Oddly Foggy Winter Locally In Vermont, And In Nation

Once again, dense fog obscures the forest behind my
St. Albans, Vermont home today. Dense fog has been
unusually prevalent this winter in Vermont. It has 
also been strangely widespread across
the United States this week. 

 Once again Thursday afternoon, my area around St. Albans, Vermont was encased in a cold, dense fog.

It's happened a lot this winter. It's very strange for a season in which cold, dry, gusty air usually flushes out any trace of fog.  Even on days without dense fog this winter, a low overcast and lighter fogs and haze have given us a truly gloomy winter. 

Vermont isn't the only place in the nation to have strange bouts of dense winter fog. 

The past three days have set records for the most widespread areas in the United States to fall under dense fog advisories. This morning parts or all of 27 states were under dense fog advisories. That's 100 million people who were under those fog alerts. 

Fog was widespread in other states today, too, but in those areas like in Vermont the fog wasn't dense or widespread for long lasting enough to trigger those advisories. 

Dense fog can obviously be dangerous or at best inconvenient. The Washington Post noted that a pedestrian died when they were hit by a car in dense fog in Indiana, and a Louisiana causeway was closed because of a highway pileup in the fog.

Over the past two days, fog  is mostly responsible for the cancellation of roughly 7 percent of flights at Chicago's O'Hare airport and delayed roughly 17 percent more. 

WHY IT'S HAPPENING

The vast areas of fog in the Midwest, Plains, Ohio Valley and Great Lakes had their roots in last week's cold snap. Snow covered most of this area, and the ground got quite cold, actually freezing in many of these places.

The vast gray area on this National Weather Service
map from this morning is all dense fog advisories. 

That same fire hose of moisture I talked about the other day - the one that produced severe flooding in parts of Texas, Louisiana and Mississippi - also features a much wider, broader flow of warm, moist air from the Gulf that infiltrated most the nation east of the Rockies.

That warm, wet air passing over the cold and often snow covered ground. This moist air is chilled by the cold ground, so that moisture condenses into fog.  

Strong winds that usually blow in the winter would disperse a lot of this fog. But this flow of wet air has featured just light winds 

The past few days featured the biggest fog attack of the winter in the U.S. But there were other ones around Christmas and New Year, some of which disrupted holiday travel. 

Here in Vermont, we've had frequent bouts of slow, warm, wet air flowing over us since November, hence our many days in the fog. 

This latest bout of fog should clear up toward the weekend in most places as windier, drier air makes inroads across the Lower 48.