Graupel

Graupel (German pronunciation: [ˈɡʁaʊpəl]; English: /ˈɡraʊpəl/), also called soft hail or snow pellets,[1] is precipitation that forms when supercooled droplets of water are collected and freeze on falling snowflakes, forming 2–5 mm (0.08–0.20 in) balls of rime. The term graupel comes from the German language.

Graupel is distinct from hail, small hail and ice pellets: the World Meteorological Organization defines small hail as snow pellets encapsulated by ice, a precipitation halfway between graupel and hail.[2] The METAR code for graupel is GS.

Formation

A handful of graupel pellets
Falling graupel

Under some atmospheric conditions, 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 often 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.[3] 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.[4]

Microscopic structure

Graupel encasing and hiding a snow crystal from view
Rime on both ends of a columnar snow crystal

The frozen droplets on the surface of rimed crystals are difficult to see, 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 (0.002 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 form of the original snow crystal, thereby giving rise to a graupel particle.[3]

Graupel and avalanches

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 (7.9–11.8 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.[5] Graupel tends to compact and stabilise ("weld") approximately one or two days after falling, depending on the temperature and the properties of the graupel.[6]

References

  1. ↑ "Graupel - Definition". Merriam-Webster Dictionary. Merriam-Webster. Retrieved 15 Jan 2012.
  2. ↑ International Cloud Atlas. Geneva: Secretariat of the World Meteorological Organization. 1975. ISBN 92-63-10407-7.
  3. 1 2 "Rime and Graupel". Electron Microscopy Unit, Beltsville Agricultural Research Center, U.S. Department of Agriculture. Public domain. URL accessed 2006-07-23.
  4. ↑ Graupel - What Is Graupel?. Weather Glossary, G, About.com.
  5. ↑ LaChapelle, Edward R. (May 1966). "The Relation of Crystal Riming to Avalanche Formation in New Snow". Department of Atmospheric Sciences, University of Washington. Archived from the original on 2008-12-06.
  6. ↑ "Graupel". avalanche.org. American Avalanche Association. Archived from the original on 2010-05-04.

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