Evolution of Atmosphere:
Stage I: The primordial atmosphere of the earth contains hydrogen and helium. This is due to the Big Bang event, which occurred 13.7 billion years ago.
Stage II: Billions of years of cooling down of primordial atmosphere led to the formation of the solid earth's interior. Due to the continuous volcanic eruption, the gases from the earth's interior are released into the atmosphere. The initial atmosphere consists of CO2, N2, CH4, and sulfur oxides in huge quantities and other gases in minimal quantities.
Stage III: During this stage unicellular organisms such as algae, cyanobacteria and phytoplanktons are formed. They carry out anaerobic respiration which takes in carbon dioxide and releases oxygen into the atmosphere. This process continued out for billions of years which led to more oxygen in the present-day atmosphere. The carbon dioxide in the atmosphere got dissolved in rainwater. Temperature decreased further causing more condensation and more rains.
Composition of present-day atmosphere:
- Nitrogen - 78%
- Oxygen - 21%
- Argon - 0.93%
- Carbon dioxide - 0.03%
Carbon dioxide:
- Meteorologically important gas.
- Transparent to incoming solar radiation and opaque to outgoing longwave terrestrial radiation. (CO2 allows short wavelength radiation but traps long wavelength radiation)
- Absorbs a part of terrestrial radiation.
- Largely responsible for the greenhouse effect.
Ozone:
- Important gas filters and absorb UV radiation from the sun.
- It prevents them from reaching the surface of the earth.
- Mainly concentrated up to 10 - 50 km from the earth's surface.
- Generally present in Stratosphere.
- Sometimes they are also found in the Troposphere - Tropospheric ozone or Ground-level ozone is one of the secondary pollutants - causes breathing difficulties.
NO2 + O2 (in the presence of sunlight) -> NO3 -> NO + O3 (Ground level Ozone)
Water Vapour:
- It is a variable gas.
- In tropical region quantity of water vapour is very high due to the high amount of evaporation.
- Water vapour decreases with latitude and found as minimal in the polar regions.
- The quantity of water vapour decreases with increasing latitude.
- Since water vapour exists only for a short period, its contribution to global warming is minimal.
Dust:
- The atmosphere has small particles which originate from different sources like sea salts, fine soil, ashes, pollens etc.
- Generally in lower layers of the atmosphere, higher concentration of dust particles found in sub-tropical and temperate regions due to the presence of dry winds in comparison to equatorial and polar regions. So the dust particles act as hygroscopic nuclei around which water vapour condenses to form water droplets.
Convection - the vertical rising of air:
The higher temperature due to sunlight heats up air particles in the earth's atmosphere and cause them to expand which lowers their density. As a result, the low-density air particles move to the upper atmosphere creating low pressure. Hence air from high-pressure areas moves to low-pressure areas. This movement of air from high-pressure areas to low-pressure areas is called wind.
Structure of Atmosphere - Categorised based on temperature, pressure, and specific features of each layer.
- Troposphere (Lower-most layer)
- Stratosphere
- Mesosphere
- Thermosphere or Ionosphere
- Exosphere (Top-most layer)
Atmosphere - consists of different layers with varying density and temperature. The density is highest near the earth's surface and decreases with altitude.
1st layer - Troposphere:
- Lowermost layer.
- Average height 13 km, extends up to 18 km in equator due to constant vertical rising of air (conventional currents) and 8 km in the pole.
- The temperature drops with increasing altitude - 6.5 degree Celcius per km (Normal Lapse Rate).
- All weathering and climatic phenomenon occur in this region.
- The turbulence of air and convection is largely present in this region.
- Ends at Tropopause which separates Troposphere and Stratosphere.
- The temperature of Tropopause is -80*C at the Equator whereas it is -45*C at the poles. This is due to the Normal Lapse Rate (As altitude increases, temperature decreases).
