The Atmosphere

• an envelop of gases that surrounds the earth and extends 500 km above the Earth's surface
  
  

Origin of the Atmosphere

• Origin: In formative years the atmosphere was composed of H and He.
• Chemical/ pre-biological era: Atmosphere formed from volcanic outgassing lead dominated by water vapor, CO2, SO2 and others.
• Microbial era: Initial O2 formed through photolysis allowed ozone layer, early microbes emitted O2 as waste product.
• Biological era: Simultaneous decrease in atmospheric CO2 and the increase in O2 due to life processes.

Present Composition of the Earth's Atmosphere

Nitrogen	78.1%
Oxygen	20.9%
Argon	0.9%
Carbon Dioxide (0.03%) Neon Helium Methane Krypton Nitrous Oxide Hydrogen Xenon Ozone	0.1%
Water Vapor	0 - 4%

Structure of the Atmosphere

• The vertical structure of the atmosphere is defined in terms of temperature.
  
  
• Troposphere (0 - 12 km)
  • temperature relationships
  • since air has weight it exerts pressure on the surface of the earth
    • low vs high pressure systems
  • warm vs. cool air
  • warm air rises
    • causes mixing of air masses
      
      
• Stratosphere (12 - 49 km)
  • Production of ozone (O₃)
    • ultraviolet rays from the Sun break O₂ molecules apart and then O atoms recombine with O₂ to form O₃

• temperature profile of the stratosphere is due to the stratosphere absorbing UV radiation and since the UV rays enter from the top of the atmosphere
• no mixing
• contains 1000 times less water vapor
  • if pollutants enter they will remain for many years
    
    
• Mesosphere and Thermosphere (49 - 500 km)
  • about 99% of the mass of the atmosphere is in the troposphere and stratosphere
  • all atmospheric circulation occurs below the mesosphere
  • above the mesosphere O₂ molecules absorb UV rays and thus heats it up (thermosphere)
    
    

NOTE: In this course we will limit our discussions to the lowest two levels of the atmosphere--the troposphere and the stratosphere. Our discussion of the atmospheric circulation will focus on the troposphere, but we will return to the stratosphere when we talk about volcanic eruptions and ozone depletion.

Functions of the Atmosphere

• Climate and Weather
  • Greenhouse Effect
  • Differential Heating of the Earth
    • sources of heat
    • patterns
      • not uniform from place to place
      • not uniform from time to time

Tropospheric Circulation

• energy is transferred between the equator and the poles by the same process that is responsible for plate tectonics (convection)
• air is heated at the equator and sinks at the poles causing pressure gradients
• air flows along these pressure gradients causing the winds
  
  
• Coriolis Effect
  • deflection of air and water currents to the right in the northern hemisphere and to the left in the southern hemisphere
  • rocket launch experiment
    
    
• Hadley Cell Circulation
  • in the northern hemisphere, as warm air flows poleward it is deflected to right by the Coriolis effect
  • at about 30north latitude the air is flowing due east
  • this mass of air cools and sinks back the Earths surface
  • the mass splits and part of it returns to the equator where the process starts again making the Hadley Cell circulation loop
  • at about 60latitude the air flowing north encounters the mass of air flowing south from the equator
  • the two masses collide and uplift to the top of the troposphere where it splits again
  • part of the mass flows north to the poles and part flows south to the Hadley cell that formed at 30
    
    

General Climate Patterns as a result of Atmospheric Circulation