Noctilucent clouds in Lithuania
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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.
Generally colorless or light blue, noctilucent clouds are distinct in their patterns of streaks, waves, and whorls. It's not fully known why they happen, though some recent studies suggest methane levels could contribute to atmospheric water vapor that rises high into the mesosphere where they form. What is known is that they were first observed in 1885.