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Mauna Kea Weather Center Meteorology Glossary/Dictionary

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• G system - Same as grid navigation. • gain - An increase or amplification. There are two general uses of the term in radar meteorology: 1) antenna gain (or gain factor), which is the ratio of the power transmitted along the beam axis to that of an isotropic radiator transmitting the same total power; and 2) receiver gain (or video gain), which is the amplification given a signal by the receiver. • gating - (Or range gating.) The use of digital or electrical methods in radar to eliminate or reject the target signals from all targets that are outside certain range limits. Such methods make it possible to measure properties of the echoes from particular targets without interference from the signals returned from closer or more distant targets. • general forecast - See weather forecast. • global model - See general circulation model. • GOES - Abbreviation for Geostationary Operational Environmental Satellite. • gradient - 1. The space rate of decrease of a function. The gradient of a function in three space dimensions is the vector normal to surfaces of constant value of the function and directed toward decreasing values, with magnitude equal to the rate of decrease of the function in this direction. The gradient of a function f is denoted by -f (without the minus sign in the older literature) and is itself a function of both space and time. The ascendent is the negative of the gradient. In Cartesian coordinates, the expression for the gradient is -f = i + j + k. For expressions in other coordinate systems, see Berry et al. (1945). 2. Often loosely used to denote the magnitude of the gradient or ascendent (i.e., without regard to sign) of a horizontal pressure field. • grading - The degree of mixing of particle size classes in a sediment. Well-graded sediments are those with a more or less uniform distribution of sizes; poorly graded implies uniformity in size or lack of a continuous distribution. • gravity - (Or force of gravity.) The force imparted by the earth to a mass that is at rest relative to the earth. Since the earth is rotating, the force observed as gravity is the resultant of the force of gravitation and the centrifugal force arising from this rotation. It is directed normal to sea level and to its geopotential surfaces. The magnitude of the force of gravity at sea level decreases from the poles, where the centrifugal force is zero, to the equator, where the centrifugal force is a maximum but directed opposite to the force of gravitation. This difference is accentuated by the shape of the earth, which is nearly that of an oblate spheroid of revolution slightly depressed at the poles. Also, because of the asymmetric distribution of the mass of the earth, the force of gravity is not directed precisely toward the earth's center. The magnitude of the force of gravity per unit mass (acceleration of gravity) g may be determined at any latitude f[&phgr;] and at any geometric height z (meters) above sea level in the free air from the following empirical formula: g = gf[&phgr;] - (3.085462 ´[×] 10-4 + 2.27 ´[×] 10-7cos 2f[&phgr;])z + (7.254 ´[×] 10-11 + 1.0 ´[×] 10-13cos 2f[&phgr;])z2 - (1.517 ´[×] 10-17 + 6 ´[×] 10-20cos 2f[&phgr;])z3 (cm s-2), where gf[&phgr;] = 980.6160 (1 - 0.0026373 cos 2f[&phgr;] + 0.0000059 cos2 2f[&phgr;]) is the sea level value of gravity (cm s-2) at latitude f[&phgr;]. This formula as applied near the earth indicates that gravity changes very little with height or latitude, so that for rough calculations a constant value of 980 cm s-2 may be used. Besides these variations in the magnitude of the force of gravity, there are more localized variations controlled by the topography of the earth's surface, and the distribution of mass beneath. The magnitude of the force of gravity is usually called either gravity, acceleration of gravity, or apparent gravity. See virtual gravity, geopotential height, standard gravity. • gravity wave - (Also called gravitational wave.) A wave disturbance in which buoyancy (or reduced gravity) acts as the restoring force on parcels displaced from hydrostatic equilibrium. There is a direct oscillatory conversion between potential and kinetic energy in the wave motion. Pure gravity waves are stable for fluid systems that have static stability. This static stability may be 1) concentrated in an interface or 2) continuously distributed along the axis of gravity. The following remarks apply to the two types, respectively. 1) A wave generated at an interface is similar to a surface wave, having maximum amplitude at the interface. A plane gravity wave is characteristically composed of a pair of waves, the two moving in opposite directions with equal speed relative to the fluid itself. In the case where the upper fluid has zero density, the interface is a free surface and the two gravity waves move with speeds where U is the current speed of fluid, g the acceleration of gravity, L the wavelength, and H the depth of the fluid. For deep-water waves (or Stokesian waves or short waves), H >> L and the wave speed reduces to For shallow-water waves (or Lagrangian waves or long waves), H << L, and c = U ±[±] (gH)½. All waves of consequence on the ocean surface or interfaces are gravity waves, for the surface tension of the water becomes negligible at wavelengths of greater than a few centimeters (see capillary wave). 2) Heterogeneous fluids, such as the atmosphere, have static stability arising from a stratification in which the environmental lapse rate is less than the process lapse rate. The atmosphere can support short internal gravity waves and long external gravity waves. The short waves (of the order of 10 km) have been associated, for example, with lee waves and billow waves. Such waves have vertical accelerations that cannot be neglected in the vertical equation of perturbation motion. The long gravity waves, moving relative to the atmosphere with speed ±[±](gH)½, where H is the height of the corresponding homogeneous atmosphere, have small vertical accelerations and are therefore consistent with the quasi-hydrostatic approximation. In neither type of gravity wave, however, is the horizontal divergence negligible. For meteorological purposes in which neither type is desired as a solution, for example, numerical forecasting, they may be eliminated by some restriction on the magnitude of the horizontal divergence. The above discussion is based upon the method of small perturbations. In certain special cases of water waves, for example, the Gerstner wave or the solitary wave, a theory of finite-amplitude disturbances exists. See shear-gravity wave. • grid - A set of points arranged in an orderly fashion on which specified variables are analyzed or predicted. Various forms of horizontal and vertical grids, each with particular characteristics, have been devised for use in numerical weather prediction. • gust - 1. A sudden, brief increase in the speed of the wind. It is of a more transient character than a squall and is followed by a lull or slackening in the wind speed. Generally, winds are least gusty over large water surfaces and most gusty over rough land and near high buildings. According to U.S. weather observing practice, gusts are reported when the peak wind speed reaches at least 16 knots and the variation in wind speed between the peaks and lulls is at least 9 knots. The duration of a gust is usually less than 20 s. 2. With respect to aircraft turbulence, a sharp change in wind speed relative to the aircraft; a sudden increase in airspeed due to fluctuations in the airflow, resulting in increased structural stresses upon the aircraft. 3. (Rare.) Same as cloudburst. 