当潮湿空气遇冷降至一定温度时，其中包含的水蒸气便聚集成小水滴或小冰晶，它们组成了云。
Clouds form when humid air cools enough for water vapor to condense into droplets or ice crystals.
这种相当常见的气象景象之所以会出现，是因为覆盖广大天空的高层薄云中含有数以百万计的小冰晶，而每一个冰晶的作用就像是小型的棱镜。
This fairly common sight occurs when high thin clouds containing millions of tiny ice crystals cover much of the sky. Each ice crystal ACTS like a miniature lens.
水分子在冰晶里的排列留下许多小空隙，而在液体水里，从统计学意义上讲，水分子们紧密地一个挨着一个，没有空隙。
The arrangement of water molecules in an ice crystal leaves small interstices, whereas the water molecules in liquid water are statistically cheek-to-jowl.
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