Illustration shows (at left) a cloud with normal foundation of rain droplets, while at right the droplets do not form readily because of the small particles caused by pollution. The water vapor condenses on the pollution particles and creates a cloud with a large number of drops that are so small that they float with the air and are slow to coalesce into raindrops or to freeze into sleet and snowflakes. Credit: Prof. Rosenfeld of the Hebrew University