in what ways does the author let you know that is it bitterly cold ?

He basically comes right out and tells us,

"Undoubtedly it was colder than fifty below—how much colder he did not know."

"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..."