• Weather Glossary (G)

    From Daryl Stout@57:57/10 to All on Sun Oct 25 00:05:25 2020
    This weather glossary contains information on more than 2000 terms,
    phrases and abbreviations used by the National Weather Service (NWS)...
    the government agency who makes weather forecasts, and issues weather advisories, watches, and warnings, for the United States, and its

    Many of these terms and abbreviations are used by NWS forecasters to communicate between each other and have been in use for many years and
    before many NWS products were directly available to the public. It is the purpose of this glossary to aid you in better understanding NWS products.


    Gusts- A rapid fluctuation of wind speed with variations of 10 knots or
    more between peaks and lulls.

    Grams per Kilogram

    In hydrologic terms,

    1) A device for indicating the magnitude or position of a thing in
    specific units, when such magnitude or position undergoes change,
    for example: The elevation of a water surface, the velocity of flowing
    water, the pressure of water, the amount or intensity of precipitation,
    the depth of snowfall, etc.

    2) The act or operation of registering or measuring the magnitude or
    position of a thing when these characteristics are undergoing change.

    3) The operation, including both field and office work, of measuring the discharge of a stream of water in a waterway.

    Gage Datum
    A horizontal surface used as a zero point for measurement of stage or
    gage height. This surface usually is located slightly below the lowest
    point of the stream bottom such that the gage height is usually slightly greater than the maximum depth of water. Because the gage datum is not
    an actual physical object, the datum is usually defined by specifying
    the elevations of permanent reference marks such as bridge abutments
    and survey monuments, and the gage is set to agree with the reference
    marks. Gage datum is a local datum that is maintained independently of
    any national geodetic datum. However, if the elevation of the gage datum relative to the national datum (North American Vertical Datum of 1988
    or National Geodetic Vertical Datum of 1929) has been determined, then
    the gage readings can be converted to elevations above the national
    datum by adding the elevation of the gage datum to the gage reading.

    Gaging Station
    In hydrologic terms, a particular site on a watercourse where systematic observations of stage/ and or flow are measured.

    An extratropical low or an area of sustained surface winds of 34 (39 mph)
    to 47 knots (54 mph).

    Gale Warning
    A warning of sustained surface winds, or frequent gusts, in the range of
    34 knots (39 mph) to 47 knots (54 mph) inclusive, either predicted or occurring, and not directly associated with a tropical cyclone.

    In hydrologic terms, a passageway within the body of a dam or abutment.

    A unit of magnetic field intensity equal to 1 x 10.0-5 Gauss; also equal
    to 1 nanotelsa (nT).

    Gamma Ray
    A type of electromagnetic radiation with a very short wavelength and
    high energy level. Generally, emitted during radioactive decay of a

    Gap Winds
    Strong winds channeled through gaps in the Pacific coastal ranges,
    blowing out into the Pacific Ocean or into the waterways of the Inside
    Passage. The winds blow through low passes where major river valleys
    issue onto the seaways when strong east-west pressure gradients exist
    between the coast and the inland areas, with low pressure over the

    Gas Laws
    The thermodynamic laws pertaining to perfect gases, including Boyle's
    law, Charles' law, Dalton's law and the equation of state.

    In hydrologic terms, a device in which a leaf or member is moved across
    the waterway from an external position to control or stop flow. There
    are many different kinds of gates used on a dam.

    The unit of magnetic induction in the cgs (centimeter-gram- second)

    Gaussian Plume Model
    A computer model used to calculate air pollution concentrations. The
    model assumes that a pollutant plume is carried downwind from its
    emission source by a mean wind and that concentrations in the plume
    can be approximated by assuming that the highest concentrations
    occur on the horizontal and vertical midlines of the plume, with the distribution about these mid-lines characterized by Gaussian- or
    bell-shaped concentration profiles.

    Gaussian Puff Model
    A model used to calculate air pollution concentrations. The model
    assumes that a continuously emitted plume or instantaneous cloud of
    pollutants can be simulated by the release of a series of puffs that
    will be carried in a time- and space-varying wind field. The puffs are
    assumed to have Gaussian or bell-shaped concentration profiles in
    their vertical and horizontal planes.

    On a buoy report, direction, in degrees clockwise from true North,
    of the GSP, reported at the last hourly 10-minute segment.

    General Environmental Meteorological Package (programming language).


    General Circulation
    The totality of large-scale organized motion for the entire global

    General Circulation Models
    (GCMs) - These computer simulations reproduce the Earth's weather
    patterns and can be used to predict change in the weather and climate.

