Experiment: Oceanic overturning circulation (the easiest version)

“The easiest” in the title of this page is to show the contrast to a “slightly more complicated” version here.


One of the first concepts people hear about in the context of ocean and climate is the “great conveyor belt”. The great conveyor belt is a very simplified concept of the global ocean circulation, which is depicted as a single current that spans the world oceans (see Figure 1 below). In this simplified view of the global circulation, water flows as a warm, global surface current towards the North Atlantic, where it cools, sinks and finally returns southward and through all the world oceans near the bottom of the ocean. Water is transported back to the surface through mixing processes and starts over its journey again as a warm surface current. While in reality some part of the conveyor belt is wind-driven and many processes come to play together, a large part of the circulation can be explained by the water sinking due to cooling at high latitudes.

Figure 1: The great conveyor belt. My sketch on top of a map from http://www.free-world-maps.com (used with permission)
The experiment

Since the global conveyor belt is such a basic concept that we come across in many different contexts, it is very useful to have a good demonstration of what is happening in the world ocean. Plus demonstrations and experiments are always fun!

I here present a very simple experiment that can be used for many different purposes. In science outreach, for example on a fair or in a talk, to catch people’s attention and raise an interest in oceanography. In schools to do the same, or to connect the fascination of the ocean to school physics and talk about density, convection, heat. At university to do all of the above, as well as to practice writing lab reports, talk about the scientific method or the validity of simplifications in theoretical or physical models.

Materials needed

All we need to run this experiment is

  • a clear plastic container
  • lukewarm water
  • red and blue food dye
  • an ice cube tray and
  • access to a freezer.

Ideally we’d also have a thermos or some other kind of insulation to keep the ice cubes frozen until we start running the experiment. To prepare the experiment, all we need to do a half a day ahead is mix some blue food dye into the water that we put in the ice cube tray, and freeze the ice cubes.

Running the experiment

To run the experiment, we start out by filling our “tank” with lukewarm water. Let it settle for a bit. Now we decide for one end of your tank to be the “equator” end. There, we add some red food dye (see Figure 1).

Figure 2: Tank with luke warm water. Red food dye added to the “warm” end of the tank.

Then we add the blue ice cubes to the “poleward” end of our tank (see Figure 3).

Figure 3: Blue ice cubes melting at the poleward end of the tank. The cold melt water sinks to the bottom of the tank and then spreads “equatorward”.

The cold melt water from the ice cubes is denser than the lukewarm water in the tank and therefore sinks to the bottom of the tank where it spreads “equatorward”, pushing below the warmer water. This can be seen where the red water is pushed upwards and “poleward”.


Of course, the processes at play here are not exactly the same as in the real ocean.

For one, deep water formation is NOT due to ice cubes melting in lukewarm water. In fact, melting of sea ice will in most cases not lead to any kind of sinking of water, since the melt water is fresh and the surrounding ocean water is salty and hence denser than the melt water. Cooling in the ocean happens through many processes at the surface of the ocean, like radiation into space and evaporation.

Heating is also represented in an extremely simplified way in this experiment. Heating in the ocean occurs mainly (with the negligible exception of thermal springs in the ocean) by radiative heating from the sun, and at the surface only. We “heat” throughout the whole depth of the ocean by filling the whole tank with lukewarm water.

Also, the mixing processes that, in the real ocean, bring deepwater back to the surface are not represented here at all. Our tank will eventually fill with a layer of cold water at the bottom (See Figure 4), and the circulation will stop once all the ice has melted.

Figure 4: Blue ice cubes melting at the poleward end of the tank. The cold melt water sinks to the bottom of the tank and then spreads “equatorward”. Slowly, the tank fills with cold water.
Why use the experiment?

Even with all the simplifications described above, this experiment is a great first step in becoming intrigued by the ocean, and towards understanding ocean circulation. Seeing the melt water sink from the ice cubes is fascinating no matter how little interest one might have in the physics that cause it. Sliding a finger up and down the side of the tank lets you experience – feel! – how the temperature changes from warm near the surface to very cold near the bottom. Actually physically feeling this is a lot more impressive than just watching the experiment or even just being shown temperature sections of the ocean. And the experiment invites you to play: What if you added little pieces of paper on the water surface, would you see them move with the flow towards the cold end of the tank? Or if you dropped a dye crystal in the middle of the tank, would the dye ribbon that forms be deformed by the currents in the tank? And what if you added twice as many ice cubes, would the currents be twice as fast?

This is pretty much the easiest experiment you can imagine. If you are afraid of what food dye might do in the hands of your participants, you don’t even need to let them handle it themselves, even when they are working in small groups with individual tanks: just go around dripping the dye in and then add the dyed ice cubes yourself. While someone might still tip over a tank and spill the water, this has yet to happen to me. Especially since, before running the experiment, you will have pointed out that they need to make sure not to bang against the tables as to not disturb the experiment. And now apart from making sure that the ice cubes are frozen when you want to run this experiment, there is nothing that can go wrong. So why not try this experiment next time you want to talk about global ocean circulation?

Watch a video of the experiment here:

What would I do differently next time?

Next time, I would pay attention to which end of my tank will represent the equatorward and poleward side of the ocean. Not that it matters much, but in most graphics of sections through the North Atlantic, the northern end will be on the right side and the southern end on the left. If the experiment is set up the other way round (as on all pictures and movies above) you will need to remember to point it out (or even mark it on the tank with a sharpie or such).

Still scared of the hassle of running experiments?

And for all of you who hesitate doing awesome experiments because it looks like you need so much equipment: No you don’t. Here is a “making of” shot from how I did this experiment on my coffee table while sitting on my couch. The background is the back of an old calendar sheet, clipped to the back of a chair. And that’s it.

Screen shot 2015-11-02 at 3.41.24 PM
The setup for my experiment.

P.S.: This text originally appeared on my website as a page. Due to upcoming restructuring of this website, I am reposting it as a blog post. This is the original version last modified on November 4th, 2015.

I might write things differently if I was writing them now, but I still like to keep my blog as archive of my thoughts.

Experiment: Eddy in a jar

Rotating experiments in your kitchen.

Eddies, those large, rotating structures in the ocean, are pretty hard to imagine. Of course, you can see them on many different scales, so you can observe them for example in creeks, as shown below:

Eddies in the Pinnau river, and their dark “shadows”.

If you can’t really spot them in the image above, check out this post for clues and a movie.

So how can you create eddies to observe their structure?

Dye spiral caused by an eddy in a jar

I took a large cylindrical jar, filled it with water, stirred, let it settle down a little bit and then injected dye at the surface, radially outward from the center. Because the rotating body of water is slowed down by friction with the jar, the center spins faster than the outer water, and the dye streak gets deformed into a spiral. The sheet stays visible for a very long time, even as it gets wound up tighter and tighter. And you can see the whole eddy wobble a bit (or pulsate might be the more technical term) because I introduced turbulence when I stopped stirring.

Watch the movie below if you want to see more. Or even better: Go play yourself!

P.S.: This text originally appeared on my website as a page. Due to upcoming restructuring of this website, I am reposting it as a blog post. This is the original version last modified on November 27th, 2015.

I might write things differently if I was writing them now, but I still like to keep my blog as archive of my thoughts.

Experiment: Demystifying the Coriolis force

Mirjam S. Glessmer & Pierré D. de Wet


Even though experiments – whether demonstrated to, or personally performed by students – have been part of training in STEM for a long time, their effectiveness as an educational tool are sometimes questioned. For, despite students’ ability to produce correct answers to standard questions regarding these laboratory exercises, probing deeper often reveals a lack of conceptual understanding.

