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New South Wales Tide Glossary

Index


Amplitude (H)

One-half the range of a constituent tide. By analogy, it may be applied also to the maximum speed of a constituent current.

Angular velocity of the Earth's rotation

Time rate of change of angular displacement relative to the fixed stars. It is equal to 0.729,211 x 10^-4 radian/second.

Annual inequality

Seasonal variation in water level or current, more or less periodic, due chiefly to meteorological causes.

Automatic tide gauge

An instrument that automatically registers the rise and fall of the tide. In some instruments, the registration is accomplished by recording the heights at regular time intervals in digital format.

Bench mark (BM)

A fixed physical object or mark used as reference for a vertical datum. A tidal bench mark is one near a tide station to which the tide staff and tidal datums are referred. A primary bench mark is the principal (or only) mark of a group of tidal bench marks to which the tide staff and tidal datums are referred.

Bubbler tide gauge

Same as gas purged pressure gauge.

Chart datum

The datum to which soundings on a chart are referred. It is usually taken to correspond to a low-water elevation, and its depression below mean sea level is represented by the symbol Z

Coastal boundary

The mean high water line (MHWL) or mean higher high water line (MHHWL) when tidal lines are used as the coastal boundary. Also, lines used as boundaries inland of and measured from (or points thereon) the MHWL or MHHWL.

Coastal zone (legal definition for coastal zone management)

The term coastal zone means the coastal waters (including the lands therein and thereunder) and the adjacent shorelands (including the waters therein and thereunder), strongly influenced by each and in proximity to the shorelines of the several coastal states, and includes islands, transitional and inter-tidal areas, salt marshes, wetlands, and beaches.

Constituent

One of the harmonic elements in a mathematical expression for the tide-producing force and in corresponding formulas for the tide or tidal current. Each constituent represents a periodic change or variation in the relative positions of the Earth, Moon, and Sun. A single constituent is usually written in the form y = A cos (at + à), in which y is a function of time as expressed by the symbol t and is reckoned from a specific origin. The coefficient A is called the amplitude of the constituent and is a measure of its relative importance. The angle (at + à) changes uniformly and its value at any time is called the phase of the constituent. The speed of the constituent is the rate of change in its phase and is represented by the symbol a in the formula. The quantity a is the phase of the constituent at the initial instant from which the time is reckoned. The period of the constituent is the time required for the phase to change through 360° and is the cycle of the astronomical condition represented by the constituent.

Declination

Angular distance north or south of the celestial equator, taken as positive when north of the equator and negative when south. The Sun passes through its declinational cycle once a year, reaching its maximum north declination of approximately 23-½° about June 21 and its maximum south declination of approximately 23-½° about December 21. The Moon has an average declinational cycle of 27-1/3° days which is called a tropical month. Tides or tidal currents occurring near the times of maximum north or south declination of the Moon are called tropic tides or tropic currents, and those occurring when the Moon is over the Equator are called equatorial tides or equatorial currents. The maximum declination reached by the Moon in successive months depends upon the longitude of the Moon's node, and varies from 28-½° when the longitude of the ascending node is 0°, to 18-½° when the longitude of the node is 180° . The node cycle, or time required for the node to complete a circuit of 360° of longitude, is approximately 18.6 years. See epoch (2).

Diurnal

Having a period or cycle of approximately one tidal day. Thus, the tide is said to be diurnal when only one high water and one low water occur during a tidal day, and the tidal current is said to be diurnal when there is a single flood and a single ebb period of a reversing current in the tidal day. A rotary current is diurnal if it changes its direction through all points of the compass once each tidal day. A diurnal constituent is one which has a single period in the constituent day. The symbol for such a constituent is the subscript 1. See type of tide.

