Science / Tides and Currents Glossary

Geodetic Datum: See National Geodetic Vertical Datum of 1929 (NGVD 1929) and North American Vertical Datum of 1988 (NAVD 1988).

Geopotential: The unit of geopotential difference, equal to the gravity potential of 1 meter squared per second squared, m2 / s2, or 1 joule per kilogram, J / kg.

Geopotential (Equipotential) Surface: A surface that is everywhere normal to the acceleration of gravity.

Geopotential Anomaly: The excess in geopotential difference over the standard geopotential difference [at a standard specific volume at 35 parts per thousand (‰) and 0 degrees C] between isobaric surfaces. See ge . . . View Full Definition

Geopotential Difference: The work per unit mass gained or required in moving a unit mass vertically from one geopotential surface to another. See geopotential, geopotential anomaly, and geopotential topography.

Geopotential Topography: The topography of an equiscalar (usually isobaric) surface in terms of geopotential difference. As depicted on maps, isopleths are formed by the intersection of the isobaric surface with a s . . . View Full Definition

Geostrophic Flow: A solution of the relative hydro-dynamic equations of motion in which it is assumed that the horizontal component of the Coriolis force is balanced by the horizontal component of the pressur . . . View Full Definition

Gradient Flow: A solution of the relative hydrodynamic equations of motion in which only the horizontal Coriolis, pressure gradient, and centrifugal forces are considered.

Gravity Wave: A wave for which the restoring force is gravity.

Great Diurnal Range (GT): The difference in height between mean higher high-water and mean lower low water. The expression may also be used in its contracted form, diurnal range.

Great Tropic Range (GC): The difference in height between tropic higher high water and tropic lower low water. The expression may also be used in its contracted form, tropic range.

Greenwich Argument: Equilibrium argument computed for the meridian of Greenwich.

Greenwich Interval: An interval referred to the transit of the Moon over the meridian of Greenwich, as distinguished from the local interval which is referred to the Moon's transit over the local meridian. The . . . View Full Definition

Gregorian Calendar: The modern calendar in which every year divisible by 4 (excepting century years) and every century year divisible by 400 are bissextile (or leap) years with 366 days. All other years are com . . . View Full Definition

Guiana Current: An Atlantic Ocean current setting northwestward along the north-east coast of South America.

Guinea Current: An Atlantic Ocean current setting eastward along the west central coast of Africa. A continuation of the Equatorial Counter Current of the Atlantic Ocean.

Gulder: Local name given to the double low water occurring on the south coast of England. See double tide.

Gulf Coast Low Water Datum (GCL WD): A tidal datum. Used as chart datum from November 14, 1977, to November 27, 1980, for the coastal waters of the Gulf coast of the United States. GCLWD is defined as mean lower low water when . . . View Full Definition

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