Class | Subject | Book | Chapter | Session | Questions covered |
|---|---|---|---|---|---|
XI | Geography | Fundamentals of Physical Geography (NCERT) | 7 – Composition and Structure of Atmosphere | 2026-27 | 11 of 11 |
Chapter at a glance
Topic | Key points |
|---|---|
Gases | Nitrogen 78.08 %, oxygen 20.95 %, argon 0.93 %, carbon dioxide ~0.04 %, plus neon, helium, ozone, hydrogen |
Water vapour | Up to 4 % in warm, wet tropics; decreases with altitude; absorbs heat |
Dust particles | Sea salt, pollen, ash, smoke, soot, fine soil — act as condensation nuclei |
Layers | Troposphere (weather; ~8 km at poles, ~18 km at equator), stratosphere (ozone), mesosphere (up to 80 km), thermosphere/ionosphere (80–400 km), exosphere |
Elements of weather and climate | Temperature, pressure, winds, humidity, clouds, precipitation |
Exercise Questions
Question 1 (i)
Which one of the following gases constitutes the major portion of the atmosphere?
(a) Oxygen (b) Nitrogen (c) Argon (d) Carbon dioxide
Answer
- Nitrogen
Question 1 (ii)
Atmospheric layer important for human beings is:
(a) Stratosphere (b) Mesosphere (c) Troposphere (d) Ionosphere
Answer
All weather phenomena occur in the troposphere, and it contains most of the air, water vapour and dust.
- Troposphere
Question 1 (iii)
Sea salt, pollen, ash, smoke soot, fine soil — these are associated with:
(a) Gases (b) Dust particles (c) Water vapour (d) Meteors
Answer
- Dust particles
Question 1 (iv)
Oxygen gas is in negligible quantity at the height of atmosphere:
(a) 90 km (b) 100 km (c) 120 km (d) 150 km
Answer
- 120 km
Question 1 (v)
Which one of the following gases is transparent to incoming solar radiation and opaque to outgoing terrestrial radiation?
(a) Oxygen (b) Nitrogen (c) Helium (d) Carbon dioxide
Answer
Carbon dioxide lets short-wave solar radiation pass but absorbs long-wave terrestrial radiation — the greenhouse effect.
- Carbon dioxide
Question 2 (i)
What do you understand by atmosphere?
Answer
The atmosphere is the envelope of air — a mixture of gases, water vapour and dust particles — that surrounds the earth and is held by gravity. It extends to thousands of kilometres but is densest near the surface. It provides oxygen for life, protects from harmful ultraviolet rays and keeps the earth warm.
The layer of air (gases, water vapour, dust) surrounding the earth, held by gravity, which sustains life and regulates temperature.
Question 2 (ii)
What are the elements of weather and climate?
Answer
The main elements are temperature, pressure, winds, humidity, clouds and precipitation.
Temperature, pressure, winds, humidity, clouds and precipitation.
Question 2 (iii)
Describe the composition of atmosphere.
Answer
The atmosphere consists of gases (nitrogen 78 %, oxygen 21 %, argon 0.93 %, carbon dioxide about 0.04 %, and traces of neon, helium, ozone, hydrogen), water vapour (0–4 % by volume) and dust particles (salt, pollen, ash, smoke, fine soil).
Gases (mainly nitrogen and oxygen, with argon, CO₂ and traces of others), variable water vapour and dust particles.
Question 2 (iv)
Why is troposphere the most important of all the layers of the atmosphere?
Answer
The troposphere is the lowest layer (up to 8 km at the poles, 18 km at the equator). It contains about three-quarters of the air mass, almost all the water vapour and dust, and all weather phenomena — clouds, rain, storms — occur here. Life exists within it.
Because it contains most of the air, water vapour and dust, all weather occurs in it, and it supports life.
Question 3 (i)
Describe the composition of the atmosphere.
Answer
The atmosphere is composed of gases, water vapour and dust particles.
Gases (by volume, in dry air):
Answer
Gas | Percentage |
|---|---|
Nitrogen | 78.08 |
Oxygen | 20.95 |
Argon | 0.93 |
Carbon dioxide | about 0.04 |
Neon, helium, ozone, hydrogen, krypton, xenon, methane | traces |
- Proportions change with height; oxygen becomes almost negligible at 120 km, and carbon dioxide and water vapour are found only up to about 90 km.
- Carbon dioxide is meteorologically important: it is transparent to incoming solar radiation but absorbs outgoing terrestrial radiation, warming the atmosphere (greenhouse effect). Its share has been rising due to burning fossil fuels.
- Ozone, found mainly between 10 and 50 km, absorbs ultraviolet rays and protects life.
2. Water vapour: a variable gas — up to 4 % in warm, humid tropics, less than 1 % in dry and cold areas; decreases with altitude. It absorbs part of insolation and preserves earth’s heat (acts like a blanket) and is the source of clouds and precipitation.
3. Dust particles: sea salt, fine soil, smoke, soot, ash, pollen, meteor particles. Concentrated in the lower layers, more in subtropical and temperate regions. They act as hygroscopic nuclei around which water vapour condenses to form clouds.
Mostly nitrogen (78 %) and oxygen (21 %), with argon, CO₂ (greenhouse gas) and ozone (UV shield) in small amounts; variable water vapour (0–4 %) that supplies precipitation and retains heat; and dust particles that act as condensation nuclei.
Question 3 (ii)
Draw a suitable diagram for the structure of the atmosphere and label it and describe it.
Answer
The atmosphere has five layers based on temperature:
- Troposphere: the lowest layer; average height 13 km (about 8 km at the poles, 18 km at the equator, thicker due to convection). Contains dust and water vapour; all weather changes occur here. Temperature decreases at 1°C for every 165 m of height (normal lapse rate). The boundary is the tropopause, where temperature is about −80°C over the equator and −45°C over the poles.
- Stratosphere: up to about 50 km. Contains the ozone layer, which absorbs ultraviolet radiation; temperature rises with height. Calm conditions favour jet aircraft.
- Mesosphere: from 50 to 80 km; temperature falls with height, reaching about −100°C at 80 km (mesopause). Meteors burn up here.
- Thermosphere/Ionosphere: from 80 to 400 km; contains electrically charged (ionised) layers that reflect radio waves back to earth; temperature rises with height.
- Exosphere: the uppermost layer, with very thin air, gradually merging into space.
Five layers: troposphere (weather, temperature falls 1°C/165 m), stratosphere (ozone, to 50 km), mesosphere (to 80 km, coldest), thermosphere/ionosphere (80–400 km, reflects radio waves) and exosphere (outermost).
Topic checklist
- Composition: gases, water vapour, dust
- Importance of carbon dioxide and ozone
- Structure of the atmosphere: five layers
- Elements of weather and climate