Snowballing Doorway

Sited in an existing aperture, two archway forms of the same size pass air back and forth all day long.  As one arch inflates the other collapses and air to fill the engorging arch is sucked from the other.  The two arches are in mirrored orientation, participants can pass through the bottom arch until it becomes volatile and the upper up-side-down arch starts to displace its counterpart.