Everything about Graupel totally explained
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 doesn't 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 isn't 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.
Further Information
Get more info on 'Graupel'.
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