How cold is it suppose to be in the story?

Fifty degrees below zero seemed the norm but the man speculates that it is even colder.

"Fifty degrees below zero was to him just precisely fifty degrees below zero...Undoubtedly it was colder than fifty below—how much colder he did not know. But the temperature did not matter. He was bound for the old claim on the left fork of Henderson Creek, where the boys were already.."