So far we have been considering winds near the suface, but they are only half the story. There is another but related pattern of winds aloft in the troposphere. Different winds at various levels are shown by movements of cirrus cloud in directions quite distinct from those of low-level clouds.
We shall consider winds at two levels especially, where the pressures are around 850 hPa (at about 1,500 m) and about 300 hPa (at about 9 km), respectively. The first represents conditions in the lower troposphere, but well
Plate 12.1 The patterns of cloud over Africa associated with the global circulation of winds, seen from Apollo 17 on its way to the Moon on 9 December 1972. Antarctica was surrounded by cloud, with several frontal disturbances shaped like inverted commas. There was a band of cloud along the ITCZ over the Indian ocean, and orographic cloud along the east coast of Madagascar, fronting the Trade winds.
clear of the PBL, while the second represents winds not far below the tropopause (Figures 1.9-1.11). The region above the tropopause is separate because of the stability of the stratosphere and the consequent absence of vertical motion (Section 7.6).
There are several ways of observing winds aloft. For instance, a high cloud is identified from satellites by the coldness of the cloud top (deduced from the wavelength of the infra-red radiation detected by radiometers on the satellite), and the winds carrying such a cloud can be inferred from its shift in position over time. Other information on winds at high levels is obtained from the sideways movements of ascending radiosondes (Section 1.6) and from aircraft reports. In addition, winds aloft can be inferred from surface winds combined with theory based on measured temperature profiles (Notes 12.E and 12.F).
But why consider winds away from the surface, whose climates are the main concern of this book? The reason is that the troposphere's circulation consists of both upper and lower winds, interacting with each other, and therefore upper winds considerably influence the lower winds which largely determine the advection of heat and moisture and thus affect climates. In particular, winds aloft help moderate the difference between equatorial and polar climates. Second, differences between higher and lower winds imply circulations with compensating regions of ascent and subsidence. These vertical motions are important because they are responsible for cloud and rain. They cannot be measured, as they move at only a few millimetres per second within the turbulence of horizontal winds which are hundreds or thousands of times bigger, but they can be inferred from a comparison of upper and lower winds. An understanding of the vertical circulations of which high-level winds form part explains why some parts of the world have mostly subsiding air and are consequently arid, for example.
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