When water becomes warm enough, it evaporates as vapor into the air. When a mass of air quickly cools to its saturation point, the water vapor condenses into clusters of tiny water droplets and frozen water crystals. We call these clusters clouds. Over time, the droplets and crystals that make up a cloud can attract more water to themselves. When water droplets grow heavy enough, gravity pulls them down as raindrops. If the air is cold enough, the ice crystals can remain frozen and grow large enough to fall as snow, sleet, freezing rain or hail.
Answered by: Discovery Channel