This MCQ module is based on: Layered Structure — Troposphere to Exosphere & Exercises
TOPIC 15 OF 29
Layered Structure — Troposphere to Exosphere & Exercises
🎓 Class 11
Social Science
CBSE
Theory
Ch 7 — Composition and Structure of Atmosphere
⏱ ~28 min
🌐 Language: [gtranslate]
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7.5 Structure of the Atmosphere — Five Stacked Layers
The atmosphere consists of different layers with varying density and temperature. Density is highest near the surface of the earth and decreases with increasing altitude — that is why mountaineers carry oxygen cylinders. The column of atmosphere is divided into five different layers, depending mainly on their temperature behaviour with height. From the surface upward they are: troposphere → stratosphere → mesosphere → thermosphere (with the ionosphere within it) → exosphere. Geographers are most directly concerned with the lowermost two layers because almost everything that affects daily life happens there.
📖 Definition — Atmospheric Layer
An atmospheric layer is a vertically defined zone in which temperature changes with height in a characteristic way — either decreasing, increasing, or remaining constant. The boundary between two layers is named after the layer below it (e.g. tropopause? separates the troposphere from the stratosphere) and is marked by a reversal in the temperature trend.
The Five Layers — Cross-Section with Temperature Profile
7.6 Troposphere — The Living Layer
The troposphere is the lowermost layer of the atmosphere. Its average height is 13 km, but it extends roughly to a height of 8 km near the poles and about 18 km at the equator. The thickness of the troposphere is greatest at the equator because heat is transported to great heights by strong convectional currents rising from the warm equatorial surface. Towards the poles, those currents are weak, so the layer is thinner.
This layer contains the dust particles and water vapour studied in Part 1. All changes in climate and weather — clouds, rainfall, thunderstorms, cyclones, fog, dew — take place in this layer. The temperature decreases at the rate of approximately 1 °C for every 165 m of height, a value known as the normal lapse rate. Because virtually all biological activity, food production, breathing and weather happen here, the troposphere is rightly called the most important layer for life.
🔑 Quick Number
Average thickness 13 km; 8 km over poles; 18 km over equator. Lapse rate ≈ 1 °C / 165 m. All weather happens here.
Tropopause — The Cold Lid
The zone separating the troposphere from the stratosphere above is known as the tropopause. The air temperature at the tropopause is about −80 °C over the equator and about −45 °C over the poles (the equatorial tropopause is colder because the air column rises higher and therefore cools more). The temperature in this thin transitional zone is nearly constant with height, and that is why it is called the "tropo-pause" — pause meaning a flattening out of the temperature profile.
THINK ABOUT IT — Why Is the Tropopause Colder over the Equator?
L4 Analyse
The equator is the hottest part of the surface, yet the tropopause above the equator is colder (−80 °C) than the tropopause above the poles (−45 °C). Explain.
Strong equatorial convection lifts warm air to a much greater height (about 18 km) before it stops rising; over the poles, the weak convection halts at only about 8 km. Because the temperature falls at roughly 1 °C per 165 m, an air parcel that rises 18 km has cooled far more than one rising only 8 km. So the equatorial tropopause is the higher and therefore the colder of the two — a counter-intuitive result that makes perfect sense once you remember where the air finally stops rising.
7.7 Stratosphere — Calm, Ozone-Rich and Aviation-Friendly
The stratosphere is found above the tropopause and extends up to a height of 50 km. One important feature of the stratosphere is that it contains the ozone layer?. This layer absorbs the ultra-violet radiation streaming in from the Sun and shields life on the earth from this intense and harmful form of energy.
The stratosphere has two notable behavioural traits that make it strikingly different from the troposphere:
- The lower stratosphere is nearly isothermal — temperature stays roughly constant with height — and the upper stratosphere shows a temperature increase caused by absorption of UV by ozone. So the lapse rate is the opposite of what we saw in the troposphere.
- Because there are no convectional currents and no water vapour worth speaking of, the air is extremely stable — no weather, no clouds (except very thin polar stratospheric ones), no turbulence. This is exactly why commercial jet aircraft cruise in the lower stratosphere, where the air is calm and fuel-efficient.
7.8 Mesosphere — The Meteor-Burning Zone
The mesosphere lies above the stratosphere and extends up to a height of 80 km. In this layer, once again, temperature starts decreasing with the increase in altitude and reaches a minimum of about −100 °C at the height of 80 km — the coldest part of the entire atmosphere. The upper limit of the mesosphere is known as the mesopause.
