- 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.
