Tag Archives: Kiel fjord

Refraction of light in moving water — why stuff seems to be jumping around

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I was waking along Kiel fjord one morning and noticed a stone “jump” on the ground as waves went over it (and actually, that observation was the motivation to dive into stuff from the last post, too).

I think the stone only looked so curious because the rest of the ground was uniformly sandy and hence didn’t seem to move.

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So seeing that jumping stone made me want to draw the optical path, which I’ve animated for you here:

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Funny. I think in physics class in school, I would absolutely have hated it had I gotten the task to draw all those different diagrams, and here I really enjoyed it. Maybe because of that jumping stone? Would the right motivation have helped me as a kid to get interested in this? I think it wasn’t that I was not interested in physics, but it would never have occurred to me to sit down on my own to sketch optical paths or anything like that. Now if I could figure out what changed for me, maybe we could use that to make other people interested in physics, too?

Those foam stripes parallel to the coast — again!

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I think I might be getting obsessed with those stripes parallel to the coast. We saw them as foam stripes, eel grass stripes and now today: leaf stripes!

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Or should it be leaves stripes instead of leaf stripes?

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Interestingly enough, that day there wasn’t just one stripe, but in some places there were even two. It’s a little difficult to see in the pictures, but it was very clear in person.

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See?

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A little further downwind foam also appeared, but only inshore of the innermost leaf stripe.

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And then a little further downwind, several parallel foam stripes appeared. Now this I could imagine being Langmuir circulation. And all the other stripes must be on individual convergence zones, too?

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Someone should hire a PhD student to figure this out, it is really bugging me that there is a phenomenon that we can observe pretty much every time we look at Kiel fjord, yet I can’t find anything on what is going on there.

Luckily, the day I took those pictures, my famous oceanographer friend J was with me, and it was bugging her almost as much as it was bugging me :-) We decided the most likely explanation was that someone had pulled all those leaves on thin strings and put them out in the water just to see whether someone would notice…

Interference of one wave field with its reflection and a second wave field

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All you regular readers of my book and my blog surely recognize what’s going on below?

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Yes! A wave field comes in at an angle to the pier and gets reflected, leading to a chequered pattern. And a second wave field comes in with wave crests pretty much parallel to the pier, adding a little more interest to the pattern.

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I love watching these kinds of waves! But it is really difficult to take good pictures. Sorry about the over-exposed background…

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But it is so beautiful!

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Wave-watching

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Do you know the feeling when you, even on the most beautiful of days, want to get out of the pretty parks as quickly as possible so you can finally see the water?

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Especially when it’s foggy?

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And it is so worth it, there is always something to see. For example on that day: what a nice field of shallow water waves!

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And what an awesome criss-crossing of waves being reflected on the sea wall on which I was standing when taking that picture.

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And how sad that this lake was frozen over! :-)

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P.S.: Still looking for a christmas present for your nerdy friend, your niece, anyone who should spend more time looking at water? Check out my book :-)

Wavelength dependency of wave-object-interactions

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Wavelength dependency of wave-object-interactions. What a title! :-) But that is exactly what I observed over the enormous timespan of three full minutes (as shown by the time stamps of my camera) when I went strolling along Kiel Fjord one Saturday morning.

First, I saw this old, overgrown tyre in a wave field that was dominated by small wind waves. We clearly see how they are diffracted around the tyre and how there is a nice interference pattern downwind of the tyre (to the left in the picture below).

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Tyre in the Kiel Fjord and diffraction of small wind waves around it, leading to a beautiful interference pattern to the left of the tyre

Also I quite like how there are absolutely no waves inside the tire, where the wind is shaded off by the tyre and the stuff growing on it.

Then, a really short while later, the wave field was dominated by longer waves running in from the distance. Below, we still see remnants of the old interference pattern to the left of the tyre, but also how the longer waves run around it. In the picture below, the wave crest that was broken up by the tyre is about to rejoin.

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And then, only an instant later, this is what the wave field looked like. Hardly and ripples caused by local wind, but many short waves. No real interaction between tyre and waves visible any more.

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Isn’t that fascinating? And it all happened within three minutes! :-)

P.S.: Still looking for a christmas present for your nephew, your friends’ kid, your geeky friend? Check out my book! :-)

Reflections on reflections

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When we think about reflections in water, we usually think of calm lakes and trees on the shore opposite to us. Or clouds. Or at least that’s what I think of: Everything is so far away, that it seems to be reflected at an axis that is a horizontal line far away from us.

