nacreous cloud

nacreous cloud

noun
a rarely seen, luminous, iridescent cloud shaped like a cirrus or altocumulus, approximately 15 miles (24 km) above the earth, and of unknown composition.
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American Heritage
Science Dictionary
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.
The American Heritage® Science Dictionary
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