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Composition and Structure of Atmosphere – Class 11 Geography Chapter 7 Notes & NCERT PDF

Class 11GeographyChapter 7NCERT book: Fundamentals of Physical Geography - Geography Class 11

Notes and a simple summary of Chapter 7 of the NCERT Class 11 Geography book, with the official chapter PDF, flashcards, an MCQ quiz and an AI tutor for your doubts.

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Chapter 7: Composition and Structure of Atmosphere

Introduction

Can a person live without air? We can survive for some time without food and water, but not even a few minutes without breathing. The atmosphere is a mixture of gases that envelops the earth; it contains life-giving oxygen for humans and animals and carbon dioxide for plants. 99 per cent of the atmosphere's total mass lies within 32 km of the earth's surface. Air is colourless and odourless and can be felt only when it blows as wind. This chapter opens Unit IV: Climate.

Key Concepts

Composition of the atmosphere

The atmosphere is made of gases, water vapour and dust particles. The proportion of gases changes with height: oxygen becomes almost negligible at 120 km, while carbon dioxide and water vapour are found only up to 90 km. The present atmosphere is chiefly nitrogen and oxygen (nitrogen forms the major portion).

Gases

  • Carbon dioxide — meteorologically very important: transparent to incoming solar radiation but opaque to outgoing terrestrial radiation. It absorbs part of the terrestrial radiation and reflects some back to the surface, so it is largely responsible for the greenhouse effect. Other gases stay constant in volume, but CO₂ has been rising in recent decades, mainly from burning fossil fuels, raising air temperature.
  • Ozone — found between 10 and 50 km; acts as a filter, absorbing ultra-violet rays from the sun.

    • Water vapour

  • A variable gas that decreases with altitude and from the equator towards the poles.
  • Up to four per cent of air by volume in warm, wet tropics; less than one per cent in dry, cold deserts and polar regions.
  • Absorbs part of the insolation and preserves the earth's radiated heat — acts like a blanket, keeping the earth neither too cold nor too hot. Contributes to stability and instability of air.

    • Dust particles

  • Sources: sea salts, fine soil, smoke-soot, ash, pollen, dust and disintegrated meteor particles.
  • Concentrated in the lower layers but convection can lift them high; higher concentration in subtropical and temperate regions (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

    Density is highest near the surface and decreases with height. Based on temperature, the atmosphere has five layers:

  • Troposphere — the lowermost layer; average height 13 km — about 8 km at the poles and 18 km at the equator (thickest at the equator because strong convection carries heat higher). Contains dust and water vapour; all weather and climate changes occur here. Temperature falls at 1°C for every 165 m of height. Most important layer for biological activity.

    • - Tropopause — the zone separating troposphere and stratosphere; temperature about −80°C over the equator and −45°C over the poles; nearly constant, hence the name.

  • Stratosphere — above the tropopause up to 50 km; contains the ozone layer, which absorbs ultra-violet radiation and shields life.
  • Mesosphere — up to 80 km; temperature again decreases with altitude, reaching −100°C at 80 km. Its upper limit is the mesopause.
  • Ionosphere/Thermosphere — 80 to 400 km; contains electrically charged ions; reflects radio waves back to the earth; temperature increases with height.
  • Exosphere — the uppermost layer; very little is known; contents extremely rarefied; merges gradually with outer space.

    • Geographers are mainly concerned with the first two layers.

    Elements of weather and climate

    Temperature, pressure, winds, humidity, clouds and precipitation — the elements that change and influence human life (detailed in Chapters 8–10).

    Diagrams to Know

  • Figure 7.1 — Structure of atmosphere: draw a vertical column with height in km: troposphere (0–13 km average; 8 km poles, 18 km equator), tropopause, stratosphere with ozone (to 50 km), mesosphere (to 80 km, −100°C), ionosphere (80–400 km), exosphere. Add a temperature curve: falling in the troposphere, rising through the stratosphere, falling in the mesosphere, rising again above.