The mesosphere is where most meteors burn up by friction with air molecules, leaving behind the bright streaks we call "shooting stars". Without this layer, meteor impacts at the surface would be vastly more frequent. The mesosphere also hosts rare luminous clouds called noctilucent clouds, which can sometimes be seen on summer nights at high latitudes.
7.9 Thermosphere & Ionosphere — The Radio-Reflecting Layer
The ionosphere? is located between 80 and 400 km above the mesopause. (The same volume of the atmosphere is also called the thermosphere because temperature in it rises sharply with height — the names emphasise different properties of the same region.) This zone contains electrically charged particles known as ions, and that is why it is named the ionosphere.
The presence of ions has a far-reaching consequence for human communication. Radio waves transmitted from the earth are reflected back to the earth by this layer. Without the ionosphere, AM radio and short-wave radio could not bounce around the globe — it is the ionosphere that turned the planet into a "wireless world" before satellites took over.
The temperature here starts increasing with height, opposite to the mesosphere — the tenuous gas absorbs powerful X-ray and ultra-violet radiation directly from the Sun, raising kinetic temperatures to extreme values (although the air is so thin that an astronaut would not actually feel "hot"). The ionosphere is also where the aurora — the northern and southern lights — flicker, when charged particles from the Sun spiral down along the earth's magnetic field lines and excite atmospheric atoms into glowing.
Ionosphere — How Radio Waves Reach Far-off Cities
LET'S EXPLORE — Why Are Auroras Confined to High Latitudes?
L4 Analyse
Auroras (northern and southern lights) are seen mainly in places like Alaska, Canada, Norway and Antarctica — rarely from India. Use what you know about the ionosphere and the earth's magnetic field to explain.
Auroras are produced when high-energy charged particles from the Sun (the "solar wind") collide with atoms in the ionosphere and excite them into glowing — green from oxygen, red and pink from nitrogen. The earth's magnetic field channels these particles down along its lines of force, which dive into the atmosphere near the magnetic poles. India lies near the magnetic equator, far from the funnel zones, so auroras are very rare here. Only during exceptionally strong solar storms — like the one in 2003 — do faint auroras reach lower latitudes such as Ladakh.
7.10 Exosphere — Atmosphere Fading into Space
The exosphere is the uppermost layer of the atmosphere above the thermosphere — extending from about 400 km outward. This is the highest layer, but very little is known about it. Whatever contents are there are extremely rarefied, and the layer gradually merges with outer space. Hydrogen and helium atoms — light enough to escape the earth's gravity — slowly leak away into interplanetary space from this region. Although all layers of the atmosphere must be exercising influence on us, geographers are mainly concerned with the first two layers (troposphere and stratosphere).
7.11 Comparing the Five Layers — At a Glance
| Layer | Altitude (km) | Temperature trend with height | Hallmark |
|---|---|---|---|
| Troposphere | 0 – 13 (8 km poles, 18 km equator) | Decreases at ≈ 1 °C / 165 m | All weather, water vapour, dust |
| Tropopause | ≈ 13 | Near constant (−80 °C eq; −45 °C poles) | Boundary "lid" |
| Stratosphere | 13 – 50 | Lower part isothermal, upper part rising | Ozone layer; calm; aviation |
| Mesosphere | 50 – 80 | Decreases to −100 °C at 80 km | Meteors burn up |
| Mesopause | ≈ 80 | Coldest temperatures of atmosphere | Upper limit of mesosphere |
| Thermosphere / Ionosphere | 80 – 400 | Increases sharply with height | Reflects radio waves; aurora |
| Exosphere? | 400 + | Effectively undefined | Merges with outer space |
Temperature vs altitude profile of the atmosphere
The "S"-shape of the temperature curve — falling, rising, falling, rising — is what defines the four temperature layers. Each reversal marks a "pause".
Troposphere
Where you live. Weather, clouds, life. Temp drops with height.
Stratosphere
Ozone shield. Calm, dry — perfect for jet aircraft.
Mesosphere
Coldest layer (−100 °C). Meteors burn up here.
Ionosphere
Reflects radio. Hosts the aurora. Temperature soars.
Exosphere
Air fades to vacuum. Satellites orbit. Atmosphere ends.
