ANNUAL INEQUALITY---Seasonal variation in the
water level or current, more or less periodic, due chiefly to meteorological
causes. .
APOGEAN TIDES OR TIDAL CURRENTS---Tides of decreased
range or currents of decreased speed occurring monthly as the result
of the Moon being in apogee (farthest from the Earth).
AUTOMATIC TIDE GAGE---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
intervals in digital format, in others by a continuous graph in
which the height, versus corresponding time of the tide, is recorded.
BENCH MARK (BM)---A fixed physical object or
marks 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 geodetic bench mark identifies a surveyed
point in the National Geodetic Vertical Network.
CHART DATUM---The tidal datum to which soundings
on a chart are referred. It is usually taken to correspond to a
low water elevation of the tide, and its depression below mean sea
level is represented by the symbol Zo.
CURRENT---Generally, a horizontal movement of
water. Currents may be classified as tidal and nontidal.
Tidal currents are caused by gravitational interactions between
the Sun, Moon, and Earth and are a part of the same general movement
of the sea that is manifested in the vertical rise and fall, called
tide. Nontidal currents include the permanent currents in
the general circulatory systems of the sea as well as temporary
currents arising from more pronounced meteorological variability.
CURRENT DIFFERENCE---Difference between the time
of slack water (or minimum current) or strength of current in any
locality and the time of the corresponding phase of the tidal current
at a reference station, for which predictions are given in the
Tidal Current Tables.
CURRENT ELLIPSE---A graphic representation of
a rotary current in which the velocity of the current at different
hours of the tidal cycle is represented by radius vectors and vectorial
angles. A line joining the extremities of the radius vectors will
form a curve roughly approximating an ellipse. The cycle is completed
in one-half tidal day or in a whole tidal day according to whether
the tidal current is of the semidiurnal or the diurnal type. A current
of the mixed type will give a curve of two unequal loops each tidal
day.
CURRENT METER---An instrument for measuring the
speed and direction or just the speed of a current. The measurements
are usually Eulerian since the meter is most often fixed or moored
at a specific location.
DATUM (vertical)---For marine applications, a
base elevation used as a reference from which to reckon heights
or depths. It is called a tidal datum when defined by a certain
phase of the tide. Tidal datums are local datums and should not
be extended into areas which have differing topographic features
without substantiating measurements. In order that they may be recovered
when needed, such datums are referenced to fixed points known as
bench marks.
DAYLIGHT SAVING TIME---A time used during the
summer in some localities in which clocks are advanced 1 hour from
the usual standard time.
DIURNAL---Having a period or cycle of approximately
1 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 single
ebb period in the tidal day. A rotary current is diurnal if it changes
its direction through all points of the compass once each tidal
day.
DIURNAL INEQUALITY---The difference in height
of the two high waters or of the two low waters of each day; also
the difference in speed between the two flood tidal currents or
the two ebb tidal currents of each 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 an
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 day observed over a specific 19-year Metonic cycle
(the National Tidal Datum Epoch). It is obtained by subtracting
the mean of all 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 day observed over
a specific 19-year Metonic cycle (the National Tidal Datum Epoch).
It is obtained by subtracting the mean of the lower low waters from
the mean of all low waters. Tropic high water inequality
(HWQ) is the average difference between the two high waters of the
day at the times of the tropic tides. Tropic low water inequality
(LWQ) is the average difference between the two low waters of
the day at the times of the 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.
DOUBLE EBB---An ebb tidal current where, after
ebb begins, the speed increases to a maximum called first ebb; it
then 'decreases, reaching a minimum ebb near the middle of the ebb
period (and at some places it may actually run in a flood direction
for a short period); it then again ebbs to a maximum speed called
second ebb after which it decreases to slack water.
DOUBLE FLOOD---A flood tidal current where, after
flood begins, the speed increases to a maximum called first flood;
it then decreases, reaching a minimum flood near the middle of the
flood period (and at some places it may actually run in an ebb direction
for a short period); it then again floods to a maximum speed called
second flood after which it decreases to slack water.
DOUBLE TIDE---A double-headed tide, that is,
a high water consisting of two maxima of nearly the same height
separated by a relatively small depression, or a low water consisting
of two minima separated by a relatively small elevation. Sometimes,
it is called an agger.
DURATION OF FLOOD AND DURATION OF EBB---Duration
of flood is the interval of time in which a tidal current is flooding,
and the duration of ebb is the interval in which it is ebbing. Together
they cover, on an average, a period of 12.42 hours for a semidiurnal
tidal current or a period of 24.84 hours for a diurnal current.
