NCERT Solutions for Class 11 Geography Chapter 7: Composition and Structure of Atmosphere (NCERT 2026–27)

These Class 11 Geography Chapter 7 solutions cover Composition and Structure of Atmosphere from Fundamentals of Physical Geography, the first chapter of Unit IV (Climate). The chapter explains what the atmosphere is made of — gases, water vapour and dust particles — and how it is arranged in five layers (troposphere, stratosphere, mesosphere, thermosphere/ionosphere and exosphere) that differ in density and temperature. Below you get step-by-step answers to every NCERT Exercises question reproduced verbatim, plus key concepts, extra practice, MCQs, Assertion–Reason questions and FAQs, all written in CBSE exam-ready style for the 2026–27 session.

Class: 11 Subject: Geography Book: Fundamentals of Physical Geography Unit: IV – Climate Chapter: 7 Session: 2026–27
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Class 11 Geography Chapter 7 – Overview

The atmosphere is the envelope of air that surrounds the Earth and is essential to the survival of all organisms. It is a mixture of gases, water vapour and dust particles. By volume, nitrogen and oxygen form the bulk of dry air, while gases present in small quantities — especially carbon dioxide and ozone — have great meteorological importance. About 99 per cent of the total mass of the atmosphere lies within 32 km of the surface. Carbon dioxide is responsible for the greenhouse effect, ozone (10–50 km up) absorbs harmful ultra-violet rays, water vapour acts like a blanket regulating temperature, and dust particles act as hygroscopic nuclei for condensation. Vertically, the atmosphere is divided into five layers by temperature: the troposphere (weather layer), stratosphere (ozone layer), mesosphere, thermosphere/ionosphere (reflects radio waves) and exosphere. The temperature, pressure, winds, humidity, clouds and precipitation that change in these layers form the elements of weather and climate.

Key Concepts & Terms

Atmosphere: the envelope of gases, water vapour and dust particles that surrounds the Earth; 99% of its mass lies within 32 km of the surface, and it can be felt only when it blows as wind.

Carbon dioxide: meteorologically important because it is transparent to incoming solar radiation but opaque to outgoing terrestrial radiation; largely responsible for the greenhouse effect. Its volume has been rising due to the burning of fossil fuels.

Ozone: found between 10 and 50 km above the surface; acts as a filter, absorbing ultra-violet rays and preventing them from reaching the Earth.

Water vapour: a variable gas that decreases with altitude and from the equator towards the poles (up to 4% by volume in warm wet tropics, under 1% in dry/cold regions); absorbs insolation, preserves radiated heat and influences stability of air.

Dust particles: tiny solid particles (sea salt, fine soil, smoke-soot, ash, pollen, meteor fragments) concentrated in lower layers; act as hygroscopic nuclei around which water vapour condenses to form clouds.

Troposphere: lowermost layer, average height 13 km (8 km at poles, 18 km at equator); contains dust and water vapour; all weather occurs here; temperature falls 1°C per 165 m (the normal lapse rate).

Tropopause: the zone separating the troposphere from the stratosphere, where temperature is nearly constant (about −80°C over the equator, −45°C over the poles).

Stratosphere: extends up to 50 km; contains the ozone layer that absorbs UV radiation and shields life on Earth.

Mesosphere: extends up to 80 km; temperature falls again with height to about −100°C; its upper limit is the mesopause.

Ionosphere (thermosphere): located between 80 and 400 km; contains electrically charged ions; reflects radio waves back to Earth; temperature rises with height.

Exosphere: the highest, extremely rarefied layer that gradually merges with outer space.

Elements of weather and climate: temperature, pressure, winds, humidity, clouds and precipitation.

“Exercises” — Full Solutions

All questions below are reproduced verbatim from the NCERT textbook’s end-of-chapter Exercises section. Answers are original, written in exam-ready style.

1. Multiple choice questions.

(i) Which one of the following gases constitutes the major portion of the atmosphere? (a) Oxygen    (b) Nitrogen    (c) Argon    (d) Carbon dioxide

ANSWER (b) Nitrogen. Nitrogen makes up about 78 per cent of the volume of dry air, the largest single share, while oxygen (about 21 per cent) is the second largest. Argon and carbon dioxide are present only in very small proportions.

(ii) Atmospheric layer important for human beings is: (a) Stratosphere    (b) Mesosphere    (c) Troposphere    (d) Ionosphere

ANSWER (c) Troposphere. It is the lowermost layer where all weather phenomena (clouds, rain, fog, etc.) occur and which contains the dust and water vapour needed for life. It is the most important layer for all biological activity.