Diurnal inequality

The difference in height of the two high waters or of the two low waters of each tidal day; also, the difference in speed between the two flood tidal currents or the two ebb currents of each tidal day. The difference changes with the declination of the Moon and, to a lesser extent, with the declination of the Sun. In general, the inequality tends to increase with increasing declination, either north or south, and to diminish as the Moon approaches the Equator. Mean diurnal high water inequality (DHQ) is one-half the average difference between the two high waters of each tidal day observed over the National Tidal Datum Epoch. It is obtained by subtracting the mean of all the high waters from the mean of the higher high waters. Mean diurnal low water inequality (DLQ) is one-half the average difference between the two low waters of each tidal day observed over the National Tidal Datum Epoch. It is obtained by subtracting the mean of the lower low waters from the mean of all the low waters. Tropic high water inequality (HWQ) is the average difference between the two high waters of each tidal day at the times of tropic tides. Tropic low water inequality (LWQ) is the average difference between the two low waters of each tidal day at the times of tropic tides. Mean and tropic inequalities, as defined above, are applicable only when the type of tide is either semidiurnal or mixed. Diurnal inequality is sometimes called declinational inequality.

Duration of rise and duration of fall

Duration of rise is the interval from low water to high water, and duration of fall is the interval from high water to low water. Together they cover, on an average, a period of 12.42 hours for a semidiurnal tide or a period of 24.84 hours for a diurnal tide. In a normal semidiurnal tide, duration of rise and duration of fall each will be approximately equal to 6.21 hours, but in shallow waters and in rivers there is a tendency for a decrease in duration of rise and a corresponding increase in duration of fall.

Equatorial tides

Tides occurring semimonthly as a result of the Moon being over the Equator. At these times the tendency of the Moon to produce a diurnal inequality in the tide is at a minimum.

Estuary

An embayment of the coast in which fresh river water entering at its head mixes with the relatively saline ocean water. When tidal action is the dominant mixing agent it is usually termed a tidal estuary. Also, the lower reaches and mouth of a river emptying directly into the sea where tidal mixing takes place. The latter is sometimes called a river estuary.

Extreme high water

The highest elevation reached by the sea as recorded by a tide gauge during a given period.

Extreme low water

The lowest elevation reached by the sea as recorded by a tide gauge during a given period. The National Ocean Service routinely documents monthly and yearly extreme low water for its control stations.

Float well

A stilling well in which the float of a float-actuated gauge operates.

Flood current (flood)

The movement of a tidal current toward the shore or up a tidal river or estuary. In the mixed type of reversing current, the terms greater flood and lesser flood are applied respectively to the two flood currents of greater and lesser speed of each day. The expression maximum flood is applicable to any flood current at the time of greatest speed.

Flushing time

The time required to remove or reduce (to a permissible concentration) any dissolved or suspended contaminant in an estuary or harbor.

Fortnight

The time elasped between the new and full moons. Half a synodic month of 14.765,294 days.

Gas purged pressure gauge

A type of analog tide gauge in which gas, usually nitrogen, is emitted from a submerged tube at a constant rate. Fluctuations in hydrostatic pressure due to changes in tidal height modify the emission rate for recording. Same as bubbler tide gauge.

Great diurnal range (Gt)

The difference in height between mean higher high water and mean lower low water. The expression may also be used in its contracted form, diurnal range.

Half-tide level

A tidal datum. The arithmetic mean of mean high water and mean low water. Same as mean tide level.

Harmonic analysis

The mathematical process by which the observed tide or tidal current at any place is separated into basic harmonic constituents.

Harmonic constants

The amplitudes and epochs of the harmonic constituents of the tide or tidal current at any place.

Harmonic function

In its simplest form, a quantity that varies as the cosine of an angle that increases uniformly with time. It may be expressed by the formula: y = A cos at in which y is a function of time (t), A is a constant coefficient, and a is the rate of change in the angle at.

Harmonic prediction

Method of predicting tides and tidal currents by combining the harmonic constituents into a single tide curve. The work is usually performed by electronic digital computer.

Harmonic tide plane

Same as Indian spring low water.

Head

The difference in water level at either end of a strait, channel, inlet, etc.

Head of tide

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

High water (HW)

The maximum height reached by a rising tide. The high water is due to the periodic tidal forces and the effects of meteorological, hydrologic, and/or oceanographic conditions. For tidal datum computational purposes, the maximum height is not considered a high water unless it contains a tidal high water.

High water mark

A line or mark left upon tide flats, beach, or along shore objects indicating the elevation of the intrusion of high water. The mark may be a line of oil or scum on along shore objects, or a more or less continuous deposit of fine shell or debris on the fore shore or berm. This mark is physical evidence of the general height reached by wave run up at recent high waters. It should not be confused with the mean high water line or mean higher high water line.

