http://medheadlines.com/2008/03/01/new-evidence-germs-cause-rain/ Scientists have long known that a particle of some sort, often dust or soot, is needed to form an ice crystal in the atmosphere. Moisture in the atmosphere clings to the tiny ice crystal and freezes, too. The ice crystal gets bigger and bigger until it falls to the ground as snow and rain. Christner’s research has disclosed that bacteria that lives on plants can also get swept high into the air, where moisture clings to it, forming an ice crystal that grows until it, too, falls as rain or snow. He and his colleagues have identified the rain-generating bacteria on every continent around the globe.