CBSE Class 9 · Atmosphere and Climate

Layers of the Atmosphere Class 9: Composition, Heights and Features

Updated 17 October 2026 · Fact-checked

The atmosphere is a blanket of gases held by gravity: nitrogen 78 per cent, oxygen 21 per cent, argon 0.93 per cent, carbon dioxide 0.04 per cent and others 0.03 per cent. It has five layers: troposphere (about 12 km, weather), stratosphere (to 50 km, ozone), mesosphere (to 80 km, meteors burn), thermosphere (80 to 700 km, radio waves and auroras) and exosphere (the thin outer layer).

Notice that the troposphere is the thinnest layer at the bottom, where clouds and weather occur, and layers get higher and thinner going up.

Illustration drawn for QuestionBuddy.

Understand Composition and Layers of the Atmosphere

The atmosphere is a blanket of air around the Earth held in place by gravity. It does three jobs: it protects us from the Sun's harmful radiation, including ultraviolet rays; it keeps us warm by trapping part of the Sun's energy; and it makes weather and climate through temperature, humidity and pressure. We do not feel air pressing on us because the pressure inside our bodies is equal and balances it.

Composition (NCERT pie chart).

Gas	Share
Nitrogen	78 per cent
Oxygen	21 per cent
Argon	0.93 per cent
Carbon dioxide	0.04 per cent
Others (helium, neon, krypton, xenon, ozone, hydrogen)	0.03 per cent

Together they make 100 per cent. The air also holds water vapour (generally 0.1 to 0.4 per cent), which forms clouds and rain, and tiny dust particles. Composition changes with altitude.

Layers. They are defined by changes in temperature and density with height; air is densest near the ground and thins upward.

Layer	Height	Key features
Troposphere	average about 12 km	The air we breathe; most water vapour and clouds; nearly all weather; temperature falls with height. Top: tropopause.
Stratosphere	up to 50 km	Free of clouds and weather, ideal for aeroplanes; contains the ozone layer that filters harmful UV rays. Top: stratopause.
Mesosphere	up to 80 km	Temperature falls with height; most meteorites burn up here.
Thermosphere	80 to 700 km	Temperature rises rapidly; gases absorb X-rays and short-wave UV; reflects radio waves back to Earth; contains the ionosphere; auroras occur here.
Exosphere	top layer	Very thin air; light gases such as helium and hydrogen drift into space because gravity is weak.

Temperature falls with height only in the troposphere and mesosphere; it rises in the stratosphere (where ozone absorbs ultraviolet rays) and in the thermosphere.

Auroras. Charged particles from the Sun (the solar wind) are guided toward the magnetic poles; when they hit gases each gas glows in its own colour. The northern display is the Aurora Borealis and the southern the Aurora Australis.

Why the layers matter. The troposphere supports life and weather; the ozone in the stratosphere protects living things from UV; the mesosphere protects the surface from most meteorites; the thermosphere makes long-distance radio communication possible.

Composition and Layers of the Atmosphere: key facts

Composition of air: Nitrogen 78 per cent
Oxygen 21 per cent
Argon 0.93 per cent
Carbon dioxide 0.04 per cent
Others 0.03 per cent; Adds to 100 per cent.
Layers bottom to top: Troposphere about 12 km
Stratosphere to 50 km
Mesosphere to 80 km
Thermosphere 80 to 700 km
Exosphere; With boundaries tropopause and stratopause.
Temperature with height: Falls: troposphere, mesosphere | Rises: stratosphere, thermosphere; Do not generalise.
Layer and feature: Troposphere weather
Stratosphere ozone
Mesosphere meteors burn
Thermosphere radio waves, auroras
Exosphere light gases escape; Matching.
Auroras: Solar wind guided to magnetic poles
Aurora Borealis in the north
Aurora Australis in the south; In the thermosphere.

Troposphere	weather
Stratosphere	ozone
Mesosphere	meteors burn
Thermosphere	radio waves, auroras
Exosphere	light gases escape

How to answer Composition and Layers of the Atmosphere questions

Questions ask you to name layers in order, state heights, match features to layers, explain why a phenomenon occurs in a particular layer and work out thicknesses from the heights.

1Write layers bottom to top with heights; thicknesses are differences of the upper limits (for example 50 - 12).
2For a feature question, link the phenomenon to the layer (weather: troposphere; ozone: stratosphere).
3For temperature questions, remember it falls in the troposphere and mesosphere but rises in the thermosphere.
4For percentages, add them to check the total 100 per cent.
5End with a function: protection, warmth or communication.

Composition and 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.

Why a jet cruises high

Suppose a long-distance aircraft climbs above the clouds to a cruising height in the stratosphere, where the air is smooth.

Why is the stratosphere suitable for such flights?

Try it first, then show the solutionSolution

It is free of clouds and weather.
Nearly all weather occurs in the troposphere below.
So flights are smoother.

Answer: The stratosphere has no clouds and weather, so flights there are steadier. Nearly all weather phenomena take place in the troposphere below it.

