Air is a mixture of gases and is one of the main abiotic components of the environment. Air is an extremely important natural resource, as living organisms breathe in air. Air is found everywhere. Air in motion is called wind. It is also present in water and soil.

Composition of Air

Air is a mixture of nitrogen, oxygen, carbon dioxide, water vapour and a few other gases. Some dust particles may also be present in it. 

  • Nitrogen (N2): 78.03%
  • Oxygen (O2) 20.09%
  • Argon (Ar): 0.94%
  • Carbon dioxide (CO2): 0.033%
  • Inert gases: 0.0020%

Concentration of water in air varies drastically from location to location.

Oxygen, nitrogen and carbon dioxide are directly or indirectly useful for human beings, other animals and plants. Without oxygen and nitrogen it is impossible for living beings to survive. Water vapour also plays a very important role in our life.

Air Pressure

Air is a mixture of gases and molecules of these gases have weight due to gravity. Anything that has weight, pushes and presses against other objects. The envelop of air that surrounds earth (atmosphere) exerts a force which acts downwards on the surface of the earth.

The force of air column acting per unit area of a surface results in a pressure exerted by atmosphere. This pressure is called atmospheric pressure. The atmospheric pressure is about 1 kg cm2 or 10 ton m2.

The atoms and molecules of the gases in the atmosphere like those of all other matter are subject to earth’s gravitational pull. As a consequence, the atmosphere is much denser near the surface of earth than at higher altitudes. In fact, the density of air decreases very rapidly with increasing distance from earth. Therefore, atmospheric pressure also decreases with altitude. Often at higher altitudes, people find their nose bleeding because blood pressure of the body is much more than the pressure outside (i.e. atmospheric pressure).

Air pressure is measured by the instrument known as barometer.


The envelope of air that surrounds the earth is known as atmosphere. Atmosphere is essential for life on earth. It protects us and all living organism from harmful radiations of the sun like ultraviolet rays, etc. Atmosphere is divided into different layers according to temperature, pressure variation and composition. The main layers of the atmosphere from the surface of earth upward are:

  • Troposphere (0-10 km)
  • Stratosphere (10-50 km)
  • Mesosphere (50-85 km)
  • Thermosphere (85-500 km)

The most active region of the atmosphere is the troposphere, the layer of the atmosphere, which contains about 18% of the total mass of air and practically all the atmosphere’s water vapour. It is the thinnest layer of atmosphere and all the dramatic events of weather (such as rain) occur here.

Aquatic animals use dissolved air in water for respiration. Plants and animals depend on each other for exchange of oxygen and carbon dioxide from air.


Moving air is called the wind. Air moves from the region where the air pressure is high to the region where the pressure is low. The greater the difference in pressure, the faster the air moves.

The air exerts pressure. Increased wind speed is accompanied by a reduced air pressure.

The warm air is lighter than the cold air, hence it rises at a place. The air pressure at that place is lowered. The cold air from the surrounding areas rushes in to fill its place. This sets up convection in air.

Uneven Heating on the Earth

Uneven heating between the equator and the poles

Regions close to the equator get maximum heat from the Sun. The air in these regions gets warm. The warm air rises, and the cooler air from the regions in the 0-30 degrees latitude belt on either side of the equator moves in. These winds blow from the north and the south towards the equator.

At the poles, the air is colder than that at latitudes about 60 degrees. The warm air at these latitudes rises up and the cold wind from the polar regions rushes in, to take its place. In this way, wind circulation is set up from the poles to the warmer latitudes.

Uneven heating of land and water

In summer, the land warms up faster and most of the time the temperature of the land is higher than that of water in the oceans. The air over the land gets heated and rises. This causes the winds to flow from the oceans towards the land. These are monsoon winds. The winds from the oceans carry water and bring rain.

In winter, the direction of the wind flow gets reversed. It flows from the land to the ocean.


Thunderstorms develop in hot, humid tropical areas like India very frequently. The rising temperatures produce strong upward rising winds. These winds carry water droplets upwards, where they freeze, and fall down again. The swift movement of the falling water droplets along with the rising air create lightning and sound.


Before cloud formation, water takes up heat from the atmosphere to change into vapour. When water vapour changes back to liquid form as raindrops, this heat is released to the atmosphere. The heat released to the atmosphere warms the air around. The air tends to rise and causes a drop in pressure. More air rushes to the centre of the storm. This cycle is repeated. The chain of events ends with the formation of a very low-pressure system with very high-speed winds revolving around it.

Air Pollution

Air pollution is the introduction of harmful chemicals, biological wastes, and particulate matter into the atmosphere. Pollution has harmful effects on humans as well as on all other living beings. Pollutants can be classified into two main categories:

  1. Primary pollutants which are directly emitted into the atmosphere such as carbon monoxide from exhaust of a motor vehicle.
  2. Secondary pollutants which are not emitted directly into atmosphere but are formed in air when primary pollutants interact.

Major primary pollutants include:

Carbon monoxide (CO) is produced by incomplete combustion of fuels like petrol, natural gas, coal or wood. It is a colourless and odourless gas but very poisonous in nature.

Carbon dioxide (CO2) is produced by complete combustion of fuels in motor vehicles and various industries. It is a colourless, odourless and non-toxic gas. (A person dies in atmosphere of carbon dioxide due to lack of oxygen and not due to its toxic nature).

Sulphur oxides (mainly sulphur dioxide, SO2) are produced by combustion of coal and petroleum and also produced in volcanoes. It is also produced in various industrial processes. Oxidation of sulphur dioxide (SO2) to sulphur trioxide (SO3) results in formation of sulphuric acid (H2SO4) which causes acid rain.

Nitrogen oxides especially nitrogen dioxide, NO2 is a reddish brown gas with pungent smell. It catalyses the oxidation of SO2 to SO3 and indirectly causes acid rain.

Volatile organic compounds (VOCs) include methane, benzene, toluene and xylene. While methane is a major green house gas, others are suspected to be carcinogens (cancer inducing).

Particulate matter consists of tiny particles of solids or liquids suspended in air. These are also called ‘suspended particulate matter (SPM)’. The major sources for these include volcanoes, dust storms and burning of fuels. These can cause heart and lung diseases and breathing disorders.

Chloro-fluorocarbons (CFCs) are used as refrigerants in air conditioners and refrigerators and are harmful to the ozone layer which protect us from harmful ultraviolet rays.

Major secondary pollutants include:

Photochemical smog (smoke + fog) formed by the action of ultraviolet light from the sun on particulate matter or formed due to burning of coal and petrol in an atmosphere containing SO2. It prevents dissipation of pollutants and causes breathing disorders.

Ground level ozone (O3) is formed from NOx and VOCs. It is a constituent of smog. Normally ozone occurs in stratosphere and prevents UV radiations from reaching earth’s surface. At ground level, when inhaled, it is harmful for health of humans and animals.