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They're thin, wispy clouds that can only be seen in the summer at high latitudes.
它們是稀薄的纖細(xì)的云只有在高緯度地區(qū)的夏天才能看見
Beginning at nearly 50 miles high,
從50英里(85公里)處開始
there's the fourth layer, the thermosphere.
是第四個氣層：熱氣層
Here, the atmosphere is so thin that beyond 50 miles
這里的大氣如此稀薄以至于50英里以外
we approach the beginnings of space.
就可以肯定地認(rèn)為是宇宙的開始
The space shuttle orbits the Earth in the thermosphere.
正是在這個氣層，航天飛機(jī)繞著地球飛行
It's also where nitrogen and oxygen
也是在這里，氮?dú)夂脱鯕?/p>
interact with the Sun's lethal solar wind,
被太陽風(fēng)輻射
creating the aurora around the Earth's poles.
創(chuàng)造了環(huán)繞地球磁極的極光
There's another way of looking at the atmosphere.
但是這有另一種看待大氣層的方式
If you could unwrap the atmosphere from the surface of the Earth
如果你把地球表面的大氣層展開
and put it all into a ball, this is what it would look like.
并把它變成一個球，就像我們所看到的這樣
Its weight is equal to a layer of water 34 feet deep, covering the Earth.
其重量只相當(dāng)于覆蓋地球34英尺深的水
The layer that we spend our lives in, the troposphere, is just a narrow band.
我們生命所依賴的平流層只是一個窄帶
It's a finely balanced mixture of different gases essential for life
它由生命必須的不同氣體平衡混合而來
but it also has a physical presence.
但它也有實體存在
