万物简史 第362期:进入对流层(3)

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　　After you have left the troposphere the temperature soon warms up again, 离开对流层以后，由于臭氧层的吸收作用

　　to about 40 degrees Fahrenheit, thanks to the absorptive effects of ozone³ (something else de Bort discovered on his daring 1902 ascent). （这是博尔特在1902年那次勇敢的攀升中的又一发现），温度很快又会上升到大约4摄氏度。

　　It then plunges⁴ to as low as -130 degrees Fahrenheit in the mesosphere 到了中间层，温度又聚降到零下90摄氏度，

　　before skyrocketing to 2,700 degrees Fahrenheit or more in the aptly named but very erratic⁵ thermosphere, 然后到了那个顾名思义的热层又一下子上升到1500摄氏度以上，

　　where temperatures can vary by a thousand degrees from day to night—though it must be said that "temperature" at such a height becomes a somewhat notional concept. 而且热层的日夜温差可达500摄氏度以上——必须指出。在这样高度的温度多少已经成了个理论概念。

　　Temperature is really just a measure of the activity of molecules⁷. 温度其实只是盘度分子活动程度的一个标准。

　　At sea level, air molecules are so thick that one molecule⁶ can move only the tiniest distance, 在海平面高度，空气分子密度很大，一个分子只要运动极小的距离，

　　about three-millionths of an inch, to be precise—before banging into another. 说得确切一点，大约百万分之一厘米一就会砰地撞上另一个分子。

　　Because trillions of molecules are constantly colliding, a lot of heat gets exchanged. 由于几万亿个分子在不停撞击，大量的热量得到交换。

　　But at the height of the thermosphere, at fifty miles or more, 但是，在热层的高度，即在80公里以上的高度，

　　the air is so thin that any two molecules will be miles apart and hardly ever come in contact. 空气那么稀薄，两个分子相隔数公里，几乎没有接触的机会。

　　So although each molecule is very warm, there are few interactions between them and thus little heat transference. 因此，虽然每个分子的动能都很高，但彼此之间几乎没有影响，因此没有多少热量传递。这对卫星和宇宙飞船来说是个好消息。

　　This is good news for satellites and spaceships because if the exchange of heat were more efficient any man-made object orbiting at that level would burst into flame. 这是因为，要是热量交换的频率较高，在那个高度运行的任何人造物体都会熊熊起火。

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