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Layers of Atmosphere and Gases Class 11 Geography

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GeographyCBSE Class 11 · Composition and Structure of Atmosphere

Atmospheric Gases and the Layers of the Atmosphere (Class 11 Geography, Fundamentals of Physical Geography, Chapter 7)

Updated 17 October 2026 · Fact-checked

The atmosphere is a mixture of gases, water vapour and dust; 99 per cent of its mass lies within 32 km. Carbon dioxide causes the greenhouse effect, ozone (10-50 km) filters ultraviolet rays and water vapour acts as a blanket. Temperature divides it into troposphere, stratosphere, mesosphere, thermosphere with the ionosphere, and exosphere.

Understand Atmospheric Gases, Water Vapour, Dust and the Five Layers of the Atmosphere

Why this topic matters. Layer-wise facts (height, temperature, feature) are standard 3-mark items.

1. Importance. The atmosphere is a mixture of gases enveloping the earth; it holds oxygen for animals and carbon dioxide for plants. It is part of the earth's mass: 99 per cent lies within 32 km. Air is colourless and odourless and felt only as wind.

2. Gases. Proportions change with height: oxygen is almost negligible at 120 km, and carbon dioxide and water vapour occur only up to 90 km. Near the surface nitrogen is the major gas.

3. Carbon dioxide. Transparent to incoming solar radiation but opaque to outgoing terrestrial radiation, so it is largely responsible for the greenhouse effect. Its volume has risen in recent decades mainly through burning fossil fuels.

4. Ozone. Found between 10 and 50 km; it absorbs ultraviolet rays and prevents them from reaching the surface.

5. Water vapour. Variable gas, up to 4 per cent in warm wet tropics and less than 1 per cent in dry, cold deserts and polar areas; it decreases from the equator towards the poles and with height. It absorbs part of insolation and preserves radiated heat, acting like a blanket.

6. Dust. From sea salts, fine soil, smoke-soot, ash, pollen and meteor particles; concentrated in lower layers and more in subtropical and temperate regions. Dust and salt act as hygroscopic nuclei for condensation and cloud formation.

7. Layers.

LayerHeightFeatures
TroposphereAverage 13 km; 8 km poles, 18 km equatorAll weather; temperature falls 1 degree C per 165 m
TropopauseBoundaryAbout -80 C over equator, -45 C over poles; nearly constant
StratosphereTo 50 kmOzone layer absorbs ultraviolet
MesosphereTo 80 kmTemperature falls to -100 C; mesopause at top
Thermosphere (ionosphere)80-400 kmIons reflect radio waves; temperature rises
ExosphereAboveExtremely rarefied; merges into space

The troposphere is thickest at the equator because strong convectional currents carry heat to great heights.

8. Elements of weather and climate. Temperature, pressure, winds, humidity, clouds and precipitation.

Atmospheric Gases, Water Vapour, Dust and the Five Layers of the Atmosphere: key facts

Gas heights
  • 99 per cent of mass within 32 km
  • Oxygen negligible at 120 km
  • Carbon dioxide and water vapour up to 90 km
  • Ozone 10-50 km
Numbers.
Troposphere
  • Height: average 13 km, 8 km at poles, 18 km at equator
  • Temperature falls 1 C per 165 m
  • All weather occurs here
Key layer.
Layers above
Stratosphere to 50 kmozone
Mesosphere to 80 km-100 C
Ionosphere 80-400 kmradio waves
Exospheremerges with space
Order.
Tropopause temperatures
About -80 C over the equator | About -45 C over the poles
Compare.

How to answer Atmospheric Gases, Water Vapour, Dust and the Five Layers of the Atmosphere questions

Layer questions need height, temperature trend and a feature.

  1. 1Name the layer and its height.
  2. 2State the temperature trend.
  3. 3Add the key feature.
  4. 4Compare poles and equator for the troposphere.
  5. 5Link greenhouse gases to temperature.

