Updated NCERT Solutions for Class 11 Geography Chapter 7: Composition and Structure of Atmosphere | Important Questions 2026-27
Welcome, students! This comprehensive guide covers everything you need to know about the **Composition and Structure of Atmosphere**. We'll break down complex topics into simple points, making this chapter easy to master for your CBSE board exams and even competitive exams like CUET and UPSC. Let's explore the invisible blanket protecting our Earth!
Chapter NameComposition and Structure of Atmosphere
SubjectGeography (Book 1: Fundamentals of Physical Geography)
Class11
BoardCBSE
Important TopicsGases of the Atmosphere, Water Vapour, Dust Particles, Troposphere, Stratosphere, Mesosphere, Thermosphere.
Difficulty LevelEasy to Medium
Exam Weightage4-6 Marks (Includes MCQs, short answer, and can be part of a long answer question)
Learning Objectives
After completing this chapter, students will be able to:
- List the major gases that make up our atmosphere and understand their significance.
- Explain the crucial role of water vapour and dust particles.
- Describe the five distinct layers of the atmosphere based on temperature.
- Differentiate between the characteristics of the Troposphere, Stratosphere, Mesosphere, Thermosphere, and Exosphere.
- Understand the difference between weather and climate.
Key Concepts and Definitions
- Atmosphere: A thick gaseous envelope that surrounds the Earth, held in place by gravity. It protects life by absorbing harmful solar radiation and regulating temperature.
- Composition of Atmosphere: The atmosphere is primarily composed of Nitrogen (78.08%), Oxygen (20.95%), Argon (0.93%), and trace amounts of other gases like Carbon Dioxide, Neon, Helium, and Ozone.
- Greenhouse Gas: Gases like Carbon Dioxide (CO2) that are transparent to incoming solar radiation but opaque to outgoing terrestrial radiation, thus trapping heat and warming the Earth's surface.
- Troposphere: The lowermost layer of the atmosphere where all weather phenomena occur. Temperature decreases with an increase in height.
- Tropopause: The boundary zone separating the Troposphere from the Stratosphere.
- Stratosphere: The layer above the troposphere, famous for containing the Ozone Layer. Temperature increases with height in this layer.
- Ozone Layer: A region in the stratosphere with a high concentration of Ozone (O3) that absorbs most of the Sun's harmful ultraviolet (UV) radiation.
- Mesosphere: The layer above the stratosphere where temperature again decreases with height. Most meteors burn up in this layer.
- Thermosphere: The layer above the mesosphere where temperature rises very rapidly with height. It contains the Ionosphere.
- Ionosphere: A region within the thermosphere containing electrically charged particles (ions) that help in reflecting radio waves back to Earth, enabling long-distance communication.
- Exosphere: The outermost layer of the atmosphere, which gradually merges with outer space.
- Normal Lapse Rate: The rate at which temperature decreases with an increase in altitude in the troposphere, which is approximately 6.5°C per 1,000 metres.
Full NCERT Solutions for Class 11 Geography Chapter 7
Here are the detailed, easy-to-understand answers to all the questions from your NCERT textbook exercise.
1. Multiple choice questions.
(i) Which one of the following gases constitutes the major portion of the atmosphere?
(a) Oxygen
(b) Nitrogen
(c) Argon
(d) Carbon dioxide
Answer: (b) Nitrogen.
Explanation: The atmosphere is composed of approximately 78.08% Nitrogen, making it the most abundant gas. Oxygen is the second most abundant at about 20.95%.
Explanation: The atmosphere is composed of approximately 78.08% Nitrogen, making it the most abundant gas. Oxygen is the second most abundant at about 20.95%.
(ii) Atmospheric layer important for human beings is:
(a) Stratosphere
(b) Mesosphere
(c) Troposphere
(d) Ionosphere
Answer: (c) Troposphere.
Explanation: The troposphere is the most important layer for humans because it contains the air we breathe (Oxygen), and all weather phenomena like rain, clouds, and storms that affect our lives occur here.
Explanation: The troposphere is the most important layer for humans because it contains the air we breathe (Oxygen), and all weather phenomena like rain, clouds, and storms that affect our lives occur here.
(iii) Sea salt, pollen, ash, smoke soot, fine soil — these are associated with:
(a) Gases
(b) Dust particles
(c) Water vapour
(d) Meteors
Answer: (b) Dust particles.