Higher high water (HHW)

The highest of the high waters (or single high water) of any specified tidal day due to the declination Al effects of the Moon and Sun.

Higher low water (HLW)

The highest of the low waters of any specified tidal day due to the declination Al effects of the Moon and Sun.

Hydrographic datum

A datum used for referencing depths of water and the heights of predicted tides or water level observations. Same as chart datum.

Indian spring low water

A datum originated by Professor G. H. Darwin when investigating the tides of India. It is an elevation depressed below mean sea level by an amount equal to the sum of the amplitudes of he harmonic constituents M2, S2, K1, and O1.

Intertidal zone (technical definition)

The zone between the mean higher high water and mean lower low water lines.

Inverse barometer effect

The inverse response of sea level to changes in atmospheric pressure. A static reduction of 1.005 mb in atmospheric pressure will cause a stationary rise of 1 cm in sea level.

J1

Smaller lunar elliptic diurnal constituent. This constituent, with M1, modulates the amplitudes of the declinational K1, for the effect of the Moon's elliptical orbit. Speed = T + s + h - p = 15.585,443,3° per solar hour.

K1

Lunisolar diurnal constituent. This constituent, with O1, expresses the effect of the Moon's declination. They account for diurnal inequality and, at extremes, diurnal tides. With P1, it expresses the effect of the Sun's declination. Speed = T + h = 15.041,068,6° per solar hour.

K2

Lunisolar semi diurnal constituent. This constituent modulates the amplitude and frequency of M2 and S2 for the declination Al effect of the Moon and Sun, respectively. Speed = 2T + 2h = 30.082,137,3° per solar hour.

L2

Smaller lunar elliptic semi diurnal constituent. This constituent, with N2, modulates the amplitude and frequency of M2 for the effect of variation in the Moon's oribital speed due to its elliptical orbit. Speed = 2T - s + 2h - p = 29.528,478,9° per solar hour.

Lambda

Smaller lunar evectional constituent. This constituent, with v2, u2, and (S2), modulates the amplitude and frequency of M2 for the effects of variation in solar attraction of the Moon. This attraction results in a slight pear-shaped lunar ellipse and a difference in lunar orbital speed between motion toward and away from the Sun. Although (S2) has the same speed as S2, its amplitude is extremely small. Speed = 2T - s + p = 29.455,625,3° per solar hour.

Low water (LW)

The minimum height reached by a falling tide. The low water is due to the periodic tidal forces and the effects of meteorological, hydrologic, and/or oceanographic conditions. For tidal datum computational purposes, the minimum height is not considered a low water unless it contains a tidal low water.

Low water line

The intersection of the land with the water surface at an elevation of low water.

Lower high water (LHW)

The lowest of the high waters of any specified tidal day due to the declination Al effects of the Moon and Sun.

Lower low water (LLW)

The lowest of the low waters (or single low water) of any specified tidal day due to the declination Al effects of the Moon and Sun.

Lower low water datum (LLWD)

An approximation of mean lower low water that has been adopted as a standard reference for a limited area and is retained for an indefinite period regardless of the fact that it may differ slightly from a better determination of mean lower low water from a subsequent series of observations. Used primarily for river and harbor engineering purposes.

Lunar day

The time of the rotation of the Earth with respect to the Moon, or the interval between two successive upper transits of the Moon over the meridian of a place. The mean lunar day is approximately 24.84 solar hours in length, or 1.035 times as great as the mean solar day.

Lunar tide

That part of the tide on the Earth due solely to the Moon as distinguished from that part due to the Sun.

Lunisolar tides

Harmonic tidal constituents K1, and K2, which are derived partly from the development of the lunar tide and partly from the solar tide, the constituent speeds being the same in both cases. Also, the lunisolar synodic fort nightly constituent MSf.

M1

Smaller lunar elliptic diurnal constituent. This constituent, with J1, modulates the amplitude of the declinational K1, for the effect of the Moon's elliptical orbit. A slightly slower constituent, designated (M1), with Q1, modulates the amplitude and frequency of the declinatianal O1, for the same effect. Speed = T - s + h + p = 14.496,693,9° per solar hour.

