What is the Coriolis Effect and does it really affect my toilet water?

By now, everyone’s heard that the water that goes down the drain in their sinks and toilets spins clockwise in the northern hemisphere and counterclockwise in the southern hemisphere because of The Coriolis Effect. Most of us have been using little factoids, like this, about the Earth to impress our friends or would-be boyfriends/girlfriends since the days when recess was the highlight of our afternoon. Is there any truth to the claim, and what the heck is the Coriolis Effect, anyway?

By definition, the Coriolis Effect (TCE) is an apparent deflection of the path of an object that moves within a rotating coordinate system. For us, the rotating coordinating system is the Earth, and the objects are various things like hurricanes or airplanes. When something, like an airplane, is set into motion the rotation of the Earth literally moves the ground from underneath the plane. Because of this, pilots need to adjust their course so they can navigate to the correct destination, otherwise the Earth would have moved so far underneath them that they would not arrive at the correct destination. Now, imagine the pilot disables the adjustments and lets the plane fly from point A to point B as if the Earth weren’t moving underneath it. If we were watching the plane from the ground it would look to us as if the pilot were steering the plane to purposely miss the target, but that’s just an illusion because the Earth is what “veered off course”, not the plane.

Another way to observe TCE is on a merry-go-round with a few friends and an inflated ball. Sit across from a friend on the merry-go-round and have another friend keep the merry-go-round going at a slow and constant speed. Now, try to throw the ball to your friend that’s sitting in front of you (as if you didn’t already know about TCE), and observe how it seems to veer off course and go to one side instead of to your friend. When something (your ball) is set into motion it will travel in a straight line unless otherwise disturbed. In this case, the disruption is the merry-go-round moving underneath the ball so that the ball misses the target (your friend). It’s almost as if the merry-go-round thinks your playing dodge ball, so it’s jerking your friend out of the way before the ball can get to them.

If you’re a visual learner like me, you’ll enjoy this short YouTube video. It describes the basics of TCE with helpful graphics.

TCE also affects long-range shooting weapons because the Earth will move the target away from the bullet’s path. Check out this video to see what kind of changes TCE can have on long-range weapons:

TCE affects the wind and ocean currents, as well, affecting the weather systems we experience here on the ground. Because of TCE, hurricanes in the northern hemisphere spin to the right and their path is in a spiral that curves to the right. In the southern hemisphere, the hurricanes spiral to the left and their path is in a spiral that curves to the left. TCE changes its effect on things the closer or farther away the object is from either pole. This change in effect is caused by something physicists refer to as the Conservation of Angular Momentum. For instance, if you are spinning around in circles in your office chair, tucking in your arms and legs will cause you to spin faster while extending your arms and legs will cause you to spin slower. In this scenario, imagine that the crown of your head is the North Pole of the earth, and the center of the bottom of the chair is the South Pole of the Earth. The “line” that connects these two points is the center of the spin, or the axis. Now, imagine the Earth spinning and see how the equator is farther from the center of the spin than the poles. When something is set into motion at the equator its speed will increase as it gets closer to the North Pole because it is getting closer to the center of the spin, like when you pulled in your arms and legs to gain speed in the chair.

Here is a YouTube video that explains how TCE affects the weather with helpful graphics:

Now that you are more familiar with the Coriolis Effect and how it affects us here on Earth, let’s revisit the original question. Does the Coriolis Effect really affect the direction my toilet water spins when I flush, or the direction that my bath water spins when I pull the plug? The short answer is no, not in any way that you can measure with your eyes. The scale is just too small, but remember that episode of Spongebob Squarepants where he finds a drain in the bottom of the ocean and pulls the plug? If that scenario were real, if there were actually a drain pipe in the ocean and someone pulled it, the direction that the water would flow down that drain would absolutely be affected by TCE because the scale is much larger.

Because of the copious use of the words “affect” and “effect”, please forgive me if I got them in the wrong spot a couple of times. Also, my understanding of TCE is very general, no matter how many times I’ve watched the included videos or read the webpages listed in the links below. If anyone knows of something I missed, or just has something to add, your comments are welcome!

