Glossary terms G-L



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. ]


(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)


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


"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.


(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).


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.


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


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.



See hPa


See "What is Helicity?"

Horizontal vorticity

Even the classical 'vertical' vorticity term (q.v.) has some upward / downward component, but this is usually ignored for practical synoptic-scale meteorology. However, when coming down a scale or two, to local / mesoscale development, ( e.g. severe convective storms ), then vorticity about a horizontal axis is most important. It is often assessed in the lowest 3 km of atmosphere, and is 'driven' by two terms: vertical speed shear (increase / decrease of wind with increasing altitude) and directional ('twisting') shear, the change of direction with increasing altitude. If, in the lowest 3 km of atmosphere (up to 700 hPa), there is both a sharp increase of wind speed and a directional veer of wind with height, then horizontal vorticity will be potentially significant, provided it is coupled to the vigour of a developing cumulonimbus complex. (See also Vorticity; Vertical vorticity and "What is Helicity?"


(abbr) Hectopascal - equivalent to a millibar (q.v.). An attempt to use SI units without doing away with the idea of millibars (from the c.g.s. system). [ 1 hPa=100 Pa (or N/m2)]


See "What is the difference between humidity and relative humidity?"

Hurricane Force

This term (in UK Met Office use) is only used in shipping bulletins and associated Gale/Storm warnings in the form "Hurricane Force 12", from the modified Beaufort scale. It is strictly defined as a mean (10 minute) wind of 64 knots or more. (Gusts not defined) (See also comments at Severe Gale).

[ Please note carefully that just because an area of low pressure produces winds to 'hurricane' force as defined here, it does NOT make that feature a Hurricane! For more on this, see this question on the October 1987 storm ]


Haze: used in METAR/TAF reports etc., when visibility is reduced in a 'dry' atmosphere. (visibility > = 1km, relative humidity roughly < 90%).



(abbr) Ice crystals (also known as diamond dust); used in METAR/aviation reports.


When used in aviation weather reports/forecasts, implies aircraft superstructure icing.

Ice day

A period of 24hr (conventionally beginning 0900UTC), during which the air temperature is less than 0 degC.

Ice fog

Visibility reduced to less than 1000 m by suspension of minute collection of ice crystals in high concentration. The crystals will glitter and may give rise to optical phenomena. (NB: this is NOT the same as freezing fog, which is composed of water droplets - see definition elsewhere.)


The process whereby a model 'analysis' is produced by utilising model fields from an earlier run, and integrating synoptic, and asynoptic observations to produce the 'initial state' at t=0. The model analysis may not be the same (in detail), as a hand-drawn analysis, and intervention (q.v.) is sometimes needed as a result to preserve some small scale features which can influence the forecast run.


Radiant energy received from the sun on any particular surface. Often used when discussing receipt of infra-red radiation on the surface of the earth.

Instant Occlusion

[ or pseudo-occlusion ] The name that has been coined to label the cloud mass associated with an active trough in the cold air, that comes close to, and interacts with a pre-existing baroclinic zone, forming a pattern that looks superficially as if it was part of a traditional occlusion process.

Intertropical Convergence Zone

(usually abbr. ITCZ) A zone (often rather broad, but sometimes quite narrow), which separates the 'air-masses' brought together by the low-level outflow from the sub-tropical high pressure belts north and south of the equator. Over the oceans, the zone can be well marked; over land, sensible heating usually leads to 'breaks' or other anomalies, and the regional-scale monsoon circulations also distort, or swamp the idealised structure of the ITCZ. Cloudiness (and hence precipitation activity) can vary sharply over a period of 24hr. Day-to-day change of position is often small, but the zone migrates north & south through the course of a year, roughly in sympathy with the changing position of the sun.


A process where forecasters force acceptance of a report rejected in the model initialisation routine ('supporting'), or use 'bogus' observations to input a conceptual model observed in imagery.


(abbr) Intensifying (as used in SIGMETs for a phenomenon becoming more intense or extensive).

Inversion (of temperature)

A layer in the atmosphere (usually very shallow < 0.4 km), where temperature rises with increasing height. Two of the best-known in operational meteorology are the nocturnal inversion (formed due to strong cooling of land surfaces after sunset), and the subsidence inversion (due to descent & adiabatic warming of air associated with anticyclones). Another near-surface type is that formed when warm air travels over a cold surface (e.g. cold seas or ice/snow).


(abbr) Isentropic Potential Vorticity - the product of the absolute vorticity of an air parcel, and its static stability, calculated along a constant surface of 'theta' (potential temperature), hence the 'isentropic'. Anomalies in IPV around the level of the tropopause (and hence in the region of the driving jet stream) can be related to developments through the troposphere, leading to cyclogenesis. Because IPV is a highly conservative property for any sample of air, it is found to be particularly useful for tracking the path that stratospheric air (high IPV values) will take as it enters the upper troposphere during rapid cyclogenesis events. NWP models can be programmed to output the height of a particular IPV value - defined such that it 'samples' air in the model stratosphere. These patterns are then overlaid on water vapour imagery, and any mis-match between model and reality are quickly seen and allowed for. (See also Potential Vorticity) and also this article on Water Vapour Imagery.


