From the very beginning of the story, imagery is used (like many of London's stories) to convey the feeling of the place and at times to also foreshadow events to come. In the first paragraph, London writes:
There was no sun nor hint of sun, though there was not a cloud in the sky. 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.
The fact that this absence of sun does not concern our intrepid traveler is a clue as to what will happen to him down the trail. It also helps to build the idea that this "intangible pall" will likely bring about some future tragedy that no one can quite put a finger on yet, but will become clear.
London uses images like this throughout the story.