Pressure Belts

The horizontal distribution of air pressure across the latitudes is characterized by high or low pressure belts.
This is however, a theoretical model because pressure belts are not always found as such on the earth.
These pressure belts are:

(i) The Equatorial Low Pressure Belt;

(ii) The Sub tropic High Pressure Belts;

(iii) The Sub-polar Low Pressure Betts;

(iv) The Polar High Pressure Belts

(i) The Equatorial Low Pressure Belt

The sun shines almost vertically on the equator throughout the year.
As a result the air gets warm and rises over the equatorial region and produce equatorial low pressure.
This belt extends from equator to 10º N and 10 º S latitudes.
Due to excessive heating horizontal movement of air is absent here and only conventional currents are there.
Therefore this belt is called doldrums (the zone of calm) due to virtual absence of surface winds.
These are the regions of convergence because the winds flowing from sub tropical high pressure belts converge here.
This belt is also known as-Inter Tropical Convergence Zone (ITCZ).

(ii) The Sub-tropical High Pressure Belts

The sub-tropical high pressure belts extend from the tropics to about 35 º latitudes in both the Hemispheres.
In the northern hemisphere it is called as the North sub-tropical high pressure belt and in the southern hemisphere it is known as the South sub-tropical high pressure belt.
The existence of these pressure belts is due to the fact that the up rising air of the equatorial region is deflected towards poles due to the earth’s rotation.
After becoming cold and heavy, it descends in these regions and get piled up. This results in high pressure.
Calm conditions with feeble and variable winds are found here.
In olden days vessels with cargo of horses passing through these belts found difficulty in sailing under these calm conditions.
They used to throw the horses in the sea in order to make the vessels lighter. Henceforth these belts or latitudes are also called ‘horse latitudes’.
These are the regions of divergence because winds from these areas blow towards equatorial and subpolar low pressure belts.

(iii) The Sub-polar low Pressure Belts

The sub-polar low pressure belts extend between 45 º N and the Arctic Circle in the northern hemisphere and between 45°S and the Antarctic Circle in the southern hemisphere.
They are known as the North sub-polar low and the South sub-polar low pressure belts respectively.
Winds coming from the sub-tropical and the polar high belts converge here to produce cyclonic storms or low pressure conditions.
This zone of convergence is also known as polar front.

(iv) The Polar High Pressure Belts

In polar regions, sun never shines vertically.
Sun rays are always slanting here resulting in low temperatures.
Because of low temperature, air compresses and its density increases. Hence, high pressure is found here.
In northern hemisphere the belt is called the North polar high pressure belt while it is known as the South polar high pressure belt in the southern hemisphere.
Winds from these belts blow towards sub-polar low pressure belts.

Shifting of Pressure belts

This system of pressure belts is a generalised picture.
In reality, the location of these pressure belts is not permanent.
They shift northward in July and southward in January, following the changing position of the sun’s direct rays as they migrate between the Tropics of Cancer and Capricorn.
The thermal equator (commonly known as the belt of highest temperature) also shifts northwards and southwards of the equator.
With the shifting of thermal equator northwards in summer and southwards in winter, there is also a slight shift in pressure belts towards north and south of their annual average location.