Explanation: These are all examples of solid particles, collectively known as aerosols or dust particles, which are suspended in the atmosphere. They play a vital role as condensation nuclei for cloud formation.
Explanation: These are all examples of solid particles, collectively known as aerosols or dust particles, which are suspended in the atmosphere. They play a vital role as condensation nuclei for cloud formation.
(iv) 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
Answer: (d) Carbon dioxide.
Explanation: Carbon dioxide is a greenhouse gas. It allows the sun's shortwave radiation to pass through but traps the Earth's outgoing longwave radiation, leading to the greenhouse effect that keeps our planet warm.
Explanation: Carbon dioxide is a greenhouse gas. It allows the sun's shortwave radiation to pass through but traps the Earth's outgoing longwave radiation, leading to the greenhouse effect that keeps our planet warm.
(v) The scale used for measuring the intensity of the earthquakes is known as:
(Note: This question is from a different chapter in some old NCERT editions but sometimes appears. The correct answer is related to earthquakes, not the atmosphere.)
(a) Mercalli scale
(b) Richter scale
(c) Number scale
(d) Metre scale
Answer: (b) Richter scale.
Explanation: The Richter scale measures the magnitude or energy released by an earthquake. The Mercalli scale measures the intensity or observed effects of an earthquake.
Explanation: The Richter scale measures the magnitude or energy released by an earthquake. The Mercalli scale measures the intensity or observed effects of an earthquake.
2. Answer the following questions in about 30 words.
(i) What do you understand by atmosphere?
Answer: The atmosphere is a multi-layered gaseous envelope that surrounds the Earth, attached to it by gravitational force. It is essential for life, protecting us from the sun's harmful UV radiation, regulating temperature, and containing vital gases like oxygen for breathing and carbon dioxide for photosynthesis.
(ii) What are the elements of weather and climate?
Answer: The main elements of weather and climate are:
- Temperature: The degree of hotness or coldness of the air.
- Pressure: The force exerted by the weight of the air.
- Wind: The horizontal movement of air.
- Humidity: The amount of water vapour present in the air.
- Clouds and Precipitation: The visible mass of water droplets and the forms in which water falls to the Earth (rain, snow, etc.).
(iii) Describe the composition of the atmosphere.
Answer: The atmosphere is a mixture of numerous gases, water vapour, and dust particles. The major gases by volume are:
- Nitrogen (N₂): 78.08%
- Oxygen (O₂): 20.95%
- Argon (Ar): 0.93%
- Carbon Dioxide (CO₂): ~0.04%
(iv) Why is the troposphere the most important of all the layers of the atmosphere?
Answer: The troposphere is the most crucial layer for life on Earth because:
1. It contains almost all the water vapour, which leads to the formation of clouds and precipitation.
2. All weather phenomena that directly impact us occur here.
3. It contains the oxygen we breathe.
4. It has a suitable temperature and pressure for most living organisms to survive.
1. It contains almost all the water vapour, which leads to the formation of clouds and precipitation.
2. All weather phenomena that directly impact us occur here.
3. It contains the oxygen we breathe.
4. It has a suitable temperature and pressure for most living organisms to survive.
3. Answer the following questions in about 150 words.
(i) Describe the composition of the atmosphere.
Answer: The atmosphere is a dynamic mixture of gases, water vapour, and aerosols (dust particles) that surround our planet. Its composition is crucial for sustaining life.
Gases: The atmosphere is predominantly made up of a few major gases.
Dust Particles (Aerosols): The atmosphere holds tiny solid particles like sea salts, fine soil, smoke, ash, and pollen. These particles are crucial as they act as hydroscopic nuclei around which water vapour condenses to form clouds. They also scatter solar radiation, which is why we see colour in the sky at sunrise and sunset.
Gases: The atmosphere is predominantly made up of a few major gases.
- Nitrogen (78.08%): It is the most abundant gas. While not directly used by most organisms for respiration, it is a vital component of proteins and is made available to plants through nitrogen fixation.
- Oxygen (20.95%): It is the life-giving gas, essential for respiration by all living beings and for combustion.
- Argon (0.93%): An inert gas with limited direct biological or chemical role but used in industrial applications like light bulbs.
- Carbon Dioxide (~0.04%): Although a small percentage, it is meteorologically very important. It is used by plants for photosynthesis and acts as a major greenhouse gas, absorbing terrestrial radiation and keeping the Earth warm.
