The Greenhouse Effect

1
The Greenhouse Effect

• Ananda Guneratne
• (Part A)

2
Topics Covered in this Presentation

• Finding the Intensity of Radiation on a planet
• 8.5.1 - Calculate the intensity of the Sun's
  radiation incident on a planet.
•  
• Albedo
• 8.5.2 - Define albedo.
• 8.5.3 - State factors that determine a planet's
  albedo.
•  
• Black-Body Radiation
• 8.5.8 - Outline the nature of black-body
  radiation.
• 8.5.9 - Draw and annotate a grap of thee emission
  spectra of black bodies at different temperatures
• 8.5.10 - State the Stefan-Boltzmann Law and apply
  it to compare emission rates from different
  surfaces.
• 8.5.11 - Apply the concept of emissivity to
  compare the emission rates from the different
  surfaces.

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Intensity of Solar Radiation
8.5.1

• How to calculate the solar radiation incident on
  any planet
•  
• Imagine the planet as a disc
• The intensity of the radiation is measured in
  watts per square meter.
• Thus, the intensity of solar radiation incident
  on a planet is

       Fig. 0.0 - Discs and Spheres
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Example - Earth
8.5.1
Average energy of solar radiation entering
Earth's atmosphere Thus,         The average
radiation incident at the surface of the Earth is
342 watts per meter squared.
5
What is Albedo? 
8.5.2

• Definition   
•  
• The albedo of a surface is the percentage of
  incoming radiation that is reflected (rather than
  absorbed) by that surface.
• Albedo
•  

     Radiation reflected    
Total incoming Radiation
Fig. 1.0 - Differences between albedos
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What Determines Albedo?
8.5.3

• Color
• Caused by reflection
• Angle
• Latitude
• Season
• Cloud cover
• Daily variance

Some Common Albedo Ranges Fresh Snow/Ice       
   0.60-0.90 Old melting snow    
   0.40-0.70 Clouds                        
0.40-0.90 Desert Sand              
 0.30-0.50 Soil                             
0.05-0.30 Tundra                        
0.15-0.35 Grasslands                 
0.18-0.25 Forest                         
0.05-0.20 Water                         
0.05-0.10   Global Annual Mean   0.30
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What Determines Albedo?
8.5.3
Fig. 1.1 - The Seasons
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What Determines Albedo?
8.5.3
Fig. 1.2 - Seasonal and Latitudinal Albedo Changes
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Albedo Loss - "The Slippery Slope to Hell"
Fig. 1.3 - Polar Icecaps and Albedo
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What is a black body?
8.5.8

• Hypothetical
• Perfect absorber
• Perfect emitter
• Produces black-body radiation
• Emitted from a black body
• Properties depend solely on temperature

Fig. 2.0 - Black Body
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The Ultraviolet Catastrophe
8.5.8

• Classical Physics
• (James Maxwell)
• Energy is proportional to frequency

• Quantum Physics
• (Max Planck)
• Ehf, energy emitted in quanta

Fig. 2.1 Black body raditation under classical
theory
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The Blackbody Spectrum
8.5.9
Fig. 2.2 - Blackbody spectra at various
temperatures
13
The Stefan-Boltzmann Law
8.5.10

• The Stefan-Boltzmann Law states that the amount
  of energy radiated per unit area per second is
  proportional to the temperature of the object
  raised to the fourth power.
•  
• General Formulation
• Q Energy transferred per unit time (W)
• b Constant
• A Area
• T1 Absolute temperature of radiating body
• T2 Absolute temperature of absorbing body

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Stefan-Boltzmann Constant
8.5.10
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Example
8.5.10
Surface A has a temperature of 50.0 K and a
constant b of 0.021.  Surface B has a temperature
of 100. K and a constant b of 0.016.  Assume both
surfaces have an equal area, and that the
surrounding temperature is 25.0 K, which surface
has a higher rate of emission?   Since we do not
know area, we will be finding our answer in rate
of emission per unit area.  Thus,               
                                                
             and           We can thus see that
Surface B will emit electromagnetic radiation at
a higher rate.
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Emissivity
8.5.11

• Emissivity is the ratio of the radiation emitted
  by a surface to the radiation of a blackbody at
  the same temperature and under similar
  conditions.
• Kirchoff's law
•     At a given temperature, the rate of emission
  of electromagnetic energy by an object is equal
  to the rate at which the object absorbs
  electromagnetic energy of the same wavelength.

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Example
8.5.11
Surface A has an emissivity of 0.8.  Surface B
has an emissivity of 0.2.  If the rate of
emission of a black body under similar conditions
is 5 kJ per second, find the rates of emmission
of Surface A and Surface B. A 5(0.8) 4 kJ
per second B 5(0.2) 1 kJ per second
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Sources

• B.N. Taylor, P.J. Mohr, D.B. Newell,CODATA
  Reccomended Values of the Fundemental Physical
  Constants 2006,(National Institute of Standards
  and Technology,Gaithersburg, Maryland, 2006).
• J.R. Gribbin, Q is for Quantum Particle Physics
  From A-Z, 1st ed. (Weidenfield Nicolson, Great
  Britain, 1998), p. 44-45, 201, 286, 376.
• Van Nostrand Reinhold Company, Van Nostrand's
  Scientific Encyclopedia, 5th ed. (Van Nostrand
  Reinhold Company, New York, 1976), p. 948, 1249.
• W.F. Ruddiman, Earth's Climate Past and Future,
  1st ed. (W.F. Freeman Co., New York, 2000), p.
  19-20, 22-25.

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Picture Sources

• Fig. 0.0 http//www.windows.ucar.edu/earth/climat
  e/images/earth_insolation_disk_sphere_big.jpg
• Fig. 1.0 http//www.uwsp.edu/geo/faculty/ritter/i
  mages/atmosphere/energy/albedo.gif
• Fig. 1.1 http//www.dkimages.com/discover/preview
  s/784/563581.JPG
• Fig. 1.2 http//photojournal.jpl.nasa.gov/jpeg/PI
  A04378.jpg
• Fig. 1.3 http//www.zeeburgnieuws.nl/nieuws/image
  s/albedo_effect_sea_ice_loss.jpg
•                http//www.zeeburgnieuws.nl/nieuws/
  images/sea_ice_yearly_decline.jpg
• Fig. 2.0 http//www.egglescliffe.org.uk/physics/a
  stronomy/blackbody/animation.gif
• Fig. 2.1 http//www.haverford.edu/physics/songs/c
  avendish/uvcat.jpg
• Fig. 2.2 http//rip.physics.unk.edu/Larsen/blackb
  ody_spectra.jpg