by Ranjana Banerjee

Created on: November 14, 2008   Last Updated: November 18, 2008

Air is the essential source for almost all living being. It is around us above us below us in one word it is wrapping the whole planet. The Greek Philosophers once considered it as one of the four basic elements. It is not too long, when early chemists began to learn that air was not an element neither a compound instead a mixture of different gases though the process of understanding these gases individually and also to learn their correlation took several more decades.

Lavoisier was the first to prove that air is composed mainly of two gases with his theory of combustion in 1770. The gas he named Azote is known today as Nitrogen identified by Daniel Rutherford and the reactive one in Lavoisier's experiment was credited to Presley for discovering and naming it as Oxygen. In late 1700 Cavendish confirmed the composition of air as78% of nitrogen and 21% of oxygen with traces of carbon dioxide, argon, water vapor and other elements; he also proved by performing hundred more samples to confirm that the composition of air was the same irrespective of its geographic origin.

Alas it now seems to be a fairy tale, as modern science invented tools to rule over nature and Cavendish's sample became an extinct and air lost its purity making its composition a variable one regarding on the quality of locality and also other geological factors.

However the standard composition of dry air by volume is as below:

Nitrogen: 78.08%, Oxygen 20.95%, Argon 0.93%, Carbon Dioxide 0.38%, Neon 0.0018%,Helium 0.0005%, Krypton 0.0001%, Hydrogen 0.00005%, Xenon 0.0000087%.

There is also trace amount of water vapor or moisture varying primarily with temperature. It would be noteworthy that the average air temperature diminishes at the rate of 6 degree c for each 100.00 m vertical height. Now let's see why and how the composition differs. To understand the diverse nature of air we must take a quick tour through our Earth's atmosphere.

Our Earth's atmosphere is composed of several layers:

First layer is called troposphere. This is our neighbor layer and the air we breathe in flows here. This is the layer where all weather takes place. It is the region of rising and falling packets of air. It is closest to the Earth's surface and of air approximately 10 to 15 kilometers thick that is constantly in motion. The conditions in this layer determine practically all of the Earth's weather patterns. "Tropos," in Greek means "turning" or "mixing." The constant motion in this layer is significant in understanding

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