How cold was it in "To Build a Fire"?
In "To Build a Fire," the air temperature is seventy-five degrees below zero. The protagonist knows that it is colder than fifty degrees below because his spittle freezes mid-air when he spits, but he does not know precisely how cold it is.
This is the protagonist’s first winter in the Yukon, and he seems to believe, at first, that it is only about fifty degrees below zero. He spits into the air, knowing that spittle will freeze on the snow when the air temperature is fifty below. However, his spittle actually freezes in mid-air, before it even hits the ground. This fact helps him to realize that it is much colder than fifty below, though he does not know precisely how much colder.
It is the narrator who tells us that the temperature is actually greater than sixty below and greater even than seventy below: it is a full seventy-five degrees below zero when the protagonist sets out on his ill-advised journey. The protagonist does reflect that an “old-timer” in these parts once told him that no man should ever travel alone when the temperature is colder than fifty degrees below zero, but the protagonist’s arrogance and overconfidence leads him to undertake this trip alone, and it absolutely leads to his downfall. His fingers freeze too quickly when he has to stop moving to build a fire and dry his footgear after he steps into high water. When his first fire goes out, it is essentially a death sentence for the man because he cannot keep his fingers working to succeed again.
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