说热空气“持有”更多的水分在技术上是不正确的,但是是一种常见的口语。让我们将其分解技术。让我们考虑一杯水和一个真空(没有空气)上面。会发生什么呢?最顶部的分子层的水会蒸发。以什么速度将水蒸发?更好的是,蒸发是什么?蒸发是当水分子获得足够的动能(他们振动速度)将持有的债券。动能是依赖于温度。所以分子振动速度,打破他们的债券,并进入真空蒸汽。 Some molecules will stay as a vapor in the vacuum, but others will reenter the liquid. When the molecules enter the liquid as fast as they are leaving, then it is saturated. If the air is cooled down, then the rate at which molecules leave the liquid slows down. The molecules entering the liquid do not slow down at the same rate, causing the liquid to grow toward it's initial state. Note that I specifically said it is a vacuum. Instead of a glass of water, picture the water as little drops. The atmosphere can act to warm or cool these drops, and vice-versa. In the more nitty-gritty aspect of this, the equation that describes the vapor pressure as a function of temperature is called the Clausius-Clapeyeron equation/relation. The American Meteorological Society has [one approximate solution][1], but [I prefer this equation][2]: $$e_{sat}(T)=611 Pa \exp[\frac{L_v}{R_v}(273.15^{-1}-T^{-1})]$$, where $L_v$ is the [latent heat of vaporization][3], $R_v$ is the [specific gas constant for water vapor][4], and $T$ is the absolute temperature in Kelvin. Combined with the ideal gas law for water vapor (assuming saturation) $$e_{sat}(T)V=m_vR_vT$$, and given the volume ($V$) we can write an expression for the mass of water vapor $m_v$. The equation comes out to $$m_v=611 Pa \exp[\frac{L_v}{R_v}(273.15^{-1}-T^{-1})]V R_v^{-1}T^{-1}$$ To answer your final question, the molecules are approximated as being infintessimally small, per [the ideal gas law][5]. To be more specific, one molecule of water is about [7.08$\times$ 10$^{-19}$ cubic feet][6] (after some math), so the added volume is considered negligible. In short, the molecules are treated as point masses. [1]: http://glossary.ametsoc.org/wiki/Clausius-clapeyron_equation [2]: http://www.theweatherprediction.com/habyhints2/646/ [3]: https://en.wikipedia.org/wiki/Enthalpy_of_vaporization#Other_common_substances [4]: http://glossary.ametsoc.org/wiki/Gas_constant [5]: https://en.wikipedia.org/wiki/Ideal_gas_law [6]: http://www.mc3cb.com/pdf_chemistry/What%20is%20the%20diameter%20of%20a%20water%20molecule.pdf