Accidental double-diffusive mixing

When setting up the stratification for the Nansen “dead water” demo (that we’ll show later today, and until then I am not allowed to share any videos, sorry!), I went into a meeting after filling in layer 4 (the then lowest). When I came back, I wanted to fill in layer 5 as the new bottom layer. For this experiment we want the bottom four layers to have the same density (so we would actually only have one shallow top layer and then a deep layer below [but we can’t make enough salt water at a time for that layer, so I had to split it into four portions]), and I had mixed it as well as I could. But two things happened: a) my salinity was clearly a little fresher than the previous layer, and b) the water in the tank had warmed up and the new water I was adding with layer 5 was cold tap water. So I accidentally set up the stratification for salt fingering: warm and salty over cold and fresh! Can you spot the darker pink fingers reaching down into the slightly lighter pink water? How cool is this??? I am completely flashed. Salt fingering in a 6 meter long tank! :-D

 

Communicating Climate Change — a book you should definitely know about!

In a presentation about science communication I gave on Monday, I recommended a couple of resources for scientists interested in science communication. For example the amazing climatevisuals.org for advice on which images to use to communicate about climate change (plus lots of images that even come with explanations for what purpose they work well, and why!). And of course my #scicommchall to get people inspired to try out a new micro scicomm format every month.

But here is an (open access!) book I wish I had known about then already but only came across two days after my presentation: “Communicating Climate Change” by A. K. Armstrong, M. E. Krasny, J. P. Schuldt (2018).

This is a book aimed at educators who want to communicate climate change in a literature-based and effective manner. It consists of four parts: A background, the psychology of climate change, communication, and stories from the field, which I will briefly review below (and you should definitely check out the real thing!). It’s nice and easy to read, and there are “bottom line for educators” at the end of each chapter as well as recaps at the end of each part, making it easy to get a quick overview even if you might not have the time to read the whole thing in detail.

Background

This part of the book begins with an introduction to climate change science, reporting state-of-the-art science on climate, greenhouse gases, evidence for climate change, and climate impacts. It then moves to how climate change can be addressed: by mitigating or adapting to its effects, how it is important to reduce greenhouse gas emissions, and how that can be achieved both on an individual level and by collective action. It ends with a “bottom line for educators” summary that stresses that climate change is real, that misinformation campaigns are an unfortunate reality, and that educators can contribute to solving the problem.

The next chapter then deals with what is known on attitudes and knowledge about climate change in different audiences internationally and at different ages, explaining that attitudes are actually a pretty bad predictor for behaviour, but nevertheless important to know about if you are an educator! For example, if you want teens to be concerned about climate change, a useful approach might be to involve their parents along with them, since what family and friends believe about climate change is very important to what an individual teenager believes, as is how often they discuss climate topics with their friends and family. Again, the “bottom line for educators” breaks this down into advice, for example to focus on different topics depending on how concerned about climate change a given audience already is, or to focus on areas in which a common ground between them and their audiences exists in order to generate a constructive and positive dialogue even though there might still be areas in which they do not agree with their audiences (which they should think about beforehand, hence the importance to know about the audience’s attitudes).

The next chapter suggests possible outcomes for climate change education — how do we know if a climate change communication activity was successful? — and stresses the importance of defining these goals in the first place. Outcomes can be defined on the level of individuals, of communities, of the environment, or of resilience of all of the above. For individuals, outcomes could for example be literacy (understanding essential principles, knowledge of how to assess scientifically credible information, ability to communicate, ability to make informed and responsible decisions) of climate change, or attitudes and emotions, the feeling of confidence that you can reach your goals, or environmentally friendly behaviour. For communities, outcomes could be positive development of youth, building of social capital (e.g. trust or positive action), the belief that the community can reach a goal together, or action taken together by the whole community. Focussing on the environment, an outcome could be adaptation to, or mitigation of, climate change.

The next chapter then presents three climate change vignettes — three examples of how different educators address different audiences in different settings — and a discussion of why they chose to design their activity a certain way and react to questions or comments the way they did.

