I consider fog as a medium of sensory deprivation*. It limits visibility, one's orientation possibilities, and makes one focus on something else: on the sound of silence, on the tactile joy from touching the air, on the feeling of endless space, on the damp smell of grass, on the bleached color of reality.
*Sensory deprivation is the deliberate reduction or removal of stimuli from one or more of the senses. All experiments and researches proved that sensory deprivation leads to certain psychological and physiological changes, positive or negative depending on the duration and goals of the isolation. Short-term sessions of sensory deprivation are described as relaxing and conducive to meditation; however, extended or forced sensory deprivation can result in extreme anxiety, hallucinations, bizarre thoughts, and depression. Some people deprived of light, sound or ability to move are known to have gone insane.