Some fog layers are very thin and you're up above them at 50' or 100'. I've seen mornings in FL when the airports were reporting near 0/0 conditions (0' ceiling, 0 visibility) due to fog but when flying over, you could see the tops of multi-story buildings and towers sticking up above it because it was only about 50' thick.
Other locations can have fog banks that are significantly thicker (depends on the type of fog - advection, radiation, upslope, precipitation-induced, etc.) Please be careful if you do what you're suggesting and make sure you're nowhere near airports or approach paths where you could be a hazard to manned aircraft operating IFR with low / no visibility. Also keep in mind that if you're operating Part 107 you are supposed to have 3 statute miles visibility and keep your aircraft in view at all times; these requirements would likely prohibit such an operation unless you can take off in an area of better visibility and fly over the edge of a nearby fog bank all while keeping your aircraft in view the whole time (I used to see situations like this in coastal CA all the time with "marine layer" fog that would sock in the beach up to about 200' AGL, but it'd be clear blue and sunny ½ mile away inland, for example...)
Flying (in manned aircraft) skimming right along the tops of clouds or fog banks is probably one of the coolest and most fun things you can do, as well as being a beautiful, sublime experience. I've had the pleasure several times (my assigned altitude just happened to put me right at the tops). Most times you don't get much sense of movement / speed while flying but in those cases, very much so! Very cool stuff. Watch the beginning of "The Right Stuff" for an idea of what it's like. You get a similar effect (for a shorter time) when "punching in" or out of clouds / IMC. Not recommended for drones however...
