a rarely seen, luminous, iridescent cloud shaped like a cirrus or altocumulus, approximately 15 mi. (24 km) above the earth, and of unknown composition.
nacreous cloud (nā'krē-əs) Pronunciation Key
A cloud resembling a cirrus, showing iridescent coloration when the Sun is several degrees below the horizon. These clouds appear mostly during winter in regions of high latitude.
(nā'krē-əs) Pronunciation Key
