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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