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datum of tabulation—A permanent base elevation at
a tide station to which all water level measurements are
referred. The datum is unique to each station and is
established at a lower elevation than the water is ever
expected to reach. It is referenced to the primary bench
mark at the station and is held constant regardless of
changes to the water level gauge or tide staff. The datum of
tabulation is most often at the zero of the first tide staff
installed.
Davidson Current—A North Pacific Ocean counter-
current setting northward between the California Current
and the coasts of California, Oregon, and Washington
during the winter months.
day—The period of rotation of the Earth. There are
several kinds of days depending on whether the Sun, Moon,
or other object or location is used as the reference for the
rotation. See constituent day, lunar day, sidereal day, and
solar day.
daylight saving time—A time used during the summer
months, in some localities, in which clocks are advanced 1
hour from the usual standard time.
decibar—The practical unit for pressure in the ocean,
equal to 10 centibars, and is the approximate pressure
produced by each meter of overlying water
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 27-½ 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).
declinational inequality—Same as diurnal inequality.
declinational reduction—A processing of observed
high and low waters or flood and ebb tidal currents to obtain
quantities depending upon changes in the declination of the
Moon; such as tropic ranges or speeds, height or speed
inequalities, and tropic intervals.
s,t,p
density, in situ (D )—Mass per unit volume. The
reciprocal of specific volume. In oceanography, the density
of sea water, when expressed in gm/cm , is numerically
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equivalent to specific gravity and is a function of salinity,
temperature, and pressure. See specific volume anomaly,
thermosteric anomaly, sigma-t, and sigma-zero.
deviation (of compass)—The deflection of the needle
of a magnetic compass due to masses of magnetic metal
within a ship on which the compass is located. This
deflection varies with different headings of the ship. The
deviation is called easterly and marked plus if the deflection
is to the right of magnetic north, and is called westerly and
marked minus if it is to the left of magnetic north. A
deviation table is a tabular arrangement showing the amount
of deviation for different headings of the ship. Each
compass requires a separate deviation table.
digital tide (water level) gauge—See automatic tide
(water level) gauge.
direct method—A tidal datum computation method.
Datums are determined directly by comparison with an
appropriate control, for the available part of the tidal cycle.
It is usually used only when a full range of tidal values are
not available. For example: Direct Mean High Water, when
low waters are not recorded.
direction of current—Same as set.
direction of wind—Direction from which the wind is
blowing.
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 stationary wave theory and 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