Jack London spent time in the same area that "To Build a Fire" takes place: Yukon in Alaska. The temperature in that area in the winter has gotten as low as 120 degrees below zero. In the story, the weather drops to about 75 degrees below zero which is life threatening.
The two characters in the story are an unnamed man who is traveling to another mining camp and his companion, a large husky dog. A newcomer to the area and the weather--the man has chosen to ignore advice from old-timers to never travel alone in this kind of weather.
Setting and Atmosphere
The atmosphere of the story is based on the situation in which the man finds himself. The first sentence of the story describes the day as dark and gray. Later, the mention of the lack of sun and its melacholy atmosphere sets the mood for the story.
It was a clear day, and yet there seemed an intangible pall over the face of things, a subtle gloom that made the day dark, and that was due to the absence of sun. This fact did not worry the man. He was used to the lack of sun. It had been days since he had seen the sun.
Obviously, with the initial descriptive words, London was utilizing the weather to create and prepare the reader for the challenges that the protagonist was going to encounter as he travels through the unbelievably cold day. The foolish man is ready to avoid the natural signs, and in his hubris, attempt what should not have been tried.
Another important aspect of the story to understand is the narration of the third person omniscient point of view. The narrator is able to see into the thoughts of not only the man but the dog as well.
The man faces a conflict of with nature. Nature is not out to get the man; however, it does nothing to help him. The environment is neutral. Man makes his own problems and must find his own solutions. When the man spits his tobacco juice, it is so cold that it freezes in mid air and cracks apart.
If the man had only listened to the old-timer, he would have waited and survived. Instead, he tries it alone and dies.