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Noctilucent 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 by gravity 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 mesosphere.