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"Stunning noctilucent clouds over a misty Stonehenge during twilight this morning."
Posted by @ST0NEHENGE on twitter.
Photo credit Nick Bull

Noctilucent clouds are composed of tiny crystals of water ice up to 100 nm in diameter and exist at a height of about 76 to 85 km, higher than any other clouds in Earth's atmosphere.
They are only visible during astronomical twilight, with the Sun below the horizon.
Noctilucent roughly means "night shining" in Latin.

https://en.wikipedia.org/wiki/Noctilucent_cloud
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