What is Graupel

Friday, February 7, 2020 - 2:27pm

During the early afternoon hours today i started getting reports of hail and sleet occurring in portions of the county. Based on the atmospheric conditions and lack of certain supporting factors that could lead to hail or sleet. I had to come to the conclusion of the precipitation being spotted as graupel. You may be asking yourself "What is Graupel?"

Graupel forms high in the atmosphere where it’s common to have liquid water with a temperature much colder than 32°F. This is called super-cooled water. When a snowflake interacts with a layer of super-cooled water, it instantaneously freezes into tiny ice crystals on the surface of the snowflake in a process called accretion. Because the ice crystals form quickly, they stack atop one another in a disorganized order. This creates lot of tiny air pockets, which in turn creates a fluffy looking round ball. As the process continues, the graupel grows in size and eventually falls to the ground.

For hail to occur there needs to be an updraft which typically comes with thunderstorms like we experienced yesterday. The updraft lifts moisture above the freezing line in the atmosphere. depending on how strong the updraft is a hail stone could get tossed back into the atmosphere a few times making it larger and larger before it finally falls to the ground. We simply do not have that lift or updraft potential in the atmosphere today over the county. 

For sleet to have been the culprit this would have needed to occur. There must be a layer of air near the ground whose temperature is below freezing, where water turns to ice. Above this layer of freezing air must be a layer of warmer air. As snow falls through the warm air, it melts or partially melts into raindrops. As the melted snow falls through the cold layer of air, it re-freezes. It forms ice pellets, or sleet, before hitting the ground.

During the timeframe the weather reports for hail and sleet were coming in the layer of cold air causing the issue was very shallow around 370 feet in the atmosphere. Typically with hail to form the freezing line must be around 20,000 feet. There certainly hasn't been clouds topping out that high today around the county. 

 

The above image shared by a follower on my Facebook page today as one of the reports is a perfect picture of what Graupel looks like. 

 

Image result for small hail

The above image is a good example of what hail looks like

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The above image is an example of sleet. 

The crazy thing with Graupel is its not a normal occurrence. Unless you are typically in really high altitudes. This just happens to be another crazy weather day here in Rowan County!

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