Characteristics of Air Masses
The characteristics of an air mass are acquired in
the source region, which is the surface area over which
the air mass originates. The ideal source region has a
uniform surface (all land or all water), a uniform
temperature, and is an area in which air stagnates to
(temperature and moisture content) an air mass
acquires in its source region are dependent upon a
number of factorsthe time of year (winter or
summer), the nature of the underlying surface (whether
land, water, or ice covered), and the length of time it
remains over its source region.
ARCTIC (A) AIR.There is a permanent
high-pressure area in the vicinity of the North Pole. In
this region, a gentle flow of air over the polar ice fields
allows an arctic air mass to form. This air mass is
characteristically dry aloft and very cold and stable in
the lower altitudes.
ANTARCTIC (A) AIR.Antarctica is a great
source region for intensely cold air masses that have
continental characteristics. Before the antarctic air
reaches other land areas, it becomes modified and is
properly called maritime polar. The temperatures are
colder than in the arctic regions. Results of Operation
temperatures in the world to be in the Antarctic.
continental polar source regions consist of all land
areas dominated by the Canadian and Siberian
high-pressure cells. In the winter, these regions are
covered by snow and ice. Because of the intense cold
and the absence of water bodies, very little moisture is
taken into the air in these regions. Note that the word
polar, when applied to air mass designations, does not
mean air at the poles (this area is covered by the words
arctic and antarctic). Polar air is generally found in
latitudes between 40 and 60 degrees and is generally
warmer than arctic air. The air over northern and central
Asia are exceptions to this.
MARITIME POLAR (mP) AIR.The maritime
polar source regions consist of the open unfrozen polar
sea areas in the vicinity of 60° latitude, north and south.
Such areas are sources of moisture for polar air masses;
consequently, air masses forming over these regions are
moist, but the moisture is sharply limited by the cold
CONTINENTAL TROPICAL (cT) AIR.The
significant land areas lying in the tropical regions;
generally these tropical regions are located between
latitudes 25°N and 25°S. The large land areas located in
these latitudes are usually desert regions (such as the
Sahara or Kalahari Deserts of Africa, the Arabian
Desert, and the interior of Australia). The air over these
land areas is hot and dry.
maritime tropical source regions are the large zones of
open tropical sea along the belt of the subtropical
anticyclones. High-pressure cells stagnate in these
areas most of the year. The air is warm because of the
low latitude and can hold considerable moisture.
EQUATORIAL (E) AIR.The equatorial source
region is the area from about latitudes 10°N to 10°S. It
is essentially an oceanic belt that is extremely warm
and that has a high moisture content. Convergence of
the trade winds from both hemispheres and the intense
insolation over this region causes lifting of the unstable,
moist air to high levels. The weather associated with
these conditions is characterized by thunderstorms
throughout the year.
SUPERIOR (S) AIR.Superior air is a high-level
air mass found over the south central United States.
This air mass occasionally reaches the surface; because
of subsidence effects, it is the warmest air mass on
record in the North American continent in both seasons.
Southern Hemisphere Air Masses
Hemisphere differ little from their counterparts in the
Northern Hemisphere. Since the greater portion of the
Southern Hemisphere is oceanic, it is not surprising to
The two largest continents of the Southern
Hemisphere (Africa and South America) both taper
from the equatorial regions toward the South Pole and
have small land areas at high latitudes. Maritime polar
air is the coldest air mass observed over the middle
latitudes of the Southern Hemisphere.
In the interior of Africa, South America, and
Australia, cT air occurs during the summer. Over the
predominating air masses are mP, mT, and E air. The
structure of these air masses is almost identical with
those found in the Northern Hemisphere.