Morkus from Orkus
Fog, like foliage, comes in ranges of thickness. It can be so thick that you cannot see break lights a few feet in front of you. You get 100+ car pileups that way. Or it can be light and you can see for hundreds of yards. One of those is heavily obscured and the other is not. If you do have a heavy fog and it's light around the edges, then the edges are not heavily obscured. Same with foliage. Foliage heavy enough to be heavy obscurement is the kind that's so thick you have to chop your way through every foot of it. The kind with holes to see through is not heavy, it's moderate and gives light obscurement.Yes, opaque is used as an intensifying descriptor for fog. I don't think that establishes opacity as a necessary condition for something to constitute a heavily obscured area. Also, fog isn't actually opaque. It might appear so from a distance (like dense foliage), but at its edges it is quite translucent or even transparent, especially to an observer looking out into an area under daylight.