Rare self-rolling snowballs show up in Fond du Lac County

FRIENDSHIP - Several people stopped their cars Thursday to take in a rare phenomenon rolling in the fields and lawns along rural roads in northern Fond du Lac County.

Thousands of round, hay bale-shaped snowballs littered the countryside, each with a unique trail showing its south-west journey.

A rare anomaly, these self-made balls are known as “snow rollers.”

According to Tim Halbach of The National Weather Service in southeast Wisconsin, a very specific set of conditions need to come together for snow rollers to form.

RELATED: Finally, it's going to feel like spring. Get ready for melting snow

RELATED: Man leads high-speed chase before getting stuck in a snowbank

“A light layer of snow with a certain moisture content needs to be on top of a harder packed layer of snow,” Halbach said. “The top layer needs to be light enough to be blown by the wind, but wet enough to stick to itself as it travels over more snow.”

Halbach also said the winds needed to be around 30 miles per hour — “strong enough to push some snow, but not so strong that it blows the bale of snow apart.”

Once a bale forms, the wind has more surface to push which can keep the bale going for several yards, getting larger as it travels, picking up more snow.

“They usually happen when the temperature is a couple degrees above freezing,” Halbach said.

The rolls of snow are not expected to stick around for long as sun and higher temperatures are forecast for today and later this week.

The snow rollers come on the heels of Wednesday's snowfall, which added about three inches to the 11 dumped on the Fond du Lac area over the weekend.