Memory hook: Weather below, smooth sky above.

A radio signal at night

Suppose a radio operator finds that signals travel far across the country by bouncing back from the sky.

Which layer helps and how?

Try it first, then show the solutionSolution

The thermosphere includes the ionosphere.
It reflects radio waves back to the Earth.
This lets signals travel long distances.

Answer: The thermosphere reflects radio waves back to Earth, so signals can travel beyond the horizon.

Memory hook: The sky as a mirror for radio waves.

Tricks to remember Composition and Layers of the Atmosphere

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

Layers: T-S-M-T-E
Tiny Students Make Terrific Estimates: Troposphere, Stratosphere, Mesosphere, Thermosphere, Exosphere.
Use it when: Order of layers.
78-21
Nitrogen 78, Oxygen 21: the rest is under 1.
Use it when: Composition.
Temperature turns
Down, up, down, up: tropo falls, strato rises, meso falls, thermo rises.
Use it when: Temperature with height.

Composition and Layers of the Atmosphere: solved examples

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

Example 1: Which layer? · 1 mark · MCQBoard level

The layer that contains the ozone layer is the:
(a) troposphere
(b) stratosphere
(c) mesosphere
(d) exosphere

Try it first, then show the solutionSolution

The ozone layer that filters harmful UV rays lies in the stratosphere (1 mark).

Answer: (b) stratosphere

Example 2: Functions (2 marks) · 2 marks · Very short answerBoard level

Give two functions of the atmosphere. (2 marks)

Try it first, then show the solutionSolution

Protects from harmful radiation such as UV rays (1 mark).
Traps part of the Sun's energy to keep the Earth warm (1 mark).

Answer: The atmosphere protects living beings from the Sun's harmful radiation, including ultraviolet rays, and it traps part of the Sun's energy, keeping the Earth warm.

Example 3: Thickness of layers (3 marks) · 3 marks · Short answerHarder board level

Using the heights given in the chapter, find the thickness of the stratosphere (taking the troposphere as 12 km deep), the mesosphere and the thermosphere. Which is the thickest? (3 marks)

Try it first, then show the solutionSolution

Stratosphere: 50 - 12 = 38 km (1 mark).
Mesosphere: 80 - 50 = 30 km; thermosphere: 700 - 80 = 620 km (1 mark).
Thermosphere is the thickest (1 mark).

Answer: Stratosphere: 50 - 12 = 38 km. Mesosphere: 80 - 50 = 30 km. Thermosphere: 700 - 80 = 620 km. The thermosphere is the thickest of the three. (These are derived from the heights; the troposphere height of 12 km is an average.)

Example 4: Assertion-reason: temperature trends · 1 mark · Assertion-reasonHarder board level

Assertion (A): Temperature rises with height throughout the atmosphere.
Reason (R): In the thermosphere, gases absorb X-rays and short-wave ultraviolet radiation.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.

Try it first, then show the solutionSolution

Temperature falls in the troposphere and mesosphere, so A is false.
R is true: the thermosphere's rise is due to absorption (1 mark).

Answer: (d) A is false, but R is true.

Example 5: Compare two layers (5 marks) · 5 marks · Long answerHOTS / competency

Compare the troposphere and the stratosphere on height, temperature trend, weather and importance to life. (5 marks)

Try it first, then show the solutionSolution

Height: troposphere average about 12 km; stratosphere up to 50 km (1 mark).
Temperature falls with height in the troposphere, but rises with height in the stratosphere because ozone absorbs ultraviolet rays (1 mark).
Weather: nearly all weather, clouds and most water vapour in the troposphere; stratosphere is free of clouds and weather (1 mark).
Importance: troposphere has the air we breathe and the water for rain (1 mark).
Stratosphere holds the ozone layer filtering UV rays; ideal for aeroplanes (1 mark).

Answer: The troposphere extends to an average of about 12 km and the stratosphere to 50 km. In the troposphere temperature falls with height, whereas in the stratosphere temperature rises with height, as ozone absorbs ultraviolet rays. Nearly all weather and clouds, with most of the water vapour, are found in the troposphere, while the stratosphere has no clouds or weather. The troposphere contains the air we breathe and gives rain. The stratosphere contains the ozone layer, which filters harmful ultraviolet rays, and its calm air is ideal for aeroplanes. Together they make life and travel possible.

Example 6: Case: northern lights · 4 marks · Case-basedHOTS / competency

Read the case and answer the questions.

A traveller near the Arctic sees green and red lights moving across the night sky. A guide explains that charged particles from the Sun stream toward the Earth and are guided towards the magnetic poles, where they strike gases in the upper atmosphere.

(i) What are these lights called in the north and the south? (1)
(ii) In which layer do they occur? (1)
(iii) Why do different colours appear, and why can the same layer also help radio communication? (2)

Try it first, then show the solutionSolution

(i) Aurora Borealis (north) and Aurora Australis (south) (1 mark).
(ii) Thermosphere (1 mark).
(iii) Each gas glows in its own colour; the thermosphere (ionosphere) reflects radio waves back to Earth (2 marks).

