s Rate of change (as of January 1, 1900) in mean longitude of Moon.s = 0.549,016,53° per solar hour. S1-Solar diurnal constituent.S1Solar diurnal constituent. Speed = T = 15.000,000,0° per solar hour.S2Principal solar semidiurnal constituent. This constituent represents the rotation of the Earth with respect to the Sun. Speed = 2T = 30.000,000,0° per solar hour.S4, S6Shallowwater overtides of the principal solar constituent. Speed of S4 = 2S2 = 4T = 60.000,000,0° per solar hour. Speed of S6 = 3S2 = 6T = 90.000,000,0° per solar hour.SaSolar annual constituent. This constituent, with Ssa, accounts for the nonuniform changes in the Sun's declination and distance. In actuality, they mostly reflect yearly meteorological variations influencing sea level. Speed = h = 0.041,068,64° per solar hour.SsaSolar semiannual constituent. See Sa. Speed = 2h = 0.082,137,3° per solar hour.salinity (S)The total amount of solid material in grams contained in 1 kilogram of sea water when all the carbonate has been converted to oxide, the bromine and iodine replaced by chlorine, and all organic matter completely oxidized. The following is approximate. S(‰) = 1.806,55 x Cl (‰) Where Cl(‰) is chlorinity in parts per thousand. See chlorinity.Sargasso SeaThe west central region of the sub-tropical gyre of the North Atlantic Ocean. It is bounded by the North Atlantic, Canary, North Equatorial, and Antilles Currents, and the Gulf Stream. It is characterized by the absence of any well-marked currents and by large quantities of drifting Sargassum, or gulfweed.SarosA period of 223 synodic months corresponding approximately to 19 eclipse years or 18.03 Julian years, and is a cycle in which solar and lunar eclipses repeat themselves under approximately the same conditions.sea level datum (SLD)An obsolete term. See National Geodetic Vertical Datum of 1929 and mean sealevel.secondary control tide stationA tide station at which continuous observations have been made over a minimum period of 1 year but less than 19 years. The series is reduced by comparison with simultaneous observations from a primary control tide station. This station provides for a 365-day harmonic analysis including the seasonal fluctuation of sea level. See tide station, primary control tide station, tertiary tide station, and subordinate tide station (1).secular trendSee apparent secular trend as preferred term.seicheA stationary wave usually caused by strong winds and/or changes in barometric pressure. It is found in lakes, semi-enclosed bodies of water, and in areas of the open ocean. The period of a seiche in an enclosed rectangular body of water is usually represented by the formula: Period (T) = 2L / √gd in which L is the length, d the average depth of the body of water, and g the acceleration of gravity. See standing wave.seismic sea waveSame as tsunami.semidiurnalHaving a period or cycle of approximately one-half of a tidal day. The predominant 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. A semidiurnal constituent has two maxima and two minima each constituent day, and its symbol is the subscript 2. See type of tide.sequence of currentThe order of occurrence of the four tidal current strengths of a day, with special reference as to whether the greater flood immediately precedes or follows the greater ebb.sequence of tideThe order in which the four tides of a day occur, with special reference as to whether the higher high water immediately precedes or follows the lower low water.set (of current)The direction towards which the current flows.shallow water constituentA short-period harmonic term introduced into the formula of tidal (or tidal current) constituents to account for the change in the form of a tide wave resulting from shallow water conditions. Shallow water constituents include the overtides and compound tides.shallow water waveA wave is classified as a shallow water wave whenever the ratio of the depth (the vertical distance of the still water level from the bottom) to the wave length (the horizontal distance between crests) is less than 0.04. Such waves propagate according to the formula: C = √gd where C is the wave speed, g the acceleration of gravity, and d the depth. Tidal waves are shallow water waves.shearA quasi-horizontal layer moving at a different velocity relative to the layer directly below and/or above.shoreline (coastline)The intersection of the land with the water surface. The shoreline shown on charts represents the line of contact between the land and a selected water elevation. In areas affected by tidal fluctuations, this line of contact is the mean high water line. In confined coastal waters of diminished tidal influence,the mean water level line may be used. See coastline.sidereal dayThe time of the rotation of the Earth with respect to the vernal equinox. It equals approximately 0.997,27 of a mean solar day. Because of the precession of the equinoxes, the sidereal day thus defined is slightly less than the period of rotation with respect to the fixed stars, but the difference is less than a hundredth part of a second.sidereal monthAverage period of the revolutionof the Moon around the Earth with respect to a fixed star, equal to 27.321,661 mean solar days.sidereal timeThis is usually defined by astronomers as the hour angle of the vernal equinox. The sidereal day is the interval between two successive upper transits of the vernal equinox. It is to be noted that when applied to the month and year the word sidereal has reference to motion with respect to the fixed stars, while the word tropical is used for motion with respect to the vernal equinox. Because of the precession of the equinox there is a slight