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.