Answer: (i) Aurora Borealis in the north and Aurora Australis in the south. (ii) They occur in the thermosphere. (iii) Different gases glow in different colours when struck by the charged particles. The same layer includes the ionosphere, which reflects radio waves back to Earth, so it also helps radio communication.

Where students lose marks in Composition and Layers of the Atmosphere

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

Mistake: Wrong order of layers
Mixing mesosphere and stratosphere.
Wrong working
Mesosphere lies below the stratosphere.
Right working
Stratosphere comes before the mesosphere.
Do this: Use the T-S-M-T-E sentence.
Usually costs: 1-2 marks.
Mistake: Percentages not adding
Careless pie chart.
Wrong working
N 78, O 22, others 1.
Right working
N 78, O 21, argon 0.93, CO2 0.04, others 0.03.
Do this: Add to 100.
Usually costs: 1 mark.
Mistake: Weather in the wrong layer
Assuming clouds extend higher.
Wrong working
Clouds form in the stratosphere.
Right working
Weather and clouds are in the troposphere.
Do this: Link weather with troposphere.
Usually costs: 1 mark.
Mistake: Temperature generalisation
One rule for all layers.
Wrong working
Temperature always falls with height.
Right working
It falls in the troposphere and mesosphere, rises in the thermosphere.
Do this: Write by layer.
Usually costs: 1 mark.

Exam tips

Draw the pie chart with labels and percentages that add up to 100.
Write each layer with height and one feature; this earns full marks in a 3-mark answer.
Mark the boundaries (tropopause and stratopause) when asked for a diagram.
Use the word "derived" when you compute thicknesses from the heights.

Practice questions on Composition and 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.

Q1 · 1 mark · MCQBoard level
Most meteors burn up in which layer?
(a) Troposphere
(b) Stratosphere
(c) Mesosphere
(d) Thermosphere
Hint
Between 50 and 80 km.
Answer
(c) Mesosphere
Q2 · 2 marks · Very short answerBoard level
Name the two main gases of the atmosphere and their percentages. (2 marks)
Hint
Pie chart.
Answer
Nitrogen, about 78 per cent, and oxygen, about 21 per cent.
Q3 · 3 marks · Short answerHarder board level
If 1,000 litres of dry air is taken, find the volume of nitrogen, oxygen and carbon dioxide using NCERT's percentages. (3 marks)
Hint
Percent of 1,000.
Answer
Nitrogen: 78% of 1,000 = 780 litres. Oxygen: 21% of 1,000 = 210 litres. Carbon dioxide: 0.04% of 1,000 = 0.4 litre. These are derived from the percentages in the chapter.
Q4 · 1 mark · Assertion-reasonHarder board level
Assertion (A): The exosphere loses light gases to space.
Reason (R): Gravity is weak in the exosphere.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Hint
Helium and hydrogen.
Answer
(a) Both A and R are true, and R is the correct explanation of A.
Q5 · 5 marks · Long answerHarder board level
Describe the ozone layer's location and importance and name the layers above and below it. (5 marks)
Hint
Stratosphere between troposphere and mesosphere.
Answer
The ozone layer lies in the stratosphere, which extends to about 50 km above the troposphere. It filters harmful ultraviolet rays from the Sun, protecting living beings. The layer below the stratosphere is the troposphere, which holds the weather and the air we breathe, and the layer above it is the mesosphere, where most meteorites burn up. The stratosphere is free of clouds, which also makes it suitable for aeroplanes. Without ozone, more ultraviolet radiation would reach the surface.
Q6 · 3 marks · Short answerHOTS / competency
Suppose a student claims, "Because temperature falls as you go up, it must be coldest at the top of the atmosphere." Evaluate her claim using the layers. (3 marks)
Hint
Where does temperature rise?
Answer
Her claim is wrong. Temperature falls with height only in the troposphere and the mesosphere. In the thermosphere it rises rapidly because gases absorb X-rays and short-wave ultraviolet radiation, so the upper layers are not simply colder.

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.Which gas is the most abundant in air?Show answer
Nitrogen (78 per cent).
2.Which layer has most of the weather?Show answer
The troposphere.
3.Where is the ozone layer?Show answer
In the stratosphere.
4.Which layer reflects radio waves?Show answer
The thermosphere (ionosphere).
5.What are the southern lights called?Show answer
Aurora Australis.

Composition and Layers of the Atmosphere: frequently asked questions

What are the layers of the atmosphere?

Troposphere, stratosphere, mesosphere, thermosphere and exosphere.

What is the composition of the atmosphere?

Nitrogen 78 per cent, oxygen 21 per cent, argon 0.93 per cent, carbon dioxide 0.04 per cent and other gases 0.03 per cent.

Why is the stratosphere good for aeroplanes?

It is free of clouds and weather.

Why do auroras occur?

Charged particles from the Sun hit gases near the magnetic poles, and each gas glows in its own colour.