The psychology of climate change

This part of the book presents psychology research on why knowledge about climate change is not sufficient to actually change behaviours.

Identity research especially is very helpful, as it explains how in order to feel like you are part of a group (something that we as humans are hard-wired to crave) we tend to conform with our group’s norms and values. We might be part of different groups at different times as well as simultaneously (for example our family as one and our colleagues as another, or inhabitant of a city, or student of oceanography), and contexts trigger specific identities that might even not be completely congruent with each other. When new information is presented, we interpret it in a way that does not threaten our identity in the context the information is presented in. Therefore, in order to not threaten anybody’s identity and making it impossible for them to take on our message, it is important to make sure that climate change is not communicated as something polarising or political, but rather choose to trigger identities that are inclusive, like for example “inhabitant of place x”, and focus on outcomes that benefit that community independent of what other identities might exist, by for example protecting a local beach.

Psychological distance is another lens through which climate change communication can be viewed. The more distant a problem seems, the less important it is perceived. Therefore focussing on local relevance rather than global, on places that are important to people, on communities they care about, might in some cases be helpful — although not always; the results of the research on this are not conclusive yet.

Then a few other relevant psychological research areas are discussed, like for example “terror management theory”. This leads to the recommendation to avoid “doom and gloom” presentations of climate change that might kick people into a defence mechanism of ignoring the topic to protect their emotional well-being in the moment, and to focus on hope and positive action instead. Then there is the “cognitive dissonance theory”, according to which we try to ignore information that conflicts with what we think we already know or threatens other goals we might have. The recommendation here is to give people ideas of easy things they can do to combat climate change to combat cognitive dissonance.

Communication

This part of the book presents three aspects of communicating climate change: How we frame it, which analogies and metaphors we use, and how we, as a messenger, can build trust.

“Framing” is about how a message is featured in a story line to help the audience interpret it in a certain way, by making certain aspects of it especially visible, for example economic aspects or tipping points. When thinking about framing a climate change message, it is important to think about audiences and their identities and to avoid wording that will trigger identities which make it difficult to accept the message. Depending on the desired outcomes, climate change communications could, for example, be framed for solutions, hope, or values. There are ways to build entire climate change communication programs around those frames, and there are several examples given for how this might be done.

The next chapter focusses on analogies and metaphors. For example, “osteoporosis of the sea” (which I had never heard in use before) has been found to be a successful metaphor for ocean acidification. However, as all metaphors, it only highlights similarities between issues and neglects to mention the dissimilarities which makes them tricky to use because it’s hard to make sure people don’t take a metaphor so far that it breaks down. In fact, to address this problem, the authors recommend to explicitly talk about where the analogy or metaphor will break down.

Establishing trust in the climate change messengers: This is tricky as people tend to trust other people that hold values similar to their own. Therefore it is helpful to think about the messenger and to use trusted middle persons. [There is are actually some very interesting work on trust out there, for example by Hendriks, Kienhues and Bromme (2015) that isn’t mentioned in the book, but that I’d be happy to summarise for you if anyone is interested!]

Stories from the field

The book ends with a part called “stories from the field” in which examples of different climate change communication activities, focussing on different goals, audiences, messenges and happening in very different settings, are given and the design choices that were made explained in detail. Also for each of the story, an example is given how the message is phrased in actual interaction with the target audience. All of this is super interesting to read because all the theory the book provided in the previous chapters is applied to real world cases, which makes it easy to see how they might be applied to your own climate change communication activities. Also these best practice examples are inspiring to see and give me a sense of hope.

To sum up: I really enjoyed reading this book! So much so that continuing reading it was more important than getting a good Instagram pic of my latte while writing this blogpost. I would really recommend anyone interested in climate change communication to check it out! When I finished my talk on Monday, on my second to last slide I put the African proverb along the lines of “if you think you are too small to make a difference, try going to sleep with a mosquito in the room”. I used this to talk about using messages of hope in climate change communication, and then also applied it to science communication — don’t think you are too small to make a difference there, either! And that’s a message that this book conveys really well, too, providing a good idea of what one could do and how one might go about it, and inspiring one — or at least me — to do so, too.

