Geoscience Reference
In-Depth Information
Upper Airflow and the 500-mb
Pressure Surface
An important example of a pressure surface within the atmo-
sphere is the 500-mb surface, which occurs at a specific altitude
at any given place on Earth. This altitude typically varies from
one place to another. Recall that the average surface air pressure
at sea level is 1013.2 mb and that air pressure decreases with
altitude. Thus, the 500-mb pressure surface is at a relatively
high altitude compared to the surface air pressure. From a me-
teorologic standpoint, the 500-mb pressure surface is important
because it vertically divides the atmosphere in two, if we as-
sume that the average surface air pressure is 1013 mb and 0 mb
of air pressure occurs at the top of the atmosphere. In addition,
the wind patterns at the 500-mb surface exert a strong steering
influence on the circulatory patterns at the surface.
To understand the significance of the 500-mb pressure
surface and how it is important to airflow, consider Figure 8.5.
Figure 8.5a shows two columns of air (Air columns 1 and 2)
that contain the same mass of atmosphere. Given that each col-
umn contains the same number of molecules, the surface pres-
sure is the same. In addition, the altitude of the 500-mb surface
in both columns is equal, in this case 5460 m (17,900 ft).
What happens if we warm one of the columns of air, say
Air column 2, relative to the other (Figure 8.5b)? When this
warming occurs, the column of air stretches vertically—
because warm air rises—and the height of the 500-mb surface
in that column moves to a higher altitude; let's say 5760 m
(18,900 ft). At that same altitude in Air column 1, the air pres-
sure is 480 mb. Air column 1 has this relatively lower pressure
at 5760 m (18,900 ft) because the elevation of the 500-mb sur-
face in that column did not change from the previous example
(Figure 8.5a). This pressure/altitude difference between the two
air columns reflects the fact that the warmer column of air now
has a lower overall density than the first (cooler) one. No change
occurs in the mass of Air column 2, which is why the surface air
pressure is still 1000 mb. Another way of comparing the two air
masses in the second example is to say that Air column 2 has a
shallower vertical pressure gradient than Air column 1.
The same kind of altitude change in the 500-mb surface
also occurs when air pressure at the surface varies between
two regions. Using Figure 8.5a again as a hypothetical baseline
scenario, let's say that the surface air pressure in Air col-
umn 1 decreases to 990 mb at the same time it increases to
1005 mb in Air column 2 (Figure 8.5c). What happens? No-
tice that the altitude of the 500-mb surface drops to, let's say,
5350 m (17,500 ft) in Air column 1, whereas it rises to 5600 m
(18,400 ft) in Air column 2. This change occurs because the
mass of air beneath the 500-mb surface decreases in Air column
1 at the same time it increases in Air column 2.
This example shows how pressure can vary within two
distinct columns of air. Now examine how the altitude of
the 500-mb surface might vary horizontally in the continu-
ous medium of the actual atmosphere. Figure 8.6a presents a
geography was examined in Chapter 6, so you might want to
review Figure 6.21.
Let's begin this review in the summer when high latitudes
receive high amounts of solar radiation. During this time of
year, the polar front and jet stream are confined to the poles
and exhibit a zonal-flow pattern much like that illustrated in
Figure 6.21a. This confinement of the polar front to these high
latitudes occurs, of course, because temperatures are warm well
into the higher middle latitudes. Although midlatitude cyclones
may develop along the polar front during this time of year, they
are generally weak systems because no strong temperature con-
trast occurs north and south of the polar front.
As fall approaches and daily insolation at high latitudes
decreases, large masses of cP air begin to develop north of
the polar front. This increase of cold, dry air causes the polar
front and associated jet stream to move south where it begins
to encroach upon the warmer air. As this migration proceeds,
the path of the jet stream gradually changes from a zonal
pattern to an undulating meridional pattern with well-devel-
oped Rossby waves in the upper atmosphere (see Figure 6.21
again). This development occurs because the atmosphere is
reacting to the growing temperature difference on either side
of the polar front. These waves allow tongues of warm air
(south of the front) to penetrate northward at the same time
that wedges of cold air (north of the front) expand to the
south.
On a global scale, the development of Rossby waves is how
the atmosphere begins the process of mixing cold and warm air.
The most vigorous mixing of these air masses occurs on a more
regional scale, within midlatitude cyclones that form along dis-
tinct sections of the polar front in any given Rossby wave. The
term
cyclone
is sometimes confusing because tropical storms
near southern Asia are referred to by the same name. In addi-
tion, tornadoes are sometimes called
cyclones
. Although these
specific kinds of storms are investigated later in this chapter, for
now the term
cyclone
is used only in the context of midlatitude
circulatory processes.
Formation of midlatitude cyclones is called
cyclogenesis
and encompasses a complex set of processes most often asso-
ciated with an undulating polar jet stream pattern in the up-
per atmosphere and the interaction of these winds with airflow
at ground level. To better understand this interaction, consider
how the barometric pressure within the atmosphere changes
from the surface to the top. In this context, it can be viewed
as containing many different pressure surfaces, with the high-
est air pressure exerted at ground level and progressively lower
air pressure at progressively higher altitudes. Just as a particu-
lar isobar (a line of equal air pressure) can be followed on the
ground, it can also be traced horizontally within different levels
of the atmosphere.
Cyclogenesis
The sequence of atmospheric events along the
polar jet stream that produces midlatitude cyclones.
