Graupel

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Graupel (

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. ^[3] The METAR code for graupel is GS.

Formation

Under some

As graupel falls, it often deforms into a conical shape. This conical shape, in turn, determines which direction it falls and how far it travels as it falls. Small graupel particles with a base diameter less than 1mm generally fall with the conical base down, but if the particle is between 1mm and 3mm persistent oscillations around the center of the conical base appear, and if larger than 3mm the graupel particle will start to tumble. As the base diameter increases, conical graupel particles generally further horizontally from where it initially fell.[5]

Graupel was formerly referred to by meteorologists as "soft hail." Graupel is distinguishable from true hail in both the shape and strength of the pellet and, in some cases, the circumstances in which it falls. Ice from

However, observations of snow crystals with a low-temperature scanning electron microscope (LT-SEM) clearly show frozen 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 eventually obscures the form of the original snow crystal, thereby giving rise to graupel.^[4]

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

Graupel and avalanches

Graupel commonly forms in high-altitude^[quantify] 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 (8–12 in) or higher 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 or otherwise making surfaces slippery.^[7] Graupel tends to compact and stabilise ("weld") approximately one or two days after falling, depending on the temperature and the properties of the graupel.^[8]

Gallery

• 
  Snowflakes can turn into graupel
• 
  Almost graupel
• 
  Graupel in shape of snowflake

See also

• Sleet – term variously used for frozen precipitation
• Freezing rain
• Ice pellets

References

External links

Look up graupel in Wiktionary, the free dictionary.

Wikimedia Commons has media related to Graupel.

Dictionaries

• 3 results for:graupel. Dictionary.com, accessed September 12, 2006.
• Graupel. Merriam-Webster Online Dictionary, accessed September 12, 2006.

Weather glossaries

• Weather Glossary, G. The Weather Channel, accessed September 12, 2006.
• All About Snow. National Snow and Ice Data Center (NSIDC), accessed September 12, 2006.
• Terms used by meteorologists, forecasters, weather observers, and in weather forecasts. National Oceanic and Atmospheric Administration (NOAA), accessed September 12, 2006.
• About.com Weather Glossary. Weather at
  About.com
  , accessed December 21, 2008.