M2

Principal lunar semidiurnal constituent. This constituent represents the rotation of the Earth with respect to the Moon. Speed = 2T - 2s + 2h = 28.984,104,2° per solar hour.

M3

Lunar terdiurnal constituent. A shallow water compound constituent. See shallow water constituent. Speed = 3T - 3s + 3h = 43.476,156,3° per solar hour.

M4, M6, M8

Shallow water overtides of principal lunar constituent. See shallow water constituent.
Speed of M4 = 2M2 = 4T - 4s + 4h = 57.968,208,4° per solar hour.
Speed of M6 = 3M2 = 6T - 6s + 6h = 86.952,312,7° per solar hour.
Speed of M8 = 4M2 = 8T - 8s + 8h = 115.936,416,9° per solar hour.

Mean diurnal tide level (MDTL)

A tidal datum. The arithmetic mean of mean higher high water and mean lower low water.

Mean high water (MHW)

A tidal datum. The average of all the high water heights observed over the National Tidal Datum Epoch. For stations with shorter series, simultaneous observational comparisons are made with a control tide station in order to derive the equivalent datum.

Mean high water line (MHWL)

The line on a chart or map which represents the intersection of the land with the water surface at the elevation of mean high water.

Mean higher high water (MHHW)

A tidal datum. The average of the higher high water height of each tidal day observed over the National Tidal Datum Epoch. For stations with shorter series, simultaneous observational comparisons are made with a control tide station in order to derive the equivalent datum.

Mean higher high water line (MHHWL)

The line on a chart or map which represents the intersection of the land with the water surface at the elevation of mean higher high water.

Mean low water line (MLWL)

The line on a chart or map which represents the intersection of the land with the water surface at the elevation of mean low water.

Mean low water springs (MLWS)

A tidal datum. Frequently abbreviated spring low water. The arithmetic mean of the low water heights occurring at the time of spring tides observed over the National Tidal Datum Epoch. It is usually derived by taking an elevation depressed below the half-tide level by an amount equal to one-half the spring range of tide, necessary corrections being applied to reduce the result to a mean value.

Mean lower low water line (MLLWL)

The line on a chart or map which represents the intersection of the land with the water surface at the elevation of mean lower low water.

Mean range of tide (Mn)

The difference in height between mean high water and mean low water.

Mean water level (MWL)

A datum. The mean surface elevation as determined by averaging the heights of the water at equal intervals of time, usually hourly. Mean water level is used in areas of little or no range in tide.

Mean water level line (MWLL)

The line on a chart or map which represents the intersection of the land with the water surface at the elevation of mean water level.

Meteorological tides

Tidal constituents having their origin in the daily or seasonal variations in weather conditions which may occur with some degree of periodicity. The principal meteorological constituents recognized in the tides are Sa, Ssa, and S1. See storm surge.

Mf

Lunar fort nightly constituent. This constituent expresses the effect of departure from a sinusoidal declination Al motion. Speed = 2s = 1.098,033,1° per solar hour.

Mm

Lunar monthly constituent. This constituent expresses the effect of irregularities in the Moon's rate of change of distance and speed in orbit. Speed = s - p = 0.544,374,7° per solar hour.

MSf

Lunisolar synodic fort nightly constituent. Speed = 2s - 2h = 1.015,895,8° per solar hour.

mu

Variational constituent. See lambda. Speed = 2T - 4s + 4h = 27.968,208,4° per solar hour.

N

Rate of change (as of January 1, 1900) in mean longitude of the Moon's node. N = 0.002,206,41° per solar hour.

N2

Larger lunar elliptic semi diurnal constituent. See L2 Speed = 2T - 3s + 2h + p = 28.439,729,5° per solar hour.

2N2

Lunar elliptic semi diurnal second-order constituent. Speed = 2T - 4s + 2h + 2p = 27.895,354,8° per solar hour.

Neap tides or tidal currents

Tides of decreased range or tidal currents of decreased speed occurring semimonthly as the result of the Moon being in quadrature. The neap range (Np) of the tide is the average range occurring at the time of neap tides and is most conveniently computed from the harmonic constants. It is smaller than the mean range where the type of tide is either semi diurnal or mixed and is of no practical significance where the type of tide is predominantly diurnal. The average height of the high waters of the neap tide is called neap high water or high water neaps (MHWN) and the average height of the corresponding low waters is called neap low water or low water neaps (MLWN).