In a normal semidiurnal tidal current, the duration of flood and
duration of ebb will each be approximately equal to 6.21 hours,
but the times may be modified greatly by the presence of a nontidal
flow. In a river the duration of ebb is usually longer than the
duration of flood because of the freshwater discharge, especially
during the spring when snow and ice melt are the predominant influences.
DURATION OF RISE AND DURATION OF FALL---Duration
of rise is the interval from low water to high water, and duration
of fall is the interval from high water to low water. Together they
cover, on an average, a period of 12.42 hours for a semidiurnal
tide or a period of 24.84 hours for a diurnal tide. In a normal
semidiurnal tide, the duration of rise and duration of fall will
each be approximately equal to 6.21 hours, but in shallow waters
and in rivers there is a tendency for a decrease in the duration
of rise and a corresponding increase in the duration of fall.
EBB CURRENT--- the movement of a tidal current
away from shore or down a tidal river or estuary. In the mixed type
of reversing tidal current, the terms greater ebb and lesser ebb
are applied respectively to the ebb tidal currents of greater and
lesser speed of each day. The terms maximum ebb and minimum ebb
are applied to the maximum and minimum speeds of a current running
continuously ebb, the speed alternately increasing and decreasing
without coming to a slack or reversing. The expression maximum ebb
is also applicable to any ebb current at the time of greatest speed.
EQUATORIAL TIDAL CURRENTS---Tidal currents occurring
semimonthly as a result of the Moon being over the Equator. At these
times the tendency of the Moon to produce a diurnal inequality in
the tidal current is at a minimum. EQUATORIAL TIDES-Tides occurring
semimonthly as the 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.
FLOOD CURRENT---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 flood currents of greater and lesser
speed of each day. The terms maximum flood and minimum flood are
applied to the maximum and minimum speeds of a flood current, the
speed of which alternately increases and decreases without coming
to a slack or reversing. The expression maximum flood is also applicable
to any flood current at the time of greatest speed.
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.
GULF COAST LOW WATER DATUM---A chart datum. Specifically,
the tidal datum designated for the coastal waters of the Gulf Coast
of the United States. It is defined as mean lower low water when
the type of tide is mixed and mean low water when the type of tide
is diurnal.
HALF-TIDE LEVEL---See mean tide level.
HIGH WATER (HW)---The maximum height reached
by a rising tide. The height may be due solely to the periodic tidal
forces or it may have superimposed upon it the effects of prevailing
meteorological conditions. Use of the synonymous term, high tide,
is discouraged.
HIGHER HIGH WATER (HHW)---The higher of the two
high waters of any tidal day.
HIGHER LOW WATER (HLW)---The higher of the two
low waters of any tidal day.
HYDRAULIC CURRENT---A current in a channel caused
by a difference in the surface level at the two ends. Such a current
may be expected in a strait connecting two bodies of water in which
the tides differ in time or range. The current in the East River,
N.Y., connecting Long Island Sound and New York Harbor, is an example.
KNOT---A speed unit of I international nautical
mile (1,852.0 meters or 6.076.11549 international feet) per hour.
LOW WATER (LW)---The minimum height reached by
a falling tide. The height may be due solely to the periodic tidal
forces it may have superimposed upon it the effects of meteorological
conditions. Use of the synonymous term, low tide, is discouraged.
LOWER HIGH WATER (LHW)-The lower of the two high waters of any tidal
day.
LOWER LOW WATER (LLW)---The lower of the two
low waters of any tidal day.
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 long, or 1.035 times
as long as the mean solar day.
LUNAR INTERVAL---The difference in time between
the transit of the Moon over the meridian of Greenwich and over
a local meridian. The average value of this interval expressed in
hours is 0.069 L, in which L is the local longitude in degrees,
positive for west longitude and negative for east longitude. The
lunar interval equals the difference between the local and Greenwich
interval of a tide or current phase.
LUNICURRENT INTERVAL---The interval between
the Moon's transit (upper or lower) over the local or Greenwich
meridian and a specified phase of the tidal current following
the transit. Examples: strength of flood interval and
strength of ebb interval, which may be abbreviated to
flood interval and ebb interval, respectively. The
interval is described as local or Greenwich according to whether
the reference is to the Moon's transit over the local or Greenwich
meridian. When not otherwise specified, the reference is assumed
to be local.
LUNITIDAL INTERVAL---The interval between the
Moon's transit (upper or lower) over the local or Greenwich meridian
and the following high or low water. The average of all high water
intervals for all phases of the Moon is known as mean high water
lunitidal interval and is abbreviated to high water interval
(HWI). Similarly the mean low water lunitidal interval
is abbreviated to low water interval (LWI). The interval
is described as local or Greenwich according to whether the reference
is to the transit over the local or Greenwich meridian. When not
otherwise specified, the reference is assumed to be local.
