The chances are that the last time you stood in line to pay for a few supermarket staples right before a winter storm, you and the person ahead of you didn’t exchange concerns about a graupel on its way. Most Americans haven’t even heard of the word. In fact, a graupel is a weather term for certain types of precipitation.
Definition of graupel
According to The Weather Channel©, graupel is a type of frozen precipitation. It consists of snowflakes or ice crystals with super-cooled water drops that have frozen together and look like white pellets. A more common term for the frozen mass is snow pellets or tapioca snow.
Graupel can be as big as a quarter, Skyview Weather reports. And it’s very easy to mistake it for hail. Sometimes the only way to differentiate the two is to touch the frozen ball. If it’s graupel, the pellets usually fall apart when they hit the ground or are touched. Hail is much harder.
How graupel forms
Graupel occurs across the United States, not just in the coldest regions of the country. Super-cooled water droplets that exist at temperatures as low as -40 degrees F create an unusual scenario when they encounter snow crystals. According to Wikipedia, when the two meet and mingle, liquid droplets freeze onto the surface of the crystals.
Snow crystals form as one of four types: plates, dendrites, columns and needles. During the formation of graupel, an affected snow crystal grows to accommodate the process of droplets freezing to its surface, which is known as accretion. Once the process has progressed to the point that the original snow crystal can no longer be identified, the morphed body is known as graupel. The actual crystals with frozen droplets on their surfaces are said to be rimed.
Link to avalanches
When graupel is present on the slopes, meteorologists and outdoor experts have a concern about the possibility of avalanches. This is because graupel typically forms at higher altitudes. Once it’s rimed, it’s denser and has a grainier texture than snow. This makes fresh graupel layers somewhat unstable on the ground. The highest risk for an avalanche occurs when the layers of graupel on the ground reach 20 to 30 cm.
Sometimes relatively thin layers of graupel form underneath accumulating layers of snow. The more recent snowfall therefore sits on a relatively slippery surface. As snow continues to fall, the chance of an avalanche escalates. However, most graupel stabilizes within two days after forming.
