Graupel (also called snow pellets) refers to precipitation that forms when supercooled droplets of water condense 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 the same. Graupel does not include other frozen precipitation such as snow, hail, ice pellets or diamond dust. The METAR code for graupel is GS.
FormationUnder some atmospheric conditions, snow crystals may encounter supercooled cloud droplets. These droplets, which have a diameter of about 10 µm, can exist in the liquid state at temperatures as low as −40 °C, 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.
Microscopic structureThe 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. Graupel tends to compact and stabilise approximately one or two days after falling, depending on the the temperature and the properties of the graupel.
- 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.
graupel in German: Graupel
graupel in Esperanto: Grajlo
graupel in French: Neige roulée
graupel in Italian: Neve tonda
graupel in Japanese: 霰
graupel in Polish: Krupy (opad atmosferyczny)
graupel in Russian: Крупа (осадки)
graupel in Finnish: Lumirae
graupel in Chinese: 霰