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CBSE Class 11 Geography★ 4-6 marks · CBSE Class 11 Geography (Fundamentals of Physical Geography) Chapter 7

Composition and Structure of Atmosphere

The air we breathe is just a thin envelope around the Earth — but it makes life possible. This chapter covers what the atmosphere is made of (nitrogen, oxygen, trace gases), how it's structured into 5 layers from the troposphere to the exosphere, and why each layer matters. CBSE Class 11 Geography (Fundamentals of Physical Geography) Chapter 7.

⏱ 18 min readEasy difficulty✓ Free · NCERT-aligned

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By the end of this chapter you'll be able to…

1List permanent and variable gases of the atmosphere with proportions
2Identify the 5 atmospheric layers with altitude, temperature trend, and key features
3Explain the ozone layer: location (stratosphere) and function (UV absorption)
4Explain why ALL weather occurs in the troposphere
5Understand why temperature INCREASES in the stratosphere (ozone absorbs UV)

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Why this chapter matters

Five atmospheric layers with temperature trends is a guaranteed question. Ozone layer location and function. Troposphere as the weather layer. Permanent vs variable gases composition.

Composition and Structure of Atmosphere

"The atmosphere is a thin, fragile veil — but without it, Earth would be as dead as the Moon."

1. Chapter Overview

The atmosphere is the GASEOUS ENVELOPE surrounding the Earth, held by GRAVITY. This chapter covers: (1) the COMPOSITION of the atmosphere — what gases it's made of, and (2) the STRUCTURE — how it's organised into FIVE DISTINCT LAYERS, each with different temperature behaviours.

2. Composition of the Atmosphere

Permanent Gases (Constant Proportion)

Gas	Percentage
Nitrogen (N₂)	78.08%
Oxygen (O₂)	20.95%
Argon (Ar)	0.93%
Others (Ne, He, Kr, Xe)	Trace

Variable Gases (Varying Proportion)

Gas	Role
Water Vapour (H₂O)	Varies 0-4%. Absorbs heat. Source of clouds and precipitation. DECREASES with altitude.
Carbon Dioxide (CO₂)	~0.04% (420 ppm). Greenhouse gas. Absorbs outgoing heat. INCREASING due to human activity.
Ozone (O₃)	In STRATOSPHERE: ABSORBS UV radiation (protects life). In TROPOSPHERE: pollutant.

Particulates (Dust, Salt, Smoke)

Solid and liquid particles suspended in air
Sources: volcanic eruptions, dust storms, sea spray, industrial emissions
Act as condensation nuclei (particles around which water droplets form → clouds)

3. Structure of the Atmosphere — Five Layers

Based on TEMPERATURE change with altitude

Layer	Altitude Range	Temperature Trend	Key Features
Troposphere	0–13 km (avg; varies from 8 km at poles to 18 km at equator)	DECREASES with height at ~6.5°C/km (normal lapse rate)	ALL WEATHER occurs here. Contains 75% of atmospheric mass and nearly ALL water vapour. Top = TROPOPAUSE.
Stratosphere	13–50 km	INCREASES with height	Contains OZONE LAYER (15-35 km). Ozone absorbs UV → heats up. Aircraft fly here (stable, no weather). Top = STRATOPAUSE.
Mesosphere	50–85 km	DECREASES with height	Coldest layer (-90°C at top). Meteors BURN UP here. Top = MESOPAUSE.
Thermosphere (Ionosphere)	85–600 km	INCREASES with height (up to 1500°C+)	Temperature rises sharply (absorbs UV/X-rays). RADIO WAVES reflected → long-distance communication. AURORA occurs here.
Exosphere	600+ km	Gradual transition to space	Lightest gases (H, He) escape into space. EXTREMELY thin atmosphere. Gradual boundary between atmosphere and SPACE.

Why Temperature INCREASES in the Stratosphere?

OZONE in the stratosphere absorbs ULTRAVIOLET (UV) radiation from the Sun
Absorption of UV → HEAT → temperature RISES with altitude (INVERSION)
This is crucial: ozone ABSORBS UV that would otherwise reach the surface and harm life

4. The Tropopause — Where Weather Stops

Boundary between troposphere and stratosphere
Weather is CONFINED to the troposphere
Jet streams (fast winds) occur near the tropopause

5. Exam Focus

Composition — permanent vs variable gases (nitrogen, oxygen, water vapour, CO₂, ozone)
Five layers — names, altitude ranges, temperature trends, key features
Ozone layer location and function (stratosphere, absorbs UV)
Why the troposphere contains nearly all weather
Tropopause significance

6. Common Mistakes

Temperature always decreases with altitude — NO. Temperature DECREASES in the troposphere and mesosphere, but INCREASES in the stratosphere (ozone absorbs UV) and thermosphere.
The ozone layer is in the upper atmosphere (thermosphere) — NO. The ozone layer is in the STRATOSPHERE (15-35 km). Different layers, different functions.
Water vapour is a permanent gas — It's VARIABLE (0-4%) and decreases with altitude. Nearly ALL water vapour is in the troposphere.

