Tag Archives: wave watching


Ha, I was so keen on getting to my morning swim that I scared one of the ducks so it didn’t just swim away (like the other one), but took flight for a meter or so, before it plopped back down into the water! Can you see it in the waves?

This one is for you @manelriveracamacho: an interference checkerboard pattern in capillary waves! And at the same time a really interesting case of total reflection: the whole green-yellow-ish part in the foreground is where we look directly at the slope of one side of a wave, and can look into the water because the angle is steep enough. For the whole blue area, including the one side of the capillary waves in the yellow-green-ish area, the angle is smaller, and in fact so small that we can’t look into the water any more, but instead see the reflection of the sky!

More super awesome capillary waves. See how they act like lenses and deform the straight-ish ripples in the sand into zigzags?

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“Evaluating shallow water waves by observing Mach cones on the beach” — guest post by Felipe Veloso on his recent #WaveWatching article!

Super excited to share a guest post today: Felipe is writing about his recent #WaveWatching article on “Evaluating shallow water waves by observing Mach cones on the beach”. I came across this article and was going to write a summary, but how much cooler is it to hear from Felipe himself? Thank you for being here! :)

My name is Dr Felipe Veloso1 and I tremendously appreciate Dr Mirjam Glessmer invitation to write this post and letting me contribute to the terrific #WaveWatching collection!!

One of the spectacular things of #WaveWatching is that the observations are ubiquitous. It doesn’t matter if you live in Germany, USA, Japan or Chile. Oscillations and waves are there, whether you observe swimming pools, lakes, sea, or even a relaxing bathtub ready for you. In all cases, the water is always naturally oscillating in a comfortable dance combining up-and-down and back-and-forth movements. If you enjoy these natural phenomena like I do, invest some of your time and take a look to the wonderful #WaveWatchingWednesday and #KitchenOceanography collections that Mirjam has gathered for us. But there are some occasions that these wave phenomena are obscured to our naked-eye observations and a more careful revision is needed to figure out where these oscillations are hidden. A turbulent river coming down of a hill, or the simple passing of fast water flow in front of our eyes are some examples of “waves hidden at first sight”. Such situation occurred to me in the latest family vacations we had as a break from the lockdowns imposed by the pandemia. In particular, this situation became the reason of an article in Physics Education, and also the reason  of why I am writing these lines.

In an attempt to run away from the contaminated air of Santiago (the Chilean capital city, surrounded by mountains), we drove ~90 minutes to Viña del Mar city, to enjoy one week in the beach side. In this place, with the appropriate weather and personal calmness, families can enjoy the waves crushing the beach, the rising of children as “sand engineers”, and the “continuous fight” between these children and the ocean waves to avoid the destruction of the sand fortresses by the water. It is in this relaxing and family-friendly environment where my story begins.

My kids are playing in the sand and my feet are partially covered by water. After long time, we are able to come out from our houses after several months of mandatory quarantines, pandemic stress, and online teaching activities. In this particular moment, watching waves looks like a perfect panorama for me. Suddenly, the voice of my daughter Pilar wakes me up and asked me two questions: “Dad, what are you looking in the water?… and dad, why does the water creates those conical shapes at the end of the undertow current?” The first answer was easy. I was #WaveWatching. But the second answer was not so simple. What about those conical shapes?

Mach cones observed in the surface of undertow water produced by stationary millimeter grains/seashells in sand. Those feet belong to my daughter Pilar and myself. Image taken from the article.

Before her question, I haven’t thought on that. Rapidly, I realized I was observing a wave phenomena in a different and non-standard way. We were observing shock waves in the shape of Mach cones!! These cones appear when an object moves inside of a fluid with a relative velocity larger than the natural oscillation velocity of the fluid. In these situations, there is a shock occurring in the fluid itself. The tip of the cone (or V-) shape arises from the relative movement of the object, whereas the radial expansion of the wave creates the sides of the cone. This explains the formation of V-shapes in the water when a ship travels in a river, or when ducks swim in the lake. In the case of beach observations, the cones were originated by stationary small seashells or larger grains buried in the sand when the undertow water current returned back to the sea with depth not sufficient to immerse my toes.

Now, I am not really sure if my 8 years-old daughter or my 11 years-old son understood completely my explanations of waves and Mach cones. But, I am sure they understood that observing nature can be a fun and relaxing activity to enjoy in family vacations. As an exercise, I taught them how to compute the wave velocity by measuring these Mach cones. I also show them that we did not need any fancy or expensive equipment to accurately evaluate it. We only require interest and fascination on looking for an explanation of a natural phenomena… a phenomena that they could observe while enjoying the beach, the sand and the family time.

