Noctilucent clouds

It's around this time of year when some lucky people get to witness these rare, wondrous clouds. Known as noctilucent, or 'night-shining,' clouds, they're the highest clouds in our sky and are only visible during summer. They're made up of icy dust glowing at the edge of space, roughly 50 miles above the planet's surface. The trick to seeing them is to gaze up into the sky at twilight, when sunlight is not reaching the Earth's surface, but is still shining through the high-altitude noctilucent clouds. These clouds occur more often at high latitudes but have been seen lower than 50° north and south.
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