Maritime Tropical (mT) Air in Winter
Maritime tropical air that arrives over Europe
usually originates over the southern portion of the
North Atlantic under the influence of the Azores
anticyclone. Maritime tropical air is marked by
pronounced stability in the lower layers and typical
warm-mass cloud and weather conditions. Relatively
high temperatures accompany the influx of mT air, and
the moisture content is greater than in any other air
mass observed in the middle latitudes of Europe.
Visibility is, as a rule, reduced because of the presence
of fog and drizzle, which are frequently observed with
an influx of mT air. Maritime tropical air in winter
exists only in western Europe. By the time it reaches
Russia, it is generally found aloft and greatly modified.
Maritime Tropical (mT) Air in Summer
In general, mT air has the same properties as its
counterpart in winter with the exception that it is less
stable over land because of surface heating.
Additionally, this air mass loses its maritime
characteristics soon after passing inland.
Over water, mT air is still a typical warm air mass.
Sea fog frequently occurs in the approaches to the
English Channel during the spring and early summer.
Visibility in mT air is generally better in summer than
in winter, particularly over land where convection
currents usually develop.
Mediterranean in summer usually changes to a cold
mass, since the water temperature of the Mediterranean
is then slightly higher than that of the air. Weak
convection currents prevail, usually sufficiently strong
to form cumulus clouds but seldom sufficiently strong
to produce showers.
Continental Tropical (cT) Air in Winter
The continental tropical air that arrives over Europe
in winter originates over North Africa. By the time it
reaches central Europe, it differs little from mT air. In
general, a cT air mass is much more prevalent over
southern Europe than over central or western Europe.
Although the moisture content of cT air is less than that
observed in mT air, the visibility is not much better,
primarily because of the dust that cT air picks up while
over Africa. This air mass constitutes the major source
of heat for the development of the Mediterranean
cyclonic storms, most common during the winter and
Continental Tropical (cT) Air in Summer
The cT air usually develops over North Africa, Asia
Minor, and the southern Balkans. At its source region,
the air is dry and warm as well as unstable. The North
African air mass is the hottest air mass on record in the
world. In its northward flow over southern Europe, cT
air absorbs moisture and increases its convective
instability. The summer showers and thunderstorms
observed over southern Europe are often produced by a
modified cT air mass. This air mass is much more
prevalent over southern Europe than is its winter
AIR MASSES IN THE SOUTHERN
The air masses of the Southern Hemisphere are
predominantly maritime. This is because of the
overwhelming preponderance of ocean areas. Great
meridional (south-north and north-south) transports of
air masses, as they are known in the Northern
Hemisphere, are absent because the westerlies are
much more developed in the Southern Hemisphere than
in the Northern Hemisphere. Except for Antarctica,
there are no large landmasses in the high latitudes in the
Southern Hemisphere; this prevents sizable invasions
of antarctic air masses. The large landmasses near the
equator, on the other hand, permit the extensive
development of warm air masses.
The maritime tropical air masses of the Southern
Hemisphere are quite similar to their counterparts of
the Northern Hemisphere. In the large area of Brazil,
there are two air masses for consideration. One is the
regular air mass from the Atlantic, which is composed
of unmodified mT air. The other originates in the
Atlantic; but by the time it spreads over the huge
Amazon River basin, it undergoes two important
changesthe addition of heat and moisture. As a result
of strong summer heating, a warm, dry continental
tropical (cT) air mass is located from 30° south to 40°
The maritime polar air that invades South America
is quite similar to its counterpart in the United States.
Maritime polar air occupies by far the most territory in
the Southern Hemisphere, encircling it entirely.
Australia is a source region for continental tropical
air. It originates over the vast desert area in the interior.
Except along the eastern coast, maritime tropical air
does not invade Australia to a marked degree. This air is
brought down from the north, particularly in the