GeographyICSE Class 9 · Composition and Structure of the Atmosphere

Layers of the Atmosphere, Ozone Depletion and Global Warming (ICSE Class 9 Geography)

Updated 17 October 2026 · Draft, not yet fact-checked

The atmosphere is a mixture of gases (about 78 per cent nitrogen and 21 per cent oxygen), water vapour and dust. Its layers are the troposphere (weather, temperature falling with height), stratosphere (ozone layer), ionosphere (reflects radio waves) and exosphere. CFCs thin the ozone layer, and rising greenhouse gases cause global warming.

Understand Layers of the Atmosphere, Ozone Depletion and Global Warming

1. Composition. Dry air by volume: nitrogen about 78 per cent, oxygen about 21 per cent, argon about 0.93 per cent, carbon dioxide about 0.04 per cent, and traces of neon, helium, methane and ozone. Water vapour (variable, up to about 4 per cent) and dust are also present; dust acts as nuclei for condensation.

2. Troposphere. The lowest layer, about 18 km high at the equator and about 8 km at the poles (average 13 km). Temperature falls about 6.5 degrees C per 1,000 m (normal lapse rate). It holds most of the water vapour, so all weather occurs here. Its upper boundary is the tropopause.

3. Stratosphere. Extends up to about 50 km. The air is dry and clear, so jet aircraft fly in its lower part. The *ozone layer* (roughly 15 to 35 km) absorbs harmful ultraviolet rays, so temperature rises with height.

4. Ionosphere and exosphere. From about 80 to 400 km the ionosphere contains charged particles (ions), reflects radio waves and displays auroras. The exosphere is the outermost layer, very thin and merging into space.

5. Ozone depletion. Chlorofluorocarbons (CFCs) from refrigerators, air conditioners, aerosols and foam release chlorine atoms in the stratosphere that destroy ozone; a seasonal ozone hole forms over Antarctica. Effects: more ultraviolet reaching the earth, causing skin cancer, eye cataract and damage to crops and plankton. Remedy: phasing out CFCs, as under the Montreal Protocol.

6. Global warming. Greenhouse gases such as carbon dioxide, methane and nitrous oxide trap the earth's heat (the greenhouse effect). Burning fossil fuels and cutting forests raise their level, increasing temperature; effects include melting ice, rising sea level and changing rainfall. Remedies: renewable energy, afforestation and less fuel use.

Layers of the Atmosphere, Ozone Depletion and Global Warming: key facts

Composition: Nitrogen 78 per cent
Oxygen 21 per cent
Argon 0.93 per cent
CO2 0.04 per cent
Troposphere: Height 8 km poles, 18 km equator
Falls 6.5 degrees C per 1,000 m
All weather
Layers in order: $T r o p os p h er e ∣ S t r a t os p h er e ∣ I o n os p h er e ∣ E x os p h er e$
Ozone and CO2: CFCs thin ozone
Ozone absorbs UV
CO2 and methane cause warming

Layers of the Atmosphere, Ozone Depletion and Global Warming 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.

A mountain trip

Suppose a hiker starts a trek at 30 degrees C at the base and climbs 2,000 m over a day.

Estimate the temperature at the top.

Try it first, then show the solutionSolution

Fall = 6.5 degrees C per 1,000 m.
Fall over 2,000 m = 13 degrees C.
Temperature = 30 - 13 = 17 degrees C.

Answer: About 17 degrees C.

Memory hook: 6.5 down per km.

Tricks to remember Layers of the Atmosphere, Ozone Depletion and Global Warming

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

TSIE
Troposphere, Stratosphere, Ionosphere, Exosphere.
Use it when: Order of layers.
CFC = Cuts From Cover
CFCs cut the ozone cover.
Use it when: Ozone depletion.

Layers of the Atmosphere, Ozone Depletion and Global Warming: solved examples

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

Example 1: Composition · 3 marks · Short answerBoard level

State the percentage of nitrogen and oxygen in dry air and name one use of oxygen. [3]

Try it first, then show the solutionSolution

Nitrogen about 78 per cent.
Oxygen about 21 per cent.
Oxygen is used for breathing and burning.

Answer: Nitrogen 78 per cent, oxygen 21 per cent; oxygen is used for breathing.

Example 2: Lapse rate · 3 marks · Short answerHarder board level

A place is at 3,000 m and the temperature at sea level is 28 degrees C. Estimate the temperature at the place using the normal lapse rate. [3]

Try it first, then show the solutionSolution

Fall = 6.5 x 3 = 19.5 degrees C.
Temperature = 28 - 19.5.
= 8.5 degrees C.

Answer: 8.5 degrees C.

Example 3: Troposphere thickness · 3 marks · Short answerHarder board level

Why is the troposphere thicker at the equator than at the poles? [3]

Try it first, then show the solutionSolution

Strong heating at the equator causes intense convection.
Convection carries air to greater heights.
Poles are cold with weak convection.

Answer: Strong convectional heating at the equator lifts the air higher.