    • Key-Facts Sheet

  • 99% of atmospheric mass within 32 km
  • Oxygen negligible at 120 km; CO₂ and water vapour up to 90 km
  • Ozone: 10–50 km
  • Troposphere: avg 13 km (8 km poles, 18 km equator); lapse rate 1°C per 165 m
  • Tropopause: −80°C (equator), −45°C (poles)
  • Stratosphere to 50 km; mesosphere to 80 km (−100°C); ionosphere 80–400 km

    • Exam Focus

  • MCQs: gas forming the major portion (nitrogen); layer important for humans (troposphere); sea salt, pollen, ash, soot, fine soil (dust particles); oxygen negligible at 120 km; gas transparent to incoming and opaque to outgoing radiation (carbon dioxide).
  • 30-word answers: define atmosphere; elements of weather and climate; composition; why the troposphere is most important (all weather changes, water vapour, dust, biological activity).
  • 150-word answers: composition of the atmosphere (gases, water vapour, dust); labelled diagram of atmospheric structure with description of each layer.

    • Key Terms

  • Atmosphere (वायुमंडल)
  • Troposphere (क्षोभमंडल)
  • Tropopause (क्षोभसीमा)
  • Stratosphere (समतापमंडल)
  • Mesosphere (मध्यमंडल)
  • Ionosphere (आयनमंडल)
  • Exosphere (बहिर्मंडल)
  • Ozone layer (ओज़ोन परत)
  • Water vapour (जलवाष्प)
  • Hygroscopic nuclei (आर्द्रताग्राही नाभिक)
  • Greenhouse effect (हरितगृह प्रभाव)
  • Terrestrial radiation (पार्थिव विकिरण)

    • Common Mistakes

  • Saying the troposphere is thickest at the poles — it is thickest at the equator (18 km) due to convection.
  • Placing the ozone layer in the troposphere — it is in the stratosphere (ozone found between 10 and 50 km).
  • Saying temperature keeps falling through all layers — it rises in the stratosphere and ionosphere.
  • Calling water vapour a constant gas — it is variable.

    • 💡 Key Learning Points

    • ✓The atmosphere is a mixture of gases, water vapour and dust particles; 99% of its mass lies within 32 km of the surface.
    • ✓Carbon dioxide is transparent to incoming solar radiation but opaque to outgoing terrestrial radiation, causing the greenhouse effect; ozone (10–50 km) filters ultra-violet rays.
    • ✓Water vapour is variable (up to 4% in tropics, under 1% in deserts and polar areas) and acts like a blanket; dust particles serve as hygroscopic nuclei.
    • ✓The atmosphere has five layers — troposphere, stratosphere, mesosphere, ionosphere/thermosphere and exosphere — defined by temperature.
    • ✓The troposphere (8 km at poles, 18 km at equator) holds all weather; temperature falls 1°C per 165 m.

    👨‍🏫 Teaching Tips

    • →Begin with a breath-holding challenge (safely, a few seconds) to show why air matters.
    • →Draw the layered structure as a tall wall chart with heights and a temperature curve.
    • →Use a car parked in the sun with closed windows as an everyday example of the greenhouse effect.
    • →Compare humid coastal days and dry winter days to discuss variable water vapour.
    • →Show how radio broadcasts reach far places by reflection from the ionosphere.
    • →Have students calculate temperature drop up a 3,300 m hill using 1°C per 165 m (20°C).

    📋 Assessment Questions

    1. Can the student describe the three components of the atmosphere?
    2. Can the student explain the role of carbon dioxide and ozone?
    3. Can the student draw and label the layers of the atmosphere with heights?
    4. Can the student explain why the troposphere is thickest at the equator and most important for life?
    5. Can the student list the elements of weather and climate?

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    Keep learning

    All chapters in Fundamentals of Physical Geography - Geography Class 11

    1. 1. Geography as a Discipline
    2. 2. The Origin and Evolution of the Earth
    3. 3. Interior of the Earth
    4. 4. Distribution of Oceans and Continents
    5. 5. Geomorphic Processes
    6. 6. Landforms and their Evolution
    7. 7. Composition and Structure of Atmosphere
    8. 8. Solar Radiation, Heat Balance and Temperature
    9. 9. Atmospheric Circulation and Weather Systems
    10. 10. Water in the Atmosphere
    11. 11. World Climate and Climate Change
    12. 12. Water (Oceans)
    13. 13. Movements of Ocean Water
    14. 14. Biodiversity and Conservation