Then the other day I walked along Kiel Fjord and it hit me that I had never actually consciously observed reflection of things that are located close to my position, and especially things who are not pretty much equidistant to me, but where one end is a lot closer than another one. Consider the picture below: Do you notice something that looks kinda odd to you (while at the same time looking super familiar)?

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If you are wondering what I mean, I marked it in red in the picture below: The rope and its reflection! It’s embarrassing to say that (as someone who has been sailing A LOT since the age of 7) this was the first time I really noticed, but it struck me how the maximum of the parable of the reflected rope isn’t right below the minimum of the parable of the rope, but seems shifted to the left. Of course this is exactly how it should be if we think about the optics, but I was really shocked that I had never noticed before and never thought about it before! I bet if I had had to draw the reflection I would have done it wrong and probably not even noticed…

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Here is another picture to show you what I mean. This is what it looks like:

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Below I’ve drawn in the original objects in blue, the axis of reflection in red and then the reflection in green:

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So far, so good, everything looking the way it’s supposed to look. Right? Then look at the picture below:
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Sorry if this seems obvious to you, but I’m fascinated with this right now :-)

But it leads to another interesting thought: Asking people to draw stuff in order to both check their understanding and also make them reflect on their understanding. I recently had the opportunity to observe a class of master students draw the SST of the mean state of the Pacific Ocean (which was an exercise that I had suggested in connection with a class on El Nino. I thought it would be neat to have them draw the mean state and then later the anomalies of El Nino and La Nina to activate prior knowledge) and it was surprising how difficult that was even though I’m sure they would all have claimed to know what the mean state looks like. Having to draw stuff really confronts us with how sure we are of things we just assumed we knew…

And then I’m pretty sure that once we’ve drawn something that we have constructed ourselves from what we knew (rather than just copied a drawing from the blackboard or a book, although I think that also helps a lot), we are a lot less likely to forget it again.

Anyway, this is a type of exercise I will use — and recommend — a lot more in the future!

Surface films dampen waves

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On Tuesday I talked about sea weed accumulating close to the sea wall in Kiel fjord, and I showed you the picture below.

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One thing that I found really interesting about that is how the floating sea weed dampens out all the waves. You can see it in the image above, bit it might be easier to see in the picture below: Can you see how there are a lot of ripples near the top of the image and none close to the sea wall at the bottom of the image?

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It’s not quite a surface film, yet its impact is the same.

Foam stripes parallel to the coast, take 2

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I recently got a new comment on my blog post on foam stripes parallel to the coast, and since you guys hardly ever comment on my blog (I like getting your emails! Really! But why not comment on here? ;-)) it spiked my interest enough to look out for more foam stripes. So about a month ago, I saw this in Kiel: Yes! A foam stripe parallel to the coast!

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I’m actually pretty sure that they are there most of the time, at least when there is some wave action going on, but I just never noticed since they are so close to the sea wall and it’s easy to just look out over the fjord and never look down.

But again, as much as I tried, I could not see how the position of the stripe related to the wave field.

But now that I was intrigued, I went back the next morning to take a look. A lot less wind than the night before, and shorter waves. And what do we see?

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At first glance, there is no foam stripe, but instead there is a stripe where floating sea weed accumulated (indication of a convergence zone? Can you see it? Sorry about the bad picture).

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But then in other spots, there is a little bit of foam, too, where the sea weed accumulated. And this time I could actually see where it came from: That is the area where most of the wave breaking happens when reflected waves meet incoming waves. Mind, though, I could not observe that on previous occasions!

Plus, waves break when they meet the sea wall, and that creates more foam that sits between the foam stripe and the sea wall.

Since at that point I was really intrigued, I went back around lunch time the same day. And what do we see?

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Foam stripes are gone, but there is a lot more sea weed now! And all confined to a narrow stripe along the coast.

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Actually,  if we look really carefully, we can see that there still is a foam stripe parallel to the coast, but very very close to the sea wall now.

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And looking down the other direction, there are even two stripes with sea weed, and only the one closer to the coast also has foam on it. It gets weirder and weirder :-)

Anyway. I guess what we need for foam stripes is enough choppy wave action that waves break (waves alone are not enough as you can see in this post on standing waves which happens to talk about the exact same spot), because if waves weren’t breaking, where would the foam come from? Although sea weed could still accumulate, I guess?

I will investigate further. In the meanwhile, does anyone have any more ideas of what is going on? Do you now see those foam stripes everywhere, too? :-)

P.S.: Kiel peeps, btw, you probably know exactly when I took those pictures, since there is the Sweden ferry coming in on one and then the Norway ferry going out on the other… :-)