Gamma Ray Bursts (TGFs): X-Ray/Gamma Ray Bursts occur from the tops of large lightning-producing cumulonimbus clouds. The acronym, TGF, comes from Terrestrial Gamma-ray Flashes. TFGs are short blasts of ionizing gamma-ray energy associated with thunderstorms. They only last a 1-2 milliseconds and can only be detected by satellites orbiting Earth. NASA scientists inadvertently discovered TFGs while monitoring gamma-ray energy from deep space in 1991. Between the years 1991 and 1999, about 70 TGFs were recorded, however this number would certainly be much higher if satellites used to detect deep-space gamma rays were less restricted in their orbital paths. It is believed that these gamma rays are scattered by the inner atmosphere before reaching outer space. Lightning is not powerful enough to generate TGFs, however TGFs are often seen with powerful thunderstorms, red sprites, blue jets and ELVES. For this reason, scientists believe these phenomena (collectively termed, "Transient Luminous Events" or TLEs) are somehow related. Research is still underway...

General Circulation of the Atmosphere: Large-scale atmospheric motions over the entire surface of Earth.

Geopotential Height: A modification of the true height to take account for the variation of gravity at the surface of the globe. If g = 9.8 m/sec²then geopotential and true height are the same. When the value of 'g' is increased, the geopotential height is greater.

Geostationary Satellite (GOES): A satellite that orbits Earth at the same rate that Earth rotates and thus remains over a fixed place above the equator or other position above Earth. There are many such satellites in orbit. Because these satellites must orbit at distances of about 35,000km, images from this type of satellite are not as detailed as those from polar satellites.

Geostrophic Departure: The vector difference between the actual wind and the geostrophic wind.

Geostrophic Wind: A horizontal wind blowing parallel to the isobars when the pressure gradient is exactly balanced by Coriolis Effect.

Glaze: A transparent layer of ice formed when supercooled droplets of cloud or rain freeze on contact with an object.

Glitter Path: A trivial, yet nice, effect of the sunlight reflecting off a water surface with ripple waves on it, creating a light path on the water from the horizon down to the observer. If the waves are not directly perpendicular to the line sun-observer, the glitter path will appear angled to one side.

Global Scale: The largest scale of atmospheric motion. Also called the Planetary Scale.

Global Average Temperature Lapse Rate: Roughly 6.5ºC/1 km in the troposphere. This represents a state of the atmosphere lying between perfectly dry and saturated conditions. This atmospheric state does not exist everywhere on the globe, nor does it actually exist at very many individual places. Rather, the global average lapse rate would represent the mean state of the troposphere if it could be geographically averaged.

Glory: A series of rings of colored light most commonly appearing around the shadow of an airplane that is projected on clouds below. Harald E. Edens adds: The glory is the name for colored diffraction rings around the antisolar point (i.e. where the shadow of the observer's head is). The rings occur when the sun (or moon or any light source) shines on a cloud deck or fog bank. The radius of the diffraction rings is dependent on the size of the cloud droplets: the smaller the droplets, the larger the radii. Moreover, the droplets have to be of much uniform size. The glory is believed to be caused by backscattered light rays which skim the edges of a water droplet. Diffraction takes care of the multiple colored rings, like with the corona.

Gradient: The slope of a surface. It may refer to the ground or an invisible feature such as a pressure surface defined mathematically.

Gradient Wind: A modification of the geostrophic wind which takes centrifugal force into account when the isobars are curved. With cyclonic curvature, the gradient wind is less than the geostrophic wind; with anticyclonic curvature, it is greater.

Graupel: A term used for ice pellets or small hail (diameter <5mm).

Gravity Current: Also called Density Current or Buoyancy Current. A mainly horizontal current caused by gravity acting on volumes of air or other fluid with different densities. Examples are sea-breezes, thunderstorm outflows and bores on rivers and estuaries.

Gravity Wave: Waves caused by gravity and the buoyancy of air. Lee waves are a type of gravity wave.

Green Flash: A small green color that occasionally appears on the upper part of the Sun as it rises or sets. When the sun is near the horizon, its light must penetrate a thick section of atmosphere, resulting in refraction of light with purple and blue light bending the most, and red light the least. Because of this bending, more blue light should appear along the top of the Sun. But because the atmosphere selectively scatters blue light, very little reaches us, and we see green light instead.

Harald E. Edens adds: The green flash is the effect of the last bit of solar disk coloring green when the sun sets below horizon. The effect is due to atmospheric refraction of light; the refraction angle for the green end of the light spectrum (the shorter wavelengths) is slightly larger than the red part. As a result, the spectrum of the solar disk is spread over a small vertical distance (a few sec. of arc), and when the sun sets, the green part is the last to disappear. During sunrise, the effect can also be seen, but this is a lot harder because one does not know exactly where to look.

The green flash is quite rare; it can only be seen when the horizon is absolutely clear, like it is over a sea. Also, the atmospheric temperature variation has to be suitable for enough refraction to occur. The green flash may, however, also be seen when the sun is low and is being obscured by a sharply defined cloud.
Variations of the green flash are the green rim at the upper part of the solar disk, and the green segment, a piece of green flash when a small part of the upper solar disk is separated from the main disk by atmospheric refraction through different temperature layers or inversion layers.

Greenhouse Effect: See Atmospheric Greenhouse Effect.

Grid: A network, often consisting of squares, drawn on a chart to enable the position of geographic features to be identified by a set of numbers.

Grid Length: The distance between grid points. Large squares are said to have a course mesh while small squares have a fine mesh.

Ground Fog: See Radiation Fog.

Ground Frost: A temperature below 0°C measured an several centimeters
above the ground. On clear nights, this is several degrees colder than the air temperature, which is measured in a screen at a height of 4 feet. (Here again, the definition uses both metric and US Standard units) :(

Growing Degree-Day: A form of the degree-day used as a guide for crop planting and for estimating crop maturity dates. A growing degree-day for a particular crop is defined as a day on which the mean daily temperature is one degree above the base temperature (also known as the zero temperature)--the minimum temperature required for growth of that crop. For sweet corn, the base temperature is 50°F and, for peas it is 40°F. The following is a chart adapted from Ahrens "Essentials of Meteorology":

Crop (Variety, Location)	Base Temperature (°F)	Growing Degree-Days to Maturity
Beans (Snap, South Carolina)	50	1200-1300
Corn (Sweet, Indiana)	50	2200-2800
Cotton (Delta Smooth Leaf, Arkansas)	60	1900-2500
Peas (Early, Indiana)	40	1100-1200
Rice (Vegold, Arkansas)	60	1700-2100
Wheat (Indiana)	40	2100-2400

Gulf Stream: A warm, swift, narrow ocean current flowing along the east coast of the USA. Also the name of a type of jet. :)

Gust: A sudden and brief increase in wind speed, often caused when the flow becomes turbulent over rough ground. A prolonged increase of wind is called a squall.

Gust Front: A boundary that separates a cold downdraft of a thunderstorm from warm, humid surface air. On the surface its passage resembles that of a cold front.