Behind the Forecast: Why the Coriolis Effect doesn’t affect your toilet

Listen to Science Behind the Forecast with Meteorologist Tawana Andrew every Friday on 89.3 WFPL at 7:45 a.m.
Published: May. 24, 2019 at 9:34 AM EDT
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LOUISVILLE, Ky. (WAVE) - First to bust a long-standing myth: The rotation of the earth and the Coriolis effect have no impact on what direction your toilet or swimming pool drains.

They do, however, impact the weather systems that affect the planet daily. From hurricanes to mid-latitude cyclones (the low-pressure systems that consistently traverse the United States), the Earth’s rotation has a significant effect on our day to day weather.

Our planet rotates faster at the Equator than at the poles since it’s much wider around it’s midsection. There’s bit of math involved in calculating the Earth’s rotational speed at different points across the planet; NASA has a great explanation here. The Earth’s circumference is around 40,075 kilometers, or just under 25,000 miles, at the equator. Here the earth speeds along at 1670 kilometers/hour or about 1,000 miles per hour. We don’t feel ourselves hurtling through the universe because everything around us is also racing along at that same speed. Near the Poles, the Earth meanders along at 0.0005 mph.

The Coriolis Force works perpendicularly to the planet’s axis of rotation. Since the planet spins from west to east, the force occurs in a northerly/southerly direction.

The effect of the Coriolis Force, which is named after Gaspard Gustave de Coriolis, is more significant over long distances and at higher wind speeds. It’s effect on objects moving horizontally is greatest at the Poles and consistently decreases until it is nonexistent at the Equator. Looking from above in the Northern Hemisphere, the Coriolis force deflects objects to the right of their direction of motion. This gives mid-latitude cyclones and hurricanes their counterclockwise rotation in the Northern Hemisphere. In the Southern Hemisphere, the deflection is to the left giving cyclones their clockwise circulations. These rotating systems can not exist at the Equator or cross it because the Coriolis effect is non-existent there.

Hurricane Florence approaching the United States' eastern coast.
Hurricane Florence approaching the United States' eastern coast.(Lauren Dauphin data from DSCOVR EPIC)

Just to clarify, there is no actual physical force deflecting the air currents.

The Coriolis Force affects wind patterns worldwide. As warm air rises near the Equator it flows towards the Poles. In the northern Hemisphere, this air is deflected east as it pushes north. These air currents move back south around 30° latitude. The southward flowing air is deflected west as it approaches the equator. This air pattern is called the trade winds.

Back to the toilet and the swimming pool. The way fluids move in these containers is based on their design and other outside forces such as a strong breeze or swimmers taking laps in a pool.

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