Family picture in Viña del Mar. My beautiful wife Alicia, my kids Diego and Pilar and myself. And of course, our dear dog Chewbacca trying to run away from the camera.

Further details can be found in the paper: Felipe Veloso (2021) “Evaluating shallow water waves by observing Mach cones on the beach” Phys Education 56, 054001.

  1. @fvelosoe in Instagram and Twitter


The ultimate guide to #WaveWatching, gooseling edition

I’ve been fascinated by gooselings recently, mainly because they are super cute. But I can tell you about what makes wave watching special when the waves are made by geese — it is an extra challenging type of wave watching! (Check out this post for a more general — and likely much more useful — guide to wave watching)

I dug through my phone (all these pictures are from this year, but they aren’t posted chronologically as you’ll see from the size of the gooselings) and found quite a collection. This is for you, Yasmin and Maike!

Let’s start out with a couple of pics that work well for wave watching purposes, but that already show hints of the challenges discussed below.

Here all three gooselings and the goose that is still closer to the shore are swimming fast and are making beautiful wakes; those feathery, V-shaped waves that are caused by them swimming faster than the speed at which waves can spread. The goose swimming in the front is swimming slower than that critical speed: See how there is a wave the goose is pushing in front of itself, but how it doesn’t break into these feathery structures? And the background wave field, those large curved waves, were likely caused when one of the large geese jumped into the water, or at least something close to the lower left corner of the picture falling at about the time it would have taken the front goose to swim  to where it is now at a slow (but typical) speed.

And here, the geese are doing what I’m always hoping I’ll snap a good picture of: Swimming exactly in a row. This should be energetically really good, because the goose swimming in front already creates the wake and lowers the resistance for all the other geese swimming in its wake. You see that both families are doing it in this pic, but then the duck still takes the price for the prettiest wake.

Here is another example of the front goose doing the hard work and the gooselings following in its wake. For some reason, geese never have such nice distinct wakes as ducks! Maybe it’s their bow shape in addition to their erratic swimming behaviour?

This picture gives you a glimpse of the problems we are about to discuss below. See the resting gooselings on the tree trunk in the right? Cute, but not helpful for wave watching. And the ones that are swiming, although making comparatively pretty waves, are changing speed and direction so much that the wave field looks quite messy. You can see they all started out fom the left side of the partly submerged tree trunk (the one the other family is sitting on on the right)

Here again: They CAN make petty waves if they choose to, but they are often moving quite erratically. Speeding up, slowing down, changing direction… But the right family is making pretty waves!

So why is it so difficult to get good pictures of geese-made waves? Continue reading

#WaveWatching as “transformative experience”? (Based on articles by Pugh et al. 2019, 2011, 2010)

I was reading an article on “active learning” by Lombardi et al. (2021), when the sentence “In undergraduate geoscience, Pugh et al. (2019) found that students who made observations of the world and recognized how they might be explained by concepts from their classes were more likely to stay in their major than those who do not report this experience” jumped at me. Something about observing the world and connecting it to ideas from class was so intriguing, that I had to go down that rabbit hole and see where this statement was coming from, and if it might help me as a theoretical framework for thinking about #WaveWatching (which I’ve been thinking about a lot since the recent teaching conversation).

Going into that Pugh et al. (2019) article, I learned about a concept called “transformative experience”, which I followed back to Pugh (2011): A transformative experience happens when students see the world with new eyes, because they start connecting concepts from class with their real everyday lives. There is quote at the beginning of that article which reminds me very much of what people say about wave watching (except that in the quote the person talks about clouds): that once they’ve started seeing pattern because they understood that what they look at isn’t chaotic but can be explained, they cannot go back to just looking at the beauty of it without questioning why it came to be that way. They now feel the urge to make sense of the pattern they see, everytime they come across anything related to the topic.

This is described as the three characteristics of transformative experiences:

  • they are done voluntarily out of intrinsic motivation (meaning that the application of class concepts is not required by the teacher or some other authority),
  • they expand peception (when the world is now seen through the subject’s lens and looks different than before), and
  • they have experiential value (meaning the person experiencing them perceives them as adding value to their lives).

And it turns out that facilitating such transformative experiences might well be what distinguishes schools with higher student retention from those with lower student retention in Pugh et al.’s 2019 study!

But how can we, as teachers, facilitate transformative experiences? Going another article further down the rabbit hole to Pugh et al. (2010), this is how!

