What Is Graupel?

Winter precipitation is a diverse and not always clear-cut category. You know a snowflake when you see one, but what about sleet versus hail? Even more confusing is graupel, another form of icy rain to send you into a state of head-scratching confusion.

This oddly named and little-recognized form of precipitation is essentially a mix of snow and hail. In fact, it's sometimes called soft hail because of their similarities. It's also often described as snow pellets, tapioca snow, rimed snow, and ice balls.

Learn more about this mystifying category of weather you may have seen but probably never heard of.

How Does Graupel Form?

Although "rimed snow" is decidedly less fun to say than "graupel," it's quite descriptive of how the precipitation forms.

When the atmospheric conditions are just right, snow crystals may come in contact with super-cooled water droplets called rime, a form of frost. The official term "super-cooled" means droplets are still in a liquid form at minus 40 degrees (when the temperature is this low, Fahrenheit or Celsius are the same). Only when these frigid droplets make contact with snow crystals do they begin to freeze. The result is a rimed snow crystal.

As the freezing process continues, the original shape and form of the snow crystal becomes lost to its new frozen nature. This final product is graupel.

Sleet, Snow, Hail, or Graupel?

There are so many frozen forms of precipitation it's hard to know what's actually falling from the sky in the winter. Graupel is most often confused with hail, and while both tend to be spherical, hail is generally larger and always more solid than graupel.

Sleet is also sturdier than graupel; it bounces when it hits a surface. Graupel will either simply land on the surface like snow or break apart fairly easily if you touch it. Their formation process is also different, with sleet being the result of snow melting and refreezing before it hits the ground.

Graupel does not take the form of a flake like snow.

Graupel and Avalanches

Graupel is so soft it doesn't cause the property damage that hail can cause, but it can lead to bigger problems than a few dents in your car.

Thanks to its denser nature and larger size than regular snow, graupel can contribute to the formation of slab avalanches. Either the graupel functions as the lubricating layer that leads to a slip or it becomes the dense slab layer itself, growing 20 to 30 centimeters thick.

A slab avalanche occurs when layers underneath densely packed snow and/or ice are weak and collapse, sending everything on top of it sliding down the mountain. The avalanche says graupel creates a risk of this because the layer it creates contains "a great deal of pore space and poor bonding."