The beautiful Noctilucent clouds (photo above by Kevin Cho) are slightly misnamed. The name means “night-shining clouds,” but in fact they are only visible between the end of civil twilight and the end of astronomical twilight (and at latitudes north of 45°N or south of 45°S).
Noctilucent clouds are the visible form of polar mesospheric clouds, made up out of ice crystals in the normally very dry almost-vacuum at extremely high altitudes, around 80–90 km up (see NASA image of a polar mesospheric cloud above).
Noctilucent clouds were first identified as a distinct atmospheric phenomenon in 1885 (see the Google ngram below for uses of the phrase). The ice crystals from which noctilucent clouds are formed require both water vapour and dust for crystal growth nuclei. The sources of these ingredients is still mysterious, although at least some of the dust may come from meteors.
Yet another atmospheric phenomenon for the bucket list!