Observing waves in a tank

So you thought filling water into a tank was boring? Not on my watch!

This is how we fill up the tank: Through a hole at the bottom. Which leads to a very nice fountain that slowly submerges as the water level rises:

…and to tons of nice waves, which are great to observe!

Propagation of waves

Below you see waves propagating. Can you spot the water’s orbital movement, i.e. water particles moving in circles, even though the wave phase is propagating from left to right?

Standing waves

After a while, waves are reflected at the end of the tank and propagate back, setting up a different, very cool, pattern:

Now the wave phase does not seem to travel any more! Instead, there are fixed points in space where water levels oscillate between maximum and minimum, and in between there are other points where the water level stays more or less the same. How cool is that?!

…And this is just filling the tank. Just wait how cool it gets when we are actually running our demonstrations! :-)

Did you know that uranine starts fluorescing again when the ice cubes melt?

Yes, I had to test that! :-D

Top pic: Ice cubes (left) and leftover uranine solution from when I was preparing those ice cubes Bottom left pic: Melting the ice cubes in a water bath to see if they start glowing again when melting. Bottom right pic: Yes, they do! :-)

Top pic: Ice cubes (left) and leftover uranine solution from when I was preparing those ice cubes Bottom left pic: Melting the ice cubes in a water bath to see if they start glowing again when melting. Bottom right pic: Yes, they do! :-)

Why do I find this exciting? Because that means that a phase change of the water switches fluorescence as a tracer of that water on and off. Or the other way round: Seeing water fluoresce (or not) tells you what state the water is in without having to figure out anything else about that water.

…and funny how that the apprenticeship as chemical technician that I started in parallel to finishing high school (and didn’t finish because other stuff — oceanography — became more interesting) does come into play in the most unexpected moments :-)

Did you know uranine doesn’t fluoresce when frozen?

Something else I found out when checking on my ice cubes yesterday:

I had frozen a second tray* with ice cubes dyed with uranine (you know, the green stuff I found in the lake near my house the other day?). Like the other ones, they clearly froze back-of-the-freezer-forward, and the most interesting thing to me: Only the parts that weren’t frozen were fluorescing in the fridge!

For comparison, here a picture of the leftover water (so same dye concentration as the ice cubes) in the beaker, and the tray with frozen ice cubes next to it. You can see the beam of the UV lamp on the table* so you believe me that the ice cubes were lit with UV light, too, they just don’t fluoresce!

*Ailin and Steffi, did you see I got you a gold colored ice cube tray? :D Although, had I thought that I would be looking at the ice cubes themselves this much, I would probably have gotten two whites instead of a white and a gold one… But now you at least have a pretty ice cube tray in the lab :-)

**and a stain that I am only now noticing, but it wasn’t me; I was working super carefully since today I am not dressed for the lab and I don’t want that stuff on my clothes!

Update on freezing ice cubes and the temperature distribution in our freezer

After writing the blog post on sea ice formation, brine release and what ice cubes can tell you about your freezer earlier today, I prepared some more ice cubes (because you can never have too many ice cubes for kitchen oceanography!), and then happened to look into the freezer a couple of hours later. And this is what I found:

Isn’t that beautiful?

Top pic shows the ice cubes “in situ”, clearly showing the cold back wall of the freezer where they were sitting.

Bottom left pic shows a top view of those ice cubes and it is very obvious that they have been starting to freeze from the back wall of the freezer forward: The upper row of ice cubes in the pic has formed clear ice in the direction towards that wall and has pushed the dye forward, whereas the bottom row in the pic is still not completely frozen and ice cubes seem to be freezing from all sides towards the middle and not as distinctly from back to front.

Bottom right pic: The rest of the water I prepared for the ice cubes that I left sitting on the counter for future use — still looks well mixed, no sinking of the dye to be observed!

