Noctilucent clouds occur at a height of about 80 km and consist of very tiny ice crystals. In order for noctilucent clouds to form, the temperature has to drop below -120 °C, which only happens for some weeks around summer solstice. Because of their extreme height, noctilucent clouds still receive light from the sun after sunset and before sunrise. This is why they look like white shapes againts the dark blue background. More information about these photos.
[ visible light || IR light ] ⇛ Interestingly, the noctilucent clouds do not show up in when they are photographed in infrared! On the other hand, some crepuscular rays are clearly visible in infrared, while they are hardly visible in visible light. (2014-07-12)
(2014-07-12)
(2014-07-12)
(2015-08-04)
(2015-08-04)
(2016-07-24)
(2016-08-09)
(2018-08-09)