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← Fundamentals of Physical Geography — all chaptersComposition and Structure of Atmosphere
Start here: why this chapter matters
In simple wordsWe can live weeks without food and days without water, but only minutes without air. The atmosphere is the thin envelope of air wrapped round the earth — like the peel of an orange compared with the fruit. This chapter answers two questions: what is air made of, and how is the atmosphere arranged in layers?
This short chapter opens Unit IV, ‘Climate’. Everything you study next — sunlight and temperature (Chapter 8), pressure and winds (Chapter 9), humidity, clouds and rain (Chapter 10) and climate change (Chapter 11) — happens inside the atmosphere described here, mostly in its lowest layer. Learn its gases and layers well: they are the vocabulary of the whole unit.
- Opens Unit IV (Climate) and leads into Chapters 8–11.
- Links to environment topics: greenhouse gases, ozone depletion, air pollution, and to space technology (radio communication, satellites).
- UPSC: Prelims has asked the order of layers (1984, 1998), the ozone layer (1982, 1986, 1987), the ionosphere and radio waves (1996, 2011), why jets fly in the lower stratosphere (2011), water vapour (2003, 2024), troposphere thickness (2004, 2024) and dust (2025). Mains GS1 asked why the troposphere is so significant for weather (2022).
| Word | What it means here |
|---|---|
| Atmosphere | The envelope of gases, water vapour and dust round the earth |
| Insolation / solar radiation | Energy coming in from the sun (mostly short-wave) |
| Terrestrial radiation | Heat given out by the earth (long-wave, infrared) |
| Transparent / opaque | Lets radiation pass / blocks or absorbs it |
| Ultraviolet (UV) rays | Harmful high-energy rays in sunlight |
| Hygroscopic nuclei | Tiny particles (dust, salt) that attract water vapour to condense on them |
| Ion | An electrically charged particle |
| Lapse rate | The rate at which temperature falls with height |
| -pause | The boundary at the top of a layer (tropopause, stratopause, mesopause) |
At a glance — the six ideas to carry away
- 99% of the atmosphere's mass lies within 32 km; oxygen is negligible by 120 km; CO₂ and water vapour reach 90 km.
- CO₂: transparent to incoming sunlight, opaque to outgoing heat → greenhouse effect; rising with fossil-fuel burning.
- Ozone (10–50 km) absorbs ultraviolet rays; water vapour (≤ 4%) is a heat blanket; dust and salt are cloud nuclei.
- Five layers by temperature: troposphere → stratosphere → mesosphere → thermosphere (ionosphere) → exosphere.
- Troposphere: all weather; ~8 km at the poles, ~18 km at the equator; temperature falls 1°C per 165 m.
- Ionosphere (80–400 km) reflects radio waves; geographers care most about the first two layers.
1. What air is made of: the gases
In simple wordsAir is like a big pot of dal: mostly lentils and water (nitrogen and oxygen), with a pinch of salt and spices (argon, carbon dioxide, ozone). The pinches are tiny, but they change the taste completely — carbon dioxide and ozone, though very small in amount, control the earth's warmth and protect life.
The atmosphere is a mixture of gases, water vapour and dust particles that envelops the earth. It contains life-giving gases — oxygen for humans and animals, carbon dioxide for plants. Air is part of the earth's mass: 99% of the atmosphere's total mass lies within 32 km of the surface. Air is colourless and odourless and is felt only when it blows as wind. The proportions change with height: oxygen is almost negligible at 120 km, and carbon dioxide and water vapour are found only up to 90 km.
