GeographyISC Class 11 · Composition and Structure of the Atmosphere

Composition and Layers of the Atmosphere (ISC Class 11 Geography)

Updated 17 October 2026 · Fact-checked

Dry air is about 78.1 per cent nitrogen, 20.9 per cent oxygen, 0.93 per cent argon and 0.04 per cent carbon dioxide, with variable water vapour and dust. By temperature trend the layers are the troposphere (falls), stratosphere (rises), mesosphere (falls), ionosphere or thermosphere (rises) and the exosphere.

Notice the temperature line falls, rises, falls, then rises with height, and that nitrogen and oxygen make up about 99 per cent of dry air.

Illustration drawn for QuestionBuddy.

Understand Composition and Layers of the Atmosphere

Composition (dry air by volume). Nitrogen about 78.1 per cent; oxygen about 20.9 per cent; argon about 0.93 per cent; carbon dioxide about 0.04 per cent, rising from burning fossil fuels and deforestation, a greenhouse gas that absorbs outgoing long-wave radiation; traces of neon, helium, methane, krypton, hydrogen and ozone. Water vapour is variable, from almost nil in cold deserts to about 4 per cent in warm humid tropics, decreasing with height and towards the poles; it forms clouds and releases latent heat. Solid particles (dust, sea salt, smoke, pollen, volcanic ash) act as condensation nuclei and scatter sunlight.

Troposphere. Up to about 8 km at the poles and 18 km at the equator (average about 13 km). Temperature falls about 6.5 degrees C per km (normal lapse rate). It holds about three-quarters of the atmosphere's mass and nearly all water vapour and dust; all weather occurs here. The tropopause is about minus 80 degrees C over the equator and about minus 45 degrees C over the poles; jet streams blow near it.

Stratosphere. To about 50 km. Temperature is nearly constant at the bottom then rises to about 0 degrees C at the stratopause because ozone absorbs ultraviolet. Dry and calm, so jet aircraft cruise in its lower part.

Mesosphere. About 50 to 80 km; temperature falls to about minus 100 degrees C at the mesopause, the coldest part; most meteors burn up here.

Ionosphere (thermosphere). About 80 to 400 km; temperature rises sharply (over 1,000 degrees C but air very thin); ionised layers reflect radio waves; auroras occur; many satellites orbit.

Exosphere. Above about 400-500 km; light gases escape into space.

Composition and Layers of the Atmosphere: key facts

Dry air by volume: N2 about 78.1%
O2 about 20.9%
Ar about 0.93%
CO2 about 0.04%; Water vapour 0-4%.
Layers and heights: Troposphere 0-8/18 km
Stratosphere to 50 km
Mesosphere 50-80 km
Ionosphere 80-400 km
Exosphere above
Temperature trend: $F a l l s ∣ R i ses ∣ F a l l s ∣ R i ses$; Tropo, strato, meso, thermo.
Lapse rate: $F a l l = 6.5 d e g r ees C x h e i g h t ink m ∣ T r o p os p h er e$

How to answer Composition and Layers of the Atmosphere questions

Questions mix lists (2 to 3 marks), layer descriptions (4 marks) and lapse-rate sums.

1List gases with percentages in order.
2For each layer give height, temperature trend and a key feature.
3Show the lapse-rate formula and substitute in km.
4Link the layer to uses: weather, aircraft, radio, meteors.

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.

Hill station

Suppose a plain is at 30 degrees C and a hill resort lies 2.4 km higher.

Estimate the resort temperature.

Try it first, then show the solutionSolution

Fall = 6.5 x 2.4 = 15.6 degrees C.
Temperature = 30 - 15.6 = 14.4 degrees C.

Answer: 14.4 degrees C.

Memory hook: 6.5 per km.

Radio at night

Suppose a distant radio station is heard clearly on a medium wave set at night.

Which layer helps and how?

Try it first, then show the solutionSolution

The ionosphere has charged particles.
It reflects radio waves back to earth.

Answer: The ionosphere reflects radio waves.

Memory hook: Sky mirror for radio.

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.

Fall, Rise, Fall, Rise
Troposphere falls, stratosphere rises, mesosphere falls, thermosphere rises.
Use it when: Temperature trend.
78-21-1
Nitrogen 78, oxygen 21, the rest about 1.
Use it when: Gas percentages.

Composition and Layers of the Atmosphere: solved examples

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

Example 1: Clouds and rain · 1 mark · MCQBoard level

Clouds, rain and storms are confined mainly to which layer?
(a) Stratosphere
(b) Troposphere
(c) Mesosphere
(d) Exosphere

Try it first, then show the solutionSolution

Contains vapour and dust.

Answer: (b) Troposphere.

Example 2: Gases · 3 marks · Short answerBoard level

State the percentage of nitrogen and oxygen in dry air and name two variable constituents.

Try it first, then show the solutionSolution

Nitrogen about 78.1 per cent; oxygen about 20.9 per cent.
Variable: water vapour and dust (or ozone).

Answer: N2 about 78.1%, O2 about 20.9%; water vapour and dust.

