Fog

Don't think that snow is the only weather condition that gets a shout-out. Fog is a key player, too—particularly on the night of Carl Heine's death. First and foremost, it's notable from a plotting standpoint, as it created poor visibility out on the water that night. The fog was ultimately the reason the freighter ended up crossing the waters where Carl was fishing, and it obscured Carl's accident from the view of others.

Beyond its basic significance to the plot, though, the fog becomes symbolic of all things unknowable and ghostly, representing (in a larger sense) knowledge that is beyond our grasp. You kind of get that vibe in the narrator's description of Kabuo's fishing trip on the night of Carl's death, just before he runs into Carl:

Tonight, he knew, was what old-timers called ghost time, with fog as immobile and dense as buttermilk. A man could run his hands through such a fog, separating it into tendrils and streamers that gathered themselves languidly once more into the whole and disappeared seamlessly without a trace. […] It was possible on such a night to become as disoriented as a man without a torch in a cave. (27.32)

Not only does this reference to "ghost time" provide some bangin' foreshadowing of Carl's unfortunate demise, it also links the fog (and all the blindness and obscurity that it creates) to the great beyond—you know, the big question mark in the sky. Man, the weather really does do some symbolic heavy lifting in this book, no?