Composition and Structure of Atmosphere, Class 11 Geography

What Composition and Structure of Atmosphere is about

Weightage: This chapter is part of Unit IV, Climate, of Fundamentals of Physical Geography, which carries 8 of the 70 theory marks in the school annual exam together with Solar Radiation, Heat Balance and Temperature, Atmospheric Circulation and Weather Systems and Water in the Atmosphere. Map work carries a further 5 marks for each book.Source: CBSE Geography (029) syllabus 2026-27, Class XI course structure

Key concepts

The atmosphere and why it matters

NCERT section Composition and Structure of Atmosphere (introduction)

• The atmosphere is a mixture of gases that envelops the earth on all sides.
• It contains life-giving gases: oxygen for humans and animals, carbon dioxide for plants.
• It is an integral part of the earth's mass; 99 per cent of its total mass is within 32 km of the surface.
• Air is colourless and odourless; we feel it only when it blows as wind.
• Air is essential to all organisms: people may survive a while without food or water but not even a few minutes without breathing.
• Without ozone, harmful ultraviolet rays would reach the surface (the chapter's question to think about).

Worked example

What fraction of the atmosphere's mass lies within 32 km of the earth's surface, and what does this suggest about air at high altitudes? (2 marks)

Try it first, then show the solutionSolution

1. 99 per cent of the atmosphere's mass is confined to 32 km above the surface.
2. So above 32 km only about 1 per cent of the mass is spread over a huge height: air there is extremely thin, and density falls rapidly with height.

Answer: 99 per cent; the air above is very thin, showing density decreases with height.

Gases: carbon dioxide and ozone

NCERT section Composition of the Atmosphere

Change with height: proportions of gases change in higher layers. Oxygen becomes almost negligible at 120 km; carbon dioxide and water vapour are found only up to 90 km. Near the surface, nitrogen is the gas that makes up the major portion of the atmosphere (with oxygen next).

Carbon dioxide:

• Meteorologically very important.
• Transparent to incoming solar radiation, opaque to outgoing terrestrial radiation.
• Absorbs part of the terrestrial radiation and reflects some back to the surface.
• Largely responsible for the greenhouse effect.
• The volume of other gases is constant, but CO2 has been rising in the past few decades, mainly from burning fossil fuels, which has also raised air temperature.

Ozone:

• Found between 10 and 50 km above the surface.
• Acts as a filter: absorbs ultraviolet rays from the sun and prevents them from reaching the surface.

Worked example

Why is carbon dioxide called 'transparent' to sunlight but 'opaque' to terrestrial radiation, and what is the result? (2 marks)

Try it first, then show the solutionSolution

1. It lets incoming solar radiation pass through (transparent) but absorbs and partly reflects back the outgoing radiation from the earth (opaque).
2. Heat is therefore kept near the surface: the greenhouse effect; more CO2 from burning fossil fuels has raised air temperature.

Answer: It lets sunlight in but traps outgoing heat, causing the greenhouse effect.

Water vapour and dust particles

NCERT section Composition of the Atmosphere

Water vapour:

• A variable gas; decreases with altitude.
• Up to 4 per cent of air by volume in warm, wet tropics; less than 1 per cent in dry, cold desert and polar areas.
• Decreases from the equator towards the poles.
• Absorbs part of the insolation and preserves the earth's radiated heat: acts like a blanket, keeping the earth from becoming too cold or too hot.
• Contributes to stability and instability of air.

Dust particles:

• Sources: sea salts, fine soil, smoke-soot, ash, pollen, dust, disintegrated meteor particles.
• Concentrated in the lower layers, though convectional currents can lift them high.
• Higher concentration in subtropical and temperate regions (dry winds) than in equatorial and polar regions.
• Particles of dust and salt serve as hygroscopic nuclei: water vapour condenses on them, forming clouds.

Worked example

Why are dust particles more concentrated in subtropical and temperate regions than at the equator? Why does this matter for rain? (2 marks)

Try it first, then show the solutionSolution

1. Subtropical and temperate regions have dry winds that lift and carry fine soil and dust; equatorial and polar regions have less of this.
2. Dust and salt particles act as hygroscopic nuclei on which water vapour condenses to form clouds, so they are needed for cloud formation.

Answer: Dry winds raise more dust there; dust provides nuclei for condensation and clouds.