One way to help students make sense of experiments is to use them in combination with an elicit-confront-resolve approach. With this approach, before the experiment demonstrating a specific concept is run, students are asked to discuss the expected outcome in groups. In so doing, should (specific) misconceptions be harbored about the underlying concept, these are elicited. Incorrect student feedback (feedback illustrating that a misconception is present) is not corrected at this stage. As the demonstration plays out, a mismatch between observation and hypothesis confronts students with their misconceptions. Finally, repetition of the experiment and peer discussion as well as discussion with the instructor lead to resolving of the misunderstandings.

Here, we apply the elicit-confront-resolve approach to a standard demonstration in introductory dynamics, namely the interplay of a rotating frame of reference, movement of particles observed from outside that frame of reference and the resulting fictitious forces. The efficacy of the elicit-confront-resolve approach for this purpose is discussed. Additionally, recommendations are given on how to modify instruction to further aid students in interpreting and understanding their observations.

Key words

Coordinate system, frame of reference, fictitious force, hands-on experiment, elicit-confront-resolve


In many STEM disciplines, demonstrations and hands-on experimentation have been part of the curriculum for a long time. However, whether students actually learn from watching demonstrations and conducting lab experiments, and how their learning can be best supported by the instructor, is under dispute (Hart et al, 2000). There are many reasons why students might fail to learn from demonstrations (Roth et al, 1997). For example, separating the signal to be observed from the inevitable noise can be difficult, and inference from other demonstrations might hinder interpretation of a specific experiment. Sometimes students even “remember” witnessing outcomes of experiments that were not there (Milner-Bolotin, Kotlicki, and Rieger (2007)).

Even if students’ and instructors’ observations were the same, this does not guarantee congruent conceptual understanding and conceptual dissimilarity may persist unless specifically treated. However, helping students overcome deeply rooted notions is not simply a matter of telling them which mistakes to avoid. Often they are unaware of the discrepancy between the instructors’ words and their own thoughts (Milner-Bolotin, Kotlicki, and Rieger (2007)).

One way to address misconceptions is by using an elicit-confront-resolve approach (McDermott, 1991). Posner et al. (1982) suggested that dissatisfaction with existing conceptions, which in this method is purposefully created in the confront-step, is necessary for students to make major changes in their concepts. As shown by Kornell (2009), this approach enhances learning by confronting the student with their lack of an answer to a posed question. Similarly, Muller et al. (2007) find that learning from watching science videos is improved if those videos present and discuss common misconceptions, rather than just presenting material textbook-style.

In this article we look at how an elicit-confront-resolve approach can further student engagement and learning. This is done by using a typical introductory demonstration in geophysical fluid dynamics, namely the effect of rotation on the movement of a ball as seen from within and from outside the rotating system. The motivation for the choice of experiment is dual: the rising popularity of rotating tables in undergraduate oceanography instruction (Mackin et al, 2012), and the difficulties students display in anticipating the movement of an object on a rotating body when they themselves are not part of the rotating system.


The Coriolis force as example for the instructional method

On a rotating earth, all large-scale motion is subject to the influence of the fictitious Coriolis force, and without a solid understanding of the Coriolis force it is impossible to understand the movement of ocean currents or weather systems. Furthermore, the Coriolis force forms an important part of classical oceanographic theories, such as the Ekman spiral, inertial oscillations, topographic steering and geostrophic currents. A thorough understanding of the concept of fictitious forces and observations in rotating vs. non-rotating systems is thus essential in order to gain a deeper understanding of these systems. Therefore, most introductory books on oceanography, or more generally geophysical fluid dynamics, present the concept in some form or other (cf. e.g. Cushman-Roisin (1994), Gill (1982), Pinet (2009), Pond and Pickard (1983), Talley et al. (2001), Tomczak and Godfrey (2003), Trujillo and Thurman (2013)). Yet, temporal and spatial frames of reference have been described as thresholds to student understanding (Baillie et al., 2012).

The frame of reference is the chosen set of coordinate axes relative to which the position and movement of an object is described. The choice of axes is arbitrary and usually made such as to simplify the descriptive equations of the object under regard. Any object can thus be described in relation to different frames of reference. When describing objects moving on the rotating Earth, the most commonly used frame of reference would be fixed on the Earth (co-rotating), so that the motion of the object is described relative to the rotating Earth. Alternatively, the motion of the same object could be described in an inert frame of reference outside of the rotating Earth. Even though the movement of the object is independent of the frame of reference used to describe it, this independence is not immediately apparent. Objects moving on the rotating Earth seemingly experience a deflecting force when viewed from the co-rotating reference frame. Comparison of the expressions for the movement of a body on the rotating Earth in the inert versus rotating coordinate systems, shows that the rotating reference frame requires additional terms to correctly describe the motion. One of these terms, introduced to convert the equations of motion between the inert and rotating frames, is the so-called Coriolis term (Coriolis, 1835).

Ever since its first mathematical description in 1835 (Coriolis, 1835) this concept is most often taught as a matter of coordinate transformation, rather than focusing on its physical relevance (Persson, 1998). Students are furthermore taught that the Coriolis force is a “fictitious” force, resulting from the rotation of a system and that its influence is not visible when observed from outside the rotating frame of reference. It is therefore often perceived as “a ‘mysterious’ force resulting from a series of ‘formal manipulations’” (Persson, 2010).

In many oceanography programs, the difficult task of helping students gain a deeper understanding of these systems is approached by presenting demonstrations, either in the form of videos or simulations (e.g. a ball being thrown on a merry-go-round, showing the movement both from a rotating and a non-rotating frame, Urbano & Houghton (2006)), or in the lab as demonstration, or as a hands-on experiment. While helpful in visualizing an otherwise abstract phenomenon, using a common rotating table introduces difficulties when comparing the observed motion to the motion on Earth. This is, among other factors, due to the table’s flat surface (Durran and Domonkos, 1996), the alignment of the (also fictitious) centrifugal force with the direction of movement of the ball (Persson, 2010), and the fact that a component of axial rotation is introduced to the moving object when launched. Hence, the Coriolis force is not isolated. Regardless of the drawbacks associated with the use of a (flat) rotating table to illustrate the Coriolis effect, we see value in using it to make the concept of fictitious forces more intuitive, and it is widely used to this effect.

During conventional instruction, students are exposed to simulations and after instruction, students are able to calculate the influence of the Coriolis term. Nevertheless, they have difficulty in anticipating the movement of an object on a rotating body when confronted with a real-life situation where they themselves are not part of the rotating system. When asked, students report that they are anticipating a deflection depending on the rotation direction and rate. Contextually triggered, these knowledge elements are invalidly applied to seemingly similar circumstances and lead to incorrect conclusions. Similar problems have been described for example in engineering education (Newcomer, 2010).


The Coriolis demonstration

A demonstration observing a body on a rotating table from within and from outside the rotating system was run as part of the practical experimentation component of the “Introduction to Oceanography” semester course. Students were in the second year of their Bachelors in meteorology and oceanography at the Geophysical Institute of the University of Bergen, Norway. Similar experiments are run at many universities as part of their oceanography or geophysical fluid dynamics instruction.



