Parts of the South awoke to a display of jagged rime ice Wednesday morning, the result of “freezing fog” that descended across the area. Alerts were issued in Alabama, Georgia, Tennessee, Southwest Virginia and parts of the Carolinas, where the National Weather Service advised of low visibility and possible slick roads.
Freezing fog is exactly what it sounds like — fog that coats objects on the ground with a slippery layer of ice. It forms at temperatures below freezing, when moisture in the air takes the form of supercooled water droplets but remains in a liquid state.
Only when those droplets have something to freeze onto, like a tree, vehicle or road overpass, are they able to create telltale prickles of ice.
In the case of Wednesday’s episode across the South, surface temperatures remained below freezing long enough to support up to eight hours of freezing fog. In Huntsville, Ala., temperatures dropped as low as 21 degrees with 92 percent humidity just before sunrise Wednesday.
Daytime temperatures above freezing meant that roadways were sufficiently warm to prevent widespread glazing, but overpasses, which cool rapidly at night since they’re not in contact with the ground, were at risk of becoming covered in ice.
Northern Alabama was under a freezing-fog advisory again Thursday morning, as were parts of northeast Maryland and Delaware.
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“The most significant event we had was actually [Wednesday] morning,” said Dan Dixon, a meteorologist at the Weather Service’s Huntsville office. “I’ve been working the day shift, and when I came into work, it was pretty widespread across our entire area. Not only lakes and rivers … we had dense fog under ridge tops widespread across the Tennessee Valley.”
Dixon said that freezing fog isn’t rare in northern Alabama and adjacent eastern Tennessee, but the coverage of this particular event was impressive.
“It’s not as uncommon as you would think, because we have a lot of river valleys in this area,” he explained. “The Tennessee River is the biggest, but we’ve got a lot of tributaries and a lot of lakes and rivers. When we see temperatures in the upper 20s or lower 30s, we stand a high chance of seeing freezing fog in very localized areas. But in terms of how widespread this was over the land areas, that’s very unusual.”
Dixon said his car was coated with ice, thicker than a normal frost and difficult to scrape off.
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Freezing fog can lend itself to producing some of the most aesthetically striking winter landscapes thanks to the rime ice associated with it. It resembles small fractal-like branches of ice that build on one another, and is especially common on trees, shrubs and other vegetation.
Patchy soft rime ice formed in many areas throughout the South and Southeast, with feathery or spiky tendrils of ice jutting out from branches. Up close, the crystalline structure of the ice is visible. From far away, the trees appear dusted with powdered sugar.
It’s sometimes less prevalent on grass if ground temperatures are marginal, leading to a stark contrast that can make for even more serene scenes.
Rime ice is often confused with hoar frost, which is similar in appearance but fundamentally different. Rime ice forms when supercooled water droplets in the liquid state are suspended in the atmosphere; hoar frost requires water vapor, a gas, to be present in the vicinity of subfreezing surface objects.
Freezing fog and rime ice can occur anywhere where temperatures fall below freezing but is most common at the mid-latitudes, where a sufficient overlap of low temperatures and moist air can occasionally support its formation.
Rime ice also was spotted along with freezing fog in parts of Canada and Britain last week.
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