Science / Tides and Currents Glossary

2N2: Lunar elliptic semi diurnal second-order constituent. Speed = 2T– 4s + 2h + 2p = 27.895,354,8° per solar hour.

2Q1: Lunar elliptic diurnal, second order, constituent. Speed = T – 4s + h + 2p = 12.854,286,2° per solar hour.

Absolute Mean Sea Level Change: An eustatic change in mean sea level relative to a conventional terrestrial coordinate system with the origin at the center of mass of the Earth.

Accepted Values: Tidal datums and Greenwich high and low water intervals obtained through primary de-termination or comparison of simultaneous observations with a control tide station in order to derive the . . . View Full Definition

Acoustic Doppler Current Profiler (ADCP): Current measuring instrument employing the transmission of high frequency acoustic signals in the water. The current is determined by a Doppler shift in the backscatter echo from plankton, s . . . View Full Definition

ADR Gauge: Analog to Digital Recording water level gauge. A float or pressure-actuated water level gauge that records heights at regular time intervals in digital format.

Age Of Diurnal Inequality: The time interval between the maximum semimonthly north or south declination of the Moon and the maximum effect of declination upon range of tide or speed of the tidal current. The age may b . . . View Full Definition

Age Of Moon: The time elapsed since the preceding new Moon

Age Of Parallax Inequality: The time interval be tween perigee of the Moon and the maximum effect of parallax upon range of tide or speed of the tidal current. This age may be computed from the harmonic constants by th . . . View Full Definition

Age Of Phase Inequality: The time interval between new or full Moon and the maximum effect of these phases upon range of tide or speed of the tidal current. This age may be computed from the harmonic constants by th . . . View Full Definition

Agulhas Current: An Indian Ocean current setting southwestward along the southeast coast of Africa.

Air Acoustic Ranging Sensor: A pulsed, acoustic ranging device using the air column in a tube as the acoustic sound path. The fundamental measurement is the time it takes for the acoustic signal to travel from a transmi . . . View Full Definition

Alaska Current: A North Pacific Ocean current setting counterclockwise along the coasts of Canada and Alaska in the Gulf of Alaska.

Alaskan Stream: A North Pacific Ocean current setting westward along the south side of the Aleutian Islands. It is an extension of the Alaska Current.

Amphidromic Point: A point of zero amplitude of the observed or a constituent tide.

Amphidromic Region: An area surrounding an am phidromic point from which the radiating cotidal lines progress through all hours of the tidal cycle.

Amplitude (H): One-half the range of a constituent tide. By analogy, it may be applied also to the maximum speed of a constituent current.

Analog: A continuous measurement or a continuous graphic display of data. See ADR gauge and marigram.

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