From Evernote:

Bacterial origin of clouds and rain

Ok, there is a lot to say and I don't have the time now to think about it in detail, but consider this a place holder.  Basically there is a conjecture floating around that bacteria act as condensation nuclei for cloud formation [http://www.nytimes.com/2010/05/25/science/25snow.html?partner=rss&emc=rss].  First I guess that we have to state that there is a plausible idea, and probably more than that. There have been some recent studies that found bacteria in cloud drops, and while I have been a little skeptical of these results, I guess it is time to look at them more seriously. Second, though, I think it is important to keep in mind that a lot of this work is being driven by the biological side of the story, without a lot of input from the cloud physics side. Whether the bacterial origin can explain much of global cloudiness is up for debate.  For now, the argument for bacteria being very important for global climate is on the much weaker side. Deails to come soon.