This bacteria found in clouds:

[Pseudomonas syringae](http://en.wikipedia.org/wiki/Pseudomonas_syringae)

I find frost in Ireland in our farm, though it is definitely not 0 degree (I thought I was imagining things):

[The Rain-Making Bacteria](http://www.hardydiagnostics.com/articles/Ice-Forming-Bacteria.pdf)

>As P. syringae commonly inhabits plant surfaces, its ice nucleating nature incites frost development at warmer temperatures than 0C,freezing the buds of the plant and destroying the crop.

In other research papers, evidence found that this bacteria moves between the clouds and crust of earth via certain transportation which affects the weather, due to its ability to nucleate water.