Earth's Atmosphere & Weather Notes

Earth System

  • Earth system science studies the interconnections between Earth's components (land, water, air, life).
  • A system consists of interacting parts forming a complex whole.

Formation of the Earth

  • Earth formed 4.6 billion years ago via the Nebular Theory from an accretion disk.
  • Nebular Theory: Material in a circumstellar disk spirals inward towards a central body (star) due to gravity.

Earth's Atmosphere Evolution

  • 1st Atmosphere:
    • Primarily H and He, no oxygen.
    • Solar winds swept away light molecules due to lack of magnetic field.
  • 2nd Atmosphere:
    • Formed after Earth's differentiation and crust formation.
    • Volcanic activity released gases: methane (CH4), ammonia (NH3), water (H2O).
    • Formation of magnetic field deflected solar wind.
  • 3rd Atmosphere (4-3.8 billion years ago):
    • Chemical reactions converted CH4 to CO2 and NH3 to N2.
    • High concentrations of CO2, H2O, and N2; greenhouse effect prevented freezing.
    • Water condensed to form oceans; CO2 dissolved into oceans, forming carbonates and limestone.
    • Nitrogen (N2) became the major atmospheric component.
  • Life and Oxygen:
    • Cyanobacteria (3.5 billion years ago) carried out photosynthesis, producing oxygen.
    • Eukaryotes emerged, increasing photosynthesis efficiency.
    • Ozone layer formation allowed life to emerge outside oceans.

Evidence of Early Photosynthesis

  • Stromatolites:
    • Microbial reefs created by cyanobacteria via sediment trapping and binding.
    • Earliest known life forms with evidence of photosynthesis.
  • Banded Iron Formations (BIF):
    • Iron in rocks reacted with atmospheric O2 to form rust bands.
    • Alternating bands indicate seasonal aerobic/anaerobic cycles.

Modern Atmosphere

  • Formed around 400 million years ago.
  • Photosynthesis reduced CO2 and increased O2; ozone layer strengthened.
  • Nitrogen (inert and heavy) remained dominant.
  • Composition: Mostly Nitrogen, with Oxygen, CO2, and trace gases.

Atmospheric Gases

  • Permanent Gases: Long residence times (e.g., N2, O2).
  • N_2 residence time = ~42,000,000 years
  • O_2 residence time = ~3,700,000 years
  • Variable Gases: Shorter residence times (e.g., H2O, CO2).
  • H_2O residence time = ~10 days
  • CO_2 residence time = ~150 years

Importance of Variable Gases

  • CO2 and water vapor are major greenhouse gases.
  • Water exists in three phases, driving atmospheric circulation.
  • O3 protects against harmful UV radiation.

Earth's Magnetosphere

  • Deflects high-energy solar winds and cosmic rays.

Vertical Structure of the Atmosphere

  • Pressure decreases with increased altitude.
  • Temperature varies depending on the layer.

Atmospheric Layers

  • Troposphere:
    • Temperature decreases with altitude.
    • Environmental lapse rate: 6.5° C/km (variable).
    • Most weather occurs here; contains 75% of atmospheric mass and 99% of water vapor.
    • Tropopause marks the top (around -55 °C).
  • Stratosphere:
    • Temperature increases with altitude due to ozone absorbing UV rays.
    • Ozone layer is concentrated here.
    • Stratopause marks the top (about -3°C).
  • Mesosphere:
    • Temperature decreases with altitude.
    • Meteors burn up here.
    • Mesopause is the coldest naturally occurring place (below -90 °C).
  • Thermosphere:
    • Temperature increases with altitude (up to 2,000°C).
    • Thermopause is the atmospheric boundary.
  • Exosphere:
    • Outermost layer, gradually fading into space.
  • Ionosphere:
    • Located in the upper mesosphere and thermosphere.
    • Nitric oxide absorbs solar energy and becomes ionized (during the day).
    • Blocks high-energy electromagnetic radiation; enables radio wave reflection.
  • Homosphere: Composition of the atmosphere is uniform.
  • Heterosphere: Atmosphere is layered by molecular weight and electric charge.
  • Auroras (borealis and australis) occur in the ionosphere due to trapped solar particles.