Dust Particles (Aerosols): The atmosphere holds tiny solid particles like sea salts, fine soil, smoke, ash, and pollen. These particles are crucial as they act as hydroscopic nuclei around which water vapour condenses to form clouds. They also scatter solar radiation, which is why we see colour in the sky at sunrise and sunset.
(ii) Draw a suitable diagram for the structure of the atmosphere and label it and describe it.
Answer:
[Image Suggestion: A clear, well-labelled diagram showing the 5 layers of the atmosphere on the Y-axis (Altitude in km) and temperature on the X-axis (in °C). The line graph should show temperature decreasing in the troposphere, increasing in the stratosphere, decreasing in the mesosphere, and increasing in the thermosphere.]
The atmosphere is vertically divided into five distinct layers based on temperature variations.
1. Troposphere:
[Image Suggestion: A clear, well-labelled diagram showing the 5 layers of the atmosphere on the Y-axis (Altitude in km) and temperature on the X-axis (in °C). The line graph should show temperature decreasing in the troposphere, increasing in the stratosphere, decreasing in the mesosphere, and increasing in the thermosphere.]
The atmosphere is vertically divided into five distinct layers based on temperature variations.
1. Troposphere:
- This is the lowest and densest layer, extending from the Earth's surface up to an average altitude of 13 km (about 8 km at the poles and 18 km at the equator).
- All weather phenomena like clouds, fog, rain, and storms occur in this layer.
- Temperature decreases with an increase in altitude at a rate called the normal lapse rate (approx. 6.5°C per km).
- This layer contains 75% of the total mass of the atmosphere and almost all the water vapour and dust particles.
- The upper limit is the Tropopause.
- It lies above the troposphere and extends up to 50 km.
- The lower part of this layer has a constant temperature, but then the temperature begins to increase with altitude. This is because of the presence of the Ozone layer, which absorbs the sun's ultraviolet (UV) radiation.
- This layer is free from clouds and weather disturbances, making it ideal for flying jet aircraft.
- The upper limit is the Stratopause.
- Extending from 50 km to 80 km, this is the third layer.
- In this layer, the temperature again decreases with increasing altitude, reaching as low as -100°C at its top.
- Most meteors entering the Earth's atmosphere burn up in the mesosphere due to friction with air particles.
- The upper limit is the Mesopause.
- Located above the mesosphere, from 80 km to about 400 km.
- The temperature rises very sharply with increasing height.
- It contains the Ionosphere, a sub-layer with electrically charged particles (ions). These ions reflect radio waves back to Earth, facilitating long-distance wireless and radio communication.
- This is the uppermost layer of the atmosphere, extending beyond the thermosphere.
- The air is extremely thin (rarefied), and the layer gradually merges with outer space.
- Light gases like helium and hydrogen are found here.
Extra Important Questions (Board Style) for 2026-27
(A) Multiple Choice Questions (MCQs)
1. The normal lapse rate is the rate at which temperature drops in the troposphere. What is its approximate value? (Easy)
(a) 1°C per 165 m
(b) 6.5°C per 1,000 m
(c) 10°C per km
(d) Both (a) and (b)
Answer: (d) Both (a) and (b). They represent the same rate.
2. Radio waves are reflected back to the Earth by which layer? (Easy)
(a) Stratosphere
(b) Exosphere
(c) Ionosphere
(d) Mesosphere
Answer: (c) Ionosphere. This layer, part of the thermosphere, contains ions that enable radio communication.
3. Which of the following is NOT a primary greenhouse gas? (Medium)
(a) Carbon Dioxide
(b) Methane
(c) Water Vapour
(d) Oxygen
Answer: (d) Oxygen. While essential for life, it does not trap heat in the same way as greenhouse gases.
4. The blue colour of the sky is a result of which process involving dust particles? (Medium)
(a) Reflection
(b) Refraction
(c) Scattering
(d) Absorption
Answer: (c) Scattering. Dust particles and gas molecules scatter the shorter blue wavelengths of sunlight more than other colours.
5. Why does temperature increase with height in the Stratosphere? (Hard)
(a) It is closer to the sun.
(b) Due to the absorption of UV radiation by the ozone layer.
(c) Because of chemical reactions involving ions.
(d) Due to the absence of clouds and water vapour.
Answer: (b) Due to the absorption of UV radiation by the ozone layer. This absorption process releases heat, warming the upper parts of the stratosphere.
