Generally the influx of mT air is carried aloft by a
rapidly occluding frontal system somewhere over
southern
California,
producing
the
heaviest
precipitation recorded in that area. Occasionally mT air
is seen above the surface with pronounced storm
developments over the Great Basin. Since large, open,
warm sectors of mT air do not occur along the west
coast, representative air mass weather is not
experienced. Flying conditions are generally restricted
when this air is present, mainly because of low frontal
clouds and reduced visibility in precipitation areas.
Maritime Polar (mP) Air Atlantic in Winter
Maritime polar air, which originates in the Atlantic,
becomes significant at times along the east coast. It is
not nearly so frequent over North America as the other
types because of the normal west-east movement of all
air masses. This type of air is observed over the east
coast in the lower layers of the atmosphere whenever a
cP anticyclone moves slowly off the coast of the
maritime provinces and New England. (See fig. 4-10.)
This air, originally cP, undergoes less heating than its
Pacific counterpart because the water temperatures are
colder and also because it spends less time over the
water. This results in the instability being confined to
the lower layers of this air. The intermediate layers of
this air are very stable. Showers are generally absent;
however, light drizzle or snow and low visibility are
common. Ceilings are generally about 700 to 1,500 feet
with tops of the clouds near 3,000 feet. Marked
subsidence above the inversion ensures that clouds
caused by convection will not exist above that level.
The synoptic weather condition favorable to mP air
over the east coast is usually also ideal for the rapid
development of a warm front with maritime tropical air
to the south. Maritime tropical air then overruns the mP
air and a thick cloud deck forms. Clouds extending
from near the surface to at least 15,000 feet are
observed. Ceilings are near zero and severe icing
conditions exist in the cold air mass. Frequently,
freezing rain and sleet are observed on the ground.
Towering cumulus clouds prevail in the warm air and
often produce thunderstorms.
Flying conditions are rather dangerous with mP air
because of turbulence and icing conditions present near
the surface. Poor visibility and low ceilings are
additional hazards. The cloudiness associated with the
mP air mass usually extends as far west as the
Appalachians.
Maritime Tropical (mT) Air Atlantic in Winter
Temperature and moisture content are higher in mT
air masses than in any other American air mass in
winter. In the southern states, along the Atlantic coast
4-11
AG5f0409
mP
Figure 4-9.—Trajectory of mT air over the Pacific in winter.
AG5f0410
cP
mP
NOTE: The representation of international
boundaries on this chart is not necessarily
authoritative.
Figure 4-10.—Trajectory of mP air over the Atlantic in winter.