MEAN HIGH WATER (MHW)---A tidal datum.The average
of all the high water heights observed over the National Tidal Datum
Epoch. (See High Water.) For stations with shorter series, simultaneous
observational comparisons are made with a control tide station in
order to derive the equivalent of a 19-year datum. MEAN HIGHER HIGH
WATER (MHHW)- A tidal datum. The average of the highest 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 of a 19-year datum.
MEAN HIGHER HIGH WATER LINE (MHHWL)---The intersection
of the land with the water surface at the elevation of mean higher
high water.
MEAN LOW WATER (MLW)---A tidal datum. The average
of all the low water heights observed over the National Tidal Datum
Epoch. (See Low Water.) For stations with shorter series, simultaneous
observational comparisons are made with a control tide station in
order to derive the equivalent of a 19-year datum.
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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 the spring tides
observed over a specific 19-year Metronic cycle (the National Tidal
Datum Epoch).
MEAN LOWER LOW WATER (MLLW)---A tidal datum.
The average of the lowest low 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 of a 19 year datum.
MEAN RANGE OF TIDE (Mn)---The difference in height
between mean high water and mean low water.
MEAN RIVER LEVEL---A tidal datum. The average
height of the surface of a tidal river at any point for all stages
of the tide observed over a 19-year Metonic cycle (the National
Tidal Datum Epoch), usually determined from hourly height readings.
In rivers subject to occasional freshets the river level may undergo
wide variations, and for practical purposes certain months of the
year may be excluded in the determination of tidal datums. For charting
purposes, tidal datums for rivers are usually based on observations
during selected periods when the river is at or near low water stage.
MEAN SEA LEVEL (MSL)---A tidal datum. The arithmetic
mean of hourly water elevations observed over a specific 19-year
Metonic cycle (the National Tidal Datum Epoch). Shorter series are
specified in the name; e.g., monthly mean sea level and yearly mean
sea level.
MEAN TIDE LEVEL (MTL)---Also called half-tide
level. A tidal datum midway between mean high water and mean low
water.
MIXED TIDE---Type of tide with a large inequality
in the high and/or low water heights, with two high waters and two
low waters usually occurring each tidal day. In strictness, all
tides are mixed but the name is usually applied to the tides intermediate
to those predominantly semidiurnal and those predominantly diurnal.
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 semidiurnal range occurring
at the time of neap tides and is most conveniently computed from
the harmonic constants. It is smaller than the mean range where
the type of tide is either semidiurnal or mixed and is of no practical
significance where the type of tide is diurnal. The average height
of the high waters of the neap tides 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) .
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 or nearest the Earth. The
perigean range (Pn) of tide is the average semidiurnal range
occurring at the time of perigean tides and is most conveniently
computed from the harmonic constants. It is larger than the mean
range where the type of tide is either semidiurnal or mixed, and
is of no practical significance where the type of tide is diurnal.
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. Where the
type of tide is diurnal the mean range is the same as the diurnal
range. For other ranges, see great diurnal, spring, neap, perigean,
apogean, and tropic tides.
REFERENCE STATION---A tide or current station
for which independent daily predictions are given in the Tide
Tables and Tidal Current Tables, and from which corresponding
predictions are obtained for subordinate stations by means of differences
and ratios.
REVERSING CURRENT---A tidal current which flows
alternately in approximately opposite directions with a slack water
at each reversal of direction. Currents of this type usually occur
in rivers and straits where the direction of flow is more or less
restricted to certain channels. When the movement is towards the
shore or up a stream, the current is said to be flooding, and when
in the opposite direction it is said to be ebbing. The combined
flood and ebb movement including the slack water covers, on an average,
12.42 hours for the semidiurnal current. If unaffected by a nontidal
flow, the flood and ebb movements will each last about 6 hours,
but when combined with such a flow, the durations of flood and ebb
may be quite unequal. During the flow in each direction the speed
of the current will vary from zero at the time of slack water to
a maximum about midway between the slacks.
ROTARY CURRENT---A tidal current that flows continually
with the direction of flow changing through all points of the compass
during the tidal period. Rotary currents are usually found &shore
where the direction of flow is not restricted by any barriers. The
tendency for the rotation in direction has its origin in the Coriolis
force and, unless modified by local conditions, the change is clockwise
in the Northern Hemisphere and counterclockwise in the Southern.
The speed of the current usually varies throughout the tidal cycle,
passing through the two maxima in approximately opposite directions
and the two minima with the direction of the current at approximately
90° from the direction at time of maximum speed.
