Graupel

April 21, 2015: If you happened to be out early this morning, you may have seen what looked like little balls of styrofoam falling from the sky.

A look at graupel from South Beloit this morning, courtesy of Suzan Laws

A look at graupel from South Beloit this morning, courtesy of Suzan Laws

These little balls of ice are called “graupel”, and they are similar to hail and sleet. We had conditions ideal enough to form these little pellets of ice.  Here’s how we get those:

 

Graupel starts out as snow crystals

Graupel starts out as snow crystals

Basically, the upper atmosphere has to be pretty cold.  Cold enough to create snow crystals.

The snow crystals fall through supercooled water droplets. The droplets freeze on to the snow crystals.

The snow crystals fall through supercooled water droplets. The droplets freeze on to the snow crystals.

As the snow crystals fall through the clouds and toward the ground, they fall through a layer of supercooled water drops. “Supercooled” means the water is at a temperature below water’s freezing point. Water needs to have some structure on which an ice crystal can form; if the water is “pure” and without dust, etc. for the ice to form onto, it will stay as a liquid.

Round balls of ice form as the freezing water droplets build up.

Round balls of ice form as the freezing water droplets build up.

So, as the snow crystals fall through this area of supercooled water droplets, the water droplets form ice on the snow crystals as the crystals come in contact with them.  This ice builds up to the point where the snow crystals are covered and resemble “pellets” or “balls” of ice.

The balls of ice or graupel fall to the surface after the ice builds up enough.

The balls of ice or graupel fall to the surface after the ice builds up enough.

This all falls to the surface, resembling bouncing balls of styrofoam as they fall. How do you tell the difference between graupel and sleet, hail, etc.? Graupel is brittle and falls apart easily.  Graupel is also pretty small, especially compared to hail.

We usually do not see graupel except in thunderstorms, or when the atmosphere is below freezing in a deep layer, similar to the cold conditions we had today.

-Alex

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Posted under weather

This post was written by Alex Kirchner on April 21, 2015

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