6.1 Introduction to the Atmosphere

The Atmosphere

The atmosphere is a layer of gases surrounding Earth, held by gravity, essential for sustaining life by regulating temperature, filtering radiation, and enabling chemical reactions. It is the outer limit of the biosphere and its composition and processes support life on Earth. Differential heating of the atmosphere creates the tricellular model of atmospheric circulation that redistributes the heat from the equator to the poles. This circulation disperses the heat across the planet, reducing the heat at the equator and increasing the temperature in higher latitudes.

Composition:

• Nitrogen (N₂): ~78%
• Oxygen (O₂): ~21%
• Argon (Ar): ~0.93%
• Trace Gases: CO₂, CH₄, H₂O vapour, and others.

Atmospheric Layers

• Troposphere:
  • 0–12 km; weather occurs here; temperature decreases with height.
• Stratosphere:
  • 12–50 km; contains the ozone layer; temperature increases with height due to UV absorption.
• Mesosphere:
  • 50–85 km; meteors burn here; temperature decreases with height.
• Thermosphere:
  • 85–600 km; auroras occur here; temperature increases due to solar radiation.
• Exosphere:
  • Above 600 km; merges with space; contains very few particles.

Physical and Chemical Processes in the Atmosphere

• Physical Processes:
  • Advection: Horizontal transport of heat and moisture by wind.
  • Convection: Rising of warm air and sinking of cooler air, driving weather patterns.
  • Condensation: Water vapour cools to form clouds, releasing latent heat.
• Chemical Processes:
  • Ozone Formation: UV radiation splits oxygen molecules (O₂) into single atoms, which combine with O₂ to form ozone (O₃) in the stratosphere.
  • Carbon Cycle: CO₂ is absorbed by plants during photosynthesis and released through respiration, decomposition, and combustion.
  • Nitrogen Cycle: Atmospheric nitrogen (N₂) is fixed into biologically usable forms by lightning or microorganisms.

The greenhouse effect

The average temperature of the Earth is 15 degrees Celsius. Without this warmth, life cannot exist on Earth. The temperature of the Earth is maintained by the atmosphere.

In the atmosphere, certain gases (greenhouse gases) and aerosols trap radiation that heats the surface of the Earth, increasing the temperature. Atmospheric gases allow incoming short-wave radiation to pass through. Greenhouse gases (GHGs) and aerosols either trap or reflect the outgoing long-wave radiation. This process is known as radiation trapping. This is mainly done by water vapor and carbon dioxide but methane, nitrous oxide, and ozone are also involved. These gases and aerosols create a thermal blanket that maintains a suitable temperature to support life on Earth.