The setting is immediately indicated in Jack London's "To Build a Fire":
Day had broken cold and gray, exceedingly cold and gray,....There was no sun or hint of sun, though there was not a cloud in the sky. It was a clear day....[with] an intangible pall over the face of things, a subtle gloom that made the day dark....lack of sun.
The foreshadowing of the man's stepping into the frozen river comes in the second paragraph:
The Yukon lay...hidden under three feet of ice. On top of this ice were as many feet of snow...pure white...where the ice jams of the freeze-up had formed....the mysterious, far-reaching hair-line trail, the absence of sun from the sky, the tremendous cold, and the strangeness and weirdness of it all....
(Adjectives are in italics)
Fifty degrees below zero was to him [the man] just precisely fifty degrees below zero.
This factual description of the gray sky absent of any sun, a temperature that means only exactly what degree it is, the frozen river hidden under three feet of snow suggest the ominousness of Nature in such a severe form. The man who "has no imagination" does not realize that he is in jeopardy against this cold and indifferent Nature. Here, London's Naturalism is clearly evident in the exposition of his story. For, the man is "surprised" at the cold while the husky possesses an "instinct [that] told it a truer tale than was told to the man by the man's judgment.
In a subtle, but telling way, London suggest the conflict of Nature vs. the living creatures. Only a creature in tune with Nature can survive; the man "without imagination" is at its mercy.