Backcountry Science: Rime Ice

This past weekend formed one of the first major examples this season of a somewhat common winter phenomenon in the central Appalachian high county: rime ice. Often mistaken for “snow” in trees and other objects exposed above the surface, rime ice (sometimes shortened to just “rime”) instead occurs most often not in a snowstorm but when thick clouds or fog envelop our higher summits. When conditions are right, these supercooled water droplets rapidly freeze on exposed objects, particularly trees and other standing vegetation. When riming occurs with the passage of a cool front, as it did late this past week, it can leave behind a striking crust of brilliantly white ice contrasted against a clear, cobalt blue sky. Check out Mount Rogers and Whitetop, Virginia’s two highest summits, this past Saturday, as well as High Knob, Virginia, below. Can you tell in each photo how far down the mountain the cloud deck made it the previous night?

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Rime ice in mid-December 2016 on Mount Rogers (left) and Whitetop (right) – Virginia’s two highest mountain summits
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Rime on High Knob, Virginia

There’s a little more to riming than just the ice, though. Rime ice often forms when somewhat consistent winds blow airborne water droplets across exposed surfaces. These winds then create a “flag-type” effect, where ice deposits on an object like a tree branch or stem in the direction of the wind. Check out the image the below of rime ice formed in this fashion on the summit of Wise County, Virginia’s High Knob, or see this process in real time via this video from Mount Washington, New Hampshire.

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Rime “flagging” on vegetation atop southwest Virginia’s High Knob

Believe it or not, rime can also contribute to keeping our higher mountain summits more moist than surrounding areas. Since rime often forms without significant precipitation (such as snow) falling from the cloud layer, this means that high elevations that are colder and more persistently in the clouds can see higher amounts of water deposited in the form of rime ice…while valleys see little to no added moisture at all. And since that rime eventually has to melt, the water it contains goes back into the mountaintop soils and ultimately feeds the streams and rivers that cascade below. It’s these traits and the unique formation of rime that makes it much, much more than simply “snow” high up in the treetops.

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Rime coating thousands of trees on the High Knob Massif of southwest Virginia, eventually feeding the headwaters of Big Stoney Creek with meltwater.
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