AIR MASSES AND FRONTS

coincident with the greatest temperature contrasts. Two
of the most important frontal zones are those over the
north Pacific and north Atlantic Oceans. In winter, the
arctic front, a boundary between polar and arctic air,
forms in high latitudes over northwest North America,
the north Pacific, and near the Arctic Circle north of
Europe (fig. 4-25). In summer, the arctic front mainly
disappears, except north of Europe. (See fig. 4-26.)
The polar front, on the other hand, is present the
year round, although it is not as intense in the summer
as in the winter because of a lessening temperature
contrast between the opposing air masses. The polar
front forms wherever the wind flow and temperature
contrast is favorable. Usually this is the boundary
between tropical and polar air, but it may form between
maritime polar and continental polar air. It also may
exist between modified polar air and a fresh outbreak of
polar air. The polar front is common over North
America in the continental regions in winter in the
vicinity of 50°N latitude.
The polar front in winter is found most frequently
off the eastern coasts of continents in areas of 30° to 60°
75
60
45
30
15
0
15
30
45
60
LOW
1010
MP
1020
HIGH
MT
cA
SOURCE
(ITCZ) ARCTIC FRONT
MT
CP
1020
HIGH
POLAR FRONT
1020
HIGH
1000 MP
1020
HIGH
MT
POLAR FRONT (ATLANTIC)
L
(I C
cA MA
SOURCE
LOW ARCTIC FRONT
T Z)
MT
1020
HIGH
SO. PACIFIC POLAR FRONT SO. ATLANTIC POLAR FRONT
4-26
latitude. It is also found over land; but since the
temperature contrasts are greater between the continent
and the oceans, especially in winter, the coastal areas
are more favorable for formation and intensification of
the polar front.
The intertropical convergence zone (ITCZ), though
not truly a front but a field of convergence between the
opposing trades, forms a third semipermanent frontal
type. This region shows a seasonal variation just as do
the trade winds.
FRONTAL CHARACTERISTICS
LEARNING OBJECTIVE: Describe the
frontal elements and general characteristics of
fronts.
FRONTAL ELEMENTS
From our previous discussion and definitions of
fronts, it was implied that a certain geometrical and
meteorological consistency must exist between fronts
180 150 120 90 60 30 0 30 60 90 120 150 180
AG5f0425
POLAR FRONT (PACIFIC)
MP CP
MED POLAR FRONT
1020
H CT
SOURCE
CT
LOW
MT
POLAR FRONT
CT
(ITCZ)
POLAR FRONT (SO.
1020
HIGH
CA
SOURCE
ASIA ARCTIC FRONT
1032
CP
SOURCE
INDIAN OCEAN)
CT
POLAR FRONT
Figure 4-25.—Chart showing world air masses, fronts and centers of major pressure systems in January.
LOW
CA
LOW
JANUARY
75
60
45
30
15
0
15
30
45
60