Key takeaways
- Graupel is soft, round, white pellets formed when supercooled droplets freeze onto snowflakes (riming).
- It looks like tiny Styrofoam balls — distinct from hard, layered hail and from ordinary snow.
- A layer of round graupel can act like ball bearings in the snowpack.
- That makes graupel a sneaky weak layer other snow can slide on — an avalanche concern.
From German Graupel, 'soft hail'.
What graupel is
Graupel is a form of precipitation consisting of soft, round, white pellets, formed when supercooled water droplets freeze onto falling snowflakes (a process called riming), creating small balls of rimed snow. It looks like tiny Styrofoam balls and crumbles when squeezed.
Graupel vs hail vs snow
Graupel is soft and crumbly, small and round — it falls apart when pressed. Hail is harder, often larger, layered solid ice from thunderstorms. Snow falls as crystalline flakes. Graupel is essentially heavily rimed snow, sitting between the two, common in convective winter showers and mountain storms.
During a squally storm, a ski tourer notices the snow falling as soft round pellets — graupel — piling up in a distinct layer. Days later, after more snow buries it, they treat that buried graupel layer as a suspect weak layer when assessing whether slopes might slab avalanche.
Why it matters for avalanches
Because its small, round pellets don’t bond well, a buried graupel layer can act like ball bearings in the snowpack — a weak layer overlying snow can slide on. Graupel that falls and then gets buried can create a sneaky, hard-to-detect weak layer contributing to slab avalanches, sometimes a persistent slab concern. Recognizing it during or after a storm helps assess danger.
The bottom line
Graupel is soft, round, white pellets — rimed snow formed when supercooled droplets freeze onto snowflakes, looking like tiny Styrofoam balls (distinct from hard hail and crystalline snow). Beyond being a curiosity underfoot, buried graupel can act like ball bearings in the snowpack, forming a sneaky weak layer that other snow slides on — making it a real avalanche concern worth recognizing.
Frequently asked questions
What is graupel?
Graupel is a type of precipitation made of soft, round, white pellets. It forms when supercooled water droplets in a cloud freeze onto a falling snowflake — a process called riming — building up into a small ball of rimed snow. Graupel looks like tiny Styrofoam or foam balls and crunches softly, unlike hard hail.
How is graupel different from hail and snow?
Graupel is soft and crumbly — it falls apart when you squeeze it — and is small and round. Hail is harder, often larger, and consists of layered solid ice, typically from thunderstorms. Regular snow falls as crystalline flakes. Graupel is essentially heavily rimed snow, sitting between snow and hail, and is common in convective winter showers and mountain storms.
Why does graupel matter for avalanches?
Because its small, round, hard-ish pellets don't bond well together or to the snow around them, a layer of graupel buried in the snowpack can act like ball bearings — a weak layer that overlying snow can slide on. Graupel that falls during a storm and then gets buried by more snow can create a sneaky, sometimes hard-to-detect weak layer, contributing to slab avalanches. So recognizing graupel during or after a storm is useful for assessing avalanche danger.
Sources
- Snow science & avalanches — American Avalanche Association
- Precipitation types — National Weather Service