Class 11 Geography Notes Chapter 8 (Composition and structure of atmosphere) – Fundamental of Physical Geography Book

Detailed Notes with MCQs of a crucial chapter for your understanding of physical geography and for your government exam preparation: Chapter 8 - Composition and Structure of the Atmosphere from your NCERT Class 11 textbook. This chapter lays the foundation for understanding weather, climate, and various atmospheric phenomena. Pay close attention to the details.

Chapter 8: Composition and Structure of Atmosphere - Detailed Notes

Introduction:
The atmosphere is a vast envelope of gases surrounding the Earth, held in place by gravity. It's essential for life, protecting us from harmful solar radiation, regulating temperature, and providing gases necessary for respiration and photosynthesis. Understanding its composition and structure is fundamental.

I. Composition of the Atmosphere:

The atmosphere is a mixture of numerous gases, water vapour, and particulate matter (aerosols).

• A. Gases:

  • Permanent Gases: Their proportion remains relatively stable in the lower atmosphere.
    • Nitrogen (N₂): ~78.08%. Largely inactive but crucial for plant life through nitrogen fixation. Dilutes oxygen, regulates combustion.
    • Oxygen (O₂): ~20.95%. Essential for respiration (breathing) for most living organisms and for combustion.
    • Argon (Ar): ~0.93%. An inert gas.
    • Others (Neon, Helium, Krypton, Xenon, Hydrogen): Present in trace amounts.
  • Variable Gases: Their concentration varies significantly in time and space, primarily in the lower atmosphere.
    • Water Vapour (H₂O): 0-4% by volume. Highly variable. Decreases rapidly with altitude and from equator to poles.
      • Significance: Acts as a powerful greenhouse gas (absorbs terrestrial radiation), carries latent heat influencing weather, source of clouds and precipitation.
    • Carbon Dioxide (CO₂): ~0.04% (currently rising due to human activities).
      • Significance: Essential for photosynthesis by plants. A major greenhouse gas, transparent to incoming solar radiation but absorbs outgoing terrestrial (longwave) radiation, contributing significantly to Earth's heat budget and global warming. Concentrated in lower layers.
    • Ozone (O₃): Found primarily in the Stratosphere (10-50 km altitude, peak concentration around 25 km).
      • Significance: Absorbs harmful Ultraviolet (UV) radiation from the sun, protecting life on Earth. A small amount near the surface acts as a pollutant.
    • Methane (CH₄), Nitrous Oxide (N₂O), etc.: Trace greenhouse gases.

• B. Particulate Matter (Aerosols):

  • Tiny solid or liquid particles suspended in the air.
  • Sources: Sea salts, fine soil/dust, smoke/soot (combustion), volcanic ash, pollen, disintegrated meteors.
  • Significance:
    • Act as hygroscopic nuclei around which water vapour condenses to form clouds.
    • Scatter and absorb incoming solar radiation, affecting visibility and energy balance.
    • Can influence air quality and health.
    • Responsible for optical phenomena like red/orange colours of sunrise/sunset (scattering of blue light).

II. Structure of the Atmosphere:

The atmosphere is divided into distinct layers based primarily on temperature variations with altitude.

• 1. Troposphere:

  • Altitude: Extends from the Earth's surface up to an average height of ~13 km. Height varies - thicker at the equator (~18 km) due to strong convection, thinner at the poles (~8 km).
  • Temperature: Decreases with increasing altitude at an average rate of 6.5°C per 1000 meters (1 km). This is called the Normal Lapse Rate.
  • Key Features:
    • Contains ~75-80% of the atmospheric mass and almost all water vapour and aerosols.
    • All weather phenomena (clouds, rain, storms, fog, etc.) occur within this layer.
    • Most turbulent layer due to convection currents.
  • Boundary: The upper limit is the Tropopause, where the temperature stops decreasing and becomes relatively stable. Acts as a lid, limiting vertical mixing with the layer above.

• 2. Stratosphere:

  • Altitude: Extends from the Tropopause up to ~50 km.
  • Temperature: Remains nearly constant in the lower part, then increases with altitude. This is known as a temperature inversion. The increase is due to the absorption of UV radiation by the ozone layer.
  • Key Features:
    • Contains the Ozone Layer (Ozonosphere), which has the maximum concentration of O₃.
    • Very dry, with almost no water vapour or clouds (except occasional Polar Stratospheric Clouds).
    • Stable layer with minimal vertical mixing, making it ideal for long-distance jet aircraft travel.
  • Boundary: The upper limit is the Stratopause, where the temperature reaches its maximum in this layer (around 0°C).

