FRONTOLYSISThe dissipation of a front or frontal
GENERAL CIRCULATION(also called planetary
circulation) In its broadest sense, the complete
statistical description of atmospheric motions over
GEOSTROPHIC FLOWA form of gra-dient flow
where the Coriolis force exactly balances the
horizontal pressure force.
GEOSTROPHIC WIND The wind velocity for which
the Coriolis acceleration exactly balances the
horizontal pressure force. The geostrophic wind is
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
device used for the determination of the speed of
the geostrophic wind from the isobar or contour
line spacing on a synoptic chart.
GRADIENTThe space rate of decrease of a
function. It is often used to denote the magnitude of
pressure change in the horizontal pressure field.
GRADIENT WINDAny 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 acceleration 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.
exerted by the atmosphere upon Earth by virtue of
the fact that the atmosphere (mainly, its water
vapor) absorbs and re-emits infrared radiation. In
detail: The shorter wavelengths of insolation are
transmitted rather freely through the atmosphere to
be absorbed at Earths 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.
HALOAny one of a large class of atmospheric
optical phenomena (luminous meteors) that appear
as colored or whitish rings and arcs about the Sun
or Moon when seen through an ice crystal cloud or
in a sky filled with falling ice crystals. The halos
experiencing prismatic coloration are produced by
refraction of light by the ice crystals, and those
exhibiting only whitish luminosity are produced by
reflection from the crystal faces.
HEAT BALANCEThe equilibrium, which exists on
the average, between the radiation received by
Earth and its atmosphere and that emitted by Earth
and its atmosphere.
HEATING DEGREE-DAYA form of degree-day
used as an indication of fuel consumption; in
United States usage, one heating degree-day is
given for each degree that the daily mean
temperature departs below the base of 65°F.
HEAT TRANSFERThe transfer or ex-change of
heat by radiation, conduction, or convection in a
fluid and/or between the fluid and its surroundings.
The three processes occur simultaneously in the
atmosphere, and it is often difficult to assess the
contributions of their various effects.
HIGHAn area of high pressure, refer-ring to a
dimensions (closed isobars) on the synoptic surface
chart, or a maximum of height (closed contours) on
the constant-pressure chart. Highs are associated
with anticyclonic circulations, and the term is used
interchangeably with anticyclone.
HORSE LATITUDESThe belts of latitude over the
oceans at approximately 30 to 35 degrees north and
south where winds are predominantly calm or very
light and the weather is hot and dry.
located near Iceland (mainly between Iceland and
southern Greenland) on mean charts of sea-level
pressure. It is a principal center of action in the
INACTIVE FRONT(or passive front) A front
or portion thereof that produces very little
cloudiness and no precipitation, as opposed to an