万物简史 第360期:进入对流层(1)

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　　Thank goodness for the atmosphere. It keeps us warm. 谢天谢地，我们有了大气。它使我们有了个温暖的环境。

　　Without it, Earth would be a lifeless ball of ice with an average temperature of minus 60 degrees Fahrenheit¹. 没有大气，地球会是个没有生气的冰球，平均温度只有零下50摄氏度。

　　In addition, the atmosphere absorbs or deflects² incoming swarms³ of cosmic rays, charged particles, ultraviolet rays, and the like. 而且，大气吸收或挡开大盘射来的宇宙射线、带电粒子、紫外线等等。

　　Altogether, the gaseous⁴ padding of the atmosphere is equivalent to a fifteen-foot thickness of protective concrete, 总的来说，厚厚的大气相当于4.5米厚的保护性混凝土，

　　and without it these invisible visitors from space would slice through us like tiny daggers⁵. 没有它，来自太空的这些无形访客会像小小的匕首那样捅进我们的身体。

　　Even raindrops would pound us senseless if it weren't for the atmosphere's slowing drag. 没有大气的牵制作用，连雨点也会把我们打昏在地。

　　The most striking thing about our atmosphere is that there isn't very much of it. 最引人注目的是，我们的大气并不很多。

　　It extends upward for about 120 miles, which might seem reasonably bounteous⁶ when viewed from ground level, 它往上伸展至大约190公里处，从地面上看，它也许显得很多，

　　but if you shrank the Earth to the size of a standard desktop⁷ globe it would only be about the thickness of a couple of coats of varnish⁸. 但如果把地球缩小到书桌上地球仪的大小，大气不过是大约一两层漆的厚度。

　　For scientific convenience, the atmosphere is divided into four unequal layers: 科学上为了方便起见，把大气分成4个厚度不等的层次：

　　troposphere, stratosphere, mesosphere, and ionosphere (now often called the thermosphere). 对流层、平流层、中间层和电离层（现在往往又称热层）。

　　The troposphere is the part that's dear to us. 对流层对我们来说是个十分宝贵的部分。

　　It alone contains enough warmth and oxygen to allow us to function, though even it swiftly becomes uncongenial to life as you climb up through it. 仅对流层包含的热量和氧气就足以使我们活下去，虽然你往上穿越对流层的时候，它很快会变成一个对生命来说是不合意的地方。

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