David Sands

I am happy to say the world's expert in bio-precipitation, David Sands, talked to me for 1/2 hour on the phone. I am wondering about inter-bacteria signalling, and later emailed: "If it starts to rain all at once, then the bacteria must be signalling each other." During the conversation, I mentioned fibers made from polymers, he mentioned fibers of DNA/RNA almost 10' long in one bacterium. That'll tangle a cloud together for ya. Also lightening! I am thrilled.
Update: This is the 2nd email I sent him:

Several different observations support the idea that clouds are being held together by bacteria and are a bit "sticky".  

The best information comes from comparing normal clouds with jet trails - which are effectively sterile clouds. Jet trails dissipate quickly in the same sky where clouds persist. More significantly, one observes an occasional jet trail intersecting with a cloud. The part of the trail outside the cloud continues to dissipate while the part inside the cloud does not. Hence there must be forces holding the cloud together that are protecting the enclosed jet trail from dissipation. Those forces cannot simply be "water-to-water" forces.

There are other less compelling reasons to think clouds are a bit sticky.(1) One observes a fast cloud overtake a slower one, tear off a piece of the slower cloud, and continue on its way. (2) The upwind side of a cloud tends to bunch together in a way that looks more like tissue paper than like disconnected and homogeneously dispersed water particles. It just looks like a fibrous mat being rolled up. Over and over one sees the visual appearance of a non-uniform, non-homogenous, fibrous, and sticky structure.

Bacteria do contain long fibres (DNA and RNA, as you suggested in our phone converstation) and they are capable of producing oils, polymers, and other products. It would make clouds sticky. Many types of bacteria live in biofilms.

I do not know how to test this more quantitatively. One might try to:
(a) Locate free-floating bi-products, also in the cloud
(b) Sample over a surface, over a very short time span, and then do an assay to study the physical distribution of the organisms, and bi-products within the cloud. It might be easier to do this with fog at ground level.

Also on the subject of bacterial bi-products, is inter-bacteria signalling important for rain formation? Certain bacterial group behaviors like disease and bioluminesce, require a given population density before being triggered. The question is: what signals, if any, do rain forming bacteria (pseudomonas) use or require for rain formation? Could one suppress or enhance ice formation with the right signals?