Example 3: Lapse rate sum · 4 marks · Long answerHarder board level

The surface temperature is 32 degrees C. At what height is the air temperature 0 degrees C, and what is the temperature at 4.2 km?

Try it first, then show the solutionSolution

Height for 0 degrees C = 32 / 6.5 = 4.92 km, about 4.9 km.
Fall at 4.2 km = 6.5 x 4.2 = 27.3 degrees C.
Temperature = 32 - 27.3 = 4.7 degrees C.
Check: 4.2 km is below 4.9 km, so above freezing.

Answer: About 4.9 km; 4.7 degrees C at 4.2 km.

Example 4: Tropopause comparison · 4 marks · Long answerHarder board level

Explain why the troposphere is thicker over the equator than over the poles, and compare the tropopause temperatures.

Try it first, then show the solutionSolution

Strong convection at the equator carries heat high.
So the layer reaches about 18 km, against about 8 km at the poles (a 10 km difference).
The higher tropopause over the equator is colder: about minus 80 degrees C versus minus 45 degrees C.
Temperature at the top depends on height reached.

Answer: Convection makes it about 18 km versus 8 km; equatorial tropopause is colder, minus 80 versus minus 45 degrees C.

Example 5: Layer reasoning · 5 marks · Case-basedHOTS / competency

Case: A passenger aircraft cruises in calm dry air and the pilot hears radio from a distant city, while a meteor streaks across the sky above. (a) Name the layer for cruising. (b) Name the layer for radio reflection. (c) Name the layer where the meteor burns and explain why it burns there.

Try it first, then show the solutionSolution

(a) The lower stratosphere (calm, dry, no weather).
(b) The ionosphere.
(c) The mesosphere, where friction with air makes meteors burn.

Answer: (a) Stratosphere (b) ionosphere (c) mesosphere, friction burns meteors.

Where students lose marks in Composition and Layers of the Atmosphere

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

Mistake: Rounding gas percentages carelessly
Nitrogen is written as 80 per cent.
Do this: Use about 78.1 per cent and 20.9 per cent.
Usually costs: 1 mark.
Mistake: Applying lapse rate in metres
6.5 degrees C per km is multiplied by m.
Wrong working
Fall = 6.5 x 3,600
Right working
Fall = 6.5 x 3.6 = 23.4 degrees C
Do this: Convert height to km first.
Usually costs: 2 marks.
Mistake: Placing weather in the stratosphere
Layers confused.
Do this: All weather is in the troposphere.
Usually costs: 1 mark.

Exam tips

Quote heights and trends for each layer.
Show 6.5 degrees C per km with km units.
Mention latent heat for water vapour.
Do not give a single tropopause height.

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
The coldest part of the atmosphere is the:
(a) tropopause
(b) mesopause
(c) stratopause
(d) thermopause
Hint
About minus 100 degrees C.
Answer
(b) Mesopause.
Q2 · 3 marks · Short answerHarder board level
A mountain camp is 3.6 km above a valley at 25 degrees C. Estimate the camp temperature.
Hint
6.5 x height.
Answer
Fall = 6.5 x 3.6 = 23.4 degrees C; temperature = 25 - 23.4 = 1.6 degrees C.
Q3 · 4 marks · Long answerHarder board level
Explain why water vapour and dust are important though small in quantity.
Hint
Weather and scattering.
Answer
Water vapour forms clouds and precipitation and releases latent heat; dust provides condensation nuclei and scatters sunlight, giving red sunsets.
Q4 · 4 marks · Long answerHarder board level
Compare the stratosphere and ionosphere on temperature trend, air density and use.
Hint
Ozone warms; ions reflect.
Answer
Stratosphere: temperature rises to about 0 degrees C due to ozone, air dry and calm, jets cruise. Ionosphere: temperature rises sharply, air very thin, ionised layers reflect radio waves and auroras occur.
Q5 · 4 marks · Case-basedHOTS / competency
Case: A student notes that temperature falls up to the tropopause, then rises, then falls and rises again. (a) Name the layers in this sequence. (b) Explain why temperature rises in the stratosphere. (c) Explain why the ionosphere is hot but would not feel hot.
Hint
Ozone and thin air.
Answer
(a) Troposphere, stratosphere, mesosphere, thermosphere. (b) Ozone absorbs ultraviolet. (c) Gases absorb high-energy radiation but the air is so thin little heat is transferred.

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.Coldest layer part?Show answer
Mesopause, about minus 100 degrees C.
2.Layer reflecting radio?Show answer
Ionosphere.
3.Normal lapse rate?Show answer
6.5 degrees C per km.
4.Where do meteors burn?Show answer
Mesosphere.
5.Tropopause over equator?Show answer
About minus 80 degrees C.

Composition and Layers of the Atmosphere: frequently asked questions

Why is the stratosphere calm?

It has little vertical mixing and almost no water vapour.

Why is CO2 a greenhouse gas?

It absorbs outgoing long-wave radiation.

What is the ozonosphere?

The ozone-rich part of the stratosphere.