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