The fog functions in several ways within this play, which spans the length of one day in the life of the Tyrone family. It serves as a marker of time as the play progresses, it creates dramatic atmosphere, and it works symbolically in developing the characters and the theme.

When the play begins, the day is new and bright. The fog has lifted from the night before, and the Tyrones' mood seems happy and optimistic. As the day wears on, family turmoil and personal torments are slowly revealed, and in Act III, night is falling and the fog has returned. Its presence, which is noted by the family, creates an atmosphere of darkness and isolation. A distant foghorn is heard, sounding like a moan. As the fog rolls in, it envelopes the house as the Tyrones move farther into a darkness of their own where despair lives beneath the surface of their lives.

The fog works symbolically also to represent peace in escaping from painful truths. Edmund, the Tyrones' tortured younger son, speaks of wanting to lose himself in the fog, to dissolve into it where he can be alone and separate from the real world. Mary Tyrone, Edmund's mother, seeks escape by losing herself in the fog of her drug addiction.