7.12 Elements of Weather and Climate
The main elements of the atmosphere which are subject to change and which influence human life on the earth are temperature, pressure, winds, humidity, clouds and precipitation. These elements are studied in detail in Chapters 8, 9 and 10 of this textbook. Each of them is rooted in the physics of the troposphere — its composition, density structure and the way it interacts with the Sun's energy. With this preparation, the reader is ready to engage with the climate chapters that follow.
🌍 Bridge to the Next Chapters
Insolation (Ch. 8) explains how solar energy is intercepted and distributed; Atmospheric circulation (Ch. 9) explains why winds blow; Water in the atmosphere (Ch. 10) explains evaporation, condensation and precipitation. Every one of those processes operates on the foundation laid by Chapter 7 — the composition and structure of the air we breathe.
7.13 NCERT EXERCISES — Full Model Answers
1. Multiple Choice Questions
(i) Which one of the following gases constitutes the major portion of the atmosphere?
(a) Oxygen (b) Nitrogen (c) Argon (d) Carbon dioxide
Answer: (b) Nitrogen. By volume, nitrogen makes up about 78.08 per cent of dry air — easily the most abundant gas. Oxygen is second at about 20.95 per cent.
(ii) Atmospheric layer important for human beings is —
(a) Stratosphere (b) Mesosphere (c) Troposphere (d) Ionosphere
Answer: (c) Troposphere. All breathing air, water vapour, weather and biological activity are confined to the troposphere. Without this 13-km thick lowermost layer, life as we know it could not exist.
(iii) Sea salt, pollen, ash, smoke soot, fine soil — these are associated with —
(a) Gases (b) Dust particles (c) Water vapour (d) Meteors
Answer: (b) Dust particles. All five items are sources of suspended solid particles in the atmosphere. They serve as hygroscopic (condensation) nuclei essential for cloud and rainfall formation.
(iv) Oxygen gas is in negligible quantity at the height of atmosphere —
(a) 90 km (b) 120 km (c) 100 km (d) 150 km
Answer: (b) 120 km. NCERT explicitly states that oxygen is almost in negligible quantity at the height of 120 km. CO₂ and water vapour are similarly limited to the lowest 90 km.
(v) Which one of the following gases is transparent to incoming solar radiation and opaque to outgoing terrestrial radiation?
(a) Oxygen (b) Nitrogen (c) Helium (d) Carbon dioxide
Answer: (d) Carbon dioxide. This selective behaviour is exactly why CO₂ is the chief greenhouse gas — sunlight passes through, terrestrial heat is trapped, and the lower atmosphere warms.
2. Answer in about 30 words
(i) What do you understand by atmosphere?
The atmosphere is a mixture of different gases, water vapour and dust particles that envelopes the earth on all sides. It contains life-giving oxygen and CO₂; about 99 per cent of its mass lies within the lowest 32 km.
(ii) What are the elements of weather and climate?
The main elements of weather and climate are temperature, atmospheric pressure, winds, humidity, clouds and precipitation. They change daily (weather) and average out over long periods (climate), shaping life on the earth's surface.
(iii) Describe the composition of atmosphere.
The atmosphere is composed of three constituents: gases — chiefly nitrogen (78.08%), oxygen (20.95%), argon (0.93%) and traces of CO₂, neon, helium, hydrogen, methane; water vapour (0–4%, variable); and dust particles from sea salt, soil, ash, pollen and meteors. Ozone is concentrated between 10 and 50 km.
(iv) Why is troposphere the most important of all the layers of the atmosphere?
All weather phenomena — clouds, rainfall, storms, fog, dew — and almost all biological activity occur in the troposphere. It contains water vapour, dust and breathable air; temperature falls 1 °C per 165 m. Without it, life on the surface would not exist.
3. Answer in about 150 words
(i) Describe the composition of the atmosphere.
The atmosphere is a mechanical mixture of gases, water vapour and dust particles.
Gases: By volume of dry air, nitrogen (78.08%) and oxygen (20.95%) together account for nearly 99 per cent. Argon supplies 0.93%; carbon dioxide 0.039%; and neon, helium, krypton, xenon, hydrogen and methane occur in trace amounts. Although CO₂ is tiny in volume, it is meteorologically important — transparent to solar radiation, opaque to terrestrial radiation — and is the chief greenhouse gas. Ozone, between 10 and 50 km, absorbs UV and protects the surface.
Water vapour: Highly variable; up to 4% in warm wet tropics, less than 1% in deserts and polar regions. It decreases with altitude and from equator to poles. Water vapour absorbs solar and terrestrial radiation, releases latent heat, and contributes to atmospheric stability or instability.