(iii) Sea salt, pollen, ash, smoke soot, fine soil — these are associated with: (a) Gases    (b) Dust particles    (c) Water vapour    (d) Meteors

ANSWER (b) Dust particles. Sea salt, fine soil, smoke-soot, ash, pollen and disintegrated particles of meteors are all examples of solid dust particles suspended in the atmosphere; they act as hygroscopic nuclei for cloud formation.

(iv) Oxygen gas is in negligible quantity at the height of atmosphere: (a) 90 km    (b) 120 km    (c) 100 km    (d) 150 km

ANSWER (b) 120 km. The proportion of gases changes in the higher layers, and oxygen becomes almost negligible at a height of about 120 km from the Earth’s surface.

(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 (d) Carbon dioxide. Carbon dioxide allows incoming short-wave solar radiation to pass through but absorbs and reflects back the outgoing long-wave terrestrial radiation, making it largely responsible for the greenhouse effect.

2. Answer the following questions in about 30 words.

(i) What do you understand by atmosphere?

ANSWER The atmosphere is the envelope of air that surrounds and is held to the Earth by gravity. It is a mixture of gases, water vapour and dust particles, and is essential for the survival of all living organisms.

(ii) What are the elements of weather and climate?

ANSWER The main elements of weather and climate are temperature, pressure, winds, humidity, clouds and precipitation. These changeable elements of the atmosphere directly influence human life on the Earth.

(iii) Describe the composition of atmosphere.

ANSWER The atmosphere is composed of gases (mainly nitrogen and oxygen, with small but important amounts of carbon dioxide and ozone), water vapour (a variable gas decreasing with altitude) and dust particles such as sea salt, soot and pollen.

(iv) Why is troposphere the most important of all the layers of the atmosphere?

ANSWER The troposphere is the most important layer because all weather and climatic changes — clouds, rainfall, fog, storms — occur in it. It contains dust particles and water vapour and is the layer essential for all biological activity.

3. Answer the following questions in about 150 words.

(i) Describe the composition of the atmosphere.

ANSWER The atmosphere is made up of three components: gases, water vapour and dust particles. Gases: By volume, nitrogen (about 78%) and oxygen (about 21%) dominate, while argon, carbon dioxide and other gases occur in small amounts. The proportion of gases changes with height — oxygen becomes negligible at about 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 opaque to outgoing terrestrial radiation, causing the greenhouse effect, and its volume is rising due to the burning of fossil fuels. Ozone (between 10 and 50 km) absorbs harmful ultra-violet rays. Water vapour: A variable gas, up to 4% by volume in warm wet tropics and under 1% in dry cold regions; it decreases with altitude and towards the poles, and acts like a blanket regulating the Earth’s temperature. Dust particles: Tiny solid particles — sea salt, fine soil, soot, ash and pollen — concentrated in the lower atmosphere; they act as hygroscopic nuclei around which water vapour condenses to form clouds.

(ii) Draw a suitable diagram for the structure of the atmosphere and label it and describe it.

ANSWER In the examination, draw a vertical column of the atmosphere divided into five horizontal layers, labelling from bottom to top: Troposphere → Tropopause → Stratosphere (with ozone layer) → Mesosphere → Mesopause → Thermosphere/Ionosphere → Exosphere, marking the approximate heights (13 km, 50 km, 80 km, 400 km) and the rise and fall of temperature. The layers can be described as follows: The atmosphere is divided vertically into five layers on the basis of temperature; density is highest near the surface and decreases with altitude. 1. Troposphere — the lowermost layer, average height 13 km (8 km at the poles, 18 km at the equator). All weather occurs here; temperature falls 1°C every 165 m. It ends at the tropopause, where temperature is nearly constant (about −80°C over the equator, −45°C over the poles). 2. Stratosphere — above the tropopause, extends up to 50 km; contains the ozone layer that absorbs ultra-violet radiation and shields life on Earth. 3. Mesosphere — extends up to 80 km; temperature again decreases with height, reaching about −100°C; its upper limit is the mesopause. 4. Thermosphere / Ionosphere — located between 80 and 400 km; contains electrically charged ions that reflect radio waves back to Earth; temperature increases with height. 5. Exosphere — the highest, extremely rarefied layer, which gradually merges with outer space.

Extra Practice Questions

Short Answer Type Questions

Q1. What is the normal lapse rate in the troposphere?

