PrepYodhaClass Notes · Geography
Geography · World · 06

The Atmosphere & Its Layers

The atmosphere is the vast envelope of air that completely surrounds planet Earth, held in place by the gravitational pull of the Earth. It extends upward to about 1000 km, though 99% of its mass is concentrated within just 32 km of the surface. Among all planets in the solar system, only Earth possesses this life-sustaining atmospheric blanket. Understanding its composition and its five distinct layers is a high-frequency topic in SSC CGL, CHSL, and Railway exams.

☁️ 9 topics🎯 69+ points📝 self-test
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Topic 01

Extent & Importance of the Atmosphere

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The atmosphere reaches far above us but is much denser closer to the surface, which is why the bulk of its gases stay near the ground.

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Key Point
The atmosphere extends upward to 1000 km, but 99% of its total composition is found within 32 km of the surface.
  • The atmosphere is held close to Earth by the Gravitational pull of the Earth.
  • Only our planet Earth has an atmosphere capable of supporting life.
  • The atmosphere shields Earth from harmful solar radiation, regulates temperature, and enables weather processes.
📝 Quick self-test 2 MCQs · 2 fill-ups

Up to what height does the atmosphere extend upward from the Earth's surface?

  1. 100 km
  2. 500 km
  3. 1000 km
  4. 5000 km
✔ C. 1000 km — The atmosphere extends upward to about 1000 km.

What holds the atmosphere close to the Earth?

  1. Magnetic field
  2. Gravitational pull of the Earth
  3. Air pressure
  4. Solar wind
✔ B. Gravitational pull of the Earth — The atmosphere is held close to Earth by the gravitational pull of the Earth.

About 99% of the atmosphere's total composition is found within of the surface.

✔ 32 km

Only the planet has an atmosphere capable of supporting life.

✔ Earth
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Topic 02

Composition of the Atmosphere

Air is a mixture of several gases, with two dominating. Traces of dust, water vapour, and other particles are also present.

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Key Point
Nitrogen (78%) and Oxygen (20.95%) together make up almost 99% of the atmosphere.
Gas composition table
GasApproximate %
Nitrogen (N₂)78%
Oxygen (O₂)20.95%
Argon (Ar)0.93%
Carbon Dioxide (CO₂)0.04%
Neon (Ne)0.0018%
Helium (He)0.0005%
Ozone (O₃)0.0006%
Hydrogen (H₂)0.00005%
  • CO₂ is present in very small quantity but plays a crucial role in the greenhouse effect.
  • Dust particles in the atmosphere intercept and reflect insolation (incoming solar radiation).
  • Dust in the atmosphere is responsible for the red and orange colours of sunrise and sunset.
📝 Quick self-test 2 MCQs · 2 fill-ups

Which gas is the most abundant in the atmosphere?

  1. Oxygen
  2. Carbon Dioxide
  3. Nitrogen
  4. Argon
✔ C. Nitrogen — Nitrogen makes up about 78% of the atmosphere, the highest of any gas.

What is responsible for the red and orange colours of sunrise and sunset?

  1. Ozone
  2. Dust particles
  3. Water vapour
  4. Nitrogen
✔ B. Dust particles — Dust in the atmosphere is responsible for the red and orange colours of sunrise and sunset.

Nitrogen and Oxygen together make up almost of the atmosphere.

✔ 99%

The percentage of Oxygen in the atmosphere is about .

✔ 20.95%
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Topic 03

Greenhouse Gases

Greenhouse gases trap outgoing infrared radiation from the Earth's surface, warming the planet. Their build-up due to human activity intensifies the natural greenhouse effect.

