What is graupel? Explaining the weird snow that fell in parts of Boise on Tuesday evening

Shaun Goodwin
·2 min read

The first round of heavy snow came and went through Boise Wednesday morning, dropping 5.2 inches on the city and leaving behind a threat of up to another 18 inches in the coming days.

But before the snow moved in, something else fell from the sky on Tuesday afternoon. It kind of fell like snowflakes but would bounce hard off surfaces before finally settling and melting away.

It’s a weather phenomenon called graupel, or as National Weather Service meteorologist Jaret Rogers put it to the Idaho Statesman, “soft hail.”

“As precipitation forms and freezes, it doesn’t turn into big hail just because there’s not a lot of stability (in the atmosphere),” Rogers said. “So it falls as soft hail; sometimes it can resemble snow pellets.”

Graupel typically occurs when temperatures are around freezing, and there’s an unstable atmosphere.

Individual snowflakes are formed in the upper atmosphere and begin to fall, as typical snowflakes do. But with enough instability in the atmosphere, supercooled water droplets then freeze onto the snowflake as part of a process called riming.

“Instability is what causes thunderstorms in the summer, warmer air moisture,” Rogers said. “We can still get that in the winter; it’s just not very much of it. But it still changes the way the precipitation forms. So instead of just rain or snow, graupel forms.”

Rogers noted that there was so much instability in the atmosphere on Tuesday that thunder and lightning were recorded near Mountain Home.

Riming is the same process that also creates something called rime ice or hoarfrost, but that occurs on foggy, still mornings when moisture close to the ground freezes upon solid objects like fences and branches.

Rime ice on a fence at the J.A. & Kathryn Albertson Family Foundation Bike Park at the base of Boise’s Military Reserve. Rime ice forms on cold, still mornings when there is fog or mist in the air.
Rime ice on a fence at the J.A. & Kathryn Albertson Family Foundation Bike Park at the base of Boise’s Military Reserve. Rime ice forms on cold, still mornings when there is fog or mist in the air.

The riming process on the snowflakes results in graupel, which is what was seen in parts of Boise on Tuesday afternoon. Graupel is also possible on Thursday, with the next round of snow expected to arrive on Thursday night and temperatures floating around freezing.