Composition and Structure of the Atmosphere ICSE Class 9

What Composition and Structure of the Atmosphere is about

Weightage: CISCE publishes no chapter weightage and no question-paper layout for ICSE Class 9 Geography. Class IX is examined by your school: one 80-mark paper of 2 hours plus 20 marks of internal assessment (a record file and one project). Composition and Structure of the Atmosphere is topic 4(a), Atmosphere, of the 2028 syllabus.Source: CISCE ICSE 2028 Geography syllabus, Class IX

Key concepts

Composition of the atmosphere

The atmosphere is a mixture of gases, water vapour and dust particles.

Permanent gases (by volume of dry air):

• Nitrogen: about 78%. Dilutes oxygen; needed by plants (fixed in the soil by bacteria and lightning).
• Oxygen: about 21%. Needed for breathing and burning.
• Argon: about 0.93%, an inert gas.
• Carbon dioxide: about 0.04% and rising. Used by plants in photosynthesis; absorbs heat radiated by the earth (a greenhouse gas).
• Others in tiny amounts: neon, helium, ozone, hydrogen, methane.

Variable components:

• Water vapour: up to about 4% of the air in warm, wet tropics, less than 1% in cold, dry areas; source of clouds and precipitation; absorbs heat.
• Dust particles (sea salt, fine soil, smoke, ash, pollen): act as nuclei on which water vapour condenses; scatter sunlight, giving red and orange sunrises and sunsets.

Most of the mass of the atmosphere lies close to the surface; the air becomes thinner with height, which is why climbers on high mountains need oxygen cylinders.

Worked example

(2 marks) Give the proportions of the two main gases in the atmosphere and state the importance of dust particles.

Try it first, then show the solutionSolution

1. Nitrogen about 78%, oxygen about 21% - 1 mark.
2. Dust particles act as nuclei for condensation and scatter sunlight - 1 mark.

Answer: Nitrogen makes up about 78% and oxygen about 21% of dry air. Dust particles act as nuclei around which water vapour condenses to form clouds and fog, and they scatter sunlight, colouring sunrise and sunset.

Troposphere

The troposphere is the lowest layer of the atmosphere.

• Height: about 18 km at the equator and about 8 km at the poles (average about 13 km). It is thicker at the equator because strong heating there carries air upward by convection.
• Temperature falls with height at an average rate of 6.5 °C per 1,000 m (the normal lapse rate), because the air is heated mainly from below by the earth.
• It contains almost all the water vapour and dust, so all weather (clouds, rain, storms, winds) occurs here. 'Tropo' means 'turning' or 'change'.
• Most of the mass of the atmosphere is in this layer.
• Tropopause: the boundary at the top of the troposphere, where the temperature stops falling. It is extremely cold, about −80 °C over the equator and about −45 °C over the poles.

Worked example

(3 marks) Why is the troposphere called the 'weather layer'? Why is it thicker over the equator than over the poles?

Try it first, then show the solutionSolution

1. It contains nearly all the water vapour and dust needed for clouds and rain - 1 mark.
2. So clouds, rain, storms and winds all occur in it - 1 mark.
3. It is thicker at the equator because strong heating sets up convection currents that carry air higher - 1 mark.

Answer: The troposphere contains nearly all the water vapour and dust, so all weather phenomena such as clouds, rain, storms and winds occur in it. It is thicker over the equator (about 18 km) than over the poles (about 8 km) because intense heating there sets up strong convection currents that carry air to greater heights.

Stratosphere and the ozone layer

The stratosphere lies above the tropopause, up to about 50 km.

• Temperature: stays almost constant in its lower part and then rises with height, because ozone absorbs ultraviolet (UV) radiation from the sun and is warmed by it.
• It is almost free of clouds, water vapour and weather disturbances, and has steady horizontal winds; long-distance jet aircraft often fly in its lower part for a smooth ride.
• The ozone layer lies in the stratosphere, roughly between 15 and 35 km. Ozone (O₃) is a form of oxygen with three atoms.

Importance of ozone: it absorbs most of the sun's harmful ultraviolet rays, protecting living things from skin cancer, eye damage and harm to plants and plankton. It is therefore called the earth's protective shield.

Stratopause: the upper boundary at about 50 km.

Mesosphere (above the stratosphere, about 50-80 km): temperature falls again with height to about −100 °C; most meteors burn up here.

Worked example

(2 marks) Why does temperature rise with height in the stratosphere?