The “Teaching for Transformative Experiences” model consists of three methods acting together:

  • framing content in a way that the “experiential value” becomes clear, meaning making an effort to explain the value that perceiving the world in such a way adds to our lives. This can be done by expressing the feelings it evokes or usefulness that it adds. For #WaveWatching, I talk about how much I enjoy the process, but also how making sense of an aspect of the world that first seemed chaotic is both satisfying and calming to me. But framing in terms of the value of the experience can also be done by metaphors, for example about the tales that rocks, trees, or coastlines could tell. Similarly, when I speak about “kitchen oceanography”, I hope that it raises curiosity about how we can learn about the ocean in a kitchen.
  • scaffolding how students look at the world by helping them change lenses step by step, i.e. “re-seeing”, for example by pointing out specific features, observing them together, talking through observations or providing opportunities to share and discuss observations (so pretty much my #WaveWatching process!).
  • modeling transformative experiences, i.e. sharing what and how we perceive our own transformative experiences, in order to show students that it’s both acceptable and desirable to see the world in a certain way, and communicate about it. I do this both in person as well as whenever I post about #WaveWatching online.

So it seems that I have been creating transformative experiences with #WaveWatching all this time without knowing it! Or at least that this framework works really well to describe the main features of #WaveWatching.

Obviously I have only just scratched the literature on transforming experiences, but I have a whole bunch of articles open on my desktop already, about case studies of facilitating transformative experiences in teaching. And I cannot wait to dig in and find out what I can learn from that research and apply it to improve #WaveWatching! :)

Lombardi, D., Shipley, T. F., & Astronomy Team, Biology Team, Chemistry Team, Engineering Team, Geography Team, Geoscience Team, and Physics Team. (2021). The curious construct of active learning. Psychological Science in the Public Interest, 22(1), 8-43.

Pugh, K. J., Phillips, M. M., Sexton, J. M., Bergstrom, C. M., & Riggs, E. M. (2019). A quantitative investigation of geoscience departmental factors associated with the recruitment and retention of female students. Journal of Geoscience Education, 67(3), 266-284.

Pugh, K. J. (2011). Transformative experience: An integrative construct in the spirit of Deweyan pragmatism. Educational Psychologist, 46(2), 107-121.

Pugh, K. J., Linnenbrink-Garcia, L., Koskey, K. L., Stewart, V. C., & Manzey, C. (2010). Teaching for transformative experiences and conceptual change: A case study and evaluation of a high school biology teacher’s experience. Cognition and Instruction, 28(3), 273-316.

An iEarth teaching conversation with Kjersti Daae and Torgny Roxå on #WaveWatching

iEarth is currently establishing the new-to-me format of “teaching conversations”, where two or more people meet to discuss specific aspects of one person’s teaching in a “critical friend” setting. Obviously I volunteered to be grilled, and despite me trying to suggest other topics, too (like the active lunch break and the “nerd topic” intro in a workshop), we ended up talking about … #WaveWatching. Not that I’m complaining ;-)

After the conversation, I wrote up the main points as a one-pager, which I am sharing below. Thank you, Kjersti and Torgny, for an inspiring conversation!

I use #WaveWatching in introductory courses in oceanography and in science outreach both on social media and in in-person guided tours. #WaveWatching is the practice of looking at water and trying to make sense of why its surface came to look the way it does: What caused the waves (e.g. wind, ships, animals)? How did the coastline influence the waves (e.g. shelter it from wind in some places, or block entrance into a basin from certain directions, or cause reflection)? What processes must be involved that we cannot directly observe (e.g. interactions with a very shallow area or a current)? Kjersti Daae (pers. comm.) suggests an analogy to explain #WaveWatching: Many people enjoy a stir-fry for its taste, like we enjoy looking at water, glittering in the sun, without questioning what makes it special. But once we start focusing on noticing different ingredients and the ways they are prepared, it is a small change in perspective that changes our perception substantially, and leads to a new appreciation and deeper understanding of all future stir-fries (and possibly other dishes) we will encounter.

I teach #WaveWatching using a cognitive apprenticeship leaning (Collins et al., 1988) approach*: By drawing on photos of selected wave fields (in the field using a drawing app on a tablet), I model my own sensemaking (Odden & Russ, 2019). I coach students to engage in the process, and slowly fade myself out. Students then engage in #WaveWatching practice anywhere they find water – in the sink, a puddle in the street, a lake, the ocean. Since waves are universally accessible, this works perfectly as hyper-local “excursions” in virtual teaching: Students work “in the field” right outside their homes.

