Being outside only with T-shirt on during winter is actually nothing. You just have to keep moving and you can't stand still.
I have been in the forest and worked as a woodcutter(I was helping my grandfather..) and it was about -20 - -25 Celsius all that time. Sometimes even colder. The thing is that if you work hard then you don't feel cold. And I didn't wear almost anything special.
Anyway, I doubt that anyone would fight during winter in snow when it's freezing like hell outside!
But wearing nothing when it's just -5 - -10 Celsius isn't that insane. But you do need a nice day without a wind, though.