Composition and Structure of Atmosphere – Class 11 Geography notes, important questions and MCQs

Chapter overview

The atmosphere is a mixture of gases, water vapour and dust particles surrounding the Earth. Nitrogen (about 78%) and oxygen (about 21%) make up most of the dry air, while argon, carbon dioxide and other gases are present in small amounts. The atmosphere is divided into layers based on temperature: the troposphere, stratosphere, mesosphere, thermosphere and exosphere. Weather occurs mainly in the troposphere, and the ozone layer in the stratosphere protects life from ultraviolet radiation.

Composition of the atmosphere

• Nitrogen forms about 78% and oxygen about 21% of dry air by volume; argon is about 0.93% and carbon dioxide about 0.04%.
• Carbon dioxide is important for photosynthesis and absorbs outgoing heat; ozone, concentrated at 10–50 km, absorbs ultraviolet rays.
• Water vapour varies from nearly zero in deserts to about 4% in humid tropics; dust particles act as nuclei for condensation.

Structure of the atmosphere

• The troposphere extends to about 8 km at the poles and 18 km at the equator; temperature falls about 6.5°C per 1,000 m (normal lapse rate); almost all weather occurs here, and the tropopause marks its upper limit.
• The stratosphere extends to about 50 km and contains the ozone layer; temperature increases with height.
• The mesosphere extends to about 80 km with temperatures falling to about −100°C; the thermosphere includes the ionosphere, which reflects radio waves; the exosphere is the outermost layer.

Key terms

Troposphere

the lowest layer of the atmosphere, where weather occurs

Stratosphere

the layer above the troposphere containing the ozone layer

Ozone layer

a layer of ozone gas that absorbs ultraviolet radiation

Ionosphere

a part of the thermosphere with charged particles that reflect radio waves

Normal lapse rate

the fall in temperature with height, about 6.5°C per 1,000 m

Important questions with answers

Quick revision

• Gases, water vapour, dust.
• Troposphere: weather; lapse rate 6.5°C/km.
• Stratosphere: ozone.
• Mesosphere, thermosphere (ionosphere), exosphere.

Exam tips

• Draw the layers of the atmosphere with heights and temperature trends.
• Remember: N2 78%, O2 21%, Ar 0.93%, CO2 about 0.04%.

Practice MCQs

1. The most abundant gas in the atmosphere is

   1. oxygen
   2. nitrogen
   3. carbon dioxide
   4. argon
   Show answer

   Answer: (b) nitrogen. It forms about 78% of dry air.

2. Weather phenomena occur mainly in the

   1. stratosphere
   2. exosphere
   3. mesosphere
   4. troposphere
   Show answer

   Answer: (d) troposphere. It contains most water vapour.

3. The ozone layer is found mainly in the

   1. troposphere
   2. exosphere
   3. thermosphere
   4. stratosphere
   Show answer

   Answer: (d) stratosphere. It absorbs ultraviolet rays.

4. The normal lapse rate is about

   1. 1°C per 1,000 m
   2. 10°C per 100 m
   3. 6.5°C per 1,000 m
   4. 2°C per km
   Show answer

   Answer: (c) 6.5°C per 1,000 m. Temperature falls with height in the troposphere.

5. The layer that reflects radio waves back to the Earth is the

   1. ozonosphere
   2. tropopause
   3. troposphere
   4. ionosphere
   Show answer

   Answer: (d) ionosphere. It lies in the thermosphere.

6. The height of the troposphere is greatest at the

   1. poles
   2. tropic of Capricorn only
   3. 60° latitude
   4. equator
   Show answer

   Answer: (d) equator. Strong convection carries heat higher.

7. Why are dust particles important in the atmosphere

   1. they act as nuclei on which water vapour condenses
   2. they produce oxygen
   3. they absorb all ultraviolet rays
   4. they form the ozone layer
   Show answer

   Answer: (a) they act as nuclei on which water vapour condenses. Clouds need condensation nuclei.

8. In the stratosphere temperature

   1. decreases with height
   2. equals surface temperature
   3. stays at −100°C
   4. increases with height because ozone absorbs ultraviolet radiation
   Show answer

   Answer: (d) increases with height because ozone absorbs ultraviolet radiation. Ozone heats this layer.