Chicago Weather: Mushy ice pellets may fall Thursday

Graupel shown on a car windshield.

October 18, 2012 12:23:39 PM PDT
What do you get when a snowflake collides with a rain droplet? A slushy ice pellet- called graupel.

If you see ice pellets falling from the sky on Thursday or Friday, it's not sleet. It's graupel.

"Graupel ice pellets tend to be white, small and more mushy than typical thunderstorm hail or sleet," according to a statement from the National Weather Service Forecast Office in Chicago.

It can be difficult to distinguish between sleet and graupel just by appearance, according to statement. What matters is how the precipitation forms.

Graupel is a result of a rapid drop off in temperatures and increase in elevation. Snowflakes high in the clouds encounter water droplets as they fall, and the snow causes the water to instantly freeze into ice or snow pellets.

Those pellets generally melt quickly, as temperatures on the ground are above freezing.

Sleet forms when snowflakes partially melt as they fall and then refreeze. Ground temperatures then are almost always above freezing.