Rime Ice & Hoar Frost - Different Expressions Of How, In Winter, Water Can Be Beautiful

Jan 7, 2021

If you’ve been outside or just looked out your window, you may have seen a frosty or icy display of winter weather. Depending on where you live, that's hoar frost or rime ice. They look a lot alike. The major difference is how they're formed.

Jessie Crow Mermel is an educator and naturalist with Severson Dells Nature Center. She said when conditions are cold, clear and calm, hoar frost "forms like dew."

"And it's when the water vapor -- which is in the gas form -- turns into the solid state, which is ice crystals directly landing on objects that are below freezing," Mermel said. "The water in this process never actually goes through the liquid form."

On the other hand, Mermel said, rime ice forms in the fog.

"We've been having a lot of foggy conditions the last few days," she said, "which is why we're seeing rime ice in the mornings and throughout the day."

Mermel explained the process.

"It's actually when 'super-cooled' liquid water droplets in the fog turn solid when they land on surfaces like trees, fences and whatnot," she said.

These densely clustered ice crystals tend to look a little more spiky than hoar frost and they have been sticking around longer than usual.

"We do see them both from time to time," she said. "What's rare about it this time around is the duration that we're seeing it [rime ice] for. We've had these foggy conditions for a few days now and so the fact that we're able to enjoy it for as long as we are, that's more rare."

Rime ice in downtown Rockford.
Credit Connie Kuntz

Usually, by the time the sun comes out, the ice crystals start to melt.

In terms of predicting rime ice or hoar frost, Mermel said the conditions in which both occur can be predicted, but the presence of ice crystals cannot be guaranteed.

"But," she said, "if we have a foggy night and the temperatures are below freezing, I am definitely going to make sure that my camera is charged and I can go out in the morning and try to captures some of the winter wonderland."

Mermel said that seeing the rime ice is a reminder of the beauty and wonder of all the different expressions of the natural world in the winter.

"It's just a great reminder to get outside and experience that."