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They're thin, wispy clouds that can only be seen in the summer at high latitudes.
它们是稀薄的纤细的云只有在高纬度地区的夏天才能看见
Beginning at nearly 50 miles high,
从50英里(85公里)处开始
there's the fourth layer, the thermosphere.
是第四个气层：热气层
Here, the atmosphere is so thin that beyond 50 miles
这里的大气如此稀薄以至于50英里以外
we approach the beginnings of space.
就可以肯定地认为是宇宙的开始
The space shuttle orbits the Earth in the thermosphere.
正是在这个气层，航天飞机绕着地球飞行
It's also where nitrogen and oxygen
也是在这里，氮气和氧气
interact with the Sun's lethal solar wind,
被太阳风辐射
creating the aurora around the Earth's poles.
创造了环绕地球磁极的极光
There's another way of looking at the atmosphere.
但是这有另一种看待大气层的方式
If you could unwrap the atmosphere from the surface of the Earth
如果你把地球表面的大气层展开
and put it all into a ball, this is what it would look like.
并把它变成一个球，就像我们所看到的这样
Its weight is equal to a layer of water 34 feet deep, covering the Earth.
其重量只相当于覆盖地球34英尺深的水
The layer that we spend our lives in, the troposphere, is just a narrow band.
我们生命所依赖的平流层只是一个窄带
It's a finely balanced mixture of different gases essential for life
它由生命必须的不同气体平衡混合而来
but it also has a physical presence.
但它也有实体存在
