Atmosphere: Composition and Layers

Earth’s atmosphere is mainly consisted of nitrogen, oxygen, and argon, which together constitute the major gases of the atmosphere. The remaining gases are often referred to as trace gases. The below table shows the composition of Dry atmosphere.

Composition of Earth’s Atmosphere 
Nitrogen (N2)78.08%
Oxygen (O2)20.95%
Argon (Ar)0.93%
Carbon dioxide (CO2)0.04%
Neon (Ne)0.00%
Helium (He)0.00%
Methane (CH4)0.00%
Krypton (Kr)0.00%
Hydrogen (H2)0.00%
Nitrous oxide (N2O)0.00%
Carbon monoxide (CO)0.00%
Xenon (Xe)0.00%
Ozone (O3)0 to 7×10−6%
Nitrogen dioxide (NO2)0.00%
Iodine (I2)0.00%
Ammonia (NH3)trace

The upper boundary of the atmosphere is not clearly defined. For differentiation of aeronautics and astronautics, the Kármán line at 100 kilometers from sea level is used.  Below around 100 kilometers or so, the atmosphere behaves like a fluid. The outermost layer of Earth’s atmosphere is mainly composed of hydrogen and helium. The particles are so far apart that they can travel hundreds of kilometers without colliding with one another. Since the particles rarely collide, the atmosphere no longer behaves like a fluid. These free-moving particles follow ballistic trajectories and may migrate into and out of the magnetosphere of the Earth. 

The atmosphere has been divided into several layers on the basis of change in height and some other factors such as change in climate etc. These include the Troposphere (the lowermost), Stratosphere (stratified), Mesosphere, Thermosphere, Exosphere (outer space). Between individual spheres there are usually distinguished transitory layers, called ‘PAUSES’ where temperature varies but little with height The character and composition of the atmosphere changes as we go higher and higher. Thus, there are  4 important spheres, with 3 pauses as follows:

  • Troposphere with tropopause
  • Stratosphere with stratopause
  • Mesosphere with mesopause, and
  • Ionosphere or thermosphere.