Polar Stratospheric Clouds - Nacreous Clouds 3
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.
Photo credit - Alan Light