Nacreous Clouds or Polar Stratospheric
As the clouds are
so high they can often receive light from
below the horizon due to the curvature of
the earth so appearing very bright at dusk
These pictures were taken
in the Antarctic winter when the sun didn't
appear above the horizon for long and much
of the time any natural light was twilight.
The winter gives ideal conditions for the
formation of such clouds and also for observation
of such clouds.
Picture taken 23/08/2009
12.41 - McMurdo Antarctica.