What in the World Is Graupel?

Getting pelted with graupel is like being hit with soft hail or hard snow. Jne Valokuvaus/Shutterstock

Winter weather is something we all recognize. Snow's easy. We may struggle with the difference between sleet and hail, but we basically known it's ice raining down from the sky that can damage our cars.

But would you know graupel if it pelted you on a cold winter's day? Have you even heard of graupel before now?

This type of winter weather precipitation is a mix of snow and hail. In fact, it's often called soft hail, among other names, including snow pellets, tapioca snow, rimed snow and ice balls.

Rimed snow is actually a pretty solid name for graupel, even though it's far less fun to say. The name helps explain how graupel forms.

When the atmospheric conditions are just right, snow crystals may come in contact with super-cooled water droplets called rime. And by "super-cooled," we mean the droplets are still in a liquid form at minus 40 degrees Fahrenheit or Celsius (they're the same). Once the droplets make contact with the crystals, however, they begin to freeze. The result is that the snow crystal is now rimed, hence the name rimed snow. As the freezing process continues, the original shape and form of the snow crystal becomes lost to its new frozen nature.

The result is graupel.

The progression of a snow crystal to graupel
From left, you can see the formation of graupel on this particular snow crystal. LiWei/Wikimedia Commons

From left, you can see the formation of graupel on this particular snow crystal. (Photos: LiWei/Wikimedia Commons)

How do you know if you're dealing with graupel or sleet? Sleet is definitely sturdier than graupel; it bounces when it hits a surface. Graupel will either simply land on the surface, much like snow, or break apart fairly easily if you touch it, according to the World Atlas. Additionally, their formation process is different as well, with sleet being the result of snow melting and then freezing again before it hits the ground.

Graupel also won't really hurt you, or anything for that matter, as it falls. It feels more like you're being pelted in a very tentative manner with something not quite soft and not quite hard. It's an odd, but weirdly pleasant, sensation.

It can, however, become a danger when it comes to avalanches. Thanks to their denser nature and larger sizes than regular snow, graupel can contribute to the formation of slab avalanches, according to a 1966 avalanche study performed by the University of Washington. Either the graupel functions as a "lubricating layer" that encourages avalanches, or it becomes the "dense, cohesive slab layer" which, when it becomes 20 to 30 centimeters thick, is primed for a slab avalanche.

So unless you're near avalanche-prone areas, graupel isn't likely to cause too many problems that you wouldn't otherwise experience during an regular snowfall.