(The horizontal distribution of air pressure)
These pressure belts are:
(1) The Equatorial Low-Pressure Belt
(2) The Subtropic High Pressure Belts
(3) The Sub-polar Low-Pressure Betts
(4) The Polar High-Pressure Belts
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 100N and 100S 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).
The Sub-tropical High-Pressure Belts:
- The sub-tropical high-pressure belts extend from the tropics to about 35° 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.
- 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.
The Sub-polar low-Pressure Belts:
- The sub-polar low-pressure belts extend between 45°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.
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.
Pressure belts also shift northwards and southwards with the shift of thermal equator (commonly known as the belt of highest temperature).
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