7. Conclusion

The atmosphere is a layered, life-sustaining envelope:

Composition: 78% N₂, 21% O₂, trace variable gases (H₂O, CO₂, O₃)
Troposphere: Where we live, where weather happens
Stratosphere: Where the ozone layer protects us from UV
Mesosphere: Where meteors burn up
Thermosphere: Where auroras dance and radio waves bounce

Five layers. One atmosphere. All life depends on this thin gaseous blanket.

Key formulas & results

Everything you need to memorise, in one card. Screenshot this for revision.

Permanent gases

N₂ 78%, O₂ 21%, Ar 0.93%

Variable gases

H₂O (0-4%, decreases with altitude), CO₂ (~0.04%, greenhouse), O₃ (stratosphere = protector)

5 Layers (↑ temp)

Troposphere (↓ temp, weather, 0-13km) → Stratosphere (↑ temp, OZONE LAYER, 13-50km) → Mesosphere (↓ temp, meteors, 50-85km) → Thermosphere (↑ temp, aurora, radio, 85-600km) → Exosphere (→ space, 600+km)

Ozone layer

Stratosphere, 15-35 km. Absorbs UV radiation → protects life on Earth. Also: HEATS the stratosphere (why temp increases).

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Common mistakes & fixes

These are the exact errors that cost students marks in board exams. Read them once, save yourself the trouble.

WATCH OUT

✗ Temperature always decreases as you go up

✓ Temperature DECREASES in troposphere and mesosphere, but INCREASES in stratosphere (ozone absorbs UV → heats) and thermosphere. The temperature TREND is what distinguishes each layer.

WATCH OUT

✗ Ozone layer is in the thermosphere / upper atmosphere

✓ Ozone layer is in the STRATOSPHERE (15-35 km). The thermosphere has auroras and radio reflection — a completely different layer with different functions.

NCERT exercises (with solutions)

Every NCERT exercise from this chapter — what it covers and how many questions to expect.

Ex 7

Exercise 7

Composition of atmosphere, five layers, ozone, temperature trends

Questions

Practice problems

Try each one yourself before tapping "Show solution". Active recall > rereading.

Q1MEDIUM

Why does temperature INCREASE as you go up through the stratosphere, when temperature decreases in the troposphere?

Q2MEDIUM

Name the five atmospheric layers, giving the altitude range and ONE key feature of each.

Q3MEDIUM

Explain the composition of the atmosphere, distinguishing between permanent and variable gases. Why is the distinction important for understanding climate?

5-minute revision

The whole chapter, distilled. Read this the night before the exam.

•Permanent: N₂ 78%, O₂ 21%, Ar 0.93%. Variable: H₂O (0-4%), CO₂ (0.04%, greenhouse), O₃ (stratosphere).
•5 layers by temperature trend: Troposphere (↓, weather) → Stratosphere (↑, OZONE) → Mesosphere (↓, meteors) → Thermosphere (↑, aurora/radio) → Exosphere.
•Troposphere: 0-13km avg. 75% mass, ALL water vapour, ALL weather. Tropopause at top.
•Stratosphere: 13-50km. Ozone layer (15-35km) absorbs UV → temperature INCREASES. Stratopause at top.
•Mesosphere: 50-85km. Coldest (-90°C at mesopause). Meteors burn up.
•Thermosphere (Ionosphere): 85-600km. UV/X-ray absorption → high temps. Aurora. Radio wave reflection.
•Exosphere: 600+km. Lightest gases escape. Transition to space. NO clear upper boundary.

CBSE marks blueprint

Where the marks come from in this chapter — so you can plan your prep.

