Graupel is the fragile, ball-like winter precipitation you have likely mistaken for hail
In short: Often called soft hail or snow pellets, graupel forms when supercooled water droplets freeze onto falling snowflakes. This accretion process creates opaque, granular balls that look like polystyrene beads. While they resemble hail, these fragile crystals are distinct in their formation, structure, and the specific conditions under which they fall.
Graupel, or soft hail, originates through a process called accretion. As snow crystals descend through the atmosphere, they encounter supercooled water droplets—liquid water that remains unfrozen at temperatures as low as -40 °C. When these droplets, which average about 10 μm in diameter, collide with a snowflake, they freeze instantly upon contact. This coating of rime continues to accumulate until the original structure of the snow crystal is completely obscured, resulting in a rounded, ball-like particle typically measuring 2–5 mm in diameter.
The physical properties of graupel differ significantly from true hail or ice pellets. While hail consists of hard, uniform layers of ice and is primarily associated with intense thunderstorms, graupel is fragile and easily crumbles under pressure. Meteorologists classify these particles using the METAR code GS. Because of their unique, often conical shape, graupel particles exhibit complex flight dynamics; smaller particles fall with their base downward, while those larger than 3 mm begin to tumble as they descend, which influences how far they travel horizontally from their point of origin.
Beyond its appearance, graupel poses a significant hazard in mountainous regions. Because it is denser and more granular than ordinary snow, it creates unstable layers on slopes. Fresh accumulations of 20–30 cm can trigger dangerous slab avalanches. Even thinner layers are treacherous, acting like ball bearings beneath subsequent snowfalls and creating extremely slippery surfaces. While graupel typically begins to weld and stabilize within one or two days, its presence remains a critical factor for avalanche safety in high-altitude climates, where it frequently appears in winter storms or alongside ice pellets.
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- Summary by Intellect · source: Graupel
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