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Glossary: G's

  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
The arbitrary zero datum elevation which all stage measurements are made from.
Gage Height
The water-surface elevation referred to some arbitrary gage datum. Gage height is often used interchangeably with the more general term stage, although gage height is more appropriate when used with a reading on a gage.
Gage Zero
The elevation of zero stage. (Same as gage datum.)

Gaging Station
A particular site on a river, stream, canal, or body of water where systematic observations of stage and/or flow are measured.
Gale Warning
The National Weather Service will issue marine warnings for 1-minute sustained winds between 34 (39 mph) and 47 knots (54 mph) are expected at end of downwind fetch (nearshore or open waters).
A passageway within the body of a dam or abutment.
  1. 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. Some include:  Bulkhead, Crest (or Spillway), Emergency, Fixed Wheel, Flap, Flood, Guard, Outlet, Radial, Regulating, and Slide Gates.
  2. A radar term. See range gate.
Gating (Range Gating)
The use of electric circuits in radar to eliminate or discard the target signals from all targets falling outside certain desired range limits.
That branch of hydrology relating to subsurface, or subterranean waters.
The study of the physical characteristics and properties of the earth; including geodesy, seismology, meteorology, oceanography, atmospheric electricity, terrestrial magnetism, and tidal phenomena.
Geostationary 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.
Geostrophic Wind
The horizontal wind for which the Coriolis acceleration (caused by the Earth's rotation) exactly balances the horizontal pressure force. In practice it is assumed that this marks the upper limit of frictional influence of the Earth's surface. The geostrophic wind blows along the contours on a constant pressure surface. The speed of the geostrophic wind is dependent upon how close your pressure contour are together. Thus, when your pressure contours are close together, you will see a strong geostrophic wind. The opposite occurs when your pressure contours are far apart.
An acronym for Global Forecast System.
Bodies of land ice that consist of recrystallized snow accumulated on the surface of the ground, and that move slowly downslope.
Glacier Dammed Lake
The lake formed when a glacier flows across the mouth of an adjoining valley and forms an ice dam.
Ice formed by freezing precipitation covering the ground or exposed objects.
Global Forecast System (GFS)
NCEP MRF and AVN forecast models were combined into a single system and renamed the Global Forecast System (GFS). The GFS produces forecasts out to 16 days, four times per day.
Global Maritime Distress and Safety System (GMDSS)
An internationally agreed upon communication system for distributing safety information, including weather warnings and forecasts, to mariners.
Global Ocean Model
Another global model used by NCEP is the Global Ocean Model. The ocean model forecasts seasonal changes in oceanic variables, such as sea surface temperature and ocean currents. The ocean model is coupled with an atmospheric model to help determine how forecasted changes in oceanic variables, such as sea surface temperature, will affect the atmosphere. This model tandem is not used to give detailed daily forecasts for the ocean or the atmosphere like some of the other models. Instead it is mainly used to help forecast seasonal or yearly variations of the ocean and the atmosphere. The ocean model coupled with the atmospheric model is used to forecast events such as an El Niño warming event in the Pacific Ocean and in long range seasonal outlooks.
Global Warming
An overall increase in world temperatures which may be caused by additional heat being trapped by greenhouse gases.
Geostationary Operational Environmental Satellite.
The time rate or spatial rate of change of an atmospheric property.
Small pellets of ice created when supercooled water droplets coat, or rime, a snowflake. The pellets are cloudy or white, not clear like sleet, and often are mistaken for hail.
Gravity Wave
A wave disturbance in which buoyancy acts as the restoring force on parcels displaced from hydrostatic equilibrium. Waves on the ocean are examples of gravity waves.
Gravity Dam
A concrete structure proportioned so that its own weight provides the major resistance to the forces exerted on it.
Greenhouse Effect
The heating effect caused by gases in the atmosphere absorbing heat (solar radiation) instead of letting it escape back into space. There are 2 types:
  1. Natural - It is what keeps the Earth's average temperature at 59°F instead of 0°F. In this case, the most abundant greenhouse gas is water vapor.
  2. Anthropogenic - Additional warming caused by having too much carbon dioxide (CO2).
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. It is usually more noticeable at night when the radar beam is encountering superrefractive conditions.
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. Ground fog has little vertical extent (usually 20 feet or less).
Grounded Ice
Ice that has run aground or is contact with the ground underneath it.
Ground Receiver Site
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 processing.
Ground Water
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
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
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 flow.
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 Mining
Pumping ground water from a basin where the safe yield is very small, thereby extracting ground water which had accumulated over a long period of time.
Ground Water Outflow
That 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
The part of runoff, caused by precipitation and/or snowmelt, that passes into the ground, becomes ground water, and gets discharged into a stream or river as spring or seepage water.
Grout Curtain
A barrier produced by injecting grout into a vertical zone, usually narrow (horizontally), and in the foundation to reduce seepage under a dam.
Growing Degree Days
The number of degrees that the average temperature is above a baseline value. For example, 40°F for canning purposes; 45°F for potatoes; and 50°F 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.
Gulf Stream
A warm, swift, narrow ocean current flowing along the East Coast of the United States.
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.
A brief sudden increase in wind speed. Generally the duration is less than 20 seconds and the fluctuation greater than 10 mph.
Gust Front
Formed when the down draft and rain-cooled air of a thunderstorm reach the ground, and then spread out along the ground. Usually marked by a sudden wind shift, sharply falling temperatures, and possibly heavy downpours and/or hail. If two or more of these gust fronts intersect each other, a new thunderstorm could possibly develop. Sometimes it is associated with a shelf cloud or roll cloud. Also, see downburst, gustnado, and outflow boundary. .
Slang for a 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.
A circular or spiral motion, primarily referring to water currents.
A device used for measuring changes in direction. Often used in antenna stabilization.


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