What distinguishes graupel from hail and sleet?

Weather Blog

Dear Tom,
What distinguishes graupel from hail and sleet?
—Paul Lockwood, Woodstock
Dear Paul,
Falling snow occasionally encounters supercooled water droplets (liquid droplets remaining as liquid at subfreezing temperatures) as it descends. The droplets freeze on contact with snowflakes in a process called riming. If this process continues long enough, the shape of the snow crystals becomes unidentifiable and the resulting crystals are referred to as graupel. Hail falls from cumulonimbus clouds (thunderstorms) or other convective clouds and consists of hard, rather uniform layers of ice. Sleet forms when snow aloft falls into above-freezing air and melts into water drops, then continues down into below-freezing air and refreezes into ice pellets before reaching the ground.

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