And with these exciting updates I’ll leave you for now, so start playing with your own ice cubes! :-)

Sea ice formation, brine release, or: What ice cubes can tell you about your freezer

Many of my kitchen oceanography experiments use dyed ice cubes, usually because it makes it easier to track the melt water (for example when looking at how quickly ice cubes melt in freshwater vs salt water, or for forcing overturning circulations).

But the dyed ice cubes tell interesting stories all by themselves, too!

Salt water doesn’t freeze

“Salt water doesn’t freeze”? Then how do we get sea ice in the Arctic, for example?

When freshwater freezes, the water molecules arrange in a hexagonal crystal structure. If there is salt (or anything else) in the water, however, the ions don’t fit into the regular structure. Ice freezes from the water molecules, and all the disturbances like salt get pushed in the last remaining bits of liquid water, which therefore gets higher and higher concentrations of whatever was dissolved in it. As those little pockets with high concentrations of salt get cooled further, more and more water molecules will freeze to the surrounding freshwater ice, leading to even higher concentrations of salt in the remaining liquid water. So the freshwater is freezing, while rejecting the salt.

Of course if you cool for long enough, also the last bit of remaining water will freeze eventually, but that takes surprisingly long (as you can try by freezing salt water in some of the cups ice cube trays and freshwater in others, for comparison. Also the structures of freshwater vs saltwater ice look very different and are interesting to look at, see how here).

“Brine release”

When the ocean freezes, this rejection of high-salinity water leads to interesting phenomena: Even when you melt it again to include all the pockets of high salinity water, sea ice will have salinities way lower than the water it froze from. This is because of a process called brine release. Since you are cooling the ocean from above, sea ice also forms from the surface downwards. This means that it is easy for the salty water to be pushed, “released”, or “rejected”, downwards, into the liquid ocean below. That ocean will then of course get more salty right below the ice!

In the picture below you see something similar happening in the left pictures. Instead of salt, I have used blue food dye for visualization purposes. In the top left, you see an ice cube exactly as it looked when I took it out of the ice cube tray it froze in, and in the bottom left you see the same one after I let it melt a little bit so the surface got smoother and it got easier to look inside (a lot more difficult to hold on to, though!).

Do you see how the top part of the ice cube is pretty much clear, while the bottom part is blue? That’s because it froze top-to-bottom and the dye got pushed down during the initial freezing process!

Stuck in an ice cube tray

Something else that you see in the top left picture is the effect of the ice cube being stuck in the ice cube tray as it froze: Pores filled with blue dye that had nowhere to escape!

Had I taken out those ice cubes earlier, when they had just frozen half way through, we would have found a clear ice layer floating on a cold, blue ocean. Maybe I should do that next time!

Checking on the temperature distribution of your freezer

Something else fun we can observe from the right pictures: Here, the dye was concentrated towards the center of the ice cube rather than the bottom! How did that happen?

My theory is that those ice cubes were located in an area of the freezer that was cooling from all sides (more or less) equally, whereas the ones shown on the left must have been placed somewhere where cooling happened mainly from the top.

So if you ever want to know where the cooling in your freezer happens, just put lots of dyed little water containers everywhere and check from which side the dye gets rejected — that’s the cooling side! Actually, I might check that for the freezer below just for fun. Would you be interested in seeing that done?

Now it’s your turn!

Let’s look back at the ice cubes I froze yesterday in the picture above. I’ve now written about a lot of things I see when I look at them. What else do you see? Do you think it’s interesting to use with kids, for example? I’ve used those experiments with first year university students, too, I think there is plenty to observe and explain here!

Please discuss: Experimental setup for Nansen’s “dead water”

During my last visit to Bergen in August, we set up a nice “dead water” experiment. However, there are nice experiments, and then there are awesome experiments, and since Elin wants to use this experiment in her teaching of the ocean and atmosphere dynamics class, we are going for the latter!

So I’ve done some reading and this is what I have come up with (and I am posting this before we’ve actually run the experiment as basis for discussion with Elin and anyone else who might be interested in discussing this. If you have any comments to share, please do! This is by no means final and I am really happy about any kind of input I am getting!)

