My long time Twitter friend Anne shared these beautiful pictures and I absolutely had to do a #friendlywaves post where I explain other people’s wave pictures.
Take a moment to admire the beautiful picture below. Wouldn’t you love to be there? I certainly would!
What can we learn from this picture? First — it’s a windy day! Not stormy, but definitely not calm, either. See how the water outside of the surf zone is dark blue and looks a little choppy? That’s the local wind doing that.
And then there are the waves that we see breaking in the foreground. Without knowing where the picture was taken, I would think that they traveled in from a large water body where there was a long fetch so they could built up over quite some distance. And then they meet the coast!
You see breaking waves of two kinds: the one marked with red ovals below, where there is hardly any buildup of the wave before it meets a rock and breaks into white, foamy turbulence. The other type of breaking waves, the ones where I marked the crests with green lines, build up over a short distance before they break because there is a more gradual decrease in water depth. The stope is still quite steep so the waves change from deep water (where they can’t feel the sea floor and have a fairly low amplitude, so we can’t distinguish wave crests further offshore than the two I marked in green) to shallow water waves that feel the sea floor and build up to break.
In contrast, let’s look at the lovely picture below.
Here, we have a sandy beach on which the waves can run out. There slope right at the water’s edge is not very steep, but seeing that we can only really spot two wave crests there has to be a change in gradient. About where the offshore wave crest is in the picture below, or possibly a little further offshore, the water depth must suddenly increase, otherwise there would be more wave crest visible further offshore. Since there aren’t any, water must be a lot deeper there.
But what I found really cool about the picture above are the trains of standing waves in the little stream that flows into the sea here. I find it so fascinating to see standing waves break in the upstream direction — so completely unintuitive, isn’t it? So much so that I dug out some pics from January for you and posted them last Friday in preparation for today’s post. Sometimes I actually plan my posts, believe it or not!
Standing waves don’t move in space because the flow of the current they are sitting on is exactly as fast as they are moving, only in the opposite direction. What is happening in the picture is that in those standing waves sit on ripples in the sand. The waves become so steep that they are constantly falling back down onto the current, get carried up the ripples again, in an endless loop. So fascinating!
A #friendlywaves post: you send me the pictures, I talk about physics! Today: My friend A sent me these lovely pictures from Lofoten, knowing I love wave watching. And there is so much to see!
Let’s begin with the picture above, where we are looking out over the stern of a ship towards a bridge. There are two different kind of things that jump out to me: The ship’s wake and the tidal current.
The ship’s wake consists of two parts: The turbulent wake we see right in the middle of the picture, behind the A-frame crane (in between the red lines below), and the feathery V-shaped wake (some of the individual “feathers” are marked in green).
And then there is the turbulent backwater behind the bridge’s pylons that’s caused by the tidal current going through underneath the bridge. Pretty cool, isn’t it?
And now on to the next picture, that is one of the most beautiful wave pictures I’ve seen the last couple of weeks: Now we are sailing in the wake of a second ship.
We are following the other ship a bit off to the side, therefore the perspective is a little confusing. Between the red lines, we see the other ship’s turbulent wake. Additionally, it has an interesting V-shaped wake that actually consists of two stacked Vs, a bit like this: <<
One of the Vs is the actual bow wave radiating from where the ship’s bow cuts through the water, the second one detaches further backwards from the ship. Both Vs are marked in dark green below. But to the left of the picture, in light green, I marked some of the individual “feathers”, wavelets that make up the V-shaped wake.
A reader of my blog, Rocío*, sent me this beautiful image from Arnao beach (Castrillón- Asturias-Spain), and I asked if I could use it in a #friendlywaves post. He agreed, so here we go!
First, let’s check out the original image in all its beauty, before I start scribbling on it. What features of the waves stand out to you that you find especially interesting?
For me, what I think is especially awesome here, is how the behaviour of the waves lets you draw conclusions about the sea floor underneath. Look at all the wave crests coming in nice and parallel. Far offshore, it’s difficult to even see wave crests (marked orange, for example), only when they come closer to the shore and the sea gets shallower, they start to build up, get a distinct shape. Yet in some places they become a lot steeper and start breaking a lot further offshore (red marks) than in others — why?