  • Rotating table with a co-rotating video camera (See Figure 1. For simpler and less expensive setups, please refer to “Possible modifications of the activity”)
  • Screen where images from the camera can be displayed
  • Solid metal spheres
  • Ramp to launch the spheres from
  • Tape to mark positions on the floor
Figure 1A: View of the rotating table. Note the video camera on the scaffolding above the table and the red x (marking the catcher’s position) on the floor in front of the table, diametrically across from where, that very instant, the ball is launched on a ramp. B: Sketch of the rotating table, the mounted (co-rotating) camera, the ramp and the ball on the table. C: Student tracing the curved trajectory of the metal ball on a transparency. On the screen, the experiment is shown as filmed by the co-rotating camera, hence in the rotating frame of reference.



Time needed:

About 45 minutes to one hour per student group. The groups should be sufficiently small so as to ensure active participation of every student. In our small lab space, five has proven to be the upper limit on the number of students per group.


Student task:

In the demonstration, a metal ball is launched from a ramp on a rotating table (Figure 1A,B). Students simultaneously observe the motion from two vantage points: where they are standing in the room, i.e. outside of the rotating system of the table; and, on a screen that displays the table, as captured by a co-rotating camera mounted above it. They are subsequently asked to:

  • trace the trajectory seen on the screen on a transparency (Figure 1C),
  • measure the radius of this drawn trajectory; and
  • compare the trajectory’s radius to the theorized value.

The latter is calculated from the measured rotation rate of the table and the linear velocity of the ball, determined by launching the ball along a straight line on the floor.


Instructional approach

In years prior to 2012, the course had been run along the conventional lines of instruction in an undergraduate physics lab: the students read the instructions, conduct the experiment and write a report.

In 2012, we decided to include an elicit-confront-resolve approach to help students realize and understand the seemingly conflicting observations made from inside versus outside of the rotating system (Figure 2). The three steps we employed are described in detail below.

Figure 2: Positions of the ramp and the ball as observed from above in the non-rotating (top) and rotating (bottom) case. Time progresses from left to right. In the top plots, the position in inert space is shown. From left to right, the current position of the ramp and ball are added with gradually darkening colors. In the bottom plots, the ramp stays in the same position, but the ball moves and the current position is always displayed with the darkest color.
  1. Elicit the lingering misconception

1.a The general function of the “elicit” step

The goal of this first step is to make students aware of their beliefs of what will happen in a given situation, no matter what those beliefs might be. By discussing what students anticipate to observe under different physical conditions before the actual experiment is conducted, the students’ insights are put to the test. Sketching different scenarios (Fan (2015), Ainsworth et al. (2011)) and trying to answer questions before observing experiments are important steps in the learning process since students are usually unaware of their premises and assumptions. These need to be explicated and verbalized before they can be tested, and either be built on, or, if necessary, overcome.


1.b What the “elicit” step means in the context of our experiment

Students have been taught in introductory lectures that in a counter-clockwise rotating system (i.e. in the Northern Hemisphere) a moving object will be deflected to the right. They are also aware that the extent to which the object is deflected depends on its velocity and the rotational speed of the reference frame.

A typical laboratory session would progress as follows: students are asked to observe the path of a ball being launched from the perimeter of the circular, not-yet rotating table by a student standing at a marked position next to the table, the “launch position”. The ball is observed to be rolling radially towards and over the center point of the table, dropping off the table diametrically opposite from the position from which it was launched. So far nothing surprising. A second student – the catcher – is asked to stand at the position where the ball dropped off the table’s edge so as to catch the ball in the non-rotating case. The position is also marked on the floor with insulation tape.

The students are now asked to predict the behavior of the ball once the table is put into slow rotation. At this point, students typically enquire about the direction of rotation and, when assured that “Northern Hemisphere” counter-clockwise rotation is being applied, their default prediction is that the ball will be deflected to the right. When asked whether the catcher should alter their position, the students commonly answer that the catcher should move some arbitrary angle, but typically less than 90 degrees, clockwise around the table. The question of the influence of an increase in the rotational rate of the table on the catcher’s placement is now posed. “Still further clockwise”, is the usual answer. This then leads to the instructor’s asking whether a rotational speed exists at which the student launching the ball, will also be able to catch it him/herself. Ordinarily the students confirm that such a situation is indeed possible.


  1. Confronting the misconception

2.a The general function of the “confront” step

For those cases in which the “elicit” step brought to light assumptions or beliefs that are different from the instructor’s, the “confront” step serves to show the students the discrepancy between what they stated to be true, and what they observe to be true.


2.b What the “confront” step means in the context of our experiment

The students’ predictions are subsequently put to the test by starting with the simple, non-rotating case: the ball is launched and the nominated catcher, positioned diametrically across from the launch position, seizes the ball as it falls off the table’s surface right in front of them. As in the discussion beforehand, the table is then put into rotation at incrementally increasing rates, with the ball being launched from the same position for each of the different rotational speeds. It becomes clear that the catcher need not adjust their position, but can remain standing diametrically opposite to the student launching the ball – the point where the ball drops to the floor. Hence students realize that the movement of the ball relative to the non-rotating laboratory is unaffected by the table’s rotation rate.

This observation appears counterintuitive, since the camera, rotating with the system, shows the curved trajectories the students had expected; circles with radii decreasing as the rotation rate is increased. Furthermore, to add to their confusion, when observed from their positions around the rotating table, the path of the ball on the rotating table appears to show a deflection, too. This is due to the observer’s eye being fooled by focusing on features of the table, e.g. cross hairs drawn on the table’s surface or the bars of the camera scaffold, relative to which the ball does, indeed, follow a curved trajectory. To overcome this latter trickery of the mind, the instructor may ask the students to crouch, diametrically across from the launcher, so that their line of sight is aligned with the table’s surface, i.e. at a zero zenith angle of observation. From this vantage point the ball is observed to indeed be moving in a straight line towards the observer, irrespective of the rate of rotation of the table.

To further cement the concept, the table may again be set into rotation. The launcher and the catcher are now asked to pass the ball to one another by throwing it across the table without it physically making contact with the table’s surface. As expected, the ball moves in a straight line between the launcher and the catcher, who are both observing from an inert frame of reference. However, when viewing the playback of the co-rotating camera, which represents the view from the rotating frame of reference, the trajectory is observed as curved.


  1. Resolving the misconception

3.a The general function of the “resolve” step

Misconceptions that were brought to light during the “elicit” step, and whose discrepancy with observations was made clear during the “confront” step, are finally corrected in the “resolve” step. While this sounds very easy, in practice it is anything but. The final step of the elicit-confront-resolve instructional approach thus presents the opportunity for the instructor to aid students in reflecting upon and reassessing previous knowledge, and for learning to take place.


3.b What the “resolve” step means in the context of our experiment

The instructor should by now be able to point out and dispel any remaining implicit assumptions, making it clear that the discrepant trajectories are undoubtedly the product of viewing the motion from different frames of reference. Despite the students’ observations and their participation in the experiment this is not a given, nor does it happen instantaneously. Oftentimes further, detailed discussion is required. Frequently students have to re-run the experiment themselves in different roles (i.e. as launcher as well as catcher) and explicitly state what they are noticing before they trust their observations.


Possible modifications of the activity:

We used the described activity to introduce the laboratory activity, after which the students had to carry out the exercise and write a report about it. Follow-up experiments that are often conducted usually include rotating water tanks to visualize the effect of the Coriolis force on the large-scale circulation of the ocean or atmosphere, for example on vortices, fronts, ocean gyres, Ekman layers, Rossby waves, the General circulation and many other phenomena (see for example Marshall and Plumb (2007)).