SEMIDIURNAL---Having a period or cycle of approximately
one-half of a tidal day. The predominating type of tide throughout
the world is semidiurnal, with two high waters and two low waters
each tidal day. The tidal current is said to be semidiurnal when
there are two flood and two ebb periods each day.
SET (OF CURRENT)---The direction towards
which the current flows.
SLACK WATER---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 is also 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 standing tidal waves,
slack water occurs near the times of high and low water, while for
progressive tidal waves, slack water occurs midway between high
and low water.
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 semidiurnal 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 semidiurnal or mixed, and is of no practical significance
where the type of tide is diurnal. The mean of the high waters of
the spring tide is called spring high water or mean high
water springs (MHWS), and the average height of the corresponding
low waters is called spring low water or mean low water
springs (MLWS).
STAND OF TIDE---Sometimes called a platform tide.
An interval at high or low water when there is no sensible change
in the height of the tide. The water level is stationary at high
and low water for only an instant, but the change in level near
these times is so slow that it is not usually perceptible. In general,
the duration of the apparent stand will depend upon the range of
tide, being longer for a small range than for a large range, but
where there is a tendency for a double tide the stand may last for
several hours even with a large range of tide.
STANDARD TIME---A kind of time based upon the
transit of the Sun over a certain specified meridian, called the
time meridian, and adopted for use over a considerable area. With
a few exceptions, standard time is based upon some meridian which
differs by a multiple of 15° from the meridian of Greenwich.
STRENGTH OF CURRENT---Phase of tidal current
in which the speed is a maximum; also the speed at this time. Beginning
with slack before flood in the period of a reversing tidal current
(or minimum before flood in a rotary Current) , the speed gradually
increases to flood strength and then diminishes to slack before
ebb (or minimum before ebb in a rotary current), after which the
current turns in direction, the speed increases to ebb strength
and then diminishes to slack before flood completing the cycle.
If it is assumed that the speed throughout the cycle varies as the
ordinates of a cosine curve, it can be shown that the average speed
for an entire flood or ebb period is equal to 2/π
or 0.6366 of the speed of the corresponding strength of current.
SUBORDINATE CURRENT STATION---(1) A current station
from which a relatively short series of observations is reduced
by comparison with simultaneous observations from a control current
station. (2) A station listed in the Tidal Current Tables for which
predictions are to be obtained by means of differences and ratios
applied to the full predictions at a reference station.
SUBORDINATE TIDE STATION---(1) A tide station
from which a relatively short series of observations is reduced
by comparison with simultaneous observations from a tide station
with a relatively long series of observations. (2) A station listed
in the Tide Tables for which predictions are to be obtained by means
of differences and ratios applied to the full predictions at a reference
station.
TIDAL CURRENT TABLES---Tables which give daily
predictions of the times and speeds of the tidal currents. These
predictions are usually supplemented by current differences and
constants through which additional predictions can be obtained for
numerous other places.
TIDAL DIFFERENCE---Difference in time or height
of a high or low water at a subordinate station and at 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 the 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
the motion as tidal current.
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 additional predictions can be obtained for numerous other
places.
TIME MERIDIAN---A meridian used as a reference
for time.
TROPIC CURRENTS---Tidal currents occurring semimonthly
when the effect of the Moon's maximum declination is greatest. At
these times the tendency of the Moon to produce a diurnal inequality
in the current is at a maximum.
TROPIC RANGES---The great tropic range (Gc) ,
or tropic range, is the difference in height between tropic higher
high water and tropic lower low water. The small tropic range (Sc)
is the difference in height between tropic lower high water and
tropic higher low water. The mean tropic range (Mc) is the mean
between the great tropic range and the small tropic range. The small
tropic range and the mean tropic range are applicable only when
the type of tide is semidiurnal or mixed. Tropic ranges are most
conveniently computed from the harmonic constants.
TROPIC TIDES---Tides occurring semimonthly when
the effect of the Moon's maximum declination is greatest. At these
times there is a tendency for an increase in the diurnal range.
The tidal datums pertaining to the tropic tides are designated as
tropic higher high water (TcHHW), tropic lower high water (TcLHW),
tropic higher low water (TcHLW), and tropic lower low water
(TcLLW).
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.
VANISHING TIDE---In a mixed tide with very large
diurnal inequality, the lower high water (or higher low water) frequently
becomes indistinct (or vanishes) at time of extreme declinations.
During these periods the diurnal tide has such overriding dominance
that the sernidiurnal tide, although still present, cannot be readily
seen on the tide curve.
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