agger -- See double tide.
chart datum -- (or datum, datum plane, hydrographic datum, plane of reference, reference
plane, tidal datum, tidal datum plane). The permanently established
surface from which soundings or tide heights are referenced (usually low water). The surface is called a tidal datum when referred
to a certain phase of the tide. In order to provide a factor of
safety, some level lower than mean sea level is generally selected,
such as mean low water or mean lower low water.
conjunction -- The situation of two celestial bodies with the same celestial longitude
(the angular distance measured east of the vernal equinox along the ecliptic);
for example, conjunction occurs when the moon and the sun are directly
in line with the earth and the moon is between the earth and the sun.
datum -- 1. Any numerical or geometrical quantity or set of such quantities which
may serve as a reference or base for other quantities. 2. See chart
datum
datum plane -- See chart datum.
diurnal tide -- A tide in which there is only one high water and one low water each lunar day.
double high water -- See double tide.
double tide -- (or agger, double high water, gulder). A high
water consisting of two maximums of nearly the same height separated
by a relatively small depression, or a low water consisting of two
minimums separated by a relatively small elevation.
flood tide -- See rising tide.
GMT -- See Greenwich mean time
Greenwich mean
time -- (abbreviated GMT; also called zulu time) Local
mean time at the Greenwich meridian. [Local note:
To convert GMT(zulu) to Central Standard Time (CST), subtract 6 hours (
subtract 5 hours during Daylight Saving Time ). ] gulder -- A double low water occurring on the south coast of England. See double tide.
HHW -- See higher high water
high tide -- See high water.
higher high water -- (abbreviated HHW). The higher of two high waters occurring
during a tidal day where the tide exhibits mixed characteristics. See mixed tide.
high water -- (abbreviated HW; also called high tide). The highest limit of
the surface water level reached by the rising tide. High water is
caused by the astronomic tide-producing forces and/or the effects
of meteorological, hydrologic and/or oceanographic conditions.
HW -- See high water hydrographic datum -- See chart datum.
limb -- The outer edge of the apparent disk of a celestial body.
LLW -- See lower low water
lower low water -- (abbreviated LLW). The lower of two low waters of any tidal
day where the tide exhibits mixed characteristics. See mixed
tide.
low tide -- See low water.
low water -- (abbreviated LW; or low tide). The lowest limit of the surface
water level reached by the lowering tide. Low water is caused by the astronomic tide-producing forces and/or the effects of meteorological, hydrologic
and/or oceanographic conditions.
lunar day -- (or tidal day). The interval between two successive upper transits of the moon over a local meridian. The period of the mean
lunar day, approximately 24.84 solar hours, is derived from the rotation
of the earth on its axis relative to the movement of the moon about the
earth.
LW -- See low water mean
higher high water -- (abbreviated MHHW). The average height
of all the daily higher high waters recorded over a 19-year period,
or a computed equivalent period. It is usually associated with a tide
exhibiting mixed characteristics. See mixed tide.
mean high water -- (abbreviated MHW). The average height of all the high waters
recorded over a 19-year period, or a computed equivalent period.
mean
lower low water -- (abbreviated MLLW). The average height
of all the lower low waters recorded over a 19-year period, or
a computed equivalent period. It is usually associated with a tide exhibiting
mixed characteristics. See mixed tide.
mean sea level -- (abbreviated MSL; or sea level datum). The mean surface water
level determined by averaging heights at all stages of the tide over a
19-year period. Mean sea level is usually determined from hourly height
readings measured from a fixed predetermined reference level (chart
datum).
MHW -- See mean high water
MHHW -- See mean higher high water
mixed tide -- The type of tide in which a diurnal wave produces large inequalities
in heights and/or durations of successive high and/or low waters.
This term applies to the tides intermediate to those predominantly
semidiurnal and those predominantly diurnal.
MLLW -- See mean lower low water
moonrise -- The time when the upper limb of the moon appears above the sensible horizon (no adjustment made for elevation of observer or atmospheric
refraction).
moonset -- The time when the upper limb of the moon disappears below the sensible horizon (no adjustment made for elevation of observer or atmospheric
refraction).
MSL -- See mean sea level
National Tidal Datum Epoch -- (abbreviated NTDE). A period of 19 years adopted by the National Ocean Service as the period over which observations of tides are to be taken and reduced to average values for tidal datums. This 19-year period covers all variations in the path of the moon about the sun (regression of the moon’s nodes). The most recent NTDE of 1983-2001 was implemented in April 2003 to reflect the latest variations in Mean Sea Level (MSL) along the nation's coasts.
neaps -- See neap tide.
neap tide -- (or neaps). Tide of decreased range which occurs about
every two weeks when the moon is in quadrature.
