F. ZAGURY
516
The aerosol extinction term e in Equation (1) is
probably small and could not be disentangled, in these
observations, from the change of slope due to Rayleigh
extinction. It can only diminish the steepness of the 1/λ4
rise of Rayleigh scattering, and the impression of red
with it, which is not what is observed. But the analysis of
sun occultation spectra [20] proves that before the sun
completely sets its light is progressively replaced by
sunlight forward-scattered by aerosols. If this light was in
turn scattered by nitrogen in the direction of the observer,
the wavelength dependence of the spectrum at small
wave-numbers would be 1/λ5 instead of 1/λ4. The red
side of the spectrum would be steepened, and the im-
presssion of red increased. In a cloudy sunrise or sunset
aerosol scattering that follows Rayleigh extinction would
have the same effect. Therefore the possibility that aero-
sols contribute to the red color of the sky at the very be-
ginning of a sunrise, in the latest part of a sunset, or under a
cloudy sky, cannot be ruled out. Evidence of this phe-
nomenon would require observations with a larger wave-
length coverage of the near-infrared spectrum.
p
b
The analytical representation of the spectra and the
way they transform one into the other excludes any sig-
nificant participation of multiple scattering in the color of
the sky. This is not surprising given that Rayleigh scat-
tering is, in contradistinction with forward scattering by
aerosols, nearly isotropic. The lost of photons over all
directions after each scattering renders multiple Rayleigh
scattering a particularly inefficient process.
Molecular absorption by molecules other than ozone
consists in local interruptions of the spectra, is marginal
except for the strong water bands, and was neglected.
Water vapors are important at sunrise (spectra (4)-(6))
but do not appear to influence significantly the color of
the sky.
These observations could be improved in several ways,
especially by extending the wavelength coverage in the
infrared. By combining photography with spectral ob-
servations of the sky and a more systematic planning of
observations, the link between spectra and the palette of
colors of the sky would be better captured. Finally a long
lasting and controversial issue, the green-flash, which is
also observed when the sun is low on the horizon, could
hopefully be solved with spectroscopy. All these issues
suggest interesting research projects that can be studied
from the college level on with a relatively inexpensive
equipment.
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