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Layers of Atmosphere

The atmosphere can be divided into different layers according to composition, density, pressure and temperature variations.

Based on Composition:

According to its composition, broadly it is divided into two layers – homosphere and heterosphere.

Homosphere:

  • It extends from the earth’s surface up to the altitude of 80km. even though the atmosphere rapidly decreases in density with increasing altitude, the composition of the gases remains uniform in the homosphere.
  • The exceptions in the homosphere are the concentration of Ozone (O3) in stratosphere from almost 19-50 km and variation of water vapour and dust particles in the lower atmosphere.
  • This uniform composition was attained approximately 600 million years ago.

Heterosphere:

  • the gases in this layer are not evenly mixed.
  • It begins over 80km and extends upto 10,000 km. however, for all scientific purposes the upper limit of atmosphere is taken as 480km as earth’s gravitational pull becomes negligible after it.
  • The atmosphere above it is called exosphere and it contains individual atoms of light gases like hydrogen, helium etc.

Based on Change in temperature:

On the basis of change in temperature the atmosphere is broadly divided into four layers:

Troposphere:

  • It is the lower most layer of atmosphere.
  • It extends up to 18km at equator, 13 km at mid latitude and about 8km at poles.
  • It contains approximately 90% of the total mass of the atmosphere.
  • The entire weather phenomenon takes place in this layer.
  • It contains all the water vapour, dust particles, clouds etc.
  • In troposphere the temperature decreases with increase in height.
  • The average rate of decrease of temperature with height is called normal lapse rate and it is equal to 6.4o C/km. the rate of decrease of temperature is not constant everywhere.
  • The local rate of decrease is called local lapse rate.
  • The minimum temperature attained in this layer is -57 degree C.

Stratosphere:

  • It lies above the troposphere and extends uniformly across the globe up to 50km.
  • In this layer the temperature increases with increase in height.
  • The temperature varies from -57 to 0 degree C.
  • This layer is characterized with the presence of Ozonosphere.
  • Ozone is highly reactive oxygen molecule made up of three atoms.
  • Ozone absorbs the high frequency ultra violet radiations.
  • Because of this absorption the temperature of the layer increases.
  • The energy absorbed is used in chemical reactions causing the formation of ozone gas.
  • Ultra violet rays are highly harmful for living organism including plants, animals as well as humans.
  • Absorbing these radiations ozone layers makes a protective layer around us.

Mesosphere:

  • The mesosphere extends from 50 – 80 km.
  • The temperature again decreases in this layer and reaches its minimum mark averaging -90o C.
  • Although this temperature can vary.
  • The homogenous layer extends up to the mesosphere.
  • At the upper boundary of mesosphere there exists a layer of ions extending in the other layer.
  • This layer of ions or charged particles is helpful in reflecting the radio waves and helps in telecommunication.

Thermosphere:

  • This is a region extending from 80km to 480km.
  • It contains the functional ionosphere.
  • The temperature rises very sharply in this layer as the gas molecules absorbs the short wave radiations coming from the sun.
  • The temperature can reach as high as 1200o C, but despite such high temperature the thermosphere is not as ‘hot’ as we expect it to be.
  • As the density of air is so low in this layer, the energy is not easily transferred; hence the hotness is not felt.

Ionosphere:

  • This is the zone containing charged particles called ions.
  • It lies from upper mesosphere to thermosphere.
  • The charged particles are ionized by absorption of cosmic rays, gamma rays, X-rays and shorter wavelengths of ultraviolet rays.
  • It is in this layer that incoming space vehicles and meteorites begin to heat due to friction.
  • Above this layer i.e. above 480km, atomic oxygen is prevalent and beyond that first helium is more common and then hydrogen atoms predominate.

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