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Key Point
The six major greenhouse gases are: Methane (CH₄), Chlorofluorocarbons (CFCs), Carbon Dioxide (CO₂), Nitrous Oxide (NO₂), Ozone (O₃), and Fluorinated Gases.
  • Methane (CH₄) — released from paddy fields, cattle, landfills, and natural gas leaks.
  • Chlorofluorocarbons (CFCs) — synthetic compounds that also destroy the ozone layer in the stratosphere.
  • Carbon Dioxide (CO₂) — primary greenhouse gas; produced by burning fossil fuels.
  • Nitrous Oxide (NO₂) — released from agricultural soils and fertilisers.
  • Ozone (O₃) — acts as a greenhouse gas in the troposphere; protective shield in the stratosphere.
  • Fluorinated Gases — powerful industrial greenhouse gases with very long atmospheric lifetimes.
📝 Quick self-test 2 MCQs · 2 fill-ups

How many major greenhouse gases are recognised in the notes?

  1. Four
  2. Five
  3. Six
  4. Seven
✔ C. Six — The six major greenhouse gases are Methane, CFCs, CO2, Nitrous Oxide, Ozone, and Fluorinated Gases.

Which greenhouse gas is released from paddy fields, cattle, and landfills?

  1. Methane (CH4)
  2. Carbon Dioxide (CO2)
  3. Nitrous Oxide (NO2)
  4. Ozone (O3)
✔ A. Methane (CH4) — Methane is released from paddy fields, cattle, landfills, and natural gas leaks.

The primary greenhouse gas produced by burning fossil fuels is .

✔ Carbon Dioxide

Synthetic compounds that also destroy the ozone layer in the stratosphere are .

✔ Chlorofluorocarbons
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Topic 04

Layers of the Atmosphere — Overview

The atmosphere is divided into five distinct layers based on temperature variation with altitude. The mnemonic "Trust me in The Exam" (Troposphere → Stratosphere → Mesosphere → Thermosphere → Exosphere) helps recall the order.

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Key Point
There are 5 layers of the atmosphere: Troposphere, Stratosphere, Mesosphere, Thermosphere, and Exosphere.
Summary of the five layers
LayerAltitude (approx.)Key Feature
Troposphere0–12 kmWeather occurs here; temperature decreases with height
Stratosphere12–50 kmOzone layer; temperature increases with height; jet aircraft fly here
Mesosphere50–80 kmColdest layer; meteors burn up here; temperature decreases with height
Thermosphere80–513 kmHottest layer; also called Ionosphere; auroras occur here
Exosphere513–960 kmOutermost layer; very thin; gases escape to space
Layers from Earth's Surface Upward
1
Troposphere (0–12 km)
1st layer — all weather; temperature decreases; 90%+ of atmospheric gases
2
Stratosphere (12–50 km)
2nd layer — ozone layer absorbs UV rays; temperature increases; jet planes fly here
3
Mesosphere (50–80 km)
3rd layer — coldest layer (down to −90°C); meteors burn up
4
Thermosphere (80–513 km)
4th layer — hottest layer (up to 2500°C); Ionosphere; radio waves reflected here
5
Exosphere (513–960 km)
5th (outermost) layer — temperature −300°C to 1650°C; gases escape to space
📝 Quick self-test 2 MCQs · 2 fill-ups

How many distinct layers is the atmosphere divided into?

  1. 3
  2. 4
  3. 5
  4. 6
✔ C. 5 — The atmosphere has 5 layers: Troposphere, Stratosphere, Mesosphere, Thermosphere, and Exosphere.

Which layer is the outermost layer of the atmosphere?

  1. Thermosphere
  2. Mesosphere
  3. Stratosphere
  4. Exosphere
✔ D. Exosphere — The Exosphere is the outermost layer, where gases escape to space.

The atmospheric layer where all weather occurs is the .

✔ Troposphere

The coldest layer of the atmosphere, where meteors burn up, is the .

✔ Mesosphere
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Topic 05

Troposphere

The troposphere is the lowest and densest layer of the atmosphere — the layer where all life exists and all weather events take place.

