Geoscience Reference
In-Depth Information
Low-Pressure Systems
Low-pressure systems are often referred to as
cyclones
. If you
observe a low-pressure system from the side (Figure 6.5), notice
that the vertical flow of air consists of rising air, with the most
vigorous upward flow in the center of the system. As a result,
the central part of the system has the lowest pressure and is des-
ignated with the letter “L” on a weather map. In the Northern
Hemisphere, the horizontal low of air around the center of a low
is counterclockwise (looking at it from above) as the air flows
toward the core of the system. In the Southern Hemisphere, the
horizontal flow of air is clockwise. This inward flow occurs be-
cause the rising air at the center of the system creates a void into
which air must flow. Low-pressure centers are generally asso-
ciated with cloudy or stormy weather because rising air cools
with altitude and thus squeezes water out of the atmosphere. This
relationship will be discussed in much greater detail in Chapter 7.
36,000
118,098
34,000
111,537
Research balloons,
32,000 m
32,000
104,976
30,000
98,415
28,000
91,854
Military spy planes,
30,000 m
26,000
85,293
Stratosphere
24,000
78,732
22,000
72,171
20,000
65,610
18,000
59,049
Commercial airliner,
12-15,000 m
16,000
52,488
14,000
45,927
Tr
opopause
12,000
39,366
10,000
32,805
Mt. Everest, 8,850 m
Cirrus
clouds,
6-12,000 m
8,000
26,244
6,000
19,683
High-Pressure Systems
In contrast to low-pressure systems, large areas of relatively high
air pressure are referred to as
anticyclones
. In these systems,
the vertical flow of air consists of descending air, which diverges
(spreads apart) at the surface (Figure 6.5). Because air sinks most
vigorously in the center of a high, the highest pressure is in the
central part of the system, which is designated with the letter “H.”
The horizontal airflow around the center of a high is clockwise in
the Northern Hemisphere and counterclockwise in the Southern
Hemisphere, just opposite to the direction of low around a low-
pressure system. High-pressure centers are generally associated
with fair weather because descending air warms as it approaches
the surface. As mentioned earlier, warm air can hold more mois-
ture than cold air, and this makes precipitation less likely. Again,
more will be said about this relationship in Chapter 7.
In general, high- and low-pressure systems (anticyclones
and cyclones, respectively) create large-scale circulatory sys-
tems that are interconnected by airflow. Look again at Figure 6.5
and see how the air flows horizontally, in a process called
advection
, from the high-pressure system to the low-pressure
system at the surface. You can see clearly the geographic pat-
tern of low- and high-pressure systems on a barometric pressure
map. Figure 6.6 illustrates pressure variations in the upper part
of the atmosphere across the North Atlantic. The (gray) lines on
the map are the isobars. The red arrows illustrate the way that
air is moving relative to the pressure systems.
Notice in Figure 6.6 that the isobars in the eastern Atlantic
form a rough oval or egg-shaped area. This region is the center of
a low-pressure system (or the area of lowest air pressure) at this
particular point in time. From the wind direction arrows, observe
4,000
13,122
Tr
oposphere
2,000
0
6,561
Denver
0
0
200
600
Pressure (millibars)
400
800
1000
Figure 6.4 Atmospheric pressure and altitude.
Average
atmospheric pressure decreases with increasing elevation and
altitude above the Earth's surface.
of the air is 1013.25 mb. In contrast, at Denver, Colorado
(5280 ft—“the Mile High City”), the air pressure is 840 mb.
At a still higher elevation, such as at the top of Mount Everest,
the Earth's tallest mountain at 8850 m (29,035 ft), the average
air pressure is only 320 mb.
Atmospheric Pressure Systems
In addition to the pressure changes that occur with respect to al-
titude, air pressure also varies horizontally across the Earth's sur-
face, with distinct zone of high air pressure and low air pressure.
A
high-pressure system
is a circulating body of air that exerts
relatively high pressure on the surface of Earth because air de-
scends (toward the surface) in the center of the system. In contrast,
a
low-pressure system
is a circulating body of air where rela-
tively less pressure exists on the Earth's surface because the air is
rising (away from the surface) in the system's core. This portion
of the chapter focuses on the nature of these large-scale pressure
systems and describes how air flows within and between them.
High-pressure system
A rotating column of air that descends
toward the surface of Earth where it diverges.
Low-pressure systems.
Cyclones
High-pressure systems.
Anticyclones
Low-pressure system
A rotating column of air where air
converges at the surface and subsequently lifts.
The horizontal transfer of air.
Advection