Try it first, then show the solutionSolution

1. The ozone layer lies in the stratosphere - 1 mark.
2. Ozone absorbs the sun's ultraviolet rays and is heated, so the upper stratosphere is warmer - 1 mark.

Answer: The stratosphere contains the ozone layer. Ozone absorbs ultraviolet radiation from the sun and is warmed by it, so temperature increases with height in the stratosphere.

Ionosphere and exosphere

Ionosphere: from about 80 km to about 400 km above the earth.

• It contains electrically charged particles (ions), formed when the sun's high-energy radiation strips electrons from gas atoms.
• It reflects radio waves back to the earth, making long-distance radio communication possible.
• Temperature rises with height.
• Auroras (the Northern and Southern Lights) are glowing displays produced here when charged particles from the sun strike the upper atmosphere near the poles.

Exosphere: the outermost layer, above the ionosphere.

• The air is extremely thin, made mainly of light gases such as hydrogen and helium.
• It gradually merges into outer space; there is no clear upper boundary.
• Many satellites orbit at these heights.

Diagram checklist (layers of the atmosphere): a vertical bar or column with heights marked (8-18 km, 50 km, 80 km, 400 km); label troposphere, tropopause, stratosphere with ozone layer, mesosphere, ionosphere, exosphere; add a temperature line falling in the troposphere, rising in the stratosphere, falling in the mesosphere and rising in the ionosphere; small symbols for clouds (troposphere), jet plane (lower stratosphere), meteor (mesosphere), radio wave reflected (ionosphere), satellite (exosphere).

Worked example

(2 marks) State one important feature each of the ionosphere and the exosphere.

Try it first, then show the solutionSolution

1. Ionosphere: has electrically charged particles that reflect radio waves - 1 mark.
2. Exosphere: outermost, very thin air of light gases (hydrogen, helium), merging into space - 1 mark.

Answer: The ionosphere (about 80-400 km) contains charged particles that reflect radio waves back to the earth, aiding communication. The exosphere is the outermost layer, with very thin air of light gases such as hydrogen and helium, which gradually merges into space.

Ozone depletion

Ozone depletion is the thinning of the ozone layer in the stratosphere by man-made chemicals.

Causes:

• Chlorofluorocarbons (CFCs), formerly used in refrigerators, air conditioners, aerosol sprays and foam packaging. They rise slowly to the stratosphere, where UV rays break them up and release chlorine atoms.
• One chlorine atom can destroy thousands of ozone molecules before it is removed.
• Halons (used in fire extinguishers), methyl bromide (a pesticide) and some other chemicals also destroy ozone.

The ozone hole: a severe seasonal thinning of ozone over Antarctica, mainly in the southern spring (September to November); its discovery was announced by British scientists in 1985.

Effects of depletion: more UV rays reach the surface, causing skin cancer, cataracts (eye damage) and weaker immunity in people; damage to crops and to plankton, the base of ocean food chains.

Action: the Montreal Protocol (1987) is an international agreement to phase out ozone-depleting substances. 16 September is the International Day for the Preservation of the Ozone Layer. Because the use of CFCs has fallen sharply, scientists expect the ozone layer to recover slowly over the coming decades.

Worked example

(3 marks) How do CFCs damage the ozone layer? State two effects of ozone depletion.

Try it first, then show the solutionSolution

1. CFCs from refrigerators, ACs and sprays rise into the stratosphere - 1 mark.
2. UV rays break them and release chlorine, which destroys ozone molecules - 1 mark.
3. Effects: skin cancer, cataracts, damage to crops or plankton (any two) - 1 mark.

Answer: CFCs released from refrigerators, air conditioners and aerosol sprays drift up to the stratosphere, where ultraviolet rays break them down and free chlorine atoms; each chlorine atom destroys many ozone molecules. As a result more UV rays reach the earth, increasing skin cancer and cataracts and damaging crops and plankton.

Global warming and its impact

The greenhouse effect: the earth's surface absorbs sunlight and gives out heat (long-wave radiation). Gases such as carbon dioxide, water vapour, methane and nitrous oxide absorb part of this heat and send some of it back to the surface, as the glass of a greenhouse traps warmth. This natural effect keeps the earth's average temperature at about 15 °C instead of well below freezing.

Global warming is the gradual rise in the average temperature of the earth's surface and lower atmosphere, caused by the increase of greenhouse gases due to human activities. The world is now more than 1 °C warmer than before industrialisation.

