Earth’s Atmosphere: Definition, Layers, Composition

What is the Atmosphere?

  • An atmosphere is a layer of gases surrounding a planet or other material body of sufficient mass that is held in place by the gravity of the body.
  • The envelope of air that completely surrounds the earth is known as the atmosphere. The atmosphere extends to about 1000 from the surface of the earth. But 99% of the total mass of the atmosphere is found within 32 km. This is because the atmosphere is held by the gravitational pull of the earth.


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  • Nitrogen – 78%
  • Oxygen – 20.93% ~ 21%
  • Argon -0.93%
  • Carbon-dioxide – 0.03%
  • Neon – 0.0018%
  • Helium – 0.0005%
  • Ozone – 0.0006%
  • Hydrogen – 0.00005%

Carbon dioxide is present in small quantity in the atmosphere

It is an important constituent of air because it has the ability to absorb heat and thus keep the atmosphere warm, thereby, balancing the heat of the earth.

Dust intercepts and reflects incoming insolation.

The polluted particles present in the air not only absorb larger amount of insolation but also greatly absorb the terrestrial radiation. Dust in the atmosphere contributes to the red and orange color of sunrise and sunset.

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Layers of the Atmosphere

There are five distinct layers of the atmosphere –

  1. Troposphere
  2. Stratosphere
  3. Mesosphere
  4. Thermosphere / Ionosphere
  5. Exosphere


  • This is the first layer of the atmosphere.
  • It extends to a height of 18 km at the equator and 8 km at the poles.
  • In this layer, temperature decreases with height. This is due to the fact that the density of air decreases with height and so the heat absorbed is less.
  • It contains more than 90% of the gases in the atmosphere. Since most of the water vapor forms clouds in this layer, all-weather changes occur in the troposphere(“Tropo” means “change”).
  • The height at which the temperature stops decreasing is called Tropopause. Here the temperature may be as low as -58 degrees Celsius.


  • This is the second layer of the atmosphere.
  • It extends from the tropopause to about 50 km.
  • Temperature increases due to the absorption of the ultraviolet radiation of the Sun by Ozone present in this layer.
  • The temperature slowly increases to 4 degrees Celsius.
  • This layer is free from clouds and associated weather phenomena. Hence, it provides ideal flying conditions for large jet planes.


  • Above the stratosphere lies the Mesosphere.
  • The mesosphere extends to a height of 80 km.
  • Here the temperature decreases again, falling as low as -90 degrees Celsius. The end of this layer is known as the Mesopause.


  • This layer extends to a height of about 640 km.
  • This increase in temperature is due to the fact that the gas molecules in this layer absorb the X-rays and Ultraviolet radiation of the Sun.
  • The electrically charged gas molecules of the thermosphere reflect radio waves from the Earth back into space.
  • Thus, this layer also helps in long-distance communication.
  • The Thermosphere also protects us from meteors and obsolete satellites because its high Temperature burns up nearly all the debris coming towards the Earth.


  • The Exosphere extends beyond the Thermosphere up to 960 km.
  • It gradually merges with interplanetary space.
  • The temperatures in this layer range from about 300 degrees Celsius to 1650 degrees Celsius.
  • This layer contains only traces of gases like oxygen, nitrogen, argon, and helium because the lack of gravity allows the gas molecules to escape easily into space.

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