Beyond NCERT — the missing gas tableEarlier editions of this chapter had a table of permanent gases; the current reprint has dropped it, yet exercise Q1(i) still asks which gas forms the major part of the atmosphere. The standard values for dry air by volume: nitrogen ≈ 78.08%, oxygen ≈ 20.95%, argon ≈ 0.93%, carbon dioxide ≈ 0.04% (about 420 parts per million today, up from about 280 before industrialisation), and traces of neon, helium, krypton, xenon, hydrogen and ozone.
| Gas | What NCERT says | Why it matters |
|---|---|---|
| Carbon dioxide | Meteorologically very important. Transparent to incoming solar radiation but opaque to outgoing terrestrial radiation; absorbs part of the earth's radiation and sends part back to the surface. Its volume has been rising in recent decades, mainly from burning fossil fuels, which has raised air temperature | Largely responsible for the greenhouse effect; other gases' volumes are constant |
| Ozone | Found between 10 and 50 km; acts as a filter, absorbing ultraviolet rays from the sun | Without it, harmful UV would reach the surface |
| Oxygen | Negligible by 120 km | Breathing for humans and animals |
⤵ How more CO₂ means warmer air
Sunlight passes through the airCO₂ is transparent to it
The ground warms and gives off heatlong-wave terrestrial radiation
CO₂ absorbs part of that heatand sends part back to the surface
Less heat escapes to spacethe greenhouse effect
Burning fossil fuels adds CO₂→ air temperature has risen
Beyond NCERTNCERT says CO₂ ‘reflects back’ part of the terrestrial radiation; more precisely it absorbs and re-emits it in all directions, part of it downwards. The ozone layer is densest at about 15–35 km. Its thinning by chlorofluorocarbons (CFCs) — the Antarctic ‘ozone hole’ — led to the Montreal Protocol (1987), the treaty to phase out ozone-depleting substances (UPSC 2015). Ozone near the ground, by contrast, is a pollutant in smog.
Exam trapNitrogen, not oxygen, is the largest part of air (exercise Q1-i). The gas transparent to incoming but opaque to outgoing radiation is CO₂ (Q1-v). Ozone absorbs ultraviolet, not infrared (UPSC 1987); CO₂ absorbs infrared (UPSC 2012).
2. Water vapour and dust particles
In simple wordsWater vapour is the earth's quilt: on a humid night the air stays warm, on a dry desert night it turns cold fast. Dust is the seed of every raindrop — water vapour needs a speck of dust or salt to cling to before it can form a cloud droplet.
Water vapour is a variable gas. It decreases with altitude and from the equator towards the poles. In the warm, wet tropics it may be about 4% of the air by volume; in dry, cold deserts and polar regions less than 1%. It absorbs part of the incoming insolation and keeps the earth's radiated heat, acting like a blanket so the earth gets neither too hot nor too cold. It also affects the stability and instability of air.
Dust particles come from many sources: sea salt, fine soil, smoke-soot, ash, pollen, dust and disintegrated meteors. They are concentrated in the lower layers, though convection can carry them high. Concentration is higher in subtropical and temperate regions because of dry winds, than in equatorial and polar regions. Dust and salt particles act as hygroscopic nuclei around which water vapour condenses to form clouds.
⤵ From a speck of dust to a cloud
Dust and salt particles in the airfrom soil, sea spray, smoke, volcanoes, meteors, pollen
Air rises and coolsit can hold less water vapour
Vapour condenses on the particleshygroscopic nuclei
Tiny droplets gathera cloud forms (Chapter 10)
Exam trapWater vapour is maximum in the tropics, not at the poles, and decreases with altitude (UPSC 2024). Dust is more over subtropical and temperate areas because of dry winds — not ‘less dry’ winds (UPSC 2025 made exactly this switch). Sea salt, pollen, ash, soot and fine soil are all dust particles (exercise Q1-iii).
Beyond NCERTWarm air can hold far more water vapour than cold air — roughly doubling for every 10°C rise. That is why moisture is linked to latitude (UPSC 2003 assertion–reason: both true, and the reason explains the assertion).