Waves are not an integral part of the general curriculum in physical oceanography. While some wave processes are relevant for specific research questions, for typical large-scale oceanography they are not. And the concepts used in #WaveWatching are not even new to students, they are just an application of high-school optics to a new context.

Nevertheless, #WaveWatching helps work towards several goals that are important to me:

  1. Using “authentic data” acts as motivation to engage with theory because the connection with the real world makes it feel more interesting and engaging (Kjelvik & Schultheis, 2019).
  2. Engaging in sensemaking and gaining experience on what can (and cannot!) be concluded from an observation are highly relevant skills and this is an opportunity for practice.
  3. Building an identity as oceanographer – seeing the world through a new lens, joining a community of practice (Wenger, 2011), but also being able to demonstrate newfound expertise and identity to friends and family outside of that new community by talking about this new lens – are otherwise rare in socially distant times.

After being exposed to #WaveWatching, people tell me that they can’t look at water in the same way they did before. They are now seeing pattern they never noticed, and they try to explain them or ask themselves what I would see. They often send me photos of their observation years after our last interaction, and ask if I agree with their interpretations. #WaveWatching might thus be a threshold concept, “a portal, opening up a new and previously inaccessible way of thinking about something” and where “the change of perspective […] is unlikely to be forgotten” (Meyer & Land, 2003).


  • Collins, A., Brown, J. S., & Newman, S. E. (1988). Cognitive apprenticeship: Teaching the craft of reading, writing and mathematics. Thinking: The Journal of Philosophy for Children8(1), 2-10.
  • Kjelvik, M. K., & Schultheis, E. H. (2019). Getting messy with authentic data: Exploring the potential of using data from scientific research to support student data literacy. CBE—Life Sciences Education18(2), es2.
  • Meyer, J. H. F., and Land, R. (2003) “Threshold Concepts and Troublesome Knowledge: Linkages to Ways of Thinking and Practising” in Improving Student Learning: Ten Years On. C. Rust (Ed), OCSLD, Oxford.
  • Odden, T. O. B., & Russ, R. S. (2019). Defining sensemaking: Bringing clarity to a fragmented theoretical construct. Science Education103(1), 187-205.
  • Wenger, E. (2011). Communities of practice: A brief introduction.

*more on that in this post (that comes online on 21.5.2021).

Fun #WaveWatching today! Reflections all the way….and then the boundary suddenly ends!

You might remember this edge here and the reflection situation.

More details in this recent post, but in a nutshell: The wave crests marked in red are approaching the beach and wooden edge, and where they hit the wooden edge, they get reflected and converted into the green wave crests which propagate away from the edge again.

And this is what the other side of the edge looks like: The reflections end where the edge stops!

Again, the red wave crests are the incoming waves, and the green the reflections. Waves always travel perpendicularly to their crests, so you see how they propagate away from the boundary and appear to be cut on the right side where the boundary suddenly stopped and no reflection could happen any more.


When you meet a hard boundary, the reflection starts

When I look at the picture above, I see basically three different zones on the surface of the lake.

The yellow zone, which is under the direct influence of the wind, where the water is full of small waves, and then two other zones.

In the red zone, the water isn’t under direct influence of the wind any more, we see clear, parallel wave crests propagating towards the shore. I’ve marked some of them below.

While they are still to the left of the wooden edge, not much happens. But once they hit the edge, we enter the “green zone”: The incoming wave crests get reflected at the wooden edge. They start propagating out onto the lake, getting longe and longer over time, while the red wave crests continue running further and further into the green zone, so we get interference between the incoming and reflected wave crests. Pretty cool! :)

DIY #WaveWatching “fortune teller” on #WaveWatchingWednesday

(Download in English || Download in Spanish; thanks to Felipe Veloso for translating!)

How about a little wave watching game to celebrate #WaveWatchingWednesday?

The minute I saw Andrea Lopez Lang’s tweet, where she made a “fortune teller” (no idea that’s what they were called) as going-away and please-remember-what-you-learned gift for her class, I HAD to make something like that!

Unfortunately I’m not teaching a class right now where I could easily see how this could be done, but luckily there is always wave watching!

Grafic with instructions to build a wave-watching-themed "fortune teller"

Click to get the pdf!

And Kjersti had a great idea for how this could be used right away: To send students out with these toys and ask them to discover one example for each of the waves shown on the toy. Plus then of course document it, and share on social media… ;-)

Waves are traditionally taught in a theoretical and very dry manner, and the transfer to the real world is hardly happening at all (especially since the large tank in the basement at GFI has been demolished, which still breaks my heart), so this is a fun way to get students outside and try & find contents from their lecture in real life.

