Atmosphere of Earth:
The atmosphere consists of a blanket of gases that surrounds the earth. It absorbs harmful ultraviolet radiations from sun. Atmosphere has mass and exerts pressure. It has a total mass of 5×108 Kg. Life is not possible without atmosphere. It contains oxygen which is essential for life. Carbon dioxide in atmosphere is essential for Photosynthesis. The atmosphere is composed of various gases and water vapors. The dry air in the lower atmosphere contains nitrogen, oxygen, argon, carbon dioxide, neon, helium, hydrogen, methane, ozone, carbon monoxide etc.
The major constituents of dry air are
| Gases | % by Volume |
| Nitrogen | 78.084 |
| Oxygen | 20.946 |
| Argon | 0.934 |
| Carbon dioxide | 0.0397 |
Structure of atmosphere:
The atmosphere can be divided into five different layers based on temperature profile.
Troposphere: It is the lower most portion of the atmosphere which extends up to about 8 km at poles and 16 km at the equator with some variation due to weather. In troposphere, the decrease in temperature with increase in altitude is fairly uniform. The end of troposphere boundary is marked by tropopause. Troposphere consists of 80% of the atmosphere mass.
Stratosphere: It extends roughly from 12 km to up to 50 to 55 km above earth surface. It is separated from Troposphere by Tropopause. It contains ozone layer. It defies a layer in which temperature increases with increase in altitude. It is due to the absorption of ultraviolet radiations from the sun by ozone layer. The outer limit of stratosphere is stratopause having temperature of 0°C.
Mesosphere: It extends up to 85 km from earth and 30-35 km beyond stratosphere. In Mesosphere, the temperature decreases slowly with altitude and then sharply to a minimum of about -75°C. In mesosphere most meteorites burn upon entrance due to experience of increasing friction.
Thermosphere: This layer lies in between Mesosphere and Exosphere. It is the biggest layer in the atmosphere. It begins at 85 km from earth surface. The temperature increases with altitude to a very high value and it approaches 2000°C and even more depending upon solar activity.
Exosphere: Exosphere is the outer most layer of earth’s atmosphere. It contains lightest gases like hydrogen. It also contains some gases like atomic oxygen, helium and carbon dioxide near its base (exobase). There is no fixed boundary between the outer space and exosphere so sometimes it is considered as part of outer space.