Also known as Snow Pellets or Soft Hail, graupel is precipitation that forms when supercooled water droplets are collected and freeze on falling snowflakes, forming 2–5 mm balls of rime ice. Graupel is distinct from hail, which is larger and harder, and ice pellets, which are frozen water droplets and are also called sleet. Hail is produced in thunderstorms and is more common in the spring and summer, while graupel typically falls in winter storms. The METAR code for graupel is GS.
Under some atmospheric conditions, snow crystals may encounter supercooled water droplets. These droplets, which have a diameter of about 10 μm (0.00039 in), can exist in the liquid state at temperatures as low as −40 °C (−40 °F), 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 often 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. Graupel was formerly referred to by meteorologists as soft hail. However, graupel is easily distinguishable from hail in both the shape and strength of the pellet and the circumstances in which it falls. Ice from hail is formed in hard, relatively uniform layers and usually falls only during thunderstorms. Graupel forms fragile, oblong shapes and falls in place of typical snowflakes in wintry mix situations, often in concert with ice pellets. Graupel is also fragile enough that it will typically fall apart when touched.
Graupel in the shape of a snowflake. Source: Wikicommons; Author: LiWei, 2018.
Flight Safety implications
Graupel that collects on aircraft surfaces should be treated in the same way as snow. Particular attention should be paid to clearing Graupel that has collected in the joints next to control surfaces. Since Graupel is associated with winter storms, it may be a precursor to more dangerous types of precipitation such as Freezing Rain.