Geoscience Reference
In-Depth Information
of the globe - which provide much of the climatic
variation and consistency in the world. We have already
indicated that the westerly winds dominate the climate of
the temperate latitudes; similarly, the equatorwards
movement of air in the regular easterly trade winds has a
prevailing influence on tropical climates.
within that area the steady trade winds provide a stable
and relatively constant climate. At the surface the winds
have a component towards the equator, this being from
the north-east in the northern and from the south-east in
the southern hemisphere. Above the surface friction layer
the winds become more easterly.
Viewed from the air, the oceanic trade winds contain
innumerable uniform small clouds, all with a similar base
and depth (see Plate 7.4 ). These are the visible expression
of the transfer of latent heat from the sea surface, through
evaporation, before condensation at higher levels.
As we have noted, the seasonal movement of the
equatorial trough is slight over the oceans, so the oceanic
tropical areas are dominated by the trades. On the
continents the trades are far more restricted in extent,
and the equatorial westerlies and monsoons are more
important. The two belts interact closely; it is the
convergence of moisture in the trade winds that feeds the
equatorial trough. The shift in the position of the trough
thus determines the relative extent of the easterlies and
westerlies. When the trough is farther north, with the
overhead sun in July, the trades are restricted in the
northern hemisphere, particularly over land. In January
the trough is at its most southerly position and the trades
extend to the equator. In the southern hemisphere less
marked variations occur, for the predominance of ocean
means that the southern limit of the trough remains close
to the equator.
Surface winds
Four main surface wind belts can be distinguished.
Around the equator, in the low-pressure equatorial
trough, occurs a zone of convergence where the north-
easterly trade winds blowing from the Tropic of Cancer
meet the south-easterly trade winds blowing from the
Tropic of Capricorn. Either side of the equatorial trough
these winds dominate, giving the trade-wind belt. Pole-
wards of the tropics, in the temperate latitudes, we find a
zone of prevailing westerlies, while around the poles
occurs a belt of easterlies. We will examine each of these
zones separately, but as we do so it is important to
remember that, in reality, these wind belts do not operate
in isolation. They are closely interrelated ( Plate 6.2 ).
Equatorial trough
The equatorial trough, or Intertropical Convergence Zone
(ITCZ), is a shallow trough of low pressure generally
situated near the equator. Over the oceans it is fairly static,
because seasonal temperature changes are small. This can
be seen in Figure 6.1 where the wind direction changes
little between seasons. In the Pacific, for example, its
average position varies by no more than 5
Westerlies
In comparison with the winds of the tropics, the westerlies
of the mid-latitudes seem unreliable and fickle. They are
westerlies only on average.
Polewards of the subtropical anticyclones, rotating
storms are the main mechanism of energy transfer. Unlike
hurricanes, these systems cover vast areas and can be seen
clearly from space, identified by their characteristic spiral
of cloud s ( Plates 6.1 an d 7.2 ). In the northern hemisphere
they tend to move north-eastwards, although individual
directions may vary from north to south-east. Typically,
they follow an evolutionary pattern which we shall be
examining more closely in the next chapter. The storms
are initiated in areas of strong temperature gradients,
such as off Newfoundland, where the cold Labrador
current and warm Gulf Stream are in close proximity,
forming roughly circular patterns of low pressure and a
rotational movement of winds, which are often strong
( Figure 6.14 ). They are known as lows, cyclones or
depressions. As they evolve they become initially more
intense - the central pressure has been known to fall as
low as 930 hPa - before filling as the storm declines. On
of latitude
within the course of a single year. The situation is very
different over the continents. During summer in
continental areas the trough sweeps polewards, reaching
30
latitude over eastern China. Behind the
trough the winds are predominantly westerly and are the
main rain-bearing winds to most of those areas. Where
they reach into higher latitudes they are called monsoons
(an Arabic word meaning 'season') and they show an
almost complete reversal of direction from summer to
winter, a change that tends to occur with uncanny
regularity about the same dates each year.
With the exception of the monsoon, the winds in the
equatorial trough tend to be light and variable and,
because sailors often found themselves becalmed there, the
area became known as the Doldrums.
or even 40
Trade winds
The trade-wind belts lie between the equatorial trough
and the subtropical highs ( Figures 6.1 and 6.15 ) . This zone
occupies nearly half the globe, much of it ocean, and
 
 
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