Geoscience Reference
In-Depth Information
5760 m
(18,900 ft)
480 mb
500 mb
50
0
mb
50
0
m
b
50
0
m
b
52
0
mb
5460 m
(17,900 ft)
5460 m
(17,900 ft)
1000 mb
1000 mb
1000 mb
1000 mb
0 m (ground)
0 m (ground)
Air column 1
Air column 2
Air column 1
Air column 2
(warmed)
(a)
(b)
5600 m
(18,400 ft)
500 mb
500 mb
5350 m
(17,500 ft)
Figure 8.5 Vertical changes in air pressure.
(a) The height of the 500-mb
pressure surface in two columns of air with the same mass and temperature.
(b) If one column of air (column 2) is warmed relative to the other, then the
atmosphere stretches vertically and the height of the 500-mb surface rises.
(c) The height of the 500-mb surface also fluctuates when the surface air
pressure changes.
990 mb
1005 mb
0 m (ground)
Air column 1
Air column 2
(c)
typical isoline map that shows the variation in altitude of the
500-mb surface across the United States on a particular day.
This reflects the fact that surface air pressure varies across
the nation in a manner similar to Figure 8.5c. The map shows
a strong high-pressure system (anticyclone) in the western
United States and two areas of low pressure—one over the
northeastern part of the country and another off the Pacific
Northwest coast. You can see how this information trans-
lates into an actual topographic map of the 500-mb surface
in Figure 8.6b. Notice that the height of the pressure surface
is higher and forms a ridge-like feature where the anticyclone
occurs. Meteorologists call such an atmospheric feature a
Figure 8.6 Mapping the 500-mb pressure surface.
(a) Isolines of constant
pressure across the United States on a particular day. (b) A topographic map
showing the variation in the 500-mb surface.
HIGH
LOW
HIGH
5760 m
(18,900 ft)
5760 m
(18,900 ft)
LOW
5460 m
(17,900 ft)
Motion of
system
H
H
0
800 mi
L
L
5460 m
(17,900 ft)
0
1200 km
(a)
(b)
