Noctilucent clouds over St Mary's lighthouse in Whitley Bay, on the North East coast of England. (Credit: Owen Humphreys/PA Images)
Noctilucent clouds are the highest clouds in the Earth's atmosphere, forming in the mesosphere about 50 miles above the surface. They only appear in the UK during the summer months, when the mesosphere becomes cold enough to allow ice to form on suspended dust particles floating in the atmosphere, and the sun only just sets below the horizon at night time.
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