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Key Point
The Troposphere is the 1st (lowest) layer of the atmosphere and the site of all weather phenomena.
  • In the troposphere, temperature decreases with increasing height at an average rate of about 6.5°C per km.
  • It contains more than 90% of the total gases of our atmosphere.
  • Cloud formation, rainfall, thunderstorms, cyclones — all weather changes — occur in the Troposphere.
  • The word "Tropo" means change, which is why all weather changes happen here.
  • The upper boundary of the troposphere is called the Tropopause.
  • At the Tropopause, temperature stops decreasing and can reach as low as −58°C.
📝 Quick self-test 2 MCQs · 2 fill-ups

In the troposphere, temperature decreases with height at an average rate of about:

  1. 6.5°C per km
  2. 10°C per km
  3. 1°C per km
  4. 3.5°C per km
✔ A. 6.5°C per km — Temperature decreases at about 6.5°C per km in the troposphere.

What percentage of the atmosphere's total gases does the troposphere contain?

  1. More than 50%
  2. More than 70%
  3. More than 90%
  4. More than 30%
✔ C. More than 90% — The troposphere contains more than 90% of the total gases of the atmosphere.

The upper boundary of the troposphere is called the .

✔ Tropopause

The word 'Tropo' means , which is why all weather changes happen here.

✔ change
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Topic 06

Stratosphere & Ozone Layer

Above the tropopause lies the stratosphere — a calm, cloud-free zone where temperature rises due to UV absorption by ozone.

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Key Point
The Stratosphere is the 2nd layer; it extends from the Tropopause up to 50 km and contains the vital ozone layer.
  • Temperature in the stratosphere increases slowly with altitude, reaching about 4°C at the top, due to ultraviolet (UV) rays from the Sun being absorbed by ozone.
  • The stratosphere is free from clouds and other weather phenomena, making it ideal for large jet planes and airplanes to fly in.
  • The ozone layer (concentrated around 20–35 km) absorbs harmful UV radiation and protects life on Earth.
  • The upper boundary of the stratosphere is called the Stratopause.
📝 Quick self-test 2 MCQs · 2 fill-ups

Why does temperature increase with altitude in the stratosphere?

  1. Friction from meteors
  2. Absorption of UV rays by ozone
  3. Reflection of radio waves
  4. Escape of light gases
✔ B. Absorption of UV rays by ozone — Ultraviolet rays absorbed by ozone cause the temperature to rise in the stratosphere.

Why is the stratosphere ideal for large jet planes to fly in?

  1. It is the hottest layer
  2. It is free from clouds and weather phenomena
  3. It reflects radio waves
  4. Meteors burn up there
✔ B. It is free from clouds and weather phenomena — The stratosphere is free from clouds and other weather phenomena, making it ideal for jet planes.

The vital ozone layer is concentrated in the layer of the atmosphere.

✔ Stratosphere

The upper boundary of the stratosphere is called the .

✔ Stratopause
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Topic 07

Mesosphere

The mesosphere lies above the stratosphere and is the layer in which most meteors burn up as they enter from space.

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Key Point
The Mesosphere is the 3rd layer; it extends up to 80 km and is the coldest layer of the atmosphere.
  • Temperature decreases again in the mesosphere due to no significant heat source; it falls to about −90°C at the top.
  • Most meteors (shooting stars) burn up in the mesosphere due to friction with atmospheric gases.
  • The upper boundary of the mesosphere is called the Mesopause.
📝 Quick self-test 2 MCQs · 2 fill-ups

Which is the coldest layer of the atmosphere?

  1. Troposphere
  2. Stratosphere
  3. Mesosphere
  4. Exosphere
✔ C. Mesosphere — The Mesosphere is the coldest layer, falling to about -90°C at the top.

What burns up in the mesosphere due to friction with atmospheric gases?

  1. Satellites
  2. Meteors
  3. Jet planes
  4. Radio waves
✔ B. Meteors — Most meteors (shooting stars) burn up in the mesosphere.

The temperature at the top of the mesosphere falls to about .

✔ -90°C

The upper boundary of the mesosphere is called the .

✔ Mesopause
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Topic 08

Thermosphere & Ionosphere

Above the mesopause, the thermosphere is the hottest layer — where the Sun's most energetic rays are absorbed.

