By Nic Loyd, WSU meteorologist, and Linda Weiford, WSU News
Which is why much of Spokane was taken by surprise when small white Styrofoam-like beads coated the area on a chilly Sunday afternoon, Dec. 4.
Graupel – pronounced like grovel, but with a “p” instead of a “v” – is basically the mingling of snow and hail. Also called snow pellets or tapioca snow, it is created when snow falling high in the atmosphere encounters super-cooled water droplets that freeze around each snowflake, forming a delicate crust of ice crystals.
If you squeeze graupel between your fingers, it will crumble because snowflakes are inside the crust instead of solid ice. This makes it more dense and granular than snow but more slushy than hail.
Graupel is more common in high elevation areas such as Gunnison, Colo., and Laramie, Wyo. Why, then, did it unload on Spokane in early December?
As a storm was moving into the region, the cloud-level air was very cold while the surface air temperatures were relatively warm. In these unstable atmospheric conditions, falling snowflakes met up with super-cooled water droplets. In a process called accretion, the cold water instantly froze around the snowflakes to create gritty balls of white.
At higher altitudes, graupel is less likely to melt and break apart because it has less distance to travel before hitting the ground. Because of its density, it can trigger dangerous slab avalanches when a cohesive layer of graupel slides downslope over snowpack.
Though it sounds a bit like a German dessert, graupel is derived from the German term for cereal grain, according to the English Oxford Dictionary. The next time these not-quite-snow-but-not-quite-hail pellets fall onto lawns and roadways, keep in mind that it’s a weather phenomenon not often seen in our part of the world.
Weathercatch is a bimonthly column that appears in The Spokesman Review. Nic Loyd is a meteorologist with Washington State University’s AgWeatherNet. Linda Weiford is a WSU news writer and weather geek. Contact: firstname.lastname@example.org.