Direction of Water Drains

Coriolis Force

• The Coriolis force results from the Earth’s rotations on its axis. As the Earth spins, this movement causes certain effects to occur in weather patterns as well as in the direction of water flows. The Earth rotates easterly at a constant rate. Since the Earth has both north and south poles, different areas along the north/south axis will travel or rotate at different rates. For example, land regions along the equator line move faster than northern regions because of the tilt and spin of the Earth.

Regional Effects

• Even though the centrifugal effects of the Coriolis force remain active at all times, other more local forces affect the direction of water flows in drains. In theory, the differences in latitude between the North and South hemispheres would cause water to drain in opposite directions. Because of the tilt and spin of the Earth, the Northern Hemisphere would experience counterclockwise water flows while water in the Southern Hemisphere would drain clockwise. To a certain extent, the Coriolis force does affect water flows; however even more localized effects exist within the confines of a home’s plumbing and draining systems.

Large-Scale Effects

• When comparing differences in water flows between the Northern and Southern hemispheres, the Coriolis force becomes more apparent within large-scale events. In the case of cyclones and hurricanes, these storms naturally follow counterclockwise rotations in the Northern Hemisphere and clockwise rotations in the Southern Hemisphere. These differences in water and air flow result from collisions between the storm and the air masses that naturally follow the Earth’s rotation. In effect, large-scale events become more susceptible to the large-scale movements exerted by the Coriolis force.

Small-Scale Effects

• As water flows down a sink or bathtub drain, other factors, such as gravity, the shape of the drain and existing water currents alter the effects of the Coriolis force. For smaller scale events, the Earth’s rotational force only amounts to one ten-millionth of the force exerted by gravity, according to the Cooperative Institute for Meteorological Satellite Studies. Gravitational forces coupled with the shape of the actual drain and the initial current created by draining water overpowers any effects exerted by the Coriolis force. In the absence of these overpowering influences, the direction of water in drains would follow the clockwise versus counterclockwise pattern found in the Southern versus Northern Hemispheres.