In Episode 4 of the Shedding Light on the Sun and Earth series, The Coriolis Effect, we describe how tropical cyclones form and examine their huge power. We explain what the Coriolis Effect is and how it makes tropical cyclones rotate the way that they do. We also travel to three continents to demonstrate how the Coriolis Effect affects water draining from a small container. Does water really swirl in a different direction depending on which hemisphere it’s in?
An extended excerpt from the program that covers the controversial topic of the science of draining water!
The Question Sheet for students.
Part A: Introduction: Cyclones always rotate clockwise in the Southern Hemisphere and they always rotate anticlockwise in the Northern Hemisphere. But does water draining from a container do the same thing? Well, that’s what we’re going to find out.
Part B: Melbourne in the Southern Hemisphere: We use a small round container to investigate how the Coriolis Effect affects draining water in Melbourne.
Part C: What is the Coriolis Effect? The Earth is turning so anything on the equator is moving faster than anything near the north and south poles. It’s not normally noticeable, but it affects the way winds move and water drains!
Part D: Paris in the Northern Hemisphere: We head to Paris to repeat the experiment that we performed in Melbourne.
Part E: From Winds to Storms: Tropical cyclones are incredibly destructive. We look at how they form and at the damage that they can do.
Part F: Home via Singapore: We head to Singapore to repeat the experiment we performed in Melbourne and Paris.
Part G: Clockwise AND Anticlockwise (at the same time): Cyclones in the northern hemisphere actually spin the same way as cyclones in the southern hemisphere. It’s just our point of view that is different!
The Transcript (which also features low-resolution screen shots)
Part A: Introduction
It’s a question that has been debated for decades now. Does water draining from a container of some sort, like a bathtub, always drain in a clockwise direction in the Southern Hemisphere and does it always drain in an anticlockwise direction in the Northern Hemisphere?
This water is starting to swirl clockwise, so according to popular belief, I must be in the Southern Hemisphere, and in fact I am. I’m in Australia. But one draining bath in one location doesn’t prove anything.
We know that the winds in low-pressure weather systems like tropical cyclones as they’re called in Australia (they’re called hurricanes in the Americas), do in fact always rotate clockwise in the Southern Hemisphere and they always rotate anticlockwise in the Northern Hemisphere. The reason for this is what’s called the Coriolis Effect, and it has to do with the fact that the Earth is spinning.
The Coriolis Effect was named after French scientist Gaspard-Gustave de Coriolis (that’s the best French accent I can do) who studied energy flow in moving systems.
Given that this series is about the Sun and Earth, I’ll talk about how tropical cyclones form and how the Coriolis Effect makes them spin a little later.
But, does the Coriolis Effect apply to water draining from a fairly small container like a bathtub, or a sink, or a container? Some people say that the Coriolis Effect is too small to be measured with small bodies of water, and that if we see any swirling, it has to do more with either the shape of the container or the movement the water already had before it started draining. They say that your location has nothing to do with it.
So, to find out the truth once and for all, I’m going to perform the experiment with the same container, literally exactly the same container, both in the Southern Hemisphere here in Melbourne, Australia, and in the Northern Hemisphere in Paris, France.
Our trip will also take us to Singapore which is as close to the equator as I’m going to get (on the ground anyway) to see if being close to the equator makes any difference. Singapore is 1° north of the equator. Melbourne is 38° south of the equator, and Paris is 49° north of the equator.
Part B: Melbourne in the Southern Hemisphere
Right now, I’m in Melbourne, about 38 degrees south of the equator. This is the equipment I’m going to use: 2 containers, one of which I’ve drilled a small hole into, some blu-tack, two rulers, and some small pieces of cork.
For each experiment, I will plug the hole in the container with a bit of blu-tack, sit it on top of another container using two rulers and then fill it with water. I will let it sit still for a minimum of five minutes so that the water can come to a complete stop, and then I’ll reach underneath the container to remove the blue tack. This will allow me to set the water draining without actually touching the water.
I’ll let the water drain for about 2 minutes, before placing a small piece of cork into the water. The cork isn’t big enough to disrupt any currents that have been established. It took a total of about 3½ minutes for the water to drain completely. After each trial, I just poured the water from the bottom container back into the top container and started again.
