Skip navigation.
Home
uk.sci.weather resources

G

Gale

The word 'gale' is used in everyday speech in a rather loose way to describe any strong wind, for example ..." it's blowing a gale outside", when it may be just a 'strong blow' in inland areas of the southern Britain. Meteorologists must work to a strict definition of a gale. For operational forecasting (UK Met Office practice) both for land and sea use, a gale [ Force 8 on the Beaufort scale ] is defined as a mean wind (over 10 minutes) of 34 knots (39mph, 63 km/hr, 17 m/s ) or more, or gusts of 43 knots (49 mph, 79 km/hr, 22 m/s) or more. This definition is also used for verifying Shipping Forecasts and Gale Warnings. Isolated gusts accompanying squalls or thunderstorms are not counted. However, for climatological purposes (i.e. post-event analysis), only the mean wind is considered, i.e. a mean wind of 34 knots or more, as specified in the Beaufort wind scale (q.v.). [ see also definitions for Severe Gale, Storm, Violent Storm and Hurricane Force. ]

GC

(abbr)[ground-to-cloud lightning flash] Used when describing lightning which branches from the ground to the cloud. The upward branching often results in an appearance like a trident, etc. This is an uncommon type of lightning. (See also CA, CC & CG)

GEFS

(abbr) Global Ensemble Forecast System (of NCEP / q.v.)

Geopotential

"Potential energy per unit mass of a body due to the earth's gravitational field, referred to an arbitrary zero" (The Meteorological Glossary/UK; in meteorology, mean sea level is the reference level). A geopotential metre (by this definition) is related to the dynamic metre (straightforward unit of length) by the expression: 1 gpm=0.98 dynamic m. Geopotential height differs from geometric height where the value of the gravitational 'constant' (g) departs from 9.8 m/s^2. Gravity does vary, both by altitude and latitude, but for practical purposes, when looking at NWP output on the web, you can ignore these slight differences. Geopotential heights are used in meteorology because flow along a geopotential surface involves no loss or gain of energy, whereas flow along a geometric surface may do so - so for strict physical / mathematical calculations within computer models, the distinction between the two must be maintained.

Geostrophic wind

Defined as the (theoretical) wind that would blow on a rotating planet which results from a balance between the pressure gradient causing the initial displacement of the air, and the apparent (to us on the earth) deflecting force due to the planetary rotation. Many corrections are needed to find the 'true' wind vector amongst which are the effects of friction and the several forces involved when the pressure pattern changes - which is the usual case. However, by this definition we get the general statement that the speed of the geostrophic wind is proportional to the pressure gradient, or inversely proportional to the distance between isobars/contours. Curvature of the flow must also be taken into account ... see Gradient wind.

GFS

(abbr) Global Forecast System

Global Forecast System

The primary forecast model (NWP) from the US NCEP service (q.v.). The model suite is run to T + 384 hr, in two 'bursts'; one to T + 120 (5 days) then a further run to the 16 days (384hr). The model is run four times daily, though not all WWW sites hold all runs (or full output for each run).

GR

Hail (abbr. from French) dia: > = 0.5 cm; used in METAR / aviation reports etc.

Gradient wind

When the path that an air parcel takes is curved (relative to the earth's surface), as so often in meteorology, that airflow is subject to an additional force necessary to maintain a curved path. For cyclonic flow, the 'true' wind that blows will be less than the theoretical/geostrophic wind; for anticyclonic flow the true wind is greater, subject to a limiting maximum. This is why, for example, around what initially looks like a dramatically intense depression, the wind may not be quite so excessive: cyclonic curvature will account for substantial negative correction to the theoretical value. Around a surface ridge, the wind is often surprisingly stronger than might be implied by isobaric spacing.

GS

Small hail dia: <0.5cm; used in METAR/aviation reports etc.

Gust

Given that the wind in the surface boundary level varies markedly about the mean wind (q.v.), it is often necessary to report the accompanying instantaneous maximum (or gust speed) in a defined period. For METAR reports, then the period over which this peak wind is reported is between 2 and 10 minutes (depending upon the country). For SYNOP reports, the period is either the last hour (most likely in NW Europe), or the period covered by the past weather group in the report - reference to the accompanying amplifying groups will usually sort this out.