(Photo courtesy of NOAA and NWS)
Freezing fog isn't something we get to talk about all that much across the Tennessee Valley. Because of that, you may be wondering what in the wide world of sports freezing fog even is. Freezing fog is simply fog that forms when the temperatures are cold enough for the water droplets to freeze onto objects. Those water droplets in the atmosphere remain in a supercooled liquid state despite the air temperature being below freezing because they don't have anything for ice crystals to use as nuclei to develop onto (other ice crystals in the air, dust particles, etc.,). This is how you can get cloud droplets instead of crystals in the atmosphere with a temperature below the freezing point, and it's why you need moisture in a snow situation to extend into the layer of the atmosphere that's at least -10C to -12C in order for snow to form.
The big concern is then when those supercooled water droplets are able to land onto a surface that is at or below freezing. The water droplet's temperature is already below freezing. It just doesn't have something to connect with in order to form ice crystals. Once those water droplets come into contact with grass, tree limbs, bridge and road surfaces, etc., that may be below freezing, ice forms on contact. Road temperatures across all of our viewing area during these early morning hours are down into the mid to upper 20s, with air temperatures in the low to mid 20s. Any fog that forms this morning will not only reduce visibility, but may leave a thin layer of ice or frost on road surfaces, especially the elevated surfaces like bridges, overpasses, etc., that cool easier because air travels overtop and underneath them.
It is for this reason the National Weather Service offices have placed all of our Alabama counties in a Freezing Fog Advisory this morning until 9:00am. Our Tennessee and Mississippi counties are not (as of this moment) included in the advisory because the fog will be much more patchy or isolated in nature compared to in Alabama. That's because of the heavier snow and heavier rain totals that Alabama got back on Monday. They have a bit more in the way of moisture added to the atmosphere from evaporation on Tuesday than what the other areas do. However, patchy dense freezing fog is possible through mid to late morning across ALL of our coverage area. Be sure to be extra cautious as you travel this morning, especially if you travel through shaded areas or across elevated road surfaces like bridges or overpasses. This fog, like fog usually does, will burn off toward late morning as temperatures rise, winds begin to stir, and the temperature rises away from the dewpoint and the air desaturates. The icing from the freezing fog will also begin melt during the mid morning, even before air temperatures get above freezing, because of how the solar radiation acts upon the ice as the sun comes up.