Nu (v2)

Larger lunar evectional constituent. See lambda. Speed = 2T - 3s + 4h - p = 28.512,583,1° per solar hour.

O1

Lunar diurnal constituent. See K1. Speed = T - 2s + h = 13.943,035,6° per solar hour.

OO1

Lunar diurnal, second-order, constituent. Speed = T + 2s + h = 16.139,101,7° per solar hour.

Overtide

A harmonic tidal (or tidal current) constituent with a speed that is an exact multiple of the speed of one of the fundamental constituents derived from the development of the tide-producing force. The presence of overtides is usually attributed to shallow water conditions. The overtides usually considered in tidal work are the harmonics of the principal lunar and solar semi diurnal constituents M2 and S2, and are designated by the symbols M4, M6, M8, S4, S6, etc. The magnitudes of these harmonics relative to those of the fundamental constituents are usually greater in the tidal current than in the tide.

P

Rate of change (as of January 1, 1900) in mean longitude of lunar perigee. P = 0.004,641,83° per solar hour.

p1

Rate of change (as of January 1, 1900) in mean longitude of solar perigee. p1 = 0.000,001,96° per solar hour.

P1

Solar diurnal constituent. See K1. Speed = T - h = 14.958,931,4° per solar hour.

Perigean tides or tidal currents

Tides of increased range or tidal currents of increased speed occurring monthly as the result of the Moon being in perigee. The perigean range (Pn) of tide is the average range occurring at the time of perigean tides and is most conveniently computed from the harmonic constants. It is larger than the mean range where the type of tide is either semi diurnal or mixed, and is of no practical significance where the type of tide is predominantly diurnal.

Perigee

The point in the orbit of the Moon or man-made satellite nearest to the Earth. The point in the orbit of a satellite nearest to its companion body.

Perihelion

The point in the orbit of the Earth (or other planet, etc.) nearest to the Sun.

Phase

Any recurring aspect of a periodic phenomenon, such as new Moon, high water, flood strength, etc. A particular instant of a periodic function expressed in angular measure and reckoned from the time of its maximum value, the entire period of the function being taken as 360°. The maximum and minimum of a harmonic constituent have phase values of 0° and 180°, respectively.

Phase inequality

Variations in the tides or tidal currents due to changes in the phase of the Moon. At the times of new and full Moon the tide-producing forces of the Moon and Sun act in conjunction, causing the range of tide and speed of the tidal current to be greater than the average, the tides at these times being known as spring tides. At the times of the quadratures of the Moon these forces are opposed to each other, causing neap tides with diminished range and current speed.

Pressure Sensor

A pressure transducer sensing device for water level measurement. A relative transducer is vented to the atmosphere and pressure readings are made relative to atmospheric pressure. An absolute transducer measures the pressure at its location. The readings are then corrected for barometric pressure taken at the surface.

Ql

Larger lunar elliptic diurnal constituent. See M1. Speed = T - 3s + h + p = 13.398,660,9° per solar hour.

2Q1

Lunar elliptic diurnal, second order, constituent. Speed = T - 4s + h + 2p = 12.854,286,2° per solar hour.

Quadrature of Moon

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

R2

Smaller solar elliptic constituent. This constituent, with T2, modulates the amplitude and frequency of S2 for the effect of variation in the Earth's orbital speed due to its elliptical orbit. Speed = 2T + h - p1 = 30.041,066,7° per solar hour.

Range of tide

The difference in height between consecutive high and low waters. The mean range is the difference in height between mean high water and mean low water. The great diurnal range or diurnal range is the difference in height between mean higher high water and mean lower low water. For other ranges see spring, neap, perigean, apogean, and tropic tides; and tropic ranges.

