Graupel

Graupel, also known as soft hail, corn snow, hominy snow, or snow pellets, is a type of precipitation that forms when supercooled water droplets freeze onto snowflakes, creating small, opaque pellets of ice. Graupel typically has a diameter ranging from 2 to 5 millimeters (0.08–0.20 in) and is characterized by its soft, granular texture. It looks like tiny, broken up, foam packing peanuts.

This precipitation occurs in certain meteorological conditions, often associated with convective snow showers or in the vicinity of thunderstorms. The formation of graupel begins with the presence of supercooled water droplets in a cloud. Supercooled water droplets are liquid water droplets that exist at temperatures below freezing but have not yet frozen. When these supercooled droplets come into contact with snowflakes,

which act as ice nuclei, they freeze onto the snowflakes and form graupel. The process is facilitated by a mechanism known as riming, where supercooled droplets adhere to the surface of the snowflake, gradually building up layers of ice. Microscopically, graupel exhibits a porous and irregular structure.

The pellets often consist of a combination of ice and air pockets, contributing to their characteristic softness. The internal structure of graupel can vary depending on the specific atmospheric conditions during its formation. Graupel plays a role in the context of avalanches, especially in snowpack stability assessments.

In mountainous regions with significant snow accumulation, the presence of graupel layers within the snowpack can contribute to instability. The irregular shape and porous structure of graupel pellets can create weak layers within the snowpack, making it prone to fracture and failure. This, in turn, increases the risk of avalanches, particularly when additional snowfall or external triggers such as skiers or snowmobilers add stress to the already precarious snow layers.

Understanding the formation and microscopic characteristics of graupel is crucial for meteorologists and avalanche forecasters in assessing and predicting the potential avalanche risk in snow-covered regions. Monitoring weather conditions conducive to graupel formation and recognizing its presence within the snowpack aids in the accurate evaluation of avalanche hazards.