Have you ever seen little balls of snow falling from the sky and wondered what they were? They look like hail, but they aren’t. They also kind of look like snow, but again they aren’t. What exactly is this stuff?
It’s called graupel, also referred to as snow pellets. These little balls of snow are composed of snowflakes and super-cooled water. Simply touching the graupel can cause it to melt and fall apart, unlike hail which is more durable, heavy and solid. In fact, if you pinch a graupel ball, you can compress it quite a bit. It’s kind of strange stuff.
Graupel forms when snowflakes interact with super-cooled water in a process called accretion. In the upper atmosphere, it’s quite common to have water in liquid form that is well below its freezing point of 32°F, and as the super-cooled water touches the snowflakes, the water instantaneously freezes. Because this happens so quickly, the tiny ice crystals form without combining with one another. They instead form atop one another. As the process continues, the original snowflake gets coated in the frozen water crystals and takes on a fluffy or “airy” look. Hailstones on the other hand, form when rain drops slowly freeze after they have collided with a dust particle, creating an ice stone made of layers.
In Colorado, graupel is fairly common due to the higher altitude, but it can form anywhere that it snows. It’s just more difficult at lower altitudes because there is a greater height separation between the super-cooled water high above and the ground below. At higher altitudes, like Denver and Colorado Springs, the graupel has less distance to travel before it hits the ground. Conversely, graupel will have to fall an additional mile after forming before it reaches the ground in a location that’s closer to sea level.