Well I do know that they sometimes fall as rain, but my question is why don't the droplets fall as soon as they condense from steam to cloud.

Clouds are white by the process of Mie scattering so the particles are on the order of light wavelength ~$0.5 \mu m$, although small would a particle of this size not slowly fall to the ground?

When a cloud does rain many of the rain drops fall at the same time what is physically happening to the droplets to encourage them to join into large drops that fall?

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    $\begingroup$ this ppt presentation goes into the details of drop formation, duratation and fall google.gr/… $\endgroup$ – anna v Feb 18 '14 at 15:39

Many clouds are sustained by upward currents, either thermals or generated by a front, that also determine their vertical extension. However this would be an incomplete answer.

Look at the clouds as regions where temperature and pressure are such that water molecules can condensate. If a water drop leaves that region without being big enough, it just evaporates again. At the same times new water molecules coming from outside, condensate into that region as long as the "cloud condition" stays there. Look at this or this time-lapse videos of wonderful lenticular clouds where the wind blows but due to the turbulence of a mountain, the cloud region stays fixed!

Talking about volcanic ash clouds, where you cannot have re-evaporation, be sure that particles will fall down sooner or later.

Regarding your second question, the answer is about the amount of water that is taken into the cloud. Cumulonimbus, for instance, are generated by strong thermal currents that pushes up there a lot of humidity. The same process on a much weaker scale leads to cumulus that do not bring any rain. Only when a lot of droplets are there, they can start to collide effectively and increase their mass until they are able to leave the cloud without evaporating.

An extreme case in this scenario is the virga, this phenomenom happens when the rain is capable to leave the clouds, but evaporates before hitting the ground. It is not so uncommon: if you live at low altitude, you may be able to see it in hot summer days!

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  • $\begingroup$ "If a water drop leaves that region without being big enough, it just evaporates again." Is this a general principle of most clouds, or just specific to the lenticular clouds you mentioned? $\endgroup$ – Cory Klein Jun 16 '17 at 20:17
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    $\begingroup$ @CoryKlein I used lenticular clouds as a striking evidence, but the principle is general. $\endgroup$ – DarioP Jun 17 '17 at 7:20

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