I would say that even though the freezing point of water is 0c, to keep water frozen takes a colder temperature, especially if snow falls onto something that isn't pure, or even ever so slightly above 0c. That's why your freezer is at about -18c (though I'm aware that's also to freeze things quickly). That snow came from a much colder place than the ground.
Typing out loud, prepared to be corrected.
edit: in hindsight, that wasn't a great post, sorry