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DEEPENING.—A decrease in the central pressure of a low-pressure system.

DISPERSION.—The process in which radia-tion is separated into its component wavelengths. It results when an optical process, such as diffrac-tion, refraction, or scattering, varies according to wavelength. All of the coloration displayed by atmospheric optical phenomena is the result of dispersion.

DOLDRUMS.—A nautical term for the equatorial trough, with special reference to the light and variable nature of the winds.

DOWNWIND.—The direction toward which the wind is blowing; with the wind.

DROPSONDE.—A radiosonde that is dropped by parachute from an aircraft for the purpose of obtaining a sounding of the atmosphere below.

DRY AIR.—In atmospheric thermodynamics and chemistry, air that contains no water vapor.

DYNAMIC TROUGH.—(also called lee trough) A pressure trough formed on the lee side of a mountain range across which the wind is blowing almost at right angles; often seen, on U.S. weather maps, east of the Rocky Mountains, and sometimes east of the Appalachians, where it is less pronounced.

EASTERLIES.—Any winds with components from the east, usually applied to broad currents or patterns of persistent easterly winds; the "easterly belts," such as the equatorial easterlies, the tropical easterlies (trade winds), and the polar easterlies.

EASTERLY WAVE.—A migratory wave-like disturbance of the tropical easterlies. Easterly waves occasionally intensify into tropical cyclones.

ELECTROMAGNETIC WAVES.—Disturb-ances in electric and magnetic fields in space or in material media, resulting in the propagation of electromagnetic energy (radiation).

EQUATORIAL TROUGH.—The quasi-continuous belt of low pressure lying between the subtropical high-pressure belts of the Northern and Southern hemispheres. The region is one of very homogeneous air, probably the most ideally barotropic region of the atmosphere. The posi-tion of the equatorial trough is fairly constant in the eastern portions of the Atlantic and Pacific, but it varies greatly in the western portions of those oceans and in southern Asia and the Indian Ocean. It moves into or toward the hemisphere experiencing summer.

EQUINOX.—(1) Either of the two points of intersection of the Sun’s apparent annual path and the plane of Earth’s equator. (2) Popularly, the time at which the Sun passes directly above the equator; the "time of the equinox." In the North-ern Hemisphere, the vernal equinox falls on or about 21 March, and the autumnal equinox on or about 22 September. These dates are reversed in the Southern Hemisphere.

EVAPORATION.—The physical process by which a liquid or solid is transformed to the gaseous state.

EXTRATROPICAL CYCLONE.—Typically, any cyclonic-scale storm that forms poleward of the tropical easterlies, i.e., the migratory frontal cyclones. Tropical cyclones that move poleward out of the tropical easterlies and take on extratropical characteristics (air mass discon-tinuity) are reclassified as extratropical.

FILLING.—An increase in the central pressure of a pressure system on a constant-height chart, or an analogous increase in height on a constant-pressure chart; the opposite of deepening.

FRONT.—The interface or transition zone between two air masses of different density. Since temperature distribution is the most important regulator of atmospheric density, a front almost invariably separates air masses of different temperature.

FRONTAL INVERSION.—A temperature inversion in the atmosphere, encountered upon vertical ascent through a sloping front.

FRONTAL SURFACE.—Refers specifically to the warmer side of the frontal zone. 

FRONTAL SYSTEM.—Simply, a system of fronts as they appear on a synoptic chart. This is used for (a) a continuous front and its characteristics along its entire extent, including its warm, cold, stationary, and occluded sectors, its variations of intensity, and any frontal cyclones along it; and (b) the orientation and nature of the fronts within the circulation of a frontal cyclone.

FRONTAL ZONE.—The transition zone be-tween two adjacent air masses of different den-sities bounded by a frontal surface.

FRONTOGENESIS.—The initial formation of a front or frontal zone.

FRONTOLYSIS.—The dissipation of a front or frontal zone.

GENERAL CIRCULATION.—(also called planetary circulation) In its broadest sense, the complete statistical description of atmospheric motions over Earth.

GEOPOTENTIAL.—The potential energy of a unit mass relative to sea level, numerically equal to the work that would be done in lifting the unit mass from sea level to the height at which the mass is located; commonly expressed in terms of dynamic height or geopotential height.

GEOPOTENTIAL HEIGHT.—The height of a given point in the atmosphere in units propor-tional to the potential energy of a unit mass (geopotential) at that height, relative to sea level.

GEOSTROPHIC FLOW.—A form of gra-dient flow where the Coriolis force exactly balances the horizontal pressure force.

GEOSTROPHIC WIND.—That horizontal wind velocity for which the Coriolis acceleration exactly balances the horizontal pressure force. The geostrophic wind is directed along the contour lines on a constant-pressure surface (or along the isobars in a geopotential surface) with low pressure to the left in the Northern Hemisphere and to the right in the Southern Hemisphere.

GEOSTROPHIC-WIND SCALE.—A graph-ical device used for the determination of the speed of the geostrophic wind from the isobar or contour-line spacing on a synoptic chart.

GRADIENT.—The space rate of decrease of a function. It is often used to denote the magnitude of pressure change in the horizontal pressure field.

GRADIENT WIND.—Any horizontal wind velocity tangent to the contour line of a constant-pressure surface (or the isobar of a geopotential surface) at the point in question. At such points, where the wind is gradient, the Coriolis accelera-tion and centripetal acceleration together exactly balance the horizontal pressure force.

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

GREENHOUSE EFFECT.—The heating effect exerted by the atmosphere upon Earth by virtue of the fact that the atmosphere (mainly, its water vapor) absorbs and re-emits infrared radia-tion. In detail: The shorter wavelengths of insolation are transmitted rather freely through the atmosphere to be absorbed at Earth’s surface. Earth then re-emits this as long-wave (infrared) terrestrial radiation, a portion of which is absorbed by the atmosphere and again emitted as atmospheric radiation. The water vapor (cloud cover) acts in the same way as the glass panes of a greenhouse; the heat gained during the day is trapped beneath the cloud cover, and the counter-radiation adds to the warming of Earth.

GROUND CLUTTER.—The pattern of radar echoes from fixed ground targets near the radar. This type of clutter tends to hide or confuse the echoes returned from nearby moving or precipita-tion targets. Ground clutter can be significantly increased during periods of superrefraction.

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