Composition and Structure of the Atmosphere

Introduction to the Atmosphere

• The atmosphere is a layer of gases surrounding the Earth, held in place by gravity.
• It is essential for life as it provides oxygen, regulates temperature, and protects against harmful solar radiation.
• Example: Without the atmosphere, Earth would be as barren and lifeless as the Moon.

Composition of the Atmosphere

The atmosphere is composed of a mixture of gases, water vapor, and aerosols.

Gases in the Atmosphere

• Permanent Gases:
  • Nitrogen (N₂): 78% of the atmosphere; inert and dilutes oxygen.
    • Example: Nitrogen is crucial for plant growth as it is a key component of amino acids and proteins.
  • Oxygen (O₂): 21% of the atmosphere; essential for respiration and combustion.
    • Example: Oxygen supports aerobic respiration in humans and animals.
  • Argon (Ar): 0.93%; inert gas.
  • Carbon Dioxide (CO₂): 0.04%; crucial for photosynthesis and a greenhouse gas.
    • Example: CO₂ levels have increased due to human activities like burning fossil fuels, contributing to global warming.
  • Other trace gases: Neon, Helium, Methane, Krypton, Hydrogen, etc.
• Variable Gases:
  • Water Vapor (H₂O): 0-4%; varies with temperature and location.
    • Example: High water vapor content in tropical regions leads to high humidity and frequent rainfall.
  • Ozone (O₃): Found in the stratosphere; absorbs harmful ultraviolet (UV) radiation.
    • Example: The ozone layer over Antarctica has been depleted due to chlorofluorocarbons (CFCs), causing the “ozone hole”.
  • Aerosols: Tiny solid or liquid particles (dust, pollen, smoke, sea salt) that influence climate and weather.
    • Example: Volcanic eruptions release large amounts of aerosols, which can cool the Earth’s surface by reflecting sunlight.

Structure of the Atmosphere

The atmosphere is divided into layers based on temperature variations:

A. Troposphere

• Altitude: 0 to 12 km (varies with latitude; 8 km at poles, 18 km at equator).
• Characteristics:
  • Contains 75% of the atmosphere’s mass.
  • Temperature decreases with altitude (6.5°C per km; Environmental Lapse Rate).
  • Weather phenomena occur here (clouds, rainfall, storms).
    • Example: Cyclones, thunderstorms, and monsoons occur in the troposphere.
  • Upper boundary is called the Tropopause.

B. Stratosphere

• Altitude: 12 to 50 km.
• Characteristics:
  • Temperature increases with altitude due to ozone absorption of UV radiation.
    • Example: The ozone layer absorbs UV-B and UV-C rays, protecting life on Earth.
  • Contains the Ozone Layer (15-35 km).
  • Calm and stable; ideal for jet aircraft.
    • Example: Commercial jets fly in the lower stratosphere to avoid turbulence.
  • Upper boundary is called the Stratopause.

C. Mesosphere

• Altitude: 50 to 80 km.
• Characteristics:
  • Temperature decreases with altitude (coldest layer; up to -90°C).
  • Meteors burn up in this layer.
    • Example: The “shooting stars” we see are meteors disintegrating in the mesosphere.
  • Upper boundary is called the Mesopause.

D. Thermosphere

• Altitude: 80 to 400 km.
• Characteristics:
  • Temperature increases with altitude (up to 1200°C).
  • Contains the Ionosphere (80-400 km), which reflects radio waves.
    • Example: Radio communication over long distances is possible due to the ionosphere.
  • Aurora phenomena occur here.
    • Example: The Northern Lights (Aurora Borealis) and Southern Lights (Aurora Australis) occur in the thermosphere.

E. Exosphere

• Altitude: 400 km and above.
• Characteristics:
  • Outermost layer; merges with outer space.
  • Contains light gases like hydrogen and helium.
    • Example: Satellites orbit the Earth in the exosphere.

Importance of the Atmosphere

• Supports Life: Provides oxygen and carbon dioxide for respiration and photosynthesis.
  • Example: Plants use CO₂ for photosynthesis, and humans rely on oxygen for breathing.
• Regulates Temperature: Greenhouse gases maintain Earth’s temperature.
  • Example: Without the greenhouse effect, Earth’s average temperature would be -18°C instead of the current 15°C.
• Protects from Solar Radiation: Ozone layer absorbs harmful UV rays.
  • Example: UV radiation can cause skin cancer and cataracts in humans.
• Weather and Climate: Drives weather patterns and climatic conditions.
  • Example: The Indian monsoon is a result of atmospheric circulation patterns.