COMPOSITION AND STRUCTURE OF ATMOSPHERE  

Atmosphere And Its Importance  

i). Atmosphere is a mixture of different gases and it envelops the earth all round. It contains life-giving gases like oxygen for humans & animals and carbon dioxide for plants.  

ii). Air is essential to the survival of all organisms.  

iii). The average height of the Atmosphere: The air is an integral part of the earth’s mass and 99 percent of the total mass of the atmosphere is confined to the height of 32 km from the earth's surface. The air is colourless and odourless and can be felt only when it blows as wind.  

iv). In the absence of Ozone, life is not possible on the earth surface.

Composition Of The Atmosphere  

i). The atmosphere is comprised of several gases, dust particles, and water vapour.    

ii). The presence of oxygen becomes insignificant at the height of 120 km from the surface of the earth with respect to the composition of the atmosphere.    

iii). Water vapour and Carbon dioxide occur only up to 90 km from the surface of the earth.  

iv). Permanent gases of the atmosphere: Nitrogen(78.08), Oxygen(20.95), Argon(0.93), Carbon dioxide(0.036) and others.  

  

Gases  

Carbon dioxide: It is meteorologically a very important gas as it is transparent to the Incoming solar radiation but opaque to the outgoing terrestrial radiation. It absorbs a part of terrestrial radiation and reflects back some part of it towards the earth’s surface. It is largely responsible for the greenhouse effect. The volume of carbon dioxide has been rising in the past few decades mainly because of the burning of fossil fuels. This has also increased the temperature of the air. 

Ozone:  It is another important component of the atmosphere found between 10 and 50 km above the earth’s surface and acts as a filter and absorbs the ultra-violet rays radiating from the sun and prevents them from reaching the surface of the earth.  

  

Water Vapour  

i) Water vapour is a variable gas, declines with altitude.   

ii) It also drops towards the poles from the equator.    

iii) It acts like a blanket letting the earth to neither to become too hot nor too cold.    

iv) It also contributes to the stability and instability in the air.  

  

Dust Particles  

i) Dust particles are in higher concentration in temperate and subtropical regions due to dry winds in contrast to the Polar Regions and equatorial regions.  

ii) They act as hygroscopic nuclei over which water vapour of atmosphere condenses to create clouds. 

Structure Of The Atmosphere  

i) The atmosphere consists of different layers with varying density and temperature. Density is highest near the surface of the earth and decreases with increasing altitude.  

ii) The column of atmosphere is divided into five different layers depending upon the temperature condition. They are: Troposphere, Stratosphere, Mesosphere, Thermosphere and Exosphere.  

  

  

Troposphere  

i) It is considered as the lowest layer of Earth’s atmosphere.  Its average height is 13 km and extends roughly to a height of 8 km near the poles and about 18 km at the equator.  

ii) Thickness of the troposphere is greatest at the equator because heat is transported to great heights by strong convectional currents.  

iii) All weather occurs within this layer. This layer has water vapour and mature particles.    

iv) Temperature decreases at the rate of 1 degree Celsius for every 165 m of height. 

v) Tropopause separates the Troposphere and Stratosphere. The air temperature at the tropopause is about minus 80 degree celsius over the equator and about minus45 degree celsius over the poles. The temperature here is nearly constant, and hence, it is called the tropopause.  

  

Stratosphere  

i) It is found above the tropopause and extends up to a height of 50 km.  

ii) This layer is very dry as it contains little water vapour.    

iii) This layer provides some advantages for flight because it is above stormy weather and has steady, strong, horizontal winds.    

iv) The ozone layer is found in this layer. The ozone layer absorbs UV rays and safeguards earth from harmful radiation.    

v) Stratopause separates Stratosphere and Mesosphere. 

Mesosphere  

i) The mesosphere lies above the stratosphere, which extends up to a height of 80 km.   

ii) In this layer once again, temperature starts decreasing with the increase in altitude and reaches up to minus 100°C at the height of 80 km.  

iii) The upper limit of the mesosphere is known as the mesopause.  

  

Thermosphere  

i) This layer is found above Mesopause from 80 to 400 km.    

ii) Radio waves which are transmitted from the earth are reflected back by this layer.   

iii) The temperature increases with height.  

iv) Aurora and satellites occur in this layer.  

v)  Ionosphere: The lower thermosphere is called the Ionosphere. The Ionosphere consists of electrically charged particles known as ions. This layer is defined as the layer of the atmosphere of Earth that is ionized by cosmic and solar radiation. It is positioned between 80 and 400 km above the Mesopause.  

  

Exosphere  

i) It is the outermost layer of the atmosphere. 

ii) The zone where molecules and atoms escape into space is mentioned as the exosphere.    

iii) It extends from the top of the thermosphere up to 10,000. 

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