Atmosphere denotes the gaseous envelope surrounding the planets of the solar system. This may divided into layers. This plays a significant role for each planet. Likewise atmosphere of earth is divided into some layers. Each has its different characteristics and effectiveness. Atmosphere absorbs parts of solar energy and maintain the balance of temperature on the earth's surface.


Origin of the Word 'Atmosphere'

The word has originated from ancient Greek 'atmos' meaning 'vapour' and 'sphaira' meaning 'ball, globe', and modern Latin word 'atmosphaera'.

Layers of Earth's Atmosphere

Atmosphere extends vertically almost upto 1,000 km. from the erath surface. This total gaseous layer is not present with uniform characteristics at all. There are some specific criteria upon which the layering of the earth's atmosphere depends on. The basic divisions of atmospheric layers is based on temperature differentiation.

Considering the temperature differentiation, earth's atmosphere is classified into five divisions, namely, Troposphere, Stratosphere, MesosphereThermosphere, and Exosphere.  Description of these layers are mentioned in the following :

1. Troposphere

It is the lowermost layer of atmosphere adjacent to the earth's surface. Greek word 'tropos' means 'mixing'. Characteristics of this layer are as follows:
  • Troposphere extends almost upto 10-11 km. from earth surface.
  • Here mixing of almost all gaseous elements, dust particles, water vapour, etc. is observed.
  • In troposphere, as the altitude increases, temperature decreases alongside. The normal lapse rate is 6.4°C/1 km. For which, at the extreme portion of troposphere, atmospheric temperature drops at about –70°C.
  • Inversion of temperature is also observed in this region of atmosphere.
  • All types of meteorological disturbances (e.g., cyclones, low-pressure areas, depression, etc.) are also observed.
  • The extreme portion of troposphere is called 'Tropopause' where the stability of temperature prevails.
Note to the fact, the term 'Troposphere' was first used by famous scientist Tesserence-de-Bost.

2. Stratosphere

Second layer of atmosphere, extends upto almost 50 km. from earth surface. Characteristics of this layer are as follows:
  • In stratosphere, temperature increases alongside the altitude increases. Temperature becomes around 0°C in the extreme portion of stratosphere, whereas it is –70°C in the tropopause.
  • There is a dense layer of ozone gas in the stratosphere, between 15 to 35 km. altitude. This layer is called 'Ozone Layer'. Incoming solar radiation and ultraviolet rays are absorbed in the ozone layer. For this reason, temperature increases rapidly in stratosphere.
  • Stratosphere is quite stable. Meteorological disturbances are not observed in this layer. For which the aeroplanes and jets go through this atmospheric layer.
  • Sometimes water vapour enters into the stratosphere due to the weather disturbances in troposphere and creates cirrus cloud. As this type of cloud does not contain dust particles, it cannot occur  any type of precipitation. This clouds are called 'Mother of Pearl'.
  • The top limit of stratosphere is called 'Stratopause' where the stability of temperature and wind pressure prevails.
Upward from the stratopause, temperature again starts to decrease.

Fig. : Classification of atmospheric layers based on temperature differentiations

3. Mesosphere

Mesosphere is the third atmospheric layer of earth. Greek word 'meso' means 'middle'. This layer extends approximately middle portion to the total extent of atmosphere. Characteristics of this layer are as follows:
  • Mesosphere extends almost upto 80 km. from earth surface.
  • In mesosphere, as the altitude increases, temperature decreases alongside. At the extreme portion of mesosphere, atmospheric temperature becomes about –90°C. Even temperature of –110°C have been reported.
  • Mesosphere is the coldest layer among other atmospheric layers.
  • Here the air pressure remains very low (almost 0.01 milibar).
  • Turbulences and atmospheric waves develop in the mesosphere.
  • Mesosphere is one of the most important layer of earth's atmosphere as the meteorites disintegrate before clashing with earth surface.
  • The upper limit of mesosphere is called 'Mesopause'. It is an isothermal region, where the temperature does not increase or even decrease.
  • Basically mesosphere is the end of Homosphere. (It is important to note that, based on chemical composition, earth's atmosphere is divided into two divisions as Homosphere, and Heterosphere).


4. Thermosphere

Thermosphere is the fourth atmospheric layer of earth, located above the mesosphere and below the exosphere. Followings are the main characteristics of thermosphere :
  • This layer extends from mesopause at 80 km. to approximately 480 or 500 km.
  • Thermosphere is characterized by rapidly temperature increase, reaching 500°C at 120 km. altitude, 700°C at 200 km. and almost 1232°C at 480 km. Actually its temperature changes with the effect of solar activity.
  • Ionization and the chemical reactions of the gaseous elements take place in this layer due to the presence of 𝛾-ray and x-ray from solar wind.
  • At an altitude of 90 to 400 km. effect of ionization process is the most. Radio waves reflect from this layer and back to the earth surface because of the presence of electrons in thermosphere.
  • Priority of nitrogen molecules at 90–120 km., and oxygen atoms at 200–1100 km. can be noticed.
  • In thermosphere at the polar regions of the earth, ionized particles which develop due to the effect of solar wind are trapped in the magnetic field of this layer, giving rise to the 'Aurora' (a type of glow in the sky). In the north pole it is called 'Aurora Borealis', whereas in the south pole, it is called 'Aurora Australis'.
  • The boundary between thermosphere and the next layer exosphere is called 'Thermopause'. It is also an isothermal region, located between 480 to 500 km.

5. Exosphere

Outer boundary of earth's atmosphere. The following characteristics of exosphere can be observed:
  • Exosphere extends from 500 km. to almost 1,000 km.
  • It is assumed that the ionized nitrogen and oxygen layer of Heterosphere is included in exosphere.
  • This layer acts as a boundary between earth's atmosphere and outer space. So, exosphere is considered as the part of the space.
  • Presence of gaseous molecules are very less than other atmospheric layers.
  • From exosphere, the gaseous molecules, ions, atoms dissipate into the space.

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