River valley fog in the Upper Midwest region
The low-lying areas of the Wisconsin River and other tributaries that drain into the upper Mississippi River are favored areas for nocturnal valley fog formation, as shown by the MODIS “fog/stratus product” and topography image comparison (above).
AWIPS images of the MODIS and GOES-12 fog/stratus product from 29 July 2007 (below) demonstrated important differences in the detection of these narrow fingers of river valley fog (yellow to orange enhancement) that were forming during the nighttime hours over parts of southwestern Wisconsin, southeastern Minnesota, and northeastern Iowa. The 1-km resolution MODIS fog/stratus product at 07:50 UTC (below, left) was able to give a more precise indication of the areas where fog was beginning to form, while the corresponding 4-km resolution GOES-12 fog/stratus product at 08:01 UTC (below, right) could only provide a vague signal that fog was starting to develop over portions of the region.
The fog continued to increase in depth and areal coverage later that night, with surface visibility eventually dropping to 0.15 miles at Wisconsin sites Lone Rock KLNR and Boscobel KOVS (0.01 inch of precipitation was also recorded at each location as mist developed) — and as the fog thickened, the GOES-12 fog/stratus product did begin to exhibit a better signal of the fog structure (especially over the lower Wisconsin River valley: Java animation).
A comparison of the AWIPS GOES-12 fog/stratus product and GOES-12 visible channel image at 13:15 UTC (below) shows that much of the river valley fog was still present after sunrise; however, note that after sunrise the fog features on the GOES-12 fog/stratus product began to change in appearance from yellow or orange enhanced features (11-3.9Âµm brightness temperature difference values around -7Âº to -8Âº C) to darker gray enhanced features (11-3.9Âµm brightness temperature difference values of around -4Âº to -5Âº C) — this is due to the fact that the 3.9Âµm channel is very sensitive to solar radiation reflected off the tops of water droplet cloud features (such as fog), which increases the brightness temperature value sensed by the 3.9Âµm detectors (and therefore decreases the 11-3.9Âµm brightness temperature difference that constitutes the fog/stratus product). Because of the solar reflection issue, the fog/stratus product is not valid during daytime hours.