Birds sense slight changes in barometric pressure in their bones because they are hollow. This is the same way an altimeter in an airplane works. Think about it this way... If you let a balloon rise through the atmosphere it will get bigger and bigger as it ascends until it pops because as altitude increases pressure drops. As pressure lowers outside the balloon, the pressure inside the balloon becomes higher than the pressure surrounding the balloon so it expands. I suspect that organs, bones, and air inside animals (and people) reacts the same way. If you ever broke a bone you can probably sense this same change in barometric pressure as your snake does.