page_title: Noctilucent cloud
section_title: General
hierarchical_section_title: Noctilucent cloud / General
caption_attribution_description: English: Noctilucent clouds over Uppsala, SwedenSvenska: Årsta i Uppsala vid 4H gården
context_page_description: Noctilucent clouds, or night shining clouds, are tenuous cloud-like phenomena in the upper atmosphere of Earth. They consist of ice crystals and are only visible during astronomical twilight. Noctilucent roughly means "night shining" in Latin. They are most often observed during the summer months from latitudes between ±50° and ±70°. They are visible only during local summer months and when the Sun is below the observer's horizon, but while these very high clouds are still in sunlight. Recent studies suggest that increased atmospheric methane emissions produce additional water vapor once the methane molecules reach the mesosphere – creating, or reinforcing existing noctilucent clouds.
  They are the highest clouds in Earth's atmosphere, located in the mesosphere at altitudes of around 76 to 85 km. They are too faint to be seen in daylight, and are visible only when illuminated by sunlight from below the horizon while the lower layers of the atmosphere are in Earth's shadow.
context_section_description: No confirmed record of their observation exists before 1885, although they may have been observed a few decades earlier by Thomas Romney Robinson in Armagh.  Doubts now surround Robinson's out-of-season records, following observations, from several points around high northern latitudes, of NLC-like phenomena following the Chelyabinsk superbolide entry in February 2013 (outside the NLC season) that were in fact stratospheric dust reflections visible after sunset.
  Noctilucent clouds can form only under very restricted conditions during local summer; their occurrence can be used as a sensitive guide to changes in the upper atmosphere. They are a relatively recent classification. The occurrence of noctilucent clouds appears to be increasing in frequency, brightness and extent.
caption: Noctilucent clouds over Uppsala, Sweden
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