P.S.: It’s not as difficult as it might seem at first once you start observing and get a little creative. Nobody said that the rock that makes the ring waves had to have been there when you got there, and wakes can be created by ships or bird or even if you pull a stick through the water…


Even though I haven’t done a #WaveWatchingWednesday in a looong time, there has of course been a lot of wave watching going on. But the longer I wait with copying all the Instagram posts into a blog post, the more work it gets, the longer I put it off. Vicious circle! But here we go today. Plenty of interesting and plenty of beautiful pics! Enjoy!

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7 years of blogging! Celebratory wave watching with A LOT OF #friendlywaves

It doesn’t feel like it, but today marks the 7-year-anniversary of my first blog post on my “Adventures in Oceanography and Teaching”! To celebrate, I sent out this call to action (and please feel free to respond, no matter when you are reading this):

Below, I am sharing the pictures that people sent me plus my thoughts on them, newest on top. Pictures that reached me after August 28th 2020 will be posted in follow-up posts! (Keep them coming, I love it!)

23:58 — Kristina

Oooh, I love the internal waves on the interface between the milky part of the coffee and the not-yet-milky part! Looking at them moving very slowly always feels like being caught in slow motion, when of course the phase speed has to be very slow because of reduced gravity

21:55 — Phil (San Francisco)

Ooooh, this is awesome! Phil writes “photo of an awesome phenomenon that I took just before landing at SFO. San Mateo Bridge, San Francisco Bay on an evidently quickly rising tide!” and I am love the vortex streets! Count 4 pylons up from the bottom — that’s such a nice and clear von Kármán vortex street, absolutely beautiful! And then the bottom one does seem to show som shear instability. Now. In some spots vortex streets develop, while in others they don’t. But why?

21:06 — Clark (Bay of Fundy)

Clark wrote an entire thread explaining this awesome observation in the Bay of Fundy. You should totally check out the whole thread & explanations on Twitter, but I had to share this video so you can see what an exciting situation it is!

20:51 — Elin (Bergen)

Oh, nice one! Lots of tiny air bubbles in a vase or jug filled with water. Now. Is the line crossing through the water level? It kinda looks like it at first glance, because it seems to show a meniscus. On the other hand, there are bubbles sitting above that line, and they don’t look qualitatively different from the ones “under water”, so that seems unlikely. But then light seems to be refracted substantially differently above and below that line, so maybe we are looking at the water surface at some weird angle? Intuitively I would say that we are looking in from underneath the water level, but see the water line on the opposite side of the vessel. But that doesn’t seem to make sense with the (most likely level) table top? I will have to think about this some more!

18:14 — Simone (Hamburg)

How beautiful — in this drop we see the tip of the leaf it is hanging on pointing uo from the lower end of the drop! Nice example of how drops act as lenses and show us the world upside-down!

15:43 — Dong

Oh! Nice long waves coming in from far offshore. From the shape of the wave profiles I would guess that this was taken close to shore in shallow-ish water. This seems to be confirmed by the waves changing direction from further away coming towards us, which would indicate that the depth is changing over that distance

Unfortunately I could only screenshot that gif, check it out in the original tweet! What I find fascinating here is how there are two waves breaking offshore running towards us, but then there are several wave crests even closer to us that are still distinct and pointy (so the waves clearly didn’t loose all their energy breaking), but not breaking. I think there must be a shallow area where those two breaking waves are, making them break, and then deeper water again so that they are only temporarily tripping up and breaking, but then propagating further towards the shore without too much of an interruption

15:18 — Nena (Bodensee)

Oh, waves and a piece of driftwood! I wonder if it’s floating, it looks like it’s grounded. Otherwise I’d expect it to move with the waves, creating a dipole pattern when the ends take turns coming further out and then sinking deeper into the water.

And now a larger wave, breaking as it reaches the shore and also washing over the driftwood. I love how the water that came washing over the driftwood “in one piece” then disintegrates into drops when it falls down the other side!

15:13 — Jeffrey (Boulder)

Wow, this video is super tricky! Please check it out — volume up!

At first, I thought that the periodicity was set by eddies shedding periodically after water had washed over the obstacle. But after about the 50th time I looked at the video, the obstacle (is it driftwood?) seemed to start moving. If it is actually moving, the periodicity makes sense: The wood is trapped in place (you see that on the far side of the river) but it can move a little. It’s bopping on the water, floating at whatever height the waterlevel is at, but at the same time acting as a dam and trapping water on its upstream side, thus influencing the waterlevel. So this is basically a recharge-discharge oscillator. Maybe. Or maybe not. Any ideas, anyone? This is really tricky!

