Geoscience Reference
In-Depth Information
(
d
- a function of day of year), and the
sunset hour angle
, (
w
s
- a function of
latitude and day of year).
Atmospheric attenuation
: typically 25% of solar radiation is absorbed for
clear skies and 75% for overcast skies (attenuation is by scattering by gas
molecules and dust, and absorption by radiatively active gases and cloud):
solar radiation at the ground can be estimated either from fractional cloud
cover or from the fraction of daytime hours with bright sunshine.
●
Net longwave radiation
: longwave exchange with the surface can be
estimated using a version of the
Stefan-Boltzmann Law
that includes an
effective emissivity (dependent on humidity) and a cloud cover correction
factor (estimated from the attenuation of solar radiation) that is different for
humid and arid conditions.
●
Net radiation
: is obtained by adding the net solar radiation (allowing for
albedo) to the net longwave radiation; at the daily time scale it has a tempera-
ture dependent negative (longwave) offset upon which is superimposed a
positive solar radiation input whose magnitude depends on latitude, day of
the year, fractional cloud cover, and time of day.
●
References
Gates, D.M. (1980)
Biophysical Ecology
. Springer-Verlag, New York.
Monteith, J.L. & Unsworth, M.H. (1990)
Principles of Environmental Physics
. Edward
Arnold, London, UK.