Example 4: Ozone depletion · 4 marks · Long answerHarder board level

Explain how CFCs destroy ozone and name two harmful effects. [4]

Try it first, then show the solutionSolution

CFCs rise to the stratosphere.
UV rays break them and release chlorine atoms.
Chlorine destroys ozone molecules repeatedly.
Effects: more UV, causing skin cancer and cataracts.

Answer: Chlorine from CFCs breaks down ozone; effects include skin cancer and cataract.

Example 5: Case: warming town · 6 marks · Case-basedHOTS / competency

Case. A coastal town records that its average temperature has risen and the sea has begun to flood its low streets. (i) Name the process. [1] (ii) Two human causes. [2] (iii) Why does sea level rise? [2] (iv) One remedy. [1]

Try it first, then show the solutionSolution

(i) Global warming.
(ii) Burning fossil fuels and deforestation.
(iii) Ice melts and warm water expands.
(iv) Use renewable energy or plant trees.

Answer: (i) Global warming (ii) Fossil fuels and deforestation (iii) Melting ice and thermal expansion (iv) Renewable energy.

Where students lose marks in Layers of the Atmosphere, Ozone Depletion and Global Warming

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

Mistake: Writing that ozone is in the troposphere
Layer confusion.
Do this: The ozone layer is in the stratosphere.
Usually costs: 1 mark.
Mistake: Mixing ozone depletion and global warming
Causes swapped.
Do this: CFCs deplete ozone; CO2 warms.
Usually costs: 2 marks.
Mistake: Lapse rate in wrong units
Per 100 m used.
Do this: 6.5 degrees C per 1,000 m.
Usually costs: 1 mark.

Exam tips

Draw a labelled vertical section of the atmosphere with heights.
Show lapse-rate calculation steps.
Name the gas and its source for each problem.
Mention a remedy in every environmental question.

Practice questions on Layers of the Atmosphere, Ozone Depletion and Global Warming

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
Clouds, rain and storms are all confined to the layer that holds most of the water vapour. It is the:
(a) stratosphere
(b) troposphere
(c) ionosphere
(d) exosphere
Hint
Water vapour.
Answer
(b) troposphere.
Q2 · 1 mark · Very short answerBoard level
Which layer reflects radio waves? [1]
Hint
Ions.
Answer
The ionosphere.
Q3 · 2 marks · Very short answerBoard level
Name two greenhouse gases. [2]
Hint
CO2.
Answer
Carbon dioxide and methane.
Q4 · 1 mark · Assertion-reasonHarder board level
Assertion (A): Jet aircraft prefer the lower stratosphere.
Reason (R): The stratosphere has little water vapour and few disturbances.
(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
Smooth.
Answer
(a).
Q5 · 2 marks · Very short answerBoard level
Explain why the temperature in the stratosphere rises with height. [2]
Hint
Ozone.
Answer
Ozone absorbs ultraviolet radiation and warms the air.
Q6 · 3 marks · Short answerHarder board level
Distinguish between the ozone hole and global warming. [3]
Hint
Different gases.
Answer
The ozone hole is thinning of ozone due to CFCs, letting in more UV; global warming is a rise in temperature due to extra greenhouse gases.
Q7 · 3 marks · Short answerHarder board level
State three measures to reduce ozone depletion and three to reduce global warming. [3]
Hint
Remedies.
Answer
Stop CFC use, use ozone-friendly coolants, recycle old fridges; use renewable energy, plant trees and save fuel.
Q8 · 2 marks · Very short answerHarder board level
Why does the presence of dust in the lower atmosphere help rainfall? [2]
Hint
Nuclei.
Answer
Dust particles act as nuclei on which water vapour condenses.
Q9 · 4 marks · Case-basedHOTS / competency
Case. A city bans aerosol sprays containing CFCs. (i) Which layer benefits? [1] (ii) Why does this take decades to show results? [2] (iii) Give one benefit to people. [1]
Hint
Slow.
Answer
(i) Stratospheric ozone layer. (ii) CFCs already released stay in the air for long. (iii) Less skin cancer.
Q10 · 3 marks · Long answerHOTS / competency
Case. A student says that the greenhouse effect is entirely harmful. Evaluate with reference to the natural effect. [3]
Hint
Natural.
Answer
The natural effect keeps the earth warm enough for life; the problem is the extra greenhouse gases from human activity that increase warming.

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.Percentage of oxygen in air?Show answer
About 21.
2.Layer with ozone?Show answer
Stratosphere.
3.Height of troposphere at the equator?Show answer
About 18 km.
4.What do CFCs destroy?Show answer
Ozone.
5.Why does ionosphere help radio?Show answer
It reflects radio waves.

Layers of the Atmosphere, Ozone Depletion and Global Warming: frequently asked questions

Is ozone good or bad?

It is protective in the stratosphere but harmful at ground level.

What is the tropopause?

The boundary between the troposphere and stratosphere.

Is CO2 harmful?

It is vital for plants, harmful in excess as a greenhouse gas.