SMOOTHING—An averaging of data in space or
time, designed to compensate for random errors or
fluctuations of a scale smaller than that presumed
significant to the problem at hand; the analysis of a
sea-level weather map smoothes the pressure field
on a space-scale more or less systematically
determined by the analyst by taking each pressure
as representative not of a point but of an area about
the point.
SOLAR CONSTANT—The rate at which solar
radiation is received outside Earth’s at-mosphere
on a surface normal to the incident radiation, and at
Earth’s mean distance from the Sun.
SOLSTICE—(1) Either of two points on the Sun’s
apparent annual path where it is displaced farthest,
north or south, from Earth’s equator. The Tropic of
Cancer (north) and Tropic of Capricorn (south) are
defined as the parallels of latitude that lie directly
beneath a solstice. (2) Popularly, the time at which
the Sun is farthest north or south; the “time of the
solstice.” In the Northern Hemisphere, the summer
solstice falls on or about 21 June, and the winter
solstice on or about 22 December. The reverse is
true in the southern latitudes.
SPECIFIC HEAT—The heat capacity of a system per
unit mass. That is, the ratio of the heat absorbed (or
released) by unit mass of the system to the
corresponding temperature rise (or fall).
SPECIFIC HUMIDITY—In moist air, the ratio of the
mass of water vapor to the total mass of the system.
For many purposes it may be approximated by the
mixing ratio.
SQUALL LINE—Any non-frontal line or narrow
band of active thunderstorms.
STANDARD
ATMOSPHERE—A
hypothetical
vertical distribution of atmospheric temperature,
pressure, and density which, by international
agreement, is taken to be representative of the
atmosphere for purposes of pressure altimeter
calibrations, aircraft performance calculations,
aircraft and missile design, ballistic tables, etc. The
air is assumed to obey the perfect gas law and the
hydrostatic equation, which, taken together, relate
temperature, pressure, and density variations in the
vertical. It is further assumed that the air contains
no water vapor and that the acceleration of gravity
does not change with height.
STRATOSPHERE—The atmospheric shell above the
troposphere and below the mesosphere. It extends,
therefore, from the tropopause to the height where
the temperature begins to increase in the 20- to
25-km region.
SUBLIMATION—The transition of a substance from
the solid phase directly to the vapor phase, or vice
versa, without passing through an intermediate
liquid phase.
SUBSIDENCE—A descending motion of air in the
atmosphere, usually with the implication that the
condition extends over a rather broad area.
SUBSIDENCE
INVERSION—A
temperature
inversion produced by the adiabatic warming of a
layer of subsiding air. Vertical mixing of the air
layer below the inversion enhances this inversion.
SUBTROPICAL HIGH—One of the semi-permanent
highs of the subtropical high-pressure belt. They
appear as centers of action on mean charts of
surface pressure. They lie over oceans and are best
developed in summer.
SUBTROPICAL HIGH-PRESSURE BELT—One
of the two belts of high atmospheric pressure that
are centered, in the mean, near 30°N and 30°S
latitudes.
SUNSPOT—A relatively dark area on the surface of
the Sun, consisting of a dark central umbra
surrounded by a penumbra, which is intermediate
in
brightness
between
the
umbra
and
the
surrounding photosphere.
SUPERADIABATIC
LAPSE
RATE—An
environmental
lapse
rate
greater
than
the
dry-adiabatic
lapse
rate,
such
that
potential
temperature decreases with height.
SUPERCOOLING—The reduction of temperature of
any liquid below the melting point of that
substance’s solid phase, that is, cooling beyond its
nominal freezing point.
SUPERIOR AIR—An exceptionally dry mass of air
formed by subsidence and usually found aloft but
occasionally
reaching
Earth’s
surface
during
extreme subsidence processes.
SUPERIOR MIRAGE—A spurious image of an
object formed above its true position by abnormal
refractive conditions; opposite of inferior mirage.
AI-8
