Possibly these are subject to the Coriolis effect.


The reason that small scale phenomena don't show a noticeable correlation to Coriolis forces is that the Coriolis forces are small compared to other conditions. In the case of the sink drain, filling the sink from an off center tap will set up motion in the water that will not damp out before the drain is opened. If the water is circulating clockwise it will go down the drain clockwise. In the case of tornadoes, local topography will have a greater effect on the air flow than Coriolis forces will. The same is true for any aerodynamic forces that the bats will experience.