英國學(xué)生科學(xué)讀本 Lesson 89 云和雨(2)

Every day the sun's rays change thousands of tons of liquid water into water-vapour; and this water-vapour mingles with the air, and is carried along by the wind. While it remains in the form of vapour, however, we cannot see it, because it is quite colourless and transparent.

每天, 太陽光都能夠?qū)?shù)千噸的液態(tài)水轉(zhuǎn)化為水蒸氣;隨后, 水蒸氣與空氣混合在一起并隨風(fēng)飄動。然而, 只要這些水分保持蒸汽狀態(tài), 我們就無法看到, 因?yàn)樗菬o色透明的。

The air near the surface of the earth is warmer than that which is higher up, so when the water-vapour rises very high above the ground, it is cooled. We have learned that when water-vapour is cooled it is condensed. So the water-vapour is first changed into those very tiny specks of liquid water which have been called water-dust, and appears as clouds; and then these specks of water join together to form drops of rain.

靠近地球表面的空氣溫度比高空的空氣溫度要高一些, 因此, 當(dāng)水蒸氣漂浮到高空時(shí)就會被冷卻。我們知道, 水蒸氣遇冷之后就會產(chǎn)生冷凝。因此, 水蒸氣首先變成了非常細(xì)小的水粒——也就是我們所說的水霧——并構(gòu)成了云朵, 然后這些水粒匯聚在一起成為了雨滴。

You must at one time or another have tasted drops of rain as they blew against your face, and so you know that the rain-water is fresh water. But the water of the sea, from which that rain-water came, is quite salt. How is it that rain-drops of fresh water can come from the salt sea?

你一定淋過雨而且嘗到過雨水的味道, 因此你知道, 雨水屬于淡水。然而, 雨水來自海水, 而海水中卻含有很多鹽分。那么, 海里的咸水怎么會變成不含鹽的雨水呢?

In last Lesson you were told that salt water can be made fresh by distilling it. Now rainwater is really distilled water. If you boil salt water in a small saucepan until the water has all passed away as steam, the salt that was dissolved in it will all be left behind as a white crust on the bottom of the saucepan. In exactly the same way the heat of the sun turns the water of the sea into vapour, but not the salt that is dissolved in it. So when this vapour is changed back into water again, it forms pure fresh water.

通過上一課的學(xué)習(xí), 你了解到, 咸水可以通過蒸餾而變成淡水。實(shí)際上, 雨水是名符其實(shí)的蒸餾水。如果你用燉鍋來煮沸咸水, 等到所有的水分都以蒸汽的方式蒸發(fā)之后, 咸水中溶解的鹽分就會變成鍋底上一層白色的粉末。太陽的熱量正是通過這種方法將海水變成蒸汽, 而海水中溶解的鹽分則留在了海洋之中。因此, 當(dāng)水蒸氣再次變成水滴的時(shí)候, 淡水應(yīng)運(yùn)而生。

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