Why we want to do an experiment

The ocean & atmosphere dynamics course is really theoretical. It would be nice to add something practical! At least for me it really helps to raise motivation to buckle down and think about the theory if I have observed something and I learn theory in order to understand or manipulate what I observed rather than just for the sake of learning theory.

What I want students to get out of the activity

Yay, learning outcomes! I know, people hate it when I start talking about those, but I really think they are the best starting point. So here we go:

  • Read (authentic) scientific literature, extract relevant information, apply it to an experiment and modify parameters accordingly
  • Get an intuitive understanding of the behaviour of internal waves
  • Explain qualitatively (and quantitatively?) how the speed of the boat and the phase velocity of internal waves relate to the drag on the boat

Why this experiment

  • Internal wave experiments get complex very very quickly. This is a two-layer system that should be comparatively easy to both control (Ha! I wish…) and interpret (Ha!! Yes. I know…).
  • This is a very nice historical example, too, going back to Nansen’s Fram expeditions. Nansen is a national hero in Norway, the Bjerknes Centre for Climate research which I am currently visiting is named after Bjerknes, who was involved in figuring this out. So lots of local references!

Setup of the experiment

Stratification

John Grue’s (2018) article “Calculating Fram’s Dead Water” uses the historical observations described by Nansen in “Farthest North” (1897) to quantify the conditions that led to Nansen’s observations: Nansen found a reduction of speed down to 1/5th of the expected speed, and Grue relates this to a density stratification, specifically a pycnocline depth. I’m using the Grue (2018) article as basis for our stratification in the tank, which we set up to best resemble the one the Fram experienced.

Layer depths

Grue describes a strong wave wake and force for a ratio of the ship’s draught (b0) to upper layer depth (h0) close to 1. For our model “Fram”, b0 is 5cm, which leads to an h0 of 5cm, too.

Grue used a ratio of h0/h1 of 1/18, which would lead to h1 of 90 cm. This is unfortunately not possible since our tank is only 50cm deep (of which the upper two cm cannot be used because of braces needed to stabilise the tank, and the water level needs to be another 3 or so cm lower because the ship will need to be able to pass below the braces. Hence our max h1 is approximately 40cm, leading to a ratio of h0/h1 of 1/8. No idea if this makes a difference? Something for students to discuss…

We could obviously also use a smaller model ship with half the drought and we’d be fine. Maybe we should do that just to figure out if it makes a difference.

Density stratification

To set up the density, we can manipulate both temperature and salinity of the water we are using.

For practical reasons, the temperature the water in our tank should be room temperature (so the tank can sit all set up, waiting for class, without equilibration with the room messing things up). Temperature in the teaching lab was T0=20.5°C when I checked this morning.

To minimize the amount of salt we need to use, we’ll use the freshest possible setup, with the upper layer having a salinity of S0=0g/l.

Grue describes a density difference between the layers of ρ0/ρ1 = 1/1.028. Using the density ρ0=0.998 g/l (calculated from T0 and S0 as above), this ratio leads to a density of ρ1=1.026g/l. For T1=T0=20.5°C, S0 thus needs to be 36g/l. (Phew! And seeing that I typically use 0 for “fresh” and 35 for “salty” anyway, this was a lot of thinking to come to pretty much the same result ;-))

How to move the boat

After just pulling it by hand in previous experiments (which was surprisingly difficult, because you need to pull veeery slowly, without jerking on the string), we’ve been thinking about different ways to move the boat.

First we thought we should program an Arduino to really slowly pull the ship through the tank, and use a dynamometer (you know, one of those spiral feathers that shows you how much force is applied by how far it stretches. Or the easy version, a rubber band) to figure out the drag of the ship.

But as I looked a little more into the experiment, and I found a really neat website by Mercier, Vasseur and Dauxois (2009) describing the experiment and the weight drop setup they used. They make the point that the dead water phenomenon is actually not about a constant speed evolution, it’s about applying a constant force and seeing how the boat reacts to that. Which I find convincing. That way we see the boat being slowed down and accelerating again, depending on its interactions with the internal waves it is creating which is a lot more interesting than seeing a feather or rubber band stretch and contract.

