Composition and Structure of Atmosphere

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. Prelims has asked why the troposphere is thicker at the equator; Mains has asked how the troposphere determines weather.

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: NCERT says oxygen becomes negligible by 120 km, and carbon dioxide and water vapour are found only up to 90 km. These are NCERT's school-level limits for the composition of the air.

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.

Prelims draws on these numbers; Mains asks why the troposphere controls the weather.

PART 1 — Quick Reference

Table 1: Layers of the atmosphere

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

Sources: NASA NSSDC, Earth Fact Sheet (last updated 15 November 2024; Wayback copy); 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

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.

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.

Why the troposphere controls weather

This is NCERT's exercise 2-(iv). The answer combines NCERT's points:

• It contains almost all the water vapour and dust, the raw materials of clouds and precipitation (NCERT: CO₂ and water vapour are found only up to 90 km; 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.

The ozone layer

NCERT says that ozone is found between 10 and 50 km above the earth's surface and acts as a filter, absorbing the sun's ultraviolet rays. NASA's figures agree: 90% of the atmosphere's ozone lies in the stratosphere, "between about 10 and 50 kilometers", with the highest concentrations between 30 and 35 km. The details below go beyond NCERT.

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

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.

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.
• NCERT's limits: 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's five layers are troposphere, stratosphere, mesosphere, ionosphere and exosphere.
• 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

Practice (UPSC-pattern, not past papers). Questions 1 to 5 are NCERT's own exercise MCQs.

Prelims:

1. Which one of the following gases constitutes the major portion of the atmosphere?
   (a) Oxygen
   (b) Nitrogen
   (c) Argon
   (d) Carbon dioxide

2. The atmospheric layer important for human beings is:
   (a) Stratosphere
   (b) Mesosphere
   (c) Troposphere
   (d) Ionosphere

3. Sea salt, pollen, ash, smoke soot and fine soil are associated with:
   (a) Gases
   (b) Dust particles
   (c) Water vapour
   (d) Meteors

4. Oxygen gas is in negligible quantity at the height of (as NCERT gives it):
   (a) 90 km
   (b) 120 km
   (c) 100 km
   (d) 150 km

5. 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

6. Consider the following statements about the troposphere: 1. It is thicker at the equator than at the poles because strong convection carries heat to great heights there. 2. The temperature at the tropopause is lower over the equator than over the poles. 3. The temperature in the troposphere falls by about 1°C for every 165 m of height. 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

7. Consider the following statements, as given in NCERT: 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
   The tropopause is about −80°C over the equator and −45°C over the poles.

Mains (practice, 150 words each):

1. Why is the troposphere the most important of all the layers of the atmosphere, and how does it determine weather processes?
2. Draw a suitable diagram of the structure of the atmosphere, label it and describe it. (NCERT Ex. 3(ii)) Approach: the figure above gives the layers, heights and the temperature curve.
3. Describe the composition of the atmosphere. (NCERT Ex. 3(i))

📦 Revision Capsule

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 (last updated 15 November 2024; Wayback copy): nssdc.gsfc.nasa.gov.
• NASA Ozone Watch, "Ozone facts": ozonewatch.gsfc.nasa.gov; NOAA Space Weather Prediction Center, "Ionosphere": swpc.noaa.gov.
• UN Treaty Collection, Montreal Protocol status: treaties.un.org.
• 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.