Todays lecture objectives:

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ATMS 455 Physical Meteorology

• Todays lecture objectives
• Absorptivity and emissivity (WH 6.5)
• Building the foundation for understanding the
  greenhouse effect

http//www.artcyclopedia.com/feature-2001-08.html
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ATMS 455 Physical Meteorology

• Todays lecture topics
• Absorptivity and emissivity (WH 6.5)
• Kirchhoffs law
• Selective absorbers and emitters
• Disposition of monochromatic radiation

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Introduction

• Blackbody radiation upper limit to the amount
  of radiation that a real substance may emit at a
  given temperature

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Introduction

• emissivity a measure of how strongly a body
  radiates at a particular wavelength. A blackbody
  has an emissivity equal to one.

http//ceos.cnes.fr8100/cdrom-97/ceos1/science/ba
phygb/chap3/chap3.htm
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Introduction

• If we neglect the dependence of emissivity on the
  wavelength (color) of radiation, we define a
  gray body emissivity

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Introduction

• We also define the quantities absorptivity (al)
  and gray body absorptivity (a) which are measures
  of the ratio of the irradiance absorbed by a
  particular body to that which is incident upon
  it. The absorptivity of a blackbody is equal to
  one at all wavelengths of radiation.

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Kirchhoffs law

• Proof of Kirchhoffs law
• Two parallel plates, infinite in extent
• Blackbody plate on left
• Gray body plate on right with absorptivity (a)
  and emissivity (e)
• Plates are in radiative and thermal equilibrium

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Kirchhoffs law

• In qualitative terms
• Materials which are strong absorbers at a
  particular wavelength are also strong emitters at
  that wavelength
• Materials which are weak absorbers at a
  particular wavelength are also weak emitters at
  that wavelength

? The validity of Kirchhoffs law is not
dependent upon whether a body is in thermal and
radiative equilibrium
? Applies for atmospheric gases up to altitudes
of 60km
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Selective absorbers and emitters

• In the moon example (Prob. 6.6, p. 292), the
  radiative equilibrium temperature of the gray
  surface is independent of the absorptivity
• When absorptivity is a function of wavelength of
  radiation, the situation for radiative
  equilibrium temperature can be quite different ?
  Prob. 6.7, p. 293

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Selective absorbers and emitters

• Snow is a relatively weak absorber at visible and
  near-IR wavelengths, but is a strong absorber in
  the IR wavelengths

http//news.bbc.co.uk/1/hi/england/2639171.stm
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Selective absorbers and emitters

• How does a selectively absorbing gas behave? ?
  Prob. 6.8, p. 293

http//www.eballoon.org/pictures/alpine.html
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Selective absorbers and emitters

• Calculate the radiative equilibrium temperature
  of the earths surface
• Atmosphere is a thin layer with a 0.1 for solar
  radiation and a 0.8 for terrestrial radiation
• Earths surface behaves as a blackbody

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Selective absorbers and emitters

• Iterative method for solving Prob. 6.8, p. 293
• Atmosphere is a thin layer with a 0.1 for solar
  radiation and a 0.8 for terrestrial radiation

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Selective absorbers and emitters

• Compare the surface temperatures for an earth
  system
• Without an atmosphere (Prob. 6.5), Te 255 K
• With an atmosphere (Prob. 6.8), Te 286 K

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Selective absorbers and emitters

• Whenever a gas that is a weak absorber in the
  visible and a strong absorber in the IR is a
  constituent of a planetary atmosphere, it
  contributes toward raising the surface
  temperature of the planet (greenhouse effect)
• In order to satisfy radiation balance, the
  surface must compensate by emitting more
  radiation than it would in the absence of such an
  atmosphere (must radiate at a higher temperature)

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Disposition of monochromatic radiation

• incident upon an opaque (non-transparent)
  surface

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Disposition of monochromatic radiation

• Strong reflectors are weak absorbers (e.g. snow
  at visible wavelengths)
• Weak reflectors are strong absorbers (e.g.
  asphalt at visible wavelengths)

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Disposition of monochromatic radiation

• Shortwave reflectivity of various surfaces

Petty 2004
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Disposition of monochromatic radiation

• incident upon a non-opaque (semi-transparent)
  layer

http//www.worldstar.com/collmer/
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