[Neeraja Raghavan]

Neeraja Raghavan asked the Naked Scientists:
   
Dear Chris:
 
If cloud formation requires the presence of dust particles as a seed, does this mean that in a dust-free atmosphere there will be no cloud formation, and consequently no rainfall?
 
Thanks and regards,
Neeraja.

What do you think?

[paul.fr]

It's not only dust that acts as CCN (Cloud Condensation Nuclei). salt, pollen, smoke particles and aerosols also seed clouds.

[chris]

True. Even dandruff can trigger cloud formation (as cloud samples have confirmed!).

To address the original question, water will condense without a nucleating centre, but to make it do so requires much lower temperatures. However, once the process has started i.e. some small particles have formed, these will act as nucleating sites for other particles and so the process is likely then to continue.

Hence I think the difference probably wouldn't be that dramatic.

Chris

[paul.fr]

To address the original question

Ah, I knew there was something I missed!

water will condense without a nucleating centre, but to make it do so requires much lower temperatures.

I believe clouds (could) also form if the RH is measured in the extremes, many hundreds of percent. Just don't ask me how that's possible.

Here is a nice article: (pdf file)
http://ams.allenpress.com/archive/1520-0469/58/8/pdf/i1520-0469-58-8-915.pdf