If it was indeed fog it is only found on the ground level; you will never find fog more than 50m above the ground. If the fog becomes to thick you run into a problem with VLOS.
Fog represents condensed water droplets found at the ground level, or in other words, a low-level cloud. It usually brings precipitation in the form of drizzle and will cause low visibility conditions at ground level. This is of high concern for RPAS operations in VLOS, as direct visual contact will be greatly reduced in fog. Fog is dynamic, thus conditions at takeoff might change during the operation and cause a threat to the RPA, manned aircraft, and the public.
I flew this morning. Checked my weather for my area, Airdata weather and UAVforcast. All said I had 10 miles plus visibility. Turns out they were all wrong. After flying over a swampy area I hit fog patches. Still having VLOS but my camera lens fogged over to the point it was not worth flying any more. Returned home and you could see the moisture on the camera lens.