Composition and Structure of the Atmosphere: Gases, Layers and Ozone Depletion, ISC Class 11

What Composition and Structure of the Atmosphere is about

Weightage: CISCE gives no Class XI unit weightage for ISC Geography. Composition and Structure of the Atmosphere is topic 12 of the Class XI syllabus (Atmosphere) and can be asked anywhere in the 70-mark Paper I (Theory, 3 hours). Class XI is examined by the school, so your school sets the paper and decides how many marks each topic carries.Source: CISCE ISC Geography (853) syllabus, examination year 2028, Class XI

Key concepts

Composition of the atmosphere

Gases in dry air (by volume)
1. Nitrogen: about 78.1 per cent; fairly inert; enters living things through nitrogen-fixing bacteria and lightning.
2. Oxygen: about 20.9 per cent; needed for respiration and burning.
3. Argon: about 0.93 per cent; an inert gas.
4. Carbon dioxide: about 0.04 per cent (over 400 parts per million), rising because of burning of fossil fuels and deforestation. It lets most incoming sunlight pass but absorbs outgoing long-wave (terrestrial) radiation, so it is the main greenhouse gas after water vapour.
5. Trace gases: neon, helium, methane, krypton, hydrogen, ozone and others.

Water vapour: a variable gas, from almost nil in cold deserts to about 4 per cent by volume in warm, humid tropics; it decreases with height and from the equator towards the poles. It absorbs heat, forms clouds and precipitation, and releases latent heat when it condenses.

Solid particles (aerosols): dust, sea salt, smoke and soot, pollen, volcanic ash and meteor dust, concentrated in the lower layers. They act as hygroscopic (condensation) nuclei around which water vapour condenses, and they scatter sunlight, producing red skies at sunrise and sunset.

Heavier gases are concentrated lower down: oxygen becomes very scarce above about 120 km, and carbon dioxide and water vapour are found only in the lower 90 km or so.

Worked example

[4] Explain the importance of carbon dioxide, water vapour and dust particles in the atmosphere. (Understanding)

Try it first, then show the solutionSolution

1. Carbon dioxide: transparent to sunlight, absorbs terrestrial radiation - 1 mark.
2. Carbon dioxide: greenhouse effect; rising with fossil fuel use - 1 mark.
3. Water vapour: absorbs heat; source of clouds and rain; latent heat - 1 mark.
4. Dust: condensation nuclei; scattering gives red skies - 1 mark.

Answer: Carbon dioxide lets sunlight pass but absorbs outgoing heat from the earth, keeping the planet warm through the greenhouse effect; its share is rising with the burning of fossil fuels. Water vapour also absorbs heat, is the source of all clouds and precipitation, and releases latent heat on condensing. Dust particles act as nuclei on which vapour condenses and scatter sunlight, colouring sunrise and sunset.

Troposphere

Height: the lowest layer, extending to about 8 km at the poles and about 18 km at the equator (average about 13 km). It is thickest at the equator because strong convection carries heat high, and thicker in summer than in winter.

Characteristics
1. Temperature falls with height at an average rate of about 6.5 degrees C per 1,000 m (the normal lapse rate), because the air is heated mainly from below by the earth's surface.
2. It contains about three-quarters of the mass of the atmosphere and almost all its water vapour and dust.
3. All weather happens here: clouds, rain, storms, winds and convection currents. It is the layer of most biological activity.
4. Tropopause: the boundary at the top, where temperature stops falling; it is about minus 80 degrees C over the equator (because it is so high) and about minus 45 degrees C over the poles. Jet streams blow near the tropopause.

The word troposphere means 'sphere of mixing' or 'turning', from the constant vertical mixing of its air.

Worked example

[4] Why is the troposphere thickest at the equator, and why is it called the weather layer? (Understanding)

Try it first, then show the solutionSolution

1. Strong heating at the equator causes powerful convection - 1 mark.
2. Convection lifts heat and air to about 18 km (8 km at the poles) - 1 mark.
3. It holds almost all water vapour and dust - 1 mark.
4. So clouds, rain, storms and winds all occur here - 1 mark.

Answer: The troposphere is thickest at the equator, about 18 km against about 8 km at the poles, because intense heating there sets up strong convection currents that carry heat and air to great heights. It is called the weather layer because it holds nearly all the water vapour and dust, so clouds, precipitation, storms and winds all form within it.