    General Wind
    Land management agency term for winds produced by synoptic-scale
    pressure systems on which smaller-scale or local convective winds are superimposed.

    In hydrologic terms, the branch of hydrology relating to subsurface,
    or subterranean waters.

    Geomagnetic Elements
    In solar-terrestrial terms, the components of the geomagnetic field at
    the surface of the earth. In SESC use, the northward and eastward
    components are often called the H and D components, where the D
    component is expressed in gammas and is derived from D (the declination
    angle) using the small angle approximation.

    Geomagnetic Field
    The magnetic field observed in and around the earth. The intensity of
    the magnetic field at the earth's surface is approximately 0.32 gauss
    at the equator and 0.62 gauss at the north pole.

    Geomagnetic Storm
    In solar-terrestraial terms, a worldwide disturbance of the earth's
    magnetic field, distinct from regular diurnal variations.

    Minor Geomagnetic Storm: A storm for which the Ap index was greater than
    29 and less than 50.

    Major Geomagnetic Storm: A storm for which the Ap index was greater than
    49 and less than 100.

    Severe Geomagnetic Storm: A storm for which the Ap index was 100 or more.

    Initial Phase: Of a geomagnetic storm, that period when there may be an increase of the middle-latitude horizontal intensity (H).

    Main Phase: Of a geomagnetic storm, that period when the horizontal
    magnetic field at middle latitudes is generally decreasing.

    Recovery Phase: Of a geomagnetic storm, that period when the depressed northward field component returns to normal levels.

    Geophysical Events
    In solar-terrestrial terms, flares (Importance two or larger) with
    Centimetric Outbursts (maximum of the flux higher than the Quiet Sun flux, duration longer 10 minutes) and/or strong SID. Sometimes these flares are followed by Geomagnetic Storms or small PCA. (Class M Flares).

    In hydrologic terms, the study of the physical characteristics and
    properties of the earth; including geodesy, seismology, meteorology, oceanography, atmospheric electricity, terrestrial magnetism, and tidal phenomena.

    Geopotential Height
    The height above sea level of a pressure level. For example, if a station reports that the 500 mb height at its location is 5600 m, it means that
    the level of the atmosphere over that station at which the atmospheric
    pressure is 500 mb is 5600 meters above sea level. This is an estimated
    height based on temperature and pressure data.

    Geostationary Satellite
    A satellite that rotates at the same rate as the earth, remaining over
    the same spot above the equator.

    Geostrophic Wind
    A wind that is affected by coriolis force, blows parallel to isobars and
    whose strength is related to the pressure gradient (i.e., spacing of the isobars).

    Term applied to any equatorial satellite with an orbital velocity equal
    to the rotational velocity of the earth. The net effect is that the
    satellite is virtually motionless with respect to an observer on the

    Ground Fog- Fog produced over the land by the cooling of the lower
    atmosphere as it comes in contact with the ground. Also known as
    radiation fog, and in parts of California as tule fog.

    (Global Forecast System) One of the operational forecast models run at
    NCEP. The GFS is run four times daily, with forecast output out to 384

    Geographic Information System. A computer-based graphics program that
    allows the superposition of plan-maps of thematic elements, such as
    roads, rivers, land use patterns, and the like to aid in local or
    regional planning activities.

    The transformation of cloud particles from water drops to ice crystals.
    Thus, a cumulonimbus cloud is said to have a "glaciated" upper portion.

    In hydrologic terms, bodies of land ice that consist of recrystallized
    snow accumulated on the surface of the ground, and that move slowly

    Glacier Dammed Lake
    In hydrologic terms, the lake formed when a glacier flows across the
    mouth of an adjoining valley and forms an ice dam.

    Glacier Wind
    A shallow downslope wind above the surface of a glacier, caused by the temperature difference between the air in contact with the glacier and
    the free air at the same altitude. The glacier wind does not reverse
    diurnally like slope and along-valley wind systems.

    Ice formed by freezing precipitation covering the ground or exposed

    (Open Lakes Forecast) - A National Weather Service marine forecast
    product for the U.S. waters within a Great Lake not including the
    waters covered by an existing Nearshore Waters Forecast (NSH). When
    the seasonal Nearshore forecast is not issued, the Open Lake forecast
    includes a forecast of nearshore waters.

    Global Forecast System
    (GFS)- One of the operational forecast models run at NCEP. The GFS is
    run four times daily, with forecast output out to 384 hours.

