Why this chapter matters for UPSC: This is Chapter 7, "Composition and Structure of Atmosphere", in the current rationalised NCERT Fundamentals of Physical Geography (Reprint 2026-27), the first chapter of Unit IV (Climate); it was Chapter 8 in pre-2023 editions. It is short, but its figures are exam material: the 13/8/18 km troposphere, the 1°C per 165 m fall in temperature, the 10–50 km ozone band, the 80–400 km ionosphere, oxygen negligible at 120 km and 99% of the atmosphere's mass within 32 km. UPSC asked about troposphere thickness in Prelims 2024 and about the troposphere and weather in Mains GS1 2022.

Contemporary hook: Atmospheric CO₂ at Mauna Loa reached a May 2026 monthly peak of 432.3 ppm (NOAA; Scripps reported 432.00 ppm), 1.8 ppm above May 2025 (Scripps Institution of Oceanography release, 11 June 2026). NCERT notes that CO₂ is the one major gas whose volume has been rising over the past few decades.

🧠 First Principles — Read This First

The atmosphere is the layer of air that surrounds the earth. It is a mixture of gases, with water vapour and tiny solid particles (dust) held in it. You cannot see or smell air; you feel it only when it moves as wind. NCERT points out that people can survive some time without food or water but only a few minutes without air.

Most of the air is close to the ground. NCERT says 99% of the atmosphere's mass lies within 32 km of the surface. Air gets thinner with height, and its make-up changes: oxygen becomes negligible by 120 km, and carbon dioxide and water vapour are found only up to 90 km.

A few minor ingredients do important jobs. Carbon dioxide lets sunlight through but absorbs heat radiated by the earth, which keeps the lower air warm. Water vapour does the same and is the source of clouds and rain. Ozone, found between 10 and 50 km, absorbs the sun's ultraviolet rays. Dust and salt particles give water vapour a surface to condense on, so clouds can form.

The atmosphere is divided into five layers by how temperature changes with height: troposphere, stratosphere, mesosphere, thermosphere and exosphere. In the lowest layer, the troposphere, temperature falls as you go up, and all weather happens there. Think of a hill station being cooler than the plain below it: that is the troposphere's falling temperature at work.

UPSC asks for these numbers directly in Prelims and asks why the troposphere controls weather in Mains.

PART 1 — Quick Reference

Table 1: Layers of the atmosphere

Layer	NCERT height	NCERT features	Other official figure (where it differs)
Troposphere	Average 13 km; about 8 km near the poles, about 18 km at the equator	Temperature falls 1°C for every 165 m; contains dust and water vapour; all weather and climate changes occur here; most important layer for biological activity	NOAA gives 18–20 km at the equator, about 9 km at 50°N/S and about 6 km at the poles
Tropopause	Boundary layer	About −80°C over the equator, about −45°C over the poles; temperature nearly constant	—
Stratosphere	Up to 50 km	Contains the ozone layer, which absorbs ultraviolet radiation	—
Mesosphere	Up to 80 km	Temperature falls again with height, to −100°C at 80 km; upper limit is the mesopause	NOAA: 50–85 km
Ionosphere (thermosphere)	80–400 km	Electrically charged particles (ions); reflects radio waves back to earth; temperature rises with height	NOAA: thermosphere about 85–600 km
Exosphere	Above the thermosphere	Uppermost layer, very little known; extremely rarefied; merges with outer space	NOAA: about 600–10,000 km

Sources: NCERT, Fundamentals of Physical Geography, Class XI, Chapter 7 (Reprint 2026-27), kegy207.pdf, p. 65; NOAA JetStream, "Layers of the Atmosphere", accessed 1 October 2026. For an NCERT-based answer, give NCERT's figures.