Despite their popularity in geophysical fluid dynamics instruction at the authors’ current and previous institutions, rotating tables might not be readily available everywhere. Good instructions for building a rotating table can, for example, be found on the “weather in a tank” website, where there is also the contact information to a supplier given: http://paoc.mit.edu/labguide/apparatus.html. A less expensive setup can be created from old disk players or even Lazy Susans. In many cases, setting the exact rotation rate is not as important as having a qualitative difference between “fast” and “slow” rotation, which is very easy to realize. In cases where a co-rotating camera is not available, by dipping the ball in either dye or chalk dust (or by simply running a pen in a straight line across the rotating surface), the trajectory in the rotating system can be visualized. The method described in this manuscript is easily adapted to such a setup.

Lastly we suggest using an elicit-confront-resolve approach even when the demonstration is not run on an actual rotating table. Even if the demonstration is only virtually conducted, for example using Urbano & Houghton (2006)’s Coriolis force simulation, the approach is beneficial to increasing conceptual understanding.


The authors noticed in 2011 that most students participating in that year’s lab course, despite having participated in performing the experiment, still harbored misconceptions. Despite having taken part in performing the demonstration, misunderstanding remained as to what forces were acting on the ball and what the movement of the ball looked like in the different frames of reference. This led to the authors adopting the elicit-confront-resolve approach for instruction, as described above, in 2012.

We initially considered starting the lab session on the Coriolis force by throwing the ball diametrically across the rotating table. Students would then see on-screen the curved trajectory of a ball, which had never made physical contact with the table rotating beneath it. It was thought that initially considering the motion from the co-rotating camera’s view, and seeing it displayed as a curved trajectory when direct observation had shown it to be linear, might hasten the realization that it is the frame of reference that is to blame for the ball’s curved trajectory. However the speed of the ball makes it very difficult to observe its curved path on the screen in real time. Replaying the footage in slow motion helps in this regard. Yet, removing direct observation through recording and playback seemingly hampers acceptance of the occurrence as “real”. It was therefore decided that this method only be used to further illustrate the concept once students were familiar with the general (or standard) experimental setup.

In 2012, 7 groups of 5 students each conducted this experiment under the guidance of both authors together. The authors gained the impression that the new strategy of instruction enhanced the students’ understanding. In order to test this impression and the learning gain resulting from the experiment with the new methodology, in 2013 identical work sheets were administered before and after the experiment. These work sheets were developed by the authors as instructional materials to make sure that every student individually went through the elicit-confront-resolve process even when, with future cohorts, this experiment might be run by other instructors (who might not be as familiar with the elicit-confront-resolve method) and with larger student groups (where individual conversations with every student might be less feasible for the instructor). However, it turned out to be useful for quantifying what we had previously only qualitatively noticed: That a large part of the student population did indeed expect to see a deflection despite observing from an inert frame of reference.

In total, 8 students took the course in 2013, and all agreed to let us talk about their learning process in the context of this article. One of those students did not check the before/after box on the work sheet. We therefore cannot distinguish the work done before and after the experiment, and will disregard this student’s responses in the following discussion. This student however answered correctly on one of the tests and incorrectly on the other.

In the first question, students were instructed to consider both a stationary table and a table rotating at two different rates. They were then asked to, for each of the scenarios, mark with an X the location where they thought the ball would contact the floor after dropping off the table’s surface. In the work sheet done before instruction, all 7 students predicted that the ball would hit the floor in different spots – diametrically across from the launch point for no rotation, and at increasing distances from that first point with increasing rotation rates of the table (Figure 3). This is the same misconception we noticed in earlier years and which we aimed to elicit with this question: students were applying correct knowledge (“In the Northern Hemisphere a moving body will be deflected to the right”) to situations where this knowledge was not applicable (when observing the rotating body and the moving particle upon it from an inert frame of reference).

Figure 3A: Depiction of the typical wrong answer to the question where a ball would land on a floor after rolling across a table rotating at different rotation rates. B: Correct answer to question in (A). C: Correct trajectories of balls rolling across a rotating table.

In a second question, students were asked to imagine the ball leaving a dye mark on the table as it rolls across it, and to draw these traces left on the table. In this second question students were thus required to infer that this would be analogous to regarding the motion of the ball as observed from the co-rotating frame of reference. Five students drew them correctly and consistently with the direction of rotation they assumed in the first questions, while the remaining two did not attempt to answer this question.

After the experiment had been run repeatedly and discussed until the students signaled no further need for re-runs or discussion, the students were asked to redo the work sheet. This resulted in 6 students answering both questions correctly. The remaining student answered the second question correctly, but repeated the same incorrect answer to the first question that they gave in their earlier worksheet.

Seeing as the students had extensively discussed and participated in the experiment immediately prior to doing the work sheet for the second time, it is maybe not surprising that the majority answered the questions correctly during the second iteration. In this regard it is important to note that our teaching approach was not planned as a scientific study, but rather developed naturally over the course of instruction. Had we set out to determine the longer-term impact of its efficacy, or its success in abetting conceptual understanding, we should ideally have tested the concept in a new context. As a teaching practice this is advisable.

However, the students’ laboratory reports supply additional support of the claimed usefulness of our new approach. These reports had to be submitted within seven days of originally doing the experiment and accompanying work sheets. One of the questions in their laboratory manual explicitly addresses observing the motion from an inert frame of reference as well as the influence of the table’s rotational period on such motion. This question was answered correctly by all 8 students. This is remarkable for two reasons: firstly, because in the previous year without the elicit-confront-resolve instruction, this question was answered incorrectly by the vast majority of students; and secondly, because for this specific cohort, it is one of the few questions that all students answered correctly in their laboratory reports.

Seven students most certainly make for an insufficient sample size to claim these results have any statistical significance, and this discussion only scratches the surface of what and how students understand frames of reference. However, there is preliminary indication that a) students do indeed harbor the misconception we suspected, and b) that an elicit-confront-resolve approach helped resolve the misunderstanding.


In the suggested instructional strategy, students are required to explicitly state their expectations about what the outcome of an experiment will be, even though their presuppositions are likely to be wrong. The verbalizing of their assumptions aids in making them aware of what they implicitly hold to be true. This is a prerequisite for further discussion and enables confrontation and resolution of potential misconceptions.

This elicit-confront-resolve approach has implications beyond instruction on the Coriolis force or frames of reference. Being able to correctly calculate solutions to textbook problems does not necessarily imply a correct understanding of a concept. Generally speaking, when investigating the roots of student misconceptions, the problem is often located elsewhere than initially suspected. The instructor’s awareness hereof goes a long way towards better understanding and better supporting students’ learning.

We would also like to point out that gaining (the required) insight from a seemingly simple experiment, such as the one discussed in this paper, might not be nearly as straightforward or obvious for the students as anticipated by the instructor. Again, probing for conceptual understanding rather than the ability to calculate a correct answer proved critical in understanding where the difficulties stemmed from, and only a detailed discussion with several students could reveal the scope of difficulties. We would encourage every instructor not to take at face value the level of difficulty your predecessors claim an experiment to have!


The authors are grateful for the students’ consent to present their worksheet responses in this article.

Supplementary materials

Movies of the experiment can be seen here:

Rotating case: https://vimeo.com/59891323

Non-rotating case: https://vimeo.com/59891020


Ainsworth, S., Prain, V., & Tytler, R. (2011). Drawing to Learn in Science Science, 333 (6046), 1096-1097 DOI: 10.1126/science.1204153


Baillie, C., MacNish, C., Tavner, A., Trevelyan, J., Royle, G., Hesterman, D., Leggoe, J., Guzzomi, A., Oldham, C., Hardin, M., Henry, J., Scott, N., and Doherty, J. 2012. Engineering Thresholds: an approach to curriculum renewal. Integrated Engineering Foundation Threshold Concept Inventory 2012. The University of Western Australia, < http://www.ecm.uwa.edu.au/__data/assets/pdf_file/0018/2161107/Foundation-Engineering-Threshold-Concept-Inventory-120807.pdf>


Coriolis, G. G. 1835. Sur les équations du mouvement relatif des systèmes de corps. J. de l’Ecole royale polytechnique 15: 144–154.