Mercier et al. have the boat strapped to a belt with constant tension on it, which they then force via a pulley system with a drop weight of a few milligrams (I think our friction might be higher then theirs was, so we might need a little more weight!).

Only problem here (and I am not quite sure how big a problem this really is): We can only pull the boat for a distance as long as the ceiling in the basement is high, and that’s definitely nowhere near the length of our 6m tank. That seems a waste, but maybe a shorter distance is still enough to see all we want to see (and at least we won’t have reflections from the ends of the tank interfering if we pick the stretch in the middle of the tank)? Or is there an easy way to use pulleys or something to have the weight seem to fall deeper? Any ideas, anyone?

10.10.2018 — Edited to include this idea I got on Twitter. This is so obvious yet I didn’t think of it. Thanks a lot, Ed, I will definitely try that! Also, is anyone still doubting the usefulness of social media?

11.10.2018 — Edited: Wow, as a sailor it’s really embarrassing that people have to point me to all kinds of different pulley systems to get this problem done! Only two issues I have now: 1) What I’ve been ignoring so far but can’t ignore any longer: The weight of the rope will increase with the length of the rope, hence the force won’t be constant but increasing, too. Since we are expecting to be working with weights of the order of a couple of paper clips, even a thin yarn might contribute substantially to the total force. Will definitely have to weigh the yarn to figure out how large that effect is! 2) Since we are expecting such tiny weights to be enough, all the blocks needed in a pulley system are already way too heavy, so we’ll have to figure out some light weight fix for that!

Mercier et al. also used a magnet at the back of the ship and one outside the tank to release the boat, which is a neat idea. But, as they point out, one could also just release the ship by hand, which is what I think we’ll opt for.

What we could ask students to do

Figure out the experimental setup

We could ask them to do basically what I did above — figure out, based on the Grue (2018) article, how to run a tank experiment that is as similar as possible to the situation Nansen described having experienced on the Fram.

Discuss layer depths

In the setup I described above, our ration of layer depths is 1/8 instead of the 1/18 assumed in the Grue (2018) article. Does that actually make a difference? Why would it? Do we think the differences are large enough to warrant running the experiment with the 1/18 ratio, even though that means changing the stratification and getting a new boat?

Check on how close we are to theory

For the density stratification as described above, the relationship

gives a phase velocity of the internal wave of c0=0.1m/s, meaning that it would take a wave crest 1min to cross our 6m long tank. We’ll see how that holds up when we do the experiment! And we could ask the students to do those calculations and compare them to the observations, too.

Compare dead water, deep water and shallow water cases

In their 2011 article, Mercier, Vasseur and Dauxois show the drag-speed relationships for dead water, deep water and shallow water (in Figure 1). The resistance will obviously be different for our setup since we’ll likely have a lot more friction, but qualitatively the curves should be similar. Might be fun to test! And also fun to interpret.

Even if we concentrate on the dead water case only (so we don’t have to empty and refill the tank), there is a lot to think about: Why is there a maximum in the resistance in the dead water case with both lower and higher speeds having a lower resistance? Probably related to how the ship interacts with the internal waves, but can we observe, for example, which Froude number that happens at, i.e. how fast the ship is moving relative to the phase velocity of the internal wave (which we both calculate and observe beforehand)?

Now it’s your turn!

What do you think? What’s your feedback on this? My plan is to go down to the lab tomorrow to figure out how to pull the boat with a drop weight. If you think that’s a really bad idea, now would be the time to tell me, and tell me what to do instead! :-)

Really, I welcome any feedback anyone might have for me! :-)

Fun notes that didn’t fit anywhere else

11.10.2018 — Edited: My former colleague Robinson pointed me to a research project he is involved in related to dredging the Elbe river (to make it possible for large container ships to reach the port of Hamburg) where they actually also look at how much ships are being slowed down, not by internal waves necessarily, but by the turbulence and turbidity they cause in the muddy river bed! That’s really cool! But the scaling is completely off from our experiment so their setup is unfortunately not transferable (they drag big objects with constant speed through the actual Elbe and measure the force that is needed).