So here you can see, in real time over about a minute or so, what happened. We performed the experiment at least a dozen times in Melbourne, six times at this location, obviously I’m only showing four of them here, and a heap of times at home and in every single trial the water ended up swirling clockwise as it drained out of the hole. Sometimes the cork kind of spun out towards the edge of the container and stuck to the side so I just threw another cork in. As you can see, the swirling wasn’t very fast but it was definitely clockwise every time. I suppose it’s always a good thing when you do an experiment and you get the same results every time.
After all these trials I was beginning to think that the people who say that the Coriolis Effect doesn’t apply to small bodies of water like this one have never actually tried the experiment. However, I’m guessing that if the water drains out too slowly, like a drop at a time, then there wouldn’t be enough movement to establish any kind of spiral current and if it drains out too quickly through a really large hole, then there won’t be enough time for any spiral current to form. Two litres or so of water draining in three to four minutes though seems to be about right to establish as you can see, a clockwise current flow.
It’s now been 1½ minutes of real time footage since I dropped the corks in and about 3½ minutes since the water actually started draining.
So it’s pretty consistent. As water drains out of this container in Melbourne, it always ends up swirling clockwise, which is the same as the way the winds blow in tropical cyclones in the Southern Hemisphere. What happens though if I perform exactly the same experiment with exactly the same equipment in the Northern Hemisphere? Well, let’s go to Paris and find out.
Part C: What is the Coriolis Effect?
The theory of the Coriolis Effect which I’ll explain using water draining, is that if you have a large container of water, a very large container of water in this case, touching the North Pole at one end and the equator at the other end it spins around as the earth turns but the water near the equator is moving faster than the water near the Pole.
This is quite an unusual map, but it shows what the Northern Hemisphere looks like from above.
Now, if you pull the plug the water starts draining inwards. However, near the Pole the water moves inwards in this case towards Europe from kind of a standing start, but as the Earth spins underneath it, it appears to move to the west, that is to the left on the map and ends up somewhere over the Atlantic Ocean.
The water near the equator though is already moving at high speed from west to east. So, when the plug is pulled and it starts moving northwards towards the drain hole, the easterly movement that the water already had combines with the northerly movement towards the drain hole so the water moves over the surface of the Earth at an angle between North and East so it moves to the right of the drain hole at an angle between north and east, so it moves to the right of the drain hole.
Just to illustrate, if I throw a ball from a stationary trolley, it will just move in the direction that I throw it. Pretty obvious.
However, if I throw the ball from a moving trolley (at right angles to the direction that the trolley is moving), it will move diagonally across the screen because the speed I give it combines with the speed that it already has which means that it moves away from the camera AND towards the right.
So, back to the water in our large container the water closer to the equator that moves to the north when the plug is pulled, is also flung to the east because the earth is spinning and it ends up moving at an angle between north and east.
The two motions from the North and from the South combine and the water forms an anticlockwise current in the Northern Hemisphere. The Coriolis Effect is the name given to this apparent shifting of an object’s movement due to the Earth’s rotation.
So in the Northern Hemisphere, the theory is that water should drain (and that winds will flow in cyclones) in an anti-clockwise spiral, but if you’re in the Southern Hemisphere, it’s the opposite, because you’re kind of upside down with respect to the Northern Hemisphere.
Part D: Paris in the Northern Hemisphere
Okay, so here I am in Paris in front of the Eiffel Tower. I have the same two buckets that I had in Melbourne. I’m going to fill this one up with water, let it drain into the other one, and see if in fact the water spins differently here in the northern hemisphere compared to what it did in Melbourne in the southern hemisphere. Let’s begin.
I set it up exactly as I did in Melbourne of course. After letting the water drain undisturbed for the first two minutes or so, I dropped a small piece of cork into the water, and just kept filming it so that you can watch it in real time.
And I think the results are pretty clear. We performed 6 trials in Paris, three near the Eiffel Tower which you’re watching now and three in our hotel room, and as you can see we got consistent anticlockwise swirling.
