Graupel

Graupel (also called soft hail) refers to precipitation that forms when supercooled droplets of water condense on a snowflake, forming a 2–5 mm ball of rime; the snowflake acts as a nucleus of condensation in this process.

Graupel is both denser than ordinary snow and granular, in both cases due to its rimed exterior. The combination of weight and low viscosity makes fresh layers of graupel unstable on slopes, and layers of 20-30 cm present a high risk of dangerous slab avalanches. In addition, thinner layers of graupel falling at low temperatures can act as ball bearings below subsequent falls of more naturally stable snow, rendering them also liable to avalanche. Graupel tends to compact and stabilise approximately one or two days after falling, depending on the temperature and the properties of the graupel.