Does Water Swirl the Other Way in the Southern Hemisphere?

TL;DR

Ordinary toilets, sinks, and bathtubs do not reliably drain in opposite directions across the Equator because their design and existing water motion overwhelm the Coriolis Effect. Derek and Destin revealed the effect only after letting water settle for 24 hours in five-foot-wide kiddie pools: it drained clockwise in the Southern Hemisphere and counterclockwise in the Northern Hemisphere. Read on to see how their controlled experiment separated Earth’s rotation from everyday sources of motion.

Transcript

Derek: A couple of years ago my friend Destin and I wanted to definitively answer the question: does water actually swirl the opposite direction down the drain in the other hemisphere? At the time, I was living in Sydney, Australia, and Destin was in Huntsville, Alabama. Those are about the same latitude, just opposite sides of the Equator, so we c... Read More

Key Insights

• 💦 The direction of water swirls down the drain is determined by the design of the drain, not the hemisphere.
• 🌀 The Coriolis Effect, caused by the Earth's rotation, influences the direction of water swirls in larger systems like hurricanes and cyclones.
• 🎮 Conducting controlled experiments can help eliminate variables and provide clearer results.
• 🚾 The Simpsons episode about toilet flushing in Australia perpetuated the myth of water swirling in the opposite direction down the drain depending on the hemisphere.
• 🖐️ Inconsistent results are observed when observing the direction of toilet and sink drains, indicating that other factors play a role.
• 💦 The Coriolis Effect causes water to move faster towards the equator and slower towards the poles, resulting in swirling motions.
• 💦 The speed and direction of water flow in a drain can be influenced by angular momentum and other sources of motion.

Questions & Answers

Q: Does water swirl the other way in the Southern Hemisphere?

Not reliably in toilets, sinks, or bathtubs, where drain design and existing motion determine the swirl. In a controlled experiment, however, settled water drained clockwise in the Southern Hemisphere and counterclockwise in the Northern Hemisphere because of Earth’s rotation.

Q: Why do toilets not consistently flush in different directions across the Equator?

Most toilets have jets that set the direction of the swirl, so their design matters more than the hemisphere. Crossing the Equator therefore does not make a toilet automatically change flushing direction.

Q: What determines which way water drains from an ordinary sink or bathtub?

Water in a container is never perfectly still, and its existing rotation can determine how it swirls toward the drain. Other sources of angular momentum overwhelm the tiny effect caused by Earth’s rotation.

Q: How did Derek and Destin isolate the Coriolis Effect?

They used kiddie pools about one and a half meters, or five feet, wide and initially filled them against the expected drainage direction. They then let the water settle for a complete day and opened valves at the bottoms so that pulling plugs would not introduce additional rotation.

Q: What happened in the controlled kiddie-pool experiments?

Destin’s pool in Alabama developed counterclockwise rotation even though it had been filled clockwise. Derek’s pool in Sydney drained clockwise after being filled in the anti-clockwise direction, matching the expected direction in the Southern Hemisphere.

Q: Why did they let the pool water sit for 24 hours?

They wanted motion created while filling the pools to dissipate before drainage began. Removing that competing motion made the much smaller influence of Earth’s rotation observable.

Q: How does Earth’s rotation create a swirl near a drain?

The side of the pool closer to the Equator moves faster eastward and has more momentum than the side closer to the pole. As water moves toward the drain, the faster water gets ahead while the slower water lags, producing rotation.

Q: Does the Coriolis Effect explain the direction of hurricanes?

Yes. Destin states that it is why hurricanes swirl counterclockwise in the Northern Hemisphere, and the controlled pool experiment demonstrated the same direction there.

Summary & Key Takeaways

• Derek and Destin set out to determine if water actually swirls in the opposite direction down the drain in the other hemisphere.

• They conducted experiments with a sink, toilet, and kiddie pool, and found that the direction of the swirl is determined by the design of the drain, not the hemisphere.

• The Coriolis Effect, caused by the Earth's rotation, does influence the direction of water swirls in larger systems like hurricanes and cyclones.