So that seems pretty conclusive. In Melbourne in the southern hemisphere, the water drains out the hole clockwise, just like it should, whereas here in the northern hemisphere in Paris, the water drains out anticlockwise.
So, according to my experiments, if still water far away from the equator starts draining from a round container, the Coriolis Effect is strong enough to make it start spinning. I chose a round container and drilled the hole in the middle of the container to eliminate unnecessary variables. But, bathtubs and basins are not usually round, so that might complicate the way water flows out of them. Also the hole isn’t usually in the middle. It might be on the equator side, the opposite side or somewhere in between, and that might also be a factor. I’ll need another trip to do more research!! I also suspect that if you have to put your hand in the water to remove the plug, then the movement you give the water will produce inconsistent results; the Coriolis Effect may not be strong enough to overcome the movement that the water already has.
Now just in case you’re wondering, Liacos Educational Media didn’t travel all the way to Paris just to film water draining out of a container. We actually went to England and Greece as well to shoot scenes for new programs and this is just one of them that you’re watching right now.
The only thing left to do for this program was to perform the experiment in Singapore which is only 1° north of the equator. But first, let’s take a quick look at how tropical cyclones form and at the destruction that they can bring.
Part E: From Winds to Storms
Tropical cyclones typically cover a huge area. The yellow line here represents about 600 km, while this one here represents about 1000 km.
They bring really strong winds that vary from about 120 km/hr (for a Category 1 cyclone) to more than 250 km/hr (for a Category 5, the highest you can get), a huge amount of flooding rain, and what’s called storm surge. Storm surge is the sea water that is literally blown onto land by the strong winds. Storm surge often causes more deaths and damage than the strong winds and the rain.
Tropical cyclones cause a huge amount of damage and it’s estimated that they’ve caused nearly two million deaths over the past few hundred years. Most of the deaths are associated with the flooding caused by storm surge.
However, they can also bring much-needed and welcome rain to farms that are inland and they transfer huge amounts of heat energy away from tropical regions towards temperate regions.
Tropical cyclones, which are typically called hurricanes in the Americas and typhoons in Asia, even though they’re all the same thing, form in warm tropical waters and usually move from east to west initially. So how do they form?
At a simple level, all winds are caused by uneven heating of the Earth’s surface.
Depending on cloud cover, winds that are already blowing, and ocean currents, a part of the Earth may be heated by the sun more than surrounding areas. The warm air in this region heats up and expands, causing a region of low pressure. As a result, air flows in from the side because air flows from high-pressure regions to low-pressure regions. This flowing air is the wind we feel. At the centre of the low pressure region, the warm air rises because it’s pushed upwards. Of course all this happens over huge areas not small regions like we’re showing here.
Because of the spinning of the earth though and the Coriolis Effect, the air flowing towards the low pressure area ends up being deflected and it starts to rotate.
In a container with a hole in it, the water is pulled downwards, but in a cyclone the air moving inwards is pushed upwards.
Now severe storms can form anywhere on Earth, but tropical cyclones have much stronger winds than other storms. This is because tropical water is warmer than the water everywhere else.
The vast amount of energy and the vast amount of evaporation that takes place in warm tropical waters contribute to the pressure differences involved and make the winds faster.
For water to evaporate it has to absorb energy from the surrounding water. When it then condenses after it’s risen up into the atmosphere, that energy is released into the air which actually heats the air higher up in the atmosphere even more.
These kinds of weather maps, called synoptic charts, show lines that are called isobars. Isobars are lines of equal air pressure. Air moves from higher pressure regions (often just called Highs) to lower pressure regions (often called Lows) and that movement of air is, as I said, what wind is. However, because of the Coriolis Effect, the winds get deflected and tend to move more or less parallel to the isobars. In the Southern Hemisphere, winds rotate clockwise around lows and anticlockwise around highs. When the isobars are close together, the winds are stronger since the pressure difference is greater, and when the isobars are far apart, the winds are lighter. On this particular day, the winds in Victoria were moderate, while the winds in northern Australia were light.
