Soft, pellet-like hail formed when supercooled water freezes onto snowflakes is called graupel. It is also known as soft hail, snow pellets, or granular snow.
As a snow crystal falls through a cloud, it can collide with liquid droplets that remain unfrozen below 0°C. These supercooled droplets freeze on contact, gradually covering the original crystal with a rough layer of rime. When the snowflake’s shape is hidden and the particle becomes a small, fragile ball, it is classified as graupel.
Graupel is easy to confuse with hail and sleet, but the processes differ. Hail grows in powerful thunderstorms as strong updrafts repeatedly lift and freeze water into hard, layered stones. Sleet, also called ice pellets in many regions, refreezes before reaching the ground. Graupel is softer and usually breaks apart when pressed. Tiny supercooled droplets can remain liquid at temperatures as low as about −40°C under suitable conditions.