Environmental Engineering Reference
In-Depth Information
slight shifts in position and intensity. Between about 30° N and 30° 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 3 km. On either side of this belt occurs a ring or vortex of much
stronger westerly winds.
Figure 6.16
Mean zonal wind for (a) December-February
and (b) June-August. Units are m s
−1
. Positive values denote
a westerly wind. Colour indicates easterly wind.
Source: After Newell et al. (1969).
THE UPPER WESTERLIES
These high-altitude westerly winds are a major feature of our atmosphere. They reach
their maximum speed at approximately 12 km between 30° and 40° latitude. The mean
speed is as much as 35 m s
−1
(125 km hr
−1
) and maximum speeds of several hundred
kilometres 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 15 m s
−1
(55 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
