Graupel a Mix of Snow and Hail

When you think of wintry precipitation, you probably think of snow, sleet or maybe freezing rain. But it’s likely that the word â€Å"graupel† doesn’t come to mind.  Although it sounds more like a German dish than a weather event, graupel is a type of winter precipitation thats a mix of snow and hail. Graupel is also known as  snow pellets, soft hail, small hail, tapioca snow, rimed snow and ice balls. The  World Meteorological Organization  defines small hail as snow pellets encapsulated by ice, a precipitation halfway between graupel and hail. How Graupel Forms Graupel forms when snow in the atmosphere encounters supercooled water. In a process known as accretion, ice crystals form instantly on the outside of the snowflake and accumulate until the original snowflake is no longer visible or distinguishable. The coating of these ice crystals on the outside of the snow is called a rime coating. The size of graupel is typically under 5 millimeters, but some graupel can be the size of a quarter (coin).  Graupel  pellets are cloudy or white—not clear like sleet. Graupel forms fragile, oblong shapes and falls in place of typical  snowflakes  in  wintry mix  situations, often in concert with  ice pellets. Graupel is also fragile enough that it will typically fall apart when touched. Graupel Vs. Hail To tell the difference between graupel and hail, you simply have to touch a graupel ball. Graupel pellets typically fall apart when touched or when they hit the ground. Hail is formed when layers of ice accumulate and are very hard as a result. Avalanches Graupel commonly forms in high-altitude climates and is both denser and more  granular  than ordinary  snow, due to its rimed exterior. Macroscopically, graupel resembles small beads of  polystyrene. The combination of density and low viscosity makes fresh layers of graupel unstable on slopes, and some layers result in a high risk of dangerous slab  avalanches. In addition, thinner layers of graupel falling at low temperatures can act as ball bearings below subsequent falls of more naturally stable snow, rendering them also liable to avalanche.  Graupel tends to compact and stabilize (weld) approximately one or two days after falling, depending on the temperature and the properties of the graupel. The National Avalanche Center refers to graupel as a Styrofoam ball type of snow that stings your face when it falls from the sky. It forms from strong convective activity within a storm (upward vertical motion) caused by the passage of a cold front or springtime convective showers. The static buildup from all these falling graupel pellets sometimes cause lightning as well. It looks and behaves like a pile of ball bearings. Graupel is a common weak layer in maritime climates but rarer in continental climates. Its extra tricky because it tends to roll off cliffs and steeper terrain and collect on the gentler terrain at the bottom of cliffs. Climbers and extreme riders sometimes trigger graupel avalanches after they have descended steep terrain (45-60 degrees) and have finally arrived on the gentler slopes below (35-45 degrees)—just when they are starting to relax. Graupel weak layers usually stabilize in about a day or two after a storm, depending on temperature.