Painting fog and atmospheric effects is an area where understanding the physics produces dramatically better results than guessing.

Fog is suspended water droplets that scatter light. The practical effect: contrast between objects and background decreases as distance increases. Saturation decreases. Value range compresses toward the ambient light value.

For ground-level fog: the fog is densest near the ground. Objects visible at height are clearer than objects at ground level at the same distance. Trees vanish into fog from the bottom up. Buildings are most visible at their upper floors.

For elevated fog (overcast): the light source is diffuse and comes from the entire sky. Shadows are soft and directionless. The ground is lit from above but not by a distinct point source. The effect is low-contrast, low-saturation, and visually restful.

The common painting mistake: applying fog as a uniform grey overlay. Real fog has depth variation — closer fog areas are denser (darker or lighter depending on whether you're viewing toward the light or away from it). The depth variation is what makes fog feel atmospheric rather than filtered.