This week I am taking over @IAmScicomm!

You might think that I’d be busy enough with my visiting fellowship at the Bjerknes Centre for Climate Research in Bergen this month, and you would be right.

However, I’ve always* wanted to take over one of the rotating Twitter accounts (You know? Those accounts that tweet about certain topics, but are being run by a different person every week. People follow it for the topic and get to see a new person’s view on things every week. A very cool thing when you want to be exposed to a lot more people than you usually are and that are all interested in that topic!). And when I signed up for it in spring, October still seemed so far away that it seemed to be a good idea to do it then.

So this week, I am curating @IAmScicomm! Which I am super excited about, although it is also kinda scary. 18.6k followers is a little different from my usual couple of hundred… Anyway, this is what I am proposing to talk about:

Come join the discussion! :-)

Last week I already did a similar thing already for @geoscitweeps.”Only” 4.6k followers were slightly less scary, and I had the super cool surface drifters to tweet about. @geoscitweeps is another really interesting account you should be following!

*Yes, ALWAYS. It was actually pretty much on top of my list when I started my private #scicommcall, only at that time there were no vacant weeks available that suited my schedule, so I put it off. But now here we are!

Mystery picture! Can you solve this wave riddle?

Today is a great day for a wave riddle! Below you see a picture I took on my walk home the other day.

Can you tell what caused those waves? (Solution underneath the picture!)

In the picture above, we are looking at the curb and the lid of a drain. There are two ring-shaped waves radiating outward from centres that seems to be sitting pretty much on the edge of the curb stone on both sides of the drain cover, and these are the waves we are trying to explain.

Now there are several possible explanations for ring-shaped waves:

Raindrops falling on the water

As we see from the absence of ring-shaped waves on the water surface (except for the two we are trying to explain), it wasn’t raining at the time this picture was taken, hence raindrops are not the explanation to our observed wave pattern.

Also, there are a lot of concentric rings radiating outwards from each of those two points. This doesn’t work well with a “rain drop” explanation. Raindrops do create more than one ring wave because a raindrop makes the water surface oscillate and sometimes secondary raindrops are thrown up into the air and then fall back into the same spot, creating a wave ring of their own. But still, raindrops typically do not create more than two or three rings. But as you see from the picture above, there are a lot more concentric waves!

Something other than rain dripping on the water

So if raindrops are out, since we can’t expect them all to be falling just in those two centres of the wave rings in order to create so many concentric rings, how about water dripping (or even pebbles falling, for that matter) from some defined place to create that structure?

This is actually a good explanation, except that we would expect to see some evidence of something falling. Yes, we might have just captured the picture right after the last drop or pebble or whatever else of a whole series of things dropped in the water, so we get the waves but don’t see what dropped in. But that’s pretty unlikely, isn’t it?

So on to the next explanation:

Something beneath the water surface poking at the surface from below

This is actually something we see a lot: If there are rocks or other obstacles on a shore and we have long waves washing over the obstacle, it will create wave concentric rings on the surface. This happens because when a wave trough goes over the obstacle, water is displaced in a different way than if there wasn’t an obstacle and the wave could just pass through undisturbed. And then, when a crest comes, the obstacle is in the way again, interrupting the orbital movements in the wave.

This might actually be the case in the picture above — except we don’t see any evidence of long waves on the puddle. So this explanation is out, too.

Water draining from the puddle

So now we’ve come to the last option that I can think of: Water draining from the puddle into the drain. And not only that: Water going around an obstacle and through two small-ish holes while draining underneath the drain cover! Those holes would be the centres of the wave rings. And the waves would be created by the little surges of water leaving whenever the water level was high enough, then a short pause as the reservoir filled up enough to overcome friction and surface tension, and then the next surge.

And after thinking through all this, I bent down to check, and indeed — the last solution is the correct one! Would you have guessed? :-)