Synoptic charts also often show what are called cold fronts (areas of cold air that often bring clouds and rain) and troughs (long extended regions where the air pressure is lower than the surrounding air).
This synoptic chart shows the air pressure in Australia and southern Asia. Across pretty much the whole of Australia, the air pressure varied from 1017 hectopascals (hectopascals, hPa, is the unit scientists use for air pressure) to 1011 hectopascals, a difference of only 6 hectopascals. In this tropical cyclone though, called typhoon Halong, the pressure difference between this isobar (at 1004 hPa) and the cyclone’s centre at only 905 hectopascals was 99 hectopascals, a huge difference over such a small distance. At its peak, the wind speed was about 300 km/hr. So, a tropical cyclone is basically the same as a tropical storm (and in fact any low-pressure system) but the air pressure at the centre of a tropical cyclone is especially low and so the winds are especially ferocious.
The centre of a tropical cyclone is called the eye and it’s typically about 30 to 60 kilometres wide. If you’re inside the eye, the winds are fairly light. However, as the cyclone continues to move, the eye moves on as well. The wind starts up again but blows in the opposite direction to the wind that blew before the eye passed.
When the eye of the cyclone hits land, it’s called landfall. The strongest winds are usually felt in the hours leading up to landfall. Once a cyclone makes landfall, it weakens because it is cut off from the warm water that gives it energy. The winds die down, but the rain may continue for days.
Tropical Cyclones should obviously be taken very seriously. You may think your house is strong enough to survive the winds, but it may get flooded and you may lose power and water.
We sometimes take simple things like the ability to go to the toilet for granted, but if the sewerage system isn’t working properly, or if your toilet is underwater, then what do you do?
Many people get sick, and many die, in the days after a cyclone hits because of the diseases that spread when sewerage comes up into the flooded streets and into people’s homes.
Ideally, if the authorities tell you to evacuate, you should. Unfortunately, though, it’s not always possible.
In the Southern Hemisphere, tropical cyclones are most likely to form between November and March, while in the northern hemisphere, they form mostly between June and November. These months correspond to the warmer months in each hemisphere when the ocean water is hotter.
Let’s now see what happens to draining water in Singapore.
Part F: Home via Singapore
I’m now on the way back from Europe. I’m in Singapore, I’m at the Singapore International Airport, and the Singapore International Airport is only 1° north of the equator. I’m going to do the same experiment again, same bucket same hole there, same cork, same ruler same everything and we’ll see what happens.
Unfortunately, we were only able to perform three trials in Singapore because our plane made only a brief stop in Singapore on the way back to Australia. Our shots here aren’t super steady because I had to pack the tripod into the suitcase since you’re not allowed to carry tripods into the cabin of the plane. The containers and the camera though fitted easily into our carry-on bags!
Now part of the scientific method when you’re trying to establish relationships between variables is that you can only change one variable at a time to see what effect it has on another variable. That’s why I decided to use exactly the same container and exactly the same set up. The only variable I changed was my location and I can now say with confidence that changing your location from Melbourne in the Southern Hemisphere to Paris in the Northern Hemisphere definitely affects the direction in which water will drain out of a container. I suppose I could have taken my whole bath tub to test but that would have been a little harder.
Now in Singapore, as you can see, there doesn’t seem to be any swirling of the water at all. I’m not sure why I threw two pieces of cork in on two of the trials, I’m going to blame severe sleep deprivation, but it seems to me that the Coriolis Effect near the equator is very small, certainly too small to have any effect at all on the water in this container as it drains.
So, water seems to drain clockwise in the southern hemisphere, anticlockwise in the northern hemisphere and straight down the hole near the equator.
And now I’m back in Australia and I can say that even though the container we used was quite small the results are clear. The Coriolis Effect is very small but it definitely does apply even to small bodies of draining water, not just to tropical cyclones.
Also, the Coriolis Effect isn’t some constant value in the Southern Hemisphere which then drops to zero at the equator which then becomes the opposite of that constant value in the Northern Hemisphere. The Coriolis Effect starts at zero at the equator but then gradually gets stronger and stronger as you move further and further away from the equator.
