The atmosphere is held on the earth by the gravitational pull of the earth. A column of air exerts weight in terms of pressure on the surface of the earth. The weight of the column of air at a given place and time is called air pressure or atmospheric pressure. Atmospheric pressure is measured by an instrument called barometer

Distribution of atmospheric pressure on the surface of the earth is not uniform. It varies both vertically and horizontally.

### Vertical Distribution

Air is a mixture of various gases. It is highly compressible. As it compresses, its density increases. The higher the density of air, the greater is the air pressure and vice versa. The mass of air above in the column of air compresses the air under it hence its lower layers are more dense than the upper layers; As a result, the lower layers of the atmosphere have higher density, hence, exert more pressure. Conversely, the higher layers are less compressed and, hence, they have low density and low pressure.

Air pressure decreases with increase in altitude but it does not always decrease at the same rate. Dense components of atmosphere are found in its lowest parts near the mean sea level. Temperature of the air, amount of water vapour present in the air and gravitational pull of the earth determine the air pressure of a given place and at a given time.

The normal rate of decrease in air pressure is 34 millibars per every 300 metres increase in altitude. The effects of low pressure are more clearly experienced by the people living in the hilly areas as compared to those who live in plains. In high mountainous areas rice takes more time to cook because low pressure reduces the boiling point of water. Breathing problem such as faintness and nose bleeding are also faced by many trekkers from outside in such areas because of low pressure conditions in which the air is thin and it has low amount of oxygen content.

### Horizontal Distribution

The distribution of atmospheric pressure over the globe is known as horizontal distribution of pressure. It is shown on maps with the help of isobars. An isobar is a line connecting points that have equal values of pressure. The horizontal distribution of atmospheric pressure is not uniform in the world. It varies from time to time at a given place; it varies from place to place over short distances. The factors responsible for variation in the horizontal distribution of pressure are:

• Air temperature
• The earth’s rotation
• Presence of water vapour

### Pressure Belt

The horizontal distribution of air pressure across the latitudes is characterized by high or low pressure belts. These pressure belts are:

1. The Equatorial Low Pressure Belt
2. The Sub tropic High Pressure Belts
3. The Sub-polar Low Pressure Betts
4. The Polar High Pressure Belts

### 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. 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).

### Sub-tropical High Pressure Belts

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 get piled up. This results in high pressure. Calm conditions with feeble and variable winds are found here.

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 sub-polar low pressure belts.

### Sub-polar Low Pressure Belts

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.

### 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. Winds from these belts blow towards sub-polar low pressure belts.