万物简史 第375期:进入对流层(16)

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　　Much higher cirrus clouds are composed of ice, and the zone between the edge of the cloud and the air beyond is not so clearly delineated, 而高得多的卷云由冰和该云的边缘与外面的空气之间的区域组成，

　　which is why they tend to be blurry² at the edges. 故而它的边缘往往是模模糊糊的）

　　For all the heft and fury of the occasional anvil-headed storm cloud, the average cloud is actually a benign³ and surprisingly insubstantial thing. 偶尔出现的砧头状雷雨云尽管气势汹汹，实际上在一般情况下都是温和而不实的东西。

　　A fluffy⁴ summer cumulus several hundred yards to a side may contain no more than twenty-five or thirty gallons of water— "about enough to fill a bathtub," as James Trefil has noted⁵. 夏天一朵绒毛状的积云，虽然每一边都伸展数百米，所含的水分却不足100-150升——“大约够注满一个浴缸”，正如詹姆斯·特雷菲尔说的。

　　You can get some sense of the immaterial quality of clouds by strolling through fog—which is, after all, nothing more than a cloud that lacks the will to fly. 要是你想知道云是一种华而不实的东西，你可以在雾里走一走——雾只不过是一种没有决心远走高飞的云。

　　To quote Trefil again: "If you walk 100 yards through a typical fog, you will come into contact with only about half a cubic inch of water—not enough to give you a decent drink." 我们再引特雷菲尔的话： “要是你在普通的雾里走91米，你只会接触到大约8立方厘米的水——还不够你好好喝一口。”

　　In consequence, clouds are not great reservoirs of water. 因此，云不是大水库。

　　Only about 0.035 percent of the Earth's fresh water is floating around above us at any moment. 在任何时候，地球上只有大约0.035%的淡水飘浮在我们的头顶。

　　Depending on where it falls, the prognosis for a water molecule varies widely. 水分子的结局差别很大，取决于它落在哪里。

　　If it lands in fertile soil it will be soaked up by plants or reevaporated directly within hours or days. 要是它落在肥沃的土壤里，它会被植物吸收，或在数小时或数天内再次直接蒸发。

　　If it finds its way down to the groundwater, however, it may not see sunlight again for many years—thousands if it gets really deep. 然而要是进人了地下水，它也许在好多年里——好几千年里，如果它流到确实很深的地方的话——再也见不着太阳。

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