Precipitation

Although large quantities of liquid water will freeze as the temperature drops below 32°F (0°C), cloud droplets sometimes are "supercooled;" that is, they may exist in liquid form at lower temperatures down to about -40°F (-40°C). At temperatures below -40°F (-40°C), even very small droplets freeze readily, but at intermediate temperatures (between -40 and 32°F or -40 and 0°C), particles called ice nuclei initiate the freezing of droplets. An ice nucleus may already be present within a droplet, may contact the outside of a droplet and cause it to freeze, or may aid in ice formation directly from the vapor phase. Ice nuclei are considerably more rare than cloud condensation nuclei and are not as well understood.

Once initiated, ice crystals will generally grow rapidly because air that is saturated with respect to water is supersaturated with respect to ice; i.e., water vapor will condense on an ice surface more readily than on a liquid surface. The habit, or shape, of an ice crystal is hexagonal and may be plate-like, column-like, or dendritic (similar to the snowflakes cut from paper by children). Habit depends primarily on the temperature of an ice crystal's formation. If an ice crystal grows large enough to fall through air of varying temperatures, its shape can become quite intricate. Ice crystals can also grow to large sizes by aggregation (clumping) with other types of ice crystals that are falling at different speeds. Snowflakes are formed in this way.

Clouds that contain both liquid water and ice are called mixed clouds. Supercooled water will freeze when it strikes another object. If a supercooled droplet collides with an ice crystal, it will attach itself to the crystal and freeze. Supercooled water that freezes immediately will sometimes trap air, forming opaque (rime) ice. Supercooled water that freezes slowly will form a more transparent substance called clear ice. As droplets continue to collide with ice, eventually the shape of the original crystal will be obscured beneath a dense coating of ice; this is how a hailstone is formed. Hailstones may even contain some liquid water in addition to ice. Thunderstorms Hailstones are often composed of concentric layers of clear and opaque ice. This is thought to be the result of the stone traveling up and down within the cloud during its formation. Opaque layers would be created in the upper, colder parts of the cloud, where the water droplets are small and freeze rapidly, forming ice with numerous air enclosures. In the warmer, lower parts of the cloud the water droplets would spread over the surface of the hailstone so that little air is trapped and the ice is transparent. © Astrid & Hanns-Frieder Michler/Science Photo Library, National Audubon Society Collection/Photo Researchers, Inc.

are dramatic examples of vigorous mixed clouds that can produce high precipitation rates. The electrical charging of precipitation particles in thunderstorms can eventually cause lightning discharges.