When I woke up Saturday morning, the temperature outdoors was -40 degrees†. The wind chill was -100 degrees! It was just unbelievably, impossibly, inhumanly cold outside. Fortunately, that was on a mountaintop in New Hampshire and not where I was. I happened to have woken up on a mountaintop in North Carolina, where the temperature was a much warmer -3°F.
from Building Science http://www.greenbuildingadvisor.com/articles/dept/building-science/ventilating-home-cold-weather