It is neither the same for all the regions nor the same for one region all the time.
Factors affecting atmospheric pressure –
Altitude: Pressure increases when air descends, due to the decrease in volume.
Temperature: Pressure rises with decrease in temperature.
Earth rotation.
Pressure belts –
Equatorial low pressure belt. (doldrums)
Zone extending between 50N and 50S.
Not stationary i.e seasonal drift of this belt with summer and winter solstice.
It is thermally induced.
Represents the zone of convergence of northeast and southeast trade winds.
Due to frequent calm conditions, known as belt of calm or doldrums.
Inter tropical convergence zone. (ITCZ)
Low pressure belt at equator where trade winds converge hence here air tends to ascend.
In July it is around 200N – 250N latitude (over the Gangetic plane), sometimes called the monsoon trough. It encourages thermal low over north and north-west India.
Due to shift of ITCZ trade winds of the southern hemisphere cross the equator between 400E and 600E longitude, and starts flowing from south-west to north-east due to the Coriolis force. It becomes south-west monsoon.
In winter the ITCZ moves southwards and so the reversal of winds to south and south-west, takes place, they are called north-eastern monsoons.
Sub tropical high pressure belt.
300 – 350 in both hemispheres.
The high pressure along this belt is due to subsidence of air coming from equatorial regions and also due to blocking effect of Coriolis force.
The convergence of winds at high altitudes above this zone results in the subsidence of air from the higher altitudes.
Thus descent of wind results in concentration of their volume and ultimately causes high pressure.
This zone of high pressure is also called Horse latitude.
Sub polar low pressure belt.
600– 650 in both the hemisphere.
Does not appear to be thermally induced as there is low temperature throughout the year and as such there should have been high pressure belt instead of low pressure belt, thus it is dynamically induced.
More developed at southern hemisphere (and regular) because of the over dominance of water.
Polar high pressure belt.
High pressure persists at the poles throughout the year.
Are small in area.
Shifting of wind belts –
The entire system of pressure and wind belts follows the movement of mid day sun.
In June when the overhead sun is above Tropic of Cancer, all the belts move about 50-100 north of their average position.
In the same manner when the sun is at Tropic of Capricorn in December, the entire belt swings 50-100 south of their average position.