|
D data collection platform (DCP)—A microprocessorbased system that collects data from sensors, processes the data, stores the data in random access memory (RAM), and provides communication links for the retrieval or transmission of the data. 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 in terms of a certain phase of the tide. Tidal datums are local datums and should not be extended into areas which have differing hydrographic characteristics without substantiating measurements. In order that they may be recovered when needed, such datums are referenced to fixed points known as bench marks. See chart datum. 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 countercurrent 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-˝° about June 21 and its maximum south declination of approximately 23- ˝° about December 21. The Moon has an average declinational cycle of 27-˝ 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-˝° when the longitude of the ascending node is 0°, to 18-˝° 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. density, in situ (Ds,t,p)—Mass per unit volume. The reciprocal of specific volume. In oceanography, the density of sea water, when expressed in gm/cm3, is numerically 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 either semidiurnal or mixed. Diurnal inequality is sometimes called declinational inequality. diurnal range—Same as great diurnal range. diurnal tide level—A tidal datum midway between mean higher high water and mean lower low water. double ebb—An ebb tidal current having two maxima of speed separated by a smaller ebb speed. double flood—A flood tidal current having two maxima of speed separated by a smaller flood speed. 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 called an agger. See gulder. drift (of current)—The speed of the current. drift current—Same as wind drift. duration of flood and duration of ebb—Duration of flood is the interval of time in which a tidal current is flooding, and duration of ebb is the interval in which it is ebbing, these intervals being reckoned from the middle of the intervening slack waters or minimum currents. 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 each will be approximately equal to 6.21 hours, but the times may be modified greatly by the presence of nontidal flow. In a river the duration of ebb is usually longer than the duration of flood because of fresh water discharge, especially during spring months when snow and ice melt are 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, 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. dynamic decimeter—See geopotential as preferred term. dynamic depth (height)—See geopotential difference as preferred term. dynamic depth (height) anomaly—See geopotential anomaly as preferred term. dynamic meter (D)—The former practical unit for geopotential difference (dynamic depth), equal to 10 geopotentials (dynamic decimeters). See geopotential (dynamic depth) anomaly. dynamic topography—See geopotential topography as preferred term.
|
