Traditionally, researchers have thought that minerals or other particulates in clouds caused water droplets to glom together until they were large enough to fall as raindrops, snowflakes and hail. The new research shows that a large variety of bacteria, and even fungi, diatoms and algae, persist in the clouds and can be used as precipitation starters, a growing field of study called bioprecipitation. (In order for snow, say, to fall from clouds, particles around which ice crystals can form — called ice nuclei — are needed.)