I’ve seen videos of locals living in countries on the equator showing tourists the water swirling clockwise when they’re 10 metres south of the equator and then swirling anticlockwise when they move to a spot 10 metres north of the equator. I suspect that they set the water swirling themselves either with the way that they pour the water into the container or the way they remove the plug, because as I said, the Coriolis Effect doesn’t just turn on when you move away from the equator, it gradually gets stronger and stronger.
Interestingly, tropical cyclones never get within about 5° of the equator.
This map shows the track of every tropical storm and cyclone over a 7-decade period. A tropical storm is declared a tropical cyclone and given a name (tropical cyclone Debbie for example) when the sustained wind speeds reach about 120 km/hr. The blue lines represent storms and the yellow, orange and red lines represent tropical cyclones, with the red lines showing the most severe ones. The Coriolis Effect just doesn’t seem to have much of an effect near the equator even on the wind, so it’s probably not surprising that water draining out of a container near the equator doesn’t start to spin either.
Part G: Clockwise AND Anticlockwise (at the same time)
So we’ve seen that cyclones rotate in different directions in the two hemispheres.
However, all cyclones actually spin the same way no matter where you are. It’s just our view that is different. I have a see-through balloon here representing the Earth and I’ve stuck some arrows onto it to show the direction that air flows around cyclones in the two hemispheres, clockwise in the southern hemisphere and anti-clockwise in the northern hemisphere.
If I tilt the balloon and view it from above, you can see that the air rotates in the same direction in both hemispheres. Whether something is rotating clockwise or anticlockwise depends on where you’re looking at it from.
The Earth rotates west to east. From the north, it’s rotating anticlockwise, but from the south it’s rotating clockwise. It’s the same with cyclones.
They all rotate the same way, but southern hemisphere cyclones are just upside down compared to northern hemisphere cyclones.
What’s really happening is that any air near the equator in the northern hemisphere moving northwards towards a low-pressure region doesn’t move straight northwards because it gets flung to the east since the Earth is spinning.
The same thing happens in the southern hemisphere to any air near the equator moving southwards towards a low-pressure region; it gets flung towards the east as well.
In all cyclones, the air that’s closest to the equator moves in the same direction that the Earth is moving: west to east.
So we should always be careful about what we think we know. Is there another point of view or another way of looking at something?
The way a cyclone spins doesn’t really matter course; a cyclone is a cyclone and if one is heading your way you have to take action.
The biggest difference between the Northern Hemisphere and the Southern Hemisphere isn’t the way tropical cyclones spin, it’s the temperature differences between summer and winter.
Melbourne, where I live, gets cold but a top temperature of about 12 degrees Celsius is considered icy and everyone freaks out when it happens. To see snow we need to drive up into the mountains. This is the same for all major cities in the southern hemisphere.
However, as you can see from this NASA simulation showing snow falls, the northern hemisphere gets lots of snow in winter. This is because it gets much colder on average during winter in the northern hemisphere than it does during winter in the southern hemisphere.
So why the difference? Well that’s what we’re going to look at in our next episode. See you then.
https://en.wikipedia.org/wiki/File:Tropical_cyclones_1945_2006_wikicolor.png by Citynoise. License: Creative Commons 3.0
NOAA. The National Oceanic and Atmospheric Administration is an American scientific agency within the United States Department of Commerce that focuses on the conditions of the oceans, major waterways, and the atmosphere.
NASA. The National Aeronautics and Space Administration is an independent agency of the United States Federal Government responsible for the civilian space program, as well as aeronautics and aerospace research.
JAXA. The Japan Aerospace Exploration Agency is the Japanese national aerospace and space agency.
Synoptic Charts produced by the Bureau of Meteorology, Australia’s national weather, climate and water agency. Creative Commons (CC) Attribution 3.0 licence.
https://commons.wikimedia.org/wiki/File:Orthographic_Projection_Polar_North.jpg by Lars H. Rohwedder (User:RokerHRO). Creative Commons (CC) Attribution 3.0 license.
Hurricane Irma DEVASTATES Naples, FL 150 mph gusts! by Bob McCallan: Creative Commons license.