NCERT Solutions for Class 11 Geography Chapter 8 Composition And Structure of Atmosphere

Ans : (i) (b) Nitrogen
(ii) (c) Troposphere
(iii) (b) Dust particles
(iv) (b) 120 km
(v) (d) Carbon dioxide 

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


(ii) The elements of weather and climate are temperature, pressure, winds, humidity, clouds and
precipitation. These elements are subject to change and which influence human life on earth.


(iii) The atmosphere is composed of gases, water vapour and dust particles. Nitrogen constitutes 78.8%, oxygen constitutes 20.94% and argon constitutes 0.93%. Both gases together constitute 99% of the atmosphere. Other gases include are Carbon dioxide, Neon, Helium, Krypto, Xenon and Hydrogen.


(iv) Troposphere is the most important of all the layers of the atmosphere:
 
 All changes in climate and weather take place in this layer.
This layer contains dust particles and water vapour
All biological activities take place in this layer.
 

Ans : (i) The atmosphere is composed of gases, water vapour and dust particles. The given table specifies the constituent of atmosphere with their volume.



Nitrogen and Oxygen gases together constitute 99% of the atmosphere. Other gases include are
Carbon dioxide, Neon, Helium, Krypto, Xenon and Hydrogen. The proportion of gases changes in the higher layers of the atmosphere in such a way that oxygen will be almost in negligible quantity at the height of 120 km. Similarly, carbon dioxide and water vapour are found only up to 90 km from the surface of the earth.
Carbon dioxide 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. Ozone is another important component of the atmosphere which absorbs the ultraviolet rays radiating from the sun and prevents them from reaching the surface of the earth.


(ii) 



The atmosphere consists of different layers with varying density and temperature. The column of atmosphere is divided into five different layers depending upon the temperature condition. They are: troposphere, stratosphere, mesosphere, thermosphere and exosphere.
 Troposphere: It is the lowermost layer of the 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. This layer contains dust particles and water vapour. All changes in climate and weather take place in this layer. The temperature in this layer decreases at the rate of 1 C for every 165 m of height. This is the most important layer for all biological activity.
Tropopause: The zone separating the troposphere from stratosphere is known as the tropopause. The air temperature at the tropopause is about minus 800C over the equator and about minus 450C over the poles. The temperature here is nearly constant, and hence, it is called the tropopause.
 Stratosphere: It is found above the tropopause and extends up to a height of 50 km. It contains the ozone layer. This layer absorbs ultraviolet radiation and shields life on the earth from intense, harmful form of energy.
  Mesosphere: It lies above the stratosphere, which extends up to a height of 80 km. 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. The upper limit of mesosphere is known as the mesopause.
 Thermosphere: It is the outermost layer of the atmosphere. It is located from 80 km with no definite upper limit. The air in this layer is very hot because heat coming from the sun strikes the thermosphere first. 
 Ionosphere: The lower layer of thermosphere is called ionosphere. It is located between 80 and 400 km above the mesopause. It contains electrically charged particles known as ions, and hence, it is known as ionosphere. Radio waves transmitted from the earth are reflected back to the earth by this layer. Temperature here starts increasing with height. 
 
  Exosphere: The uppermost layer of the atmosphere above the thermosphere is known as the exosphere. This is the highest layer but very little is known about it. Whatever contents are there, these are extremely rarefied in this layer, and it gradually merges with the outer space.