Atmospheric Gases, Water Vapour, Dust and the Five Layers of the Atmosphere in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Blanket of vapour

Suppose a clear desert night feels far colder than a humid coastal night at the same latitude.

Which atmospheric component explains this?

Try it first, then show the solutionSolution
  1. Water vapour preserves the earth's radiated heat.
  2. Deserts have less than 1 per cent.

Answer: Water vapour acts like a blanket; the humid coast retains more heat.

Memory hook: Vapour is a blanket.

Radio far away

Suppose a radio signal sent from a city is picked up hundreds of kilometres away after bouncing back from the sky.

Which layer helps?

Try it first, then show the solutionSolution
  1. The ionosphere contains ions.
  2. It reflects radio waves back to earth.

Answer: The ionosphere, between 80 and 400 km.

Memory hook: Ions reflect radio.

Tricks to remember Atmospheric Gases, Water Vapour, Dust and the Five Layers of the Atmosphere

Short memory aids. Say each one aloud once, then use it on a question the same day.

  • T-S-M-T-E

    Troposphere, Stratosphere, Mesosphere, Thermosphere, Exosphere

    Use it when: Order of layers.

  • 8, 13, 18

    Pole, average, equator (km)

    Use it when: Troposphere height.

  • 10-50

    Ozone in km

    Use it when: Ozone.

Atmospheric Gases, Water Vapour, Dust and the Five Layers of the Atmosphere: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Tropopause · 1 mark · MCQBoard level

The tropopause is the zone:
(A) of highest temperature
(B) separating troposphere from stratosphere, with nearly constant temperature
(C) reflecting radio waves
(D) containing the ozone layer

Try it first, then show the solutionSolution
  1. Tropopause separates the layers (1 mark).

Answer: (B)

Example 2: Why the troposphere is thicker at the equator · 2 marks · Very short answerBoard level

Why is the troposphere about 18 km thick at the equator but 8 km at the poles? (2 marks)

Try it first, then show the solutionSolution
  1. Strong convectional currents at the equator (1 mark).
  2. Carry heat to great heights (1 mark).

Answer: Strong convectional currents at the equator carry heat to great heights, so the layer is thicker there.

Example 3: Greenhouse effect · 3 marks · Short answerHarder board level

Explain the role of carbon dioxide in the greenhouse effect. (3 marks)

Try it first, then show the solutionSolution
  1. Transparent to incoming solar radiation (1 mark).
  2. Opaque to outgoing terrestrial radiation, absorbing and reflecting some back (1 mark).
  3. Burning of fossil fuels is raising its volume and temperature (1 mark).

Answer: Carbon dioxide lets incoming solar radiation pass but absorbs part of the outgoing terrestrial radiation and reflects some back to the surface, causing the greenhouse effect. Burning fossil fuels has raised its volume in recent decades and has raised air temperature.

Example 4: Assertion and reason: ozone · 1 mark · Assertion-reasonHarder board level

Assertion (A): Without the ozone layer life on earth would be at risk.
Reason (R): Ozone absorbs ultraviolet rays from the sun.
(A) Both true, R explains A
(B) Both true, R does not explain A
(C) A true, R false
(D) A false, R true

Try it first, then show the solutionSolution
  1. Ozone filters ultraviolet rays (1 mark).

Answer: (A)

Example 5: HOTS: temperature profile · 5 marks · Long answerHOTS / competency

Describe how temperature changes with height from the ground to 100 km, naming each layer and stating whether temperature falls, stays constant or rises in it. (5 marks)

Try it first, then show the solutionSolution
  1. Troposphere: falls 1 C per 165 m (1 mark).
  2. Tropopause: nearly constant (-80 C equator, -45 C poles) (1 mark).
  3. Stratosphere: temperature rises through ozone absorption of ultraviolet (1 mark).
  4. Mesosphere: falls to -100 C at 80 km (1 mark).
  5. Thermosphere: rises with height (1 mark).

