Blackbody Radiation

1
Blackbody Radiation

• The sun and earth can be approximated as
  blackbodies

2
Gray body
Black body
Real Absorber
3
This figure was prepared by Robert A. Rohde for
the Global Warming Art project.
4
Pressure Broadening
Molecules are selective absorbers
High altitude
saturation line
Low altitude
5
Beer-Lambert Law
For small absorption, amount of light absorbed is
linear in light intensity and amount of absorber
s
sds
Absorbing substance
F(s)
F(sds)
dF -kabsraFds
distance
Absorption coefficient
Light intensity
Absorber density
F F0 exp-kabsras
6
Wavelength Dependence Gray Atmosphere
Source
Detector
Gas Cell
Filter
1cm-1
F(sds) F(s)exp-kabsrads F(s)exp-t
7
Wavelength Dependence Structured Atmosphere
Source
Detector
Gas Cell
Filter
0.5 cm-1
Now, only ½ the radiation is absorbed because of
the lineshape
8
Beer-Lambert Law in the Atmosphere
9
Infrared Radiative Transfer
Assume plane-parallel atmosphere. Is this a
good assumption?
z
dz
q
ds
Iv
10
Infrared Radiative Transfer
Schwarzchilds Equation
See e.g. Chapter 4.2 in Principles of Planetary
Climate, by R. T. Pierrehumbert
11
Vertical Structure of the Atmosphere
Major Components of the Atmosphere
Jacob, 1999
12
Barometric Law
There is a pressure-gradient force pushing air
up. This must be balanced by gravity (i.e. the
weight of the air parcel).
Scale height 7.4 km at 250 K
13
Why is water different?
Water
Most gases (e.g. CO2)
pressure
14
Vertical Structure and Optical Depth
15
Adiabatic Lapse Rate
Height
Adiabatic profile
Temperature
16
Wet Adiabatic Lapse Rate
Water complicates things b/c there is an energy
change associated with condensation and
evaporation
wet adiabatic
adiabatic
17
The Greenhouse Effect Revisited
Add CO2
Equilibrate
255 K
288 K
288 K
293 K
18
The View From Space
O3
H2O
CO2
30 N
40 N
75 S
19
(No Transcript)
20
Key Concepts

• The Earth system is in radiative equilibrium with
  incident solar radiation, but
• Each area of the Earth is in local
  radiative-convective equilibrium
• The greenhouse effect is important for
  sustaining life on Earth
• IR-active gases in the Earths atmosphere
  selectively absorb radiation, so
• Radiation is emitted to space from different
  altitudes
• The pressure/temperature structure of the
  atmosphere