Tide Tables. The difference, when applied accord- method. William Ferrel's Maxima and Minima Tide ing to sign to the prediction at the reference station, Predictor (described in Manual of Tides, U.S. Coast gives the corresponding time or height for the subordi- and Geodetic Survey, Appendix 10, Report for 1883) nate station. was the first such machine used in the United States. tidal epoch-See National Tidal Datum Epoch and Summing only 19 constituents, but giving direct read- epoch. ings of the predicted times and heights of the high and tidal estuary-See estuary. low waters, the Ferrel machine was used for the pre- tidal stream-British equivalent of United States dictions of 1885 through 1914. A second machine, tidal current. developed by Rollin A. Harris and E. G. Fischer and tidal wave-A shallow water wave caused by the summing 37 constituents, was used for the predictions gravitational interactions between the Sun, Moon, and of 1912 through 1965 (described in Manual of Har- Earth. Essentially, high water is the crest of a tidal monic Analysis and Prediction of Tides by Paul wave and low water, the trough. Tidal current is the Schuremen, U.S. Coast and Geodetic Survey Special horizontal component of the particulate motion, while Publication No. 98, 1958). Predictions are now pre- tide is manifested by the vertical component. The pared using an electronic digital computer. observed tide and tidal current can be considered the tide-producing force-That part of the gravitational result of the combination of several tidal waves, each attraction of the Moon and Sun which is effective in of which may vary from nearly pure progressive to producing the tides on the Earth. The force varies nearly pure standing and with differing periods, approximately as the mass of the attracting body and heights, phase relationships, and direction. inversely as the cube of its distance. The tide-produc- tidal zoning-The practice of dividing a hydro- ing force exerted by the Sun is a little less than graphic survey area into discrete zones or sections, one-half as great as that of the Moon. A mathematical each one possessing similar tidal characteristics. One development of the vertical and horizontal components set of tide reducers is assigned to each zone. Tide of the tide-producing forces of the Moon and Sun will reducers are used to adjust the soundings in that zone be found in Coast and Geodetic Survey Special Pub- to chart datum (MLLW). Tidal zoning is necessary in lication No. 98. order to correct for differing water level heights occur- tide reducers-Height corrections for reducing soun- ring throughout the survey area at any given time. dings to chart datum (MLLW). A tide reducer Each zone of the survey area is geographically delin- represents the height of the water level at a given eated such that the differences in time and range do place and time relative to chart datum. Tide reducers not exceed certain limits, generally 0.2 hours and 0.2 are obtained from one or more tide stations within or feet respectively; however, these limits are subject to nearby the survey area. Often, due to differing tidal change depending upon type of survey, location, and characteristics over the survey area, the tide reducers tidal characteristics. The tide reducers are derived obtained directly from a tide station must be corrected from the water levels recorded at an appropriate tide to adjust for time and range of tide differences in the station, usually nearby. Tide reducers are used to cor- various zones of the hydrographic survey area. See rect the soundings throughout the hydrographic survey tidal zoning. area to a common, uniform, uninterrupted chart da- tide rip-See rip. tum. See tide reducers. tide staff-A tide gauge consisting of a vertical tide-The periodic rise and fall of the water result- graduated staff from which the height of the tide can ing from gravitational interactions between Sun, Moon, be read directly. It is called a fixed staff when secured and Earth. The vertical component of the particulate in place so that it cannot be easily removed. A porta- 24 ble staff is one that is designed for removal from the tractive force-The horizontal component of a tide- water when not in use. For such a staff a fixed support producing force vector (directed parallel with level is provided. The support has a metal stop secured to it surfaces at that geographic location). so that the staff will always have the same elevation transit-The passage of a celestial body over a when installed for use. See electric tape gauge. specified meridian. The passage is designated as upper tide (water level) station-The geographic location at transit or lower transit according to whether it is over which tidal observations are conducted. Also, the fa- that part of the meridian lying above or below the cilities used to make tidal observations. These may polar axis. include a tide house, tide gauge, tide staff, and tidal tropic currents-Tidal currents occurring semi- bench marks. See primary control tide station, secon- monthly when the effect of the Moon's maximum dary control tide station, tertiary tide station, and declination is greatest. At these times the tendency of subordinate tide station (1). the Moon to produce a diurnal inequality in the cur- Tide Tables-Tables which give daily predictions of rent is at a maximum. the times and heights of high and low waters. These tropic inequalities-Tropic high water inequality predictions are usually supplemented by tidal differ- (HWQ) is the average difference