Image on right courtesy of JSC Imagery Services . There are two processes that allow moisture to condense to form clouds. Clouds are composed of billions of tiny water droplets that eventually combine to form rain, snow or other forms of precipitation. There are literally billions of these droplets in a single cubic meter of air. It takes about a million water droplets to form a single rain drop.
Updrafts can recycle the droplets through the cloud, increasing droplet size.
Bergeron processes occur where temperatures fall below freezing. Miniature ice crystals or supercooled water droplets may both act as condensation surfaces. However, the density of water vapor in the air prior to saturation is slightly less adjacent to ice surfaces than water surfaces. Eventually, precipitation of ice or snow occurs unless the frozen water is warmed as it falls through the atmosphere, resulting in rain. For information on how clouds are classified, see Cloud Classification .