The solar spectrum compared to a black body

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The solar spectrum compared to a black body
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Blackbody radiation curves typical for the Sun
and Earth
Sun 6000K
Earth 290K
Sun radiates a lot more energy that the Earth!
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Normalized blackbody radiation curves for Earth
and Sun

• Divide each radiation curve by its maximum value
  
• to normalize curves

• Very little overlap of the normalized radiation
  curves

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• How steady is the Suns output?
• Measurements of solar radiation from space,
• rockets, and balloons
• Note on short timescales, some large
  fluctuations are possible.

C. FRÖHLICH
IPCC, 2001
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Solar variability the sunspot cycle
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Reconstructions of solar variability over time
TSI total solar irradiance
Note the scientific trend too
IPCC, 2001
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How do we get these temperatures? -Infrared
temperatures from Aqua satellite, April 2003.
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Reflectivity (albedo) of Solar (shortwave)
radiation
September, 2005
smsc.cnes.fr/IcPARASOL
Global average 30 Albedo increases with
latitude Oceans are quite dark (low reflectivity)
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Emissivity of infrared radiation at the surface
cimss.ssec.wisc.edu/iremis/
Emissivity, e, is a measure of how well
blackbody radiation is obeyed FesT4
Emissivity/absorptivity is close to 1. This
implies a good approximation to black body in the
infrared
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Summary (important)- At visible wavelengths, the
Earth reflects about 30 of the incident
radiation. At infrared wavelengths, most natural
materials absorb almost Everything (95 to 98),
so the Earth behaves quite closely as a true
blackbody.
-Go to calculation of black body temperature
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Radiation and physical objects Any physical
material (solid, liquid, gas) interacts with
electromagnetic waves (radiation) in one of four
different ways. TRANSMISSION waves pass through
the material ABSORPTION some of the waves are
absorbed ( heat) REFLECTION some of the waves
are reflected in the direction they came
from. EMISSION Every object (above absolute
zero) emits radiation because it
possesses thermal energy Less important- SCATTE
RING waves are deflected (hence blue sky)
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Radiation and physical objects How a material
interacts with radiation (transmission,
absorption, emission, reflection) depends on what
it is made of. For example whats the
difference between the yellow light in these 3
pictures?
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A key fact for Earths climate is that gases in
the atmosphere absorb radiation.

• Molecules absorb radiation at particular
  wavelengths, depending on amount of energy
  required to cause vibration or rotation of atomic
  bond.
• Two essential things for the greenhouse effect
• The Earths atmosphere is mostly transparent to
  visible radiation (why not totally)
• The Earths atmosphere is mostly opaque to
  infrared radiation.

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The composition of the Earths atmosphere
matters...
(Plus other trace components, e.g. methane, CFCs,
ozone)

• Bi-atomic molecules (O2, N2) can only absorb
• high energy photons, meaning ultraviolet
• wavelengths and shorter.

• Tri-atomic molecules (H2O, CO2) can absorb
• lower energy photons, with wavelengths in the
• infrared

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Atmospheric absoption by atmospheric constituents
solar terrestrial emissions as a function of
wavelength
100-
CH4 N20 O2,03 CO2 H20
0-
Peixoto and Oort, 1992
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• Key things from previous slide-
• Atmosphere mostly transparent to solar radiation
  (except in uv)
• Atmosphere mostly opaque to terrestrial
  radiation (infrared)
• Water vapor is the most important greenhouse gas
  (by far)
• Carbon dioxide is a problem because of a
  window in H2O
• absorption spectrum.
• This physics is very, very well known

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• Atmospheric absorption
• Shortwave (i.e. solar) radiation measured from
  the top of
• atmosphere and from the ground.

• The (clear) atmosphere is not totally
• transparent to solar radiation
• back scatter by dust, aerosols
• absorption by constituent gases
• amount varies as a function of
• wavelength

Peixoto and Oort, 1992
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Energy pathways in the atmosphere
IPCC, 2007
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This is wrong why?
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• Greenhouse effect summary
• CO2 and H20 (and some other gasses) effectively
  absorb radiation at the same wavelengths that
  the Earth
• emits at.
• Some of that radiation is then re-emitted back
  towards the ground keeping the surface warmer
  than it would otherwise be.

Essential to remember - CO2 , H20 in the
atmosphere absorbs and re-emits infrared
radiation - It does NOT (not, not, not)
reflect radiation
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