Atmosphere

Introduction Atmosphere

  • Our earth consists mainly of three realms namely lithosphere, atmosphere, and hydrosphere.
  • Life is found on the earth in those parts only where these three realms come in contact with each other.
  • The intersection of these realms where life exists on the earth is known as Biosphere.
  • The gaseous cover of the earth is known as the atmosphere.
  • The earth is enveloped by a thick blanket of gases extending to a height of 10,000 kilometers above its surface.
  • It is held to the earth by its gravitational forces and rotates with it.
  • Near the surface of the earth, gases are dense but it gets more and more rarefied with the increase in height above sea level.
  • The density of the atmosphere is high near the surface of the earth because the number of molecules present in the air is very high.
  • As we go higher up in the atmosphere, the molecules are spread wide apart. Therefore, the air becomes rarer. Hence the upper limit of the atmosphere is estimated roughly at a height of 10,000 km.

Elements of the Atmosphere

The atmosphere is composed of all three states of matter

[a] Gases, [b] water vapour [c] dust particles.

Gases

Composition of Gases in Atmosphere

Nitrogen (N2) Oxygen (O2) Argon (Ar) Carbon dioxide (CO2) Neon (Ne) Helium (He)78.08 20.94 0.93 0.03 0.0018 0.0005Ozone (O3) Hydrogen (H2) Krypton (Kr) Xenon (Xe) Methane (Me)0.00006 0.00005 Trace Trace Trace

Water Vapour (Variable Constituents)

  • The amount of water vapour varies from place to place and from time to time.
  • It exists up to about 12 kilometres.
  • Water vapour represents about 2% of the air by volume.
  • Water vapour is added to the atmosphere by evaporation of water from seas, lakes, and rivers. other water bodies. Water vapour is also added by transpiration from the leaves of plants.
  • Though the amount of water vapour does not normally exceed 4%, it plays an important role in the atmosphere.
  • Due to the rise of temperature, the capacity of the atmosphere to hold water vapour is also increased.

Dust Particles (Variable Constituents)

  • The atmosphere also contains solid particles of dust, salt grains, pollen grains, smoke, soot. Collectively they are called aerosols. (Everywhere in the atmosphere there are substances floating about which are very tiny solid particles of dust.)

Carbon di oxide (CO2)

Carbon dioxide is an important variable gas in the atmosphere currently constituting about 0.03%. It is a potent greenhouse gas and plays a vital role in controlling the earth’s surface temperature.

An Atmospheric layer based on Chemical Composition

  • Scientists Jastrio and Nicholate of NASA [The National Aeronautics and Space Administration of USA] divided the atmosphere into two layers based on the variation in chemical composition
  • Homosphere
  • Heterosphere

Homosphere

  • The word Homo means Uniform and Sphere means Region.
  • From the earth’s surface upward to an altitude of about 80 km [Ernest S. Gates] the chemical composition of the atmosphere is highly uniform throughout in terms of the proportions of its component gases. For this, the layer is called Homosphere.
  • According to D.S. Lal, The term homosphere means the zone of homogeneous composition.
  • Up to a height of about 80 km. the mixing of the gases takes place perfectly. The troposphere, Stratosphere and Mesosphere lie in this layer of homosphere.

Heterosphere

  • The atmosphere above the homosphere up to a height of 10,000 km. is not uniform in composition.
  • This layer is non-uniform in an arrangement of spherical shells.
  • A heterogeneous character both in the proportion and chemical composition is well-marked in this layer.
  • This is why this sphere is called Heterosphere.
  • This layer is divided into the following four sub-layers
    • (I) Molecular Nitrogen layer: This layer is extended up to 200 km. above from the homosphere.
    • (II) Atomic Oxygen layer: It extends from 200 to 1100 km.
    • (III) Helium layer: It extends between 1100 to 3500 km.
    • (IV) Hydrogen layer: It extends up to 10,000 km.

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