List the forms of precipitation and the circumstances of their formation.
22. Rain - forms in supercooled clouds as ice crystals grow at the expense of cloud droplets. When the ice crystals grow large enough, they fall, often melting and reaching the ground as rain. If they do not melt, they reach the surface as snow. Rain also forms when cloud droplets of different size collide and coalesce. Atmospheric electricity is thought to be a key factor in this latter process.
Sleet - results when rain falls through a subfreezing layer of air and freezes, reaching the ground as small pellets of ice.
Glaze - occurs when rain falls through a relatively thin layer of subfreezing air. The drops become supercooled and freeze upon impact with objects and the ground, creating an often-thick coating of ice.
Hail - composed of concentric layers of ice, is the largest form of precipitation and is formed in cumulonimbus clouds. Here ice pellets grow by collecting supercooled droplets. Layer after layer will collect and freeze as the hailstones are carried by updrafts above the freezing level.