gage—See tide (water level) gauge.

Galofaro—A whirlpool in the Strait of Messina; at one time called Charybdis.

gas purged pressure gauge—A type of water level gauge in which gas, usually nitrogen, is emitted from a submerged orifice at a constant rate. Fluctuations in hydrostatic pressure due to changes in water level modify the recorded emission rate. Same as bubbler tide (water level) gauge.

gauge—See tide (water level) gauge.

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.

See geopotential and geopotential topography.

P2

)D = I *dp

P1

where p is the pressure and *, the specific volume anomaly.

P1 and P2 are the pressures at the two surfaces.

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 (equipotential) surface—A surface that is everywhere normal to the acceleration of gravity.

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 series of geopotential surfaces. Thus, the field of isopleths represents variations in the geopotential anomaly of the isobaric surface above a chosen reference isobaric surface (such as a level of no motion).

geostrophic flow—A solution of the relative hydrodynamic equations of motion in which it is assumed that the horizontal component of the Coriolis force is balanced by the horizontal component of the pressure gradient force.

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

gravitational tide—Same as equilibrium tide.

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 epoch—See epoch (1).

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 relation in hours between Greenwich and local intervals may be expressed by the formula: Greenwich interval = local interval + 0.069L where L is the west longitude of the local meridian in degrees. For east longitude, L is to be considered negative.

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 common years with 365 days. The average length of this year is, therefore, 365.242,5 days which agrees very closely with the length of the tropical year (the period of changes in seasons). The Gregorian calendar was introduced by Pope Gregory in 1582, and immediately adopted by the Catholic countries in place of the Julian calendar previously in use. In making the change it was ordered that the day following October 4, 1582, of the Julian calendar be designated October 15, 1582, of the Gregorian calendar; the 10 days being dropped in order that the vernal equinox would fall on March 21. The Gregorian calendar was not adopted by England until 1752, but is now in general use throughout the world.

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 (GCLWD)—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 the type of tide is mixed and mean low water (now mean lower low water) when the type of tide is diurnal. See National Tidal Datum Convention of 1980.

Gulf Coast Low Water Datum line—The line on a chart or map which represents the intersection of the land with the water surface at the elevation of Gulf Coast Low Water  Datum.

Gulf Stream—A North Atlantic Ocean current setting northeastward off the east coast of the United States. A segment of the Gulf Stream System, the Gulf Stream extends from the region off Cape Hatteras to an area southeast of the Grand Banks at about latitude 40° north, longitude 50° west. It continues the flow of the Florida Current to the North Atlantic Current.

Gulf Stream System—The continuous current system composed of the Florida Current, Gulf Stream, and North Atlantic Current.