Dense fog this weekend infrequent but not unusual - WXOW News 19 La Crosse, WI – News, Weather and Sports |

Dense fog this weekend infrequent but not unusual

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LA CROSSE, Wisconsin (WXOW) - The dense fog on Saturday, Sunday, and Monday morning was nearly as show stopping as the mid-60 degree highs that were recorded on Monday.

This kind of dense fog doesn't form terribly often in the Coulee Region, but fog itself is pretty common.

"Fog usually arises when we have a clear and calm night," said Alex Kirchner, Daybreak Meteorologist at WXOW. "That means the ground can cool off rapidly, we don't have a layer of clouds to help reflect some of the outgoing radiation, and that results in the air being pretty warm and then right down at the surface it's cold. That cold air condenses down, all the water vapor condenses out and we get that nice little cloud layer near the surface."

And because of the topography in the river valleys, fog is even more common here.

"Topography also doesn't help our area when it comes to fog formation because cold air can settle down in the valley. Imagine going into your basement during the summer months. You find the air is a lot cooler down there because the air settles at the lower elevation. Same with the valleys, it doesn't mix out very readily; we get a lot more fog."

This is especially true going into the fall and winter months when the warm Mississippi waters exchange moisture with cooler air above it.

But this isn't the only way fog can form. And over the weekend we saw a classic case of advection fog. This type of fog occurs when warm air moves over a colder surface causing the air to cool and water vapor to condense.

Over the weekend, warm, moist air flowing in from the south was the main cause of the fog. And an inversion in the atmosphere was one of the reasons it stuck around so long.

"Sometimes fog can last for a while because we have what is called an inversion in the atmosphere," said Kirchner. "That means the air above the surface is a lot warmer that the air right at the surface. Think of it like a seal on a piece of Tupperware. The air at the surface can't get out, can't mix up…If it's foggy within that little area of air right at the surface you're not going to have the fog clear out for hours, even days."

Fog can occur year round, and in the winter it is commonly formed by warm air moving over the snow covered ground.

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