    Global Temperature Change
    The net result of four primary factors including the greenhouse effect,
    changes in incoming solar radiation, altered patterns of ocean
    circulations, and changes in continental position, topography and/or vegetation. Three feedback mechanisms which affect global temperature
    change include cloud height and amount, snow and ice distribution, and atmospheric water vapor levels.

    Global Warming
    An overall increase in world temperatures which may be caused by
    additional heat being trapped by greenhouse gases.

    An optical effect characterized by concentric rings of color (red
    outermost and violet innermost) surrounding the shadow of an observer's
    head when the shadow is cast onto a cloud deck below the observer's
    elevation (see Brocken specter).

    (Great Lakes Storm Summary) A National Weather Service forecast product providing updated information whenever a storm warning is in effect on
    any of the Great Lakes.

    (Global Maritime Distress and Safety System)- The Global Maritime
    Distress and Safety System (GMDSS) is intended to provide more effective
    and efficient emergency and safety communications and disseminate
    Maritime Safety Information (MSI) to all ships on the world's oceans
    regardless of location or atmospheric conditions. MSI includes
    navigational warnings, meteorological warnings and forecasts, and
    other urgent safety related information. GMDSS goals are defined in
    the International Convention for The Safety Of Life At Sea (SOLAS).
    The National Weather Service participates directly in the GMDSS by
    preparing meteorological forecasts and warnings for broadcast via
    NAVTEX and SafetyNET.

    On a buoy report, the minute of the hour that the GSP occurred,
    reported at the last hourly 10-minute segment.

    Greenwich Mean Time (now known as Universal Coordinated Time).



    Geostationary Operational Environmental Satellite- Satellites
    orbiting at 22,370 miles above the Earth's surface with the same
    rotational velocity as the Earth; therefore, the satellite remains
    over the same location on the Earth 24 hours a day. Besides sending
    back satellite pictures to earth, it also relays the DCPs river and
    rainfall data back to the ground.

    Gulf Of Maine Ocean Observing System

    An acronym for Global Positioning System. A network of satellites which
    provide extremely accurate position and time information. Useful in
    remote locations or for moving platforms.

    Gradient- A rate of change with respect to distance of a variable
    quantity, as temperature or pressure, in the direction of maximum

    (abbrev. GRAD) A rate of change with respect to distance of a variable quantity, as temperature or pressure, in the direction of maximum

    Gradient High Winds
    These high winds usually cover a large area and are due to
    synoptic-scale, extra-tropical low pressure systems.

    Gradual Commencement
    In solar-terrestrial terms, the commencement of a geomagnetic storm that
    has no well-defined onset.

    In solar-terrestrial terms, the cellular structure of the photosphere
    visible at high spatial resolution.

    Same as snow pellets or small hail.

    Gravity Dam
    In hydrologic terms, a concrete structure proportioned so that its own
    weight provides the major resistance to the forces exerted on it.

    Gravity Wave
    A wave created by the action of gravity on density variations in the
    stratified atmosphere. A generic classification for lee waves,
    mountains waves, and many other waves that form in the atmosphere.

    A hypothetical "body" that absorbs some constant fraction of all electromagnetic radiation incident upon it.


    Great Circle Track
    A great-circle track is the shortest distance between two points on a
    sphere, and when viewed on a 2-dimensional map the track will appear
    curved. Swell waves travel along routes that mark out great circles.

    Great Lakes Faxback
    Dissemination systems housed at Weather Forecast Office (WFO) Cleveland
    by which Great Lakes customers request and receive hard copies of
    selected marine products.

    Great Lakes Freeze-Up/Break-Up Outlook
    (FBO) - A National Weather Service product to keep mariners informed of
    the projected freeze-up date or break-up date of ice on the Great

    Great Lakes Marine Forecast (MAFOR)
    A National Weather Service coded summary appended to each of the Great
    Lakes Open Lakes forecasts.

    Great Lakes Storm Summary
    (GLS) - A National Weather Service forecast product providing updated information whenever a storm warning is in effect on any of the Great

    Great Lakes Weather Broadcast
    (LAWEB) - A National Weather Service product containing an observation
    summary prepared to provide Great Lakes mariners with a listing of
    weather observations along or on the Lakes.

    Green Line
    The green line is one of the strongest (and first-recognized) visible
    coronal lines. It identifies moderate temperature regions of the CORONA.

    Greenhouse Effect
    Atmospheric heating caused by solar radiation being readily transmitted
    inward through the earth's atmosphere but longwave radiation less
    readily transmitted outward, due to absorption by certain gases in the atmosphere.

