TROPOPAUSE—The
boundary
between
the
troposphere and stratosphere, usually characterized
by an abrupt change of lapse rate.
TROPOSPHERE—That
portion
of
Earth’s
atmosphere extending from the surface to the
tropopause; that is, the lowest 10 to 20 km of the
atmosphere.
TROUGH—An elongated area of low atmospheric
pressure; the opposite of a ridge.
TRUE NORTH—The direction from any point on
Earth’s surface toward the geographic North Pole;
the northerly direction along any projection of
Earth’s axis upon Earth’s surface, for example,
along a longitude line. Except for much of
navigational practice (which uses magnetic north),
true north is the universal 0° (or 360°, mapping
reference).
UPSTREAM—In the direction from which a fluid is
flowing.
UPWIND—In the direction from which the wind is
blowing.
VECTOR—Any quantity, such as force, velocity, or
acceleration, that has both magnitude and direction
at each point in space, as opposed to a scalar, which
has
magnitude
only.
Geometrically,
it
is
represented by an arrow of length proportional to
its magnitude, pointing in the assigned direction.
VEERING—A change in wind direction in a
clockwise sense in the Northern Hemisphere and
counterclockwise
direction
in
the
Southern
Hemisphere.
VERNAL EQUINOX—For either hemisphere, the
equinox at which the Sun’s most direct rays
approach
from
the
opposite
hemisphere.
In
northern latitudes, this occurs approximately on 21
March; the Sun’s most direct rays are centered over
the equator and moving north.
VIRTUAL TEMPERATURE—In a system of moist
air, the temperature of dry air having the same
density and pressure as the moist air. It is always
greater than the actual temperature.
WARM-CORE HIGH—At a given level in the
atmosphere, any high that is warmer at its center
than at its periphery.
WARM-CORE LOW—At a given level in the
atmosphere, any low that is warmer at its center
than at its periphery.
WARM FRONT—Any non-occluded front or portion
thereof that moves in such a way that warmer air
replaces colder air.
WARM SECTOR—That area within the circulation of
a wave cyclone where the warm air is found. It lies
between the cold front and the warm front of the
storm; and, in the typical case, the warm sector
continually diminishes in size and ultimately
disappears (at the surface) as the result of
occlusion.
WAVE CYCLONE—A cyclone that forms and moves
along a front.
WAVE THEORY OF CYCLONES—A theory of
cyclone development based upon the principles of
wave formation on an interface between two fluids.
In the atmosphere, a front is taken as such an
interface.
WEATHER—The state of the atmosphere, mainly
with respect to its effect upon life and human
activities.
WESTERLIES—(also
known
as
circumpolar
westerlies,
counter-trades,
middle-latitude
westerlies, midlatitude westerlies, polar westerlies,
subpolar
westlies,
subtropical
westerlies,
temperate westerlies, zonal westerlies, and zonal
winds) Specifically, the dominant west-to-east
motion of the atmosphere, centered over the middle
latitudes of both hemispheres. At the surface, the
westerly belt extends, on the average, from about
35° to 65° latitude. At upper levels, the westerlies
extend farther equatorward and poleward. The
equatorward boundary is fairly well defined by the
subtropical
high-pressure
belt;
the
poleward
boundary is quite diffuse and variable.
WIND-CHILL FACTOR—The cooling effect of any
combination of temperature and wind, expressed as
the loss of body heat, in kilogram calories per hour
per square meter of skin surface. The wind-chill:
factor. is based on the cooling rate of a nude body in
the shade; It is only an approximation, because of
individual body variations in shape, size, and
metabolic rate.
AI-10