Causes:

• Burning of fossil fuels (coal, oil, natural gas) in power stations, vehicles and industry: the largest source of carbon dioxide.
• Deforestation: fewer trees to absorb carbon dioxide, and burning releases more.
• Agriculture: methane from flooded rice fields and cattle; nitrous oxide from chemical fertilisers.
• Industrial gases such as CFCs and their substitutes, and methane from rubbish dumps.

Impact:

• Melting of glaciers and ice sheets; rising sea level threatens low-lying coasts and islands such as the Maldives, the Sundarbans and Bangladesh.
• More frequent heat waves, droughts, floods and intense cyclones.
• Coral bleaching and loss of biodiversity as species cannot adapt fast enough.
• Changes in rainfall affect crop yields; Himalayan glaciers that feed rivers like the Ganga are shrinking.
• Spread of diseases such as malaria and dengue to new areas.

Remedies: use renewable energy, save energy, plant trees, use public transport; international agreements such as the Paris Agreement (2015) aim to keep warming well below 2 °C.

Worked example

(3 marks) What is global warming? State two of its causes and two impacts.

Try it first, then show the solutionSolution

1. Rise in the earth's average temperature due to increased greenhouse gases from human activities - 1 mark.
2. Causes: burning fossil fuels; deforestation (or methane from agriculture) - 1 mark.
3. Impacts: melting ice and rising sea level; extreme weather (or coral bleaching) - 1 mark.

Answer: Global warming is the rise in the earth's average temperature caused by an increase in greenhouse gases due to human activities. Two causes are burning fossil fuels and deforestation. Two impacts are melting glaciers with rising sea levels, and more frequent heat waves, floods and droughts.

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

• Layers of the Atmosphere, Ozone Depletion and Global Warming

Composition and Structure of the Atmosphere: key facts

Composition

N₂ ≈ 78%; O₂ ≈ 21%; Ar ≈ 0.93%; CO₂ ≈ 0.04%; water vapour 0-4%

Dust: condensation nuclei.

Layers (from the ground)

$Troposphere(to8-18km)\to stratosphere(to50km)\to mesosphere(to80km)\to ionosphere(80-400km)\to exosphere$

Boundaries: tropopause, stratopause.

Normal lapse rate

Temperature falls about 6.5 °C per 1,000 m in the troposphere

Air is heated from below.

Ozone

O₃ in the stratosphere (about 15-35 km) absorbs UV; destroyed by chlorine from CFCs

Montreal Protocol 1987.

Greenhouse gases

CO₂, methane, nitrous oxide, water vapour, CFCs

Fossil fuels, deforestation, agriculture.

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.

• The Silly Monkey Is Eating

  Troposphere, Stratosphere, Mesosphere, Ionosphere, Exosphere.

  Use it when: Order of the layers.

• Temperature zigzag

  Down (tropo), up (strato, ozone), down (meso), up (iono).

  Use it when: Temperature trend in each layer.

• Ozone hole is not the greenhouse

  CFCs make the Hole; CO₂ makes the Heat.

  Use it when: Separating the two problems.

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 B style)

Read the situation, then answer every part. In a school exam set on the board pattern this is a 6-mark question split into 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: Confusing the ozone hole with global warming

  Both are 'atmosphere problems'.

  Wrong working

  The ozone hole lets in heat and causes global warming.

  Right working

  The ozone hole lets in more UV; global warming is due to greenhouse gases trapping heat.

  ✓Do this: CFCs → ozone hole; CO₂ and other greenhouse gases → warming.

  Usually costs: 1 to 2 marks.

• ✗Mistake: Saying temperature always falls with height

  The troposphere rule is applied to all layers.

  ✓Do this: Falls in the troposphere and mesosphere; rises in the stratosphere and ionosphere.

  Usually costs: 1 mark.

• ✗Mistake: Giving one height for the troposphere

  The variation is forgotten.

  ✓Do this: About 8 km at the poles and 18 km at the equator, with the reason.

  Usually costs: 1 mark.

• ✗Mistake: Wrong order of layers in the diagram

  Names learnt without sequence.

  ✓Do this: Tropo, Strato, Meso, Iono, Exo; mark heights.

  Usually costs: 1 to 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 · MCQExam level

   The ozone layer is found in the:
   (a) troposphere
   (b) stratosphere
   (c) ionosphere
   (d) exosphere

   Hint

   Above the weather layer.

   Answer

   (b) stratosphere.

