Today is a cold 3 degrees C. The clouds are hanging low and visibility is also less than 200 meters. Here's an image from above: (Source). The view from below is shown in the second image, a view from my attic window (about the center of the satellite image).

What is the height of the cloudtop? The radiosonde reading for midnight last night is shown here: (University of Wyoming) and the Lifting Condensation Level is 272K at 800m.

The contrasts in the Infrared images are smaller.  The temperature difference between the top of the cloud and the surface is 3 degrees C. Another problem with detecting fog is that it does not move. So cloud motion detection software has some trouble with it.But the human eye can see it immediately.