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(a)
High
pressure
(b)
Cool air
Cool air
Low
pressure
Warm air
Warm air
High
pressure
(c)
(d)
Low
pressure
Cool air
Low
pressure
A
A ´
C
C ´
Cool air
B ´
B
Cool air
Warm air
Warm air
High
pressure
High
pressure
Figure 10.8 Evolution of a typical northern hemisphere cyclone as illustrated by surface weather maps, with the wind aloft
shown parallel to isobars of constant pressure; (a) stationary front with wind shear across the interface; (b) and (c) a frontal
wave with growing amplitude; (d) warm front being overtaken by the cold front to create an occluded front.
Warm fronts tend to move more slowly than cold fronts and they are usually less
well-defined because the interface is shallower. As the warm air moves over the
cold air, a broad band of cloud develops which may extend several hundred
kilometers ahead of the front and likely gives rise to precipitation. If the warm air
is moist and stable the precipitation increases gradually as the front approaches,
but if it is moist and unstable taller cloud will likely occur and often heavy storms
will result as well.
Because the interface between air masses tends to be unstable, it frequently
evolves into a spiraling stream as a result of the rotation of the Earth. The resulting
weather system is called a cyclone . Cyclones vary greatly but Figs 10.8 and 10.9
show a typical life cycle.
In the initial stages the winds on either side of the stationary front are blowing
in opposite directions (Fig. 10.8a). As a result of small disturbances in the shear or
other irregularities in surface roughness or surface heating, the front may gradu-
ally assume a wave-like shape (Fig. 10.8b), which may persist and increases in
amplitude. In due course, a frontal wave evolves with well-defined cold and warm
fronts that in the northern hemisphere has a counterclockwise flow pattern
(Fig. 10.8c). Because the cold front portion usually moves quicker than the warm
front portion, the cold front eventually overtakes the warm front to become an
occluded front (Fig. 10.8d), at which time the cyclone achieves maximum inten-
sity. Later the occlusion gradually disappears and a new cyclone may be formed.
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