Dust particles: Sea salt, fine soil, smoke-soot, ash, pollen and meteor debris. Concentrated in subtropical and temperate regions due to dry winds, they act as hygroscopic nuclei around which water vapour condenses to form clouds.
Gases: By volume of dry air, nitrogen (78.08%) and oxygen (20.95%) together account for nearly 99 per cent. Argon supplies 0.93%; carbon dioxide 0.039%; and neon, helium, krypton, xenon, hydrogen and methane occur in trace amounts. Although CO₂ is tiny in volume, it is meteorologically important — transparent to solar radiation, opaque to terrestrial radiation — and is the chief greenhouse gas. Ozone, between 10 and 50 km, absorbs UV and protects the surface.
Water vapour: Highly variable; up to 4% in warm wet tropics, less than 1% in deserts and polar regions. It decreases with altitude and from equator to poles. Water vapour absorbs solar and terrestrial radiation, releases latent heat, and contributes to atmospheric stability or instability.
Dust particles: Sea salt, fine soil, smoke-soot, ash, pollen and meteor debris. Concentrated in subtropical and temperate regions due to dry winds, they act as hygroscopic nuclei around which water vapour condenses to form clouds.
(ii) Draw a suitable diagram for the structure of the atmosphere and label it and describe it.
The atmosphere is divided into five layers, mainly on the basis of how temperature varies with height (refer to the SVG figure earlier in this lesson — it labels each layer with altitude limits and temperature behaviour).
(1) Troposphere (0–13 km; 8 km poles, 18 km equator): Contains all weather, water vapour and dust; temperature falls at 1 °C per 165 m. Tropopause is its top, with temperatures of −80 °C over the equator and −45 °C over the poles.
(2) Stratosphere (13–50 km): Houses the ozone layer that absorbs UV. The lower part is isothermal; the upper part shows rising temperature. Calm and stable — preferred by aircraft.
(3) Mesosphere (50–80 km): Temperature falls again, reaching −100 °C at the mesopause. Meteors burn up here.
(4) Thermosphere / Ionosphere (80–400 km): Contains electrically charged ions; reflects radio waves; hosts auroras; temperature rises sharply.
(5) Exosphere (400 km +): Extremely rarefied; merges with outer space. Geographers are mainly concerned with the troposphere and stratosphere.
(1) Troposphere (0–13 km; 8 km poles, 18 km equator): Contains all weather, water vapour and dust; temperature falls at 1 °C per 165 m. Tropopause is its top, with temperatures of −80 °C over the equator and −45 °C over the poles.
(2) Stratosphere (13–50 km): Houses the ozone layer that absorbs UV. The lower part is isothermal; the upper part shows rising temperature. Calm and stable — preferred by aircraft.
(3) Mesosphere (50–80 km): Temperature falls again, reaching −100 °C at the mesopause. Meteors burn up here.
(4) Thermosphere / Ionosphere (80–400 km): Contains electrically charged ions; reflects radio waves; hosts auroras; temperature rises sharply.
(5) Exosphere (400 km +): Extremely rarefied; merges with outer space. Geographers are mainly concerned with the troposphere and stratosphere.
4. Project Work — Suggested
Project: Track CO₂, dust load and visibility for two weeks at your home or school, and prepare a short report on what your data say about the local lower atmosphere.
Suggested approach. (1) Use a low-cost air-quality sensor (or your municipal AQI bulletin) to log daily PM2.5/PM10 and CO₂ at a fixed time each morning. (2) Note visibility — can you see a distant landmark? Note also wind direction and rainfall. (3) After 14 days, plot the values as a line chart in your notebook. (4) Connect: were dustier days windier? Did rain "wash" the air clean for a few days? Was CO₂ higher near a busy road than in a park? (5) Conclude: which constituent of the atmosphere — gases, water vapour or dust — varied most in your two-week sample, and why?
7.14 Summary & Key Terms
Composition
Gases (N₂ 78.08%, O₂ 20.95%, Ar 0.93%, CO₂ 0.039%, traces) + water vapour (0–4%) + dust particles. Ozone 10–50 km.
Water Vapour
Variable (4% tropics → <1% poles). Decreases with altitude. Absorbs radiation, releases latent heat.
Dust
Sea salt, soil, smoke, pollen, meteor debris. High in subtropics/temperate. Acts as condensation nuclei.
Ozone
10–50 km in stratosphere. Absorbs UV. Earth's biological sunscreen.
