Graupel

Graupel

[grou-puhl]
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; 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.

Formation

Under 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 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.

Graupel and avalanches

Graupel is both denser than ordinary snow and granular, in both cases due to its rimed exterior. The combination of weight and low viscosity makes fresh layers of graupel unstable on slopes, and layers of 20-30 cm 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 approximately one or two days after falling, depending on the temperature and the properties of the graupel.

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