5:59
Does Water Swirl the Other Way in the Southern Hemisphere?
Veritasium
Overview
This video debunks the common myth that water swirls in opposite directions down drains in the Northern and Southern Hemispheres due to the Coriolis Effect. While the Coriolis Effect is real and influences large-scale phenomena like hurricanes, it's too weak to affect small-scale water drainage in sinks, toilets, or bathtubs. The swirling direction in these everyday situations is primarily determined by pre-existing motion in the water or the design of the fixture. A controlled experiment with large kiddie pools, carefully settled water, and a bottom valve demonstrated the Coriolis Effect, but highlighted how easily it's overwhelmed by other factors.
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Chapters
- The idea that water swirls in opposite directions in the Northern and Southern Hemispheres is a popular myth, often depicted in media.
- Personal observations of toilets and sinks show inconsistent swirling directions, even within the same hemisphere.
- The design of the fixture, particularly internal jets in toilets, often dictates the swirl direction, not the hemisphere.
- Any existing rotation (vorticity) in the water, however small, is the primary factor determining swirl direction, not the Coriolis Effect.
Understanding this myth helps differentiate between scientifically observable phenomena and popular misconceptions, promoting critical thinking about everyday observations.
Observing that some toilets in Alabama swirl clockwise and others counterclockwise, demonstrating that hemisphere alone doesn't determine the direction.
- To isolate the Coriolis Effect, a large-scale experiment was designed to eliminate external influences.
- Two large kiddie pools were used, one in Australia (Southern Hemisphere) and one in Alabama (Northern Hemisphere).
- Water was carefully filled to minimize initial rotation and then allowed to settle for 24 hours to ensure stillness.
- A valve at the bottom was used for draining, avoiding the disturbance caused by pulling a plug.
This meticulous setup is crucial for isolating the subtle Coriolis Effect from confounding variables, allowing for a clearer observation of its influence.
Allowing the water in the kiddie pool to sit undisturbed for a full day to eliminate any pre-existing motion.
- When the water was drained from the settled pools, a distinct swirling pattern emerged.
- In the Southern Hemisphere pool, the water swirled clockwise.
- In the Northern Hemisphere pool, the water swirled counterclockwise.
- Food coloring was used to visualize the water's motion, clearly showing the rotation.
This observation provides direct evidence that the Earth's rotation (Coriolis Effect) does influence water's motion, even if it's a subtle effect.
The dye added to the water clearly showing a tornado-like, counterclockwise rotation in the Northern Hemisphere pool.
- The observed Coriolis Effect in the large pools was very weak and required extraordinary measures to detect.
- Factors like the shape of the container, water currents, and the act of draining itself introduce much stronger rotational forces.
- These stronger forces completely overwhelm the subtle Coriolis Effect in everyday situations like sinks and toilets.
- The Coriolis Effect is significant for large-scale weather systems like hurricanes and cyclones, where it dictates their spin direction.
This distinction clarifies why the Coriolis Effect is observable in large-scale phenomena but not in small-scale, everyday water drainage, preventing misapplication of the concept.
Hurricanes swirling counterclockwise in the Northern Hemisphere and cyclones swirling clockwise in the Southern Hemisphere due to the Coriolis Effect acting on massive air masses.
Key takeaways
- The direction water swirls in a sink or toilet is determined by initial water motion or fixture design, not the hemisphere.
- The Coriolis Effect is a real phenomenon caused by the Earth's rotation, but it's too weak to influence small-scale water drainage.
- Large-scale phenomena like hurricanes and cyclones are significantly influenced by the Coriolis Effect, dictating their rotational direction.
- Controlled experiments are necessary to isolate and observe subtle scientific effects like the Coriolis Effect.
- Popular depictions of water swirling differently across hemispheres are largely mythical.
- Understanding the scale at which forces operate is crucial for correctly applying scientific principles.
Key terms
Coriolis EffectVorticityHemisphereRotationAngular MomentumMythControlled ExperimentHurricaneCyclone
Test your understanding
- What factors primarily determine the direction water swirls down a drain in a sink or toilet?
- Why is the Coriolis Effect not observable in everyday water drainage but significant for weather systems?
- How did the experimenters attempt to eliminate confounding variables when demonstrating the Coriolis Effect?
- What is the Coriolis Effect, and how does it relate to the rotation of large weather systems like hurricanes?
- Explain why the popular belief about toilet water swirling differently in the Northern and Southern Hemispheres is a myth.