Homepage of Michael Theusner
Noctilucent clouds exist in Summer high up in the mesosphere, at an altitude of 76 to 85 km. They occur when temperature in the mesophere reaches its annual minimum below -120°C. NLCs usually are found in the polar regions, but, also in mid latitudes. When the sun is only slightly below the horizon in summer in mid latitudes, the NLCs are still illuminated by sunlight while it is already dark below. Then they become visible as whispy silvery clouds against the dark night sky. In Hannover, where I took the images, they can usually be seen between early June and late July. Best is the time when the sun is between 8° and 12° below the horizon.
One of the best NLC events I have so far observed. Very bright, fantastic wave structures and Kelvin-Helmholtz-Instabilities. The NLC were already faintly visible shortly before 21 UT. I then went to a field road near Lenthe to have an unobstructed view. The NLCs were still visible after 23:15 UT. However, I ran out of space on my memory card and went home. An hour later I returned to observe the morning display which did not take place...