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 tightly on 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 soft hail by meteorologists.
While graupel has the appearance of white snow pellets, its formation is closer to that of hailstones. Winter storm clouds often contain water droplets which have cooled far below 32 degrees Fahrenheit or 0 degrees Celsius without turning to ice. Sometimes those supercooled droplets contact dust particles or ice crystals and form solid hail. Other times, the supercooled droplets attach themselves to snowflakes and freeze instantly. The air currents within the storm clouds continue to push the ice-covered snow flakes through supercooled water until they become too heavy and fall to the ground as graupel.
Because graupel is white and powdery, many people consider it to be a form of snow. Others argue that its formation is similar to hail, 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 icy hail during severe thunderstorms and tornadoes. The air temperature near the ground is not as critical for graupel formation as the conditions thousands of feet in the air.
While graupel is not considered especially hazardous in its own right, it can create some dangerous conditions on the ground during the winter season. If a layer of traditional snow is followed by a substantial layer of graupel, the next layer of snow will not pack securely. This instability could lead to an avalanche in higher elevations, since the layer of graupel would act like ball bearings between two solid layers of packed snow.
Some snow skiers do welcome an occasional layer of graupel because it can increase downhill speeds as the skis slide over the pellets. Too much graupel can make conditions on the slopes too unstable, however. This type of snow does not pack together very well, which does not make it popular for other outdoor winter activities. A heavy downburst of graupel can create temporary white-out conditions for drivers, but it does not stick or build up on windshields or roads in the same way heavy traditional snowflakes can.
By Michael Pollick