Graupel (also called snow pellets or soft hail) is a type of solid precipitation formed by snow crystals coated with a layer of opaque ice through a process of riming (deposition of supercooled water droplets onto the snow crystal). The result is soft, white, opaque pellets of 2-5 mm diameter that crumble easily when crushed.
Formation and distinction
Graupel forms in cumulonimbus or deep cumulus clouds where ice crystals and supercooled water droplets (between 0 °C and -40 °C) coexist. When a snowflake falls through this zone of supercooled water, the droplets freeze instantly onto its surface, creating a rime layer that obscures the original crystal structure.
It is important to distinguish it from: snow (unrimed crystals, visible hexagonal structure); hail (dense, transparent or translucent ice, >5 mm, does not crumble easily); and ice pellets (frozen drops, transparent). Graupel bounces upon hitting hard surfaces, unlike snowflakes.
In Spain, graupel is common in spring thunderstorms and winter mountain showers. It is often popularly confused with "small hail", but its nature is entirely different. It is also a precursor to lightning: the collision of graupel with ice crystals inside clouds generates the charge separation that produces lightning. See also: hail, powder snow.