Atmospheric Pressure

A blanket of air covering the Earth’s surface is known as the atmosphere. The Atmospheric Pressure is invisible but can be felt because the air is colourless and odourless. Unlike other objects air also has weight, when this air exerts pressure on  earth’s surface it is known as Atmospheric Pressure or air pressure. 21% Oxygen, 78% Nitrogen, and one percent of other gases are present in the atmosphere.

Definition of Atmospheric Pressure

Atmospheric Pressure is explained as the force exerted on a surface per unit area above the Earth’s surface by the air molecule’s weight. All over the earth the Atmospheric Pressure is not uniformly distributed. There is an instrument known as a ‘barometer’ which is used for the measurement of Atmospheric Pressure. About 76cms of the mercury column in the barometer is the normal Atmospheric Pressure at sea level. The level of the mercury column changes as air pressure increases or decreases. Atmospheres (atm) is a unit in which meteorologists measure air pressure.

There are two types of pressure system i.e. High-pressure System and Low-pressure System

• High-pressure System –

A high-pressure system is defined as the Atmospheric Pressure at sea level that is higher than that of the air pressure of the surroundings. Anticyclone is another name for a High-pressure system. In the Northern Hemisphere the High-pressure system moves clockwise and in the Southern Hemisphere, the High-pressure system moves anti-clockwise; this is due to the Coriolis Effect.

• Low-pressure System –

The low-pressure system is defined as the Atmospheric Pressure at sea level that is lower than that of the air pressure of the surroundings. Depression is another name for a Low-pressure system. Warm air, high winds, and atmospheric lifting are associated with Low-pressure. 

Land Breeze and Sea Breeze are associated with High-pressure System and Low-pressure System-

The air above the land warms up faster than the water in the morning when the sun rises. Warm air being less heavy rises and expands creating an area of Low-pressure. Whereas, the air over the sea is cooler because the sea takes more time to heat up and as the air is cooler so it is heavier and could not rise easily, so High-pressure is created there. A conventional current is created due to the difference in pressure and air temperature. The warm air above the land gets filled up by cool ocean air and the air gets warm again and rises, this cycle continues. The wind that blows from the sea to the land is called sea breezes. During the night time the convectional current gets reversed, the land cools more quickly than the sea in the nighttime. The air above the land becomes heavy and cools down creating a High-pressure there, whereas above the sea there is Low-pressure created at night time because the sea does not cool quickly and the air above the sea is hot and less heavy. So the air from the land rushes toward the sea to fill up the Low-pressure area and gradually the air gets heated up and rises above. Therefore, the wind that blows from the land towards the sea is known as a land breeze.

The factors that control the high-pressure system and low-pressure system are thermal and dynamic. 

Atmospheric Pressure Belts

There are a total of seven pressure belts on the earth’s surface.  These are the two High-pressure Belts that are subtropical, the two Subtropical Low-Pressure Belts, The Equatorial Low-Pressure Belts, and the two High-Pressure Belts that is polar, in the Northern and Southern Hemisphere the other forms are matching pairs. 

• Subtropical High-Pressure Belts-

The area where the increasing equatorial air current decreases lies is about 30° South and 30° North of the Equator. This is an area of High-pressure. Generally, the wind blows from high-pressure to low-pressure. So, the Trade wind blows from Subtropical Region Below towards the Equator and the Westerlies blow towards Sub-Polar Low pressure. 

• Circum-Polar Low-Pressure Belts –

Circum-Polar Low-Pressure Belts are located between 60° and 70° in each of the Hemispheres. The decreasing air gets divided into two parts in the Equatorial Low-Pressure Belt, where towards the Equatorial Low-Pressure Belt one part of the air blows, and towards the Circum-Polar Low-Pressure Belt, the other part of the air blows.

• Equatorial Low-Pressure Belts –

From 0 to 5° North and South of the Equator the Equatorial Low-Pressure Belt extends. There is intense heat due to the vertical rays of the Sun. As a result of intense heating, there is a low-pressure area created.

• Polar High-Pressure Belts –

The temperatures are always intensely low in the region of 70° to 90° North and South in North and South Pole. So, as a result, there is high pressure above the poles. Permanent IceCaps are the main characteristics of these regions.

Conclusion

It is to conclude that Atmospheric Pressure is explained as the force exerted on a surface per unit area above the Earth’s surface by the air molecule’s weight. All over the earth the Atmospheric Pressure is not uniformly distributed.