Fog is used for three purposes in GoldenEye: To add atmosphere to outdoor areas, to add a distance shadowing effect to indoor areas, and to obscure unloaded rooms in large outdoor areas. The properties of fog are determined by the stage's weather block; its color comes from the color of the sky.
Like all N64 translucencies, fog is often partially or completely missing when the game is emulated.
The fog is used in a few of the outdoor levels, most noteably Surface I and II. The latter sees guards coming out of the fog and attacking you, which means the player has to have quick reflexes in order to avoid taking damage. Statue has a much similar scenario.