| 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 n. A cloud resembling a cirrus, showing iridescent coloration when the sun is several degrees below the horizon. [From nacre (from the iridescent coloring of mother-of-pearl).] |
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. |