12:38pm — Gabriela (Lüneburg)

I would be so jealous of those kids playing with the locks and weirs and water channels if they weren’t my adorable nieces. Now I just can’t wait to go visit & play with this really cool toy!

11:49am — Gabriela (Lüneburg)

Somehow raindrop photography seems to be the topic of the day today? Here we see really well how a drop shows us an upside-down image of the world. See the sky at the bottom and the shed at the top, with the wine branches in the middle?

11:39am — Gabriela (Lüneburg)

Oh, a rain barrel! I think someone touched the side (possibly kicked it) so we get waves radiating from the outside in. Could also be that someone moved the ropes, but I would expect shorter wavelength waves then

This would be sooo difficult to interpret if I didn’t have inside knowlege: There are rocks hanging at the end of those strings to keep them in the center of the barrel, to guide the rain water in. What happened here is that someone pulled one of the strings to the side and let go of it. We see that the rock that was lifted when the rope was pulled out, is pulling the rope down again. That’s why the rope still has this weird bow shape, and that’s why we get kinda a wake where the rope moved from the outside of the barrel towards the center. All the other smaller ring waves are either drops that fell from the wet rope, or the ones that are centered around the other rope are caused by that rope vibrating because it’s attached to the first rope somewhere up on top

Here, my niece is demonstrating how to make wakes by swinging weighted ropes through water

Same as above, but we nicely see the capillary waves that radiate away

11:20am — Katharina (Hamburg)

I guess I said I liked a challenge… Screenshots with comments below! And check out the sound in the movie! Volume up!

At first I thought this was going to be a movie with pretty rainbows and reflections — water can create awesome prism effects!

But no! She dropped a fizzy tablet in! Here we see the first gas bubbles bubbling up to the surface. The bubbles get bigger the closer they get to the surface, because the pressure decreases and they can therefore expand

Already a lot of bubbling and fizzing going on! On the right side of the glass we can see that the gas bubbles are released in bursts. We also hear that when listening to the sound in the movie. Also funny to see how the bubbles raise in the middle of the glass and the flow is really turbulent there. Only at the rim can bubbles persist without bursting for a little while

The tablet has almost completely dissolved now, and what little is left has floated up to the surface (upper right). Btw, that fizzing sound is not really made by the water itself, it’s all the tiny bubbles bursting. Since sound are pressure waves and bursting bubbles radiate pressure, as the gas inside of the bubble is at higher pressure than the surrounding air and expands until the bubble bursts and then the pressure equilibrates. That’s what we hear!

10:46am — Astrid (Hamburg)

Astrid has a great taste! Both in her choice of reading materials (my blog!) and in her taste of coffee, with the beautiful swirls of milk in it. Here we see that molecular diffusion is slow when it has to physically swap whole molecules around — the swirls of milk are still distinguishable in the coffee (she clearly didn’t stir). But I would be prepared to bet that the milk has the same temperature as the coffee, because molecular diffusion of heat is fast (and also because she likes her coffee hot, the milk most likely went in hot)!

10:38am — Sara (Klein Waabs)

Phew, this is a difficult one! The first structures that jump at me (besides the wind surfers, of course) are the shadow of the sail (from the board to the left) and the reflection of the sail (from the board towards us), which are kind of distracting from the waves. And the waves are kind of messy, there is no clear direction visible. There is some wind causing the waves. Not very much, but enough to make it difficult to distinguish wind waves from others caused e.g. by the surf board.

Now we have some wind! We see waves breaking at the stones offshore (and looking at how high the foam is flying, those waves were not too shabby!), but we don’t actually see a lot of those waves because everything from the wave breakers towards us is in the lee and thus sheltered. But we see some long waves that have propagated into this bay that break on the beach

10:37am — Florian (all over the world!)

Cabo da Roca in Portugal; westernmost point of the continent. This is beautiful! Waves from far offshore arrive at the beach. As they reach the shallower water, all of their properties except their frequency change: Their wavelength gets shorter, their height larger, their steepness larger. When one part of a crest is in shallower water than the rest, the crest bends towards the shallower areas. The breaking waves create a lot of foam, indicating that there was some biological activity creating a surface film

Falesia Beach, Algarve. Oh how beautiful! Breaking waves on the other side of those rocks, and only the longer waves make it into the calm, sheltered space this side of the rocks. Interesting example of a filter that only lets long wavelength come through!

