What signals warn the man that it is much colder than anticipated?

The man in this story is a newcomer to the land described. As such, he does not necessarily recognize all the signs he could possibly have interpreted to give warning as to the cold. The fact that he cannot see the sun does not concern him, because it has been several days since he has seen it.
He does, however, attempt to summon some spittle in his mouth and spit it out "as an experiment." He is surprised to find that, on each of the several times he tries this, his spittle freezes before it reaches the ground. This is significant because it indicates to him that the temperature is less than fifty degrees below zero (although he cannot tell exactly how much lower it is).
The man also notices an unexpected numbness in his face and extremities. He knows that this is the sign of extreme cold.