Precipitation
Precipitation is produced when the droplets and crystals in clouds grow large enough to fall to the ground. Clouds do not usually produce precipitation until they are more than 1 km (0.6 mi) thick. Precipitation takes a variety of forms, including rain , drizzle, freezing rain, snow , hail , and ice pellets, or sleet. Raindrops have diameters larger than 0.5 mm (0.02 in), whereas drizzle drops are smaller. Few raindrops are larger than about 6 mm (about 0.2 in), because such large drops are unstable and break up easily. Ice pellets are raindrops that have frozen in midair. Freezing rain is rain that freezes on contact with any surface. It often produces a layer of ice that can be very slippery. Snowflakes are either single ice crystals or clusters of ice crystals. Large snowflakes generally form when the temperature is near 0° C (32° F), because at this temperature the flakes are partly melted and stick together when they collide. Hailstones are balls of ice about 6 to 150 mm (about 0.