Environmental Engineering Reference
In-Depth Information
Many experiments have been conducted to determine the reasons for this pattern.
Clearly surface features play an important part, even at this height. The presence of the
Rocky Mountains and the Himalayas is believed to 'lock' the troughs at 80° W and 140°
E respectively. The distribution of land and sea is also thought to be of importance. In the
southern hemisphere there are no mountain ranges of comparable size, nor such marked
land and sea temperature contrasts. As a result the mean circulation is much more
symmetrical around the South Pole.
Even on a shorter time period, waves may exist in the upper westerlies, though their
shape is less regular. The smaller waves tend to be associated with an individual
depression and move more rapidly, perhaps up to 15° longitude per day. The longer
waves - usually between four and six are apparent - move more slowly and can even
retrogress against the westerly flow. They are linked with the major circulation features
such as the subtropical highs and Icelandic lows. The long-wave flow tends to 'steer' the
shorter waves, moving them northwards when ahead of a trough and southwards when to
the rear of a trough.
JET STREAMS
human impact
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 1).
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
Figure 1
Jet streams (J) in the upper atmosphere in relation to the
vertical temperature gradient.