3. Structure of the atmosphere: five layers
In simple wordsThink of a multi-storey building where each floor has its own ‘climate’: the ground floor is warm and busy (weather), the next floors are calm and dry, higher still it gets bitterly cold, and the top floors are oddly hot but almost empty of air. The floors are separated not by height alone but by how temperature changes as you climb.
The atmosphere has layers of different density and temperature. Density is highest near the surface and decreases with height. By temperature, there are five layers: troposphere, stratosphere, mesosphere, thermosphere and exosphere.
Reading NCERT Figure 7.1 — Structure of atmosphere (page 65)
- A graph: altitude (km) up the side, temperature (°C) from −100 to +20 along the bottom. One zig-zag line shows how temperature changes with height.
- Read the zig-zag: temperature falls through the troposphere from about +15°C to about −57°C; stays constant for a while above the tropopause, then rises through the stratosphere to about 0°C at the stratopause (50 km); falls again through the mesosphere to about −90°C at the mesopause (80 km); then rises steeply in the thermosphere, dashing on into the exosphere above about 400 km.
- Information only in the figure: the stratopause (the text never names it), and that temperature increases with height in the stratosphere — the text only hints at it with ‘once again, temperature starts decreasing’ in the mesosphere.
- The altitude axis is not to scale: 0, 10, 50, 80 and 400 km are spaced so the upper layers fit — the gap from 80 to 400 km is squeezed.
- The figure's numbers are for an average/mid-latitude atmosphere: its tropopause (~−57°C at about 11 km) and mesopause (~−90°C) differ from the text's −80°C (equator) / −45°C (poles) and −100°C. The figure labels the layer above 80 km ‘Thermosphere’; the text describes it as the ‘ionosphere’.
| Layer | Height (NCERT) | Temperature with height | Key facts |
|---|---|---|---|
| Troposphere | Average 13 km; ~8 km at poles, ~18 km at equator | Falls, 1°C per 165 m | Dust and water vapour; all weather and climate changes; most important for biological activity |
| Tropopause | Top of the troposphere | Nearly constant: about −80°C over the equator, −45°C over the poles | Boundary zone |
| Stratosphere | Up to 50 km | Rises (Figure 7.1) | Ozone layer absorbs UV and shields life |
| Mesosphere | Up to 80 km | Falls again, to about −100°C at 80 km | Top = mesopause |
| Thermosphere / ionosphere | 80–400 km | Rises with height | Ions reflect radio waves back to earth |
| Exosphere | The uppermost layer | — | Extremely rarefied; very little known; merges with outer space |
⤵ Climbing through the atmosphere
Troposphere (0 to 8–18 km)weather; temperature falls
Tropopausetemperature nearly constant
Stratosphere (to 50 km)ozone layer; temperature rises
Mesosphere (to 80 km)temperature falls to about −100°C
Thermosphere / ionosphere (80–400 km)ions; radio waves reflected; temperature rises
Exosphererarefied; merges into space
Exam trapOrder from the ground up: troposphere, stratosphere, mesosphere, ionosphere/thermosphere, exosphere (UPSC 1984, 1998). Memory hook: ‘The Strong Man Threw Eggs’. Temperature does not fall steadily with height — it falls, rises, falls, rises.
4. The troposphere and tropopause
In simple wordsThe troposphere is the ‘kitchen’ of the atmosphere: all the cooking — clouds, rain, storms, winds — happens here. Over the hot equator the kitchen has a higher ceiling, because hot air rising from the ground pushes it up.
The troposphere is the lowest layer. Its average height is 13 km — about 8 km near the poles and 18 km at the equator. It is thickest at the equator because heat is carried to great heights by strong convection currents. It holds the dust particles and water vapour, so all changes in weather and climate take place here. Temperature decreases by 1°C for every 165 m of height. It is the most important layer for all biological activity.
The tropopause separates the troposphere from the stratosphere. Its temperature is about −80°C over the equator and −45°C over the poles; temperature stays nearly constant here — hence the name (‘pause’).
