Since the earth rotates, none of the conditions described above exists on its surface. The Coriolis force deflects any mass of moving air to the right in northern hemisphere and to the left in southern hemisphere.
Naturally, therefore, any air which starts to move directly southward from the polar region towards the low latitudes as a north wind would become a north-east wind. Similarly, in the southern hemisphere a southerly wind becomes south-easterly in direction.
Thus, on a rotating earth the fictitious unicellular wind system blowing parallel to the meridians is broken. As pointed out in Chapter 25, because of thermal and dynamic causes there are in all seven alternating belts of high and low pressure on the earth’s surface.
Largely controlled by the arrangement of pressure belts, the general circulation is divided into three distinct zones in each hemisphere. In these belts the atmospheric motions are almost parallel to the circles of latitude.
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The winds in these latitudinal belts blow either from west to east or from east to west. In this way the general circulation pattern on the earth’s surface becomes zonal rather than meridional in character.