In Jack London's short story, "To Build a Fire," there are many details provided to show how cold it is over the course of the text.
Perhaps the most indirect detail to the cold is the fact that the sun has not been seen by the man for many days. The cold which would exist, "that was due to the absence of sun," could only be even more chilling a thought to those who know of the sun to be a warming object. Without the sun, the land has no chance to warm, ever.
Not only was the sun gone, the Yukon itself was. The road which the man took was "hidden under three feet of ice" and "many feet of snow." When the narrator mentions the temperature,"fifty degrees below zero meant eighty-odd degrees of frost," readers get an idea of just how cold it was in the story.
Perhaps the most chilling detail given is the one where the spit from the man freezes, or rather explodes, in mid-air.
As he turned to go on, he spat speculatively. There was a sharp, explosive crackle that startled him. He spat again. And again, in the air, before it could fall to the snow, the spittle crackled. He knew that at fifty below spittle crackled on the snow, but this spittle had crackled in the air. Undoubtedly it was colder than fifty below—how much colder he did not know.
This image is later compacted when the narrator offers a view of the man's beard.
The frozen moisture of its breathing had settled on its fur in a fine powder of frost, and especially were its jowls, muzzle, and eyelashes whitened by its crystalled breath. The man's red beard and mustache were likewise frosted, but more solidly, the deposit taking the form of ice and increasing with every warm, moist breath he exhaled.
In the end, the fact that the man freezes to death is a large hint as to the severity of the weather and cold.