Since Jan. 1, frost and fog have been a regular occurrence. John Wheeler, a meteorologist at WDAY television in Fargo, explains the difference between rime and frost and why fog is so hard to predict.

Fog 101

“Fog is not dependent on the season but on the weather pattern,” Wheeler said. “When we get the conditions that produce freezing fog like this, it doesn’t matter to physics if it’s January or March. We do get a lot of these conditions when we have a mild winter weather pattern. It has definitely been a warmer than average winter so far.”

Wheeler said fog is very hard to predict. “It’s not just a matter of it being humid,” he said. “It’s a matter of it being humid in such a way that liquid particles get suspended in the atmosphere and float there. It takes just the right conditions. The wind is usually pretty light. Fog is tricky to predict and it’s always patchy. It may be really thick in one area and 10 miles down the road you run out of it.”

One of the things that helps fog materialize is for a layer of cold air to be hovering near the ground that’s colder than the air around it and the air above it.

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“We have what’s called a temperature inversion,” he said. “Near the ground the air is colder, and up 100 to 300 feet the temperature rises. The fog will tend to form in that coldest air, if it’s calm enough. Low-lying air, like the valleys in western Minnesota, are more prone to a little bit of cold air drainage, so fog will form there. Fog will form over lakes before it will form over the hills nearby, especially in the winter when the lakes are covered in ice.”

Wheeler said, since fog does not show up on radar, meteorologists have to look at atmospheric conditions that are likely to produce fog.

“That’s very tricky because those conditions are all at the very lowest level of the atmosphere above the ground,” he said. “We don’t have a lot of instruments hanging around at 100 feet above the ground, so we’re relying a lot on estimates. The other thing about fog is that it varies so much between one area and another. You’ll drive through super-dense fog and then much lighter fog. Pockets of heavy fog float around almost at random. It’s hard to forecast.”

That’s why fog advisories issued by the National Weather Service often cover a large area. “They issue alerts on a county by county basis, and weather doesn’t always go by counties,” he said. “There was one day this week when the fog advisory ran from the Canadian border all the way down into southern Iowa. That means a big area had a lot of fog in it, but it wasn’t all pea soup.”

Rime versus frost

“When fog produces frost that lingers on trees and other surfaces, that technically is not called hoar frost, but rime,” Wheeler explained. “That’s what we’ve been seeing lately. The difference between rime and frost is that frost is formed out of water vapor in the air. It’s invisible particles of water in a gaseous state. When we get frost on the ground that comes directly from humidity in the air. On a clear calm night, moisture in the air clings to grass and other surfaces. That’s frost. When it’s especially thick, we call that hoar frost.”

He said that while hoar frost and rime look a lot alike, they are created through different processes. “It’s the process of formation that separates them,” he said. “If I were to just look at a picture with ice crystals, I couldn’t tell whether it was hoar frost or rime unless I knew what the weather conditions outside were.”

“Rime comes from liquid water,” he said. “This fog that we’ve been having lately is what we call freezing fog. It’s made up of tiny particles of supercooled water. Supercooled means it’s below freezing, but it’s still in a liquid state. The reason these fog particles can be in a liquid state when it’s below freezing is because they’re so very small. These particles of liquid need some kind of surface to cling to. So as soon as this fog comes into contact with a tree or anything rough that has points on it at all, it immediately starts forming ice crystals. The liquid then has the opportunity to freeze, and that is called rime. You’re going to get rime whenever you have freezing fog.”

Freezing fog can also make roads a little frosty at times. “It’s not like freezing drizzle because fog doesn’t fall,” he said. “It just hangs in the air.”

Wheeler said because things outside are at different temperatures, frost is not universal. “When the air temperature is near freezing, you may notice that when you get a frost it is more prevalent in some areas than others,” he said. “For example, you’ll see frost on one color rooftop and not another. Automobile windshields are notorious for losing heat, so the windshield of a car will be colder than the air around it and frost will form there.”