T—Rate of change of hour angle of mean Sun at place of observation.

T = 15° per mean solar hour.

Speed = 2T – h + p1 = 29.958,933,3° per solar hour.

tape gauge—See electric tape gauge.

telemetry—The capability of transmitting or retrieving data over long distance communication links, such as satellite, VHF radio, or telephone.

terdiurnal—Having three periods in a constituent day. The symbol of a terdiurnal constituent is the subscript 3.

tertiary tide station—A tide station at which continuous observations have been made over a minimum period of 30 days but less than 1 year. The series is reduced by comparison with simultaneous observations from a secondary control tide station. This station provides for a 29-day harmonic analysis. See tide station, primary control tide station, secondary control tide station, and subordinate tide station (1).

thermocline—A layer in which the temperature decreases significantly (relative to the layers above and below) with depth. The principal ones are designated diurnal, seasonal, and main thermocline.

thermosteric anomaly(*T, )', or )s,t)—The specific volume anomaly (steric anomaly) that would be attained if the water were changed isothermally to a standard pressure of one atmosphere. The specific volume anomaly with pressure terms omitted. See isanostere.

tidal bench mark—See bench mark.

tidal bench mark description—A published, concise description of the location, stamped number or designation, date established, and elevation (referred to a tidal datum) of a specific bench mark.

tidal bench mark state index map—A state map which indicates the locations for which tidal datums and tidal bench mark descriptions are available.

tidal bore—A tidal wave that propagates up a relatively shallow and sloping estuary or river with a steep wave front. The leading edge presents an abrupt rise in level, frequently with continuous breaking and often immediately followed by several large undulations. An uncommon phenomenon, the tidal bore is usually associated with very large ranges in tide as well as wedge shaped and rapidly shoaling entrances. Also called eagre, eager (for Tsientan, China bore), mascaret (French), pororoca (Brazilian), and bore.

tidal characteristics—Principally, those features relating to the time, range, and type of tide.

tidal constants—Tidal relations that remain practically constant for any particular locality. Tidal constants are classified as harmonic and nonharmonic. The harmonic constants consist of the amplitudes and epochs of the harmonic constituents, and the nonharmonic constants include the ranges and intervals derived directly from the high and low water observations.

tidal constituent—See constituent.

tidal current—A horizontal movement of the water caused by gravitational interactions between the Sun, Moon, and Earth. The horizontal component of the particulate motion of a tidal wave. Part of the same general movement of the sea that is manifested in the vertical rise and fall called tide. The United States equivalent of the British tidal stream. See tidal wave, tide, and current.

Tidal Current Chart Diagrams—A series of monthly diagrams to be used with the Tidal Current Charts. Each diagram contains lines that indicate the specific tidal current chart to use for a given date and time, and the speed factor to apply to that chart.

Tidal Current Charts—Charts on which tidal current data are depicted. Tidal Current Charts for a number of important waterways are published by the National Ocean Service. Each consists of a set of charts giving the speed and direction of the current for each hour or equal interval of the tidal cycle, thus presenting a comprehensive view of the tidal current movement.

tidal current constants—See current constants.

tidal current station—See current station.

Tidal Current Tables—Tables which give daily predictions of the times and velocities of the tidal currents. These predictions are usually supplemented by current differences and constants through which predictions can be obtained for numerous other locations.

tidal datum—See datum.

tidal day—Same as lunar day.

tidal difference—Difference in time or height between a high or low water at a subordinate station and 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.

tidal epoch—See National Tidal Datum Epoch and epoch.

tidal estuary—See estuary.

tidal stream—British equivalent of United States tidal current.

tidal wave—A shallow water wave caused by the gravitational interactions between the Sun, Moon, and Earth. Essentially, high water is the crest of a tidal wave and low water, the trough. Tidal current is the horizontal component of the particulate motion, while tide is manifested by the vertical component. The observed tide and tidal current can be considered the result of the combination of several tidal waves, each of which may vary from nearly pure progressive to nearly pure standing and with differing periods, heights, phase relationships, and direction.

tidal zoning—The practice of dividing a hydrographic survey area into discrete zones or sections, each one possessing similar tidal characteristics. One set of tide reducers is assigned to each zone. Tide reducers are used to adjust the soundings in that zone to chart datum (MLLW). Tidal zoning is necessary in order to correct for differing water level heights occurring throughout the survey area at any given time. Each zone of the survey area is geographically delineated such that the differences in time and range do not exceed certain limits, generally 0.2 hours and 0.2 feet respectively; however, these limits are subject to change depending upon type of survey, location, and tidal characteristics. The tide reducers are derived from the water levels recorded at an appropriate tide station, usually nearby. Tide reducers are used to correct the soundings throughout the hydrographic survey area to a common, uniform, uninterrupted chart datum. See tide reducers.

tide—The periodic rise and fall of a body of water resulting from gravitational interactions between 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 this motion as tidal current. See tidal  wave.

tide curve—A graphic representation of the rise and fall of the tide in which time is usually represented by the abscissa and height by the ordinate. For a semidiurnal tide with little diurnal inequality, the graphic representation approximates a cosine curve. See marigram.

tide datum—See datum.

tide (water level) gauge—An instrument for measuring the rise and fall of the tide (water level). See ADR gauge, automatic tide gauge, Next Generation Water Level Measurement System, gas purged pressure gauge, electric tape gauge, pressure gauge, and tide staff.