(B) Short Answer Questions
6. What are aerosols and why are they important? (Easy)
Answer: Aerosols are tiny solid or liquid particles suspended in the atmosphere, such as dust, sea salt, and smoke. They are important because they act as condensation nuclei, which are necessary for water vapour to condense and form clouds and precipitation.
7. Differentiate between Weather and Climate. (Medium)
Answer:
- Weather refers to the state of the atmosphere over a small area at any given point in time. It is short-term and changes frequently (e.g., a sunny afternoon, a rainy day).
- Climate refers to the average weather conditions of a large area over a long period of time (typically 30 years or more). It is long-term and more stable (e.g., the tropical climate of South India).
8. Why is the height of the tropopause not uniform over the Earth? (Medium)
Answer: The height of the tropopause varies because it is dependent on temperature. At the equator, strong convectional currents push the atmospheric boundary higher, so the tropopause is at about 18 km. At the poles, the cold, dense air subsides, resulting in a lower tropopause at about 8 km.
9. What would happen if there was no Carbon Dioxide in the atmosphere? (Hard)
Answer: If there were no CO2, two major things would happen:
1. Photosynthesis would stop: Plants would not be able to produce food, disrupting the entire food chain.
2. The Earth would be much colder: CO2 is a crucial greenhouse gas that traps heat. Without it, the average temperature of the Earth would drop significantly, making it too cold for most life to exist.
1. Photosynthesis would stop: Plants would not be able to produce food, disrupting the entire food chain.
2. The Earth would be much colder: CO2 is a crucial greenhouse gas that traps heat. Without it, the average temperature of the Earth would drop significantly, making it too cold for most life to exist.
10. Why do most meteors burn up in the Mesosphere? (Medium)
Answer: The Mesosphere, while having very thin air, is still dense enough to create significant friction when meteors enter it at very high speeds. This intense friction generates immense heat, causing the meteoroids to vaporize and burn up, creating shooting stars.
(C) Long Answer Questions
11. Explain the role of water vapour and dust particles in influencing the weather. (Medium)
Answer: Water vapour and dust particles, despite being variable components, play a fundamental role in weather.
- Role of Water Vapour:
1. Source of Precipitation: It is the source of all forms of condensation (clouds, fog, dew) and precipitation (rain, snow). Without it, the water cycle would not exist.
2. Greenhouse Effect: It is a powerful greenhouse gas that absorbs both incoming solar radiation and outgoing terrestrial radiation, helping to regulate Earth's temperature.
3. Latent Heat: When water vapour condenses, it releases latent heat of condensation into the atmosphere. This energy is the power source for major storms like thunderstorms and cyclones. - Role of Dust Particles:
1. Condensation Nuclei: They act as surfaces (hydroscopic nuclei) on which water vapour can condense. Without them, cloud formation would be very difficult.
2. Scattering of Insolation: They scatter sunlight, which leads to the blue colour of the sky and the red/orange hues of sunrise and sunset. This scattering also creates diffused daylight even in shaded areas.
3. Radiation Absorption/Reflection: Some dust particles can absorb or reflect incoming solar radiation, slightly affecting the temperature of the air.
12. "The atmosphere has a complex structure where each layer has a unique and vital role." Justify this statement with special reference to the Troposphere and Stratosphere. (Hard)
Answer: The statement is accurate as the atmosphere's layered structure ensures the planet is habitable. Each layer performs distinct functions crucial for life.
- The Vital Troposphere: This layer is the cradle of life.
- Biological Importance: It contains the oxygen necessary for respiration and carbon dioxide for photosynthesis.
- Weather and Climate: It is the theatre for all weather phenomena. Convection currents, winds, cloud formation, and precipitation are confined to this layer, determining the weather and long-term climate patterns that shape ecosystems and human activities like agriculture.
- Temperature Gradient: The normal lapse rate allows for vertical movement of air, which is essential for distributing heat and moisture.
- The Protective Stratosphere: This layer acts as Earth's shield.
- The Ozone Layer: Its most critical feature is the ozone layer, which absorbs 97-99% of the Sun's harmful ultraviolet (UV-B and UV-C) radiation. Without this protective shield, life on the surface would be exposed to radiation causing skin cancer, cataracts, and damage to plant life.
- Atmospheric Stability: The temperature inversion (temperature increasing with height) makes this layer very stable. This stability prevents vertical mixing with the troposphere, keeping pollutants and weather largely confined to the lower layer. This is also why it's a smooth zone for jet travel.
