NOCTILUCENT CLOUD

rare cloud form, probably composed of ice crystals and meteoric dust, that occurs at a higher altitude than any other cloud form (about 82 km [50 miles]). It is silvery or bluish white and is visible only on summer nights in high latitudes. The ice crystals form because this level is the coldest in the entire upper atmosphere; even the minute amounts of water vapour present in this thin, dry air freeze. The cloud often exhibits a tenuous, wavy pattern that indicates the existence of strong winds at that altitude.