Table 2: Composition of dry air and current trace-gas levels

Gas	Amount	As of / source
Nitrogen (N₂)	78.08% by volume	NASA Earth Fact Sheet
Oxygen (O₂)	20.95%	NASA Earth Fact Sheet
Argon (Ar)	9,340 ppm (0.934%)	NASA Earth Fact Sheet
Carbon dioxide (CO₂)	427.35 ppm	2025 annual mean, NOAA Mauna Loa
Methane (CH₄)	1,935.94 ppb (≈1.94 ppm)	2025 global annual mean, NOAA
Nitrous oxide (N₂O)	338.85 ppb (≈0.34 ppm)	2025 global annual mean, NOAA
Water vapour	Variable: up to 4% in the wet tropics, under 1% in deserts and polar regions	NCERT

Sources: NASA NSSDC, Earth Fact Sheet; NOAA GML annual means for CO₂ (Mauna Loa), CH₄ (global) and N₂O (global), all accessed 1 October 2026. The CO₂ figure is from one station (Mauna Loa); CH₄ and N₂O are global averages.

Table 3: Global warming potential (100-year) of major greenhouse gases

Gas	GWP-100, IPCC AR6
CO₂	1 (reference)
CH₄	27.0 (non-fossil); 29.8 (fossil)
N₂O	273
HFC-134a	1,530
HFC-23	14,600
SF₆	24,300

Source for GWP values: GHG Protocol, "IPCC Global Warming Potential Values" (version 2.0, 7 August 2024), compiling IPCC AR6 WG1 Chapter 7. AR5 values differ (for example CH₄ 28, N₂O 265); state which report you are quoting. HFC GWPs range widely, so do not quote a single figure for "HFCs".

PART 2 — Concepts & Narrative

Composition of the atmosphere

NCERT divides the atmosphere's contents into three groups: gases, water vapour and dust particles.

Gases. By volume, dry air is mainly nitrogen and oxygen, with argon a distant third (Table 2). NCERT's emphasis is on two minor gases. Carbon dioxide is "meteorologically a very important gas" because it is transparent to incoming solar radiation but opaque to outgoing terrestrial radiation; it absorbs part of the earth's radiation and sends some back towards the surface, and is largely responsible for the greenhouse effect. NCERT notes that the volume of other gases is constant while CO₂ has been rising, mainly from burning fossil fuels, and that this has raised air temperature. Ozone lies between 10 and 50 km and filters out the sun's ultraviolet rays.

Water vapour. A variable gas that decreases with altitude and from the equator towards the poles. It can reach 4% of the air by volume in warm, wet tropics and fall below 1% in dry, cold deserts and polar regions. It absorbs part of the insolation and preserves the heat radiated by the earth, acting like a blanket so that the earth becomes neither too cold nor too hot. It also contributes to the stability and instability of air.

Dust particles. The atmosphere holds small solid particles: sea salt, fine soil, smoke-soot, ash, pollen, dust and broken-up meteors. They are concentrated in the lower layers, though convection can lift them high. They are more abundant in subtropical and temperate regions, because of dry winds, than in equatorial and polar regions. Dust and salt act as hygroscopic nuclei around which water vapour condenses to form clouds.

Explainer

Why a gas that is 0.04% of the air matters so much

CO₂ is about 427 parts per million of the air (2025 annual mean, NOAA), roughly 0.04%. Nitrogen and oxygen are about 99%. Yet nitrogen and oxygen hardly interact with the long-wave heat the earth radiates, while CO₂ and water vapour absorb it. NCERT's phrase is that CO₂ is "transparent to the incoming solar radiation but opaque to the outgoing terrestrial radiation". So the amount of a gas matters less than what kind of radiation it absorbs. That is why a rise of a few parts per million a year in CO₂ is tracked closely, and why NCERT's exercise MCQ (v) asks which gas is transparent to incoming and opaque to outgoing radiation (answer: carbon dioxide).

