Atmosphere, Insolation & Heat Budget

1. Composition and Structure of the Atmosphere

The atmosphere is a multi-layered envelope of gases surrounding the Earth, held by gravity. It protects life by absorbing UV radiation and reducing temperature extremes.

Composition

• Gases: Nitrogen (78.08%), Oxygen (20.95%), Argon (0.93%), and Carbon Dioxide (0.036%). Trace gases include Neon, Helium, Ozone, and Hydrogen.
• Water Vapour: Decreases rapidly with altitude and from the equator towards the poles. It acts like a blanket, absorbing terrestrial radiation.
• Dust Particles: Concentrated in the lower layers. They act as hygroscopic nuclei around which water vapor condenses to form clouds.

Structure (Thermal Layers)

• Troposphere: The lowest layer (avg height 13 km; 8 km at poles, 18 km at equator). Temperature decreases with height (Normal Lapse Rate: 6.5°C per 1000m). All weather phenomena occur here. The boundary is the Tropopause.
• Stratosphere: Extends up to 50 km. Contains the Ozone layer (15-35 km) which absorbs UV rays. Temperature increases with height due to UV absorption. Ideal for flying jet aircraft.
• Mesosphere: Extends up to 80 km. Temperature decreases again, reaching the coldest point in the atmosphere (-100°C) at the Mesopause. Meteors burn up here.
• Thermosphere (Ionosphere): 80 to 400 km. Temperature rises rapidly. Contains electrically charged particles (ions) that reflect radio waves back to Earth, enabling wireless communication. Auroras occur here.
• Exosphere: The outermost layer, gradually merging into space.

2. Insolation and Temperature

Insolation (Incoming Solar Radiation) is the solar energy received by the Earth, transferred in shortwaves. The Earth intercepts only one in two-billionths of the Sun's total output.

Factors Affecting Insolation

1. Angle of Incidence: Vertical rays heat a smaller area intensely (equator). Slanting rays spread over a larger area and pass through more atmosphere, losing heat (poles).
2. Duration of Sunshine (Length of Day): Varies with seasons and latitude.
3. Transparency of the Atmosphere: Clouds, dust, and water vapor absorb, reflect, and scatter radiation.
4. Albedo: The reflectivity of a surface. Snow has the highest albedo (70-90%), while dark soil or deep oceans have low albedo.

Temperature Inversion

Normally, temperature decreases with height. Inversion is a reversal of this normal behavior, where temperature increases with height. It occurs under specific conditions: long winter nights, clear skies, calm air, and snow-covered ground. This leads to fog formation and traps pollutants (smog) near the surface.

3. Heat Budget of the Earth

The Earth neither continuously warms up nor cools down. This implies a balance (budget) between incoming shortwave insolation and outgoing longwave terrestrial radiation.

• Incoming (Total 100 units):
  • Reflected back directly (Albedo = 35 units): 27 by clouds, 2 by snow/ice, 6 scattered by space.
  • Absorbed by Atmosphere (14 units): By ozone, water vapor, etc.
  • Absorbed by Earth's Surface (51 units): 34 directly from the sun, 17 as scattered radiation.
• Outgoing (Total 51 units from Surface):
  • 17 units radiated directly to space.
  • 34 units absorbed by the atmosphere (via latent heat of condensation, sensible heat, and radiation).
• Atmospheric Output: The atmosphere eventually radiates its 48 units (14 from sun + 34 from earth) back into space.
• Total Output: 17 (from earth) + 48 (from atmosphere) = 65 units. This perfectly balances the 65 units absorbed (14+51).