Relative mean sea level change

A local change in mean sea level relative to a network of bench marks established in the most stable and permanent material available (bedrock, if possible) on the land adjacent to the tide station location. A change in relative mean sea level may be composed of both an absolute mean sea level change component and a vertical land movement change component, together.

rho

Larger lunar evectional diurnal constituent. Speed = T - 3s + 3h - p = 13.471,514,5° per solar hour.

s

Rate of change (as of January 1, 1900) in mean longitude of Moon. s = 0.549,016,53° per solar hour.

S1

Solar diurnal constituent. Speed = T = 15.000,000,0° per solar hour.

S2

Principal solar semi diurnal constituent. This constituent represents the rotation of the Earth with respect to the Sun. Speed = 2T = 30.000,000,0° per solar hour.

S4, S6

Shallow water overtides of the principal solar constituent.
Speed of S4 = 2S2 = 4T = 60.000,000,0° per solar hour.
Speed of S6 = 3S2 = 6T = 90.000,000,0° per solar hour.

Sa

Solar annual constituent. This constituent, with Ssa, accounts for the nonuniform changes in the Sun's declination and distance. In actuality, they mostly reflect yearly meteorological variations influencing sea level.
Speed = h = 0.041,068,64° per solar hour.

Ssa

Solar semiannual constituent. See Sa.
Speed = 2h = 0.082,137,3° per solar hour.

Salinity (S)

The total amount of solid material in grams contained in 1 kilogram of sea water when all the carbonate has been converted to oxide, the bromine and iodine replaced by chlorine, and all organic matter completely oxidized. S(°/oo) = 1.806,55 x Cl (°/oo) Where Cl(°/oo) is chlorinity in parts per thousand.

Seiche

A stationary wave usually caused by strong winds and/or changes in barometric pressure. It is found in lakes, semi enclosed bodies of water, and in areas of the open ocean. The period of a seiche in an enclosed rectangular body of water is usually represented by the formula: Period (T) = 2L / square root(gd) in which L is the length, d the average depth of the body of water, and g the acceleration of gravity. See standing wave.

Semidiurnal

Having a period or cycle of approximately one-half of a tidal day. The predominant type of tide throughout the world is semi diurnal, with two high waters and two low waters each tidal day. The tidal current is said to be semi diurnal when there are two flood and two ebb periods each day. A semi diurnal constituent has two maxima and two minima each constituent day, and its symbol is the subscript 2. See type of tide.

Shallow water constituent

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

Slack water (slack)

The state of a tidal current when its speed is near zero, especially the moment when a reversing current changes direction and its speed is zero. The term also is applied to the entire period of low speed near the time of turning of the current when it is too weak to be of any practical importance in navigation. The relation of the time of slack water to the tidal phases varies in different localities. For a perfect standing tidal wave, slack water occurs at the time of high and of low water, while for a perfect progressive tidal wave, slack occurs midway between high and low water.

Small diurnal range (Sl)

Difference in height between mean lower high water and mean higher low water.

Small tropic range (Sc)

Difference in height between tropic lower high water and tropic higher low water.

Solar tide

The part of the tide that is due to the tide-producing force of the Sun.
The observed tide in areas where the solar tide is dominant. This condition provides for phase repetition at about the same time each solar day.

Species of constituent

A classification depending upon the period of a constituent. The principal species are semidiurnal, diurnal, and long-period.

Speed (of constituent)

The rate f change in the phase of a constituent, usually expressed in degrees per hour. The speed is equal to 360° divided by the constituent period expressed in hours.

Spring high water

Same as mean high water springs (MHWS). See spring tides.

Spring low water

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

Spring tides or tidal currents

Tides of increased range or tidal currents of increased speed occurring semimonthly as the result of the Moon being new or full. The spring range (Sg) of tide is the average range occurring at the time of spring tides and is most conveniently computed from the harmonic constants. It is larger than the mean range where the type of tide is either semi diurnal or mixed, and is of no practical significance where the type of tide is predominantly diurnal. The average height of the high waters of the spring tides is called spring high water or mean high water springs (MHWS) and the average height of the corresponding low waters is called spring low water or mean low water springs (MLWS).

Stand of tide

Sometimes called a platform tide. An interval at high or low water when there is no sensible change in the height of the tide. The water level is stationary at high and low water for only an instant, but the change in level near these times is so slow that it is not usually perceptible. In general, the duration of the apparent stand will depend upon the range of tide, being longer for a small range than for a large range, but where there is a tendency for a double tide the stand may last for several hours even with a large range of tide.