Cushman-Roisin, B. 1994. Introduction to Geophysical Fluid DynamicsPrentice-Hall. Englewood Cliffs, NJ, 7632.


Durran, D. R. and Domonkos, S. K. 1996. An apparatus for demonstrating the inertial oscillation, BAMS, Vol 77, No 3


Fan, J. (2015). Drawing to learn: How producing graphical representations enhances scientific thinking. Translational Issues in Psychological Science, 1 (2), 170-181 DOI: 10.1037/tps0000037


Gill, A. E. 1982. Atmosphere-ocean dynamics (Vol. 30). Academic Pr.


Kornell, N., Jensen Hays, M., and Bjork, R.A. (2009), Unsuccessful Retrieval Attempts Enhance Subsequent Learning, Journal of Experimental Psychology: Learning, Memory, and Cognition 2009, Vol. 35, No. 4, 989–998


Hart, C., Mulhall, P., Berry, A., Loughran, J., and Gunstone, R. 2000. What is the purpose of this experiment? Or can students learn something from doing experiments?, Journal of Research in Science Teaching, 37 (7), p 655–675


Mackin, K.J., Cook-Smith, N., Illari, L., Marshall, J., and Sadler, P. 2012. The Effectiveness of Rotating Tank Experiments in Teaching Undergraduate Courses in Atmospheres, Oceans, and Climate Sciences, Journal of Geoscience Education, 67–82


Marshall, J. and Plumb, R.A. 2007. Atmosphere, Ocean and Climate Dynamics, 1st Edition, Academic Press


McDermott, L. C. 1991. Millikan Lecture 1990: What we teach and what is learned – closing the gap, Am. J. Phys. 59 (4)


Milner-Bolotin, M., Kotlicki A., Rieger G. 2007. Can students learn from lecture demonstrations? The role and place of Interactive Lecture Experiments in large introductory science courses. The Journal of College Science Teaching, Jan-Feb, p.45-49.


Muller, D.A., Bewes, J., Sharma, M.D. and Reimann P. 2007. Saying the wrong thing: improving learning with multimedia by including misconceptions, Journal of Computer Assisted Learning (2008), 24, 144–155


Newcomer, J.L. 2010. Inconsistencies in Students’ Approaches to Solving Problems in Engineering Statics, 40th ASEE/IEEE Frontiers in Education Conference, October 27-30, 2010, Washington, DC


Persson, A. 1998. How do we understand the Coriolis force?, BAMS, Vol 79, No 7


Persson, A. 2010. Mathematics versus common sense: the problem of how to communicate dynamic meteorology, Meteorol. Appl. 17: 236–242


Pinet, P. R. 2009. Invitation to oceanography. Jones & Bartlett Learning.


Posner, G.J., Strike, K.A., Hewson, P.W. and Gertzog, W.A. 1982. Accommodation of a Scientific Conception: Toward a Theory of Conceptual Change. Science Education 66(2); 211-227


Pond, S. and G. L. Pickard 1983. Introductory dynamical oceanography. Gulf Professional Publishing.


Roth, W.-M., McRobbie, C.J., Lucas, K.B., and Boutonné, S. 1997. Why May Students Fail to Learn from Demonstrations? A Social Practice Perspective on Learning in Physics. Journal of Research in Science Teaching, 34(5), page 509–533


Talley, L. D., G. L. Pickard, W. J. Emery and J. H. Swift 2011. Descriptive physical oceanography: An introduction. Academic Press.


Tomczak, M., and Godfrey, J. S. 2003. Regional oceanography: an introduction. Daya Books.


Trujillo, A. P., and Thurman, H. V. 2013. Essentials of Oceanography, Prentice Hall; 11 edition (January 14, 2013)


Urbano, L.D., Houghton J.L., 2006. An interactive computer model for Coriolis demonstrations. Journal of Geoscience Education 54(1): 54-60


P.S.: This text originally appeared on my website as a page. Due to upcoming restructuring of this website, I am reposting it as a blog post. This is the original version last modified on January 24th, 2017.

I might write things differently if I was writing them now, but I still like to keep my blog as archive of my thoughts.

Experiment: Influence of stratification on mixing

A wind stress is applied to the surface of a stratified and a non-stratified tank to cause mixing.

This is a pretty impressive experiment to run if you have a lot of time, or to watch the time-lapse of if you don’t. The idea is that a density stratification will make mixing harder than it would be in the unstratified case, because more energy has to be used to break up the stratification.

To look at this, we ran two experiments, one after the other.

In the first one, we took a tank full of freshwater, added dye droplets and switched on a hair dryer, set to blow pretty much along the surface of the tank, to force mixing through the wind stress. After about a minute, the tank was fully mixed.

In the second experiment, we created a density stratification: salt water with approximately 35 psu, and freshwater. We then added the dye droplets. The droplets never penetrated into the salty layer but instead layered in at the interface between the two layers. (See how there are internal waves on the interface, which is why the dye seems to penetrate much deeper on the right? If you watch the movie at the bottom of this page, you see the internal wave very clearly) We then added the hair-dryer wind stress.

After a minute, the surface layer was well mixed, but there was no mixing penetrating into the bottom layer. (We added blue dye at some point, which makes the picture below a little confusing.) To fully mix the whole depth, the wind forcing ran for 86 minutes (and I am proud to report that my hair dryer survived this ordeal! Don’t leave this experiment on its own, not every hair dryer might make this without catching fire!).

Mixing in a non-stratified tank (left) and in a stratified tank (right). See the stop watch at the bottom of the panels for an impression of the time scales involved!

This is a great demonstration of how mixing is inhibited by stratification. We had expected to see a difference, but we were really surprised that the difference was so large. Of course, the stratification in our tank was pretty harsh, but still.

Watch a short movie below and a movie containing the full time lapse even further down!

P.S.: This text originally appeared on my website as a page. Due to upcoming restructuring of this website, I am reposting it as a blog post. This is the original version last modified on November 27th, 2015.

I might write things differently if I was writing them now, but I still like to keep my blog as archive of my thoughts.

Experiment: Interference of waves

Interference of waves is something often taught either using light as a practical example, or without a practical example. Here I want to show a couple of observations as well as a simple experiment.

When talking about waves, it is often difficult to explain that wave heights of different components of a wave field can be added to each other to give a resulting wave field, but that each of those components continues to travel with its own direction and speed and comes out of the wave field basically unaltered. Students learn about constructive, destructive and complex interference (see image below), but it is hard to realize that those interactions are only momentary and that waves come out on the other side without having changed their shape.

Constructive, destructive and complex interference of waves.

In the ocean or on lakes or rivers, you are sometimes lucky enough to observe interference of waves. At a lighthouse in the southwest of Iceland, I took the image below: Two distinct fields were meeting each other at an almost 90 degree angle, interacted and left on the other side still clearly recognizable.

Two wave crests meeting at approximately 90 degree angle.

The waves met, interacted, and left the area of interaction. Watch the movie below to get an impression!

