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. 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
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