Further Reading





About Dani Johnson

I am 26 years old and I live in a college town with my boyfriend, our 2 dogs and chinchilla, 4 additional room mates and the house cat. Since I share financial responsibilities with my boyfriend I am waiting on him to finish college before I go back (he's almost done!). I will then focus my studies on Science Writing. I want to write particularly about Astrophysics, Cosmology and Planetary Science. Until then, I spend my free time listening to various podcasts about science and skepticism to inspire deeper research on potential writing topics. I also enjoy sewing, drawing, writing fiction, spending time with the boyfriend and pets, amateur astronomy and some girly things like nails, hair and makeup.
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13 Responses to What is the Coriolis Effect and does it really affect my toilet water?

  1. Reg. says:

    Hi Dani, I always read your inputs as much for their interesting content as for improving communications between Sydney Australia and my US friends. So if I mention something about your choice of words, be assured it is mainly to fix my problems not yours.

    I lived on the Tropic of Capricorn for the first 42 years of my life where cyclones brought an essential part of our yearly rainfall. Knowing the direction of their rotation was really important because their eye changed position so quickly and when the center passed over our position, the wind direction reversed.. The practical application of this was to know which windows to seal tight and which to leave open and when. This was so the house would not be inflated like a balloon and have the roof blown off.

    I am amazed that with US friends who visit Sydney, the first thing they want to do is check which way the water rotates in the wash basin. I dismissed that a particle bias when I was about 6. Though I admit to getting far more confused when I fly from the East coast of Australia in the Southern Hemisphere to reside on Tacoma on the West Coast in the Northern Hemisphere. I find myself wondering “why is the Sun rising over the Pacific in the West?”

    For utter confusion though, try and describe to your American flight companion the intricacies of the International Date Line. That in fact midnight sweeps in an unseen line from East to West and the Celestial transition from one day to the next pays absolutely no attention to our puny clocks or our invisible line on the ocean floor. Tell them the Pacific Ocean from Australia to LA is really only 7 hours wide and watch their eyes. That at one minute past 7pm in Sydney, it is one minute past midnight in LA on the SAME day.

    That where the Sun appears to cross the Ecliptic, the people who live on the equator suffer two longest days of the year and the last thing they want to hear about is Daylight #%$@#$ Saving. Fortunately being on the Equator means they don’t have to worry which way the Cyclone rotates because they know the rain always just pours straight down in torrents.

    I find your use of weren’t instead of wasn’t, or was not, a little disturbing.”if the Earth weren’t moving underneath.” Similarly that the use of the words “was not” instead of “wasn’t” tends to reduce the possibility of missing that little apostrophic reversal when speaking. That the word “where” is not a valid substitute for “we’re” nor are they even pronounced the same way.

    One day you must apply your mind to the angle of the Earth’s Axis and just what it is at an angle to. Looking forward to it.

  2. Jon Richfield says:

    Reg, I fail to follow you concerning “… as if the Earth weren’t moving…”. Looks like a perfectly correct subjunctive to me. Next you will want to spell whom without the w or the m.
    As for the international date line, it is one of the nuttiest conventions our political lords and masters have inflicted on us in the last two centuries, short of the time zones, which are one of the nuttiest conventions our political lords and masters have inflicted on us in the last two centuries, short of daylight saving time, which is one of the nuttiest conventions our political lords and masters have inflicted on us in the last several centuries, short of our year and month lengths.
    If only they had consulted me first…

  3. Reg. says:

    Oh no, I’m going to have to justify myself. Woe is me!!!! Next you’ll be asking why I should have an affection for Dani’s toilet water. (see the headline.)

    Now don’t fall off the planet too soon about the International Date Line my friend. Just pause and ponder for a while. The crooked line is merely an indication that nature does not approve of the pace of life we have adopted. If the French had grasped the lead from the English it would have been a long way East of where it is, with an entirely different set of squiggles. DLS .. meh

    By the way, a moment of scolding Jon, our “political overlords” are our friends, we made them our overlords. Now don’t argue or I’ll piss on your garden.

