I think our average winter temps are around that -15c; mind you, that is *average*, so gets higher and lower than that depending on the month!it never gets that cold here, but I understand.
If we get down into the mid teens (about -9C) it's a rare event. Most of the time it doesn't get colder than 20F here, and our winters generally stay above 25F.
15F = -9.4C
20F = -6.6C
25F = -3.8C