Vayu¹ (Air) the gaseous envelope that surrounds the earth with sufficient gravity to maintain it. On earth, air protects life forms from harmful radiation and cosmic debris. It is a mixture of gases, the most abundant of which are nitrogen and oxygen. The composition of the air is not constant. The principal variable components are moisture and carbon dioxide. The gaseous constituents of the air are nitrogen 78.09%, oxygen 20.95%, argon 0.93%, carbon dioxide 0.03%, and minute traces of neon, helium, methane, krypton, hydrogen, xenon, and ozone. The ozone concentration is highly variable. Ozone is both produced and destroyed by absorption of ultraviolet and infrared radiation. Most of it is found at great altitudes.

It is important in meteorology largely because it absorbs ultraviolet and infrared radiation. Carbon dioxide in the air reduces the loss of heat by infrared radiation from the earth. The widespread burning of fossil fuels (eg coal and oil) increased the carbon dioxide content of the air by about 12% during the first half of the 20th century. This, in turn, caused an increase in the average temperature of the earth by 1.1°C during the same period.  [Md Mahbub Murshed]