Because in those spots the sea is shallower, thus the interaction with the seafloor is a lot stronger. If you look at the yellow mark, for example: Offshore of it the wave crests are still very shallow and not pointy, and then all of the sudden they break. Here the water is deep until there is a very fast change and then it’s suddenly very shallow (and probably rocky, hence all the turbulence).
And then, if you come closer towards the shore, there is an area that has only a very gradual incline, where the shape of the waves hardly changes any more (blue marks).
And then there is a small inlet to a large puddle that acts as “slit” (albeit a fairly wide one) and lets waves radiate as half circles from where they enter through the slit.
I love how in such a beautiful image of such a beautiful landscape, there is so much physics that we can discover if we only choose to look! :-)
*I asked how I could credit the picture to Rocío, but he doesn’t have Twitter or a website and wrote “I only want you to explain it for people i love your blog and your information you are doing a great job”. Aaaaw, thank you!!! :-) And thanks for sending me this beautiful picture!
Christina writes on Twitter: “#wavewatching from a plane, approaching #Panama. @Meermini, do you know what causes those regular ‘wrinkles’?” and how could I resist writing a blog post about what I think might be the explanation?
Below is the picture Christina shared on Twitter.
Picture by Christina Oettmeier @sulfurium
What I think we see here are basically two wave fields: The regular “wrinkles” and then a lot of small crinkle.
The small crinkle are boring: local, wind-generated waves. They are not what Christina asked about.
But the wrinkles are swell: Waves that were formed in a storm far, far away and that have propagated here over a long distance. While propagating from the area where they were formed to the beach where Christina took these pictures, the waves got sorted by wave length. The longer a wave, the faster it propagates in deep water. So long waves from a distant storm will arrive first, and over hours or days the wave lengths of the waves arriving at the beach will get shorter and shorter. The wave lengths we see here seem to be about the height of the high rise buildings we see on the shore. The highest high rise in Panama is almost 300m high, so the wave lengths might not be that long, but at least 100+m.
Why do they look so “wrinkly” and not like proper breakers? When waves are in water that is shallow compared to their wave length (so say water depth would be less than 50m for these waves if we assume they are 100m long, which I think are both reasonable estimates), their shape changes from the normal sine-shape that they would show in deep water, to steep crests and loooong troughs. You might have observed waves with this shape for example in the very shallow waters of a beach on the wadden sea coast or any other beach with a really small slope, where waves look like sausages or pool noodles that are being shoved onto the beach (compare for example to pictures in this post).
What makes me confident that we are really seeing what I’ve just described above? Mainly that I can see the interaction of the waves with the sea floor. If you look at the pic above, do you see the area where the waves bend? That’s where the water is shallower. I’ve tried to sketch that below: The red lines are — in first approximation — the wave crests (I’ve only drawn in every third or so for clarity). Red dashed lines are kinda the second approximation of the wave crests: Those are the deformations that I want to talk about. And those deformations are caused by a shallower area, which I’ve drawn in with the green dashed line. This little submerged headland slows that part of the waves down that runs above it (because in shallower water the wave’s speed only depends on water depth, not on wave length any more), but not the rest of the waves that propagate towards the beach with the straight crests intact.
It’s even easier to be confident when we look at the next two pictures that Christina shared with me. Now we are a little closer to the beach and can see the area where the waves break and where it is shallow enough that the wave lengths drastically decrease (since the waves are slowed town more and more the closer they come to the beach, waves that are further out are still faster and can catch up to waves in front of them). This is very typical for the parts of a beach where the depth changes rapidly.
Picture by Christina Oettmeier @sulfurium
And on the next pic, we see even more clearly that the waves change from pool-noodle shaped offshore to breaking waves close to the beach:
Picture by Christina Oettmeier @sulfurium
In case you don’t see what I am trying to point out, here an annotated version of the pic above. Green dashed circles: Smudges on the window, or possibly reflections on the window, but nothing to do with the waves. Red circles: Here we see foam on the back side of breaking waves, so there was definitely some wave breaking going on here. And blue circle: Cool structures in the flow of water that is retracting downslope from the beach, back into the ocean.
