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Concept
Graupel
Summary
Graupel (ˈɡraʊpəl; ˈɡʁaʊpl̩), also called soft hail, hominy snow, or snow pellets, is precipitation that forms when supercooled water droplets in air are collected and freeze on falling snowflakes, forming balls of crisp, opaque rime.
Graupel is distinct from hail and ice pellets in both formation and appearance. However, both hail and graupel are common in thunderstorms with cumulonimbus clouds, though graupel also falls in winter storms, and at higher elevations as well. The METAR code for graupel is GS.
Under some atmospheric conditions, snow crystals may encounter supercooled water droplets. These droplets, which have a diameter of about on average, can exist in the liquid state at temperatures as low as , far below the normal freezing point as long as above the homogeneous nucleation point of water. Contact between a snow crystal and the supercooled droplets results in freezing of the liquid droplets onto the surface of the crystal. This process of crystal growth is known as accretion. Crystals that exhibit frozen droplets on their surfaces are often referred to as rimed. When this process continues so that the shape of the original snow crystal is no longer identifiable and has become ball-like, the resulting crystal is referred to as graupel.
Graupel was formerly referred to by meteorologists as "soft hail." However, graupel is easily distinguishable from hail in both the shape and strength of the pellet and, in some cases, the circumstances in which it falls. Ice from hail is formed in hard, relatively uniform layers and usually falls only during thunderstorms. Graupel forms fragile, soft, oblong crystals and falls in place of typical snowflakes in wintry mix situations, often in concert with ice pellets. However, graupel does also occur in thunderstorms. Graupel is also fragile enough that it will typically fall apart when pressed on.
The frozen droplets on the surface of rimed crystals are difficult to see even when zoomed in, and the topography of a graupel particle is not easy to record with a light microscope because of the limited resolution and depth of field in the instrument.
Official source
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