Planetary-scale motions in the atmosphere and ocean - Atmosphere, Weather and Climate - page 156

Figure 7.6 Schematic map of super-
imposed contours of isobaric height
and thickness of the 1000 to 500-mb
layer (in metres). G 1000 is the geostrophic
velocity at 1000 mb, G 500 that at 500 mb;
V T is the resultant 'thermal wind' blowing
parallel to the thickness lines.

Contours of 1000-mb surface

0 60 120 180

5640

Contours
of
500-mb
surface

5700

V T

G1000

G500

5760

5700 5640 5580 5520

1000-500-mb thickness

Figure 7.7 Structure of the mid-latitude
frontal zone and associated jet stream
showing generalized distribution of tem-
perature, pressure and wind velocity.
Source : After Riley and Spolton (1981).

a large westerly component in the upper winds.
Furthermore, the zonal westerlies are strongest when
the meridional temperature gradient is at a maximum
(winter in the northern hemisphere).
The total result of the above influences is that in
both hemispheres the mean upper geostrophic winds
are dominantly westerly between the subtropical high-
pressure cells (centred aloft at about 15° latitude) and
the polar low-pressure centre aloft. Between the sub-
tropical high-pressure cells and the equator the winds
are easterly. The dominant westerly circulation reaches
maximum speeds of 45 to 65 m s -1 , which even increase
to 135 m s -1 in winter. These maximum speeds are
concentrated in a narrow band, often situated at about
30° latitude between 9000 and 15000 m, called the jet
stream (see Note 2 and Box 7.2). Plate 14 shows bands

of cirrus cloud that may have been related to jet-stream
systems.
The jet stream is essentially a fast-moving ribbon of
air, connected with the zone of maximum slope, folding
or fragmentation of the tropopause; this in turn coincides
with the latitude of maximum poleward temperature
gradient, or frontal zone, shown schematically in Figure
7.7. The thermal wind, as described above, is a major
component of the jet stream, but the basic reason for the
concentration of the meridional temperature gradient
in a narrow zone (or zones) is dynamical. In essence,
the temperature gradient becomes accentuated when the
upper wind pattern is confluent (see Chapter 6B.1).
Figure 7.8 shows a north-south cross-section with
three westerly jet streams in the northern hemisphere.
The more northerly ones, termed the polar front and