(饱和蒸汽压)(http://glossary.ametsoc.org/wiki/Saturation_vapor_pressure)由美国气象学会定义:>系统的蒸汽压,在给定的温度下,物质的蒸汽平衡的平面的,物质的纯液体或固相;也就是说,系统达到饱和蒸汽压的但不过度饱和。>纯物质的饱和蒸汽压,对指定的父阶段,是物质的固有属性,是温度的函数。水蒸气的饱和压力在过冷液体明显不同于在冰。当水蒸气存在过冷液体的水,水是液体和水蒸气之间交换不断形成。当达到平衡时,在两个方向上都有平等的相变。如果空气变得饱和,没有更多的水蒸气可以添加到它,因为它会立即冷凝液体形式。在这个温度,蒸汽压力是对水的饱和蒸汽压。记住,水可以存在低于0°C -云通常不完全冻结,直到温度下降到-40°C,虽然这取决于许多因素和细节进行研究。当水蒸气存在冰的身体,有一个相交换。 At a certain temperature and pressure, the air is saturated, any water added will precipitate as ice. At this temperature, the vapour pressure is the saturation vapour pressure with respect to ice. By "Body of" you should probably not so much think of a frozen lake or liquid ocean but rather of small cloud droplets or ice particles. This graph from Vermont State College shows the saturation vapour pressure over water or ice: [![Saturation vapour pressure][1]][1] *Source: [Vermont State College](http://apollo.lsc.vsc.edu/classes/met130/notes/chapter7/eswgtesi.html)* As your source stated, this difference is important for meteorology as it affects the formation of clouds and precipitation. Now as to **why** the saturation vapour pressure is different with respect to water than with respect to ice is another question. You may find more expertise for that on Chemistry Stack Exchange than here on Earth Science. My atmospheric science textbooks in any case don't go into detail there. [1]: https://i.stack.imgur.com/9xzTF.png