incident on a slab of an absorbing material which is in local
thermodynamic equilibrium. Figure 12 summarises the radiant energy processes taking place
including reflection, absorption, re-emission and transmission.Consider a spectral radiance incident on a slab of an absorbing material which is in local
thermodynamic equilibrium. Figure 12 summarises the radiant energy processes taking place
including reflection, absorption, re-emission and transmission.
Figure 12: Summary of radiative transfer process for monochromatic radiance incident on a slab of absorbing medium. Adapted from Smith (1985), p393.
The reflected radiation is 
The transmitted radiation is 
The absorbed radiation . The emitted radiation is 
For a black body, closely approximated by the earth's surface or a dense cloud, 
so the absorptivity is 1. For a body in local thermodynamic equilibrium (LTE) conservation
of energy means that the absorbed radiation equals the incident radiation less reflection
and transmission contributions, so that:
or
The latter equation says that the processes of absorption, reflection and transmission
account for all the incident radiation falling on a body in LTE. In any atmospheric window
region for which
then
and
Monochromatic radiation incident upon an opaque surface with 
is either absorbed or
reflected, so
At any given wavelength strong reflectors are weak absorbers (e.g. snow at visible
wavelengths) and weak reflectors are strong absorbers (e.g. asphalt at visible
wavelengths). For the atmosphere, the reflection of IR radiation is negligible, since the
radiation wavelength, is large compared to the size of molecules. Therefore for the
atmosphere
Reflectances at solar radiation (visible) wavelengths for various surfaces are given in Table 7 below:
Table 7: Albedo (reflectance in percent) of various surfaces for short wave (solar)
radiation*.
* based on data in Kondratyev (1972)
Dr D C Griersmith
