MechaPilot
Explorer
This would, of course, be the point. No one would ever walk outside in Alaska in the winter without proper clothes. That would be stupid and suicidal.
So, why would you assume the characters are both stupid and suicidal just because the player, who isn't living in Alaska and might have missed the bit about cold or whatever, didn't write down "cold weather clothes" on his character sheet?
Me, I would assume that the characters are smart enough to do that, and not even bother asking as the DM. Because simply asking after the point of no return is a blindingly douche move by the DM.
I might ask after the point of no return. However, if the answer was that they didn't bring any, I'd tell them to mark off an appropriate amount of gold for winter clothing, and then keep going.