My son is flight instructing in Merced California. The fog rolls in this time of year and sometime does not burn off. He takes students up through the fog where it is clear 'on top' and does his training, when he comes back he takes over and does an instrument landing. He is showing this student how to do it for the first time. It is an 8 minute video, it starts out clear with smooth clouds (fog) below, then visibility goes to 0%. He can only spot the approach lights at the last minute. The 400ft 'ceiling' is at minimun for landing. Commercial pilots do this all the time.

He is on the right and his student is on the left.

Click the link below to see the video.