The vis/fog satellite images are generated from visible geostationary satellite images during local daylight hours, and from a derived "fog" image at night that emphasizes the low clouds (Note the saw-tooth boundary between the different images).
During the day the visible image is brightness normalized by dividing by the cosine of the solar zenith angle. This removes most of the image dependence on sun angle, such as brightness changes at sun rise and sun set and differences between winter and summer image brightness. The visible brightness depends primarily on the thickness of the cloud. The visible image is used when the sun is higher than 3 degrees above the horizon.
At night there are no visible images, so a derived "fog" image is substituted for the visible. The "fog" image is generated from the temperature difference between the 3.7 micron images and the 11 micron infrared images. The temperature difference depends primarily on emissivity differences caused by different physical characteristics of the radiating surfaces. The brightness of the image is set up so that low clouds are white, ground is gray, high clouds are black, and very high cold thunderstorm tops are a salt and pepper black and white. The "fog" image is not very sensitive to the temperature of the low clouds except for extremely cold surface temperatures below -40 degrees, when it starts to show some of the salt and pepper appearance. The ground generally shows up as a gray color, except for a few areas in the west, such as the delta of the Colorado River which shows up white because of the soil emissivity there. The white "fog" low clouds start to show up on the images when the low clouds are wider than 2 miles, and are thicker than about a hundred feet. Hence it will generally pick up widespread IFR condition, but may not show small, thin local fog or haze conditions.