Troposphere and tropopause

NCERT section Structure of the Atmosphere

Basis of layers: density is highest near the surface and falls with altitude; the column is divided into five layers by temperature conditions: troposphere, stratosphere, mesosphere, thermosphere, exosphere.

Troposphere:

• Lowermost layer; average height 13 km; about 8 km near the poles, about 18 km at the equator.
• Thickest at the equator because strong convectional currents carry heat to great heights.
• Contains dust particles and water vapour.
• All changes in weather and climate occur here.
• Temperature decreases by 1°C for every 165 m of height.
• Most important layer for biological activity.

Tropopause:

• The zone separating troposphere from stratosphere.
• Air temperature about -80°C over the equator and about -45°C over the poles.
• Temperature is nearly constant here, hence the name (pause).

Worked example

A city at sea level records 30°C. Using the chapter's rate, estimate the temperature at a hill station 1,650 m above it. (2 marks)

Try it first, then show the solutionSolution

1. Fall in temperature = 1,650 ÷ 165 = 10°C.
2. Temperature at the hill station ≈ 30 - 10 = 20°C.

Answer: About 20°C.

Stratosphere, mesosphere, thermosphere and exosphere

NCERT section Structure of the Atmosphere

Layer	Height	Key features
Stratosphere	Tropopause to 50 km	Contains the ozone layer, which absorbs ultraviolet radiation and shields life from this intense, harmful energy
Mesosphere	Above stratosphere to 80 km	Temperature decreases again with height, reaching -100°C at 80 km; upper limit is the mesopause
Ionosphere (in the thermosphere)	80-400 km, above the mesopause	Electrically charged particles (ions); reflects radio waves transmitted from the earth back to the earth; temperature increases with height
Exosphere	Above the thermosphere	Highest layer; very little known; contents extremely rarefied; gradually merges with outer space

Although all layers influence us, geographers are concerned with the first two layers (troposphere and stratosphere).

Worked example

In which layer would a radio signal be reflected back to earth, and why? (2 marks)

Try it first, then show the solutionSolution

1. The ionosphere, 80-400 km above the surface, above the mesopause.
2. It contains electrically charged particles (ions) that reflect radio waves transmitted from the earth back to the earth.

Answer: The ionosphere, because of its ions.

Elements of weather and climate

NCERT section Elements of Weather and Climate

The main elements of the atmosphere that change and influence human life are:
1. Temperature
2. Pressure
3. Winds
4. Humidity
5. Clouds
6. Precipitation

These are studied in detail in Chapters 8 (solar radiation and temperature), 9 (pressure, winds and weather systems) and 10 (water in the atmosphere).

Link to structure: all these weather changes happen in the troposphere, which holds the water vapour and dust needed for clouds and precipitation.

Worked example

A weather report says: "Hot and humid, 38°C, light south-westerly breeze, low pressure over the region, thick clouds building, heavy showers likely by evening." Pick out the elements of weather mentioned and say in which layer of the atmosphere they all operate. (2 marks)

Try it first, then show the solutionSolution

1. Temperature (38°C), humidity (humid), winds (south-westerly breeze), pressure (low pressure), clouds (thick clouds) and precipitation (showers): all six elements.
2. All changes in weather and climate take place in the troposphere.

Answer: Temperature, humidity, winds, pressure, clouds and precipitation; all operate in the troposphere.

Composition and Structure of Atmosphere in real life

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

Topics in this chapter

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

Composition and Structure of Atmosphere: key facts

Mass of the atmosphere

99% of mass within 32 km of the surface

Density highest near the surface.

Height limits of gases

O2 negligible at 120 km; CO2 and water vapour only up to 90 km; ozone 10-50 km

Composition changes with height.

Water vapour

Up to 4% by volume (warm wet tropics); < 1% (deserts, polar)

Decreases with altitude and towards the poles.

Lapse rate (troposphere)

$Temperaturefall(°C)=heightgained(m)÷165$

1°C per 165 m.

Troposphere height

Average 13 km; poles about 8 km; equator about 18 km

Thickest at the equator because of convection.

Tropopause temperature

About -80°C over the equator; about -45°C over the poles

Temperature nearly constant within it.

Layer heights

1. 1.Stratosphere to 50 km
2. 2.mesosphere to 80 km (-100°C)
3. 3.ionosphere 80-400 km
4. 4.exosphere

Five layers by temperature.

Elements

$Temperature,pressure,winds,humidity,clouds,precipitation$

Change and influence human life.

