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nacreous cloud

noun
  1. 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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Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018

nacreous cloud in Science

nacreous cloud

[nākrē-əs]
  1. 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.
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