The very dense fog Wednesday sure was a pain to drive through. Visibilities dropped to a few hundred feet at times. It was caused by mild and very moist air riding over the top of the cold ground and melting snow pack. The air was extremely saturated. I for one am glad that we don’t have fog that thick on a weekly basis around here.
Did you know that the foggiest place in the world is the Grand Banks off Newfoundland? It is the meeting place of the cold Labrador ocean current and the much warmer Gulf Stream ocean current coming up from the south. The fog that forms there, just as the type that formed around Wisconsin Wednesday is called advection fog. Other times we have what is called radiation fog. It forms on clear and calm nights when the temperature drops rapidly down to the dew point. The air becomes saturated and fog forms. It is especially common if the ground is moist from recent rains or a wet snow.
Check out the map below. It shows the average number of fog days per year acrosss the United States. You probably won’t be suprised to see the highest frequency of fog is along the seaboards, near the Great Lakes, and in mountainous areas of the eastern U.S. The areas in red average over 40 days per year with fog. Northern Wisconsin averages around 30 while southern Wisconsin is closer to 20 days per year. I noticed that the Rocky Mountain States have a pretty low number of fog days per year compared to the Appalachians. I assume that is because in general the air is much drier in the Rocky Mountain area than out east.
This post was written by Tony Schumacher on December 14, 2011