ABOVE: The Martin County Courthouse in Fairmont is just barely visible behind Profinium Financial in dense fog Monday evening.

FAIRMONT – The fog Fairmont experienced around and after Christmas is set to subside as temperatures decrease.

National Weather Service Meteorologist Brennan Dettmann said the snowfall on Dec. 19 and 20 combined triggered the fog event.

“For southern Minnesota, some places had seen up to 8 inches but generally 3 to 5 inches,” he said. “The combination of fresh snow and period of above normal temperatures above freezing created enough surface moisture with high temperatures for a long enough time to create fog.”

Lighter winds are the reason it stayed around for as long as it did. Dettmann said the same phenomenon occurs when wildfire haze stops for a day or two.

“That was the case for us the past few days,” he said. “The air mass over us was sticking. I think that caught everyone’s attention how long it stuck around.”

Dettmann said often spring and fall are when fog likely occurs, but warmer temperatures make winter fog events a more common occurrence.

“A year ago at this point, we had above-average temperatures and it was raining on Christmas day,” he said. “We had fog for a few days then, too. It’s not uncommon, just warm temperatures over cold ground and that can happen in winter.”

Tracking in the twin cities, Dettmann said they had the fourth longest stretch of half a mile or less visibility since they started tracking in 1945. For 20 hours Dec. 29 to 30 visibility was severely impacted, and Dettmann said a similar stretch likely happened across southern Minnesota.

This impacted visibility provides a recognizable hazard for traveling, but Dettmann said it can be deeper than people realize.

“Conditions can get better or worse quickly with fog so check the weather beforehand,” he said. “If you get below freezing with fog, that moisture can deposit onto roadways and you can encounter sneaky patchy slip spots. You may not think fog can create icy road conditions, but it’s good to watch out for.”

In fog conditions, Dettmann said some people are tempted to switch on high beams for increased visibility. He said this has the opposite effect, as fog refracts light and makes it harder to see.

“Be aware of the forecast if you have to travel,” he said. “Give yourself extra time and reduce speed if needed. Those are the basics.”

For now, the fog will begin to subside for most people.

“At least for the immediate future there will be nothing similar to the intense fog we had,” Dettmann said. “In river valleys and places with moisture, there could be localized fog. We’re going to have some colder, drier air moving in from the north so there’s less water in the atmosphere to create fog.”

Moving forward, Dettmann said expect the temperatures to drop gradually then level off.

“Each day you’re going to see a gradual cool down and by the weekend highs are going to be in the teens and lows by zero,” he said. “If there’s wind the temps could go below zero. It will feel cold coming off the above-average holiday temperatures so take standard precautions there.”

Besides a potential system this coming weekend, Dettmann said there are no major indicators showing precipitation possibilities.

“We don’t see any big signals on our radar right now,” he said. “There is a system coming this weekend, but it’s looking like the epicenter will be in Missouri. We might get a couple of inches.”