Dr. Meenakshi Prasad

Assistant Professor

P.G. Deptt. of Geography

M.U, Bodh Gaya

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CONTENTS

• Definition of Atmosphere

• Composition of Atmosphere

• Structure of Atmosphere

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Definition

• Atmosphere is a thick gaseous envelope which surrounds the earth and is attached to the earth’s surface by gravitational force.

• According to a study of NASA, it extends till a height of 80,000 km above the earth’s surface

• It is estimated that 97% of the effective atmosphere is confined upto a height of 29 km above the earth’s surface.

COMPOSITION OF THE ATMOSPHERE The atmosphere is composed of gases, water vapour & gas particles.

Gases :

• 95% of the total weight of the atmosphere is made by gases

• By volume dry air contains 78.08% Nitrogen, 20.95% Oxygen, 0.93% argon, 0.035% of Carbon Dioxide & a small amount of other gases such as Methane, Hydrogen, Helium, Krypton, Xenon, Neon, Sulphur Dioxide, etc.

• Thus, Nitrogen & Oxygen are major gases which constitute almost 99% of the total gaseous composition of the atmosphere till a height of 25km from the earth’s surface.

Gaseous Composition of the Atmosphere

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• This proportion of gases exists till a height of 80 km beyond which the proportion changes & the amount of Hydrogen & Helium starts increasing.

• Infact above the height of 80 km separate layers of different gases are found among which 4 layers are important :

GASEOUS LAYE HEIGHT

1 Molecular Nitrogen Layer 90-120 km

2 Atomic Oxygen Layer 200-400 km

3 Helium Layer 1100-3500 km

4 Atomic Hydrogen Layer 3500-10,000 km

Importance of Different Gases :• Nitrogen is generally chemically inactive but it

dilutes the oxygen & makes it breathable. It is also absorbed by the plant in the form of nitrate & helps in their growth.

• Oxygen makes breathing possible & is also essential for combustion.

• Carbon dioxide is used by green plants for photosynthesis & it also regulates the temperature of the earth through green house effect.

• At ground level ozone is a pollutant but the ozone layer of stratosphere protects the earth from the Ultra Violet radiation of the Sun.

Dust Particles :

• The dust particles present in the atmosphere are divided into two types – terrestrial dust particles & cosmic dust particles

• Terrestrial dust particles are mainly found upto a height of 5 km from the earth’s surface.

• They act as hygroscopic nuclei & help in the formation of clouds

• Cosmic dust particles are found mainly above the height of 50 km & they originate due to the disintegration of celestial bodies that enter the earth’s atmosphere. They help in the formation of noctilucent clouds.

• Dust particles help in the scattering of solar radiation which adds varied charming colours of red & orange to sunrise & sunset

ROLE OF DUST PARTICLES

Formation of Clouds Scattering of Solar Radiation

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Water Vapour :

• The availability of water vapour is generally upto a height of 12 km. However, some amount of water vapour is also found above the height of 50 km in the middle part of the atmosphere. The vertical currents of the atmosphere first carries it to the stratosphere & then due to an increase in the temperature of stratosphere it reaches above the height of 50 km. After crossing the ozone layer this vapour starts condensing & forms noctilucentclouds.

• Three processes are responsible for the presence of water vapour in the atmosphere. They are –

(a)Evapoation, (b)Evapo- tranpiration and (c)Sublimation

• The water vapour is responsible for various types of weather phenomena that occur in the atmosphere, such as- formation of clouds, different forms of precipitation, storms, etc.

• Water vapour also helps in heating the atmosphere as it absorbs terrestrial radiation

STRUCTURE OF THE ATMOSPHERE

As we go up higher in the atmosphere variations occur in temperature. Based on the characteristics of temperature the vertical structure of the atmosphere can be divided into following 6 concentric layers :(i)Troposphere(ii)Stratosphere(iii)Mesosphere(iv)Ionosphere or Thermosphere(v)Exosphere(vi)Magnetosphere

Troposphere :• Lower most layer of the atmosphere

• Its average height is 8-18 km, i.e. 8 km at the poles & 18 km at the equator. The reason of greater height at the equator is the presence of convectional currents due to which the air rises up vertically to a greater height.

• All kinds of weather phenomena (e.g. fog, cloud, dew, frost, rainfall, snowfall, thunder- storms, lightning, etc.) occur in this layer.

• It is the zone of radiation, conduction & convection

• Temperature decreases with increasing height in the troposphere at the rate of 6.50C per 1000 m. This is called the normal lapse rate.

• The temperature at the upper limit of the troposphere is -600C

• Jet streams are found at the upper boundary of this layer

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Stratosphere:• It is located just above the troposphere & is

separated from it by a transition belt having a thickness of 1.5 km which is known as the tropopause.

• It extends upto a height of 50 km.

• In the lower part of the stratosphere between a height of 15-35 km, heavy concentration of ozone gas is found. This is known as the ‘ozone layer’.

• Temperature gradually increases with height in this layer due to the presence of ozone layer as ozone absorbs the UV rays coming from the sun.

• Temperature at the upper limit of the stratosphere reaches 00C

• The layer is usually devoid of atmospheric disturbances

• The Jet streams occur in the lower parts of this layer as well.

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Mesosphere:• Mesosphere extends between a height of 50-80

km

• It is separated from stratosphere by a transitional belt known as stratopause.

• Temperature decreases with increasing height in the layer

• At the upper limit of mesosphere the temperature becomes -850C to -1000C

• This is the coldest layer of the atmosphere

• Noctilucent clouds & cosmic or meteoric dust are found here

Thermosphere or Ionosphere:• This layer extends upto a height of 500 km

• It is separated from mesosphere by a transition belt known as ‘mesopause’

• Temperature increases with increasing height in this belt & at the upper limit of this belt it becomes +250C

• Ionised particles are present in this layer which are capable of reflecting back radio waves

• This layer has made radio communication & telecommunication possible on earth

• When the charged particles coming from the sun strike against the ionised particles of this layer, it creates colourfullights in the sky which can been seen from within the Arctic circle & Antarctic circle. These lights are called ‘Auroras’. When seen from within Arctic circle they are called ‘Aurora Borealis’ (Northern Lights) & when seen from within the Antarctic circle they are called ‘aurora Australis’ (Southern Lights)

AURORAS

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Exosphere :

• Exosphere extends till a height of 2000 km

• In this layer temperature increases rapidly with increasing height

• At the upper limit of this layer the temperature is recorded as 10000C

• This layer is dominated by ionised particles

• Molecules & atoms of Oxygen, Hydrogen & Helium are found in this layer.

Magnetosphere:• This sphere extends beyond the height of 2000 km till a

height of 80,000 km• Magnetosphere is the region where the temperature

continuously increases with increasing height & where the effect of earth’s gravitation decreases.

• At the upper limit of this layer temperature becomes 17000C

• This layer is dominated by charged particles, i.e. electrons & protons. A dense layer of these particles is found between the height of 3000-16,000 km.

• This layer is affected by cosmic rays, solar flares & solar storms.

• Studies have shown that solar flares & solar storms affect the temperature of magnetosphere which inturninfluence the weather changes of troposphere

Conclusion

• Thus, it is clear that a lot of variations are there in the structure of the atmosphere

• Not only that, further researches are required for the upper layers of the atmosphere

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References

• Singh, S : Physical Geography, Prayag PustakBhawan, Allahabad, 2012

• Critchfield, Howard J : General Climatology, McGraw Hill Book Co, New York,1965

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