between the two high ences and constants through which predictions can be waters of the day at the times of tropic tides. Tropic obtained for numerous other locations. low water inequality (LWQ) is the average difference tide wave-See tidal wave, between the two low waters of the day at the times of tidelands-The zone between the mean high water tropic tides. These terms are applicable only when the and mean low water lines. It is identical with intertidal type of tide is semidiurnal or mixed. See tropic tides. zone (technical definition) when the type of tide is tropic intervals-Tropic higher high water interval semidiurnal or diurnal. (TcHHWI) is the lunitidal interval pertaining to the Tides ABC-A subscription software program pro- higher high waters at the time of the tropic tides. viding users (with an IBM PC, AT, or compatible Tropic lower low water interval (TcLLWI) is the computer), on demand, access to observed and pre- lunitidal interval pertaining to the lower low waters at dicted water levels (and anomalies) in graphic or tab- the time of the tropic tides. Tropic intervals are ular form. The service acquires real-time water level marked a when reference is made to the upper transit data at 6-minute intervals from selected stations of the Moon at its north declination or to the lower equipped with telephone telemetry and, with the pre- transit at the time of south declination, and are dicted tide at the location, provides a video or printed marked b when the reference is to the lower transit at display. the north declination or to the upper transit at the tidewater-Water activated by the tide generating south declination. See tropic tides. forces and/or water affected by the resulting tide, tropic ranges-The great tropic range (Gc), or trop- especially in coastal and estuarine areas. Also, a gen- ic range, is the difference in height between tropic eral term often applied to the land and water of higher high water and tropic lower low water. The estuarine areas formed by postglacial drowning of small tropic range (Sc) is the difference in height coastal plain rivers, between tropic lower high water and tropic higher low tideway-A channel through which a tidal current water. The mean tropic range (Mc) is the mean be- flows. tween the great tropic and the small tropic range. time, kinds-Time is measured by the rotation of Tropic ranges are most conveniently computed from the Earth with respect to some point in the celestial the harmonic constants. See tropic tides. sphere and may be designated as sidereal, solar, or tropic speed-The greater flood or greater ebb lunar, according to whether the measurement is taken speed at the time of tropic currents. in reference to the vernal equinox, the Sun, or the tropic tides-Tides occurring semimonthly when the Moon. Solar time may be apparent or mean, according effect of the Moon's maximum declination is greatest. to whether the reference is to the actual Sun or the At these times there is a tendency for an increase in mean Sun. Mean solar time may be local or standard, the diurnal range. The tidal datums pertaining to the according to whether it is based upon the transit of the tropic tides are designated as tropic higher high water Sun over the local meridian or a selected meridian (TcHHW), tropic lower high water (TcLHW), tropic adopted as a standard over a considerable area. Green- higher low water (TcHLW), and tropic lower low wa- wich time is standard time based upon the meridian of ter (TcLLW). Greenwich. In civil time the day commences at mid- tropical month-The average period of the revolu- night, while in astronomical time, as used prior to tion of the Moon around the Earth with respect to the 1925, the beginning of the day was reckoned from vernal equinox. Its length is approximately 27.321,582 noon of the civil day of the same date. The name days. universal time is now applied to Greenwich mean civil tropical year-The average period of the revolution time. of the Earth around the Sun with respect to the vernal time meridian-A meridian used as a reference for equinox. Its length is approximately 365.242,2 days. time. The tropical year determines the cycle of changes in total current-The combination of the tidal and the seasons, and is the unit to which the calendar year nontidal current. The United States equivalent of the is adjusted through the occasional introduction of the British flow. See current. extra day on leap years. 25 tsunami-A shallow water progressive wave, poten- variation (of compass)-Difference between true tially catastrophic, caused by an underwater earth- north as determined by the Earth's axis of rotation and quake or volcano. magnetic north as determined by the Earth's mag- Tsushima Current-A North Pacific Ocean current netism. Variation is designated as east or positive when setting northeastward in the East China Sea (in sum- the magnetic needle is deflected to the east of true mer) and Sea of Japan. A segment of the Kuroshio north and as west or negative when the deflection is to System. the west of true north. Also called magnetic declina- type of tide-A classification based on characteris- tion. tic forms of a tide curve. Qualitatively, when the two variational inequality-An inequality in the Moon's high waters and two low waters of each tidal day are motion due mainly to the tangential component of the approximately equal in height, the tide is said to be Sun's attraction. semidiurnal; when there is a relatively large diurnal velocity (of current)-Speed and set of the