Typical chapter weightage: 4-6 marks · CBSE Class 11 Geography (Fundamentals of Physical Geography Chapter 7)

Question type	Marks each	Typical count	What it tests
MCQ / VSA (1 mark)	1	1	Name the weather layer (troposphere), location of ozone layer (stratosphere), N₂ percentage (78%)
Short Answer (2-3 marks)	2	1	Name all five layers with key feature, explain why stratosphere warms with altitude
Long Answer (4-5 marks)	4	1	All five layers with altitude + temperature trend + key feature, OR composition (permanent vs variable) and climate importance

Prep strategy

Five layers: memorise the MNEMONIC for the temperature trend pattern — Down, UP, Down, UP (Troposphere ↓, Stratosphere ↑, Mesosphere ↓, Thermosphere ↑). Each 'up' has a reason: stratosphere = ozone absorbs UV; thermosphere = UV/X-ray absorption by very sparse molecules.
For each layer know: (1) altitude range, (2) temperature trend, (3) ONE key feature. The 5-mark question typically requires all three for all five layers.
Permanent gas percentages: N₂ 78%, O₂ 21%, Ar 0.93% — know these exactly. Variable gases: H₂O (weather), CO₂ (greenhouse), O₃ (UV protection). The distinction earns the analysis mark.
Ozone layer question: always include THREE facts — (1) location (stratosphere, 15–35 km), (2) function (absorbs UV radiation), (3) why important (UV would destroy DNA, prevent life on land). All three for full marks.

Where this shows up in the real world

This chapter isn't just an exam topic — it lives in the world around you.

Aviation: why planes fly in the stratosphere

Satellite orbit and atmospheric drag

Climate science: the role of trace gases

Exam strategy

Battle-tested tips from teachers and toppers for this chapter.

Five layers question: ALWAYS provide altitude range + temperature trend + ONE key feature for each layer. Missing any one component for any layer loses marks. Write as a table or numbered list for clarity.
Temperature trend pattern: Down-Up-Down-Up (T-S-M-T). For each 'UP' layer, state WHY — stratosphere: ozone absorbs UV; thermosphere: UV/X-ray absorption by sparse molecules. Stating the trend without the reason earns only half marks.
Composition question: separate PERMANENT (constant) and VARIABLE (changes). For permanent: N₂ 78%, O₂ 21%, Ar 0.93%. For variable: H₂O (weather), CO₂ (greenhouse, ~0.04%), O₃ (stratospheric UV shield). The CLIMATE IMPORTANCE of variable gases makes this a 3-mark topic.
For 'why all weather is in troposphere': three-part answer — (1) all water vapour is there, (2) unstable temperature gradient drives convection, (3) 75% of atmospheric mass is there. All three parts for full marks.

Going beyond the textbook

For olympiad aspirants and curious learners — topics that build on this chapter.

Research the MONTREAL PROTOCOL (1987) — the international agreement to phase out ozone-depleting substances (CFCs, halons, HCFCs). It is widely considered the most successful international environmental treaty in history: the ozone hole is slowly recovering (expected full recovery ~2060–2075). Compare its success with the difficulty of the Paris Agreement (climate change). What made Montreal work (scientifically unambiguous cause, alternative chemicals available, limited number of major producers) while climate negotiations have been harder? What does this comparison say about the politics of international environmental governance?
Investigate NOCTILUCENT CLOUDS (NLCs) — clouds that form in the MESOSPHERE (80–85 km altitude) at high latitudes in summer, visible from the ground as electric-blue illuminated clouds after sunset. They are made of ice crystals forming on meteor dust at the mesopause (coldest point in the atmosphere). Their increasing brightness and frequency over the past 40 years is a proposed indicator of climate change: as CO₂ increases at the surface, the MESOSPHERE actually COOLS (because CO₂ radiates heat to space more efficiently at high altitudes). Noctilucent clouds = indirect evidence of a cooling mesosphere = fingerprint of CO₂-driven climate change propagating upward through atmospheric layers.
The KÁRMÁN LINE (100 km altitude) is defined by the International Astronautical Federation as the boundary between Earth's atmosphere and outer space. But this boundary is arbitrary: the atmosphere doesn't end suddenly at 100 km — it gradually thins. At 100 km, the air is too thin for conventional aerodynamics but still thick enough to cause reentry heating. Research the debate about where 'space' actually begins and why the definition matters legally (national airspace vs international space) and commercially (space tourism: Blue Origin's suborbital flights reach ~100 km, SpaceX's orbital flights go much higher).
Research the GREENHOUSE EFFECT in detail: the natural greenhouse effect (essential for life — without it, Earth's average temperature would be -18°C instead of +15°C) vs the ENHANCED greenhouse effect (from additional CO₂, CH₄, N₂O from human activities). Which gases contribute most to the NATURAL greenhouse effect? (Answer: water vapour ~50%, CO₂ ~20%, clouds ~25%) How does this differ from the INCREMENTAL warming from human-added gases? Why is CO₂ the focus of climate policy despite water vapour being the larger natural greenhouse gas? (Answer: because water vapour is a FEEDBACK, not a FORCING — its concentration is determined by temperature; CO₂ is a FORCING — adding it directly changes the energy budget)

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