While cruising the bottom of a deep ravine, I found places where my view distance gets reduced in a manner similar to the Void Fog. Thing is, the bottom of the ravine is flat where that occurs. I just walk, seeing the walls lit with dozens of torches on multiple levels, and then as I enter certain area, my view gets shorter, distant torches dim, then it all gets cut off, I see only like thirty blocks ahead, the rest swallowed by darkness. Then a little way further the vision gradually returns to normal. There are two such places in that ravine and I didn't find anything like that elsewhere.
This is fairly deep and I wouldn't be surprised to be affected by the Void Fog, but if I understand, the intensity of the Fog is directly proportional to your depth past certain threshold - and this seems as if I were entering some... hill of the fog? Also, there are no floating particles like in the deepest layers.
So, what can cause that local reduction of view distance?