Answer: In the troposphere temperature falls by 1 C for every 165 m. At the tropopause it is nearly constant, about -80 C over the equator and -45 C over the poles. In the stratosphere, where the ozone layer absorbs ultraviolet radiation, temperature rises. In the mesosphere it falls again to -100 C at 80 km, and in the thermosphere, including the ionosphere, it rises with height. (The stratosphere trend is inferred from the role of ozone; NCERT states the layers and the mesosphere and thermosphere trends.)

Where students lose marks in Atmospheric Gases, Water Vapour, Dust and the Five Layers of the Atmosphere

The slips examiners see most often, and what to write instead.

  • Mistake: Swapping heights

    Equator and pole.

    Wrong working

    The troposphere is 8 km at the equator.

    Right working

    8 km at poles; 18 km at the equator.

    Do this: Warm = thick.

    Usually costs: 1 mark.

  • Mistake: Ozone at the surface

    Ozone pollution is known.

    Wrong working

    Ozone layer is at the ground.

    Right working

    Ozone layer lies between 10 and 50 km.

    Do this: Give heights.

    Usually costs: 1 mark.

  • Mistake: Weather in the stratosphere

    Layers confused.

    Wrong working

    Weather occurs in the stratosphere.

    Right working

    All weather occurs in the troposphere.

    Do this: Say troposphere.

    Usually costs: 1 mark.

Exam tips

  • Give a height for each layer.
  • Use the table for layers.
  • Mention both poles and equator for the troposphere.

Practice questions on Atmospheric Gases, Water Vapour, Dust and the Five Layers of the Atmosphere

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

  1. Q1 · 1 mark · MCQBoard level

    The ionosphere extends between:
    (A) 10-50 km
    (B) 50-80 km
    (C) 80-400 km
    (D) 400-800 km

    Hint

    Above the mesopause.

    Answer

    (C)

  2. Q2 · 2 marks · Very short answerHarder board level

    At what height does the temperature of the troposphere fall to about 13 C below the surface value, given the 1 C per 165 m rule (illustration)? (2 marks)

    Hint

    13 x 165.

    Answer

    13 x 165 = 2,145 m, so about 2.1 km.

  3. Q3 · 3 marks · Short answerHarder board level

    Distinguish between the composition of the atmosphere near the surface and at 120 km. (3 marks)

    Hint

    Oxygen negligible.

    Answer

    Near the surface nitrogen is the major gas, with oxygen next, plus water vapour and dust. By 120 km oxygen is almost negligible and carbon dioxide and water vapour occur only up to 90 km.

  4. Q4 · 3 marks · Short answerHarder board level

    Describe the importance of dust particles. (3 marks)

    Hint

    Hygroscopic nuclei.

    Answer

    Dust and salt particles act as hygroscopic nuclei on which water vapour condenses to form clouds. They come from sea salt, soil, smoke, ash, pollen and meteors, and are concentrated in the lower layers, especially in subtropical and temperate regions.

  5. Q5 · 4 marks · Short answerHOTS / competency

    Why do geographers concentrate on the troposphere and stratosphere? (4 marks)

    Hint

    Weather and ozone.

    Answer

    The troposphere holds the water vapour and dust and is where all weather and climate changes occur and where biological activity is greatest. The stratosphere contains the ozone layer that shields life from ultraviolet radiation. Higher layers are rarefied and affect weather far less.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

  1. 1.What fraction of atmospheric mass is within 32 km?Show answer

    99 per cent.

  2. 2.What is the mesopause?Show answer

    The upper limit of the mesosphere.

  3. 3.Which layer reflects radio waves?Show answer

    The ionosphere.

  4. 4.What are the elements of weather and climate?Show answer

    Temperature, pressure, winds, humidity, clouds, precipitation.

Atmospheric Gases, Water Vapour, Dust and the Five Layers of the Atmosphere: frequently asked questions

Is water vapour constant?

No, it is a variable gas.

Where is the greatest dust?

In subtropical and temperate regions among the lower layers.

What is the exosphere?

The highest, extremely rarefied layer that merges with space.