CDS Geography · Climatology, Atmosphere and Weather
Composition and Layers of the Atmosphere
Which gases make up the air and in what shares, why water vapour matters so much, how the atmosphere is layered by temperature, and where the ozone layer sits.
Why this matters
Twelve CDS questions from 2016 to 2026: 4 EASY, 6 MODERATE, 2 HARD. Five are about ozone, two about water vapour, and both HARD ones ask you to rank gases by their share of the air, so the NCERT gas table is worth learning in order.
Concept 1 of 4: The gases of the air and where they came from
Dry air is almost all nitrogen and oxygen. Everything else, argon, carbon dioxide and the rare gases, adds up to about one per cent. The air we have today is the third atmosphere the Earth has had: the first was lost, the second came out of the hot interior, and living things changed the third.
Definition
Permanent gases (NCERT Class 11, share by volume of dry air):
- Nitrogen 78.08 and oxygen 20.95 per cent: together about 99 per cent.
- Argon 0.93 per cent: the third gas, far ahead of the rest.
- Carbon dioxide 0.036 per cent: small, but a strong greenhouse gas (it lets sunlight in and traps outgoing heat).
- The rare gases: neon 0.002, krypton 0.001, helium 0.0005, xenon 0.00009, hydrogen 0.00005 per cent.
- Other greenhouse gases: methane, nitrous oxide, CFCs and water vapour.
Evolution of the present atmosphere, in three stages:
- Stage 1: the primordial atmosphere (mostly hydrogen and helium) was lost, stripped away by the solar wind.
- Stage 2: the hot interior of the Earth gave out gases and water vapour (degassing).
- Stage 3: living things changed the mix through photosynthesis, which added oxygen.
| Gas | Share by volume (per cent) |
|---|---|
| Nitrogen | 78.08 |
| Oxygen | 20.95 |
| Argon | 0.93 |
| Carbon dioxide | 0.036 |
| Neon | 0.002 |
| Krypton | 0.001 |
| Helium | 0.0005 |
| Xenon | 0.00009 |
| Hydrogen | 0.00005 |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2026 · CDS (II) 2026 — General Knowledge · Q32Moderate
Example 1 · Climatology, Atmosphere and Weather · Composition and Layers of the Atmosphere
Which of the following statements related to the stages of evolution of the present atmosphere are correct ?
1. The first stage is marked by the loss of primordial atmosphere
2. In the second stage, the hot interior of the earth contributed to the evolution of the atmosphere
3. Finally, the composition of the atmosphere was modified by the living world through the process of photosynthesis
Select the answer using the code given below :
Argon, not carbon dioxide, is third
Carbon dioxide gets the most attention, but argon (0.93 per cent) is about 25 times as plentiful. In any ranking of gases by volume, argon comes right after nitrogen and oxygen.
Ascending means smallest first
A ranking item that asks for ascending order wants the scarcest gas first. Write the shares beside each gas before you read the options, then check whether the option runs up or down.
Concept 2 of 4: Water vapour
Water vapour is the one gas whose amount changes from place to place and hour to hour. Warm air can hold much more of it than cold air, so the hot wet tropics have the most and cold deserts and the poles have the least. Though small in amount, it shapes the weather.
Definition
What NCERT says about water vapour:
- It is a variable gas: up to 4 per cent of the air in the warm, wet tropics, less than 1 per cent in the dry, cold deserts and polar regions.
- It decreases with altitude, because colder air holds less.
- It absorbs part of the incoming sunlight and traps the heat the Earth gives out, so it acts like a blanket: the Earth becomes neither too hot nor too cold.
- It plays a part in the stability and instability of the air, because it releases heat when it condenses.
