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Todays lecture objectives: Absorptivity and emissivity (W&H 6.5)

Building the foundation for understandingthe greenhouse effect

ATMS 455 Physical

Meteorology

http://www.artcyclopedia.com/feature-2001-08.html

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Todays lecture topics: Absorptivity and emissivity (W&H 6.5)

Kirchhoffs law Selective absorbers and emitters Disposition of monochromatic radiation

ATMS 455 Physical

Meteorology

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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.

*

E

E

http://ceos.cnes.fr:8100/cdrom-97/ceos1/science/baphygb/chap3/chap3.htm

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Introduction

If we neglect the dependence of emissivityon the wavelength (color) of radiation, wedefine a gray body emissivity :

4* T

E

E

E

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Introduction

We also define the quantities absorptivity (a ) and gray body absorptivity (a) whichare measures of the ratio of the irradianceabsorbed by a particular body to that whichis incident upon it. The absorptivity of a

blackbody is equal to one at all wavelengthsof radiation.

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Kirchhoffs law

In qualitative terms Materials which are strong absorbers at a

particular wavelength are also strong emitters atthat wavelength

Materials which are weak absorbers at a particular wavelength are also weak emitters atthat wavelength

The validity of Kirchhoffs law is not dependent upon whethera 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 theabsorptivity

When absorptivity is a function of

wavelength of radiation, the situation forradiative equilibrium temperature can bequite different Prob. 6.7, p. 293

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Snow is a relatively weak absorber atvisible 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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How does a selectively absorbing gas behave? Prob. 6.8, p. 293

http://www.eballoon.org/pictures/alpine.html

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Calculate the radiative equilibriumtemperature 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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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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Whenever a gas that is a weak absorber inthe visible and a strong absorber in the IR is

a constituent of a planetary atmosphere, itcontributes toward raising the surfacetemperature of the planet (greenhouseeffect) In order to satisfy radiation balance, the surface must

compensate by emitting more radiation than it would inthe absence of such an atmosphere (must radiate at ahigher temperature)

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1 r a

Disposition of monochromaticradiation

incident upon an opaque (non -transparent) surface:

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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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Shortwave reflectivity of various surfaces

Petty 2004

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1 r a


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

http://www.worldstar.com/~collmer/

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