    I’ve spent many happy walking hours pondering the Coriolis Effect but many more standing on the North Star looking back (?) at the North Pole as it trundles around the Sun in a Counter-Clockwise direction. Did you ever wonder why clocks go “clockwise?”

    There’s only one Earth, weren’t implies past tense plural. Go on, shoot me down. If we’d consulted you we’d have an International pile up of thousands of ships across a line in mid Atlantic. That dented your confidence eh?

  4. Jon Richfield says:

    I am instructed by her who must be obeyed, to stop NOW, Reg, if I weren’t I would first finsh the discussion. As it stands, I’ll have to continue anon. I will come back ASAP and demand the promised dose of nitrogen for my roses. The IDL is totally unnecessary, straight, crooked, or fuzzy, or even tear along the dotted…
    And weren’t not only may imply past tense plural, but also subjunctive and tense-indefinite. Check with the grammar-nazis! 😉
    Cheers or now,

    • Reg says:

      Hey hang on a cotton-pickin’-minute. That guy doing the fall of shot is calling it the Coriolis Effect. While he’s doing it East/West it’s not Coliolus Effect it’s rise or fall of the planet in the altered shot transit time due to rotation. But for this to be true he’d have to be doing it exactly on the Equator.

      The other guy could have tidied his description too by moving it to the Equator instead of being so parochial. Mentioning too that cyclones cannot cross the Equator may have sharpened interest. Then he could have spun the tyre (tire) of a bike wheel in a tub of water to illustrate the effect. Or even dragged his finger through the pool to show the two separating waves that are left behind as his finger advances.

      eeerrr Wot was that Jon, you’re an English teacher? Oh bother. “Her” is so rude, it’s “she.” “Her” is dismissive, “she” is gender respectful. You’ll be in deep poo if you casually point your thumb over your shoulder to separate “her” from the collective herd. Very bad. Even the Kiwis would know better.

      • Jon Richfield says:

        Hi Reg,
        The sheher thing is a matter of accusatives and nominatives.The original She in this matter is Ayesha and Ayesha is nominative She as complement of the verb to be. She also is the one about whom you may read after downloading the text at
        In the genitive or accusative, as in “That is Her book. Give Her Her book. Give Her book to Her.” Her honour is not besmirched by application of the applicable case. You would not say “Her says give She She’s book” would your? And a Kiwi doesn’t have a thumb, not even an alula. And its feathers are hair-like. Cute. Lovely bird.
        But the whole Coriolis thing is a matter of maintenence of momentum. It works just as well on the equator as anywhere else. And if the planet were cylindrical instead of roughly spherical, it would be all equator and poles, wouldn’t it?
        Getting sleepy, though I bet you didn’t notice.

        • Reg. says:

          Um, ah, yes I see, you are blinding me with science Jon and my BP is already 200/110 with a pulse rate of 124 and the thing still says I’m not in AF. Half a Mogadon should fix this.

          Anyhow I make things up under the authority that all Dictionaries reflect usage, they do nor decree it as so many like to think. All dictionaries are out of date by the time they’re published. I claim that SHE and HER are like WHICH and THAT, one is specific and the other a collective. THAT instead of WHO also causes unnecessary mystery. Is it a post or is it a person “about which or whom they are speaking.”

          I humbly suggest that the Coriolis effect is zero on the Equator and reaches it’s maximum at the poles which is what makes David’s structure so interesting in that it seeks a middle Latitude.

          Let me put it another way. Which way would the rotation fetishists expect to see the water going down the plug hole if they were exactly on the Equator?

          Fading..fading.. mogadon kicking in goog byeeee…..

          • Reg. says:

            Thank you fair Princess, yes that IS better. X to you too.

