Large-Scale Fog over Lake Superior
An intriguing fog event took place over Lake Superior on Sunday, 15 February. On a day that was otherwise clear and unseasonably warm, fog crept westward as though it was trying to cover the entire lake. The following animation shows the entire day of GOES-19 true color imagery as viewed by SSEC’s Real Earth. There even appears to be a hint of rotation evident as the fog against the north shore appears to rap around the center of the open water.
It’s worth looking at what might have caused this event. First, let’s look at the Day Cloud Phase Distinction RGB for a portion of this period. In this product, low-level liquid clouds (like fog) appear blue/cyan while surface water is black and surface snow and ice are green. The surface observations for this time have also been overlaid on this image.
This is a challenging time of the year to discern what conditions are like over the Great Lakes thanks to freezing conditions. Buoys have generally been removed to protect them from damage from the ice, and satellite-based wind observations are also inhibited by the ice. Thus, we are largely flying blind over the lake itself. Still, we can put some things together. First, note how cold air is pooling within the lake basin. This is especially evident when looking at the western tip of Superior. There are three ASOS stations in greater Duluth, Minnesota. The Duluth International Airport is located on the continental crust at an elevation of 1428 feet above sea level. The Richard Bong Airport in Superior is located on the mid-continental rift at a much lower elevation of 674 feet. However, Sky Harbor airport is located on a sand bar in the lake and has a much lower elevation of 609 feet. At 1926 UTC (1:26 PM local time), that lower elevation is at least 10 degrees colder than the surrounding, higher elevations. We know that the lake surface temperature is near freezing given the presence of the ice. From all of this, we can infer that there is a decently strong inversion present over the lake. Note also that the near-lake air is already close to saturation, as the Sky Harbor dew point is just 2 degrees F less than the air temperature.
A similar dynamic can be seen further up the coast at Grand Marais, Minnesota. Here, the airport is 8 miles inland and over 1,000 feet above the lake surface, where an additional set of observations is available. Note how the lake air is a full 11 degrees colder than the airport location, while both sites have the same dew point.
Next, there appears to be a weak low pressure system near the border of Wisconsin and Michigan’s Upper Peninsula, as seen in the following NOAA Weather Prediction Center surface analysis. With the low to the south of Lake Superior, we would expect weak easterly flow over the lake, which is more or less backed up by the surface wind observations above.
The fog itself is quite shallow, which we can determine by looking at the brightness temperature of the top of the fog. Since fog acts as a near-blackbody in the infrared window, the brightness temperature of the fog in Band 13 is a reasonable estimate of the temperature of the top of the fog. The AWIPS readout tool says that the IR brightness temperature in the middle of the fog is -5 C, or about 23 F. The IR brightness temperature of the clear lake is only slightly warmer at -2.6 C (27 F ,not shown).
Putting it all together, we have easterly flow, a strong inversion, and a shallow fog layer that is both thick and uniform. Based on this, it’s likely that this is an advection fog. Warmer air from the non-lake regions moved into the cold lake basin, where it cooled and reached saturation. Since air is a poor conductor of heat, the air above the surface stayed warm which was reflected in the higher temperatures of the more elevated locations.