Book: Fundamentals of Physical Geography | Chapter 7
Composition and Structure of Atmosphere
1. Learning Objectives
After reading these notes, you will be able to:
1
Define the atmosphere and understand why it is essential for life on Earth.2
Know the three components of the atmosphere — Gases, Water Vapour and Dust Particles — and their roles.3
Understand the composition of atmospheric gases with their proportions and importance.4
Describe all five layers of the atmosphere with their heights, temperatures and key features.5
Know the elements of weather and climate that influence human life.2. The Atmosphere — Introduction
The atmosphere is a mixture of different gases that envelopes the earth all round. It contains life-giving gases like oxygen (for humans and animals) and carbon dioxide (for plants). The air is colourless and odourless and can be felt only when it blows as wind.
99%
of total atmospheric mass is confined to height of 32 km from Earth’s surface
120 km
Height at which oxygen becomes almost negligible in quantity
90 km
Maximum height up to which CO₂ and water vapour are found
3 Components
Atmosphere is composed of Gases + Water Vapour + Dust Particles
⭐ Elements of Weather and Climate
The main elements of atmosphere which influence human life on earth are: Temperature, Pressure, Winds, Humidity, Clouds and Precipitation. These are dealt in detail in Chapters 8, 9 and 10.3. Composition of the Atmosphere
🧪 Mind Map — Composition of Atmosphere
Atmosphere💨 Gases
Nitrogen 78%
Oxygen 21%
CO₂, Ozone, Argon etc.
Nitrogen 78%
Oxygen 21%
CO₂, Ozone, Argon etc.
💧 Water Vapour
Variable (0–4%)
Decreases with altitude
Acts like a blanket
Variable (0–4%)
Decreases with altitude
Acts like a blanket
🌫️ Dust Particles
Sea salts, fine soil,
smoke, ash, pollen
Hygroscopic nuclei
Sea salts, fine soil,
smoke, ash, pollen
Hygroscopic nuclei
💨 A. Gases
| Gas | % by Volume | Importance |
|---|---|---|
| Nitrogen (N₂) | 78.09% | Most abundant gas in atmosphere; helps in diluting oxygen to prevent rapid combustion |
| Oxygen (O₂) | 20.95% | Essential for respiration of living organisms; supports combustion |
| Argon (Ar) | 0.93% | Inert gas; relatively constant |
| Carbon Dioxide (CO₂) | 0.036% (rising) | Transparent to incoming solar radiation but opaque to outgoing terrestrial radiation. Absorbs and reflects back terrestrial radiation. Mainly responsible for greenhouse effect. Volume increasing due to burning of fossil fuels. |
| Ozone (O₃) | Trace | Found 10 to 50 km above Earth’s surface. Acts as a filter — absorbs ultra-violet (UV) rays from the Sun and prevents them from reaching Earth’s surface. |
💧 B. Water Vapour
- A variable gas in the atmosphere — decreases with altitude.
- In warm and wet tropics: may account for up to 4% of air by volume.
- In dry and cold desert and polar regions: may be less than 1% of the air.
- Also decreases from the equator towards the poles.
- Absorbs parts of the insolation from the Sun and preserves Earth’s radiated heat.
- Acts like a blanket — allowing Earth neither to become too cold nor too hot.
- Also contributes to stability and instability in the air.
🌫️ C. Dust Particles
- Include: sea salts, fine soil, smoke-soot, ash, pollen, dust, disintegrated particles of meteors.
- Generally concentrated in the lower layers of the atmosphere; yet convectional air currents may transport them to great heights.
- Higher concentration found in subtropical and temperate regions (due to dry winds) compared to equatorial and polar regions.
- Dust and salt particles act as hygroscopic nuclei — around which water vapour condenses to produce clouds.
📌 What is a Hygroscopic Nucleus?
Hygroscopic nuclei are tiny dust or salt particles that attract water vapour to condense around them — they are essential for cloud and rain formation. Without them, condensation and cloud formation would be much less.4. Structure of the Atmosphere
The atmosphere consists of different layers with varying density and temperature. Density is highest near the surface of the earth and decreases with increasing altitude. The column of atmosphere is divided into 5 layers based on temperature conditions.