Composition and Structure of Atmosphere: diagrams to practise

Practise each one on paper: draw it from the checklist, then tick off every label.

How the school exam asks Composition and Structure of Atmosphere

The question patterns to expect, and the method that works for each.

Tricks to remember Composition and Structure of Atmosphere

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

• Layer order

  'The Smart Monkey Throws Eggs': Troposphere, Stratosphere, Mesosphere, Thermosphere, Exosphere.

  Use it when: Order of layers.

• Heights

  '13-50-80-400': top of troposphere (average), stratosphere, mesosphere, ionosphere.

  Use it when: Layer heights.

• Gas ceilings

  'CO2 and vapour quit at 90; oxygen fades by 120'.

  Use it when: Height limits MCQs.

• Elements

  'T-P-W-H-C-P': Temperature, Pressure, Winds, Humidity, Clouds, Precipitation.

  Use it when: Elements of weather.

Composition and Structure of Atmosphere: exam-level solved questions

Ordered from exam level to HOTS. Cover the solution, write your own, then compare step by step.

Case-study practice (Section E style)

Read the situation, then answer every part. In a school exam set on the board pattern this is a 4-mark question split into 1 + 1 + 2 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in Composition and Structure of Atmosphere

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

• ✗Mistake: Putting the ozone layer in the troposphere

  Students remember 'ozone protects us' and place it close to us.

  Wrong working

  The troposphere contains the ozone layer.

  Right working

  The ozone layer is in the stratosphere; ozone occurs between 10 and 50 km.

  ✓Do this: Strato = shield.

  Usually costs: 1 mark per MCQ or diagram label.

• ✗Mistake: Wrong lapse rate

  Students guess a round figure such as 1°C per 1,000 m.

  Wrong working

  Temperature falls 1°C per 1,000 m.

  Right working

  NCERT (this chapter): temperature falls 1°C for every 165 m. Chapter 8 states the normal lapse rate as 6.5°C per 1,000 m, which is nearly the same; use the rate the question names.

  ✓Do this: Write '165' on your formula sheet.

  Usually costs: 1-2 marks in a numerical.

• ✗Mistake: Reversing the troposphere thickness

  Assuming it is thicker where it is colder.

  Wrong working

  Troposphere is 18 km at the poles.

  Right working

  About 18 km at the equator, about 8 km at the poles, because of convection.

  ✓Do this: Hot air rises higher.

  Usually costs: 1 mark.

• ✗Mistake: Calling the ionosphere a separate sixth layer

  It is named separately in the text.

  Wrong working

  Six layers: troposphere, stratosphere, mesosphere, ionosphere, thermosphere, exosphere.

  Right working

  Five layers by temperature; the ionosphere lies in the thermosphere zone, 80-400 km.

  ✓Do this: Count five; ionosphere is a part.

  Usually costs: 1 mark.

• ✗Mistake: Saying carbon dioxide is opaque to sunlight

  Transparent and opaque get swapped.

  Wrong working

  CO2 blocks incoming sunlight.

  Right working

  CO2 is transparent to incoming solar radiation and opaque to outgoing terrestrial radiation.

  ✓Do this: In clear, out blocked.

  Usually costs: 1 mark.

NCERT exercise guide: Composition and Structure of Atmosphere

What each exercise in the NCERT chapter tests, how to approach it, and a question of the same kind solved step by step. Questions here are written in our own words.

Practice questions on Composition and Structure of 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 · MCQExam level

   Without tiny particles of sea salt, smoke and fine soil in the air, which of these would become much rarer?
   (a) Ozone formation
   (b) Cloud formation
   (c) Carbon dioxide absorption
   (d) The fall of temperature with height

   Hint

   Hygroscopic nuclei.

   Answer

   (b) Cloud formation: dust particles act as hygroscopic nuclei for condensation.

2. Q2 · 1 mark · MCQExam level

   Oxygen becomes almost negligible at a height of:
   (a) 90 km
   (b) 100 km
   (c) 120 km
   (d) 150 km

   Hint

   CO2 and water vapour stop at 90 km.

   Answer

   (c) 120 km.

3. Q3 · 1 mark · Assertion-reasonHarder exam level

   Assertion (A): Water vapour keeps the earth from becoming too hot or too cold.
   Reason (R): Water vapour absorbs part of the insolation and preserves the earth's radiated heat.
   Choose (a)-(d).

