why does the tremendous cold "make no impression" on the man?

The man was ignorant of how cold it really was. It was his first winter there, and he believed himself invincible. Warnings had no effect on him; he had a plan in his mind and believed himself capable of making it to his destination in a certain amount of time. He hadn't figured on the detours that would be his death.

The above answer is true and the man didn't know the number of the degrees below zero so didn't give it the weight he should have. Silly but true.