Of course, it is very hard to plan your course such that you happen to observe this out in the “real world”. But interference of waves is so easy to set up, in any pool or tub of water! If your body of water is very small, you can even create waves with only one source and have the reflection from a wall interfere with the “original” wave (actually, you’ll probably have to, because otherwise the reflected waves will mess with the ones you are creating).

Screen shot 2015-08-16 at 11.44.47 AM
Feet tapping in the lake in Ratzeburg to create a pattern of wave interference

Check out the movie below! This is so easy to do, yet so impressive if you have never observed it before.

P.S.: This text originally appeared on my website as a page. Due to upcoming restructuring of this website, I am reposting it as a blog post. This is the original version last modified on November 27th, 2015.

I might write things differently if I was writing them now, but I still like to keep my blog as archive of my thoughts.

Experiment: Ice cubes melting in fresh water and salt water

Explore how melting of ice cubes floating in water is influenced by the salinity of the water. Important oceanographic concepts like density and density driven currents are visualized and can be discussed on the basis of this experiment.



This hands-on experiment is suited for many different audiences and can be used to achieve a wealth of different learning goals. Audience ranges from first-graders over undergraduates in physical oceanography to outreach activities with the general public. Depending on the audience, this activity can be embedded in very different contexts: For children, either in their physics teaching to motivate learning about concepts like density, or in the context of learning about the climate system and ocean circulation. For college/university students the activity can either be used in physics teaching to get a different view on density; in oceanography/Earth science to talk about ocean circulation and processes that are important there; to motivate the scientific process; or to practice writing lab reports (you can be sure that students will at some point be tasting the water to make sure they didn’t accidentally swap the salt water and fresh water cup – a great teachable moment for a) Never putting anything in your mouth in a laboratory setting, and b) Always documenting exactly what you are doing because stuff that you think you will definitely remember obviously isn’t remembered that clearly after all). For the general public, this is typically a stand-alone activity.

Skills and concepts that students must have mastered

It helps if the concept of density is known, but the experiment can also be used to introduce or deepen the understanding of the concept.

How the activity is situated in the course

I use this activity in different ways: a) as a simple in-class experiment that we use to discuss the scientific method, as well as what needs to be noted in lab journals and what makes a good lab report, or density-driven circulation; b) to engage non-majors or the general public in thinking about ocean circulation, what drives ocean currents, … in one-off presentations.



Content/concepts goals for this activity

Students learn about concepts that are important not only in physical oceanography, but in any physical or Earth science: density in general; density of water in particular, depending on the water’s temperature and salinity; how differences in density can drive currents both in the model and in the world ocean; how different processes acting at the same time can lead to unexpected results; how to model large scale processes in a simple experiment. After finishing the activity, they can formulate testable hypotheses, are able to reason based on density how a flow field will develop and they can compare the situations in the cups to the “real” ocean.

Higher order thinking skills goals for this activity

Students learn about and practice the use of the scientific method: formulation of hypotheses, testing, evaluating and reformulating.

Other skills goals for this activity

Students practice writing lab reports, making observations, working in groups.


Description and Teaching Materials


(per group of 2-4 students):

  • 1 clear plastic cup filled with room-temperature salt water (35psu or higher, i.e. 7 or more tea spoons of table salt per liter water), marked as salt water (optional)
  • 1 clear plastic cup filled with room-temperature fresh water, marked as fresh water (optional)
  • 2 ice cubes
  • liquid food dye either in drop bottle, with a pipette or with a straw as plunging syphon


Before the experiment is started, students are asked to make a prediction which ice cube will melt faster, the one in salt water or the one in fresh water. Students discuss within their groups and commit to one hypothesis.
Students then place the ice cubes into the cups and start a stop watch/note the time. Students observe one of the ice cube melting faster than the other one. When it becomes obvious that one is indeed melting faster, a drop of food dye can be added on each of the ice cubes to color the melt water. Students take the time until each of the ice cubes has melted completely.


The ice cube in the cup containing the fresh water will melt faster, because the (fresh) melt water is colder than the room-temperature fresh water in the cup. Hence its density is higher and it sinks to the bottom of the cup, being replaced by warmer waters at the ice cube. In contrast, the cold and fresh melt water in the salt water cup is less dense than the salt water, hence it forms a layer on top of the salt water and doesn’t induce a circulation like the one in the fresh water cup. The circulation is clearly visible as soon as the food dye is added: While in the freshwater case the whole water column changes color, only a thin meltwater layer on top of the salt water is colored (for clarification, see images in the presentation below)


Teaching Notes and Tips

Students usually assume that the ice cube in salt water will melt faster than the one in fresh water, “because salt is used to de-ice streets in winter”. Have students explicitly state their hypothesis (“the one in salt water will melt faster!”), so when they measure the time it takes the ice cubes to melt, they realize that their experiment does not support their hypothesis and start discussing why that is the case. (Elicit the misconception, so it can be confronted and resolved!)

My experience with this experiment is that all groups behave very consistently:

  • At least 80% of your audience will be very sure that the ice cube in salt water will melt faster than the one in fresh water. The other 20% will give the correct hypothesis, but only because they expect a trick question, and they will most likely not be able to come up with an explanation.
  • You can be 100% sure that at least in one group, someone will say “oh wait, which was the salt water again?” which hands you on a plate the opportunity to say “see — this is a great experiment to use when talking about why we need to write good documentations already while we are doing the experiment!”
  • You can also be 100% sure that in that group, someone will taste the water to make sure they know which cup contains the salt water. Which lets you say your “see — perfect experiment to talk about lab safety stuff! Never ever put things in your mouth in a lab!”
  • You can also be sure, that people come up with new experiments they want to try.
    • At EMSEA14, people asked what would happen if the ice cubes were held at the bottom of the beaker.
    • At a workshop on inquiry-based learning, people asked what the dye would do if there was no ice in the cups, just salt water and fresh water. Perfect opportunity to say “try! Then you’ll know! And btw — isn’t this experiment perfect to inspire the spirit of research (or however you would say that in English – “Forschergeist” is what I mean!). This is what you see in the pictures in this blogpost.

It is always a good idea to have plenty of spare ice cubes and salt/fresh water at room temperature ready so people can run the experiment again if they decide to either focus on something they didn’t observe well enough the first time round, or try a modified experiment like the ones described above.

A reviewer of this activity asked how easily students overcome the idea that water in the cup has to have just one temperature. In my experience this is not an issue at all – students keep “pointing” and thereby touching the cups, and in the thin-walled plastic cups I typically use the temperature gradient between “cold” melt water and “warm” salt water is easily felt. The (careful!) touching of the cups can also be explicitly encouraged.

Different ways to use this experiment

This experiment can be used in many different ways depending on the audience you are working with.

