Frosted tips

Looking out for rime and hoar frost on those glum days.

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When I drove out to Blue Mound State Park with my girlfriend this Sunday, it was grey and foggy all the way—until we actually got up toward the main park road, when the air abruptly cleared to reveal a bright and almost cloudless sky. As we headed into the woods on a downhill trail, that sky became the perfect backdrop for thousands of branches coated in finely textured, shimmering ice. I took the above photo with my phone and no filters, and I’m not much of a photographer. The light and the delicately frosted trees were just that generous.

I got back to see multiple friends posting their own photos on Instagram and talking about the beauty of hoar frost. Local weather reports around the country this week seem to be awash in lessons about the distinction between hoar frost and rime ice, and Capital Times photographer Ruthie Hauge offered an in-depth look at the latter. Whichever phenomenon you’re technically witnessing, all I really knew up until this week is that it brightens the winter and that one of my favorite Sonic Youth songs is called “Hoarfrost.” Lee Ranaldo’s lines about “A view through the trees to a couple standing in the snow” are evocative, but I went in search of a more scientific perspective.

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“The water vapor is condensing, sort of similar to the way that dew condenses on a car in the summer time,” a horticulturist friend, Daniella Echeverria, explained to me. She noted that over on the west side of town, where the fog was very heavy over the weekend, and that the displays of frost were quite striking in the UW-Madison Arboretum and the West Madison Agricultural Research Station out on Mineral Point Road.

I also reached out to New Glarus resident and amateur meteorologist Scott Rippe, who runs the popular weather page Rippeology when he’s not working in his role at a local marketing firm. Rippe told me that what I saw at Blue Mound was rime, not hoar frost. “It is an atmospheric splitting of hairs, but they are different, although often confused with each other,” Rippe says. He gave me a detailed breakdown:

“The one, foolproof way to know the difference between them is to observe the conditions under which they form. Rime is the result of dense fog and the right temperatures (generally 10-15 degrees below freezing). Water vapor in the air (i.e., very high humidity and dew point) settles on a surface that is freezing—like trees and roofs and shrubs. Vapor freezes on top of vapor on top of vapor, which forms the very intricate patterns you see. Rime accumulates.

“Hoarfrost generally forms in non-fog conditions. Instead of water droplets that exist in fog, hoarfrost (or just generally, frost) is simply a crystallization of ice on the surface of objects. It is more related to the “usual” frost that we see on the grass on an early spring cold night.  Hoarfrost can accumulate on surfaces on clear nights, when heat and moisture are escaping the earth’s surface. You’ll wake up to a sunny morning and see it sparkling on trees and even roads.”

Rippe adds: “More simply: rime = water vapor trapped; hoar frost = water vapor escaping. Generally, rime will be best observed in higher elevations and hoarfrost in lower elevations.”

Given how much people seem to be enjoying the sights, Rippe offered some advice for viewing boath rime and hoar frost. “Good indications of rime, if you have a home weather station: dew point and air temperature are very close to each other about two to four hours after sunset,” Rippe says. “Cloudy conditions, high humidity—essentially meaning that the water vapor is trapped at the surface, and will settle on freezing surfaces.

“Good indications for hoarfrost are a cloudy, humid day, followed by a clear night and rapidly dropping temperatures,” he adds.

We all have to look for small sources of emotional uplift during the long and often glum winter. It cheered me to learn that something as bleak (and dangerous) as a dense snowy fog can also help to create something beautiful.