Clouds, Oslo, Norway. A
view of the 16/17th July display by Erik Schibbye.
Image ©Erik Schibbye, shown with permission.
Erik's image shows their wonderfully intricate streak
and billow structure.
That is one way of distinguishing them from lower clouds because 80-85km
high noctilucent clouds
look sharper if viewed through low-power binoculars. They are shaped
waves, undulations between atmospheric layers of
different density like waves on the sea. The waves can start
much closer to the earth's surface in wind flow over mountains and even
thunderstorms. Higher up, the jet stream is another source. The initially
small disturbances are amplified as the waves travel upwards into the
incredibly rarified and cold gas of the