http://www.livescience.com/29472-how-cirrus-clouds-form.html

It has long been a mystery exactly what causes the formation of
cirrus clouds, the wispy billows of ice that can be seen high in the sky. But
new research, detailed in the May 9 issue of the journal Science, finds that the
clouds condense and freeze, or nucleate, on very specific mineral and metal
particles high in the atmosphere. That makes cirrus clouds unique: Most other
clouds form primarily by condensing onto organic particles, said study author
Dan Cziczo, an atmospheric chemist at the Massachusetts Institute of Technology.

http://www.ouramazingplanet.com/3410-amazon-fungi-atmosphere.html

In the Amazon rain forest, salty particles from spore-launching
fungi help make clouds and rain, according to new research.

The tiny, potassium-rich specks, smaller than bacteria, waft above the forest
into the air. Once in the atmosphere, organic gases condense on the particles,
coating them with gel-like compounds. The coated particles provide a surface for
water vapor to form cloud droplets and rain.

Thanks to Ron Gleason