I’ve been noticing something lately that I seem to notice every year about late July or early August, an increase in early morning fog. It seems like day after day in late June and July we wake up to blue skies and brilliant sunshine. It works out well for me, because I get up before 4 a.m. to cover duties for Great Lakes Weather Service. It definitely helps keep the eyes open to see that bright light. However over the past 10 days, we’ve had several mornings with patchy to occasional thick fog hanging over the landscape until about 7 to 8 a.m. Being that it has been such a dry summer, I’m actually really glad to see the fog. At least it deposits a nice layer of moisture on the plants. I know it won’t save them, but it has to help a little bit, right? After all there are some forests on mountains near the ocean in parts of the world that survive mostly on fog. Of course there the fog is so persistent over weeks and months that the moisture that condenses on the needles and leaves drips down to the forest floor, acting like rain.
In any case, it takes a little while to get used to the foggy mornings. You have to imagine what everything might be doing on the other side of your yard that you can’t see. It certainly puts you in the mood to crawl back into bed. It also tends to muffle noises, and the birds don’t seem to chirp as much. There is a very good reason that fog tends to increase in frequency as we head into early August. The main reason is that the nights are getting longer. We have about 1 hour and 20 minutes more night than we did at the end of June. This allows for a few degrees of extra cooling overnight. In addition we still usually have rather humid air in Wisconsin at this time of the year, especially if some rain fell during the previous afternoon or evening. So the extra cooling overnight drops the temperature down to the dew point more quickly. Often dew points are in the 60s to near 70 around early August. The air becomes saturated and the end result is fog. We often call this radiation fog. This is especially true when the winds are calm. Light winds are common around this time of the year as weather systems and fronts are typically quite weak. River valleys and areas near lakes are more prone to fog because the warm waters infuses additional moisture into the cooler air above them. Sometimes this takes the form of streamers of steam coming off the water. As such it is called steam fog.
So fog will be a more frequent visitor in August, then not quite as much in September. That’s because cold fronts get stronger in September and usher in much drier air and gustier winds to the Upper Midwest. Well enjoy the fog that will come our way from time to time. Just be extra cautious when you must drive through it!
This post was written by Tony Schumacher on July 30, 2012