   Hint

   The 'blanket'.

   Answer

   (a). Both true; R explains A.

4. Q4 · 2 marks · Short answerHarder exam level

   Using the chapter's lapse rate, how much higher would you need to climb to experience a 15°C fall in temperature? (2 marks)

   Hint

   Multiply by 165.

   Answer

   15 × 165 = 2,475 m.

5. Q5 · 3 marks · Short answerHOTS / competency

   Why is the tropopause colder over the equator (about -80°C) than over the poles (about -45°C)? Reason from the chapter. (3 marks)

   Hint

   Height of the troposphere and the lapse rate.

   Answer

   The troposphere is about 18 km thick at the equator but about 8 km at the poles. Temperature keeps falling with height through the troposphere, so at the equator it falls over a much greater height before the tropopause is reached, making the tropopause there colder.

6. Q6 · 2 marks · Short answerExam level

   Why is carbon dioxide called meteorologically important? (2 marks)

   Hint

   Transparent and opaque.

   Answer

   It lets incoming solar radiation through but absorbs and partly reflects back outgoing terrestrial radiation, producing the greenhouse effect; its rising volume from burning fossil fuels has raised air temperature.

7. Q7 · 3 marks · Short answerHOTS / competency

   Suppose a city's air has very little dust because of strict pollution control, but humidity is high. Could clouds still form easily? Use the chapter's idea of hygroscopic nuclei. (3 marks)

   Hint

   Water vapour needs something to condense on.

   Answer

   Water vapour condenses around hygroscopic nuclei such as dust and salt particles to form clouds. With very few nuclei, condensation is less easy even if humidity is high; but sea salt, pollen and other particles from wider areas (and convection lifting particles) would still supply some nuclei, so clouds can form, though nuclei are a necessary ingredient.

8. Q8 · 5 marks · Long answerHarder exam level

   Draw and describe the structure of the atmosphere. (5 marks)

   Hint

   Five layers with heights.

   Answer

   Diagram with layers. Troposphere: average 13 km (8 km poles, 18 km equator), weather, 1°C per 165 m. Tropopause: -80°C equator, -45°C poles. Stratosphere to 50 km with ozone. Mesosphere to 80 km, -100°C, mesopause. Ionosphere 80-400 km reflects radio waves, temperature rises. Exosphere: rarefied, merges with space.

9. Q9 · 2 marks · Short answerHarder exam level

   Which two layers are geographers mainly concerned with, and why do you think so? (2 marks)

   Hint

   Weather and ozone.

   Answer

   The troposphere and stratosphere. The troposphere holds all weather and life processes; the stratosphere holds the ozone layer protecting life from ultraviolet rays.

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 share of the atmosphere's mass is within 32 km of the surface?Show answer

   99 per cent.

2. 2.Between what heights is ozone found?Show answer

   10 and 50 km.

3. 3.What is the troposphere's average height?Show answer

   13 km (about 8 km at the poles, 18 km at the equator).

4. 4.What is the temperature at the tropopause over the poles?Show answer

   About -45°C.

5. 5.What temperature does the mesosphere reach at 80 km?Show answer

   About -100°C.

6. 6.What are hygroscopic nuclei?Show answer

   Dust and salt particles around which water vapour condenses to form clouds.

7. 7.Why has carbon dioxide increased in recent decades?Show answer

   Mainly because of burning fossil fuels.

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Atmosphere: mixture of gases; 99% of mass within 32 km.
• ☐Composition: gases, water vapour, dust; O2 negligible at 120 km; CO2 and water vapour up to 90 km.
• ☐CO2: transparent to sunlight, opaque to terrestrial radiation; greenhouse effect; rising from fossil fuels.
• ☐Ozone (10-50 km) absorbs ultraviolet rays.
• ☐Water vapour: up to 4% in wet tropics, < 1% in deserts and polar areas; a blanket.
• ☐Dust: more in subtropical and temperate regions; hygroscopic nuclei for clouds.
• ☐Troposphere: 13 km average (8 poles, 18 equator); weather; 1°C per 165 m.
• ☐Tropopause: -80°C equator, -45°C poles.
• ☐Stratosphere to 50 km (ozone); mesosphere to 80 km (-100°C); ionosphere 80-400 km (radio); exosphere.
• ☐Elements: temperature, pressure, winds, humidity, clouds, precipitation.

Composition and Structure of Atmosphere: frequently asked questions