Iowa is currently grappling with an unprecedented bout of dense fog, persisting for nearly two weeks and disrupting daily drives in the morning and evening hours. The primary culprit behind this atmospheric phenomenon is a type of fog formed when a warm, moist air mass flows over a cooler surface, in this case, a snow and ice-covered landscape. The interaction between the southern warmer air mass and the cold, snow-laden ground has created the perfect conditions for fog to linger and intensify.

driving car in the fog

So, what kind of fog is afflicting the Midwest?

Advection fog has unique characteristics, distinct from other types of fog such as radiation fog and precipitation fog. Unlike radiation fog, which typically forms overnight as temperatures drop, advection fog develops when warm, moist air encounters a layer of cold air near the ground. This particular advection fog episode has been exacerbated by the record snowpack left by the last few weeks of Arctic blasts and blizzards.

What are the other common types of Fog?

Radiation Fog:  Forms when temperatures drop as the sun sets. The loss of solar heat causes the air temperature to drop to the dew point. Once the temperature reaches the dew point the air is saturated and fog forms, especially with a lack of wind.

Precipitation Fog:  Forms when rain is falling through cold air. If the air is cold and dry at the surface while rain is falling, the rain will evaporate causing the dew point to rise and the temperature to fall. Basically, the air becomes saturated and fog forms.

I get it, fog can be "scary," but when will it end?

As snow continues to melt, contributing moisture to the already saturated air, and additional rounds of rain are forecasted through the area, the fog shows no signs of dissipating in the immediate future. So, enjoy driving through soup, I guess. Basically for the fog to clear, the air needs to become less humid. This can be achieved through stronger winds, total snow melt, or a replacement of the warm, moist air mass with a colder, drier one. Unfortunately, none of these conditions seem likely to occur in the coming days which will maintain those "lovely" foggy conditions across the region.

Credit: National Weather Service
Credit: National Weather Service / NOAA

What is interesting is the unprecedented scale of this fog outbreak, underscored by dense fog advisories affecting over a third of the U.S. population, with parts of 27 states experiencing reduced visibility. Additionally, Travel disruptions, flight delays, and vehicle accidents have been reported, making this one of the foggiest periods the United States has witnessed in at least two decades.

Credit: National Weather Service / NOAA
Credit: National Weather Service / NOAA

Don't hold your breath, but relief might be on the horizon. A storm system is expected to pass through the region which could bring winds to mix out the humid air, offering a potential solution to the fog. Additionally, the rapid snowmelt in the Mid-Atlantic and Midwest is anticipated to moderate temperatures near the ground, lowering the fog potential. As always, residents and travelers are advised to exercise caution and stay up to date on weather and driving conditions.

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