When we add the effect of earth’s rotation to temperature, the Coriolis force comes into the picture. Some of the air rising near the equator and moving towards the poles in the upper atmosphere is turned aside by earth rotation or by other dynamic processes.
The result is that not all the air rising from the equator goes to the poles. Some of it settles down in cells in the subtropical regions. These subsiding air currents create subtropical high pressure cells between 25° and 35° N and S latitudes.
There are two factors which are believed to contribute to the general subsidence found here. First, when the rising air currents move away from the equatorial region, the release of latent heat of condensation keeps them warm and buoyant.
But radiational cooling results in their increased density in the upper atmosphere. This accounts for the subsidence of cold and dense air in the subtropical regions.
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Secondly, with the increasing distance from the equator, the Coriolis force gets stronger, so that the pole-ward moving winds are deflected into a nearly west-to-east flow as they get near 25° N and S latitude.
Thus, the Coriolis force produces a blocking effect causing convergence aloft. This results in general subsidence between 25 and 35 degrees of latitude. Because the subsiding air currents are relatively drier, the weather in this zone remains fair and dry.
Besides, relative humidity of the descending air currents is further reduced by the effect of adiabatic heating. This accounts for the fact that all the hot deserts of the world are situated in the subtropical high pressure belts.
Pole-ward of the subtropical high pressure cells in both the hemispheres are large belts of low pressure at about 50 to 65 degrees latitude. This is called the subpolar low pressure belt. Here the westerlies and polar easterlies meet to form a convergent zone.
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This region where the cold polar winds and the wanner west winds clash is called the polar front. Temperature does not account for the existence of this low pressure belt, which is perhaps produced by the dynamic factors involved with the rotation of earth. In Polar Regions there are polar highs caused by extremely low temperatures and subsiding air.
From the preceding discussion of the distribution pattern of surface pressure on a rotating earth with uniform surface, it emerges that there are in all seven alternating low and high pressure belts on the earth’s surface. These pressure belts are as follows:-
(1) Equatorial trough of low pressure
(2) Subtropical high pressure belt (northern hemisphere).
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(3) Subtropical high pressure belt (southern hemisphere).
(4) Sub-polar low pressure belt (northern hemisphere).
(5) Sub-polar low pressure belt (southern hemisphere).
(6) Polar high (northern hemisphere).
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(7) Polar high (southern hemisphere).
The arrangement of continuous pressure belts around the earth as referred to above would be ideal as well as hypothetic-cal, and far away from the actual conditions as they are observed on the earth’s surface. In fact, the unequal distribution of continents and oceans makes the distribution pattern more complicated.