  • Demonstration: If you want to show this experiment rather than having students conduct it themselves, using colored ice cubes is the way to go (see experiment here). The dye focuses the observer’s attention on the melt water and makes it much easier to observe the experiment from a distance, on a screen or via a projector. Dying the ice cubes makes understanding much easier, but it also diminishes the feeling of exploration a lot – there is no mystery involved any more. And remember in order for demonstrations to increase the learning outcome, they need to be embedded in a larger didactical setting, including forming of hypotheses before the experiment is run and debriefing afterwards.
  • Structured activity: For an audience with little knowledge about physics, you might want to start with a very structured activity, much like the one described above. Students are handed (non-colored) ice cubes, cups with salt water and fresh water and are asked to make a prediction about which of the ice cubes is going to melt faster. Students test their hypothesis, find the results of the experiment in support with it or not, and we discuss. This is how I usually use this experiment in class (see discussion here).The advantage of using this approach is that students have clear instructions that they can easily follow. Depending on how observant the group is, instructions can be very detailed (“Start the stop watch when you put the ice cubes in the water. Write down the time when the first ice cube has melted completely, and which of the ice cubes it was. Write down the time when the second ice cube has melted completely. …”) or more open (“observe the ice cubes melting”).
  • Problem-solving activity: Depending on your goals with this experiment, you could also consider making it a problem-solving activity: You would hand out the materials and ask the students to design an experiment to figure out which of the cups contains fresh water and which salt water (no tasting, of course!). This is a very nice exercise and students learn a lot from designing the experiment themselves.
  • Open-ended investigation: In this case, students are handed the materials, knowing which cup contains fresh and salt water. But instead of being asked a specific question, they are told to use the materials to learn as much as they can about salt water, fresh water, temperature and density.As with the problem-solving exercise, this is a very time-intensive undertaking that does not seem feasible in the framework we are operating in. Also it is hard to predict what kind of experiments the students will come up with, and if they will learn what you want them to learn. On the other hand, students typically learn much more because they are free to explore and not bound by a specific instruction from you, so maybe give it a try?
  • Problem-based learning: This experiment is also very well suited in a Problem-Based Learning setting, both to work on the experiment itself or, as we did, to have instructors experience how problem-based learning works so they can use it in their own teaching later. Find a suggested case and a description of our experiences with it here.
  • Inquiry-based learning: Similarly as with Problem-Based Learning, this experiment can be used to let future instructors experience the method of inquiry-based learning from a student perspective. For my audience, people teaching in STEM, this is a nice case since it is close enough to their topics so they can easily make the transfer from this case to their own teaching, yet obscure enough that they really are learners in the situation.

Pro tip: If you are not quite sure how well your students will be able to cope with this experiment, prepare ice cubes dyed with food coloring and use them in a demonstration if students need more help seeing what is going on, or even let students work with colored ice cubes right from the start. If ice cubes and hence melt water are dyed right away, it becomes a lot easier to observe and deduct what is happening. Feel free to bring the photos or time lapse movie below as a backup, too!

Dyed ice cubes about to be put into fresh water (left) and salt water (right)
When the ice cubes start melting, it becomes very clear that they do so in different manners. In the left cup, the cold meltwater from the ice cube is denser than the lukewarm water in the cup. Hence it sinks to the bottom of the beaker and the water surrounding the ice cube is replaced by warmer water. On the right side, the lukewarm salt water is denser than the cold melt water, hence the cold meltwater floats on top, surrounding the ice cube which therefore melts more slowly than the one in the other cup.
The ice cube in the fresh water cup (left) is almost completely gone and the water column is fairly mixed with melt water having sunk to the bottom of the beaker. The ice cube in the salt water cup (right) is still a lot bigger and a clear stratification is visible with the dyed meltwater on top of the salt water.

And here a time-lapse movie of the experiment.

Another way to look at the experiment: With a thermal imaging camera!

Cold (dark purple) ice cubes held by warm (white-ish) fingers over room-temperature (orange) cups with water
After a while, both cups show very different temperature distributions. The left one is still room temperature(-ish) on top and very cold at the bottom. The other one is very cold on top and warmer below.
When you look in from the top, you see that in the left cup the ice has completely melted (and the melt water sunk to the bottom), whereas in the right cup there is still ice floating on top.


Depending on the audience I use this experiment with, the learning goals are very different. Therefore, no one assessment strategy can be used for all different applications. Below, I am giving examples of what are possible ways to assess specific learning goals:

– Students apply the scientific process correctly: Look at how hypotheses are stated (“salt melts ice” is not a testable hypothesis, “similar-sized ice cubes will melt faster in salt water than in fresh water of the same temperature” is).

– Students are able to determine what kind of density-driven circulation will develop: Suggest modifications to the experiment (e.g. ice cubes are made from salt water, or ice cubes are held at the bottom of the cups while melting) and ask students to predict what the developing circulation will look like.

– Students can make the transfer from the flow field in the cup to the general ocean circulation: Let students compare the situation in the cup with different oceanic regions (the high Arctic, the Nordic Seas, …) and argue for which of those regions displays a similar circulation or what the differences are (in terms of salinity, temperature, and their influence on density).

In general, while students run the experiment, I walk around and listen to discussions or ask questions if students aren’t already discussing. Talking to students it becomes clear very quickly whether they understand the concept or not. Asking them to draw “what is happening in the cup” is a very useful indicator of how much they understand what is going on. If they draw something close to what is shown on slide 28 of the attached slide show, they have grasped the main points.



Don’t worry, it is totally feasible to bring all the equipment you need with you to run the experiment anywhere you want. This is what we brought to EMSEA14 to run the workshop three times with 40 participants each:

What we brought to EMSEA14 to run workshops on the ice cubes melting in fresh and salt water experiment

In one big grocery bag:

  • 4 ice cube trays
  • 4 ice cube bags (backup)
  • 2 thermos flasks (to store ice cubes)
  • 1 insulating carrier bag (left)
  • 4 empty 1.5l water bottles to mix & store salt water in
  • 1 tea spoon for measuring salt
  • 500g table salt
  • 21 clear plastic cups for experiments
  • 10 clear plastic cups to hand out ice cubes in
  • 11 straws (as pipettes)
  • 1 flask of food dye
  • 11 little cups with lids to hand out food dye in
  • nerves of steel (not shown :-))

And if you are my friend, you might also get the “ice cube special” — a pink bucket with all you will ever need to run the experiment! Below is what the ice cube experiment kit looks like that I made for Marisa, with labels and everything…

An “ice cube experiment” kit that I made for a friend. Want one, too?

References and Resources

This activity has been discussed before, for example here:

I have also written about it a lot on my blog, see posts tagged “melting ice cubes experiment“.

P.S.: This text originally appeared on my website as a page. Due to upcoming restructuring of this website, I am reposting it as a blog post. This is the original version last modified on November 4th, 2015.

I might write things differently if I was writing them now, but I still like to keep my blog as archive of my thoughts.

Cloud in a bottle experiment

Guest post by Susann Tegtmeier (written two months ago, I just never got around to posting it. Sorry!)

No one likes clouds when they bring rain, but what if you could make your own? Making a cloud inside a bottle will help us to understand how they are formed in the atmosphere. The experiment demonstrates how changes in air pressure, temperature and volume are related and how these changes can lead to the sudden appearance of tiny water droplets, or in other words, lead to the formation of a cloud.

You can do the experiment alone at home, in front of a classroom or as a hands-on experiment with all your students. I have chosen the latter option as part of my ‘Introduction to meteorology’ lecture for the first-year students in the Bachelor program ‘Physics of the Earth System’. For this class, Mirjam and I received funding from our university’s PerLe project for teaching innovations. We use the PerLe funding to consolidate the student’s physical-based understanding of the climate system through various experiments, exercises and discussions.

For the experiment you need an air-tight, transparent container that you can pump up with air (in order to increase the pressure inside the bottle). We made a simple version using materials from home including a plastic water bottle supplemented with valve from a bike tire that is attached between the bottle and the cap. Furthermore you need a pump (in our case a bike pump), water and matches.

Picture by Susann Tegtmeier

During the first round of the experiment, the students pumped up the bottles enhancing the pressure inside. During our discussion before the experiment, the students assumed correctly that the bottles would warm due to the enhanced pressure under a constant volume. By putting their hands around the bottles, it was possible for the students to feel that indeed the air inside the bottles was warming. When opening the valve slowly the opposite effect could be noticed and the bottles cooled very quickly. While the temperature change is small, it turned out to be quite fascinating and memorable for the students to see and feel the ideal gas law, they learned about earlier in class, in real life action.