(D) Case-Based Question
13. A group of mountaineers is planning an expedition to Mount Everest (approx. 8.8 km high). Their meteorologist informs them about the atmospheric conditions they will face. The temperature at sea level is 30°C. They are told to expect very strong winds and a significant drop in oxygen levels.
(i) What atmospheric layer will they be climbing in?
Answer: They will be climbing entirely within the Troposphere, as Mount Everest's peak (around 8.8 km) is well below the average height of the tropopause (13 km).
(ii) Using the normal lapse rate, estimate the approximate temperature at the summit of Everest.
Answer:
- Normal Lapse Rate = 6.5°C per 1000 m (or 1 km).
- Height of Everest = 8.8 km.
- Total temperature drop = 8.8 km * 6.5°C/km = 57.2°C.
- Estimated temperature at the summit = Temperature at sea level - Total drop = 30°C - 57.2°C = -27.2°C.
(iii) Why are oxygen levels so low at high altitudes?
Answer: Oxygen levels are low because air pressure and density decrease significantly with altitude. The force of gravity pulls most of the air molecules, including oxygen, closer to the Earth's surface. At 8.8 km, the air is much thinner, meaning there are fewer oxygen molecules in each breath.
Common Mistakes Students Make
- Confusing Temperature Profiles: A common error is forgetting how temperature changes in each layer. Remember this pattern: Decreases (Troposphere), Increases (Stratosphere), Decreases (Mesosphere), Increases (Thermosphere).
- Mixing up Weather and Climate: Weather is short-term and local. Climate is long-term and regional. Don't use them interchangeably in your answers.
- Forgetting Percentages: Not remembering the approximate percentages of Nitrogen (78%) and Oxygen (21%) can cost you marks in MCQs or short answers.
- Ignoring the Importance of Dust Particles: Students often focus only on gases and water vapour. Remember that dust particles are essential as condensation nuclei for cloud formation.
- Confusing Ionosphere and Ozone Layer: The Ozone Layer is in the Stratosphere and blocks UV rays. The Ionosphere is in the Thermosphere and reflects radio waves.
Exam Preparation Tips
- Diagram is Key: Practice drawing and labelling the structure of the atmosphere. It's a high-yield question.
- Make a Table: Create a simple table with columns for Layer, Altitude, Temperature Trend, and Key Features. This is perfect for last-minute revision.
- Focus on 'Why': Don't just memorize facts. Understand why temperature increases in the stratosphere (ozone) or why the troposphere is important (weather, life).
- Relate to Real Life: Think about why airplanes fly in the stratosphere (stability) or why we need CO2 (warmth, plants). This makes concepts stick.
- Practice PYQs: Go through past years' questions to understand the pattern and type of questions asked from this chapter.
Frequently Asked Questions (FAQs)
Q1: What are the 5 layers of the atmosphere in order?
The five layers of the atmosphere from the ground up are: Troposphere, Stratosphere, Mesosphere, Thermosphere, and Exosphere.
Q2: Which gas is most abundant in the Earth's atmosphere?
Nitrogen (N₂) is the most abundant gas, making up about 78% of the atmosphere.
Q3: Why is the ozone layer important for life on Earth?
The ozone layer, located in the stratosphere, is crucial because it absorbs the majority of the Sun's harmful ultraviolet (UV) radiation, protecting humans, animals, and plants from its damaging effects.
Q4: What is the difference between the troposphere and stratosphere?
The main differences are:
- Location: Troposphere is the lowest layer; Stratosphere is the second.
- Temperature: Temperature decreases with height in the troposphere but increases with height in the stratosphere.
- Features: All weather occurs in the troposphere, while the stratosphere contains the ozone layer.
Q5: Where can I find updated NCERT Solutions for Class 11 Geography Chapter 7?
You can find complete, easy-to-understand, and updated NCERT Solutions for Class 11 Geography Chapter 7 right here on this page, including important questions and exam tips for the 2026-27 session.
Conclusion: Understanding the Composition and Structure of the Atmosphere is fundamental to physical geography. This invisible shield does more than just provide air to breathe; it regulates our climate, protects us from radiation, and enables communication. By mastering the concepts in this chapter, you are building a strong foundation for your exams and a deeper appreciation for our planet. Revise the layers, practice the diagrams, and you'll be well on your way to scoring full marks!