Stratosphere and ozonosphere

Stratosphere
1. Height: from the tropopause up to about 50 km.
2. Temperature: nearly constant in its lower part, then rises with height to about 0 degrees C near the stratopause, because ozone absorbs ultraviolet radiation.
3. Air is dry and calm, with almost no clouds or weather (except rare nacreous or polar stratospheric clouds over polar regions in winter); horizontal winds are steady, so long-distance jet aircraft often cruise in the lower stratosphere to avoid turbulence.

Ozonosphere (ozone layer)
1. The part of the stratosphere, mainly between about 15 and 35 km, where ozone (O3) is concentrated.
2. Ozone forms when ultraviolet rays split oxygen molecules (O2) into atoms, which combine with other O2 molecules to form O3.
3. It absorbs most of the harmful ultraviolet (UV-B and UV-C) radiation of the sun, protecting living things from skin cancer, eye cataracts, damage to plants and plankton.
4. The absorbed energy warms this layer, which is why temperature rises with height in the stratosphere.

Ozone is measured in Dobson Units; an 'ozone hole' is an area where total ozone falls below about 220 Dobson Units.

Worked example

[4] Why does temperature increase with height in the stratosphere? State the importance of the ozonosphere. (Understanding)

Try it first, then show the solutionSolution

1. Ozone concentrated about 15-35 km in the stratosphere - 1 mark.
2. Ozone absorbs ultraviolet radiation, releasing heat - 1 mark.
3. So temperature rises with height up to the stratopause - 1 mark.
4. Importance: shields life from harmful UV (skin cancer, cataracts, crops) - 1 mark.

Answer: The ozone layer, concentrated mainly between about 15 and 35 km, absorbs ultraviolet radiation from the sun and converts it into heat, so temperature in the stratosphere rises with height up to the stratopause. This ozonosphere protects life by filtering out harmful ultraviolet rays that cause skin cancer and eye cataracts and damage crops and marine plankton.

Mesosphere, ionosphere (thermosphere) and exosphere

Mesosphere
1. Height: about 50 km to 80 km.
2. Temperature falls with height to about minus 100 degrees C at the mesopause, the coldest part of the atmosphere.
3. Most meteors burn up here because of friction with the air, appearing as shooting stars. Noctilucent clouds sometimes form near its top in high latitudes.

Ionosphere (thermosphere)
1. Height: about 80 km to 400 km (and beyond).
2. Temperature rises sharply with height, because the very thin gases absorb high-energy solar radiation; it can reach over 1,000 degrees C, though the air is so thin that it would not feel hot.
3. Solar radiation ionises the gas atoms into electrically charged particles (ions and electrons), which form layers (often called D, E and F layers) that reflect radio waves back to earth, making long-distance radio communication possible.
4. Auroras (aurora borealis in the north, aurora australis in the south) occur here when charged particles from the sun strike the gases near the poles.
5. Many satellites orbit within its upper part.

Exosphere
The outermost layer, above about 400-500 km, where the air is extremely thin and light gases such as hydrogen and helium gradually escape into space. Very little is known about it.

Worked example

[4] State two special characteristics each of the mesosphere and the ionosphere. (Recall)

Try it first, then show the solutionSolution

1. Mesosphere: 50-80 km; temperature falls to about minus 100 degrees C - 1 mark.
2. Mesosphere: meteors burn up - 1 mark.
3. Ionosphere: 80-400 km; ions reflect radio waves - 1 mark.
4. Ionosphere: temperature rises with height; auroras - 1 mark.

Answer: The mesosphere extends from about 50 to 80 km; temperature falls with height to about minus 100 degrees C, and most meteors burn up in it. The ionosphere extends from about 80 to 400 km; its electrically charged particles reflect radio waves back to earth, temperature rises with height, and auroras appear in it near the poles.

Ozone depletion

Meaning: thinning of the ozone layer in the stratosphere, most severe over Antarctica each spring (September to November), where an 'ozone hole' was reported in the mid-1980s.

Causes
1. Chlorofluorocarbons (CFCs) from old refrigerators, air conditioners, aerosol sprays and foam-making; halons from fire extinguishers; carbon tetrachloride and methyl chloroform used as solvents; nitrous oxide from fertilisers, and nitrogen oxides from high-flying supersonic aircraft.
2. These stable gases rise slowly into the stratosphere, where ultraviolet rays break them down, releasing chlorine and bromine atoms.
3. One chlorine atom acts as a catalyst and can destroy thousands of ozone molecules (Cl + O3 -> ClO + O2; ClO + O -> Cl + O2).
4. Over Antarctica, very cold polar stratospheric clouds in winter speed up these reactions when sunlight returns in spring.

