Environmental Engineering Reference
In-Depth Information
convergence and divergence is extremely important in determining weather events, as we
shall see in Chapter 7.
THE GLOBAL PATTERN OF CIRCULATION
With these principles in mind we can try to build up a picture of the global pattern of
circulation in Earth's atmosphere. We can start by considering a highly simplified model
of the atmospheric system: a uniform, non-rotating, smooth Earth.
As we have seen, the basic force causing atmospheric motion is the pressure gradient;
this gradient arises from the unequal heating of the atmosphere by solar radiation. At the
equator - the 'firebox' of the circulation, as it has been called - solar radiation is
converted into heat. The air expands and rises and flows out towards the poles. Cool,
dense air from the poles returns to replace it. We can readily demonstrate the pattern of
circulation by heating a dish of water at its centre. Hot water bubbles up above the heat
source and flows across the surface to the cold 'polar' areas. At depth the flow is
reversed. So long as this unequal heating is continued, the cellular flow is maintained.
Figure 6.12
Main features of atmospheric circulations at the
surface and in the atmosphere. PFJ and STJ are the polar
front jet and the subtropical jet. The surface black lines
represent frontal zones separating warm subtropical air from
cool polar air.
In reality, however, the pattern is found to be more complex, for, instead of flowing
directly to the poles at high altitudes, the warm air from the equator gradually cools and
sinks, owing to radiational cooling. Most of it reaches the surface between about 20° and
30° latitude, and this subsiding air gives rise to zones of high pressure at the tropics - the
subtropical high-pressure belts. As the descending air reaches the surface it diverges,
some returning towards the equator to complete the cellular circulation of the tropics, the
remainder flowing polewards (Figure 6.12).
Various other factors disrupt this pattern further, for Earth is not at rest, nor uniform,
as we have so far assumed. It rotates. Its surface is highly variable; it has oceans and
continents; it consists of a mosaic of mountains and plains. Moreover, the inputs of solar
radiation vary considerably both on a seasonal and on a daily basis.
