“如果我们把一些水放在阳光下，它就会蒸发。”真的吗?如果我把一些水装在一个密封的瓶子里，放在阳光下，它会蒸发吗?显然不是。为什么不呢?因为瓶子里的空气已经被水饱和了。空气的相对湿度是100%，或者接近100%。你可能已经知道了。所以你应该意识到你的问题有缺陷。水不仅在瓶子里不蒸发，而且在晚上和没有阳光的房子里也会蒸发。 A better question, imho, is: "Why isn't the Earth's atmosphere saturated with water after 4 billion years?" The atmosphere has a mass of about 5E+18 kg (that is 5 followed by 18 0s, eg 5,000,000,000,000,000,000). The water in the oceans has a mass of about 1.3E+21 kg, meaning about 250 times more mass than the atmosphere. Simply put: there's not enough atmosphere to hold all that water. The reason why the atmosphere is not saturated, does not have 100% relative humidity, is that the temperature of the air varies a lot. Both day to night, and season to season. The air is不断地被冷却，水分通过沉淀流失。当这些空气被阳光直射和与地表接触(阳光直射下温度更高)再次加热时，相对湿度就会降低。这就是能让水蒸发的空气。由于这些温度循环，空气永远没有时间达到饱和。

“如果我们把一些水放在阳光下，它就会蒸发。”真的吗?如果我把一些水装在一个密封的瓶子里，放在阳光下，它会蒸发吗?显然不是。为什么不呢?因为瓶子里的空气已经被水饱和了。空气的相对湿度是100%，或者接近100%。你可能已经知道了。所以你应该意识到你的问题有缺陷。水不仅在瓶子里不蒸发，而且在晚上和没有阳光的房子里也会蒸发。 A better question, imho, is: "Why isn't the Earth's atmosphere saturated with water after 4 billion years?" The atmosphere has a mass of about 5E+18 kg (that is 5 followed by 18 0s, eg 5,000,000,000,000,000,000). The water in the oceans has a mass of about 1.3E+21 kg, meaning about 250 times more mass than the atmosphere. Simply put: there's not enough atmosphere to hold all that water. The reason why the atmosphere is not saturated, does not have 100% relative humidity, is that the temperature of the air varies a lot. Both day to night, and season to season. The air is经常被冷却，水分通过沉淀流失。当这些空气被阳光直射和与地表接触(阳光直射下温度更高)再次加热时，相对湿度就会降低。这就是能让水蒸发的空气。由于这些温度循环，空气永远没有时间达到饱和。