images

Noctilucent clouds

by

Alex Tudorica/Novapix

Reference

t-met03-49005

Date

2009-11-05

These rare clouds are called noctilucent or "night shining" as they seem to glow in the dark. They are caused by ice crystals at about 80 km altitude. These crystals reflect sunlight when the sun is well below the horizon. They are properly known as polar mesospheric clouds and are under intense study as it remains a mystery as to how they are formed.
Image taken in july 2009 in Russia.