• 3. Mesosphere:

  • Altitude: Extends from the Stratopause up to ~80 km.
  • Temperature: Decreases rapidly with increasing altitude, reaching the coldest temperatures in the atmosphere (around -90°C to -100°C).
  • Key Features:
    • Most meteors burn up in this layer upon entering the atmosphere due to friction with sparse air molecules.
    • Very thin air.
    • Possible formation of Noctilucent Clouds (ice clouds) near the top in polar regions during summer.
  • Boundary: The upper limit is the Mesopause, the coldest point in the atmosphere.

• 4. Thermosphere:

  • Altitude: Extends from the Mesopause upwards, roughly from 80 km to about 400-600 km (boundary is ill-defined).
  • Temperature: Increases sharply with altitude due to absorption of high-energy solar radiation (X-rays, short UV) by the sparse gas molecules (mainly Oxygen and Nitrogen). Temperatures can reach very high values (1000°C+).
  • Key Features:
    • Air density is extremely low. Despite high temperatures, it wouldn't feel hot because there are too few molecules to transfer significant heat.
    • Contains the Ionosphere (lower part of Thermosphere, overlapping with Mesosphere): Gas molecules are ionized (electrically charged) by solar radiation.
      • Significance of Ionosphere: Reflects radio waves back to Earth, enabling long-distance radio communication. Divided into D, E, and F layers based on ionization density.
    • Auroras (Aurora Borealis - Northern Lights, Aurora Australis - Southern Lights) occur here, caused by interaction of solar wind particles with atmospheric gases.
  • Boundary: No clear upper boundary, sometimes referred to as the Thermopause.

• 5. Exosphere:

  • Altitude: The outermost layer, starting around 400-600 km and gradually merging with outer space.
  • Key Features:
    • Extremely thin air, density is negligible.
    • Mainly composed of the lightest gases, Hydrogen and Helium.
    • Atmospheric particles can escape Earth's gravity from this layer.

III. Elements of Weather and Climate:
The elements observed, measured, and studied within the atmosphere include: Temperature, Pressure, Wind, Humidity, Clouds, and Precipitation. These elements and their interactions drive weather patterns and define climate zones, all governed by the composition and structure described above.

Multiple Choice Questions (MCQs):

1. Which gas is the most abundant in the Earth's atmosphere?
   (a) Oxygen
   (b) Argon
   (c) Nitrogen
   (d) Carbon Dioxide

2. The layer of the atmosphere where almost all weather phenomena occur is the:
   (a) Stratosphere
   (b) Mesosphere
   (c) Troposphere
   (d) Thermosphere

3. The Ozone layer, crucial for absorbing harmful UV radiation, is primarily located in the:
   (a) Troposphere
   (b) Stratosphere
   (c) Mesosphere
   (d) Ionosphere

4. What is the average rate at which temperature decreases with altitude in the Troposphere, known as the Normal Lapse Rate?
   (a) 5.5°C per km
   (b) 6.5°C per km
   (c) 7.5°C per km
   (d) 9.8°C per km

5. Which atmospheric layer is characterized by a temperature increase with altitude due to ozone absorption of UV radiation?
   (a) Troposphere
   (b) Mesosphere
   (c) Stratosphere
   (d) Thermosphere

6. Meteors typically burn up upon entering Earth's atmosphere in which layer?
   (a) Exosphere
   (b) Thermosphere
   (c) Mesosphere
   (d) Stratosphere

7. The Ionosphere, responsible for reflecting radio waves, is primarily found within which atmospheric layer?
   (a) Stratosphere
   (b) Mesosphere
   (c) Thermosphere
   (d) Exosphere

8. Which of the following is considered a variable gas in the atmosphere, playing a significant role as a greenhouse gas and in cloud formation?
   (a) Nitrogen
   (b) Oxygen
   (c) Argon
   (d) Water Vapour

9. Aerosols (particulate matter) in the atmosphere are important because they:
   (a) Are the primary absorbers of UV radiation
   (b) Act as condensation nuclei for cloud formation
   (c) Make up the largest percentage of atmospheric gases
   (d) Cause temperature to increase steadily with altitude in the troposphere

10. The coldest temperatures in the Earth's atmosphere are typically found at the:
    (a) Tropopause
    (b) Stratopause
    (c) Mesopause
    (d) Surface during polar winter

Answer Key for MCQs:

1. (c) Nitrogen
2. (c) Troposphere
3. (b) Stratosphere
4. (b) 6.5°C per km
5. (c) Stratosphere
6. (c) Mesosphere
7. (c) Thermosphere
8. (d) Water Vapour
9. (b) Act as condensation nuclei for cloud formation
10. (c) Mesopause

Make sure you revise these points thoroughly. Understand the 'why' behind the facts, like why temperature changes occur in different layers. This will help you tackle trickier questions in your exams. Good luck with your preparation!