Algarve-Coast. What strikes me here are the beautiful colors! Near the beach, we can look into the water and see the sandy sea floor as well as some larger rocks, that seem to be partly overgrown. The further we look offshore, the bluer the sea gets, because at shallower angles the reflection from the sky increases, until at a critical angle, we only see reflections and can’t look into the water any more.The different shades of blue in those areas show where breezes rough up the water, darker areas are those where there is currently more wind and the surface roughness is higher. Another thing I am noticing are the waves that the person in the water makes — circles around them.

Timmendorfer Strand, Baltic Sea coast. On a windy day! We can see that from the whay the waves are breaking, not only right at the beach and in areas where the water is shallower, but also in smaller, more random bursts. Also waves don’t have the same regular shape as they would have had this wave field traveled to the beach from far away, it seems to be somewhat regional. Also love how far the waves run up the beach (as you see from the large area covered in foam). This must have been a fun day to play at the beach!

Maschsee Hanover — I love how you see different things in this picture: First, how stuff floating in the water dampens out the waves (see how there are a lot more waves behind the plants floating in the water than on this side). And then the wakes that this cute family of swans is making as they are swimming towards us!

9:09am — Gabriela (Lüneburg)

Honestly, what jumps at me most is my ADORABLE niece who’s saying Kaffefoto (“coffee pic”). But then there is also the puzzle of why the coffe coming out of the machine looks so much lighter than when it’s in the mug (underneath the foam)? Well, the foam is the clue here! When there are a lot of airbubbles in the coffee still, they reflect light differently (i.e. from all different directions, making it look white, rather than directional, showing either the color of the coffee or a reflection) than when the coffee has settled down and the air bubbles have gone away.

9:01am — Kristin (hiking somewhere near Bergen)

Did you ever wonder why waterfalls (or really turbulent rivers) always look white, while a lake or the ocean can look all kinds of different colors depending on what is dissolved in the water and what they are reflecting on the surface? The answer is that in waterfalls (or the really turbulent, “white water” rivers), there are tons of tiny air bubbles trapped in the water. All the surfaces of those air bubbles and also the turbulent flow itself make the water surface very irregular. Since the surface is so irregular, it reflects light from all different directions, rather than just one. As we receive all that different-colored light, our eye interprets it as white. Fun exception: Sometimes we see rainbows in water falls. Then the sun as light source is so dominant that we see the sunlight reflected in the falling water drops and a rainbow appears!

9:01am — Siddharth (Sadashivnagar)

Oh I love this! I think that what Sid doesn’t show us on the very right is a narrow connection to a second body of water, on which waves are generated by wind. (Alternatively, there might be something there at the very right just outside the frame that is making waves, such as a bird or a fountain, but I don’t think that’s the case. Birds usually don’t move this regularly for long enough to generate this wave field even before you started filming and then throughout the whole movie. Fountains usually generate concentric waves (unless there are several fountains, in which case this would be a trick question ;-))) So let’s assume that wind-generated waves from a second body of water pass through a narrow inlet onto this pond. As they pass the narrowest part, they start spreading to all directions, forming concentric waves that grow over time. Well, almost concentric, because the narrowest part isn’t a perfect point source. Therefore we don’t see diffraction at a slit, but rather at a wider opening.

8:58am — Torge (Kiel)

Not a picture, but even better: He managed to fix the problem we had been having with the co-rotating video of our rotating tank. Super excited! If I wasn’t so busy today (slightly underestimated how many pics my dear friends would send me!) I would go try it out right away!

8:51am — Sam (Manchester)

Raindrops on the window! I love how, especially with the ones in the upper part of the window, you can see the world upside down: the bright sky at the bottom of the drops, the darker trees at the top. In the drops further down the window you see the sky at the bottom, then the darker bushes and trees, and then the brighter gravel & car! So cool!

8:28am — Ronja (Nordsee)

On this beautiful picture you clearly see wave crests meeting up at an angle, even though further offshore they all seem to be coming in parallel to the beach. Why? Because there is this groyne going out perpendicular to the shore. You see some of the stones at the bottom of the picture, but from the wave field we see that it reaches further offshore and is just submerged there. Where it is submerged, the water depth suddenly decreases. Waves running close to and on top of it therefore change their speed, and with changing speed, the wave crests bend towards the obstacle (as they are being slowed down more on the obstacle side than on the deeper side). Since this happens on both sides of the groyne, we get this cool checkerboard pattern!

