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Fascinated by frost: Exploring the difference between hoar frost and rime ice, where to spot it

These recent frosty days sent me on a hunt to remind myself once again of the difference between hoar frost and rime ice — as they tend to look virtually the same until you get an up-close look.

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Heavy frost clings to the trees and foliage along the Mississippi River on Tuesday morning, Jan. 3, 2023, near Lake Irving in Bemidji.
Annalise Braught / Bemidji Pioneer
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Thanks to a combination of foggy days and a lot of excess moisture in the air over the past week, the region has once again been treated to an even more picturesque winter wonderland than usual.

I spent some time out capturing the beauty on camera on Tuesday morning as the sun came out just enough to leave the frosty foliage with a gorgeous blue backdrop.

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Heavy frost clings to the trees and foliage along the Mississippi River on Tuesday morning, Jan. 3, 2023, near Lake Irving.
Annalise Braught / Bemidji Pioneer

Every winter when we have days like these it sends me on a hunt to remind myself of the difference between hoar frost and rime ice — as they tend to look virtually the same until you get an up-close look and do some digging on the weather conditions that cause the crystals to form.

After doing some digging, it seems the main difference between the two lies in the source of the moisture.

According to the National Weather Service, hoar frost typically develops when the water vapor in the air comes into contact with solid surfaces that are already below freezing point — 32 degrees.

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Ice crystals then begin to form and the ice continues to grow as more water vapor is frozen. The size of the frost that forms depend on how much water vapor is available to “feed” the ice crystals as they grow.

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Heavy frost clings to the leftover seeds on a maple tree along the Mississippi River on Tuesday morning, Jan. 3, 2023, near Lake Irving.
Annalise Braught / Bemidji Pioneer

Hoar frost has a distinctive appearance, forming hair-like or feathery structures as it grows. When there is little to no wind, this allows for more complex lacy deposits of crystals to form.

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Hoar frost clings to the foliage along the Mississippi River on Tuesday morning, Jan. 3, 2023, near Lake Irving.
Annalise Braught / Bemidji Pioneer

With rime ice, however, the moisture comes from freezing fog — water droplets — that turn directly from a liquid state to a solid state, or through direct freezing. So, rime ice is more like accumulating freezing dew rather than thick frost and generally looks more like icy needles pointing off of subjects.

This little bit of research led me to determine that what I was photographing this week was indeed hoar frost.

I also learned that apparently the scandalous-sounding term "hoar frost" is derived from the old English word “hoary,” which means grayish white or getting on in age. AccuWeather says it comes from frost resembling an old man's beard.

I mean, it makes sense if you think about it. Much of the frost-covered foliage — especially evergreens — does look kind of "hair-like" with their white, feathery beards.

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Frost clings to the needles of a white pine tree along Paul Bunyan Drive South on Tuesday morning, Jan. 3, 2023, near the Wastewater Treatment Plant.
Annalise Braught / Bemidji Pioneer
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Heavy frost clings to the trees and foliage along the Mississippi River on Tuesday morning, Jan. 3, 2023, near Lake Irving.
Annalise Braught / Bemidji Pioneer

My favorite spot in town when it gets frosty is down by the Lake Irving boat landing near the Wastewater Treatment Plant. The frost is always super thick and you get a beautiful variety of trees, cattails and other foliage along with some open water where the Mississippi flows under the bridge and railroad tracks.

I found this interesting, too, when reading an AccuWeather article that specifically mentions how there will obviously be more moisture and humid air flowing into an area when there is a release of moisture from an unfrozen stream or lake.

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Heavy frost clings to the trees and foliage along the Mississippi River on Tuesday morning, Jan. 3, 2023, where it flows through Lake Irving.
Annalise Braught / Bemidji Pioneer

"Some of the best places to look for hoar frost on a calm, frigid morning may be downstream of a sewage treatment plant or spring around daybreak or shortly thereafter," AccuWeather Senior Meteorologist Dave Dombek said in the article.

So I guess that's one more reason why it's such a picturesque location on frosty mornings.

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Heavy frost clings to the trees and foliage along Lake Irving on Tuesday morning, Jan. 3, 2023, in Bemidji.
Annalise Braught / Bemidji Pioneer

It’s also a great spot to capture sunsets and sundogs if you’re ever looking for a good place away from the bustle of town and have a scene free of cars and powerlines, even if it might not smell the best. But hey, if you don't want to go yourself that's the advantage of looking at it in a photo right?

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Frost clings to the pine needles on evergreen trees along Paul Bunyan Drive South on Tuesday morning, Jan. 3, 2023, near the Wastewater Treatment Plant.
Annalise Braught / Bemidji Pioneer
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Foliage is covered in frost on Tuesday morning, Jan. 3, 2023, along Lake Irving.
Annalise Braught / Bemidji Pioneer
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Heavy frost clings to the trees and foliage along the Mississippi River on Tuesday morning, Jan. 3, 2023, near Lake Irving.
Annalise Braught / Bemidji Pioneer
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The trees and foliage along the railroad tracks near Paul Bunyan Drive South are coated in a layer of hoar frost on Tuesday morning, Jan. 3, 2023, near the Wastewater Treatment Plant.
Annalise Braught / Bemidji Pioneer
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The trees and foliage surrounding Lake Irving are coated in a layer of hoar frost on Tuesday morning, Jan. 3, 2023, in Bemidji.
Annalise Braught / Bemidji Pioneer
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Foliage is covered in frost on Tuesday morning, Jan. 3, 2023, along Lake Irving.
Annalise Braught / Bemidji Pioneer
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Frost clings to the needles of a white pine tree along Paul Bunyan Drive South on Tuesday morning, Jan. 3, 2023, near the Wastewater Treatment Plant.
Annalise Braught / Bemidji Pioneer
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The trees and foliage surrounding Lake Irving are coated in a layer of hoar frost on Tuesday morning, Jan. 3, 2023, in Bemidji.
Annalise Braught / Bemidji Pioneer

Annalise is the editor and a photographer at the Bemidji Pioneer. She is a Mass Communication graduate from Bemidji State University. Her favorite pastime is exploring the great outdoors and capturing its natural beauty on camera. Contact Annalise at (218) 333-9796, (218) 358-1990 or abraught@bemidjipioneer.com.
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