Why is the tropopause colder over the equator?Because it is so much higher there (~18 km vs ~8 km). Temperature keeps falling all the way up through the troposphere, so a taller troposphere ends at a colder top — even though the equator's surface is the warmest. A good ‘why’ question for Prelims statements.
Beyond NCERTJet aircraft cruise near the top of the troposphere or in the lower stratosphere, where there are almost no clouds or water vapour and little vertical air movement, so flights are smooth (UPSC 2011). The standard average fall in temperature, the normal lapse rate, is 6.5°C per km (Chapter 8).
Exam trapThe troposphere is thicker at the equator (≈18 km) than at the poles (≈8 km) because of strong convection — UPSC asked this in 2004 and 2024. The troposphere is the most important layer for humans (exercise Q1-ii), not the stratosphere.
5. Stratosphere, mesosphere, ionosphere and exosphere
In simple wordsAbove the weather kitchen is a calm attic with a sunscreen sheet (the ozone layer in the stratosphere). Above that, a freezing loft (the mesosphere), then an electrified roof that bounces radio signals back down (the ionosphere), and finally an open terrace that fades into space (the exosphere).
| Layer | NCERT facts | Beyond NCERT |
|---|---|---|
| Stratosphere | Above the tropopause up to 50 km; contains the ozone layer, which absorbs UV radiation and shields life from intense, harmful energy | Temperature rises with height because ozone absorbs UV; air is calm and dry — good for long-haul flights in its lower part |
| Mesosphere | Up to 80 km; temperature falls again, to about −100°C at 80 km; top = mesopause | The coldest part of the atmosphere; most meteors burn up here |
| Ionosphere (thermosphere) | 80–400 km, above the mesopause; electrically charged ions; reflects radio waves transmitted from the earth; temperature increases with height | Auroras (northern and southern lights) glow here; the International Space Station orbits at about 400 km |
| Exosphere | Uppermost layer above the thermosphere; very little known; extremely rarefied; merges gradually with outer space | Many satellites orbit here |
Beyond NCERT — auroras (Mains 2024)Aurora borealis (northern lights) and aurora australis (southern lights) are curtains of coloured light seen mostly near the poles. They are triggered when charged particles of the solar wind are guided by the earth's magnetic field towards the polar regions and collide with oxygen and nitrogen atoms in the thermosphere/ionosphere (roughly 100–300 km), making them glow (oxygen: green and red; nitrogen: blue and purple). Strong solar storms push auroras to lower latitudes and can disturb radio, GPS and power grids.
Exam trapRadio waves are reflected by the ionosphere (UPSC 1996) because of its electrically charged particles — not because of ozone, and not simply because radio waves are long (UPSC 2011: neither statement was correct). Geographers concentrate on the first two layers (troposphere and stratosphere).
NCERT's question: what if there were no ozone?Harmful ultraviolet radiation would reach the surface in full strength: more skin cancer and cataracts, weaker immune systems, damage to crops, plankton and other life — and the stratosphere would not warm the way it does, changing the whole temperature structure above the troposphere.
6. Elements of weather and climate
In simple wordsA weather report is a list of a few readings: how hot (temperature), how heavy the air is (pressure), which way and how fast the air moves (winds), how much moisture it holds (humidity), whether the sky is covered (clouds) and whether it is raining (precipitation). These six are the elements.
- Temperature — Chapter 8
- Pressure and winds — Chapter 9
- Humidity, clouds and precipitation — Chapter 10
NCERT calls these the main elements of the atmosphere that are subject to change and influence human life. Weather is their state at a place over a short time; climate is their average pattern over a long period (usually 30 years or more — Beyond NCERT).
NCERT exercise — answer outlines
The five multiple-choice questions are in the Self-check below. Try each written question yourself first, then open the outline to compare.
