Reading about cloud formation, I learned that to a droplet to form, the water vapour needs a Cloud Condensation Nuclei, which is an aerosol with a size in the order of 0.0001mm. And if no CCN is found temperatures as low as -13°C are needed to form the droplets. Why this is happening? What does the contact of the solid with water do to make it condense?


2 Answers 2


The correct answer is that Surface tension prevents the formation of small water droplets.

You see, as the surface energy is proportional to the droplet area ($r^2$) and the bulk energy - to its volume ($r^3$), their ratio ($1/r$) will rise to infinity as the droplet radius $r$ goes to zero. Therefore, the droplet cannot grow steadily from $r=0$. Even below 0°C there is a certain critical droplet radius $r_c(T)$, below which the Gibbs free energy of droplet is higher than that of vapor so the droplet evaporates.

Condensation nuclei are a shortcut to the droplet formation: if they are hygroscopic and have a radius above $r_c$, water surface energy will be too small to prevent condensation and the droplet will grow quickly (assuming that vapor is supersaturated).

  • $\begingroup$ Could you pls explain this intuitively? Like what happens physically? Thank you. $\endgroup$ Aug 9, 2022 at 8:15

Solids are very good conductors of heat compared to liquids and gases. The solid will absorb thermal energy from warmer vapour and unless it is very smooth it will provide nucleation sites.

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    $\begingroup$ This answer is completely wrong!! Think: if gas and a particle are in thermal equilibrium (say, at -5°C), what is going to conduct heat and where? $\endgroup$
    – gigacyan
    Feb 27, 2014 at 8:16
  • $\begingroup$ I'm sorry. I don't claim to be an expert on physics, I dropped it after my first year in uni. $\endgroup$
    – gsurfer04
    Feb 27, 2014 at 15:24

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