ANSWERThe normal lapse rate is the rate at which temperature falls with increasing height in the troposphere. In this layer the temperature decreases at the rate of 1°C for every 165 metres of ascent.

Q2. Why is carbon dioxide called a greenhouse gas?

ANSWERCarbon dioxide lets incoming short-wave solar radiation pass through but absorbs the outgoing long-wave terrestrial radiation and reflects part of it back to the surface. This traps heat near the Earth, warming it, which is why it is called a greenhouse gas.

Q3. What is the role of dust particles in the atmosphere?

ANSWERDust particles such as sea salt and fine soil act as hygroscopic nuclei — water vapour condenses around them to form clouds. They are concentrated in subtropical and temperate regions due to dry winds, and are carried to great heights by convectional currents.

Q4. Why does the thickness of the troposphere vary from the equator to the poles?

ANSWERThe troposphere is thickest at the equator (about 18 km) and thinnest at the poles (about 8 km). At the equator, strong convectional currents transport heat to great heights, increasing the thickness, while weak convection at the poles keeps it thin.

Q5. Why is the ozone layer important for life on Earth?

ANSWERThe ozone layer, found between 10 and 50 km in the stratosphere, absorbs the harmful ultra-violet radiation coming from the Sun. By filtering out this intense, harmful energy, it shields humans, animals and plants from its damaging effects.

Long Answer Type Questions

Q1. Explain the structure of the atmosphere with reference to temperature variation in each layer.

ANSWERThe atmosphere is divided into five layers by temperature. In the troposphere (up to 13 km on average) temperature decreases with height at 1°C per 165 m; it ends at the tropopause where temperature stays nearly constant. In the stratosphere (up to 50 km), temperature rises with height because the ozone layer absorbs ultra-violet radiation. In the mesosphere (up to 80 km), temperature again falls with height to about −100°C at the mesopause. In the thermosphere/ionosphere (80–400 km), temperature rises sharply with height, and its charged ions reflect radio waves back to Earth. Finally, the exosphere is the highest, extremely rarefied layer that merges with outer space. Thus, temperature alternately decreases and increases as we move up, which is the basis for dividing the atmosphere into these layers.

Q2. Discuss the meteorological importance of carbon dioxide, ozone and water vapour.

ANSWERCarbon dioxide is transparent to incoming solar radiation but opaque to outgoing terrestrial radiation; it absorbs and reflects part of the Earth’s heat back to the surface and is largely responsible for the greenhouse effect. Its volume is rising due to the burning of fossil fuels, raising air temperature. Ozone, found between 10 and 50 km, acts as a filter, absorbing harmful ultra-violet rays and preventing them from reaching the surface, thereby protecting life. Water vapour, a variable gas that decreases with altitude and towards the poles, absorbs part of the insolation and preserves the Earth’s radiated heat, acting like a blanket that keeps the planet neither too hot nor too cold; it also contributes to the stability and instability of air and is the source of all precipitation. Together these three minor components strongly control the Earth’s heat balance and weather.

Q3. Describe the troposphere and explain why it is the most significant layer of the atmosphere.

ANSWERThe troposphere is the lowermost layer of the atmosphere, with an average height of 13 km — about 8 km over the poles and 18 km over the equator, because strong convectional currents carry heat to greater heights at the equator. It contains most of the atmosphere’s dust particles and water vapour, and in it temperature decreases at 1°C for every 165 m of height. It is the most significant layer because all weather and climatic phenomena — clouds, rainfall, fog, storms and changes in temperature — take place within it. Its store of oxygen, water vapour and dust makes it essential for breathing, for condensation and for sustaining all biological activity. The layer ends at the tropopause, where the air temperature is nearly constant (about −80°C over the equator and −45°C over the poles). For these reasons the troposphere is the most important layer for life on Earth.