Storm surge

The local change in the elevation of the ocean along a shore due to a storm. The storm surge is measured by subtracting the astronomic tidal elevation from the total elevation. It typically has a duration of a few hours. Since wind generated waves ride on top of the storm surge (and are not included in the definition), the total instantaneous elevation may greatly exceed the predicted storm surge plus astronomic tide. It is potentially catastrophic, especially on low lying coasts with gently sloping offshore topography.

T

Rate of change of hour angle of mean Sun at place of observation. T = 15° per mean solar hour.

T2

Larger solar elliptic constituent. See R2. Speed = 2T - h + p1 = 29.958,933,3° per solar hour.

Tidal bore

A tidal wave that propagates up a relatively shallow and sloping estuary or river in a solitary wave form. The leading edge presents an abrupt rise in level, frequently with continuous breaking and often immediately followed by several large undulations. An uncommon phenomenon, the tidal bore is usually associated with very large ranges in tide as well as wedge shaped and rapidly shoaling entrances. Also called eagre, eager (for Tsientan, China bore), mascaret (French), pororoca (Brazilian), and bore.

Tidal characteristics

Principally, those features relating to the time, range, and type of tide.

Tidal constants

Tidal relations that remain practically constant for any particular locality. Tidal constants are classified as harmonic and non harmonic. The harmonic constants consist of the amplitudes and epochs of the harmonic constituents, and the non harmonic constants include the ranges and intervals derived directly from the high and low water observations.

Tidal current

A horizontal movement of the water caused by gravitational interactions between the Sun, Moon, and Earth. The horizontal component of the particulate motion of a tidal wave. Part of the same general movement of the sea that is manifested in the vertical rise and fall called tide.

Tidal difference

Difference in time or height between a high or low water at a subordinate station and a reference station for which predictions are given in the Tide Tables. The difference, when applied according to sign to the prediction at the reference station, gives the corresponding time or height for the subordinate station.

Tide

The periodic rise and fall of the water resulting from gravitational interactions between Sun, Moon, and Earth. The vertical component of the particulate motion of a tidal wave. Although the accompanying horizontal movement of the water is part of the same phenomenon, it is preferable to designate this motion as tidal current. See tidal wave.

Tide curve

A graphic representation of the rise and fall of the tide in which time is usually represented by the abscissa and height by the ordinate. For a semidiurnal tide with little diurnal inequality, the graphic representation approximates a cosine curve.

Tide (water level) gauge

An instrument for measuring the rise and fall of the tide (water level).

Tide-producing force

That part of the gravitational attraction of the Moon and Sun which is effective in producing the tides on the Earth. The force varies approximately as the mass of the attracting body and inversely as the cube of its distance. The tide-producing force exerted by the Sun is a little less than one-half as great as that of the Moon.

Tide Tables

Tables which give daily predictions of the times and heights of high and low waters. These predictions are usually supplemented by tidal differences and constants through which predictions can be obtained for numerous other locations.

Tsunami

A shallow water progressive wave, potentially catastrophic, caused by an underwater earthquake or volcano.

Type of tide

A classification based on characteristic forms of a tide curve. Qualitatively, when the two high waters and two low waters of each tidal day are approximately equal in height, the tide is said to be semidiurnal; when there is a relatively large diurnal inequality in the high or low waters or both, it is said to be mixed; and when there is only one high water and one low water in each tidal day, it is said to be diurnal. Quantitatively (after Dietrich), where the ratio of K1 + O1 to M2 + S2 is less than 0.25, the tide is classified as semidiurnal; where the ratio is from 0.25 to 1.5, the tide is mixed, mainly semidiurnal; where the ratio is
from 1.5 to 3.0, the tide is mixed, mainly diurnal; and where greater than 3.0, diurnal.

Universal time (UTC)

Same as Greenwich mean time (GMT).

Vanishing tide

In a predominantly mixed tide with very large diurnal inequality, the lower high water (or higher low water) becomes indistinct (or vanishes) at times of extreme declinations.

ZO

Symbol recommended by the International Hydrographic Organization to represent the elevation of mean sea level above chart datum.
 

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