U2 - Topic 3 - Pressure belts

Physical Geography (Unit 2) 

Pressure belts

Pressure belts are seasonally identical horizontal pressure variations created in the earth’s atmosphere just above the earth’s surface due to seasonal and spatial variation of energy received by the earth at different places. [Energy received determines the pressure exerted by the atmosphere at a place and variations in pressure exerted determines the direction and velocity of the wind.]

Spatial variation is due to the Geoid (Onion shape) shape of the earth, i.e. the tropics receive maximum energy as they are more or less perpendicular to the sun’s rays whereas the polar regions receive minimum as they make an angle far lesser than 90 degree with the sun’s rays. The temperate regions receive energy somewhere in between the tropics and the polar regions.

Seasonal variations are due to revolution of the earth on a tilted axis around the sun. The dynamic behaviour of pressure belts (position of the pressure belts oscillating between certain latitudes) is due to seasonal variation in insolation (sun’s energy received).

Low Pressure belt at the equator and High Pressure belt at the poles

• They are both thermally induced pressure belts (they are created due to loss or gain of heat).

Low Pressure Belt (heat gain)

• Equator receives maximum energy. This means the air at the equator becomes less denser and raises. This creates a low pressure at the equator. Winds from the surroundings gush in to fill the gap.

High Pressure Belt (heat loss)

• Here, the air in the upper levels of troposphere cools and becomes more denser. It sinks to the surface and creates a high pressure at the surface.

Sub-Tropical High Pressure belt and Sub-Polar Low Pressure belt

• They are both dynamically induced pressure belts (they are created to earth’s rotation — Coriolis Effect)

Sub-Tropical High Pressure belt

• The ait that raises to the upper levels of troposphere at the equator cools before reaching the 25–35 degree N and S latitudes. This cool dry air sinks at 25–35 degree N and S latitudes creating a high pressure at the surface.

Sub-Polar Low Pressure

• Half of air sinking at both subtropical high and polar high coverage at around 60 degree N and S latitudes and raise. This creates a low pressure belt. The air after raising to the upper levels move towards polar and subtropical regions, adding to the sinking air there.