WILLIAMSTOWN — A polar vortex has had us in its grip. As I step outside into another bone-chilling, eyelash-freezing morning, I drive to work past a huddle of cows in a frozen field. The sight makes me smile, but I also wonder: How do they survive out there? And wouldn’t they be better off in a cozy barn? “The good news about winter is that cows love the cold, fresh air,” says Sheffield dairy farmer Morven Allen.