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Graupel (German pronunciation: ['gʁaʊpɛl]; English //, also called soft hail or snow pellets) refers to precipitation that forms when supercooled droplets of water are collected and freeze on a falling snowflake, forming a 2–5 mm (0.079–0.197 in) ball of rime. Strictly speaking, graupel is not the same as hail or ice pellets.
While graupel is sometimes referred to as small hail, the World Meteorological Organization defines small hail as snow pellets encapsulated by ice, a precipitation halfway between graupel and hail.
Under some atmospheric conditions[which?], snow crystals may encounter supercooled water droplets. These droplets, which have a diameter of about 10 µm (0.00039 in), can exist in the liquid state at temperatures as low as −40 °C (−40 °F), far below the normal freezing point. 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 referred to as rimed. When this process continues so that the shape of the original snow crystal is no longer identifiable, 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 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, oblong shapes and falls in place of typical snowflakes in wintry mix situations, often in concert with ice pellets. Graupel is also fragile enough that it will typically fall apart when touched.
The frozen droplets on the surface of rimed crystals are hard to resolve 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. However, observations of snow crystals with a low-temperature scanning electron microscope (LT-SEM) clearly show cloud droplets measuring up to 50 µm (0.00197 in) on the surface of the crystals. The rime has been observed on all four basic forms of snow crystals, including plates, dendrites, columns, and needles. As the riming process continues, the mass of frozen, accumulated cloud droplets obscures the identity of the original snow crystal, thereby giving rise to a graupel particle.
Graupel commonly forms in high altitude climates and is both denser and more granular than ordinary snow, due to its rimed exterior. Macroscopically, graupel resembles small beads of polystyrene. The combination of density and low viscosity makes fresh layers of graupel unstable on slopes, and layers of 20–30 cm (7.9–12 in) present a high risk of dangerous slab avalanches. In addition, thinner layers of graupel falling at low temperatures can act as ball bearings below subsequent falls of more naturally stable snow, rendering them also liable to avalanche. Graupel tends to compact and stabilise ("weld") approximately one or two days after falling, depending on the temperature and the properties of the graupel.
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