NTDE -- See National Tidal Datum Epoch
opposition -- The situation of two celestial bodies with their celestial longitudes
(the angular distance measured east of the vernal equinox along the ecliptic)
differing by 180 degrees; for example, opposition occurs when the moon
and the sun are directly in line with the earth and on opposite sides of
the earth.
plane of reference -- See chart datum.
quadrature -- The position of the phase cycle when the two principal tide producing
bodies (moon and sun) are nearly at a right angle to the earth; the moon
is then in quadrature in its first quarter or last quarter.
reference level -- See chart datum.
reference plane -- See chart datum.
reference station -- (or standard station, standard port). A place where tide or tidal
current constants have been determined from observations, and which is
used as a standard for the comparison of simultaneous observations at a subordinate station. It is also a place for which independent daily
predictions are obtained for other locations by means of differences or
factors.
rising tide -- (sometimes called flood tide). The portion of the tide cycle
between low water and the following high water.
sea level datum -- See mean sea level.
semidiurnal tide -- The type of tide having two high waters and two low
waters each tidal day with small inequalities between successive
high and successive low water heights and durations.
sensible horizon -- the plane tangent to the earth's surface at an observers position
springs -- See spring tide.
spring tide -- (or springs). Tide of increased range which occurs about
every two weeks when the moon is at new moon or full moon (syzygy).
stand -- See stand of tide.
standard port -- British term for reference station.
standard station -- See reference station.
stand of tide -- (or stand, tidal stand). The interval at high or low water when there is no appreciable change in the height of the
tide; its duration will depend on the range of the tide, being longer when
the tide range is small and shorter when the tide range is
large. Where a double tide occurs, the stand may last for several
hours even with a large range of tide.
subordinate station -- 1. One of the places for which tide or tidal current predictions are
determined by applying a correction to the predictions of a reference
station. 2. A tide or
tidal current station at which a short series of observations has been
made, which are reduced by comparison with simultaneous observations at
a reference station.
sunrise -- The time when the upper limb of the sun appears above the sensible horizon as a result of the diurnal rotation of the earth (no
adjustment made for elevation of observer or atmospheric refraction).
sunset -- The time when the upper limb of the sun disappears below the sensible horizon as a result of the diurnal rotation of the earth (no
adjustment made for elevation of observer or atmospheric refraction).
syzygy -- The two points in the moon's orbit when the moon is in conjunction or opposition to the sun relative to the earth; time of new moon or full moon in the cycle of phases.
tidal datum -- See chart datum.
tidal datum plane -- See chart datum.
tidal day -- See lunar day.
tidal difference -- The difference in time or height of a high or low water between
a subordinate station and a reference station. The difference
is applied to the prediction at the reference station to obtain
the time or height of the tide at a subordinate station. These differences
are available in tide tables.
tidal stand -- See stand of tide.
tide -- The periodic rising and falling of the earth's oceans. It results from
the tide- producing forces of the moon and sun acting upon the rotating
earth. This disturbance actually propagates as a wave through the surface
layer of the oceans.
tide prediction -- Predetermined time and height of high or low water at
a reference station. May be computed in advance by mechanically
summing the harmonic constituents of which the tide is composed.
Used to compile tide tables.
tide range -- The difference in height between consecutive high and low
waters.
tide-producing
force(s) -- The slight local difference between the gravitational
attraction of two astronomical bodies and the centrifugal force that holds
them apart. These forces are exactly equal and opposite at the center of
gravity of either of the bodies, but, since gravitational attraction is
inversely proportional to the square of the distance, it varies from point
to point on the surface of the bodies. Therefore, gravitational attraction
predominates at the surface point nearest to the other body, while centrifugal
"repulsion" predominates at the surface point farthest from the other body.
Hence there are two regions where tide- producing forces are at
a maximum, and normally there are two tides each lunar day and solar
day.
tide tables -- Tables which give daily predictions, usually a year in advance, of the
times and heights of the tide. These predictions are usually supplemented
by tidal differences and constants by means of which additional predictions
can be obtained for numerous other places. See tide prediction, tidal difference.
transit -- The passage of the moon over the local meridian; it is designated as
upper transit when it crosses the observers meridian and as lower transit
when it crosses the same meridian but 180 degrees from the observer's location.
When specified, transit may be referred to the Greenwich meridian.
zulu time -- See Greenwich mean time |