So much for now. No idea if that made any sense to anyone except myself. Please let me know! :-)
Do you like observing wave fields and pondering what might have caused them? Why they look the way they do? What caused them? What they can tell us about the wind, the sea floor, the trajectory of a ship?
This is actually not an easy task and takes a lot of practice and a good understanding of the physics involved. If we observe the waves in real life, we can observe the wave field developing over time, turn our heads or walk a little to see what’s happening around that corner, feel the wind or hear the ship. But it might still be difficult to figure out what is going on.
If you really want to know what is going on, though, or want to send me a little friendly challenge to see if I can figure something out that you already have the solution to, you might enjoy to send me a #friendlywaves. #friendlywaves means: You take a picture of waves, send it to me via email or social media, and I write a blogpost, explaining as much as I possibly can related to the wave field. And you get to tell me if I got it right, because you were there and I wasn’t!
Astrid, #wavewatching supporter from Day 1, sent me these pictures for a #friendlywaves post. Today, I want to start with a spoiler picture (or, rather, I did start with a spoiler picture already — see above) that shows you the setting at low tide to help us explain the wave pattern that we then observe at high(er) tide.
Note the headland in the picture above? Below shows what it looks like when it is covered in water:
Astrid, as a real #wavewatching pro, also sent me a video, so I can show you the super cool interference happening here.
Wave crests from far offshore (probably caused by a storm somewhere far away) arrive in shallower water and get broken up into parts on either side of the (now submerged) headland. But on either side, the wave crests also change their shape, being refracted towards the headland. And some of the wave crests make it over the headland, now at an angle to each other, meeting waves from the other side. And where they meet, they steepen up and even break occasionally. Doesn’t it look super cool to watch waves run towards each other in such a way, creating these interference pattern?
This wave pattern always reminds me of one that I saw years ago — coincidentally with Astrid! — when we were in Iceland in 2013, the day after my dad’s heart surgery. And while watching those waves then was beautiful and calming, seeing this pattern still always reminds me of a pretty traumatic time. So I am happy that this new wave pattern will now at least partially overwrite some of those memories with a very happy day: Herzlichen Glückwunsch und alles alles Gute, liebe Simone* & family!
*That is Astrid’s friend Simone, not my own sister Simone, although of course alles Gute to her, too :-)
I love #friendlywaves! Victor sent me the picture above. He took it in 2017 in Tampa, Florida, and I think it’s so fascinating! There is so much going on, let’s try to make sense of it!
First, the most obvious thing making waves here: The two boats. Clearly they are making waves, and they might explain a lot of what we see here. But on the other hand, they might not.
Below, you see the part of the wave field that is 100% due to these two ships: Their V-shaped wakes (in red) and the turbulent wake behind one of the ships (in yellow).
The very prominent wave pattern (marked in red in the image below) might be due to these two ships as was suggested to me, but if it is, then those ships changed course quite drastically before they created the waves I marked in the previous picture (and I can see no evidence of such a change of course, usually a turn would leave a trace similar to this one).
If the boats, as I assume they did, came out from underneath the bridge and sailed in a more or less straight line (and that seems to be the case judging from their wakes as indicated in the picture above), there is no way they could have made waves that travel in front of their V-shaped wake. Similarly to how you can’t hear the supersonic aircraft before the supersonic boom (because the sound can’t travel faster than the speed of sound and the pressure signal thus gets formed into the Mach cone), waves can’t outrun their wake (which is like their 2D Mach cone). So I don’t believe that those waves were made by those two ships. Rather, I believe that they were made by a ship that is no longer visible in the area we are able to see.
So remember, this is the wave pattern we are trying to explain (Marked is only one wave crest, but you see that there are several parallel to the marked one):
We do nicely see how the wave is reflected by the straight sea walls. But what direction is it traveling in? And what caused it? Let’s speculate!
First: let’s consider the very weird shape of the body of water shown in the picture. Quick search for Tampa on Google Maps lets me believe is that the picture was taken more or less from the position of the white star and the view is the area between the two red lines. Looking at that map, we see that the water we see opens up into four different water ways: One to the north, one to the east, one to the south east, and one to the south west. The two to the south eventually open up into Tampa Bay.
