Ice Ice Maybe. Bacteria that trigger frost in grapes could also be responsible for forming raindrops in the atmosphere.

The freezing of a single young shoot in a vineyard would go unnoticed come harvest time. However, the subsequent frosting of hundreds of thousands more shoots later that night could wipe out the entire vintage. Likewise, the freezing of a single cloud droplet would have no impact on the weather. But the subsequent freezing of millions more in the same updraft could kick-start a thunderstorm. A causal agent in both scenarios may be, oddly enough, bacteria. That is, ice nucleation active (INA) bacteria, which, by freezing water at high temperatures, could be both triggering frost and making rain. The article gives an overview of current research into the behaviour and properties of ice nucleation active (INA) bacteria and the resulting effects of the bacteria on weather patterns in different climates across the world.

Citation:
Hill T.C.J. and Moffett B.F. (2008). 'Ice Ice Maybe. Bacteria that trigger frost in grapes could also be responsible for forming raindrops in the atmosphere '. Australasian Science, November 14-18..