    Greenhouse Gases
    The gases that absorb terrestrial radiation and contribute to the
    greenhouse effect; the main greenhouse gasses are water vapor, methane,
    carbon dioxide, and ozone.

    (Gridded Binary Format) - A format used for meteorological data.
    Typically used in the past for computer generated model data but will
    be used increasingly in the future for forecaster generated data.


    1) Squared off areas across the terrain used to define forecast areas.
    Often 5x5 or 2.5x2.5 kilometer in size.

    2) Digitial forecast databases for meteorological elements, including temperature, wind direction, wind speed and others. Computer programs
    read these databases to create worded and graphical forecast products
    used by the public and others.

    Ground Blizzard Warning
    When blizzard conditions are solely caused by blowing and drifting snow.

    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. Also known as "anomalous propagation".

    Ground Fog
    (abbrev. GF) Fog produced over the land by the cooling of the lower
    atmosphere as it comes in contact with the ground. Also known as
    radiation fog, and in parts of California as tule fog.

    Ground Heat Flux
    The flux of heat from the ground to the earth's surface; a component of
    the surface energy budget.

    Ground receive sites
    In hydrologic terms, a satellite dish and associated computer which
    receives signals from the GOES satellite, decodes the information, and transmits it to a another site for further processing. The GOES
    satellite ground-receive site is located at Wallops Island, VA; and
    the information is relayed to a mainframe computer at NWSH for

    Ground Stroke
    The current that propagates along the ground from the point where a
    direct stroke of lightning hits the ground.

    Ground Water
    In hydrologic terms, water within the earth that supplies wells and
    springs; water in the zone of saturation where all openings in rocks
    and soil are filled, the upper surface of which forms the water table.
    Also termed Phreatic water.

    Ground Water Divide
    In hydrologic terms, a line on a water table where on either side of
    which the water table slopes downward. It is analogous to a drainage
    divide between two drainage basins on a land surface.

    Ground Water Flow
    In hydrologic terms, streamflow which results from precipitation that infiltrates into the soil and eventually moves through the soil to the
    stream channel. This is also referred to as baseflow, or dry-weather

    Ground Water Hydrology
    The branch of hydrology that specializes in ground water; its
    occurrence and movements; its replenishment and depletion; the
    properties of rocks that control ground water movement and storage;
    and the methods of investigation and utilization of ground water.

    Ground Water Outflow
    In hydrologic terms, the part of the discharge from a drainage basin
    that occurs through the ground water. The term "underflow" is often
    used to describe the ground water outflow that takes place in valley
    alluvium (instead of the surface channel) and thus is not measure at
    a gaging station.

    Ground Water Overdraft
    Pumpage of ground water in excess of safe yield.

    Ground Water Runoff
    That part of the runoff which has passed into the ground, has become
    ground water, and has been discharged into a stream channel as spring,
    or seepage water.

    Grounded ice
    In hydrologic terms, ice that has run aground or is contact with the
    ground underneath it.

    Group Velocity
    The speed at which a particular wave front or swell train advances.

    Grout Curtain
    A barrier produced by injecting grout into a vertical zone, usually
    narrow (horizontally), and in the foundation to reduce seepage under a

    Growing Degree Day
    The number of degrees that the average temperature is above a baseline
    value. For example, 40 degrees for canning purposes; 45 degree for
    potatoes; and 50 degrees for sweet corn, snap beans, lima beans,
    tomatoes, grapes, and field corn. Every degree that the average
    temperature is above the baseline value becomes a growing degree day. Agricultural related interests use growing degree days to determine
    planting times.

    Growing Season
    The period of time between the last killing frost of spring and the
    first killing frost of autumn.

    Similar to a bergy bit, but smaller, extending less than 1 meter above
    the sea surface and occupying an area of 20 square meters or less.



    On a buoy report, maximum 5-second peak gust during the measurement
    hour, reported at the last hourly 10-minute segment.

    On a buoy report, peak 5 or 8 second gust speed (m/s) measured during
    the eight-minute or two-minute period. The 5 or 8 second period can be determined by payload.



    Gulf Stream
    Warm water current extending from the Gulf of Mexico and Florida up the
    U.S. east coast then east northeast to Iceland and Norway.

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

    (Abbrev. G) - A rapid fluctuation of wind speed with variations of 10
    knots or more between peaks and lulls.

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

    (or Gustinado) - [Slang], gust front 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.

    Oceanic current systems of planetary scale driven by the global wind
    --- SBBSecho 3.11-Win32
    * Origin: The Thunderbolt BBS - tbolt.synchro.net (57:57/10)