Beyond the Book

CO₂ and other greenhouse gases: current figures

CO₂, Mauna Loa: annual mean 424.61 ppm in 2024 and 427.35 ppm in 2025 (NOAA GML). The seasonal peak comes in May; the May 2026 monthly mean was 432.3 ppm by NOAA's measurement and 432.00 ppm by Scripps, both 1.8 ppm above May 2025 (Scripps release, 11 June 2026).
Taking the commonly used pre-industrial level of about 280 ppm, the 2025 annual mean is about 53% higher (427.35 ÷ 280 = 1.53, derived arithmetic; the 280 ppm baseline is a standard reference value, not a NOAA figure on this page).
Methane 1,935.94 ppb and nitrous oxide 338.85 ppb (2025 global annual means, NOAA GML).

Next update: NOAA revises these files as new data arrive; check the annual-mean files before quoting.

Structure of the atmosphere

The atmosphere has layers of different density and temperature. Density is highest near the surface and decreases with height. NCERT divides the column into five layers on the basis of temperature: troposphere, stratosphere, mesosphere, thermosphere and exosphere. The temperature pattern (falling, rising, falling, rising) defines the lower four; the exosphere is the outermost, rarefied fringe.

Troposphere. The lowest layer. NCERT gives an average height of 13 km, about 8 km near the poles and about 18 km at the equator. It is thickest at the equator because strong convectional currents carry heat to great heights. It contains the dust and water vapour, and all changes in weather and climate take place in it. Temperature falls at 1°C for every 165 m of height. It is the most important layer for biological activity.

NCERT's next chapter gives the normal lapse rate as 6.5°C per 1,000 m. The two figures are close (1°C per 165 m is about 6.1°C per 1,000 m) and both appear in the same book; if a question quotes either, recognise it.

Tropopause. The zone separating the troposphere from the stratosphere. Air temperature here is about −80°C over the equator and about −45°C over the poles. Because temperature is nearly constant through it, it is called the tropopause. The tropopause is colder over the equator than over the poles because it is so much higher there.

Stratosphere. Above the tropopause, up to 50 km. Its key feature is the ozone layer, which absorbs ultraviolet radiation and shields life from that intense, harmful energy. NOAA notes that temperature rises with height in the stratosphere because of heat released during ozone formation, so warmer air lies above cooler air and convection is suppressed (NOAA JetStream).

Mesosphere. Above the stratosphere, up to 80 km. Temperature falls again with height, to about −100°C at 80 km. Its upper limit is the mesopause. NOAA notes that the gases here are dense enough to slow meteors, which burn up and leave trails.

Ionosphere. NCERT places it between 80 and 400 km, above the mesopause. It contains electrically charged particles (ions), which is how it gets its name, and it reflects radio waves transmitted from the earth back to the earth. Temperature rises with height. NCERT uses "ionosphere" for this layer and calls the next one the layer "above the thermosphere", so in NCERT's usage the ionosphere sits in the thermosphere.

Exosphere. The uppermost layer, about which little is known. Its contents are extremely rarefied, and it gradually merges with outer space.

NCERT closes by noting that although all layers influence us, geographers are concerned with the first two layers (troposphere and stratosphere).

Elements of weather and climate. NCERT lists the elements of the atmosphere that change and influence human life: temperature, pressure, winds, humidity, clouds and precipitation (NCERT exercise 2-ii). They are taken up in the next three chapters.

Key Term

"-pause" and the direction of temperature change

Each boundary between layers is a "-pause": tropopause (troposphere/stratosphere), stratopause (stratosphere/mesosphere), mesopause (mesosphere/thermosphere). Remember the layers by what temperature does with height:

Troposphere: falls (1°C per 165 m in NCERT)
Stratosphere: rises (ozone)
Mesosphere: falls, to about −100°C at 80 km
Thermosphere/ionosphere: rises

A question that says "temperature increases with height in the troposphere" is describing an inversion, not the normal state.

