Is it true that water circulates counterclockwise in going down a drain, if no other forces are at work?
— Mallory Shonns, Chicago
It’s a common misconception, and it’s just not true. At the scale of motion involved with draining water, the force caused by the Earth’s rotation (the deflective Coriolis force) is too small to have any effect. Coriolis is the major reason why air circulates around high and low pressure areas as it does (clockwise out of highs, counterclockwise into lows), but the way a drain empties is more a result of the slope of the drain or residual motion of the water when the sink or tub was filled. Scientists have been able to demonstrate the Coriolis effect by draining water, but ideal laboratory conditions are needed.
It’s a common misconception that water naturally circulates counterclockwise down a drain.