Effects: more ultraviolet radiation reaches the surface, increasing skin cancer, cataracts and weakened immunity; it harms crops, phytoplankton and the marine food chain, and damages materials such as plastics.

Action: the Montreal Protocol (1987) phased out the production of CFCs and other ozone-depleting substances; its Kigali Amendment (2016) phases down HFCs, which are greenhouse gases. India is a party to the Protocol. The ozone layer is now slowly recovering; 16 September is observed as World Ozone Day.

Worked example

[4] Explain how CFCs destroy ozone and name the international agreement to control them. (Understanding)

Try it first, then show the solutionSolution

1. CFCs released at the surface rise into the stratosphere - 1 mark.
2. UV rays split CFCs, releasing chlorine atoms - 1 mark.
3. Each chlorine atom destroys thousands of ozone molecules as a catalyst - 1 mark.
4. Montreal Protocol, 1987 - 1 mark.

Answer: CFCs released from refrigerators, air conditioners and sprays are very stable, so they drift up into the stratosphere. There ultraviolet rays break them apart and free chlorine atoms, each of which acts as a catalyst and destroys thousands of ozone molecules. The Montreal Protocol of 1987 controls and phases out CFCs and other ozone-depleting substances.

Composition and Structure of the 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

• Composition and Layers of the Atmosphere
• The Ozone Layer and Ozone Depletion

Composition and Structure of the Atmosphere: key facts

Composition (dry air, by volume)

$N278.1$

Water vapour 0-4%, variable.

Normal lapse rate

$Fall=6.5degreesCxheight(km)$

In the troposphere.

Layer heights

Troposphere 0-8/18 km; stratosphere to 50 km (ozone ~15-35 km); mesosphere 50-80 km; ionosphere 80-400 km; exosphere above

By temperature trend.

Temperature trend

$Falls-rises-falls-rises$

Tropo, strato, meso, thermo.

Ozone destruction

$Cl+O3->ClO+O2;ClO+O->Cl+O2$

Chlorine is recycled; Montreal Protocol 1987.

How the school exam asks Composition and Structure of the Atmosphere

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

Tricks to remember Composition and Structure of the Atmosphere

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

• Layers bottom to top: 'The Strong Man In Ecuador'

  Troposphere, Stratosphere, Mesosphere, Ionosphere (thermosphere), Exosphere.

  Use it when: Order of layers.

• Heights: '13-50-80-400'

  Troposphere about 13 km on average, stratosphere to 50, mesosphere to 80, ionosphere to 400.

  Use it when: Layer heights.

• Zig-zag temperature

  Down, up, down, up: cools, warms (ozone), cools (coldest at mesopause), warms (sun's energy).

  Use it when: Temperature profile.

Composition and Structure of the 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 D style)

Read the situation, then answer every part. In a school exam set on the board pattern this is a 8-mark question split into 2 + 2 + 2 + 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 the Atmosphere

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

• ✗Mistake: Saying temperature falls throughout the atmosphere

  Applying the lapse rate beyond the troposphere.

  Wrong working

  Temperature keeps decreasing with height in all layers.

  Right working

  It falls, rises (ozone), falls, then rises again.

  ✓Do this: Remember the zig-zag profile.

  Usually costs: 1-2 marks.

• ✗Mistake: Giving one fixed height for the troposphere

  It varies with latitude and season.

  ✓Do this: About 8 km at the poles, about 18 km at the equator.

  Usually costs: 1 mark.

• ✗Mistake: Linking the ozone hole to global warming as the same thing

  Two different problems.

  ✓Do this: Ozone depletion lets in UV; global warming is caused by greenhouse gases trapping heat.

  Usually costs: 1 mark.

• ✗Mistake: Placing the ozone layer in the troposphere

  Ground-level ozone confused with stratospheric ozone.

  ✓Do this: The protective layer is in the stratosphere (about 15-35 km).

  Usually costs: 1 mark.

• ✗Mistake: Lapse-rate sums with wrong height difference

  Using the full altitude instead of the difference.

  ✓Do this: Always subtract the starting height first.

  Usually costs: 1-2 marks.

Practice questions on Composition and Structure 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.

1. Q1 · 1 mark · Very short answerExam level

   Which gas forms about 78 per cent of dry air?

   Hint

   Most abundant.

   Answer

   Nitrogen.

2. Q2 · 1 mark · Very short answerExam level

   Which layer reflects radio waves back to the earth?

   Hint

   Ions.

   Answer

   The ionosphere.

3. Q3 · 1 mark · MCQExam level

   Meteors mostly burn up in the:
   (a) Troposphere
   (b) Stratosphere
   (c) Mesosphere
   (d) Exosphere

   Hint

   50-80 km.

