Yes, bottom is open to ground, roughly 20-24 inches up. My strongest survivors from last winter was wide open. Our lowest recorded temp was 33 below F. All hive face south w/ 6 foot fence on NW. I do pile up hay around the front of them despite advise about mice, which likely keep SE winds from causing too much problem.. So far, so good.
In my Langs, 1 open and 2 closed (they were smaller than I'd like) all the clusters are in the bottom NW corners, including the open one. I don't know. I'm not convinced on either way yet and have only been experimenting w/ keeping them open for 3-4 years. It is odd that at least in my owm experience those that do survive with open bottoms seem to respond pretty quick come spring.