The wave field that we are trying to explain would look somewhat similar to what I drew in below (green):
My best explanation of that green wave field above is this: A boat that went on the course that I drew in in yellow:
So far, so good. Wanna know why I believe this is what happened? Then this is the picture for you!
Assuming the boat followed along the yellow track, the other lines are the wake it would have produced:
green: Those are two parts of the wave field that I marked above that I am fairly confident of: The wake propagated across the body of water, got reflected and came then over towards the photographer. Note how not all waves reach the shoreline close to the photographer yet? That’s because they are the “newer” waves that haven’t traveled for long enough to reach that spot
light blue: The “newest” waves that aren’t very long yet and are traveling in an area where we can’t clearly make out the presence or absence, let alone direction, of waves. They are fanning away from the “green waves” because the ship is turning (similar to here).
dark blue: Those is a part from the wake that originated on the other side of the ship, got reflected, and now traveled across the body of water to reach the point where the picture was taken from. They do so at an angle that looks like they might be reflections of the incoming green waves (which is another possibility which I can’t rule out with 100% certainty). Newer wakes from that side, once they’ve been reflected on the shore, will lead to waves almost parallel to the green part of the wake and would be indistinguishable from those in the picture.
orange: Those are “old” wakes that must have happened when the ship came out of that inlet, but that would not interfere with our picture because their reflection stays caught within the inlet itself.
This is the best explanation of what must have happened that I can come up with, and I have thought about this quite some time (more on that at the end of this post) :-)
But then there are tons of shorter wave length waves that we have to explain, too: See those marked in red, yellow and green below.
I am confident that the ones I marked in red are wind-driven waves coming across the open area. Their direction also agrees quite well with the wind directions the flags indicate (marked with a white arrow above). I believe that the ones I marked in yellow and in green are two separate wave fields at a slight angle, but that might be an optical illusion, I am not quite sure.
If we go back to the map, I believe the wave fields I marked above would look pretty similar to the ones I drew in below (I changed the red waves above to magenta waves below, because red was already taken. Note the wind direction marked with a white arrow: it looks pretty much perpendicular to the now-magenta wave crests):
And looking at the angles in that depiction of the waves, I could imagine that the green wave field is a reflection of the magenta wave field where that one hits the shore on the side where the picture was taken from (see light blue wave crests). As for the yellow one: I still have no idea what caused that. But maybe there need to be some mysteries left to life? ;-)
To end on something that I am confident in: The half circles near the bottom of the picture are the result of something (two buoys? two small boats?) moored on that pier, bobbing up and down in the waves, thus radiating wave rings with shorter wavelengths and higher frequency than the wave that is exciting the movement.
But after all this hard work (more on that at the bottom of this post) — let’s take a minute and look at those beautiful interference pattern again where the wave fields cross each other and create a checkerboard pattern. How amazing is this?
Phew! I love #friendlywaves, but this was quite a challenge! How did I do, Victor? :-)
If you or anyone else have any comments or suggestions — I would love to chat about alternative explanations!
P.S.: Just to give you an idea of what my process was like: It involved late night scribbles on a tea bag (because that was the best “paper” I had available on my bedside table in the hotel in Manchester) and I needed to play scenarios through in my head…
…and some sketches on my phone while I was on a train…
This is a #friendlywaves challenge, where I try to explain other people’s wave photos and they tell me how I did.
I love it when my friends see waves, think of me, whip out their cameras, take pictures, and send them to me! In this case, Nena even used a telephoto lens and took the amazing pictures below that she allowed me to share with you!
They are the perfect example for talking about wakes when a ship doesn’t just go straight ahead. Because, of cause, ships going straight ahead are the easiest case, like the one we see below.
Picture by Nena Weiler, used with permission
Here, we see the two different constituents of a wake: The turbulent wake that is the white stripe right behind the boat, that turns blue a little way behind the boat but stays a lighter color than the surrounding water.
And then there is the V-shaped wake with the boat at its tip. This V-shaped wake consists of very many individual waves that are fairly short in the direction parallel to their crests, and that are shifted slightly so the further away from the boat you look, the wider the V opens. I usually call this the “feathery” wake, since it consists of all these little “feathers”, but since I need the “feather” image for something else today, I’ll just call it the V-shaped wake here.
