The annual average sea-level pressure is represented. We find that because of the preponderance of land masses in northern hemisphere the idealized zonal pattern of surface pressure is disrupted.
The so-called pressure belts are broken into cells of high and low pressures which are elongated in shape and extend in east-west direction.
On the contrary, the southern hemisphere is pre- eminently a water hemisphere and there is a progressive diminution of land masses in higher latitudes. The only true zonal distribution of pressure exists in the region of sub-polar low in the southern hemisphere where the ocean is continuous.
The annual average sea-level pressure pattern as represented in Figure 25.8 brings out the following salient features into sharp focus:-
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(a) There is a trough of low pressure in the vicinity of the equator which is to a certain extent zonal in character; (b) The subtropical regions in either hemisphere are dominated by a number of discontinuous high pressure cells called the subtropical highs. These highs are definitely not of thermal origin; they are supposed to be induced by mechanical or dynamic means, (c) Pole-ward from the subtropical highs there is a decrease of pressure towards the sub-polar lows located in the vicinity of the Arctic and Antarctic Circles. In the northern hemisphere the sub-polar trough of low pressure consists of individual cells of low pressure over the oceans. Contrary to this, there is a continuous belt of low pressure at about the latitude of the Antarctic Circle. The sub-polar lows owe their origin to the dynamic rather than thermal causes, (d) The polar areas in either hemisphere are dominated by anticyclones that are of thermal origin.