“如果我们把一些水在阳光下蒸发。”Really? If I take some water in a closed bottle and put it in the sunlight, then it will evaporate? Obviously not. Why not? Because the air (gas) in the bottle is saturated with water. The relative humidity of that air is 100%, or close to it. You probably know this already. So you should realize that your question is flawed. Not only does water not evaporate in the bottle, but water evaporates at night and inside houses without any sunlight. 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 constantly being cooled and the water lost via precipitation. When this air is reheated, both by direct sunlight and by contact with the surface which warms up even more in direct sunlight, it is low in relative humidity. This is the air that will allow water to evaporate. Because of these temperature cycles, air never has the time to reach saturation.