   Answer

   (c) Mesosphere.

4. Q4 · 1 mark · Very short answerHarder exam level

   Name the agreement of 1987 that phased out CFCs.

   Hint

   Canada.

   Answer

   The Montreal Protocol.

5. Q5 · 4 marks · Short answerHarder exam level

   [4] 'Water vapour and dust are small in amount but very important.' Explain.

   Hint

   Clouds, heat, nuclei.

   Answer

   (1) Water vapour is at most about 4 per cent but is the source of all clouds and precipitation. (2) It absorbs heat and releases latent heat, powering storms. (3) Dust and salt particles act as nuclei on which vapour condenses. (4) Dust scatters light, giving red sunrises and sunsets and affecting how much sunlight reaches the ground.

6. Q6 · 4 marks · Short answerHarder exam level

   [4] Explain why the ozone layer is called a protective shield, and state two effects of its depletion.

   Hint

   UV.

   Answer

   (1) Ozone in the stratosphere absorbs most harmful ultraviolet radiation. (2) This stops it from reaching living things at the surface. (3) Depletion increases skin cancer and eye cataracts in people. (4) It also harms crops and marine plankton, upsetting food chains.

7. Q7 · 4 marks · Short answerHarder exam level

   [4] Distinguish between the troposphere and the stratosphere.

   Hint

   Height, temperature, weather, contents.

   Answer

   (1) Troposphere: up to 8-18 km; stratosphere: from the tropopause to about 50 km. (2) Temperature falls with height in the troposphere but rises in the stratosphere. (3) The troposphere has all weather; the stratosphere is calm and cloudless. (4) The troposphere holds water vapour and dust; the stratosphere holds the ozone layer.

8. Q8 · 4 marks · Short answerHOTS / competency

   [4] A mountaineer at a base camp at 3,000 m records 8 degrees C. Using 6.5 degrees C per km, estimate the temperature at the summit at 8,000 m and at sea level directly below.

   Hint

   Height differences.

   Answer

   (1) Summit: rise of 5,000 m; fall = 6.5 x 5 = 32.5 degrees C. (2) Summit temperature = 8 - 32.5 = -24.5 degrees C. (3) Sea level: 3,000 m lower; rise = 6.5 x 3 = 19.5 degrees C. (4) Sea-level temperature = 8 + 19.5 = 27.5 degrees C.

9. Q9 · 4 marks · Short answerHOTS / competency

   [4] The ozone hole is slowly recovering. Explain why recovery takes decades, and why the Kigali Amendment was added to the Montreal Protocol.

   Hint

   Lifetime of CFCs; HFCs.

   Answer

   (1) CFCs already released are very stable and stay in the atmosphere for decades. (2) Each chlorine atom is recycled and goes on destroying ozone for a long time. (3) CFCs were replaced by HFCs, which do not harm ozone but are powerful greenhouse gases. (4) The Kigali Amendment (2016) phases down HFCs to protect the climate too.

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 is the tropopause?Show answer

   The boundary between the troposphere and the stratosphere, where temperature stops falling with height.

2. 2.Which is the coldest part of the atmosphere?Show answer

   The mesopause, at about 80 km (about minus 100 degrees C).

3. 3.What are hygroscopic nuclei?Show answer

   Dust and salt particles around which water vapour condenses.

4. 4.In which layer do auroras occur?Show answer

   The ionosphere (thermosphere).

5. 5.What unit measures total ozone?Show answer

   The Dobson Unit.

6. 6.When is World Ozone Day?Show answer

   16 September.

Quick revision checklist

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

• ☐N2 ~78%, O2 ~21%, Ar ~0.93%, CO2 ~0.04% and rising; water vapour 0-4%; dust as nuclei.
• ☐Troposphere: 8 km (poles) to 18 km (equator); falls 6.5 degrees C/km; all weather.
• ☐Tropopause: about minus 80 degrees C (equator), minus 45 degrees C (poles).
• ☐Stratosphere to 50 km; ozone layer ~15-35 km absorbs UV; temperature rises; calm, jets fly low in it.
• ☐Mesosphere 50-80 km: falls to about minus 100 degrees C; meteors burn.
• ☐Ionosphere 80-400 km: ions reflect radio waves; auroras; temperature rises.
• ☐Exosphere: thin; light gases escape.
• ☐Ozone depletion: CFCs -> chlorine; Antarctic spring hole; Montreal Protocol 1987, Kigali 2016.

Composition and Structure of the Atmosphere: frequently asked questions