What Is Graupel?

Un oggetto prezioso
Matteo (merto87)/Flickr

When you think of wintry precipitation, you probably think of snow, sleet or maybe freezing rain. But it’s likely that the word “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 that's 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 more rare in continental climates. It's 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."