ASK TOM: Graupel pellets

Dear Tom,

As a chilly rain began the other evening, I heard that some graupel was coming. What is that?

— Eric Bronsky, Skokie


Dear Eric,


Graupel is a type of frozen precipitation that is also referred to as snow grains or snow pellets. It forms when snowflakes fall through a layer of super-cooled water (water that remains liquid in a below-freezing environment), and those supercooled droplets coat the snowflakes with a layer of ice in a process known as riming.
Graupel pellets are white, roundish, soft and spongy, and are easily crushed, bearing a resemblance to tapioca or Styrofoam. They are small, usually less than 0.2 of an inch in diameter and bounce when hitting the ground. Graupel is common in spring and fall when the freezing level is close to the ground.