Geoscience Reference
In-Depth Information
Tropopause
Stratosphere
Polar
jet stream
Subtropical
jet stream
Ferrel
cell
Polar
cell
Subtropical
high (STH)
Hadley
cell
Westerlies
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30
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H
Equatorial
trough
(ITCZ)
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°
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0
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Figure 6.19 The polar front in the Northern Hemisphere.
The polar front is the boundary between warm
air (to the south) and cold polar air (to the north). Note how air from the STH flows toward the polar front.
results in a very steep pressure gradient as well. Given the sea-
sonal effect created by the Earth-Sun geometric relationship, this
temperature/pressure gradient is very weak during the summer
months, but strengthens with the progression of winter. The re-
sulting river of rapidly moving air is known as the
polar front jet
stream
and occurs at altitudes of 10 km to 12 km (about 30,000 ft
to 40,000 ft; see Figures 6.19 and 6.20). Like the ITCZ, the polar
jet stream is often easy to see on satellite imagery because a band
of clouds forms along this line (as in Figure 6.20b).
Although the term
westerly winds
implies that midlatitude
winds are flowing straight from west to east, this is not always
the case. In fact, the smooth westward flow in the upper-air
circulatory system frequently develops distinct undulations,
called
Rossby waves
. Rossby waves along the polar front
are the mechanism through which significant temperature dif-
ferences on either side of the front are moderated, especially
during the winter months. Figure 6.21 shows how the process
develops.
Depth: 1-2 km
(3000-6000 ft)
Width: 160-500 km
(100-300 mi)
Speed: 320-480 km/h
(200-300 mi/h)
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Altitude: 10-12 km
(30,000-40,000 ft)
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(a)
(b)
Figure 6.20 Jet streams.
(a) Jet streams consist of rivers of air that flow at very high speeds. (b) Several jet streams flow in the upper
atmosphere and can frequently be identified by a band of clouds, as in this image of the Nile/Red Sea area in the Middle East.
Polar front jet stream
River of high-speed air in the upper
atmosphere that flows along the polar front.
Rossby waves
Undulations that develop in the polar front jet
stream when significant temperature differences exist between
tropical and polar air masses.
