Flying in the fog or cloud deck is very dangerous, as Vic said, you can not properly avoid manned aircraft.
However all is not lost. One place you can get awesome fog shots is in the morning when fingers climb up canyons in the hills and mountains. You can launch in the clear, stay VLOS, get some amazing images and video over the top of the fog, the fringe edges are particularly ripe for some cool pictures, virtually no chance of a manned aircraft, and usually unoccupied land below.
Find the right spot, plan, and follow the rules, and you can do some amazing fog shots without having to fly blind up through it, violate the 400ft rule, and put other aircraft at risk.