ST. LOUIS – Dense and thick fog has been the driving weather story for the last 24 hours and will continue to be with the Dense Fog Advisory extending through Thursday.

With the city painted in fog Wednesday, it made for an eerie and dangerous morning commute, as visibility dropped to a quarter of a mile or less.

“We don’t expect it to change until at least midday tomorrow. That’s the reason we’ve continued this Dense Fog Advisory through then because it may not be the entire time we see these quarter-mile or less visibilities,” Kevin Deitsch, warning coordination meteorologist for the National Weather Service, said. “But we are expecting times over the next 18–24 hours, especially overnight in the early morning hours, where those visibilities will peak below a quarter of a mile once again.”

So, what is fog and where does it come from?

Fog is microscopic water droplets in the atmosphere near the surface that reduce or obscure visibility, almost like a cloud sitting on the ground. As we were dealing with extremely cold air a few days ago, the humid air we have now is being pulled in from the Gulf of Mexico.

“Where we have that clash in airmass, this time of year especially, it’s fairly common to get widespread low clouds and those clouds can make it all the way down to the ground to give us widespread fog,” Deitsch said.

For people driving Wednesday evening through early Thursday morning, stick to these rules of thumb:

Slow down. Allow yourself extra space between you and the vehicle in front of you

Use your low-beam headlights; high beams worsen the fog

Make sure you’re in the right lane; if it’s hard to see, follow the lines of the road