- It is the source of all clouds and precipitation.
| Place | Water vapour in the air |
|---|---|
| Warm, wet tropics | Up to about 4 per cent |
| Dry, cold deserts and polar regions | Less than 1 per cent |
| Higher up in the atmosphere | Less than near the groundQ CDS 2026 (II): water vapour is variable and decreases with altitude, and is under 1 per cent in deserts and polar regions. Both true. |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2026 · CDS (II) 2026 — General Knowledge · Q34Moderate
Example 2 · Climatology, Atmosphere and Weather · Composition and Layers of the Atmosphere
Consider the following statements related to water vapours :
1. It is a variable gas in the atmosphere, which decreases with altitude
2. In the dry and cold areas of desert and polar regions, it may be less than one per cent of the air
Which of the statements given above is/are correct ?
Variable, not permanent
Nitrogen, oxygen and argon are permanent gases with a fixed share. Water vapour is variable: its share changes with place, season and height.
Concept 3 of 4: Layers of the atmosphere
The atmosphere is cut into layers by how temperature changes with height. Temperature falls through the lowest layer, rises through the ozone layer, falls again, and then rises very high up. Each turning point is a 'pause'.
Definition
From the ground up:
- Troposphere: about 8 km thick at the poles and 18 km at the equator. All weather happens here. Temperature falls with height.
- Tropopause: the top of the troposphere, where temperature stays nearly the same with height (an isothermal zone), about minus 80 °C over the equator.
- Stratosphere: up to about 50 km. Holds the ozone layer, and temperature rises with height because ozone absorbs ultraviolet rays.
- Mesosphere: about 50 to 80 km. Temperature falls again, to about minus 100 °C at 80 km, the coldest part of the atmosphere (the mesopause).
- Thermosphere (with the ionosphere, about 80 to 400 km): temperature rises again; its charged layers reflect radio waves back to Earth.
- Exosphere: the thin outermost layer.
| Layer | Height | Temperature with height | Remember it for |
|---|---|---|---|
| Troposphere | Ground to 8 km (poles) or 18 km (equator) | Falls | All weather |
| Stratosphere | Up to about 50 km | Rises | Ozone layer, calm air for jets |
| Mesosphere | About 50 to 80 km | Falls | Coldest air, near its top |
| Thermosphere and ionosphere | About 80 to 400 km | Rises | Reflects radio waves |
| Exosphere | Above the thermosphere | Very thin air | Outermost layer |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2016 · CDS (II) 2016 — General Knowledge · Q33Moderate
Example 3 · Climatology, Atmosphere and Weather · Composition and Layers of the Atmosphere
Which of the following statements concerning atmosphere of the Earth are correct?
1. In stratosphere, temperature increases with altitude.
2. In mesosphere, temperature decreases with altitude.
3. The lowest temperature of the atmosphere is recorded in the upper part of mesosphere.
4. Tropopause is an isothermal zone.
Select the correct answer using the code given below.
The troposphere is thickest at the equator
Strong heating at the equator lifts air higher, so the troposphere is about 18 km thick there and only about 8 km at the poles.
Coldest is the top of the mesosphere
The lowest temperature in the atmosphere is near the top of the mesosphere (about minus 100 °C at 80 km), not at the tropopause and not in the stratosphere.
Concept 4 of 4: The ozone layer
Ozone is oxygen with three atoms instead of two. In the stratosphere it soaks up most of the Sun's ultraviolet rays, which would otherwise burn skin and harm living things. CFC gases released by people break ozone down, and the damage is worst over the South Pole.
Definition
Facts CDS asks:
- Ozone (O₃) is a form of oxygen with three oxygen atoms, made by chemical reactions in sunlight.
- About 90 per cent of all ozone is in the stratosphere (mostly 15 to 35 km): the ozone layer.
- It absorbs ultraviolet radiation, which is why the stratosphere warms with height. Ultraviolet rays that get through cause skin cancer.
- The ozone hole is over Antarctica, caused mainly by CFCs (chlorofluorocarbons), whose chlorine destroys ozone.
