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After spending some time watching clouds, it occurred to me how interesting it is that clouds have more structure than just being homogenous, really humid layers in the sky. I see that a similar question to that has previously been addressed on the site, and, presumably, the fractal nature of the boundary of a cloud is thus due to some related dynamical system (and is maybe to some degree influenced by the molecular structure of water).

However, what I'm curious about is a related question about how clouds are not homogenous. As I understand it, precipitation occurs when the air is oversaturated with water such that the water condenses and falls. Yet, this understanding would seem to suggest that we should observe precipitation being more like, "The cloud hits a cold spot, all the vapor condenses, and the cloud falls from the sky and goes sploosh on the ground" than what we actually observe - in particular, it doesn't seem to explain why precipitation should come down in raindrops or why a single storm should be able to continue precipitating for any length of time, rather than condensing all in a short period. What factors cause precipitation to act this way? How can I think about the processes involved in a better way to address this?

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    $\begingroup$ For one thing, clouds do not consist of a homogeneous phase of water vapor (if they did, they would be optically almost transparent). Clouds consists of droplets and the surface energy of the droplets is a signifiant thermodynamic energy term, which makes droplet growth a complex thermodynamic phenomenon. Furthermore, droplets can be in near thermodynamic equilibrium with the surrounding air, but they don't have to be - that's why cloud seeding can produce rain from a seemingly blue sky. You my want to consult a theoretical textbook on meteorology, or two, this is a non-trivial topic! $\endgroup$ – CuriousOne Dec 16 '14 at 4:02
  • $\begingroup$ I'm quite sure you can find many documentaries about clouds and rain. You could also read the wikipedia entries for clouds and rain. $\endgroup$ – TZDZ Dec 16 '14 at 7:30
  • $\begingroup$ The water droplet size in clouds is small enough that the rate of fall is negligable. Are you really asking what causes these water droplets to coalesce then fall as raindrops? $\endgroup$ – John Rennie Dec 16 '14 at 16:27
  • $\begingroup$ Also note that if the cloud can condense like that after entering such a region, it would start to gradually condense when it approaches that region. So, in practice you could expect this phenomena when there is a sharp transition from one region o the other and the cloud is prevented from entering the cold region and allowed to grow big before it can enter. The cold region will then be located at high altitudes and you need a cap that suddenly give way. Such phenomena do exist, they can cause extremely severe thunderstorms. $\endgroup$ – Count Iblis Aug 27 '15 at 15:52
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Latent Heat Slows Condensing

On a micro scale, when a molecule of water condenses out of the air by joining a water droplet it transfers its higher kinetic energy to the droplet warming it. On a macro scale, if all the water tried to condense out of the air at once, the temperature of the cloud would rise to the point where much of the water would want to evaporate again. Thus the process of condensing is controlled by the process of cooling.

Air slows the cloud's descent

Once there are all these tiny droplets of water, they still fly around in the air because their weight is small compared to the air drag on them, so they just move with the wind. In a nice fluffy cloud these tiny droplets are so few and far between that they rarely ever hit another droplet and combine, so they all stay small.

In a rain cloud, condensation has created so many of these tiny droplets that they start to collide with each other every now and then. However, as they combine with each other there are fewer of them to collide with, so until more droplets form via condensation they still don't collide very often. When eventually, enough droplets collide that its weight is enough to pull it down then it falls out of the cloud, and can't collide with any more droplets, and once again the cloud has fewer droplets to combine with.

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