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Weather Glossary - G

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Gale

Winds with a mean speed of between 33 knots and 48 knots, or roughly between 61 km/h and 89 km/h.

GASP

Global Analysis and Spectral Prognosis (or Global AnalysiS and Prediction). The medium range global numerical forecast model operated by the Bureau of Meteorology. GASP is run twice daily, with model output available around six hours after each run out to 7 days. GASP has a horizontal resolution of around 85 km and 29 vertical levels.

GCM

General Circulation Model. A computer simulation of the atmosphere. They determine the future state of the atmosphere from initial known conditions. Examples include MLaps and GFS.

Geopotential height

The altitude of a layer in the atmosphere. It is used to define isobaric surfaces on upper level charts.

Geostrophic wind

The theoretical wind generated when pressure gradient forces are exactly balanced by the Coriolis force. Most atmospheric motions are not geostrophic, due to frictional and other effects.

GFS

Global Forecast System. The operational numerical weather forecasting model run by NCEP. The GFS superceded the AVN and MRF models, on which it is largely based. Forecast data from the GFS is available out to 180 hours at full resolution and 16 days at a reduced resolution.

Glaciation

The process of freezing water in cloud formation. In thunderstorms, glaciation usually indicates strong updrafts and potential for hail. Glaciation is also believed to be required for lightning formation.

Global radiation

Global (short wave) radiation includes radiation energy reaching the ground directly from the sun, and energy received indirectly from the sky, scattered downwards by clouds, dust particles etc.

GMT

Abbreviation of Greenwich Mean Time. See also Zulu, UTC.

Gradient wind

A theoretical wind that results from a balance between the pressure gradient, Coriolis and centrifugal forces. It is a better approximation than the geostrophic wind as it accounts for the curvature of real weather systems.

Graupel

Ice particles between 2 and 5mm in diameter that form in a cloud by the process of accretion.

Green flash

A small green flash of light that occasionally appears on the upper part of the sun as it rises or sets.

Greenhouse effect

A natural warming process of the Earth. When the sun's energy reaches the earth some of it is reflected back to space and the rest is absorbed. The absorbed energy warms the Earth's surface, which then emits heat energy back toward space as long wave radiation. This outgoing long wave radiation is partially trapped by greenhouse gases such as carbon dioxide, methane and water vapour, which then radiate the energy in all directions, warming the Earth's surface and atmosphere.

Without these greenhouse gases the Earth's average surface temperature would be about 33C cooler.

Ground clutter

A pattern of radar echoes from fixed ground targets (buildings, hills, etc.) near the radar. Ground clutter may hide or confuse precipitation echoes near the radar antenna.

Gunge

[Slang] Anything in the atmosphere that restricts visibility for storm spotting, such as fog, haze, precipitation (steady rain or drizzle), widespread low clouds (stratus), etc.

Gust

A sudden increase of wind speed of short duration, usually a few seconds.

Gustfront

The leading edge of gusty surface winds from thunderstorm downdrafts; sometimes associated with a shelf cloud or roll cloud. See also downburst, gustnado, outflow boundary.

Gustnado

Gustfront tornado. A small tornado, usually weak and short-lived, that occurs along the gust front of a thunderstorm. Often it is visible only as a debris cloud or dust whirl near the ground. Gustnadoes are not associated with storm-scale rotation (i.e. mesocyclones); they are more likely to be associated visually with a shelf cloud than with a wall cloud.

Gyre

A large circular, surface ocean current that results from broadscale atmospheric forcing, eg South Pacific Gyre.

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