2(i) What do you understand by atmosphere? (30 words)
The envelope of air — a mixture of gases, water vapour and dust particles — surrounding the earth, held by gravity; 99% of its mass lies within 32 km. It supplies oxygen and CO₂, filters UV and keeps the earth warm.
2(ii) What are the elements of weather and climate? (30 words)
Temperature, pressure, winds, humidity, clouds and precipitation — the changing elements of the atmosphere that influence human life.
2(iii) Describe the composition of atmosphere. (30 words)
Gases (mainly nitrogen and oxygen, with argon, CO₂, ozone and others), water vapour (0 to about 4%, decreasing with height and towards the poles) and dust particles (salt, soil, soot, ash, pollen, meteor dust).
2(iv) Why is troposphere the most important of all the layers of the atmosphere? (30 words)
It holds almost all the water vapour and dust, so all weather and climate changes — clouds, rain, storms, winds — occur in it; it is the layer we live and breathe in, vital for biological activity.
3(i) Describe the composition of the atmosphere. (150 words)
Intro: a mixture of gases, water vapour and dust; 99% of the mass within 32 km.
Gases: nitrogen ~78%, oxygen ~21%, argon ~0.93% (standard values); CO₂ — small but vital: transparent to sunlight, opaque to outgoing heat → greenhouse effect; rising with fossil fuels. Ozone (10–50 km) absorbs UV. Oxygen negligible at 120 km; CO₂ and vapour only up to 90 km.
Water vapour: variable; up to 4% in wet tropics, under 1% in deserts and poles; decreases with height and towards the poles; absorbs insolation and keeps the earth's heat — a blanket; affects stability of air.
Dust: sea salt, soil, soot, ash, pollen, meteor dust; mostly in lower layers; more over subtropical and temperate regions (dry winds); act as hygroscopic nuclei for clouds.
Conclude: minor constituents control weather and life.
Gases: nitrogen ~78%, oxygen ~21%, argon ~0.93% (standard values); CO₂ — small but vital: transparent to sunlight, opaque to outgoing heat → greenhouse effect; rising with fossil fuels. Ozone (10–50 km) absorbs UV. Oxygen negligible at 120 km; CO₂ and vapour only up to 90 km.
Water vapour: variable; up to 4% in wet tropics, under 1% in deserts and poles; decreases with height and towards the poles; absorbs insolation and keeps the earth's heat — a blanket; affects stability of air.
Dust: sea salt, soil, soot, ash, pollen, meteor dust; mostly in lower layers; more over subtropical and temperate regions (dry winds); act as hygroscopic nuclei for clouds.
Conclude: minor constituents control weather and life.
3(ii) Draw a suitable diagram for the structure of the atmosphere, label it and describe it. (150 words)
Diagram: a temperature–height graph like Figure 7.1, labelling troposphere, tropopause, stratosphere (ozone layer), stratopause, mesosphere, mesopause, thermosphere/ionosphere, exosphere.
Troposphere: average 13 km (8 km poles, 18 km equator — convection); temperature falls 1°C per 165 m; all weather.
Tropopause: constant temperature; −80°C equator, −45°C poles.
Stratosphere: to 50 km; ozone absorbs UV; temperature rises.
Mesosphere: to 80 km; temperature falls to −100°C; mesopause on top.
Ionosphere: 80–400 km; ions reflect radio waves; temperature rises.
Exosphere: rarefied, merges with space.
Conclude: layers are defined by temperature trends; geographers focus on the first two.
Troposphere: average 13 km (8 km poles, 18 km equator — convection); temperature falls 1°C per 165 m; all weather.
Tropopause: constant temperature; −80°C equator, −45°C poles.
Stratosphere: to 50 km; ozone absorbs UV; temperature rises.
Mesosphere: to 80 km; temperature falls to −100°C; mesopause on top.
Ionosphere: 80–400 km; ions reflect radio waves; temperature rises.
Exosphere: rarefied, merges with space.
Conclude: layers are defined by temperature trends; geographers focus on the first two.
