The sky close to the rising or setting sun
is colored in a red hue. Small atmospheric particles scatter
the red light of the sun favourably into the forward direction.
On the other hand blue light is scattered much more to larger angles.
Thus a considerable fraction of the blue light is missing close to
the suns direction. At larger angles, especially 90 degrees, the
effect results into the usual blue color of the clear sky
(see a wide angle image of the
twilight sky).
The air in front of the mountains is not illuminated directly by sunlight but
by the already scattered blue light of the sky overhead (multiple scattering).
This blue haze is often called opalescence, since the direct colours of
the mountains are washed out to a pale hue.
