我可以确定这可能混淆的两个部分。第一个是蒸汽压对冰,和第二个是过度饱和。首先,让我们看一下相变的方程。所描述的饱和蒸汽压可以(克劳修斯等式Clapeyeron) [1]。求解这个方程显式(即不使用博尔顿的公式中使用的链接),我们发现$ $ e_s (T) = e_s (T_0) \ exp(\压裂{L} {R_v}[\压裂{1}{T_0} - \压裂{1}{T})) $ $ e_s饱和蒸气压是美元,T_0美元是一个给定的温度(通常是273 K), L是美元转换、潜热和温度T是美元。请注意,我说的是转换潜热,所以它可以从一个阶段到另一个。因此我们可以做一个相空间图所示使用L_f(熔化潜热),美元L_v(汽化潜热),美元和L_s美元(升华热,气体固体)。这些可以找到的值(这里)[2]。[![相图][3]][3],所有三个满足(三相点)[4],但更通常被称为水的冰点。 Notice that the below it is where the liquid does not exist. Instead the gas goes directly into solid ([deposition][5]). In reality, liquid water can exist below this temperature ([supercooled water][6]), but it will eventually turn into a gas or ice. I made a graph (below), similar to the phase diagram above, but does not clip off the clausius clapeyeron curve. Notice that below 273 K, the saturation pressure of vapor w.r.t. (with respect to) ice is lower than water. This basically says that it will first turn into ice before it turns into water. Then, above 273 K, the vapor saturation curve wrt water is less than ice- meaning it is easier to turn into liquid water above 273 K than it is for ice. [![ Custom Clausius Clapeyeron curve][7]][7] Next is the concept of supersaturation. Supersaturation generically refers to a state that is above saturation. In this sense, supersaturation can be identified by having a relative humidity over 100%. While some websites may give a different definition, I think of relative humidity as being the ratio of water entering the liquid (or solid phase), to water becoming a gas. So supersaturation means that there is enough water vapor in the air, at that temperature, to form a cloud. I should note, that the process of making a cloud can be a bit more convoluted than this answer. For that, I recommend looking at [this answer][8] where I discuss [Kohler theory][9]. So the short answer is, there is sufficient water vapor in the air to form a cloud. But because it is below freezing, the cloud is formed via deposition, much like how [frost][10] is formed. [1]: http://glossary.ametsoc.org/wiki/Clausius-clapeyron_equation [2]: http://glossary.ametsoc.org/wiki/Latent_heat [3]: https://i.stack.imgur.com/p6ZdY.png [4]: https://en.wikipedia.org/wiki/Triple_point [5]: https://en.wikipedia.org/wiki/Deposition_(phase_transition) [6]: https://en.wikipedia.org/wiki/Supercooling [7]: https://i.stack.imgur.com/88wqR.png [8]: //www.hoelymoley.com/questions/5047/at-what-humidity-does-it-start-to-rain/5048#5048 [9]: https://en.wikipedia.org/wiki/K%C3%B6hler_theory [10]: https://en.wikipedia.org/wiki/Frost