- The Montreal Protocol (1987) cut CFC use. 16 September is the International Day for the Preservation of the Ozone Layer.
| Question | Answer |
|---|---|
| Where is most ozone? | In the stratosphere, about 90 per cent of itQ Asked twice: CDS 2017 (II) and 2021 (I). Not the troposphere, not the ionosphere. |
| Does the stratosphere let ultraviolet in? | No, its ozone absorbs most of itQ CDS 2017 (II): ultraviolet causes skin cancer, but the stratosphere blocks it rather than letting it in. |
| Where is the ozone hole, and what causes it? | Over Antarctica, mainly CFCsQ CDS 2024 (II): Antarctic and CFCs, not the Arctic and not carbon dioxide. |
| Ozone Day | 16 September (Montreal Protocol signed, 1987)Q CDS 2020 (II): the false statement put Ozone Day in November. |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2017 · CDS (II) 2017 — General Knowledge · Q21Easy
Example 4 · Climatology, Atmosphere and Weather · Composition and Layers of the Atmosphere
Most ozone gas (about 90%) is located in the atmospheric layer of
The hole is over Antarctica, the cause is CFCs
Options mix the pairs. The ozone hole is over the Antarctic, not the Arctic, and it is caused mainly by CFCs, not by carbon dioxide (which warms the planet but does not destroy ozone).
Ozone Day is in September
The International Day for the Preservation of the Ozone Layer is 16 September, the date the Montreal Protocol was signed. A statement putting it in November is false.
Summary — formulas & gotchas at a glance
A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.
Reference tables (4)
The gases of the air and where they came from9 rows
| Gas | Share by volume (per cent) |
|---|---|
| Nitrogen | 78.08 |
| Oxygen | 20.95 |
| Argon | 0.93 |
| Carbon dioxide | 0.036 |
| Neon | 0.002 |
| Krypton | 0.001 |
| Helium | 0.0005 |
| Xenon | 0.00009 |
| Hydrogen | 0.00005 |
Water vapour3 rows
| Place | Water vapour in the air |
|---|---|
| Warm, wet tropics | Up to about 4 per cent |
| Dry, cold deserts and polar regions | Less than 1 per cent |
| Higher up in the atmosphere | Less than near the groundQ CDS 2026 (II): water vapour is variable and decreases with altitude, and is under 1 per cent in deserts and polar regions. Both true. |
Layers of the atmosphere5 rows
| Layer | Height | Temperature with height | Remember it for |
|---|---|---|---|
| Troposphere | Ground to 8 km (poles) or 18 km (equator) | Falls | All weather |
| Stratosphere | Up to about 50 km | Rises | Ozone layer, calm air for jets |
| Mesosphere | About 50 to 80 km | Falls | Coldest air, near its top |
| Thermosphere and ionosphere | About 80 to 400 km | Rises | Reflects radio waves |
| Exosphere | Above the thermosphere | Very thin air | Outermost layer |
The ozone layer4 rows
| Question | Answer |
|---|---|
| Where is most ozone? | In the stratosphere, about 90 per cent of itQ Asked twice: CDS 2017 (II) and 2021 (I). Not the troposphere, not the ionosphere. |
| Does the stratosphere let ultraviolet in? | No, its ozone absorbs most of itQ CDS 2017 (II): ultraviolet causes skin cancer, but the stratosphere blocks it rather than letting it in. |
| Where is the ozone hole, and what causes it? | Over Antarctica, mainly CFCsQ CDS 2024 (II): Antarctic and CFCs, not the Arctic and not carbon dioxide. |
| Ozone Day | 16 September (Montreal Protocol signed, 1987)Q CDS 2020 (II): the false statement put Ozone Day in November. |
Watch out for (7)
- Argon, not carbon dioxide, is third→ The gases of the air and where they came from
- Ascending means smallest first→ The gases of the air and where they came from
- Variable, not permanent→ Water vapour
- The troposphere is thickest at the equator→ Layers of the atmosphere
- Coldest is the top of the mesosphere→ Layers of the atmosphere
- The hole is over Antarctica, the cause is CFCs→ The ozone layer
- Ozone Day is in September→ The ozone layer
Test yourself on Climatology, Atmosphere and Weather
15 past CDS questions from this chapter, timed at 15 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.