MESOSPHERE—The atmospheric shell between
about 20 km and about 70 or 80 km, ex-tending
from the top of the stratosphere to the upper
temperature minimum (the menopause). A broad
temperature
maximum
at
about
50
km
characterizes it, except possibly over the winter
polar regions.
METEOROLOGY—The study dealing with the
phenomena of the atmosphere. This includes not
only the physics, chemistry, and dynamics of the
atmosphere, but is extended to include many of the
direct effects of the atmosphere upon Earth’s
surface, the oceans, and life in general.
MICROCLIMATE—The fine climate structure of the
air space that extends from the very surface of
Earth to a height where the effects of the immediate
character of the underlying surface no longer can
be distinguished from the general local climate
(mesoclimate or microclimate).
MIGRATORY—Moving;
commonly
applied
to
pressure systems embedded in the wastrels and,
therefore,
moving
in
a
general
west-to-east
direction.
MILLBRAE—(abbreviated MB) A pressure unit of
1,000
dynes
per
centimeter,
convenient
for
reporting atmospheric pressures.
MIRAGE—A refraction phenomenon wherein an
image of some object is made to appear displaced
from its true position.
MOISTURE—A general term usually refer-ring to the
water vapor content of the atmosphere, or to the
total water substance (gas, liquid, and solid)
present in a given volume of air.
MONSOON—A name for seasonal wind. It was first
applied to the winds over the Arabian Sea, which
blow for 6 months from the northeast and 6 months
from the southwest, but it has been extended to
similar winds in other parts of the world.
MONSOON CLIMATE—The type of climate that is
found in regions subject to monsoons. It is best
developed on the fringes of the tropics.
NEUTRAL
STABILITY—The
state
of
an
unsaturated or saturated column. of air in the
atmosphere when its environmental lapse rate of
temperature is equal to the dry-adiabatic lapse rate
or the saturation-adiabatic lapse rate, respectively.
Under such conditions a parcel of air displaced
vertically will experience no buoyant acceleration.
OCCLUDED
FRONT—(commonly
called
occlusion;
also
called
frontal
occlusion)
A
composite of two fronts, formed as a cold front
overtakes a warm front or quasi-stationary front.
This is a common process in the late stages of wave
cyclone development, but it is not limited to
occurrence within a wave cyclone.
OCCLUSION—Same as OCCLUDED FRONT.
OROGRAPHIC LIFTING—The lifting of an air
current caused by its passage up and over
mountains.
OVERRUNNING—A condition existing when an air
mass is in motion aloft above another air mass of
greater density at the surface. This term is usually
applied in the case of warm air ascending the
surface of a warm or quasi-stationary front.
PARTIAL PRESSURE—The pressure of a single
component of a gaseous mixture, according to
Dalton’s Law.
PERTURBATION—Any departure introduced into
an assumed steady state of a system. In synoptic
meteorology, the term most often refers to any
departure from zonal flow within the major zonal
currents of the atmosphere. It is especially applied
to the wave-like disturbances within the tropical
easterlies.
PHOTOSPHERE—The intensely bright portion of
the Sun visible to the unaided eye. It is a shell a few
hundred miles in thickness marking the boundary
between the dense interior gases of the Sun and the
more diffuse cooler gases in the outer portions of
the Sun.
PLANETARY BOUNDARY LAYER—(also called
friction layer or atmospheric boundary layer) That
layer of the atmosphere from Earth’s surface to the
geostrophic wind level, including therefore, the
surface boundary layer and the Eckman layer.
PLANETARY CIRCULATION—The sys-tem of
large-scale disturbances in the troposphere when
viewed on a hemispheric or worldwide scale. Same
as GENERAL CIRCULATION.
POLAR AIR—A type of air whose characteristics are
developed over high latitudes, especially within the
subpolar highs. Continental polar air (cP) has low
surface temperature, low moisture content, and,
especially in its source regions, great stability in the
lower layers.
It is shallow in comparison with
arctic air.
AI-5
