I’ve grappled with the term graupel myself. It has been called, “soft hail”,
“snow pellets”, “granular snow”, and I once heard it described as “tapioca snow”. The word graupel is from German derivation meaning ‘kernel’.
The American Meteorological Society’s official definition of graupel is;
“Heavily rimed snow particles, often called snow pellets; often indistinguishable
from very small soft hail except for the size convention that hail must have a
diameter greater than 5 mm. Sometimes distinguished by shape into conical,
hexagonal, and lump (irregular) graupel.”
Graupel is precipitation that forms when a tiny droplet of water becomes
supercooled. When the still liquid droplets freeze on the surface of the snow
crystals they grow. This growth is called accretion. It is the contact between
the snow crystals and these droplets that forms these small, usually soft,
lumps of ice.
Graupel can fall from a winter storm or even a spring thunderstorm. It forms
basically the same way as hail but these very soft and small pieces of ice often break on contact with the ground and is less than 5 mm, or less than a quarter of an inch, in diameter.
Impress your family and friends next time you experience some small lumpy ice solids with rain, tell your friends that it’s called graupel and you know how it
forms. Just be sure and not use the term, “tapioca snow”.
If you have a weather question for Dan, send it to: askdan@kjrh.com