            Anyhow now that it’s (meaning, it has) worn off, let’s (meaning, it is) look at the weather in Antarctica. The Antarctic spins off the really gigantic cyclones but few people recognise them as such. The main reason being that the Southern Hemisphere does not have the numerous land masses the other one does.

            If you’ve lived in Southern Australia you know the huge changes in temperature these rotating giants can bring as they swirl clockwise around the whole globe under various guises, with the centre remaining stolidly on the South Geographic Pole. Temperature drops like 20C in twenty minutes preceded by violent electrical storms and followed by rain.

            .The Roaring Forties being one terrifying latitudinal name that extends through Bass Straight and across the North Island of NZ.

            But the influence of this rotating giant extends as far North as 30 degrees South of the Equator. Not bad eh? 60 degrees of influence globally while the terrible local Tropical Cyclones reduce to mere rain depressions at the drop of a hat, but especially if they venture further South beyond that critical 23.5 degrees. Of course this places the Latitudes between 23.5 S and 30 S in a peculiar position.

            They are neither in a Cyclonic zone nor under the influence of the mother of all cyclones from the South.

          • Jon Richfield says:

            Hi Reg, been gone.
            The Equator is only Coriolis-free as long as you don’t drop it or throw it up into the air. The poles are Coriolis-free as long as you stay over the exact pole (a mathematical point; good luck with that one!)
            Personally I suspect that the max is just off the pole, because your increase in radius is fastest there. At 45 degrees latitude the tangent of your path over the Earth’s surface is 1 — not bad, but not great. The real max of course is when you shoot up vertically at the equator. Useful for launching spacecraft, right?

  5. David McDivitt says:

    There is another effect which will cause a column of water or air to rotate while moving up or down. The effect is strongest at 45 degrees latitude. Any cross section of the column represents a free spinning disk. Consider two discs, a big one and a small one, with the small one mounted close to the outer edge of the big one. Assume the big disk is rotating, but the small disk is standing still as it goes around on the big disk. Moving the small disk toward the center of the big disk will cause it to rotate in the opposite direction. As a column of fluid falls to earth at 45 degrees latitude, because it falls at an angle it has the effect of the large disk and small disk described. Falling at the pole or equator does not have the effect because the disc relationship is negated. This is a very strong effect often mistaken for the coriolis effect, but it does indeed cause water to rotate when going down the drain. Because hurricanes, low pressure systems, and other cyclones move large columns of air, this effect causes these systems to rotate. A powerful thunderstorm can move a column of air fast enough that the described effect causes a vortex or tornado. Again, this is not the coriolis effect but is often confused with it.

    • Jon Richfield says:

      David, I liked your discussion of the effects, but could you please clarify your explanation of why those rotations are “not the Coriolos effect”?

      • David McDivitt says:

        The coriolos effect is what the video here describes. The effect I am describing involves difference in inertial change of outermost edge versus innermost edge of the small disk when moved toward the center of the big disk. That is not the description of the coriolos effect.

        If standing on opposite sides of the north pole, when throwing a ball, the ball will curve. With the effect I describe, if a column of air falls at the north pole there is no inertial change on either side of the column because the center point of the small disk is the same as the big disk. At the equator, the ball does not curve if thrown across the equator with regard to the coriolos effect. If a column of air falls at the equator, there is no inertial change on either side of the column because the small disk is perpendicular to the big disk. The optimum latitude for inertial change in a column of fluid falling toward the earth is 45 degrees.

        The coriolos effect is not what I described.

  6. Jon Richfield says:

    David, been away (far away) for a couple of weeks. (Not sulking, honest!)
    Given that the Coriolis effect is the deflection of moving objects when they shift to different radii in a rotating reference frame, either as perceived by an observer not in the same rotational frame, or in the inertial effects resulting from interaction with other objects not in the same frame as that object (which latter causes trifling effects such as cyclones and my violent motion sickness when I moved my head in the “wall of death” in a fairground) whether moving along the surface of sphere, or in a trajectory when dropped or thrown… Lost myself there… Anyway, all those are examples of Coriolis effects.
    What moved you to exclude everything but your disks?

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