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We humans are perfectly adapted to survive at ground level,
我们人类完全适合在地表上生存
where the air has all the right conditions for life.
地表大气的状况正适合生命的存在
It's the right temperature, right pressure
合适的温度，合适的气压
and it's got the right mix of gases.
合适的大气的组成成分
Down here, there's plenty of oxygen about,
下边这儿，有足够的氧气
but after about three kilometres the air's so thin,
但是大约三千米以后空气会很稀薄
if you didn't have this mask, you'd slip into unconsciousness.
所以如果没有这个防护面具，你会变的意识不清
I'd better put it on.
我最好还是戴上
121, continue your approach. Flight information...
一二一，起飞
The atmosphere that surrounds our planet
我们周围的大气
is made up of four key layers, each very different.
由四个主要的大气层组成，每一个都不同
Although I'm above the clouds, I'm still at the very bottom,
尽管是在云彩上面，我仍然是在大气层的底部
in a layer called the troposphere.
这个气层叫做对流层
It's a narrow band,
它宽度很窄
usually more than six miles thick.
通常超过6英里的厚度
The troposphere is a rich soup of oxygen-rich air.
对流层是一个充满暖湿氧气的气层
It's unstable, chaotic and unpredictable but life depends upon it.
它很不稳定，空气运动不规则而且不可预知，但是生命的存在取决于它
And in just a couple of minutes, I'll be leaving it behind.
只是在几分钟之后，它就会在我下面了
And now I'm 40,000 feet above the Earth.
我现在离地球有4万英尺高
