and this water-vapour mingles with the air, and is carried along by the wind. 随后, 水蒸气与空气混合在一起并随风飘动。

　　While it remains in the form of vapour, however, we cannot see it, because it is quite colourless and transparent. 然而, 只要这些水分保持蒸汽状态, 我们就无法看到, 因为它是无色透明的。

　　The air near the surface of the earth is warmer than that which is higher up, 靠近地球表面的空气温度比高空的空气温度要高一些,

　　so when the water-vapour rises very high above the ground, it is cooled. 因此, 当水蒸气漂浮到高空时就会被冷却。

　　We have learned that when water-vapour is cooled it is condensed. 我们知道, 水蒸气遇冷之后就会产生冷凝。

　　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; 因此, 水蒸气首先变成了非常细小的水粒——也就是我们所说的水雾——并构成了云朵,

　　and then these specks of water join together to form drops of rain. 然后这些水粒汇聚在一起成为了雨滴。

　　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. 你一定淋过雨而且尝到过雨水的味道, 因此你知道, 雨水属于淡水。

　　But the water of the sea, from which that rain-water came, is quite salt. 然而, 雨水来自海水, 而海水中却含有很多盐分。

　　How is it that rain-drops of fresh water can come from the salt sea?  那么, 海里的咸水怎么会变成不含盐的雨水呢?

　　In last Lesson you were told that salt water can be made fresh by distilling it. 通过上一课的学习, 你了解到, 咸水可以通过蒸馏而变成淡水。

　　Now rainwater is really distilled water. 实际上, 雨水是名符其实的蒸馏水。

　　If you boil salt water in a small saucepan until the water has all passed away as steam, 如果你用炖锅来煮沸咸水, 等到所有的水分都以蒸汽的方式蒸发之后,

　　the salt that was dissolved in it will all be left behind as a white crust on the bottom of the saucepan. 咸水中溶解的盐分就会变成锅底上一层白色的粉末。

　　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. 太阳的热量正是通过这种方法将海水变成蒸汽, 而海水中溶解的盐分则留在了海洋之中。

　　So when this vapour is changed back into water again, it forms pure fresh water. 因此, 当水蒸气再次变成水滴的时候, 淡水应运而生。