Noctilucent Clouds at Dubna, Russia. Dubna, some 50 miles north of Moscow, is a major centre for physics research. Image by Alex Tudorica. �Alex Tudorica, shown with permission.

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Noctilucent clouds shine by reflecting sunlight. They are 50-53 miles high and in summer they are still in sunlight long after the sun has set at ground level. They are composed of extremely small ice crystals (1/10,000 mm across) in a layer slightly below the mesopause - at less than -125 Celsius this is the coldest part of Earth�s atmosphere.