Graupel (snow)

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Graupel (also called snow pellets) refers to precipitation that forms when freezing fog condenses on a snowflake, forming a 2–5 mm ball of rime ice; the snowflake acts as a nucleus of condensation in this process. The term is derived from German "Graupel" meaning "freezing rain" or "soft hail". Graupel does not include other frozen precipitation such as snow or ice crystals. The METAR code for graupel is GS.<ref>AVIATION ROUTING WEATHER REPORT, Present Weather Group. Department of Atmospheric Sciences, Texas A&M University.</ref><ref>USA and International Code Change For Ice Pellets. Automated Surface Observing System, National Weather Service.</ref>

Image:Snowflake 300um LTSEM, 13368.jpg

Image:Graupel encasing a snow crystal.jpg

1 Formation
2 Microscopic structure
3 References
4 External links
- 4.1 Dictionaries
- 4.2 Weather Glossaries

[edit] Formation

Under some atmospheric conditions, forming and descending snow crystals may encounter and pass through atmospheric supercooled cloud droplets. These droplets, which have a diameter of about 10 μm, can exist in the unfrozen state down to temperatures near -40°C. Contact between the snow crystal and the supercooled droplets results in freezing of the liquid droplets onto the surface of the crystals. 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.<ref name="emu">"Rime and Graupel". Electron Microscopy Unit, Beltsville Agricultural Research Center, U.S. Department of Agriculture. Public domain. URL accessed 2006-07-23.</ref>

[edit] Microscopic structure

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 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.<ref name="emu"/>