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Noctilucent clouds are thin, glowing veils that appear high in the summer sky, far above ordinary weather. Forming near the edge of space, about eighty kilometres up, they are made of tiny ice crystals coating specks of dust, some of which may come from meteors. They shine only at twilight, when the Sun has set for an observer on the ground but still lights the clouds from below the horizon. First recorded in the late nineteenth century, they appear to be seen more often now, and some researchers link this trend to changes in the upper atmosphere.