Arctic air (A) is the coldest air of the Northern Hemisphere. It is very cold and
stable in the winter and not associated with significant precipitation. Because of
the land/sea configuration around the Arctic Ocean, Arctic air originates in
northern Canada, Alaska, and Siberia in the winter.
Antarctic air (AA) is the coldest air mass on Earth. In fact, it is much colder
than the Arctic air of the Northern Hemisphere. It is generated on the Antarctic
continent, which is covered by ice sheets having surfaces frequently in excess of
two kilometers above sea level. Besides being cold, the air is very stable so that
organized low pressure systems and their consequent precipitation are rare. Most
middle latitudelocationshave considerably greater snowfall totals than Antarctica
because of the inability of Antarctic air masses to hold much moisture.
Equatorial air (E) is an extremely humid air mass found in the lowest latitudes.
Maritime and continental varieties are not usually designated because both types
of sources produce similar temperatures and humidities. The temperatures tend
not to be as hot as continental Tropical and maritime Tropical air masses because
clouds and high humidities inhibit surface heating. Equatorial air masses are
present year round near the Equator with incursions of other air masses being
extremely rare. Equatorial air becomes unstable by the middle of the afternoon
and produces daily air mass thunderstorms for which the deep tropics are so well
known.
The classification scheme can also include a third letter, a lower case k or w. A k
in the air mass designation indicates the air mass is colder than the surface over
which it passes while a w denotes the air mass is warmer than the surface. Exam-
ples of this would be cPk air as it passes over North America in the summer. The
continental surface is warm, the air is cool, and this sets up instability in the lower
atmosphere. Conversely, a winter mPw air mass can be warmer than snow-covered
North America and tend to be stable.
Climatologically, air masses have preferred areas of occurrence that enlarge,
contract, and shift latitudes with seasons. Seasonally, the zones of dominance
shift. For instance, the maritime Tropical source region strengthens, enlarges,
and edges into the lower middle latitudes during the summer season. Also, there
are geographic variations associated with the positioning of continents.
Continental Tropical air vanishes from North America in winter because of the
small size of the continental surface at those latitudes. In North Africa this air
mass persists year round.
Some locations are associated with monotony of weather caused by the firm
entrenchment of air masses. If one alights in Bele ́m, Brazil in the heart of tropical
Amazonia, she/he is struck with the sameness of weather day after day as dictated
by the presence of Equatorial air. In the middle latitudes in particular, there are
places that are neither air mass origin zones nor dominated by one air mass10 Air Masses