Noctilucent clouds, the highest clouds on Earth, had been observed in portions of the Northern Hemisphere last month in Canada.

夜光云是地球大气层中最高的云，上月在北半球的加拿大境内，人们观测到了这一罕见的“云种”。

The word almost sounds like it is from space, and indeed, noctilucent clouds are different than their more familiar, terrestrial cousins. If you've never heard of noctilucent clouds, that's because they occur under strict atmospheric conditions and are visible only during a handful of weeks each year.

“夜光云”这名字听上去便带有神秘色彩，的确，夜光云与我们平时看到的云朵在性质上都大不一样，它是深曙暮期间出现于地球高纬度地区高空的一种发光而透明的波状云，只有在特定的大气条件下才会形成，一年中可观测到的时间只有短短几周。

Noctilucent clouds were first seen in 1885 after the Indonesian volcano Krakatoa erupted and hurled plumes of volcanic ash miles into the Earth's atmosphere. While they may look like cirrus clouds, noctilucent clouds form differently than the more common, icy, wispy clouds. For example, noctilucent clouds develop in a different section of the atmosphere than cirrus, cumulus and even thunderstorm clouds.

1885年，夜光云在1885年印尼喀拉喀托火山爆发后，大量火山灰冲进进入数英里高空的大气层，人们在这个时候才第一次见到夜光云大气层后才被人们意外发现。虽然夜光云的外形与常见的卷云或许类似，但是其形成原因与那些由冰晶颗粒组成的一缕缕常见的的云朵大有不同。最主要的原因，还在于它们形成的大气层有别：我们平常见到的卷云、积云和暴雨云都形成于对流层，而夜光云则是在遥远的中间层。

Additionally, cirrus clouds are often seen during the day, while noctilucent clouds develop only in the summer and are best viewed during twilight, after the sun has dropped below the horizon in the evening or before it rises above the horizon in the morning.

真正让夜光云显得特殊的，是它不同于与其他通常在白天才能看清的云朵相比，夜光云只在夏日的日出黄昏之时才露出真容。当太阳还没从地平线升起，或是太阳落山后夜幕降临落下后不久，大地处于黑暗之中，但是因为位于中间层的夜光云够高，所以它便会在“黑夜”里继续反射太阳的光线，闪现晶莹的形态，十分美丽。

The image below illustrates the layers of Earth's atmosphere. Most of our weather occurs in the lowest layer, called the troposphere, which spans from the Earth's surface to as high as 18 kilometers into the sky.

下图所描绘的便是地球大气的各个层级。人类感受到的天气变化，大部分都发生在最底层，也就是我们所在的“对流层”，根据纬度、季节等的不同，对流层的高度会发生变化，最高能达到距地面18千米。

Just above the troposphere is the stratosphere. The stratosphere is layered in temperature, with warmer layers higher and cooler layers closer to the Earth. Further above it is the mesosphere. Noctilucent clouds are exactly formed in this layer nearly 80 kilometers above the Earth’s surface. At this height in the atmosphere, so few air molecules exist that it becomes extremely difficult to produce ice crystals and then clouds, which means noctilucent clouds are very rare to form.

在对流层的上方是平流层，该层的温度形成分层，与地面越近越冷，越远则越暖和。平流层再往上便是中间层，大约位于可以达到距地面80千米的高度，夜光云便形成于此。在这里，空气稀薄，各类分子分散，很难生成冰晶和云朵，这也解释了夜光云是一种罕见现象的原因。

terrestrial: adj. 陆地上的；地球上的

wispy: adj. 一缕缕的；成束的

molecule: n. 分子