Why the troposphere controls weather

This is NCERT's exercise 2-(iv) and Mains GS1 2022 (gs1-pyq-2022-18). The answer combines NCERT's points:

It contains almost all the water vapour and dust, the raw materials of clouds and precipitation (CO₂ and water vapour are found only up to 90 km, and water vapour falls off sharply with height).
It is heated from below, so temperature falls with height. Warm surface air rises, cools, and its vapour condenses on hygroscopic nuclei. This vertical movement produces clouds, rain and storms.
Convection is confined to the troposphere (NCERT, next chapter). Above the tropopause, temperature rises with height in the stratosphere, which suppresses vertical motion.
Its thickness varies with latitude because convection is strongest at the equator.

Jet streams are a frequent trap. They are narrow bands of strong wind in the upper atmosphere, "typically occurring around 30,000 feet (9,100 meters)" and varying in height between four and eight miles (about 6–13 km), according to NOAA's JetStream school (NOAA, "The Jet Stream", accessed 1 October 2026). That places them in the upper troposphere near the tropopause, not in the stratosphere.

UPSC Connect

Prelims 2024: troposphere thickness

UPSC CSE Prelims 2024 asked a Statement-I/Statement-II item: Statement-I said the thickness of the troposphere at the equator is much greater than at the poles; Statement-II said that at the equator heat is transported to great heights by strong convectional currents. Both are NCERT sentences, and Statement-II explains Statement-I. (Site bank ID geo-085, which paraphrases the stem.)

The ozone layer

NCERT says only that ozone lies between 10 and 50 km and absorbs ultraviolet radiation. The rest of this section is beyond NCERT.

Beyond the Book

Ozone depletion and the Montreal Protocol

Mechanism: chlorofluorocarbons (CFCs) and other ozone-depleting substances are broken down in the stratosphere, releasing chlorine and bromine that destroy ozone. Paul Crutzen, Mario Molina and F. Sherwood Rowland received the 1995 Nobel Prize in Chemistry "for their work in atmospheric chemistry, particularly concerning the formation and decomposition of ozone" (nobelprize.org).
Montreal Protocol: signed in 1987, in force from 1989; it phases out the production and consumption of ozone-depleting substances (UNEP Ozone Secretariat).
Recovery (latest assessment): the 2022 WMO/UNEP Scientific Assessment of Ozone Depletion found that, if current policies stay in place, the ozone layer should return to 1980 values by about 2040 for most of the world, about 2045 over the Arctic and about 2066 over the Antarctic (WMO release, 9 January 2023; NOAA CSL Executive Summary). The next quadrennial assessment (2026) is due for release at the end of 2026; until then, the 2022 dates are the current official ones.
Two kinds of ozone: stratospheric ozone shields the surface from ultraviolet rays; ozone formed near the ground is an air pollutant.

UPSC connection: Prelims 2011 asked the main reason for the formation of the Antarctic ozone hole; the answer was the presence of a prominent polar front and stratospheric clouds, and the inflow of chlorofluorocarbons (site bank ID env-058).

Volcanic aerosols

Particles in the atmosphere can cool the surface. The US Geological Survey records that nearly 20 million tons of sulphur dioxide injected into the stratosphere by the 1991 eruptions of Mount Pinatubo (Philippines) spread around the world and lowered global temperatures temporarily, from 1991 through 1993, by about 0.5°C (USGS Fact Sheet 113-97).

PART 3 — UPSC Integration

PYQ pattern

Mains GS1 2022, 15 marks (gs1-pyq-2022-18): "The troposphere significantly determines weather processes. How?" A direct NCERT question.
Prelims 2024 (bank ID geo-085): Statement-I/II on troposphere thickness at the equator and convection. Direct NCERT text.
Prelims 2011 (bank ID env-058): reasons for the Antarctic ozone hole. Beyond NCERT, linked to the ozone layer.
Adjacent, GS3: gs3-pyq-2022-18 (global warming and the Kyoto Protocol) and gs3-pyq-2017-37 (effects of climate change on India) use this chapter's greenhouse-gas facts but are answered mainly from the climate-change chapter.

