Many gases are found in the atmosphere but it can be considered to be composed of mainly nitrogen and oxygen gases. A rough estimate of the composition of the atmosphere is given in the following table:
S.N. Gases | Percentage |
1. Nitrogen (N2) | 78.08 |
2. Oxygen (O2) | 20.94 |
3. Argon (Ar) and other rare gases | Less than 1 |
4. Carbon dioxide (CO2) | 0.03 |
5. Neon (Ne) | 0.0018 |
6. Neon (Ne) | 0.0005 |
7. Ozone (O3) | 0.00006 |
8. Hydrogen (H) | 0.00005 |
Other rare gases are hydrogen, helium and ozone. Besides water vapour, dust particles, smoke, salts and other impurities are also present in the air, in varying quantities. As a result the composition of air is never constant. It varies from time to time and place to place. However, if these variable elements are removed from the atmosphere its make-up would be fairly constant all over the earth, at least in the lower atmosphere.

Of the many constituents carbon dioxide, dust particles, water vapour and ozone are of great importance for the earth's climate conditions.
The nitrogen does not easily enter into chemical union with other substances, but gets fixed into the soil. It serves mainly as diluent or dissolves. It regulates combustion. Oxygen on the other hand combines with all the elements and is most combustible. Carbon dioxide is an important gas in the atmospheric process. It can absorb heat and thus allows the lower atmosphere to be warmed up by heat radiation coming from the sun and from the earth's surface. Green plants in the process of photosynthesis utilise carbon dioxide from the atmosphere.