🌤️ Mind Map — 5 Layers of the Atmosphere (Bottom to Top)
5 Layers of Atmosphere🌿 Troposphere
0–13 km avg
All weather here
Temp decreases with height
0–13 km avg
All weather here
Temp decreases with height
☀️ Stratosphere
13–50 km
Ozone layer here
Temp increases with height
13–50 km
Ozone layer here
Temp increases with height
❄️ Mesosphere
50–80 km
Coldest layer
Temp decreases with height
50–80 km
Coldest layer
Temp decreases with height
⚡ Thermosphere/Ionosphere
80–400 km
Radio waves reflected
Temp increases with height
80–400 km
Radio waves reflected
Temp increases with height
🌌 Exosphere
Above 400 km
Highest layer
Merges with outer space
Above 400 km
Highest layer
Merges with outer space
🌿 Layer 1 — Troposphere (Most Important)
📏 Height and Thickness
Average height: 13 km. Extends to 8 km near poles and 18 km at equator. Thickness is greatest at equator — because heat is transported to great heights by strong convectional currents.
🌡️ Temperature
Temperature decreases at the rate of 1°C for every 165 m of height (normal lapse rate). This is called the environmental lapse rate.
🌦️ Key Features
Contains dust particles and water vapour. All changes in climate and weather take place in this layer. Most important layer for all biological activity.
🔹 Tropopause
Zone separating troposphere from stratosphere. Temperature here is nearly constant — hence called ‘tropopause’. About –80°C over equator and –45°C over poles.
☀️ Layer 2 — Stratosphere
- Found above the tropopause; extends up to a height of 50 km.
- Most important feature: contains the ozone layer.
- Ozone layer absorbs ultra-violet radiation from the Sun and shields life on Earth from this intense, harmful energy.
- Upper limit of stratosphere = Stratopause (at about 50 km).
❄️ Layer 3 — Mesosphere
- Lies above the stratosphere; extends up to a height of 80 km.
- Temperature again starts decreasing with increase in altitude.
- Temperature reaches up to minus 100°C at the height of 80 km — the coldest part of the atmosphere.
- Upper limit = Mesopause.
⚡ Layer 4 — Thermosphere / Ionosphere
- Located between 80 and 400 km above the mesopause.
- Contains electrically charged particles known as ions → hence called ionosphere.
- Radio waves transmitted from Earth are reflected back to Earth by this layer.
- Temperature here starts increasing with height.
🌌 Layer 5 — Exosphere
- The uppermost layer of the atmosphere above the thermosphere.
- The highest layer but very little is known about it.
- Whatever contents are there, they are extremely rarefied (very thin).
- Gradually merges with the outer space.
📊 Complete Layers Table
| Layer | Height | Temp. Change | Key Feature |
|---|---|---|---|
| Troposphere | 0–13 km avg (8 km poles; 18 km equator) | Decreases (1°C per 165 m) | All weather; dust + water vapour; biological activity; MOST IMPORTANT |
| Tropopause | ~13 km (boundary) | Nearly constant | –80°C at equator; –45°C at poles |
| Stratosphere | 13–50 km | Increases with height | Contains OZONE LAYER (absorbs UV rays) |
| Mesosphere | 50–80 km | Decreases with height | Coldest layer; –100°C at 80 km |
| Thermosphere / Ionosphere | 80–400 km | Increases with height | Ions; reflects radio waves back to Earth |
| Exosphere | Above 400 km | — | Highest; extremely rarefied; merges with outer space |
📌 Geographers’ Focus
Although all layers of the atmosphere must be exercising influence on us, geographers are concerned with the first two layers of the atmosphere — the troposphere and the stratosphere.Summary — Quick Revision
1
Atmosphere = mixture of gases + water vapour + dust particles enveloping Earth. 99% of atmospheric mass within 32 km of Earth’s surface. Air is colourless and odourless.
2
Nitrogen (78.09%) is most abundant. Oxygen (20.95%) next. CO₂ (0.036% — rising) causes greenhouse effect — transparent to incoming solar radiation but opaque to outgoing terrestrial radiation.
3
Ozone layer found 10–50 km above Earth. Acts as filter — absorbs harmful UV rays. Oxygen becomes negligible at 120 km. CO₂ and water vapour found only up to 90 km.
4
Water vapour = variable gas (0–4%). Decreases with altitude and from equator to poles. Acts as a blanket. Tropics = up to 4%; Desert/Polar = less than 1%.
5
Dust particles = sea salts, fine soil, smoke, ash, pollen, meteor fragments. Act as hygroscopic nuclei for cloud formation. More in subtropical and temperate regions.
