Environmental Engineering Reference
In-Depth Information
70 per cent and 90 per cent of all winds blow from the
predominant quadrant, with speeds above 3·3 m s
−1
.
Source: After Crowe (1971).
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 (Figure 6.1).
THE WESTERLIES
In comparison with the winds of the tropics, the wester-lies 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 clouds (Plates 6.1 and 7.2). In the
northern hemisphere they tend to move north-eastwards, although directions 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 Newfound-land, 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 6.13). 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 average the location of maximum intensity is in the areas of
Iceland in the Atlantic and the Aleutian Islands in the Pacific. Thus climatologists speak
of the Icelandic Low and the Aleutian Low. In the southern hemisphere there are no
distinct areas of the genesis of storms, so lows form throughout a wide belt.
As a low approaches in the northern (southern) hemisphere, winds increase in
strength, initially from a southerly (northerly) direction, then become westerly and,
finally, as the low moves away, they veer to north-westerly (south-westerly) or even
northerly (southerly). The tracks of the lows reach farther poleward in summer than in
winter, so the area affected by the storms varies seasonally. Nowhere is this seasonal
pattern more clearly seen than over the Mediterranean basin, in California and at
equivalent latitudes in the southern hemisphere such as central Chile or parts of West and
South Australia. In winter, when the cyclones follow tracks at lower latitudes, they bring
rain to these areas. In summer the cyclones move away, to be replaced by the subtropical
anticyclones and dry, hot weather. The consequences will be discussed in Chapters 26
and 27.
The regular march of cyclones and anticyclones through the temperate latitudes
produces a majority of winds between north-west and south in the northern hemisphere
and between south-west and north in the southern hemisphere. This pattern is far from
invariable, however, and depending upon the precise tracks taken by the lows, and the
local topography, winds from any direction are possible. The prevailing westerlies,
therefore, are anything but prevalent. Moreover, the strong north-south component of