Same explanation as above, but another beautiful picture! :-)

8:13am — Elsa (Bergen)

Ha! Knowing where this picture was likely taken, the fist thing I notice is the reflection of the masts of the sailing ship Statsraat Lehmkuhl! And how pretty the light is looking in between the hulls of the catamaran. And how there are waves radiating away from where other waves bumped against the bottom of those tyres

Bergen is so beautiful! I love how we see the reflection of the colorful houses of Bryggen, then the dark mountain of Fløyen behind it, and then a blue sky with pink clouds! And how the structures of waves are clearest where there are strong contrasts in brightness of the reflected light, for example at the pink/blue boundaries!

More beautiful Bergen. The dark mountain’s reflection does not have a sharp edge, because the water surface isn’t flat. So depending on the angle of the surface, some spots still show mountain while others already show the sky

Same as above, PLUS isn’t it cute how the floaty bits in the foreground have their own little wave rings around them???

What jumps at me here is the turbulent (white) water, most likely caused by a speedboat

Here I love how the surface appears smooth even though there are very clearly waves on it. That means that the waves we see haven’t been caused locally by wind, otherwise there would have to be smaller wavelength stuff on them. Instead, they were generated further offshore and traveled here

7:26am — Kati (Schönbrunn, Wien)

First thing I notice here: How pretty this looks with the reflection! And beautiful weather! Hardly any waves, but there are some structures in the lower right that look like there are possibly plants growing in the water, just breaking the surface

7:07am — Marisa (Hamburg)

Looks like it’s raining in Hamburg! What I always find super cool about rain drops is how they act as tiny lenses and show us an upside-down picture of the world

6:26am — Désirée (Möhnestausee)

This is a picture taken at the Möhne reservoir — a river that has been dammed to create this lake. The water level is regulated, which you see in the horizontal stripes on the opposite side of the lake, each representing what the water level was like at some point in the past..

What an awesome pic! But wait — aren’t water drops (or anything, really) supposed to fly in a straight line, unless there is a force acting on them? There is obviously gravity acting, but why the curve? This riddle is solved by looking at the flying droplets, which are individual droplets. And even though they look as if they are following a curved path one behind each other, this is just our eye being tricked into that. Each drop is flying on a parabola (thanks, gravity) away from the spot where it detached from the end of the flying wet hair. But the drops are not following each other, they just happen to have detached from the hair along a path.

6:17am — FrozenBike (Khajoo Bridge in Isfahan)

I’m posting a couple of screenshots from that video to make it easier to discuss…

First, doesn’t this lake look absolutely beautiful? The calm surface is an almost perfect mirror of the sky because we are looking at the water at a shallow angle, thus we only see reflections and can’t look into the water

Then, we look down. See the plants in the water in the top left of the picture? They indicate quite a strong flow from top left to bottom right of the picture. But wait, there is an obstacle in the way! That stone introduces turbulence. There are waves that are created as water washes past, and there are also eddies shed because the current shear between the flowing water and the more stagnant water in the shelter of the stone is so strong. See that one water plant thingy right below the train of eddies? It seems to be moving in the turbulent flow, too!

And we reach the next obstacle! At the top left, water is still flowing slow(ish), thus the surface is flat and we can clearly recognize the tree reflected in the water. But then we reach the threshold, and all of a sudden the flow goes from laminar to turbulent. See how the whole right side of the picture seems to be full of tiny bumps? That’s where the water is influenced by the structure of the bottom underneath

I absolutely LOVE that my blog and twitter have helped me meet and connect with so many wonderful people all over the world! Here we see Khajoo Bridge in Isfahan, Iran. So beautiful! As for wave watching: In the left side of the picture we see what was the right side of the picture above, the very fast, turbulent flow down the slope. And then at the bridge, there is a jump in surface height: As the water is deeper there, the flow slows down to subcritical speed and a hydraulic jump develops

Last screenshot: The hydraulic jump shown in the last picture, only more prominently pictured. Also visible: The influence of that stone edge on the flow. See all the waves that radiate from it as it is restricting the flow? Super awesome!

6:06am — Chirine (Kiel)

What I notice first is the tiny white sliver of light, close to where the spoon breaks the coffee’s surface. That’s showing us that the surface isn’t perfectly straight, but that it is deformed where the metal of the spoon and the coffee meet. The effect — cohesion between molecules — makes the coffee rise up a little at the edgs of the cup and also where the spoon breaks the surface. You might know this from the meniscus that shows up in test tubes and makes it difficult for the untrained eye to estimate how full a test tube really is