difference.sidereal yearAverage period of the revolution of the Earth around the Sun with respect to a fixed star. Its length is approximately 365.256,4 mean solar days.sigma-t (σt)An expression of density as a function of temperature and salinity (at atmospheric pressure) in a convenient numerical form. See density. σt = (ρs,t,p - 1)1,000sigma-zero (σ0)An expression of density as a function of salinity (at atmospheric pressure and 0°C) in a convenient numerical form. See density. σ0 = (ρs,t,o - 1)1,000slack; ebb begins (slack before ebb)The slack water immediately preceding the ebb current.slack; flood begins (slack before flood) The slack water immediately preceding the flood current.slack water (slack)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 also is 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 a perfect standing tidal wave, slack water occurs at the time of high and of low water, while for a perfect progressive tidal wave, slack water occurs midway between high and low water. See slack; ebb begins and slack; flood begins. S1Different in height between mean lower high water and mean higher low water.ScDifferent in height between tropic lower high water and tropic higher low water.solar dayThe period of the rotation of the Earth with respect to the Sun. The mean solar day is the time of the rotation with respect to the mean Sun. The solar day commencing at midnight is called a civil or calendar day, but if the day is reckoned from noon it is known as an astronomical day because of its former use in astronomical calculation.solar tide(1) The part of the tide that is due to the tide-producing force of the Sun. (2) The observed tide in areas where the solar tide is dominant. This condition provides for phase repetition at about the same time each solar day.solar timeTime measured by the hour angle of the Sun. It is called apparent time when referred to the actual Sun and mean time when referred to the mean Sun. It is also classified as local, standard, or Greenwich according to whether it is reckoned from the local, standard, or Greenwich meridian. solitary wave A wave of translation consisting of a single crest rising above the undisturbed water level without any accompanying trough. The rate of advance of a solitary wave depends upon the depth of the water and is usually expressed by the formula:C = √g(d + h) in which C = rate of advance, g = acceleration of gravity, d = depth of water, and h = height of wave, the depth and height being measured from the undisturbed water level.solsticesThe two points in the ecliptic where the Sun reaches its maximum and minimum declinations; also the times when the Sun reaches these points. The maximum north declination occurs on or near June 21, marking the beginning of summer in the Northern Hemisphere and the beginning of winter in the Southern. The maximum south declination occurs on or near December 22, marking the beginning of winter in the Northern Hemisphere and the beginning of summer in the Southern.solstitial tidesTides occurring near the times of the solstices. The tropic range may be expected to be especially large at these times.Somali (East Africa Coast) CurrentAn Indian Ocean current setting southwestward along the coast of Somalia. The current reverses and sets to the northeast during the Southwest Monsoon.South Equatorial CurrentA current setting westward along and south of the Equator in the Atlantic and Pacific Oceans, and south of the Equator in the Indian Ocean. It occurs immediately south of the Equatorial Counter Current.Southwest Monsoon CurrentSame as Monsoon Current.species of constituentA classification depending upon the period of a constituent. The principal species are semidiurnal, diurnal, and long-period.specific volume anomaly, or steric anomaly (α)The excess in specific volume over the standard specific volume at 35 ‰, 0°C, and the given pressure. See thermosteric anomaly and specific volume. δ=αs,t,p - α35,o,pspecific volume, in situ (αs,t,p)Volume per unit mass. The reciprocal of density (specific gravity). The specific volume of sea water as a function of salinity, temperature, and pressure. See specific volume anomaly and thermosteric anomaly.speed (of constituent)The rate of change in the phase of a constituent, usually expressed in degrees per hour. The speed is equal to 360° divided by the constituent period expressed in hours.speed (of current)The magnitude of velocity. Rate at which the current flows. Usually expressed in knots or centimeters per second.Spitsbergen Atlantic CurrentA current setting northwestward off the southwest coast of Spitsbergen in the Greenland Sea.spring highwaterSame as mean high water springs (MHWS). See spring tides.spring low waterSame as mean low water springs (MLWS). See spring tides and mean low water springs.spring range (Sg)See spring tides.spring tides or tidal currentsTides 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 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 semi diurnal or mixed, and is of no practical significance where the type of tide is predominantly diurnal. The average height of the high waters of the spring tides 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 tideSometimes 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 methodA tidal datum computation method. Generally used for the West Coast and Pacific Island stations. Values needed are mean tide level (MTL), mean range of tide (MN), great diurnal range (GT), and mean diurnal high and low water inequalities (DHQ and DLQ) as determined by comparison with an appropriate control. From those, the following are computed: MLW = MTL - (0.5*MN) MHW = MLW + MN MLLW = MLW - DLQ MHHW = MHW + DHQstandardtimeA 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. The United States first adopted standard time in 1883 on the initiative of the American Railway Association, and at noon on November 18 of that year the telegraphic time signals from the Naval Observatory at Washington were changed to this system.standing (stationary) waveA wave that oscillates without progressing. One-half of such a wave may be illustrated by the oscillation of the water in a pan that has been tilted. Near the axis, which is called the node or nodal line, there is no vertical rise and fall of the water. The ends of the wave are called loops and at these places the vertical rise and fall is at a maximum. The current is maximum near the node and minimum at the loops. The period of a stationary wave depends upon the length and depth of the body of water and, for a simple rectangular basin, may be expressed by the formula: T = 2L / √gd in which T is the period of wave, L the length of the basin, d the depth of water, and g the acceleration of gravity. A stationary wave may be resolve d into two progressive waves of equal amplitude and equal speeds moving in opposite directions.stationary wave theoryAn assumption that the basic tidal movement in the open ocean consists of a system of stationary wave oscillations, any progressive wave movement being of secondary importance except as the tide advances into tributary waters. The continental masses divide the sea into irregular basins, which, although not completely enclosed, are capable of sustaining oscillations which are more or less independent. The tide-producing force consists principally of two parts, a semidiurnal force with a period of approximately half a day and a diurnal force with a period of approximately a whole day. Insofar as the free period of oscillation of any part of the ocean, as determined by its dimensions and depth, is in accord with the semidiurnal or diurnal tide-producing forces, there will be built up corresponding oscillations of considerable amplitude which will be manifested in the rise and fall of the tide. The diurnal oscillations, superimposed upon the semidiurnal oscillations, cause the inequalities in the heights of the two high and the two low waters of each day. Although the tidal movement as a whole is somewhat complicated by the overlapping of oscillating areas, the theory is consistent with observational data.stencilsPerforated sheets formerly used with the tabulated hourly heights of the tide or speeds of the tidal current for the purpose of distributing and grouping them into constituent hours preliminary to summing for harmonic analysis. See Coast and Geodetic Survey Special Publication No. 98, Manual of Harmonic Analysis and Prediction of Tides. This analysis is now performed on electronic digital computers.steric anomalySame as specific volume anomaly.stilling wellA vertical pipe with a relatively small opening (intake) in the bottom. It is used in a gauge installation to dampen short period surface waves while freely admitting the tide, other long period waves, and sea level variations; which can then be measured by a water level gauge sensor inside. See float well and protective well.storm surgeThe local change in the elevation of the ocean along a shore due to a storm. The storm surge is measured by subtracting the astronomic tidal elevation from the total elevation. It typically has a duration of a few hours. Since wind generated waves ride on top of the storm surge (and are not included in the definition), the total instantaneous elevation may greatly exceed the predicted storm surge plus astronomic tide. It is potentially catastrophic, especially on low lying coasts with gently sloping offshore topography. See storm tide.storm tideAs used by the National Weather Service, NOAA, the sum of the storm surge and astronomic tide. See storm surge.stray lineUngraduated portion of line connected with the current pole formerly used in taking current observations. The stray line was usually about 100 feet long and permitted the pole to acquire the velocity of the current at some distance from the disturbed waters in the immediate vicinity of the observing vessel, before the current velocity was read from the graduated portion of the current line.strength of currentPhase 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 0.636,6 of the speed of the corresponding strength of current.strength of ebbSame as ebb strength.strength of floodSame as flood strength.submerged landsLands covered by water at any stage of the tide. See tidelands.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. See current station, control current station, and reference 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. See 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. See tide station, primary control tide station, secondary control tide station, and tertiary tide station. (2) A station listed in the Tide Tables from which predictions are to be obtained by means of differences and ratios applied to the full predictions at a reference station. See reference station.summer timeBritish name for daylight saving time.synodical monthThe average period of the revolution of the Moon around the Earth with respect to the Sun, or the average interval between corresponding phases of the Moon. The synodical month is approximately 29.530,588 days in length.syzygyWith respect to tides, whenever the Moon is lined up with the Earth and Sun in a straight Sun-Moon-Earth or Sun-Earth-Moon configuration. At these times the range of tide is greater than average. See spring tides or tidal currents. |
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