We spent most of Wednesday under thick cloud cover and foggy skies. It wasn't until mid to late afternoon that some sun was able to peek through. But by Wednesday evening, the cloud cover had filled back in and fog was beginning to redevelop.
An inversion developed in the atmosphere that helped trap moisture near the surface of the earth. With nowhere for the moisture to go, the cloud cover and fog didn't 'burn off' in time like we were hoping.
An inversion develops when warm air a few thousand feet above in the atmosphere moves over a relatively cooler surface layer. Temperatures roughly 3,300ft in the atmosphere warmed to just under 40 degrees Wednesday morning, while temperatures at the surface were a little above freezing (32 degrees). The warmer air above doesn't allow the air at the surface to rise and keeps the atmosphere from mixing. Because of this, moisture is 'trapped' near the surface of the earth keeping cloud cover and fog from clearing. When this happens in the late spring and summer, the clouds and fog are quick to burn off. When it happens in the winter, the sun isn't able to heat up the atmosphere as efficiently due to the lower sun angle.
There was enough clearing to take place Wednesday evening to allow more fog to develop. It's possible that locally dense fog may occur overnight into Thursday morning. If this happens again, highs may have a hard time reaching the low 40's - just like Wednesday afternoon.