current. inequality in the high or low waters or both, it is said vernal equinox-See equinoxes. to be mixed; and when there is only one high water vulgar establishment-Same as establishment of the and one low water in each tidal day, it is said to be port. diurnal. Quantitatively (after Dietrich), where the ratio of K1+01 to M2+S2 is less than 0.25, the tide is W classified as semidiurnal; where the ratio is from 0.25 to 1.5, the tide is mixed, mainly semidiurnal; where water level gauge-See tide gauge. the ratio is from 1.5 to 3.0, the tide is mixed, mainly ater evel Teet sta Water Level Telemetry System Auxiliary diurnal; and where greater than 3.0, diurnal. Watr e r stm uiia (WLTSAUX)-The first communication package giv- ing users (with an IBM PC, AT, or compatible com- U puter), on demand, access to observed and predicted universal time (UT)-Same as Greenwich mean time water levels in tabular format. (GMT). West Australian Current-An Indian Ocean current uplands-Land above the mean high water line setting northward along the west coast of Australia. (shoreline) and subject to private ownership, as distin- West Greenland Current-A North Atlantic Ocean guished from tidelands, the ownership of which is current setting northward along the west coast of prima facie in the state but also subject to divestment Greenland. under state statutes. See tidelands. West Wind Drift-A current setting eastward ar- upwelling-An upward flow of subsurface water due ound the Antarctic Continent south of Cape Horn, the to such causes as surface divergence, offshore wind, Cape of Good Hope, Tasmania, and New Zealand. and wind drift transport away from shore. wind drift-An ocean current in which only the Coriolis and frictional forces are significant. The wind v drift embodies an Ekman spiral. V,+u-See equilibrium argument. vanishing tide-In a predominantly mixed tide with Z very large diurnal inequality, the lower high water (or Z--Symbol recommended by the International Hy- higher low water) becomes indistinct (or vanishes) at drographic Organization to represent the elevation of times of extreme declinations. mean sea level above chart datum. 26 APPENDIX A Chronology of Significant Oceanographic Events In The History of the National Ocean Service' 1807 - The Survey of the Coast established. 19222 - Format changed from sequential listing to 18302 - Tide predictions for United States began. separate columns for high and low waters. Published in The American Almanac. High 19232 - Tidal Current Tables first published sepa- water time predictions (one per day) for Bos- rately from Tide Tables (two volumes, At- ton, New York, and Charleston. Time differ- lantic Coast and Pacific Coast, North ences for 96 other stations. Spring range America). predictions for 84 stations. 19282 - Single port miniature tables introduced. 1836 - The Survey of the Coast became Coast Sur- 19322 - Last year of single port miniature tables vey. (revived from 19402 through 19442 for New 1844 - Tide notes (including lunitidal intervals) on York Harbor and vicinity only). nautical charts began. 19402 - Special restricted tables for war effort be- 1853 - Tables for obtaining tide predictions by the gan. nonharmonic lunitidal interval method first 19512 - Last year of special wartime and occupation published in the appendix to the Annual tables. Report. 19552 - Special Tide Tables for selected places in 1854 - Tidal Division formed. Greenland, Canada, and Alaska began. 1864 - Last year of tables for lunitidal interval 19582 - Format changed from separate columns for method. One thousand copies provided to high and low waters to sequential listing. Union naval forces. 19592 - Tide predictions added to Small Craft Chart 18672 - First Tide Tables published. series. 18682 - Low water predictions began for west coast 1960 - Division of Tides and Currents changed to of Florida and Pacific coast. Marine Data Division. - First tide research group formed. - EXPLORER Transfer Cruise. 1878 - Coast Survey became Coast and Geodetic 19612 - Motor drive and automatic readout installed Survey. on Harris-Fischer machine. 1885- - Last year of special Tide Tables for selected 1889 - Gulf Stream System (anchor) investigations places in Greenland, Canada, and Alaska. of vessel BLAKE. 1963 - Research Group (Oceanography) formed. 18852 - William Ferrel's Maxima and Minima Tide - Established Deep Sea Tide Program. Predictor introduced. 1964 - International Indian Ocean Expedition of 18872 - Low water predictions included for all sta- PIONEER. tions. 19652 - Last year Harris-Fischer Tide Predicting 18902 - Tidal current predictions began (New York Machine used. Harbor and vicinity). AnlgtdiiareoerAD)ie 1896' - Ex~thensoftbe tornludeueru gauges and computer processing introduced. ports throughout th ol.19662 - Electronic digital computer introduced for 1910 - Tidal Research Section formed. peitos 19122 - Harris-Fischer Tide Predicting Machine in- - pMrieDatiiinchagdtioOenog- troduced. -Mrin aphy Division.cagdtoOeng 19142 - Last year Ferrel's Maxima and Minima Tide 1 6-Grega emutEpeyditiion.o ICVR Predictor used.197-GegeaonExeiinfDICVR 11-TiaDiiincangd torrecinofTids. - Established Estuarine Flushing and Nontidal 1920 - Section of Tides and Currents changed toCurnPedcinSvc. Division of Tides and Currents. - International Symposium on Mean Sea Lev- -~ el, TAP0 and UNESCO. -Electronic digital computer introduced for harmonic analysis of tides. 1Unless otherwise noted, all items refer to National Ocean 1968 - Established Planetary Wave Project. Service activities. 