Mains frameworks

"Troposphere determines weather": location of water vapour and dust → heating from below and fall of temperature with height → convection, condensation on hygroscopic nuclei → clouds and precipitation → thicker at the equator because of strong convection → stable stratosphere above, so weather stops at the tropopause.
Composition questions: permanent gases (N₂, O₂, Ar) vs variable components (CO₂, water vapour, ozone, dust) → the role of each in radiation and condensation → the rising trend of CO₂ with dated NOAA figures.

UPSC Connect

Cross-paper relevance

GS1 (Geography): composition, layers, troposphere and weather.
GS3 (Environment): greenhouse gases and global warming; ozone depletion and the Montreal Protocol; air pollution (ground-level ozone, particulate matter).

Exam Strategy

Prelims fact-traps:

Troposphere height: NCERT 13 km average, 8 km poles, 18 km equator. Older notes give 16 km at the equator; NOAA gives 18–20 km. In an NCERT-based question, go with NCERT.
Tropopause is colder over the equator (−80°C) than over the poles (−45°C).
Ozone: 10–50 km (NCERT); it sits in the stratosphere, not the troposphere.
Jet streams are near the tropopause (upper troposphere), not in the stratosphere.
Oxygen is negligible at 120 km; CO₂ and water vapour only up to 90 km; 99% of mass within 32 km (NCERT exercise MCQ iv asks the 120 km figure).
Ionosphere: 80–400 km (NCERT); it reflects radio waves.
NCERT counts five layers.
Dust is more abundant in subtropical and temperate regions than in equatorial and polar regions.
The coldest point in NCERT's description is −100°C at 80 km (mesopause).

Mains: questions on this chapter ask "how" or "why" (why the troposphere controls weather, why it is thicker at the equator). Use NCERT's mechanism first, then add dated data.

Practice Questions

UPSC Mains GS1 2022 (15 marks; bank ID gs1-pyq-2022-18): "The troposphere significantly determines weather processes. How?"
UPSC Prelims 2024 (bank ID geo-085): Consider the following statements: Statement-I: Thickness of the troposphere at the equator is much greater as compared to poles. Statement-II: At the equator, heat is transported to great heights by strong convectional currents. Answer: Both Statement-I and Statement-II are correct and Statement-II explains Statement-I. (Statement wording as reproduced in published 2024 answer keys; the site bank paraphrases it.)
UPSC Prelims 2011 (bank ID env-058; bank wording, the published paper opens with "The formation of ozone hole in the Antarctic region has been a cause of concern"): "What could be the main reason/reasons for the formation of the ozone 'hole' in the Antarctic region?" (a) Presence of prominent tropospheric turbulence; and inflow of chlorofluorocarbons (b) Presence of prominent polar front and stratospheric clouds; and inflow of chlorofluorocarbons (c) Absence of polar front and stratospheric clouds; and inflow of methane and chlorofluorocarbons (d) Increased temperature at polar region due to global warming. Answer: (b).
Practice (UPSC-pattern, not a past paper): With reference to the atmosphere, consider the following statements: (1) Oxygen is in negligible quantity at a height of 120 km. (2) Carbon dioxide and water vapour are found up to about 90 km. (3) The air temperature at the tropopause is lower over the poles than over the equator. Which of the statements given above are correct? (a) 1 and 2 only (b) 2 and 3 only (c) 1 and 3 only (d) 1, 2 and 3. Answer: (a). The tropopause is about −80°C over the equator and −45°C over the poles.

