What is atmosphere and what is its composition?

The earth is a solid and there is a mixture of gases surrounding the earth. The mixture of gases is called air. So the mixture of gases or the air envelope which surrounds the earth is called atmosphere. The atmosphere is held to the earth by the force of gravity.

The major part of atmosphere extends up to a height of about 40 kilometers above the earth’s surface. There is about 99 percent mass of air present in the first 40 kilometer height. Above this height from the earth, the gases of air are present but in very small amount. The atmosphere helps in protecting living organisms from genetic damage by solar ultra violet radiation, solar wind and cosmic rays. The density of atmosphere decreases with height.

Composition of Atmosphere:

The atmosphere consists of a mixture of gases having a relatively uniform position in the lower layers. At the earth’s surface, the atmosphere consists mainly of two gases such as nitrogen (78%) and oxygen (21%). Thus nitrogen is the most abundant gas in the earth’s atmosphere whereas oxygen is the second most abundant gas in the atmosphere. The third most abundant gas in the earth’s atmosphere is a rare gas ‘argon’ which is about 0.9% of the air.

Apart from these major gases, the atmosphere also contains small amount of other gases such as carbon dioxide, rare gases like helium, neon, krypton, xenon, hydrogen, methane and nitrous oxide. The lower layer of atmosphere also contains water vapor and fine dust particles whereas the upper layer of atmosphere contains ozone gas.

There are several layers in the earth’s atmosphere as given below.

• Troposphere – The depth of this layer varies between 17km at equator to 7km at the poles. Three quarter of the atmosphere resides in this region.
• Stratosphere – This layer extends from top of troposphere to the bottom of mesosphere. It contains the ozone layer which ranges in altitude between 15 and 35kms. Ozone layer absorbs most of the ultraviolet radiations from the sun.
• Mesosphere – It ranges from 50 to 85km and here most of the meteors burn up.

There are other layers such as thermosphere, exosphere, and ionosphere are located above the mesosphere up to 100kms.