# Glossary

## g/kg

grams per kilogram

Georgia

## Gamma Rays

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

## Gas

one of the three basic phases of matter; a gas can expand indefinitely to completely fill its container

## Gas Constant

a constant in the Equation of State for ideal gases; for dry air, the gas constant is 287.054 joules per kilogram per Kelvin

## Gas Laws

the thermodynamic laws pertaining to perfect gases, including Boyle’s lawCharles’ lawDalton’s law and the Equation of State

general

## General Circulation

the typical atmospheric weather patterns over the earth

## General Circulation Model (GCM)

a type of numerical model used in Numerical Weather Prediction that represents a forecast of the long-term (e.g., months or years in the future) changes in the atmosphere globally

## 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 pressure is 500 mb is 5600 m above sea level. This value varies and is estimated based on temperature and pressure data.

## Geostationary Operational Environmental Satellite (GOES)

satellite that orbits the Earth and moves at the same rotational speed as the planet, so the satellite stays over the same location on the Earth 24 hours a day. See Geostationary Satellite.

## Geostationary Satellite

satellite that orbits the earth at a height and velocity that allow it to remain over a fixed place above the equator

## Geostrophic Wind

a theoretical wind that results from the balance between the horizontal pressure gradient force and the Coriolis force. It is assumed to be straight-line, frictionless flow (which is more likely to be found at higher levels in the atmosphere). In the Northern Hemisphere, low pressure is found to the left of the flow of the geostrophic wind, while in the Southern Hemisphere, low pressure is found to the right of the flow.

## Geosynchronous

a term applied to any equatorial satellite that is virtually motionless with respect to an observer on the ground. This satellite has an orbital velocity equal to the rotational velocity of the earth.

see Ground Fog

## Glaciation

the transformation of cloud droplets from water drops to ice crystals. A cloud that becomes all ice is called glaciated.

## Glaciation

the condition that exists when land is covered with glaciers

## Glacier

a large body of ice moving slowly down a slope or valley or spreading outward on a land surface

## Global Forecast System Model (GFS)

an NCEP numerical model that uses output from the Aviation model (AVN). It is run four times daily, with forecast output out to 384 hours (long range)

the total direct solar radiation and diffuse sky radiation (e.g., radiation that is reflected or scattered) received by the surface of the earth

## Global Scale

the largest scale of planetary motion; covering all or most of a latitude circle

## Global Warming

the increase in the average measured temperature of the Earth’s near-surface air and oceans since the mid-20th century and the projected continuation. It is an enhancement of the Greenhouse Effect and usually refers to human-induced changes.

## Glory

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. A glory can also be seen sometimes around a plane when it is flying above clouds.

ground

general

## GOES

in general, the spatial change of a physical quantity (e.g., temperature). A strong gradient means that the quantity changes rapidly as you move from one side of the gradient to the other. For example, a strong temperature gradient (lots of isotherms packed together) could mean that as you walk from one side to the other, the temperature will change rapidly. A weak gradient (isotherms spaced out) means that the quantity changes more gradually.

## Graupel

snow pellets, soft sleet, small hail

## Gravitational Force

keeps the molecules in the atmosphere from moving into space. Gravity’s influence is stronger near the earth’s surface and weaker aloft. The same as Gravity.

## Gravity

in particular, the force imparted by the earth that tends to draw all bodies in the earth’s sphere of influence toward the center of the earth

## Gravity Wave

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

## Graybody

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

## Greenhouse Gases

the gases that absorb terrestrial radiation and contribute to the greenhouse effect; the main greenhouse gases are water vapor, methane, Carbon Dioxide, and ozone.

## Greenwich Mean Time (GMT)

global time scale based on the local time observed on the Prime Meridian; also called Zulu (Z) time or Universal Coordinated Time (UTC)

## Ground Clutter

a pattern of radar echoes from fixed ground targets (buildings, hills, etc.) near the radar. This contamination is processed into the NEXRAD base products (base reflectivitybase velocity, and spectrum width) and affects all derived products. Ground clutter is most prevalent close to the radar at the lowest elevation slices. Ground clutter is always present around the radar and is not the same as anomalous propagation (AP), which occurs during certain atmospheric conditions.

## Ground Fog (or Radiation Fog) (GF)

fog formed when temperatures near the ground cool to near the dew point temperature. The ground cools because the sun does not warm it at night.

## Ground Stroke

the current that moves along the ground from the point where a direct stroke of lightning hits the ground.

## Growing Degree-Day

a form of degree-day used as a guide to determine the most appropriate time to plant and harvest crops

great

greatest

gusty

greater

## Gulf Stream

a warm, swift, relatively narrow ocean current that flows along the east coast of the United States

## Gust Front

a boundary between cold air from the thunderstorm downdraft and warm, humid surface air