Geoscience Reference
In-Depth Information
When the cyclone is in the open stage, it is fully mature.
This stage does not last very long, however, because the cold
front moves faster than the warm front ahead of it. These con-
trasting frontal speeds occur because the air behind the cold
front is denser than the warm air ahead of it and thus flows
more quickly. In this fashion, the cold front drives into the
warm, moist air ahead of it, whereas the warm front gradually
slides over the top of the stationary cold air ahead of it (Figure
8.9d). As a result of this difference in speeds, the cold front
begins to overtake the warm front in the
occluded stage
of de-
velopment (Figure 8.9e). Although a distinct warm sector still
exists in the southerly parts of the system, the overtaking cold
air lifts warm air aloft near the center of the low. An
occluded
front
develops when the cold front overtakes the warm front
and begins to drive the warm air at the surface to a higher alti-
tude. This process of occlusion continues as the system enters
the
dissolving stage
(Figure 8.9f), at which time all the warm
air is pinched aloft and a stationary front is reestablished.
Remember that the evolution of midlatitude cyclones
occurs within the context of the overall westerly wind pattern
in the jet stream. You can think of the jet stream as the medium
that steers cyclones across the continent. Figure 8.11 shows an
example of this migration, beginning with an open-stage cy-
clone centered in the central United States (Figure 8.11a). This
view of the system shows the position of the warm and cold
fronts and their relationship to the air mass source regions. In
the open stage of development, mT air in the warm sector is
clearly flowing north from the Gulf of Mexico into Louisiana
and Arkansas. If you were there on that day, the weather would
be warm and muggy, with southerly winds. To the north and
west of the cold front, in contrast, the air is flowing in a south-
erly direction from Canada into places like the panhandle of
Texas and Oklahoma. In these places, the temperature would
probably be cold, with clear sky and northwest winds.
cP Air
Cold Front
mT Air
Figure 8.10 A classic midlatitude cyclone.
A cyclone cen-
tered over the midwestern United States. Notice the prominent
line of clouds, which developed along a strong cold front, extend-
ing from Illinois to eastern Texas. To the east of the cold front the
air was warm and muggy, whereas west of the cold front it was
relatively cold and dry, as indicated by the clear skies in western
Kansas and eastern Colorado.
The developing cyclone is said to be in the
open stage
when the warm and cold fronts are fully separated; that is, when
warm air exists at the surface all the way to the system's core
(Figure 8.9c). This geographical area of warm air is referred to
as the
warm sector
. Imagine that such a system develops over
the northern Great Plains in the United States. In this scenario,
the air behind the cold front (i.e., to the north) would consist of
cold, dry (cP) air that originated in Canada, while the air in the
warm sector that lies between the cold and warm fronts is rela-
tively warm and moist (mT) air that may have come from the
Gulf of Mexico. At this stage the cyclone may very well look
like the one pictured in Figure 8.10.
Occluded front
The area where a cold front begins to over-
take a warm front and thus lift warm surface air aloft.
Visual ConCept CheCk
8.1
This image shows temperature in the United
States. Where is the approximate location of
the cold front?
45
a)
Western Texas.
50
55
b)
Florida.
60
c)
It cuts across the states of Washington
and Oregon.
65
70
75
d)
It extends from western Michigan to
central Texas.
80
85
