Geoscience Reference
In-Depth Information
scale
features. The mid-latitude depression is
usually associated with the convergence of con-
trasting air masses. According to the 'Norwegian
cyclone model' (see
Figure 9.7
), the interface
between these air masses develops into a wave
form with its apex located at the center of the low
pressure area. The wave encloses a mass of warm
air between modified cold air in front and fresh,
cold air in the rear. The formation of the wave also
creates a distinction between the two sections of
the original air-mass discontinuity since, although
each section still marks the boundary between
cold and warm air, the weather characteristics
found within each section are very different. The
two sections of the frontal surface are distin-
guished by the names
warm front
for the leading
edge of the wave and the warm air and
cold front
for that of the cold air to the rear (see
Figure 9.7B
).
The boundary between two adjacent air masses
is marked by a strongly baroclinic zone of large
temperature gradient, 100-200km wide (see C,
this chapter, and
Figure 9.1
). Sharp discontinuities
W
W
W
(A)
Early stage
(B)
Open stage
C
o
o
l
W
W
(C)
Occluded stage
(D)
Dissolving stage
Figure 9.7
Four stages in the typical develop-
ment of a mid-latitude depression. Satellite views
of the cloud systems corresponding to these
stages are shown in Figure 9.8.
Source: Mostly after Strahler (1965), modified after
Beckinsale.
Notes: C = cold air; W = warm air.
(A)
(B)
Cloud covered
Mostly open
L
L
(C)
(D)
(E)
L
L
L
Figure 9.8
Schematic patterns of cloud cover (white) observed from satellites, in relation to surface
fronts and generalized isobars. A, B, C and D correspond to the four stages in Figure 9.7.
Source: After Boucher and Newcomb (1962). Courtesy of the American Meteorological Society.
