Cormacs McCarthy's The Road uses much pathetic fallacy (the mirroring of external events [weather] to correspond to internal, emotional tone), giving a perpetual cloud, or haze, over the father and son as they venture to the ocean and warmer climes. The weather contributes to the father's death, and he knows it. Indeed, the sky has been turned ashy and grey after an "apocalypse."
Just look at the weather imagery connected to death:
Everything damp. Rotting. In a drawer he found a candle. No way to light it. He put it in his pocket. He walked out in the gray light and stood and he saw for a brief moment the absolute truth of the world. The cold relentless circling of the intestate earth. Darkness implacable. The blind dogs of the sun in their running. The crushing black vacuum of the universe. And somewhere two hunted animals trembling like ground-foxes in their cover. Borrowed time and borrowed world and borrowed eyes with which to sorrow it.
The religious metaphors are obvious: has God abandoned man in this wasteland? McCarthy does say that "There is no God and we are his prophets," a paradox if there ever was one. Is the sun forever gone? Will mankind survive? It is the Genesis "Fall of Man" story set in the Book of Revelation.
Not only is the sun a metaphor for God, but it is also symbolic of the son, the boy. He is the "fire" that the two are carrying: obvious Christian and Arthurian metaphors abound.
The alien sun commencing its cold transit. He saw the boy coming at a run across the fields. Papa, he called.
Really, though, the weather is a kind of red herring. The father and son expect it to get better near the ocean, but it doesn't. It remains hazy on the coast, and the ocean is a black abyss. The point is, I think, that the fire (internal weather), which is symbolic of hope and faith, is what saves the son.