Noctilucent clouds, or night-shining clouds, are tenuous cloud-like phenomena in the upper atmosphere of the Earth. They consist of ice crystals. They are the highest clouds in Earth's atmosphere, located in the mesosphere at altitudes of around 76 to 85 km (47 to 53 mi). 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 the Earth's shadow.

Noctilucent clouds are not fully understood and are a recently discovered meteorological phenomenon. 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. 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.

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