(abbr) Infra-red (used in connection with satellite imagery) See "What are various types of satellite imagery available?"


(abbr) International Standard Atmosphere. A standard reference for temperature, pressure, and relative density structure in the troposphere and lower stratosphere, used for the calibration of (pressure) altimeters.


A line on a synoptic chart joining points of equal atmospheric pressure.


Isolated (as in ISOL CB etc.)


A line connecting points of equal temperature.


Intertropical Convergence Zone (Sometimes seen as ICZ, or perhaps, erroneously, ITF [ intertropical front ]).


Jet entrance

Given a jet core, the area where the speed increases markedly upstream is referred to as the jet entrance region.

Jet exit

Given a jet core, the area where the speed decreases markedly downstream is referred to as the jet exit region.

Jet streak

A small-scale but intense core of strong winds, usually within (and moving quickly along) a well-defined jet stream (See "What are jetstreams?"). They generally do not have a length much more than two to three-hundred kilometres, and can be associated with explosive cyclogenesis events (q.v.). Difficult to pick up via conventional observations (e.g. radiosonde wind-finding), but can be detected in WV imagery. (See also "What are the various types of satellite imagery available?")

Jet Stream

A band of high winds usually found in the upper troposphere: wind speeds can exceed 90 m/s. Jet streams are also located in the stratosphere and, with lower speeds, in association with the atmospheric boundary layer. (See also "What are jetstreams?").


Katabatic wind

A wind that blows down a slope which is strongly cooled after sunset. Given the right topography and surface composition, surprisingly high speeds can be achieved in otherwise light-wind situations.


When the warm air associated with a frontal surface descends relative to the cold air, the front is a kata-front. Such fronts are usually weak/ill-defined, but often mask more complex mechanisms, including upper frontal structures, and mesoscale processes.


(abbr. kt or kn) One nautical mile per hour (for speed of wind, ship movement, depression movement etc.) [ 1 knot = 0.515 m/s = 1.85 km/hr = 1.151 mph. As a working approximation, to obtain m/s, halve the reported wind in knots. ]


Knots (nautical miles per hour, as used in METAR/TAF reports etc.)



Abbreviation used in aviation work to stand for "over-land".

Lapse rate (of temperature)

The decrease of temperature with height in the atmosphere. Confusingly, the opposite case, an increase in temperature with height, is known as a negative lapse rate.

Latent heat

The amount of energy needed to accomplish a phase change. Latent heat of fusion is the amount of energy required to melt ice, and at 0°C is 3.34 * 105J kg-1 (or about 80 cal/g). The latent heat of vaporisation is the amount of energy needed to evaporate liquid water. It is equivalent to 2.50 * 106 J kg-1 (or about 600 cal/g) at 0°C. The latent heat of sublimation is the energy needed to carry out a change from solid (ice) to gas (vapour). It is the sum of the latent heats of fusion and vaporisation, i.e. 2.83 * 106 J kg-1 (or about 680 cal/g) at 0°C. When water freezes, condenses or changes from a gas to a solid, 80 cal/g, 600 cal/g and 680 cal/g are released to the environment respectively. The processes are all reversible.

Left exit

Lies on the cold side of the jet axis, in the region of marked deceleration of flow. A preferred region for cyclonic development.

Lenticular clouds

These form within the crest(s) of orographic (or lee) wave-trains, over and downwind of hills / mountains / islands, provided of course that the air is humid enough. The clouds are formed because air cools as it is forced to rise and if condensation takes place, lens-shaped clouds are observed with clear space in between the elements. The cloud forms within the upwind leg of each wave-crest and dissipates (evaporates) on the downwind leg: the air is therefore flowing through the cloud, with the cloud itself staying quasi-stationary; change in the cloud requires an alteration in the windflow or temperature / humidity environment. Sometimes, under very special circumstances, a 'pile of plates' is observed, where lenticular clouds are stacked vertically. The most common form of wave-forced cloud is perhaps Altocumulus lenticularis (Ac len), but lenticular cloud forms are found at all levels. Standing wave motion can also lead to a previously uniform sheet of cloud developing a lenticular appearance, and on occasion, complete dispersal. (See also MTW).

Loaded gun scenario

On a day of instability through a great depth of the troposphere, and high values of CAPE (q.v.), rising surface temperatures will at some point ensure that convection parcels leave the surface, the condensation level will be reached, cloud will grow (given sufficient moisture), and a heavy shower, or even a thunderstorm will result. It sometimes happens though that although the atmosphere is markedly unstable above, say, 2 km a 'lid' opposing surface-based convection exists at or below this level, due often to a layer of warm/dry air that has become entrained in the airflow from some source. This means that surface temperatures must become very high to overcome this 'lid', often requiring additional triggers, such as low-level convergence or release of medium level potential instability by a mid-level trough, thus lifting the whole column and releasing the 'pent-up' energy in a sudden burst... and the 'loaded gun' will be 'fired', perhaps leading to a severe storm/supercell event. (see 'Spanish plume').


Layer(s) (as used in cloud forecasting in aviation products).