Have you ever seen little balls of snow that look like Styrofoam falling from the sky and wondered what they were? They look like hail, but they’re soft. They feel like snow, but they’re round. What exactly is this stuff?
It’s called graupel, also referred to as snow pellets, soft hail, small hail, tapioca snow, rimed snow, and ice balls. Simply touching the graupel can cause it to quickly melt, unlike hail which is more durable, heavy and solid. In fact, if you pinch graupel, you can compress it quite a bit. It’s kind of strange stuff.
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. On the other hand, when a raindrop gets lifted into the super-cooled water, hail forms instead.
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.
Graupel is more common at higher altitudes, but it can form anywhere it snows. It’s just more difficult at lower altitudes because there is a greater separation between the super-cooled water high above and the ground below. The more atmosphere the graupel falls through, the more time it has to melt.
We seem to get a lot of this stuff in Colorado Springs, which has an altitude of about 6,200 feet.