Geoscience Reference
In-Depth Information
Jet streams
KEY CONCEPTS
Within both westerlies and the tropical easterlies, bands of especially strong winds can be found. The existence of
these winds or jet streams was appreciated only with the increased use of aircraft during the Second World War.
Bombers heading across the Pacific towards Japan reported headwinds so strong that they could hardly advance
relative to the ground! More recent investigations have shown that speeds up to 135 m s
-1
(490 km hr
-1
) can exist
locally in a jetstream maximum. A number of major jets have been found in the troposphere - the polar front jet, the
subtropical jet and the tropical easterly jet - and others exist in the stratosphere (
Figure 6.20
).
What is a jet? Basically it is a very narrow current of air travelling at great speed. Jet streams may flow for thousands
of kilometres, but can be only a few hundred kilometres wide and a couple of kilometres deep. Jets are formed in
regions of rapid temperature gradient. Typically the westerly jets are connected with the zone of maximum slope or
fragmentation of the tropopause, which coincides with the maximum poleward temperature gradient. They can lead
to intense accelerations (and decelerations) of air in their vicinity. As we shall see later, when air is forced to change
its rate of flow, tropospheric vertical motion may be started. In turn this may influence events at lower levels. The
tropical easterly jet forms in summer near the tropopause from south-east Asia, India and into Africa. Its origins are
probably related to the warmer atmosphere to the north derived from heating over the Tibetan plateau and over the
Sahara.
Aircraft navigation makes good use of the westerly jets by avoiding them when flying east to west and flying within
them in the reverse direction. Over the North Atlantic the journey from New York to Europe is about an hour less
than from east to west. See additional case study 'Jet streams and the summer of 2007 in the UK' on the support
website at
www.routledge.com/textbooks/9780415395168.
km
hPa
Figure 6.20
Jet streams (J) in the upper
atmosphere in relation to the
vertical temperature gradient.
50
20
-80
100
15
-
200
10
-
P
OLAR
TRO
POPAUS
E
300
5
500
700
20
1000
0
90°
80°
70°
60°
50°
40°
30°
20°
10°
0°
Latitude
monthly chart, where the mean flow has a northward
component, and other areas occur where it is southwards.
Effectively the air is flowing in a series of waves around
the pole, carrying warmer air northwards on parts of its
track and cold air southwards elsewhere. These are the
Rossby waves. In January the most prominent features of
the waves are the pronounced troughs in the pressure
surface near 80
E, with a weaker trough
between 10
E and 60
E. In July the circulation is less
intense and the troughs are less well marked. Around the
W and 140