What to remember
Numbers
Share of atmospheric mass below 32 km99%
Oxygen becomes negligible at120 km
CO₂ and water vapour found up to90 km
Ozone layer10–50 km
Water vapour in wet tropicsup to ~4%
Water vapour in deserts and polar areas< 1%
Troposphere: average height13 km
Troposphere: poles / equator~8 km / ~18 km
Fall in temperature (NCERT)1°C per 165 m
Tropopause temperature: equator / poles−80°C / −45°C
Stratosphere top50 km
Mesosphere top / temperature80 km / −100°C
Ionosphere80–400 km
Layers by temperature5
Nitrogen / oxygen (standard)~78% / ~21%
One-liners
- CO₂: transparent to incoming, opaque to outgoing radiation.
- Ozone absorbs ultraviolet rays.
- Water vapour decreases with altitude and towards the poles.
- Dust and salt are hygroscopic nuclei for clouds.
- All weather happens in the troposphere.
- The troposphere is thickest at the equator because of convection.
- The ionosphere reflects radio waves.
- Geographers study mainly the troposphere and stratosphere.
Traps to avoid
- Nitrogen is the largest component of air, not oxygen.
- Ozone absorbs UV; CO₂ and water vapour absorb infrared (heat).
- Water vapour is least at the poles, not maximum.
- Dust is more in subtropical and temperate areas because of dry winds.
- The troposphere is thicker at the equator, yet the tropopause is colder there.
- Temperature rises in the stratosphere and thermosphere, falls in the troposphere and mesosphere.
- Radio reflection is due to ions, not ozone.
- NCERT's ‘1°C per 165 m’ differs slightly from the standard lapse rate of 6.5°C per km.
- Oxygen negligible at 120 km; CO₂ and vapour end at 90 km — don't swap.
How to strategise
Prelims — what UPSC asks
- Layer order and properties are perennial: the sequence of layers (1984, 1998), the ionosphere and radio waves (1996, 2011), why jets fly smoothly in the lower stratosphere (2011).
- NCERT sentences become statement pairs: troposphere thicker at the equator because of convection (2004, 2024); dust more in subtropical/temperate regions due to dry winds (2025 — Statement II was a twisted version); water vapour decreases with altitude and is not maximum at the poles (2024).
- Gases and radiation: the ozone layer's role (1982, 1986, 1987); CO₂ absorbs infrared (2012); moisture linked to latitude because warm air holds more vapour (2003).
- Watch the switch-words: increase/decrease, equator/poles, UV/infrared, transparent/opaque — UPSC traps live there.
Mains — where it appears and how to answer
- GS1: ‘Troposphere is a very significant atmospheric layer that determines weather processes. How?’ (2022). Use: height and why it varies (convection), water vapour and dust → clouds and rain, lapse rate → instability and convection, pressure and winds, jet streams near the tropopause, and the tropopause as a lid. Draw Figure 7.1.
- GS1/GS3 links: auroras and the ionosphere (2024 — Beyond NCERT box); greenhouse gases and ozone depletion connect to climate change (Chapter 11) and environment.
- Answer frame: define → composition/structure diagram → process (why) → significance for weather, life, communication → current issue (CO₂ rise, ozone recovery).
Self-check
Tap an option. NCERT's own exercise questions come first; the rest test the figures and common confusions.
1. Which one of the following gases constitutes the major portion of the atmosphere?
2. Atmospheric layer important for human beings is:
3. Sea salt, pollen, ash, smoke soot, fine soil — these are associated with:
4. Oxygen gas is in negligible quantity at the height of atmosphere:
5. Which one of the following gases is transparent to incoming solar radiation and opaque to outgoing terrestrial radiation?
6. Why is the troposphere thickest over the equator?
7. In which layer does temperature rise with height because of ozone absorbing ultraviolet rays?
UPSC questions from this chapter
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