EATS 1011 Lecture 5

 

• Co₂ has less of an impact on global warming than waper vapour, but CO₂ can be controlled, water vapour isn't something humans can control
  
• The surface temperature would be exactly the same as the effective temperature if we had no atmosphere but 30% albedo
  
• T_effective =
  
• Greenhouse effect
  
  • Dilute solar radiation heats (mostly) surface
    
  • Surface heats and emits mid-IR radiation which is absorbed(mostly) by atmospheric gases and clouds
    
    • The gases such as water vapour(clouds), carbon dioxide are what cause the greenhouse effect because they absorb radiation from the sun
      
  • Atmosphere re-radiates
    
    • Up to space
      
    • Back down to earth
      
  • Surface is heated "twice" (solar + mid-IR)
    
    • Figure 212 on slide
      
  • Red skys are caused by Rayleigh scattering
    
    • 
       
      • Where lambda is the wavelength
        
  • Albedo
    
    • Refers to visible part of the light spectrum
      
    • Albedo depends on the surface the light is directed to
      
    • Fresh snow is most reflective, 75-95% light reflected
      
    • Earth is about 30% 'albedo'
      
      • Not sure how to use the word 'albedo'
        
    • 6% reflected from atmosphere
      
    • 20% reflected by clouds
      
    • 4% is reflected naturally by the earth's surface
      
    • 51% of solar energy heats the Earth's surface
      
    • Total energy budget
      
      • 
        
    • Most infrared is absorbed by the earth's atmosphere instead of being released into space
      
    • When the sun goes down at night we don't freeze because infra red energy from the atmosphere continues heating the earth while the sun is down
      
    • 70 units of solar coming in , 50 heating the surface, after all the processing we have 70 units of energy leaving in the mid infrared
      

Earth's Energy Budget

• Top of atmosphere
  
  • 70 units of solar in balanced by 70 units of Mid-IR emitted
    
• Surface
  
  • Heated twice
    
  • Solar 51 units and 96 mid-IR units
    
  • We are heated by mid-IR at night
    
• Read slides for more details on the web
  

Seasons

• What are the drivers of the seasons
  
  • In the tropics there are no seasons
    
  • Seasons are at mid latitudes
    
• orbital parameters
  
  • ellipticity of orbits
    
    • Perihelion
      
      • 147 x 10⁶ km, 4^th january
        
    • Ephelion
      
      • 153 x 10⁶ km, 4^th July
        
    • 7% change in insolation
      
  • Insolation:
    
    • The suns energy averaged over 24 hours
      
  • Obliquity: tilt
    
    • Rotation axis – pole star
      
    • 23.5 degrees
      
    • Solstice
      
      • N pole sun 21^st June, sun overhead at noon at tropic of Cancer(23.5N)
        
      • S pole sun 21^st December, sun overhead at noon at the tropic of Capricorn (23.5S)
        
      • Regions of perpetual day and night
        
    • Equinox
      
      • Axis 90 to sun – earth line
        
      • Vernal 21^st march
        
      • Autumnal 21^st September
        
      • Effects the length of the day
        
• So what causes the seasons
  
  • Ellipticity
    
    • 7% so little impact
      
      • Except over eons
        
  • Obliquity
    
    • Large effect at mid- and polar latitudes
      
  • Latitude
    
    • In the tropics changes driven by overhead sun
      
    • Wet and dry seasons
      
    • Twice a year
      
• Length of day light for various latitudes
  
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