Now when the boat takes a turn, this messes up the structures of the waves making up the V-shaped wake (or makes them more interesting, depending on your point of view). Below, the boat has taken a right turn, which you can see from the turbulent wake that starts right behind the boat as a white stripe that then changes color to a lighter blue than the surrounding water (with a darker stripe to each side, and then the V further out).
Picture by Nena Weiler, used with permission
Now looking at the individual waves of the V-shaped wake, we see that they get bunched up on the right side of the boat’s trajectory, while they are getting fanned out on the left side.
Now imagine the boat’s trajectory as the shaft of a feather. If you have ever bent a feather, you will have observed that on the side the shaft is bent towards, the individual barbs (I looked this up: barbs are the little thingies that spread outwards from the feather’s shaft) get bunched together, while on the other side they fan open.
So far, so good. Still with me?
Now what happens as time goes on is that the V opens up — the two sides move away from each other. We don’t usually notice this because we are used to focussing on the wake relative to the ship rather than to some fixed vantage point. But if we looked at a fixed point while a ship going past, we’ll see the wake spreading over time until one side of the V reaches us.
Picture by Nena Weiler, used with permission
And this spreading of the V is what’s making interpretation of the picture below a little difficult. The picture below is showing almost the same part of the ocean as the one above (see the little white and blue moored boats in the bottom right corner of the lower picture? They are the same boats that are visible at the left of the bottom right corner above), only a little later. During the time between the two pictures, the ship moved further towards the bottom left corner, but also the wake spread further apart.
Above, you see that some “barbs” start running into each other (the ones where the bend is strongest, where there is foam on breaking waves because the waves suddenly become a lot steeper due to interference). So some time later, they have grown longer and are now crossing each other, which leads to the checkerboard pattern located right inside the bend of the boat’s trajectory. If you follow the V-shaped wake from the boat backwards, you can still make it out, even though it’s been deformed by the ship turning around.
Picture by Nena Weiler, used with permission
Tell me, Nena, is your family happy with this explanation? :-)
My friend Alice (of the awesome Instagram @scied_alice and the equally awesome blog, which you should totally follow) sent me a #friendlywaves from her trip to Cyprus. She said that this was a simple one, so I am looking forward to what else she has up her sleeve once I pass this test ;-)
So here we go with the pictures she send.
Clearly, she is on a boat trip, and she’s looking back at the wake of the ship. You see the one side of the feathery V of the wake, pretty much in the middle of the picture. On the “feather” closest to us, you can still make out the turbulent part of the breaking bow wave, where the water surface looks all crumpled up and not as smooth as it does further away from the ship. Actually, this is a really nice example to show that the waves are traveling away in the wake, but the water is not: All the other “feathers” further away have smooth surfaces as they have run away from the ship’s trajectory, while the turbulent wake traces out the exact path where the ship went (as long as there aren’t any currents moving around the water, which we’ll assume for now).
Picture by Alice Langhans, used with permission
The waves in the V-shaped wake are fairly steep, you can see them very slightly tipping over on occasion.
And Alice sent a second picture: Similar situation, except now it’s a little more windy. The turbulent wake is a little more foam-y than in the previous picture. This could be because the ship is sailing faster, or because it’s more windy. I would guess the first.
And when I say “sailing”, I am using this as the technical term for a ship driving. I am assuming that the boat Alice is on is not a sailboat. I’m thinking this because the wake looks fairly turbulent and sail boats usually don’t cause this much turbulence; also the little bit of the boat that I can see doesn’t really scream sailboat to me. We’ll have to wait to hear what she tells us, though!
Picture by Alice Langhans, used with permission
On both pictures, there is hardly any swell visible. Waves are usually not as visible when the water is deep as when they run up on a beach, and so far off shore we can assume that the water is fairly deep. But that also means that it isn’t very windy, hasn’t been very windy recently, and hasn’t been very windy anywhere near recently, either, so no large waves have traveled into the region.
So much for these #friendlywaves. How did I do, Alice? :-)