6
Troposphere (0–13 km avg): All weather here. Temp decreases 1°C/165 m. Thickest at equator (18 km). Most important layer for biological activity.
7
Stratosphere (13–50 km): Contains ozone layer (absorbs UV rays). Tropopause = –80°C equator, –45°C poles. Upper limit = Stratopause.
8
Mesosphere (50–80 km): Temperature decreases again. Coldest layer — –100°C at 80 km. Upper limit = Mesopause.
9
Ionosphere/Thermosphere (80–400 km): Ions (electrically charged particles). Radio waves reflected back to Earth. Temperature increases with height.
10
Exosphere (above 400 km): Highest layer. Extremely rarefied. Merges with outer space. Very little known about it. Geographers focus on troposphere and stratosphere.
Important Terms to Remember
- Atmosphere: A mixture of different gases (along with water vapour and dust particles) that envelopes the earth all round. Contains life-giving gases — oxygen for animals, CO₂ for plants.
- Carbon Dioxide (CO₂): Meteorologically very important gas. Transparent to incoming solar radiation but opaque to outgoing terrestrial radiation. Absorbs and reflects terrestrial radiation back — mainly responsible for the greenhouse effect. Volume rising due to burning of fossil fuels.
- Greenhouse Effect: The process by which CO₂ (and other greenhouse gases) trap outgoing terrestrial radiation and warm the Earth’s surface, similar to how a greenhouse works.
- Ozone (O₃): Found between 10 and 50 km above Earth’s surface. Acts as a filter — absorbs ultra-violet (UV) rays radiating from the Sun and prevents them from reaching Earth’s surface.
- Water Vapour: A variable gas in the atmosphere (0–4%). Decreases with altitude and from equator to poles. Absorbs insolation and preserves Earth’s radiated heat — acts like a blanket. Also contributes to stability and instability in the air.
- Dust Particles: Small solid particles in the atmosphere — include sea salts, fine soil, smoke-soot, ash, pollen, dust and disintegrated meteor particles. Act as hygroscopic nuclei around which water vapour condenses to produce clouds.
- Hygroscopic Nuclei: Tiny dust or salt particles that attract water vapour to condense around them — essential for cloud and precipitation formation.
- Insolation: Incoming solar radiation received at Earth’s surface.
- Terrestrial Radiation: Outgoing long-wave radiation emitted by the Earth back into the atmosphere.
- Troposphere: The lowermost layer of the atmosphere (0–13 km average; 8 km at poles; 18 km at equator). All changes in climate and weather take place here. Temperature decreases at the rate of 1°C for every 165 m of height. Contains dust and water vapour. Most important layer for biological activity.
- Normal Lapse Rate / Environmental Lapse Rate: Rate of decrease of temperature with altitude in the troposphere — 1°C per 165 metres of height.
- Tropopause: The zone separating the troposphere from the stratosphere. Temperature nearly constant here. About –80°C over equator and –45°C over poles.
- Stratosphere: Layer above the tropopause, extending up to 50 km. Contains the ozone layer which absorbs UV radiation. Temperature increases with height in this layer. Upper limit = Stratopause.
- Ozone Layer: A layer within the stratosphere (10–50 km) rich in ozone gas. Absorbs ultra-violet radiation from the Sun and shields life on Earth from this harmful energy.
- Mesosphere: Layer above the stratosphere, extending up to 80 km. Temperature again decreases with altitude, reaching –100°C at 80 km — the coldest part of the atmosphere. Upper limit = Mesopause.
- Ionosphere / Thermosphere: Located between 80 and 400 km above the mesopause. Contains electrically charged particles (ions). Radio waves transmitted from Earth are reflected back to Earth by this layer. Temperature increases with height.
- Ions: Electrically charged particles present in the ionosphere. Their presence allows this layer to reflect radio waves back to Earth’s surface.
- Exosphere: The uppermost layer of the atmosphere above the thermosphere (above 400 km). Extremely rarefied contents. Gradually merges with outer space. Very little is known about it.
- Elements of Weather and Climate: The main elements that change and influence human life — Temperature, Pressure, Winds, Humidity, Clouds and Precipitation.
- Weather: Atmospheric conditions at a particular place at a particular time — short-term and changeable.
- Climate: Average atmospheric conditions of a place over a long period of time (at least 30 years).