立陶宛的夜光云 Noctilucent clouds in Lithuania (© ljphoto7/Getty Images)

立陶宛的夜光云 Noctilucent clouds in Lithuania (© ljphoto7/Getty Images)

“夜光”云 ‘Night shining' clouds

Noctilucent clouds

It's around this time of year when some lucky people get to witness these rare, wonderous 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 the sun has already set on the Earth's surface, but the high-altitude noctilucent cloud is still in sunlight. These clouds occur almost exclusively at high latitudes, roughly between about 50° and 70° North or South.

Generally colorless or light blue, noctilucent clouds are distinct in their patterns of streaks, waves, and whirls. 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, making them the newest cloud type.

夜光云

大约在每年的这个时候，一些幸运的人看到了这些罕见的、奇妙的云。夜光云是我们天空中最高的云，只有在夏天才能看到。它们是由在太空边缘发光的冰尘组成的，距离地球表面大约50英里。看到它们的诀窍是在黄昏时抬头仰望天空，这时太阳已经落在地球表面，但高海拔的夜光云仍在阳光下。这些云几乎只出现在高纬度地区，大约在南北50°到70°之间。

通常无色或淡蓝色，夜光云的条纹、波浪和漩涡图案不同。虽然最近的一些研究表明，甲烷含量可能会导致大气中的水蒸气上升到它们形成的中间层，但目前还不完全清楚它们为什么会发生。已知的是，1885年首次观察到甲烷含量，使其成为最新的云类型。