2. Q2 · 1 mark · MCQExam level

   Which gas forms about 21% of the atmosphere?
   (a) Nitrogen
   (b) Carbon dioxide
   (c) Oxygen
   (d) Argon

   Hint

   We breathe it.

   Answer

   (c) Oxygen.

3. Q3 · 2 marks · Very short answerExam level

   Name two greenhouse gases and one human source of each.

   Hint

   Fuels, farms.

   Answer

   Carbon dioxide: burning coal and petrol. Methane: flooded rice fields, cattle or rubbish dumps. (Nitrous oxide: chemical fertilisers.)

4. Q4 · 2 marks · Very short answerHarder exam level

   Why is carbon dioxide important even though it forms only about 0.04% of the air?

   Hint

   Plants and heat.

   Answer

   Plants use it in photosynthesis to make food, the base of almost all food chains; and it absorbs heat radiated by the earth, keeping the planet warm (greenhouse effect).

5. Q5 · 2 marks · Very short answerHarder exam level

   State two differences between the troposphere and the stratosphere.

   Hint

   Temperature trend, weather.

   Answer

   (1) Temperature falls with height in the troposphere but rises with height in the stratosphere. (2) The troposphere has clouds and all weather; the stratosphere is nearly free of clouds and contains the ozone layer.

6. Q6 · 2 marks · Very short answerHarder exam level

   What are auroras and in which layer do they occur?

   Hint

   Charged particles.

   Answer

   Auroras are coloured light displays in the sky near the poles, produced when charged particles from the sun strike gases in the upper atmosphere. They occur in the ionosphere.

7. Q7 · 2 marks · Short answerHOTS / competency

   Why did the Montreal Protocol succeed in slowing ozone depletion?

   Hint

   Global agreement, substitutes.

   Answer

   Nearly all countries agreed to phase out CFCs and other ozone-depleting substances, and safer substitutes were available for refrigerators and sprays. So the release of these chemicals fell sharply and the ozone layer has begun to recover slowly.

8. Q8 · 2 marks · Very short answerHarder exam level

   Explain why the exosphere has no fixed upper limit.

   Hint

   Thin, light gases.

   Answer

   Its air is so thin, made of a few light atoms such as hydrogen and helium, that the density simply falls off gradually until it merges with outer space; there is no clear boundary.

9. Q9 · 3 marks · Short answerHarder exam level

   Suggest three steps your family could take to reduce its contribution to global warming.

   Hint

   Energy, transport, trees.

   Answer

   (1) Save electricity by switching off unused appliances and using LED bulbs and efficient appliances. (2) Use public transport, carpools, bicycles or walking instead of private cars. (3) Plant trees, reduce waste and use solar water heaters or other renewable energy.

10. Q10 · 3 marks · Short answerHOTS / competency

    A student says, 'Global warming is caused by the ozone hole letting in more heat.' Evaluate this statement.

    Hint

    Two different problems.

    Answer

    It is incorrect. The ozone hole lets in more ultraviolet rays, which harm health but are not the main cause of warming. Global warming is caused by greenhouse gases (mainly carbon dioxide from fossil fuels) trapping the earth's outgoing heat. The two are linked only in that CFCs are also greenhouse gases.

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.Percentage of nitrogen in the air?Show answer

   About 78%.

2. 2.Height of the troposphere at the equator?Show answer

   About 18 km.

3. 3.Layer that reflects radio waves?Show answer

   Ionosphere.

4. 4.Main cause of ozone depletion?Show answer

   Chlorofluorocarbons (CFCs).

5. 5.Treaty to protect the ozone layer?Show answer

   The Montreal Protocol (1987).

6. 6.Largest human source of carbon dioxide?Show answer

   Burning fossil fuels.

Quick revision checklist

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

• ☐Air: N₂ 78%, O₂ 21%, Ar 0.93%, CO₂ 0.04%, water vapour 0-4%, dust.
• ☐Troposphere: 8-18 km; −6.5 °C per km; all weather; tropopause.
• ☐Stratosphere: to 50 km; ozone layer; temperature rises; jets fly here.
• ☐Ionosphere: 80-400 km; charged particles; reflects radio waves; auroras. Exosphere: thin H and He, merges into space.
• ☐Ozone depletion: CFCs release chlorine; more UV; Montreal Protocol 1987.
• ☐Global warming: more greenhouse gases; melting ice, rising seas, extreme weather.

Composition and Structure of the Atmosphere: frequently asked questions