summer, by the counterclockwise circulation around
the South Pacific high.
Antarctica is a great source region for intensely
cold air masses. The temperatures are colder than in the
arctic regions. These air masses have continental
characteristics, but before the air reaches other land
areas, it becomes modified and is properly called
During the polar night the absence of insolation
causes a prolonged cooling of the snow surface, which
makes Antarctica a permanent source of very cold air. It
is extremely dry and stable aloft. This polar air mass is
referred to as continental antarctic (cA) air. In summer
the continent is not as cold as in winter because of
constant solar radiation but continues to function as a
source for cold cA air.
In both winter and summer, the air mass is
thermally modified as it flows northward through
downslope motion and surface heating; as a result, it
becomes less stable. It assumes the characteristics of
maritime antarctic air. The leading edge of this air mass
then becomes the northern boundary of the antarctic
To the north of the antarctic front is found a vast
mass of maritime polar air that extends around the
hemisphere between 40°S and 68°S in summer and
between 34°S and 65°S in winter. At the northern limit
of this air mass is found the Southern Hemisphere polar
front. During summer this mP air is by far the most
important cold air mass of the hemisphere because of
the lack of massive outbreaks of cold continental air
Different weather conditions occur with each type
of air mass. The cA air produces mostly clear skies. The
mA air masses are characterized generally by an
extensive overcast of stratus and stratocumulus clouds
with copious snow showers within the broad zone of the
antarctic front. An area of transition that extends
mainly from the coastline to the northern edge of the
consolidated pack ice is characterized by broken to
overcast stratocumulus clouds with somewhat higher
bases and little precipitation.
What is the definition of an air mass?
Name the two factors that are necessary to
produce an air mass?
What type of air mass is mTk?
What are the two modifying influences on air
What is the warmest air mass observed in the
United States at its altitude?
LEARNING OBJECTIVE: Describe the
specific parts that make up a front and identify
how a front is classified as either cold, warm,
occluded, or quasi-stationary.
A front, generally speaking, is a zone of transition
between two air masses of different density and
temperature and is associated with major weather
changes, some of which can be violent. This fact alone
is sufficient reason for an in-depth study of fronts and
their relationship to air masses and cyclones.
DEFINITIONS AND CLASSIFICATIONS
A front is not just a colorful line drawn on a surface
chart. A front is a three-dimensional phenomena with a
very specific composition. Since a front is a zone of
transition between two air masses of different densities,
there must be some sort of boundary between these air
masses. One of these boundaries is the frontal surface.
The frontal surface is the surface that separates the two
air masses. It is the surface next to the warmer air (less
dense air). In reality, however, the point at which two air
masses touch is not a nice, abrupt separation. This area
is a zone of a large density gradient. This zone is called
the frontal zone. A frontal zone is the transition zone
between two adjacent air masses of different densities,
bounded by a frontal surface. Since the temperature
atmospheric density, a front almost invariably separates
air masses of different temperatures.
At this point you should be aware of the various
types of fronts. The question in your mind should be
how a front is classified. Whether it is cold, warm, or
stationary. A front is classified by determining the
instantaneous movement. The direction of movement
of the front for the past 3 to 6 hours is often used.
Classification is based on movement relative to the
warm and cold air masses involved. The criterion is as
A cold front is one that moves in a direction in
which cold air displaces warm air at the surface. In
other words the cold (or cooler) air mass is moving