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Graupel, a form of frozen precipitation that resembles small white snowballs, is formed when supercooled water droplets freeze instantly onto snowflakes and fall to the ground. It can create dangerous conditions on the ground during winter, but some skiers enjoy it for increased speed.
Some people who live in snowy regions may notice two different types of snow during the winter storm season. One is the familiar crystalline snowflake, which falls from the sky and packs itself firmly to the ground. Another form of frozen precipitation, however, often falls during very cold weather and has the consistency of small pellets. This type of snow is known as graupel, also called hail by meteorologists.
While graupel has the appearance of white snowballs, its formation is closer to that of hailstones. Winter storm clouds often contain droplets of water that have cooled far below 32 degrees Fahrenheit or 0 degrees Celsius without turning to ice. Sometimes those supercooled droplets come into contact with dust particles or ice crystals and form solid hail. Other times, supercooled droplets stick to snowflakes and freeze instantly. Air currents within the storm clouds continue to push the ice-coated snowflakes through the supercooled water until they become too heavy and fall to the ground as graupels.
Because graupel is white and powdery, many people think of it as a form of snow. Others argue that its formation is hail-like, so graupel should be considered a form of soft hail. Although graupel typically falls during cold winter storms, it has been known to form along with frozen hail during severe thunderstorms and tornadoes. Air temperature near the ground is not as critical to graupel formation as conditions thousands of feet in the air.
While graupel isn’t considered particularly dangerous by itself, it can create some dangerous conditions on the ground during the winter season. If a layer of traditional snow is followed by a solid layer of graupel, the next layer of snow will not pack securely. This instability could lead to an avalanche at higher elevations, as the graupel layer would act like a ball bearing between two solid layers of packed snow.
Some snow skiers like an occasional layer of graupel because it can increase descent speed as the skis glide over the pellets. However, too much graupel can make conditions on the slopes too unstable. This type of snow doesn’t pack very well, which makes it unpopular for other winter outdoor activities. A heavy discharge of graupel can create temporary bleaching conditions for drivers, but it doesn’t stick or accumulate on windshields or roads the same way heavy traditional snowflakes do.
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