During the second round of the experiment, the pumping up of the bottles was repeated, but this time with a small amount of water in the bottles. Since warm air can take up more water vapor than cold air, some of the water in the bottle was evaporated during the increase of pressure and temperature. While we discussed this effect during the experiment, it was, of course, not possible to observe the formation of the invisible water vapor. The next step of the experiment, the opening of the valve and the accompanying cooling of air, can theoretically lead to the condensation of the above discussed water vapor back to water. However, to the surprise of the students, no condensing little water droplets could be seen in the bottles.

Picture by Susann Tegtmeier

In order to lift the mystery, we carried out the third part of the experiment. With the bottle open, we lit a match and a moment later threw the blown out, smoking match into the bottle. Now the bottle needs be closed quickly before the same action (pumping of bottles and opening of valve) can be repeated. Only in this last round of the experiment, the expected water droplets became visible while the air was cooling. The reason is that small condensation nuclei are necessary for water vapor to condense and form water droplets. The experiment demonstrates this effect quite nicely in the bottle, but it also holds on large scales for the formation of atmospheric clouds.

The ‘Cloud in a bottle’ experiment is a perfect class room exercise, as it leads the students within 30 min from the basic, physical principles of the ideal gas law to one of the big climate effects, the aerosol – cloud interaction.

The energy lab’s dry run. Or: I have a great team! :-)

At first, I wanted to call this blog post “behind the scenes of a school lab” until I looked through the pictures and realized that all I am showing is people sitting around my desk in my office! Actually, not only sitting — test-running all the experiments for our energie:labor!

First, here is Julian’s experiment on atmospheric CO2.

And here an experiment from Jonas’ station on the role of the ocean in the climate system. A candle below an air-filled balloon. Can you imagine what will happen next?

Yes, this.

Sorry about the jump in perspective — even though I knew what would happen I clearly didn’t expect it enough to hold the camera stable. Small consolidation that everybody else clearly jumped, too?

Anyway, the point of that experiment is to look at the heat capacity of air and water. I’ve written about this before, see here (where we also have pictures of bursting the water-filled balloon because this is how we roll. But only in the lecture theatre, not in my office… ;-)).

Below, we are looking at Jonas’ overturning experiment, apparently discussing the work sheets. It’s really great how well this team works together on developing all their materials, even though their personal styles span the whole spectrum of teaching styles!

But we were also having fun, or at least that’s what it looks like… ;-)

In the picture below, taken on the second day of our dry run a couple of days later, we are looking at Henning’s station on the ice-albedo-feedback. As Henning is sitting next to me and we are sitting around my desk, he’s unfortunately not even in the picture!

In the background of the picture above you see the next exciting station that Nicolas prepared (and big shout out to my office mate who didn’t beat an eye when she came in and the office was filled with all our equipment and smelled of vinegar and white spirit…

What the guys are doing on the picture below? Using a bike pump to increase the pressure inside that bottle to make a cloud in a bottle.

Worked really well!

And then, there comes the most sophisticated piece of equipment of the whole lab: Nicolas’ cloud chamber. I’ll only tell you this much for now: It’s awesome! And you should stay tuned for an upcoming publication on how to build it and how to use it in teaching. Because it’s that great!

Now I’m out of pictures, but there is one last thing I want to say: Thank you, team, you are awesome! :-)

My new school lab on energy in the climate system has been launched! :-)

Today was a very exciting day: We launched my new school lab on energy in the climate system! The “energie:labor” is finally up and running again!

Let me walk you through some of the stuff that is going on in the lab.

Below, you see Mirko, who leads the station on the hydrological cycle, and his group working on an experiment.

In the left jar, you see how much smoke a burning piece of paper makes. The students tried this in order to compare it with the jar on the right. Because what they see in there is not just smoke from a burning piece of paper, it’s steam from the hot water at the bottom of the jar — a cloud in the jar! And the burning paper was just added to provide aerosols as condensation nuclei for the clouds.

Another part of the energy puzzle of the hydrological cycle: How much do raindrops falling down on the ground actually heat up the ground? The students are looking at the wooden board on the floor, using a thermal imaging camera. They won’t see a lot when the bouncing ball hits, but they saw a clear signal with the heavy metal ball they used earlier! Kinda like what we did at the European Researcher’s Night (see here).

Now, they are documenting their observations.

And later, they are running an experiment looking at how much moist vs dry air heats up in that insulated container below the lamp to explore the greenhouse effect of water vapour. The setup of this experiment was developed by Julian who is leading a different station on CO2 in the atmosphere, but sadly I don’t have any good pictures of that station!

On the table next to the hydrological cycle team, there is Jonas, working on the role of the ocean in the climate system. Below, the students are dunking air-filled bottles into hot and cold water baths to watch how air expands and contracts depending on its temperature.

They seem to be having fun!

Later, the team at this station did an overturning experiment. I have tons of pictures of that experiment, because it is just super photogenic (or because I am just still fascinated every time I see it, who knows?)

They are using a very strong lamp to model the heating by the sun near the equator, and cold packs to cool near the poles.

And they seemed to enjoy playing with food dye!

We have two more stations (or three, including the CO2 station I mentioned above), one on clouds run by Nicolas, and one on ice-albedo-feeback run by Henning. Unfortunately I don’t have good pictures of those, either, but I will post pictures of our trial run soon, where they’ll be featured, too.

Let’s close this by looking at how we brought all our new experts back together (because each group only conducted one station, for which they had almost all morning) — by using the Monash Simple Climate Model! I’ve written before about how great it is in teaching (see here), and I am still a big fan!

The new “experts” on clouds, the ocean, the hydrological cycle, atmospheric CO2 and ice-albedo feedbacks explained their topics to the rest of their groups. And — surprisingly enough — in the model, you can switch on and off each of these processes individually and see what effect it has on climate!

I think this worked really well to engage students in discussions about the processes they had just explored, and how they work together. Although I want to work on the kind of questions that guide them through the model before the next class visits the school lab in January…

But all in all, I am very happy with how the launch went, and I am super grateful to my great team! Thanks, Jonas, Julian, Nicolas, Mirko, and Henning (from the left in the picture above)! Hope you are enjoying your well-deserved weekend!

And last not least: Thank you, Frank, for letting us borrow your pupils! They were the nicest group we could have hoped for!

Of cupcakes and ice cores

For a popular science presentation on climate change, I needed a simple illustration for how ice cores can be used as archives of past climates. Luckily, my sister and family were excited to do some early Christmas baking for climate science!

And playing with food colors is always fun…

I think I had too much fun playing, actually, the “ice core data” would have been a lot easier to interpret if the different layers were just laying flat!

I should probably noticed here already that the color pattern wasn’t as regular as it should have been for easy interpretation of the core data later…

But it was fun! And they rose beautifully even though we were a little afraid that the time between mixing in the baking powder and actually baking the muffins was kinda long (because we had to mix in all the different dyes…)

The really difficult part, it turns out, was the coring itself. I had wider-than-usual straws, but instead of just cutting out the core, it was really difficult to have them pierce through the crust, and they compressed the core much more than I had hoped.

You can kind of see where the core goes in the cross section, and how the different colors correspond to their old locations inside the cupcake. But somehow this worked much better in my imagination than it did for real!

And I have a new-found appreciation for food bloggers. It’s really difficult to take good pictures of food!

But in case you were wondering: They taste just like boring, non-rainbow muffins. And my niece liked them! :-)