Air on account of its weight exerts pressure.
The weight or pressure of the air is called atmospheric pressure.
The atmosphere is held on the earth by the gravitational pull of the earth.
Atmospheric pressure changes from time to time and place to place due to differential heating.
Air pressure is crucial to all forms of life and it plays a dominant role in wind movement and precipitation.
It directly and indirectly, influences the weather and climatic changes.
The atmospheric pressure is measured by an instrument called Barometer. Mercury, Aneroid and Digital barometers are commonly used to measure pressure. Air pressure is expressed by a unit called ‘millibar’ (mb).
The average atmospheric pressure at sea level is 1013.25 mb.
Factors affecting atmospheric pressure
Atmospheric pressure is influenced by various factors. Most important among them are:
Air is compressible. Earth’s gravity pulls the molecules and gases towards the ground. The lowest layer of the atmosphere gets compressed and also becomes dense. These dense layers exert great pressure and form high pressure. With the increase in altitude there is a decrease in atmospheric pressure, at the rate of 1 inch for every 900 feet.
Lower latitudes record high temperature, with that pressure falls. In the same way higher latitudes record low temperature and forms high pressure.
The amount of water vapour in the air also influences the atmospheric pressure. Water vapour is lighter than air. So more the vapour , lighter will be the air. The dry air is heavier than vapour laden. It is because of this reason the air with more amount of vapour becomes lighter and rises upwards leading to condensation and precipitation.
Rotation of the Earth:
The spinning of the earth on its axis also causes variation in the atmospheric pressure. The spinning action also causes spinning of air mass and the cold air near the poles is rarefied to form low-pressure belts. In addition, the rotation deflects the movement of air in its direction.
DISTRIBUTION OF PRESSURE BELTS
There are seven pressure belts on the globe
Equatorial low-pressure belt (1 belt)
Sub-tropical high-pressure belts (2 belts)
Sub-polar low-pressure belts (2 belts) and
Polar high-pressure belts (2 belts)
1. EQUATORIAL LOW-PRESSURE BELT
This belt extends from the equator to 5°N and 5°S
It receives direct rays of the Sun almost throughout the year. Hence the air becomes hot, expands and rises.
Thus a belt of low pressure is created in this region.
It is a ‘belt of calm’ with very little wind, therefore it is known as ‘Doldrums’ (Equatorial calm).
This belt is also known as – Inter Tropical Convergence Zone (ITCZ) because the trade winds flowing from subtropical high-pressure belts converge here.
It receives regular convectional rainfall.
2. SUB-TROPICAL HIGH-PRESSURE BELT
This belt is found between 25° and 35° latitudes in both the Hemispheres.
The existence of these pressure belts is due to the fact that the up-rising air of the equatorial region is deflected towards the poles due to the earth’s rotation.
The formation of this belt is associated with the rotation of the Earth and descending air currents.
The warm air rises at the equator, blows to higher altitude and diverts towards the poles. The wind cools down as it rises and spreads towards the poles. Gradual cooling makes this air heavy and sinks down to the surface at 30° North and South of the equator.
This results in the formation of the high-pressure belt. The trade winds and antitrade winds originate in this belt. This belt is also called ‘Horse Latitudes’.
There are two Sub-tropical high-pressure belts
North sub-tropical high-pressure belt and
South sub-tropical high-pressure belt.
Horse latitudes are generally areas of high pressure marked by calm, subsiding air that gets heated during descent. It is said that Spanish sailors ferrying horses to the West Indies were usually stuck for months in these calm waters and had to throw their horses into the water to conserve drinking water for themselves. This led to the term ‘horse latitudes.
3. SUB-POLAR LOW-PRESSURE BELT
This belt lies between 60º to 70º North and South of the equator.
A low-pressure belt is created mainly due to the rotation of the Earth, which swings the bulk of the air towards the equator.
These are the areas of storminess, especially in winter.
There are two sub-polar low-pressure belts
North Sub-polar low-pressure belt.
South Sub-polar low-pressure belt.
4. POLAR HIGH-PRESSURE BELT
The two polar regions experience low temperatures throughout the year.
Thus the cold air sinks down because of its greater density.
Sinking of cold air in the polar region results in high pressure.
Polar high-pressure belt extends from 80º to 90º latitudes in both the hemispheres.
The polar high-pressure belts record extremely cold weather conditions so air is dense and heavy.
There are two polar high-pressure belts
North Polar high-pressure belt.
South Polar high pressure belt.
SHIFTING OF PRESSURE BELTS
The above-mentioned pressure belts are neither fixed nor permanent.
The pressure belts move (5º North and South) in response to the apparent migration of the Sun.
We assume that the Sun is directly overhead on the equator on all the days of the year. But this happens only twice a year, during the Autumn (23rd September) and Spring (21st March)
Between December and June the Sun moves northwards, and between June and December it moves southwards. Therefore, the pressure belts follow the annual migration of the Sun towards the North and South. This results in shifting of pressure belt.