📦 Revision Capsule

Revision Capsule

Hard Facts

99% of the atmosphere's mass lies within 32 km; oxygen negligible at 120 km; CO₂ and water vapour only up to 90 km (NCERT).
Dry air: N₂ 78.08%, O₂ 20.95%, Ar 0.934% (NASA).
CO₂ 427.35 ppm (2025 annual mean, NOAA Mauna Loa); May 2026 peak 432.3 ppm (NOAA).
Water vapour up to 4% (wet tropics), under 1% (deserts, polar regions).
Five layers: troposphere, stratosphere, mesosphere, thermosphere, exosphere.
Troposphere: 13 km average, 8 km poles, 18 km equator; temperature falls 1°C per 165 m (NCERT Ch 7); normal lapse rate 6.5°C per 1,000 m (NCERT Ch 8).
Tropopause −80°C over equator, −45°C over poles.
Stratosphere up to 50 km, ozone 10–50 km; mesosphere up to 80 km, −100°C at 80 km; ionosphere 80–400 km, reflects radio waves.
Jet streams around 9 km (NOAA): upper troposphere near the tropopause.
Montreal Protocol 1987 (in force 1989); ozone back to 1980 values by ~2040 (most of world), ~2045 (Arctic), ~2066 (Antarctic) (2022 assessment).
GWP-100 (AR6): CH₄ 27.0/29.8, N₂O 273, SF₆ 24,300.

Core Concepts

CO₂ and water vapour pass sunlight but absorb the earth's long-wave radiation.
Dust and salt are hygroscopic nuclei: no nuclei, no cloud droplets.
Layers are defined by the direction of temperature change with height.
The troposphere holds the moisture, is heated from below and overturns by convection, so all weather happens there.

Confused Pairs

Troposphere (temperature falls, weather) vs stratosphere (temperature rises, ozone)
Ionosphere 80–400 km (NCERT) vs thermosphere ~85–600 km (NOAA)
1°C per 165 m (NCERT Ch 7) vs 6.5°C per 1,000 m (NCERT Ch 8)
Stratospheric ozone (UV shield) vs ground-level ozone (pollutant)

PYQ Pattern

Mains GS1 2022 (gs1-pyq-2022-18) on the troposphere and weather; Prelims 2024 (geo-085) on troposphere thickness; Prelims 2011 (env-058) on the Antarctic ozone hole.

Sources

Sources

NCERT, Fundamentals of Physical Geography, Textbook for Class XI, Chapter 7 "Composition and Structure of Atmosphere", Reprint 2026-27: kegy207.pdf; Chapter 8 for the 6.5°C/1,000 m lapse rate: kegy208.pdf.
NOAA Global Monitoring Laboratory annual-mean files (CO₂ Mauna Loa; CH₄ and N₂O global), accessed 1 October 2026: CO₂, CH₄, N₂O.
Scripps Institution of Oceanography / UC San Diego, "Annual Carbon Dioxide Peak Reaches 432 Parts per Million", 11 June 2026: today.ucsd.edu.
NASA NSSDC, Earth Fact Sheet: nssdc.gsfc.nasa.gov.
NOAA JetStream, "Layers of the Atmosphere" and "The Jet Stream": layers, jet stream.
GHG Protocol, "IPCC Global Warming Potential Values", v2.0, 7 August 2024 (IPCC AR4/AR5/AR6 values): PDF.
WMO, "Ozone layer recovery is on track, helping avoid global warming by 0.5°C", 9 January 2023: wmo.int; NOAA CSL, Scientific Assessment of Ozone Depletion 2022, Executive Summary: csl.noaa.gov.
UNEP Ozone Secretariat, The Montreal Protocol: ozone.unep.org; Nobel Prize in Chemistry 1995: nobelprize.org.
USGS Fact Sheet 113-97, "The Cataclysmic 1991 Eruption of Mount Pinatubo, Philippines": pubs.usgs.gov.
UPSC questions: site banks src/_data/mains/pyq/gs1.json, src/_data/pyq-2024-2025-additions.js (geo-085), src/_data/pyq-environment-updated.js (env-058).

Composition and Structure of Atmosphere