Five Layers
Troposphere → Stratosphere → Mesosphere → Thermosphere/Ionosphere → Exosphere.
| Layer | Altitude | Diagnostic feature |
|---|---|---|
| Troposphere | 0–13 km | All weather; lapse rate 1 °C / 165 m |
| Stratosphere | 13–50 km | Ozone layer; calm; aircraft cruise zone |
| Mesosphere | 50–80 km | Coldest layer (−100 °C); meteors burn |
| Thermosphere / Ionosphere | 80–400 km | Reflects radio; aurora; ions |
| Exosphere | 400 km + | Rarefied; merges with outer space |
🎯 Competency-Based Questions — Structure of the Atmosphere
Case Stem. A flight from Mumbai to Frankfurt cruises at 11 km altitude over the equator and at 9 km altitude over Europe. Mid-flight, passengers are told that the cabin lights are dimmed so that the crew can see a meteor shower outside; later, the captain mentions a "northern lights" sighting near the Arctic Circle. Use this scenario to answer Q1–Q4.
Q1. The aircraft cruises in different atmospheric layers over the equator and over Europe because —
L3 ApplyAnswer: (b). Strong equatorial convection lifts the troposphere to about 18 km, while polar convection is so weak that the troposphere ends at about 8 km. An 11-km cruise is still in the troposphere at the equator but is in the lower stratosphere by the time the plane reaches Europe.
Q2. The meteors burning up that the passengers see are doing so in which layer, and why there?
L4 AnalyseMesosphere (50–80 km). Air is dense enough here to generate strong frictional heating on incoming meteoroids, but still high enough to give them a long burning track. The mesosphere is also the coldest layer of the atmosphere (−100 °C at 80 km), making the bright streaks visually striking.
Q3. The "northern lights" near the Arctic Circle are produced in —
L5 EvaluateThe ionosphere (thermosphere; 80–400 km). Charged particles from the Sun spiral down along the earth's magnetic field lines, collide with atmospheric atoms in the ionosphere and excite them into glowing — green from oxygen, red and pink from nitrogen. The funnel of magnetic field lines reaches the atmosphere mainly near the magnetic poles, which is why auroras are concentrated in high latitudes.
HOT Q. A radio amateur in Kerala can speak to a friend in Australia using a short-wave set without a satellite link. Write a 4–5 sentence note explaining how this is physically possible, naming the layer involved and the property that makes it work.
L6 CreateHint: The conversation is possible because the ionosphere (80–400 km) contains electrically charged ions that reflect radio waves back to the earth's surface. A signal from Kerala bounces off the ionosphere over the Indian Ocean and re-emerges thousands of kilometres away in Australia. Without satellites, this "skip" propagation was the only way for inter-continental radio. Solar storms that disturb the ionosphere can therefore disrupt amateur radio just as they do GPS.
⚖️ Assertion–Reason Questions — Structure of the Atmosphere
Options:
(A) Both A and R are true, and R is the correct explanation of A.
(B) Both A and R are true, but R is NOT the correct explanation of A.
(C) A is true, but R is false.
(D) A is false, but R is true.
(A) Both A and R are true, and R is the correct explanation of A.
(B) Both A and R are true, but R is NOT the correct explanation of A.
(C) A is true, but R is false.
(D) A is false, but R is true.
Assertion (A): The thickness of the troposphere is greatest at the equator and least at the poles.
Reason (R): Heat is transported to great heights by strong convectional currents over the warm equator, while convection is weak over the cold poles.
Answer: (A) — Both true and R is the correct explanation. Strong equatorial convection lifts the troposphere to ≈ 18 km; weak polar convection halts at ≈ 8 km.
Assertion (A): Commercial jet aircraft prefer to cruise in the lower stratosphere.
Reason (R): The stratosphere has very little water vapour, almost no convection and no weather phenomena, making the air calm and fuel-efficient.
Answer: (A) — Both true and R correctly explains A. Stable, dry stratospheric air gives a smoother and safer ride than the turbulent troposphere.
Assertion (A): Radio waves transmitted from the earth can be received thousands of kilometres away even without a satellite link.
Reason (R): The ionosphere, lying between 80 and 400 km, contains electrically charged ions that reflect radio waves back to the earth's surface.
Answer: (A) — Both true and R is the correct explanation. The "skip" propagation off the ionosphere underpinned all long-range radio before the satellite era and is still used by amateur and emergency communications.
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