How did the man know it was colder than 50 below?

The man knew that is his spit hit the snow and crackled it was 50 below..... his spit, however, crackled in the air, which meant it was even colder.

And again, in the air, before they could fall to the snow, the drops of water became ice that broke with a noise. He knew that at 50 below zero water from the mouth made a noise when it hit the snow. But this had done that in the air. Undoubtedly it was colder than 50 below.