1970 - Coast and Geodetic Survey became National 2 Date refers to Tide or Tidal Current Table volume containing Ocean Survey. preditions for the stated year. 1973 - Sea Level Service began. 27 - Established National Tidal Datum Epoch. - Personal computer software introduced for - Telemetered water level measurements intro- local user access to water level telemetry duced for Great Lakes. stations. 1977 - Gulf Coast Low Water Datum adopted. 1987 - Tidal Circulation and Water Level Forecast 1978 - Water Level Telemetry System introduced Atlas introduced. for marine coasts. - Operational RADS current meter system in- 1979 - Office of Oceanography formed.troduced. 1979 - Ofice of Oeanograph formed.1988 - International Conference on Tidal Hydrody- - National Ocean Survey Sea Level Expedi- namics. tion to the Aleutian Islands of DISCOV - - Personal computer software introduced for ERER. local user access to current telemetry sta- 1980 - National Tidal Datum Convention of 1980 tions. adopted. - Operational NGWLMS field units intro- 1982 - National Ocean Survey became National duced using an air acoustic ranging sensor Ocean Service. and satellite telemetry. 28 APPENDIX B Tide and Current Organization Chiefs W. W. Gordon....................................................April 11, 1851, to April 11, 1853 Tidal Party and Tidal Division Louis F. Pourtales .................................................April 12, 1853, to June 14, 1866 Tidal Division Robert S. Avery ..............................................June 15, 1866, to September 30, 1885 Alexander S. Christie.............................................October 1, 1885, to April 19, 1893 Benjamin A. Colonna (Acting)..............................................April 20 to May 8, 1893 George A. Fairfield...................................................May 9, 1893, to June 7, 1894 Leland P. Shidy (Acting).............................................June 8, 1894, to July 17, 1895 E. B. Latham ..................................................July 18, 1895, to February 24, 1896 Leland P. Shidy (Acting).............................................February 25 to March 4, 1896 Henry L. Marindin ................................................March 5, 1896, to May 16, 1898 Leland P. Shidy (Acting).......................................November 17, 1897, to May 16, 1898 Leland P. Shidy.................................................May 17, 1898, to October 14, 1915 Section of Tides and Currents Leland P. Shidy (Acting)..........................................October 15 to December 26, 1915 Robert F. Luce................................................December 27, 1915, to July 26, 1917 Leland P. Shidy (Acting) ..........................................July 27, 1917, to March 18, 1919 Robert F. Luce ........................................................March 19 to July 19, 1919 Gilbert T. Rude ...............................................July 24, 1919, to December 14, 1920 Division of Tides and Currents Gilbert T. Rude.............................................December 15, 1920, to August 22, 1928 Paul C. Whitney...............................................August 23, 1928, to August 27, 1942 Charles K. Green ...........................................August 28, 1942, to December 30, 1946 Cornelius D. Meaney.........................................December 31, 1946, to March 31, 1950 Henry E. Finnegan .............................................April 1, 1950, to December 28, 1956 Kenneth G. Crosby..........................................January 1, 1957, to September 30, 1960 Marine Data Division Lindsay P. Disney (Acting) .........................................October I to November 10, 1960 Kenneth S. Ulm ..............................................November 11, 1960, to May, 31, 1964 William Shofnos (Acting)...................................................June I to July 12, 1964 William D. Barbee.............................................July 13, 1964, to December 19, 1965 William Shofnos (Acting) .................................................December 20 to 27, 1965 Oceanographic Division Steacy D. Hicks ...............................................December 28, 1965, to June 29, 1970 R. Lawrence Swanson (Acting) .......................................July 25, 1969, to June 29, 1970 R. Lawrence Swanson...............................................June 30, 1970, to May 17, 1973 Carl W. Fisher ...................................................May 18, 1973, to January 4, 1977 Wesley V. Hull ................................................January 5, 1977, to January 13, 1979 Office of Oceanography (Associate Director of National Ocean Survey) Wesley V. Hull (Acting).............................................January 14 to October 17, 1979 Wesley V. Hull.............................................October 18, 1979, to November 27, 1982 Ocean Requirements and Data Analysis Division Millington Lockwood (Acting) ..............................November 28, 1982, to December 31, 1983 Amor L. Lane ....................................................January 1, 1984, to June 3, 1985 29 Physical Oceanography Division John H. Cawley (Acting) ...........................................June 4, 1985, to March 29, 1986 John G. Hayes ..................................................March 30, 1986, to April 20, 1988 Harold M. Stanford (Acting) ...................................April 21, 1988, to September 23, 1989 Ledoiph Baer ......................................................September 24, 1989, to present 30