Definition of "noctilucent" [noctilucent]

• Luminous at night. Used especially of certain high clouds. (adjective)

American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

• "That route saves fuel and avoids high-altitude ice clouds, called noctilucent clouds, that form during the summer between 50 degrees and 70 degrees north and south of the equator."
• "The clouds are intended to simulate naturally-occurring phenomena called noctilucent clouds, which are the highest clouds in the atmosphere."
• "Scientists claim these "noctilucent" clouds -- formations that are similar to what was seen after the Tunguska event -- reach the poles because water vapor appears to be quickly traveling in two dimensions, rather than three."