The atmosphere in action - Fundamentals of the Physical Environment - page 123

variable and linked with the shallow polar anticyclones.
In the northern hemisphere they are often influenced by
the circulation around the northern edge of cyclones. As
a result they change direction according to the local
weather and topography.
In the southern hemisphere the vast Antarctic ice cap
controls the atmospheric circulation around the pole.
Anticyclones develop frequently over eastern Antarctica,
and strong south-easterly winds develop around the
margins of the ice plateau with consistencies similar to
those of the trades, and occasionally of great strength
( Figure 6.1 ).

per hour are not uncommon. It is not surprising that
aircraft can travel from the United States to Europe more
quickly than on the return journey.
Although these wind patterns are steady, seasonal
variations do take place, especially in the northern
hemisphere. The upper westerlies are strongest in the
winter, when the temperature difference between the
tropics and temperate latitudes is at its greatest. From June
to August temperatures in the northern hemisphere are
relatively warm, even in polar regions, so the pressure
gradient is reduced and the upper westerlies decline to
speeds of as low as 20 m s -1 (70 km hr -1 ).
As ever, changes in the southern hemisphere are less
pronounced, largely owing to the greater thermal stability
there. The vast areas of ocean absorb large quantities of
heat without any significant increase in temperature. The
ice plateau of Antarctica also stays very cool, so the
temperature gradients do not change very much from
winter to summer.
The position of the boundary between the westerlies
and easterlies (of both the upper and the surface winds)
varies throughout the year. From December to February
the polar vortex of the winter (northern) hemisphere
expands, pushing the belts southwards so that, at the
surface, the boundaries are at about 30 N and 35 S. As the
year progresses, the other polar vortex begins to expand
as winter sets in over the southern hemisphere. The
boundaries eventually reach about 35

Upper winds

Nature of the upper winds
Looking up at high clouds on a clear day, it is not unusual
to find that the direction of their movement is different
from that of the surface winds. As this implies, winds in
the upper atmosphere can be affected by forces operating
in a different direction from those at the surface and may
appear to be part of a different system of circulation. If
we were to make an ascent by balloon into these upper
wind systems we would find that the change from surface
to upper atmosphere conditions was not abrupt but
transitional. With increasing height, we would discover,
the winds tend to follow a gradually more distinct zonal
(east-west) direction and they become stronger. The main
reason for this change is the disappearance of the frictional
influence of the ground surface upon the winds. In other
words, the flow more nearly approximates to the
geostrophic winds that, it will be remembered, result from
the interaction of the pressure gradient and the Coriolis
force ( Figure 6.17 ).
The zonal flow of the upper winds can be shown on
average as a cross-section from north to south ( Figure
6.18 ). In fact variations around this average picture are
slight, except in the monsoon areas of Asia. At each season
the same basic pattern exists, with slight shifts in position
and intensity. Between about 30

S by June
to August. The separation between the two systems is not
vertical. As a result, some parts of the tropics have
easterlies in the lower atmosphere and westerlies above.
Only over a small area of the globe do easterlies occur at
all levels, whereas westerlies extend throughout the
atmosphere over a large proportion of Earth.

N and 30

Rossby waves
The pattern of easterlies and westerlies in the upper
atmosphere is only part of the total picture. In addition
to the marked zonal flows there are less apparent but none
the less important meridional flows. In the circumpolar
areas, for example, there occur wave-like patterns of
flow called Rossby waves (after C.-G. Rossby, a Swedish
meteorologist) that play a vital role in the energy exchange
between the temperate and polar areas.
It is not easy to detect these meridional flows within
the pattern of strong zonal circulation by normal methods
of depicting winds. The normal methods usually show
average conditions, so that processes which balance each
other, flowing northwards for six months, perhaps, then
southwards for the next six months, are lost. Yet that is
what happens in the case of the Rossby waves. At a

S we have a
zone of easterly winds which are relatively weak, reaching
a maximum speed of 4-5 m s -1 (about 17 km hr -1 ) at
about 900 hPa (2 km). On either side of this belt occurs
a ring or vortex of much stronger westerly winds.

N and 30

Upper westerlies
These high-altitude westerly winds are a major feature of
our atmosphere. They reach their maximum speed at
approximately 300 hPa (12 km) between 30 and 40
latitude. The mean speed is as much as 40 m s -1 (140 km
hr -1 ) and maximum speeds of several hundred kilometres