Glossary of Meteorological Terms (G)

G

Gain

An increase or amplification. There are two general usages of the term in radar meteorology: (a) antenna gain (or gain factor) is the ratio of the power transmitted along the beam axis to that of an isotropic radiator transmitting the same total power; and (b) receiver gain (or video gain) is the amplification given a signal by the receiver.

Gain factor

See gain.

Gale

Wind with a speed between 28 and 55 knots (32 and 63 mph); Beaufort scale numbers 7 through 10.

Galilei

The unit of acceleration in the centimeter-gram-second system of units, equal to one cm per sec². Commonly used in gravimetry.

Gas thermometer

A thermometer which utilizes the thermal properties of gas. There are two forms of this instrument: (a) a type in which the gas is kept at constant volume, and pressure is the thermometric property; and (b) a type in which the gas is kept at constant pressure, and volume is the thermometric property. The gas thermometer is the most accurate of all thermometers and is used as the standard instrument for measurement of temperature.

Gauge relation

An empirical curve relating stream discharge or stage at a point on a stream to discharge or stage at one or more upstream points and, possibly, to other parameters. Also called stage relation.

Gentle breeze

Wind with a speed between 7 and 10 knots (8 and 12 mph); Beaufort scale number 3.

Geostrophic wind

That horizontal wind velocity at which the Coriolis acceleration exactly balances the horizontal pressure force. It is directed along contour lines or isobars.

Geostrophic wind level

The lowest level at which the wind becomes geostrophic in the theory of the Ekman spiral. Also called gradient wind level.

Geostrophic-wind scale

A graphical device used for the determination of the speed of the geostrophic wind from the isobar or contour-line spacing on a synoptic chart.

Gerdien aspirator

An instrument used for the determination of the electrical conductivity of the atmosphere.

Glass

In nautical terminology, a contraction for "weather glass" (a mercury barometer).

Glaze

A coating of ice, generally clear and smooth, formed by the freezing of supercooled water on a surface.

Glime

An ice coating with a consistency intermediate between glaze and rime.

Global dimming

An observed reduction in the amount of sunlight reaching the surface of the Earth, thought to be caused by air pollution.

Global radiation

The total of direct solar radiation and diffuse sky radiation received by a unit horizontal surface. Global radiation is measured by pyranometers.

Goldbeater's-skin hygrometer

A hygrometer using goldbeater's skin as the sensitive element. Variations of the physical dimensions of the skin caused by its hygroscopic character indicate relative humidity. (Note: Goldbeater's skin is the prepared outside membrane of the large intestine of an ox. It is used in goldbeating to separate the leaves of the metal.)

Goniometer

An instrument used for measuring geometric angles. See radio direction-finder.

Gradient wind

Any horizontal wind velocity tangent to the contour line of a constant pressure surface (or to the isobar of a geopotential surface) at the point in question.

Gradient wind level

Same as geostrophic wind level.

Gram

A c.g.s. (centimeter-gram-second) unit of mass. Originally defined as the mass of 1 cubic centimeter of water at 4°C but now taken as the one-thousandth part of the standard kilogram, a mass preserved by the International Bureau of Weights and Measures.

Gram calorie

See calorie.

Gram-mole

See mole.

Gram-molecule

See mole.

Graphing board

Board that holds graph paper on which is plotted information obtained from a pilot-balloon observation.

Grass minimum

The minimum temperature shown by a minimum thermometer exposed in an open situation with its bulb at the level of the tops of the grass blades of short turf.

Grass temperature

The temperature registered by a thermometer with its bulb at the level of the tops of the grass blades in short turf.

Gravity wind

A wind (or component thereof) directed down the slope of an incline and caused by greater air density near the slope than at the same levels some distance horizontally from the slope. Also called drainage wind and sometimes called katabatic wind.

Gray body

A hypothetical body which absorbs some constant fraction, between zero and one, of all electromagnetic radiation incident upon it, which fraction is the absorptivity and is independent of wavelength. Compare to black body, white body.

Grid nephoscope

A direct-vision nephoscope constructed in the following manner: A grid-work of bars is mounted horizontally on the end of a vertical column and made free to rotate about the vertical axis. The observer rotates the grid and adjusts his or her position until some feature of the cloud appears to move along the major axis of the grid. The azimuth angle at which the grid is set is taken as the direction of cloud motion.

Ground-check chamber

A chamber use to check the sensing elements of radiosonde equipment.

Ground-Hog Day

February 2nd. In American folklore, a day that is popularly supposed to provide the key to the weather for the remainder of the winter. Specifically, if the ground-hog upon emerging from its hole casts a shadow, it will return underground, thereby foreboding more wintery weather. There is no convincing statistical evidence to support this belief.

Gust

A sudden brief increase in the speed of the wind, followed by a lull or slackening. Compare to peak gust.

Gustiness components

The ratios, to the mean wind speed, of the average magnitudes of the component fluctuations of the wind along three mutually perpendicular axes.

Gustiness factor

A measure of the intensity of gusts given by the ratio of the total range of wind speed between gusts and the intermediate periods of lighter wind to the mean wind speed, averaged over both gusts and lulls.

Gustsonde

An instrument, dropped from high altitude and carried by a stable parachute, used to measure the vertical component of turbulence aloft.