tide predicting machine—A mechanical analog machine especially designed to handle the great quantity of constituent summations required in the harmonic method. William Ferrel's Maxima and Minima Tide Predictor (described in Manual of Tides, U.S. Coast and Geodetic Survey, Appendix 10, Report for 1883) was the first such machine used in the United States. Summing only 19 constituents, but giving direct readings of the predicted times and heights of the high and low waters, the Ferrel machine was used for the predictions of 1885 through 1914. A second machine, developed by Rollin A. Harris and E. G. Fischer and summing 37 constituents, was used for the predictions of 1912 through 1965 (described in Manual of Harmonic Analysis and Prediction of Tides by Paul Schureman, U.S. Coast and Geodetic Survey Special Publication No. 98, 1958). Predictions are now prepared using an electronic digital computer.

tide-producing force—That part of the gravitational attraction of the Moon and Sun which is effective in producing the tides on the Earth. The force varies approximately as the mass of the attracting body and inversely as the cube of its distance. The tide-producing force exerted by the Sun is a little less than one-half as great as that of the Moon. A mathematical development of the vertical and horizontal components of the tide-producing forces of the Moon and Sun will be found in Coast and Geodetic Survey Special Publication No. 98.

tide reducers—Height corrections for reducing soundings to chart datum (MLLW). A tide reducer represents the height of the water level at a given place and time relative to chart datum. Tide reducers are obtained from one or more tide stations within or nearby the survey area. Often, due to differing tidal characteristics over the survey area, the tide reducers obtained directly from a tide station must be corrected to adjust for time and range of tide differences in the various zones of the hydrographic survey area. See tidal zoning.

tide rip—See rip.

tide staff—A water level gauge consisting of a vertical graduated staff from which the height of the water level can be read directly. It is called a fixed staff when secured in place so that it cannot be easily removed. A portable staff is one that is designed for removal from the water when not in use. For such a staff a fixed support is provided. The support has a metal stop secured to it so that the staff will always have the same elevation when installed for use. See electric tape gauge.

tide (water level) station—The  geographic location at which tidal observations are conducted. Also, the facilities used to make tidal observations. These may include a tide house, tide (water level) gauge, tide staff, and tidal bench marks. See primary control tide station, secondary control tide station, tertiary tide station, and subordinate tide station (1).

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 predictions can be obtained for numerous other locations.

tide wave—See tidal wave.

tidelands—The zone between the mean high water and mean low water lines. It is identical with intertidal zone (technical definition) when the type of tide is semidiurnal or diurnal.

tidewater—Water activated by the tide generating forces and/or water affected by the resulting tide, especially in coastal and estuarine areas. Also, a general term often applied to the land and water of estuarine areas formed by postglacial drowning of coastal plain rivers.

tideway—A channel through which a tidal current flows.

time—Time is measured by the rotation of the Earth with respect to some point in the celestial sphere and may be designated as sidereal, solar, or lunar, according to whether the measurement is taken in reference to the vernal equinox, the Sun, or the Moon. Solar time may be apparent or mean, according to whether the reference is to the actual Sun or the mean Sun. Mean solar time may be local or standard, according to whether it is based upon the transit of the Sun over the local meridian or a selected meridian adopted as a standard over a considerable area. Greenwich time is standard time based upon the meridian of Greenwich. In civil time the day commences at midnight, while in astronomical time, as used prior to 1925, the beginning of the day was reckoned from noon of the civil day of the same date. The name universal time is now applied to Greenwich mean civil time.

time meridian—A meridian used as a reference for time.

total current—The combination of the tidal and nontida  current. The United States equivalent of the British flow. See current.

tractive force—The horizontal component of a tideproducing force vector (directed parallel with level surfaces at that geographic location). transit—The passage of a celestial body over a specified meridian. The passage is designated as upper transit or lower transit according to whether it is over that part of the meridian lying above or below the polar axis.

tropic cur rents—Tidal currents occurring semimonthl 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 inequalities—Tropic high water inequality (HWQ) is the average difference between the two high waters of the day at the times of tropic tides. Tropic low water inequality (LWQ) is the average difference between the two low waters of the day at the times of tropic tides. These terms are applicable only when the type of tide is semidiurnal or mixed. See tropic tides.

tropic intervals—Tropic higher high water interval (TcHHWI) is the lunitidal interval pertaining to the higher high waters at the time of the tropic tides. Tropic lower low water interval (TcLLWI) is the lunitidal interval pertaining to the lower low waters at the time of the tropic tides. Tropic intervals are marked a when reference is made to the upper transit of the Moon at its north declination or to the lower transit at the time of south declination, and are marked b when the reference is to the lower transit at the north declination or to the upper transit at the south declination. See tropic tides.

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 and the small tropic range. Tropic ranges are most conveniently computed from the harmonic constants. See tropic tides.

tropic speed—The greater flood or greater ebb speed at the time of tropic currents.

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).

tropical month—The average period of the revolution of the Moon around the Earth with respect to the vernal equinox. Its length is approximately 27.321,582 days.

tropical year—The average period of the revolution of the Earth around the Sun with respect to the vernal equinox. Its length is approximately 365.242,2 days. The tropical year determines the cycle of changes in the seasons, and is the unit to which the calendar year is adjusted through the occasional introduction of the extra day on leap years.

trough—The lowest point in a propagating wave. See low water and tidal wave.

true direction—Direction relative to true north (0°) which is the direction of the north geographic pole. See compass direction and magnetic direction.

tsunami—A shallow water progressive wave, potentially catastrophic, caused by an underwater earthquake or volcano.

Tsushima Current—A North Pacific Ocean current setting northeastward in the East China Sea (in summer) and Sea of Japan. A segment of the Kuroshio System.

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. Quantitatively (after Dietrich), where the amplitude ratio of K1 + O1 to M2 + S2 is less than 0.25, the tide is classified as semidiurnal; where the ratio is from 0.25 to 1.5, the tide is mixed, mainly semidiurnal; where the ratio is from 1.5 to 3.0, the tide is mixed, mainly diurnal; and where greater than 3.0, diurnal.