The horizontal distribution of air pressure across the latitudes is characterised by high or low-pressure belts. They are
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 the 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 the virtual absence of surface winds.
- These are the regions of convergence because the winds flowing from subtropical 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 350 latitudes in both the Hemispheres.
- In the northern hemisphere, it is called 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 uprising 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 gets piled up. This results in high pressure.
- Calm conditions with feeble and variable winds are found here.
- In the olden days vessels with a 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 450N 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 the polar front.
The Polar High-Pressure Belts:
- In polar regions, the sun never shines vertically. Sun rays are always slanting here resulting in low temperatures.
- Because of low temperature, the air compresses and its density increases. Hence, high pressure is found here.
- In the 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.