Glossary and Acronyms: G

Gale:

wind with a speed between 34 and 40 knots (Beaufort scale. A strong gale is a wind with a speed between 41 and 47 knots (Beaufort scale wind force 9).
A gale warning is a message intended to warn those concerned of the occurence or expected occurence of a wind of Beaufort force 8 or 9 over a specific area.

GCM:

General Circulation Model. A time-dependent numerical model of the atmosphere, using the equations of motion and including radiation, photochemistry, transfert of heat, water vapour and momentum. Evolution of the model circulation is computed by time integration of those equations starting from an initial condition. The GCM can be used for weather prediction or for climate studies.

GDPS:

Global Data-processing System. The coordinated global system of meteorological centres and arrangements for the processing, storage and retrieval of meteorological information within the framework of the World Weather Watch.

Geostationary satellite:

Meteorological satellite orbiting the Earth at an altitude of approximately 36 000 km with the same angular velocity as the Earth and within the equatorial plane, thus providing nearly continuous information in an area within a range of about 50° from a fixed sub-satellite point at the Equator.

GHz:

Giga Hertz

GMDSS:

Global Maritime Distress and Safety System. An integrated communications system using satellite and terrestrial radiocommunications to ensure that no matter where a ship is in distress, aid can be dispatched. This System ensures also the provision of Maritime Safety Information (MSI), both meteorological and navigational information, on a global basis at sea.

GMS:

Geostationary Meteorological Satellite. A series of satellites operated by the Japanese Meteorological Agency. The GMS program was initiated with the launch of GMS-1 in July 1977. Multi-spectral imagery is provided by a slightly modified version of the VISSR instrument used on GOES satellites. The GMS is positioned at 140°E longitude.

GOES:

Geostationary Operational Environmental Satellites (USA, NOAA). Applies to both the satellites themselves and to the overall system of geostationary observations used by the United States. Two GOES satellites are normally in operation, one at 75° W longitude and the other at 135° W longitude. Before launch, GOES satellites are given a letter designation (e.g., GOES-J) that is changed to a number designation (e.g., GOES-9) when the satellite becomes operational. The current generation of GOES satellites supports separate imager and sounder systems, SEM and DCS. The imager is a five-channel scanning radiometer with a 1-km resolution visible channel, along with slightly lower resolution images in the midinfrared, water vapor, and thermal IR bands. The sounder has 18 thermal infrared bands and a low-resolution visible band.

Gravity wave:

also called gravitational wave. A wave disturbance in which buoyancy (or reduced gravity) acts as the restoring force on parcels displaced from hydrostatic equilibrium.

GTS:

Global Telecommunication System The coordinated global system of telecommunication facilities and arrangements for the rapid collection, exchange and distribution of observational data, processed information and related data within the framework of the World Weather Watch.

Gust:

Sudden, brief increase of the wind speed over its mean value.
Gusts are reported when the peak wind speed is at least 10 knots greater than the average wind speed which reaches itself at least 10 knots. The duration of a gust is usually less than 20 s (often 2 s).
In a tropical cyclone, gusts are estimated to be 1.41 greater than the 10-min average wind speed.