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Key Point
The Thermosphere is the 4th layer and the hottest layer of the atmosphere; it extends up to 513 km according to NASA.
  • Temperature rises dramatically to 2500°C because gas molecules here absorb X-rays and UV rays from the Sun.
  • Gas molecules in this layer break into positively (+ive) and negatively (−ive) charged ions, which is why this layer is also called the Ionosphere.
  • The Ionosphere reflects radio waves back to Earth, enabling long-distance communications (radio broadcasting, satellite signals).
  • Since the Ionosphere is extremely hot and energetic, it burns up meteors and satellites entering Earth's atmosphere.
  • Auroras (Northern Lights / Aurora Borealis and Southern Lights / Aurora Australis) occur in the thermosphere.
📝 Quick self-test 2 MCQs · 2 fill-ups

Which is the hottest layer of the atmosphere?

  1. Troposphere
  2. Thermosphere
  3. Mesosphere
  4. Stratosphere
✔ B. Thermosphere — The Thermosphere is the hottest layer, with temperatures rising to 2500°C.

By what other name is the thermosphere known due to charged ions?

  1. Tropopause
  2. Ozonosphere
  3. Ionosphere
  4. Exosphere
✔ C. Ionosphere — Gas molecules break into charged ions, so this layer is also called the Ionosphere.

The Ionosphere reflects back to Earth, enabling long-distance communication.

✔ radio waves

Northern Lights and Southern Lights, known as , occur in the thermosphere.

✔ Auroras
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Topic 09

Exosphere

The exosphere is the outermost and topmost layer of the atmosphere, gradually merging into outer space.

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Key Point
The Exosphere is the 5th and outermost layer; it extends up to 960 km.
  • Temperature ranges from −300°C at its lower boundary to 1650°C at its upper boundary.
  • The exosphere has very low density and lack of gravity, allowing light gases (hydrogen, helium) to escape into space.
  • Most artificial satellites orbit within the exosphere.
📝 Quick self-test 2 MCQs · 2 fill-ups

Up to what altitude does the exosphere extend?

  1. 513 km
  2. 800 km
  3. 960 km
  4. 1200 km
✔ C. 960 km — The Exosphere extends up to 960 km.

What mostly orbits within the exosphere?

  1. Meteors
  2. Artificial satellites
  3. Jet planes
  4. Clouds
✔ B. Artificial satellites — Most artificial satellites orbit within the exosphere.

The exosphere is the 5th and layer of the atmosphere.

✔ outermost

Due to low density and lack of gravity, light gases like hydrogen and helium into space from the exosphere.

✔ escape
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Recap

Quick Revision

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Key Point
Atmosphere extends to 1000 km; 99% of it is within 32 km — held by Earth's gravitational pull.
  • Nitrogen 78% is the most abundant gas in the atmosphere, followed by Oxygen at 20.95%.
  • CO₂ (0.04%) is a trace gas but the principal greenhouse gas; dust causes red/orange colours at sunrise & sunset.
  • Six greenhouse gases: Methane, CFCs, CO₂, Nitrous Oxide, Ozone, Fluorinated Gases.
  • Troposphere (0–12 km) — 1st layer; all weather here; temperature decreases; "Tropo" = change; contains 90%+ of gases.
  • Tropopause temperature can reach −58°C; above it lies the Stratosphere.
  • Stratosphere (12–50 km) — 2nd layer; ozone layer; temperature increases with height; jet planes fly here.
  • Mesosphere (50–80 km) — 3rd layer; coldest (−90°C); meteors burn up here.
  • Thermosphere (80–513 km) — 4th and hottest layer (2500°C); also called Ionosphere; reflects radio waves; enables long-distance communication.
  • Exosphere (513–960 km) — 5th and outermost layer; temperature −300°C to 1650°C; gases escape to space.
  • Ozone layer is found in the Stratosphere (~20–35 km); absorbs harmful UV radiation.
  • Radio wave reflection happens in the Ionosphere (Thermosphere); long-distance communication depends on it.

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