How can fog be used as a symbolic theme in The Strange Case of Dr. Jekyll and Mr. Hyde?
One of the most prominent themes in The Strange Case of Dr. Jekyll and Mr. Hyde is what critic Irving S. Saposnik calls the "contrasts between exterior modes and interior realities." The meaning of this is that inner motives, desires and vicious tendencies are hidden in civilized society behind facades (coverings) of that which is acceptable and good, like a home, a career, social standing, a marriage, etc.
In this scenario, fog symbolically represents on a natural level the thematic element of the facade that covers people's inner wrongs. In a spiritual reading, the fog becomes the veil that covers false spirituality (sort of like the minister's veil in Hawthorne's story The Minister's Veil). In a psychological reading, the fog is the conscious and/or subconscious disguises people employ to cover over their real inner psyches, the "Hyde," which some suggest Stevenson believes is in--or potentially in--each person. In other words, the fog literally and symbolically covers realities--a heavy fog can cover everything beneath it literally and therefore symbolically, which underscores the theme of exterior modes contrasting with interior realities.