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The mesosphere is has a very low density of gas molecules.

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The Highest Clouds

The Highest Clouds

Credit: NASA
Source: http://science.nasa.gov/science-news/science-at-nasa/2012/07aug_meteorsmoke/
License: CC BY-NC 3.0

Noctilucent clouds (NLCs) are the highest clouds in the sky. These shimmering, iridescent clouds appear at late twilight when the sun strikes water crystals in the upper atmosphere.

Amazing But True!

Credit: NASA
Source: http://www.universetoday.com/102536/noctilucent-clouds-electric-blue-visitors-from-the-twilight-zone/
License: CC BY-NC 3.0

Noctilucent clouds appear in the uppermost atmosphere, almost to outer space [Figure2]

  • Noctilucent are part of the polar mesospheric clouds of the upper atmosphere, appearing at 83 km above the surface.
  • Noctilucent clouds appear near the polar regions in summer: November through February over Antarctica and May through August over the Arctic.
  • Methane concentrations in the atmosphere have doubled in the past century, and may be spurring on NLC formation.

Show What You Know

With the links below, learn more about noctilucent clouds. Then answer the following questions.

  1. When and after what event were noctilucent clouds first noticed?
  2. What is the relationship between the event mentioned in #1 above and the appearance of notilucent clouds?
  3. Why do NLCs appear in the summer, but not at other times of year?
  4. How has the appearance of NLCs changed since they were first viewed?
  5. What is the relationship between NLCs and global warming?

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