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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.