As Marlow's boat is eight miles away from the inner station, he encounters a white fog. He describes the fog as "more blinding than the night," and he describes it descending around the ship as tightly as a shutter. It blocks out the land that surrounds the boat, and when Marlow and his crew hear people on the banks making loud noises, the crew fears that they will be slaughtered in the fog.

The white fog represents Marlow's blindness and the blindness of the other white Europeans who have come to the Congo in search of ivory. Just as the white fog obscures the banks, the whites who have come to Africa don't really understand the land or its people. Instead, they, like Marlow's boat, are stranded in the middle of the river, surrounded by fog, with a gulf of understanding between them and the local people.

The word fog occurs nine times in Conrad’s Heart of Darkness. All of the usages except for one occur in close proximity in section two of the novel. The color of the fog is unexpected because Marlow says that the fog is white. The reader, of course, knows the color of fog, but because the novel is so dark, the reader expects the fog to be black or grey, perhaps the color of dark smoke. That said, the fog is so thick that it causes darkness. Here is the point that Conrad is making; even things that are white on the serpentine river in the Congo cause darkness. There is no better way to show this point than to quote the text:

When the sun rose there was a white fog, very warm and clammy, and more blinding than the night.

This fog also causes alarm and fear because the passengers on the steamboat can sense a possible attack. They are sitting ducks.

I went forward, and ordered the chain to be hauled in short, so as to be ready to trip the anchor and move the steamboat at once if necessary. "Will they attack?" whispered an awed voice. "We will be all butchered in this fog," murmured another.

Hence, the fog, too, adds to the darkness of the novel. One might even say that everything and everyone is dark on the river, even something naturally white like fog.

The fog that Marlow's steamboat gets caught in is white. But the white fog in this case is not the opposite of darkness; it simply compounds darkness. Fog not only obscures but distorts: it gives one just enough information to begin making decisions but no way to judge the accuracy of that information, which often ends up being wrong. Marlow's steamer is caught in the fog, meaning that he has no idea where he's going and no idea whether peril or open water lies ahead. He and the pilgrims he is carrying are frightened they will be attacked in the fog. One man states,‘We will all be butchered in this fog". Thus the confusion and darkness of the river is intensified by the fog.