MCQs & Assertion–Reason

1. About 99 per cent of the total mass of the atmosphere is confined within a height of:

(a) 16 km    (b) 32 km    (c) 50 km    (d) 90 km

2. Ozone in the atmosphere is mainly found between:

(a) 0 and 10 km    (b) 10 and 50 km    (c) 50 and 80 km    (d) 80 and 400 km

3. The temperature in the troposphere decreases at the rate of 1°C for every:

(a) 100 m    (b) 165 m    (c) 200 m    (d) 1000 m

4. The zone separating the troposphere from the stratosphere is the:

(a) mesopause    (b) ionosphere    (c) tropopause    (d) exosphere

5. Radio waves transmitted from the Earth are reflected back by the:

(a) troposphere    (b) stratosphere    (c) mesosphere    (d) ionosphere

6. The maximum thickness of the troposphere is found over the:

(a) poles    (b) equator    (c) tropics    (d) temperate zone

7. Dust and salt particles in the atmosphere are important because they:

(a) reflect radio waves    (b) act as hygroscopic nuclei for condensation    (c) absorb UV rays    (d) cause the greenhouse effect

8. Carbon dioxide and water vapour are found up to a height of about:

(a) 50 km    (b) 90 km    (c) 120 km    (d) 400 km

9. The uppermost layer of the atmosphere, which merges with outer space, is the:

(a) mesosphere    (b) thermosphere    (c) exosphere    (d) stratosphere

10. In warm and wet tropics, water vapour may account for what share of the air by volume?

(a) 1%    (b) 2%    (c) 4%    (d) 10%

Answer key: 1-(b), 2-(b), 3-(b), 4-(c), 5-(d), 6-(b), 7-(b), 8-(b), 9-(c), 10-(c).

For each Assertion–Reason question, choose: (A) Both true and the Reason correctly explains the Assertion; (B) Both true but the Reason is not the correct explanation; (C) Assertion true, Reason false; (D) Assertion false, Reason true.

A-R 1. Assertion: The troposphere is the most important layer for biological activity.

Reason: All weather phenomena occur in the troposphere, which also contains dust particles and water vapour.

A-R 2. Assertion: Carbon dioxide is responsible for the greenhouse effect.

Reason: Carbon dioxide is transparent to incoming solar radiation but opaque to outgoing terrestrial radiation.

A-R 3. Assertion: The ozone layer is harmful to life on Earth.

Reason: The ozone layer absorbs ultra-violet radiation and shields life from its intense, harmful form.

A-R 4. Assertion: Water vapour decreases with altitude.

Reason: Water vapour is a variable gas that also decreases from the equator towards the poles.

A-R 5. Assertion: The ionosphere reflects radio waves transmitted from the Earth.

Reason: The ionosphere contains electrically charged particles known as ions.

Answer key: 1-(A), 2-(A), 3-(D), 4-(B), 5-(A).

Exam Tips & Common Mistakes

How to score full marks in this chapter

Memorise the five layers in order with their approximate heights (Troposphere 13 km → Stratosphere 50 km → Mesosphere 80 km → Thermosphere/Ionosphere 400 km → Exosphere) and whether temperature rises or falls in each. For the 150-word diagram question, always draw a clean labelled column — the diagram alone carries marks. Remember the key figures: 99% of mass within 32 km, lapse rate 1°C/165 m, ozone at 10–50 km, oxygen negligible at 120 km, CO₂ and water vapour up to 90 km. Use precise terms like tropopause, mesopause, hygroscopic nuclei and greenhouse effect to show command of the chapter.

Common mistakes to avoid

  • Writing oxygen as the most abundant gas — it is nitrogen (about 78%).
  • Confusing the lapse rate; it is 1°C per 165 m, not per 100 m or per 1 km.
  • Placing the ozone layer in the troposphere — it lies in the stratosphere (10–50 km).
  • Mixing up the layer that reflects radio waves — it is the ionosphere, not the stratosphere.
  • Forgetting that temperature rises in the stratosphere and ionosphere but falls in the troposphere and mesosphere.
  • Attempting the 150-word structure question without the labelled diagram — the diagram is compulsory.

Frequently Asked Questions

What is Chapter 7 of Class 11 Geography (Fundamentals of Physical Geography) about?

Chapter 7, Composition and Structure of Atmosphere, explains what the atmosphere is made of (gases, water vapour and dust particles), the meteorological importance of carbon dioxide, ozone and water vapour, and the five vertical layers of the atmosphere — troposphere, stratosphere, mesosphere, thermosphere/ionosphere and exosphere — classified by temperature.

Which is the most important layer of the atmosphere and why?

The troposphere is the most important layer because all weather and climatic changes occur in it, and it contains the dust particles and water vapour essential for clouds, rain and all biological activity. Its average height is 13 km, and temperature falls 1°C per 165 m within it.

How many NCERT exercise questions are there in Class 11 Geography Chapter 7?

The end-of-chapter Exercises section has three parts: five multiple choice questions, four short questions (about 30 words) and two long questions (about 150 words). All of them are reproduced verbatim and solved step by step on this page.

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